Monday 29 August
Time Amphithéâtre Salle Bellecour 1,2,3 Salle Prestige Gratte Ciel Salle Gratte Ciel 1&2 Salle Tête d'or 1&2 Salon Tête d'Or Salle Gratte Ciel 3 Exhibition Hall
08:00
08:00-09:00
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OC
Opening Ceremony

Opening Ceremony

08:00 - 08:10 EMC 2016 President. Thierry EPICIER (Keynote Speaker, LYON, France)
08:05 - 08:10 EMC 2016 Vice-President. Pascale BAYLE-GUILLEMAUD (Keynote Speaker, GRENOBLE, France)
08:10 - 08:20 In the name of Laurent Wauquiez, President of Région Auvergne Rhône Alpes. Nora BERRA (Keynote Speaker, France)
Former ministry
08:20 - 08:30 UDL President. Khaled BOUABDALLAH (Keynote Speaker, France)
08:30 - 08:35 EMS President. Roger A. WEPF (Keynote Speaker, Zürich, Switzerland)
08:35 - 08:40 Sfμ President. Guy SCHOEHN (Keynote Speaker, Grenoble, France)
08:40 - 08:50 For the Institut Lumière. Philippe OUDOT (Keynote Speaker, lyon, France)
08:50 - 09:00 Honorary Fellowship of the RMS Award. Peter NELLIST (Professor of Materials) (Keynote Speaker, Oxford, United Kingdom)
RMS President

09:00-10:00
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PL2
Plenary Lecture 2

Plenary Lecture 2

09:00 - 10:00 Plenary Lecture 2. Eric BETZIG (Plenary Speaker, USA)

10:30-12:30
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IM2-I
IM2: Micro-Nano Lab and dynamic microscopy
SLOT I

IM2: Micro-Nano Lab and dynamic microscopy
SLOT I

Chairpersons: Francisco José CADETE SANTOS AIRES (Chairperson, VILLEURBANNE CEDEX, France), Niels DE JONGE (Chairperson, Saarbrücken, Germany), Gerhard DEHM (Chairperson, Düsseldorf, Germany)
10:30 - 11:00 Following nanomaterial dynamics in their formation and application media with liquid-cell transmission electron microscopy. Damien ALLOYEAU (CNRS scientist) (Invited speaker, LMPQ, Paris Diderot, France)
Invited - Last minute change
11:00 - 11:15 #4756 - IM02-OP054 Oxidation of Carbon Nanotubes Using Environmental TEM and the Influence of the Imaging Electron Beam.
Oxidation of Carbon Nanotubes Using Environmental TEM and the Influence of the Imaging Electron Beam.

Carbon nanotubes (CNTs) can be used as field emission electron sources in X-ray tubes for medical applications [1, 2]. In a laboratory setting, field emission measurements of CNTs are usually carried out in an ultra-high vacuum system with base pressure of about 1E-7 mbar or better. Under less stringent vacuum conditions, CNTs are found to exhibit lower emission currents and reduced lifetimes [3, 4].

 

Here, we report the direct study on the structural changes in CNTs as we heated and oxidized them in situ using an aberration-corrected environmental TEM [5].  We established a protocol whereby heating and oxidation were performed without an imaging beam and changes on identifiable nanotubes were documented after purging the gas from the chamber, to ensure that they were due to the effect of gaseous oxygen molecules on the nanotubes, rather than the ionized gas species [5].  Contrary to earlier reports that CNT oxidation initiates at the end of the tube and proceeds along its length, our findings show that only the outside graphene layer is being removed and, on occasion, the interior inner wall is oxidized, presumably due to oxygen infiltrating into the hollow nanotube through an open end or breaks in the tube [5].  The CNT caps are not observed to oxidize preferentially [5, 6].

 

In the environment of an ETEM, interaction between fast electrons and gas leads to ionization of gas molecules and increased reactivity. It is very important to evaluate the results to determine or ameliorate the influence of the imaging electron beam. We found that there is a two orders of magnitude difference in the cumulative electron doses required to damage carbon nanotubes from 80 keV electron beam irradiation in gas versus in high vacuum [7]. We anticipate that experimental conditions that delineate the influence of the imaging electron beam can be established, which will enable us to study the CNT field emission process in situ in an ETEM.

 

[1]. G. Cao et al., Med. Phys. 37 (2010), pp. 5306–5312.

[2] X. Qian et al., Med. Phys. 39 (2012), pp. 2090–2099.

[3] K. A. Dean and B. R. Chalamala, Appl. Phys. Lett. 75 (1999), pp. 3017–3019.

[4] J.-M. Bonard, et al., Ultramicroscopy 73 (1998), pp. 7–15.

[5] A. L. Koh et al., ACS Nano 7(3) (2013), pp. 2566–2572.

[6] R. Sinclair et al., Advanced Engineering Materials 16(5) (2014), pp. 476-481.

[7] A. L. Koh et al., Nano Lett. 16(2) (2016), pp. 856-863.

[8] The authors acknowledge funding from the National Cancer Institute grants CCNE U54CA-119343 (O.Z.), R01CA134598 (O.Z.), CCNE-T U54CA151459-02 (R.S.) and CCNE-TD #11U54CA199075. (R.S.)   Part of this work was performed at the Stanford Nano Shared Facilities.


Ai Leen KOH (Stanford, USA), Emily GIDCUMB, Otto ZHOU, Robert SINCLAIR
11:15 - 11:30 #5548 - IM02-OP057 Nucleation of Graphene and its Conversion to Single Walled Carbon Nanotube revealed.
Nucleation of Graphene and its Conversion to Single Walled Carbon Nanotube revealed.

During catalytic chemical vapor deposition, the chirality of single wall carbon nanotubes is determined when the growing graphene nucleus wraps around the catalyst and converts into a tubular structure. Elucidating this critical process is required to develop deterministic bottom-up strategies aiming at better chiral distribution control. Direct observations of carbon nanotube growth, and theoretical modeling and simulations of the nucleation have been published but experimental atomic-resolution evidence of single-walled carbon nanotube nucleation has, until now, eluded us.

The main obstacle is that nucleation involves a few atoms only and a short time scale, thus requiring a combination of high spatial and temporal resolution for direct observation. Here, we overcome the temporal resolution constraint by reducing the growth rate in order to match the temporal resolution of our recording medium. We employ an environmental scanning transmission electron (ESTEM), equipped with an image corrector and a digital video recording system, to follow SWCNT growth using Co-Mo/MgO catalyst and acetylene (C2H2) as a carbon source (see Methods). We present atomic-resolution movies that reveal the nucleation of graphene on cobalt carbide nanoparticles followed by its transformation to a single-walled carbon nanotube. We find that the surface termination of the faceted catalyst nanoparticles regulates the nucleation of the graphene sheet and its conversion into a nanotube. Additional density functional theory calculations show that the disparity in adhesion energies for graphene to different catalyst surfaces is critical for nanotube formation: strong work of adhesion provides anchoring planes for the tube rim to attach, while weak work of adhesion promotes the lift-off of the nanotube cap (Fig. 1). [1]

[1] Nucleation of Graphene and Its Conversion to Single-Walled Carbon Nanotubes. Nano Letters. 2014, 14, 6104−6108


Matthieu PICHER (Strasbourg), Ann Lin PIN, Jose L. Gomez BALLESTEROS, Perla BALBUENA, Renu SHARMA
11:30 - 11:45 #6776 - IM02-OP074 In-situ TEM growth of single-layer boron nitride dome-shaped nanostructures catalysed by iron clusters.
In-situ TEM growth of single-layer boron nitride dome-shaped nanostructures catalysed by iron clusters.

We report on the growth and formation of single-layer boron nitride dome-shaped nanostructures mediated by small iron clusters located on flakes of hexagonal boron nitride. The nanostructures were synthesized in situ at high temperature inside a transmission electron microscope while the e-beam was blanked (Figure 1). The formation process, typically originating at defective step-edges on the boron nitride support, was investigated using a combination of transmission electron microscopy, electron energy loss spectroscopy and computational modelling. The h-BN dome-shaped nanostructure of Figure 1 was used to simulate images of BN protrusions at various angles relative to the incident electron beam, by adjusting effectively the beam direction. Figures 2 presents simulated images for beam angles of 0° (2a), 30° (2b,c) and 50° (2d), respectively, relative to the h-BN plane normal, in comparison with experimentally observed features (Figures 2e-h). The image simulations are in striking agreement with the experimental images, consistent with the circular features being protrusions formed normal to the h-BN plane, whilst the hemispheres correspond to protrusions tilted with respect to the h-BN plane.  Computational modelling showed that the domes exhibit a nanotube-like structure with flat circular caps (Figure 3) and that their stability was comparable to that of a single boron nitride layer.

       Nanostructured carbon protrusions have been studied since 2001 [1-3], but the investigation of analogous BN structures has only just begun. In the present study, we have shown that even member rings are required for the formation of h-BN dome-shaped protrusions, but not in the form of active linear defects, containing B-B and N-N bonds, as observed recently in BN monolayers under electron beam irradiation [4]. Furthermore, according to our molecular simulations result the even members rings present in the half dome structure present B-B and N-N bonds (Figure 4). The BN dome-shaped nanostructures represent a new material that perhaps by hosting metal atoms may unveil new optical, magnetic, electronic or catalytic properties, emerging from confinement effects.

[1] Sharma, R.; et al. Journal of Electron Microscopy 2005, 54, 231-237.

[2] Chamberlain, T. W.; et al. Nature Chemistry 2011, 3, 732-737

[3] Nasibulin, A. G.; et al. Nature Nanotechnolgy2007, 2, 156-161.

[4] Cretu, O.; et al Nanoletters2014, 14, 1064-1068. 


A. LA TORRE (Nottingham, United Kingdom), E. H. ÅHLGREN, M. W. FAY, F. BEN ROMDHANE, S. T. SKOWRON, A. J. DAVIES, C. PARMENTER, J. JOUHANNAUD, A. N. KHLOBYSTOV, G. POURROY, E. BESLEY, P. D. BROWN, F. BANHART
11:45 - 12:00 #6199 - IM02-OP064 Structural changes of Au nanocones during in situ cold-field emission observed by high-resolution TEM.
Structural changes of Au nanocones during in situ cold-field emission observed by high-resolution TEM.

In situ transmission electron microscopy (TEM) allows imaging on the atomic scale of complex physical phenomena, which are induced by an externally applied stimuli. This provides a directly observed correlation between material structure and properties, which promotes the understanding of the material and the triggered phenomena. Here, a Nanofactory in situ TEM biasing holder with a nanomanipulator has been used to both manipulate Au nanostructures and also to enable the studies of electron cold-field emission (CFE).

The conical shaped Au nanostructures are produced by hole-mask colloidal lithography on an electron-transparent carbon film in macroscopic short-range-ordered arrays [1]. Individual nanocones typically feature a tip radius of around 5 nm and a height of around 180 nm. The entire macro-array is then transferred to a mechanically cut Au-wire that subsequently was inserted into the in situ TEM holder (Fig. 1).

The nanomanipulator of the TEM holder can move in 3D, with both coarse and fine motion. The coarse control utilizes a slip-stick mechanism for a mm-ranged motion. The piezo-driven fine control has a range of 10 μm and a resolution on the sub-Å level. The Au nanocones in Fig. 1 were transferred to the nanomanipulator, making the configuration seen in Fig. 2. The nanomanipulator was thereafter positioned opposite a nanocone that was in direct contact with the Au-wire (Fig. 2). During the experiment, the electrical potential between the two cones was increased until the electric field around the cathode nanocone was sufficiently high (several volts per nm) to initiate CFE.

Earlier work using a similar TEM holder reports about in situ CFE experiments using carbon-based nanotips over a distance of a few hundreds of nanometer [2-4]. Here, the distance between the two Au nanocones is around 20 nm, allowing for simultaneous imaging at high resolution of both cones during CFE. This allows a better understanding of the CFE process and the effects of electron bombardment. 

At 115 V applied voltage with a CFE current, ie, of 4 μA, structural changes of the anode Au nanocone were observed. The change in structure started with a faceting at the apex of the anode nanocone. At the same time, the anode nanocone material was redistributed forming an elongated structure, making the anode nanocone thinner over a region that stretched over more than 30 nm from the tip towards the base. The elongation was a multi-stage process, taking about 5 s to complete. See images in Figs. 3a and 3b, which are separated by 0.4 s.

The electron bombardment current was kept in the μA-range and resulted in an amorphization of the outmost atomic layers of the anode apex around 7 s after the structural changes of the nanocone had occurred.

During these events, no structural changes were observed on the cathode. This indicates that the structural changes to the anode Au nanocone is an effect of electron bombardment by the emitted and accelerated electrons.

[1]      H. Fredriksson, Y. Alaverdyan, A. Dmitriev, C. Langhammer, D. S. Sutherland, M. Zäch, and B. Kasemo, Adv. Mater. 19, 4297 (2007).

[2]      L. de Knoop, F. Houdellier, C. Gatel, A. Masseboeuf, M. Monthioux, and M. J. Hÿtch, Micron 63, 2 (2014).

[3]      L. de Knoop, C. Gatel, F. Houdellier, M. Monthioux, A. Masseboeuf, E. Snoeck, and M. J. Hÿtch, Applied Physics Letters 106, 263101 (2015).

[4]      F. Houdellier, L. de Knoop, C. Gatel, A. Masseboeuf, S. Mamishin, Y. Taniguchi, M. Delmas, M. Monthioux, M. J. Hÿtch, and E. Snoeck, Ultramicroscopy 151, 107 (2015).


Ludvig DE KNOOP (Gothenburg, Sweden), Norvik VOSKANIAN, Andrew YANKOVICH, Kristof LODEWIJKS, Alexandre DMITRIEV, Eva OLSSON
12:00 - 12:15 #6859 - IM02-OP077 Shape transformations during the growth of gold nanostructures.
Shape transformations during the growth of gold nanostructures.

Liquid cell transmission electron microscopy (LCTEM) has rapidly emerged as a potent tool for understanding the dynamical processes taking place at solid / liquid interfaces. Imaging colloidal solutions with the high temporal and spatial resolutions of TEM enables understanding the growth mechanisms that control the final size and morphology of nanoparticles. Nevertheless, conclusive LCTEM experiments require understanding the effects of electron-irradiation on the nanoscale phenomena under study. Radiolytic syntheses driven by the electron beam were performed in this work to study the effects of the dose history (including the instantaneous dose rate and the cumulative dose) and the solvent nature on the shape of gold nanoparticles. The straightforward control over the concentration of reducing agents (radiolytically-produced hydrated or solvated electrons) provides mechanistic insights on the growth of highly desired nanocrystal shapes for plasmonic applications.

 

Liquid cell experiments were carried out on an aberration corrected JEOL ARM 200F operated at 200KV, by using a commercial liquid-cell holder provided by Protochips Inc. 1mM HAuCl4 in water or methanol was analyzed in a 150 nm-spacer liquid cell. Growth experiments were conducted under two extreme regimes of dose rate (over 150 electrons/Å2s and below 1 electrons/Å2s) in both TEM and STEM modes.     

 

Under high dose rate we observed the growth of dendritic nanostructures (Fig. 1a). By comparing LCTEM observations with an extended diffusion-limited aggregation model (Fig. 1b), we explicitly reveal the molecular and atomic diffusion processes that impact the shape of these dendritic nanostructures.[1] Besides the well-established link between the dose rate and the growth speed of the nanostructures,[2,3] we have demonstrated that the cumulative dose in the irradiated area can also induce drastic transitions in the growth mode of the nanostructures. For instance, high dose rate observation severely affects the concentration of precursors in and around the irradiated area, resulting in the formation of anisotropic tree-like structures over spherical nanoparticles (Fig. 1a).

 

Under low dose rate, reaction-limited growth leads to the formation of highly facetted nanoclusters. The growth is then dominated by thermodynamic effects, because the lower adsorption rate of gold atoms provides enough time for the clusters to reach an equilibrium shape that depend on intrinsic and extrinsic parameters.[3] We show that crystal defects (intrinsic parameter) or the nature of the solvent that modulates the surface energies of crystal facets (extrinsic parameters) can both drive shape transformations during the growth of the nanoparticles. Remarkably, we reveal the formation mechanisms of highly symmetric 2D and 3D nanostars enclosed by high-index facets. . These in situ studies could help in designing new seed-mediated methods or capping strategies to fabricate metallic nanostars. 

      

[1] Ahmad et al. Advanced structural and chemical imaging, submitted (2016).

[2] Woehl et al. ACS nano, 10, 8599 (2012).

[3] Alloyeau et al. Nanoletters, 15, 2574 (2015).


Nabeel AHMAD (Paris), Christian RICOLLEAU, Yann LE BOUAR, Damien ALLOYEAU
12:15 - 12:30 #6667 - IM02-OP072 Analysis to reveal dynamical and correlated atomic displacements on gold surfaces depending on various environments.
Analysis to reveal dynamical and correlated atomic displacements on gold surfaces depending on various environments.

     Gold has a wide range of important applications, such as gold nanoparticles (AuNPs) for catalyst and various gold nanostructures for sensing technology. For the applications, it is necessary to understand the chemical reaction on gold surface in actual environments, at atomic resolution, and at high time resolution. Though gold is chemically stable, it is known that the supported AuNPs of the size smaller than about 5 nm exhibit higher catalytic activity. This partially originates from the small curvature of nanoparticle surfaces, so the gold surface structures such as facets, edges and corners could change in certain environments. Here, we analyze in-situ images of the surface of bulk gold with different curvatures that are acquired using spherical aberration (Cs)-corrected environmental transmission electron microscopy (ETEM) to derive dynamical and correlated atomic displacements in various environments.

     TEM characterization is carried out by Cs-corrected Titan ETEM G2 apparatus [1], where the accelerating voltage is 300 kV. Figure 1 shows the (E)TEM images of the gold surface with relatively small curvature in various environments (vacuum, oxygen, hydrogen, and nitrogen). In vacuum, the facets of {100} and {111} and the step edge are seen clearly. In contrast, the gold surface is rough in oxygen (oxygen partial pressure: PO2 = 100 Pa), where the surface gold atoms move continuously. In hydrogen and nitrogen (PH2, PN2 = 100 Pa), the surface is facetted as well as that in vacuum and the gold atoms hardly move on the surface.

     To shed light on the change of the gold surface in oxygen, we further investigated the dynamics of surface gold atoms in oxygen by high resolution in-situ ETEM observation with an advanced image acquiring system (Figure 2). By tracking the individual gold atoms in time-lapse images, we found that gold atoms at the step edge readily migrate on the surface compared to those of the terrace surface. We further found that as the oxygen partial pressure decreases, the gold surface becomes more stable structure. We also investigated the electron irradiation effect behind the dynamical changes of surface structures in gas environments, where the current density of the electron beam is varied from 25 A/cm2 to 0.1 A/cm2. As the current density of the electron beam decreases, the migration of the gold atoms in the surface moderates. The gold surface remains rough even in the very small current density of 0.1 A/cm2. Though the electron beam affects the structural changes of the gold surface in oxygen, the analysis result suggests that the surface of bulk gold could interact with oxygen gas molecules to some extent regardless of the electron beam.

     The dynamic surface structures of metals in actual environments most likely originate from the interaction between gas molecules and metal atoms on the surface. Full understanding of the dynamical behavior of metal surface in various environments crucially important for application to the nanomaterials and nanodevices. To this end, it is useful to detect the behavior of individual metal atoms at higher temporal resolution with high detection efficiency of electrons in atomic resolution ETEM. We have already successful in capturing the extraordinary atomic migration on gold surfaces by advanced in-situ Cs-ETEM. We will show some movies in our presentation that show the dynamics of individual surface gold atoms in various environments by time resolution better than 50 ms.


Reference

[1] S. Takeda, Y. Kuwauchi, H. Yoshida, Ultramicroscopy, 151 (2015) 178.


Ryotaro ASO (Ibaraki, Japan), Yohei OGAWA, Hideto YOSHIDA, Seiji TAKEDA

14:00-16:00
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IM2-II
IM2: Micro-Nano Lab and dynamic microscopy
SLOT II

IM2: Micro-Nano Lab and dynamic microscopy
SLOT II

Chairpersons: Francisco José CADETE SANTOS AIRES (Chairperson, VILLEURBANNE CEDEX, France), Niels DE JONGE (Chairperson, Saarbrücken, Germany), Gerhard DEHM (Chairperson, Düsseldorf, Germany)
14:00 - 14:30 #8381 - IM02-S35 In situ TEM for understanding electrical and thermal transport properties on nano and atomic scales.
In situ TEM for understanding electrical and thermal transport properties on nano and atomic scales.

In situ electron microscopy allows studies of transport of charges and matter in complex structures as well as thermal properties. We can study mechanically and thermally induced changes of charge transport properties using holders designed to enable different stimuli allowing the direct observation and correlation between material structure and properties. The direct correlation between structure and properties on the small scale involving individual interfaces, defects and atoms provides access to new information about which microstructural constituents that are active in determining the material properties on the macro, micro, nano and atomic scale. This talk addresses examples of in situ electrical, mechanical and thermal studies. A few examples are briefly described below.

 

The nanoscale dimensions of semiconducting nanowires (NWs) provide extended strain relaxation capability between lattice-mismatched materials and enable the fabrication of single-NW p-i-n junction solar cells on low-cost substrates. Due to its sub-wavelength dimension, semiconductor NWs can function as optical antennas and exhibit a “self-concentrating” effect that enhances optical absorption. The strain relaxation capability and the enhanced absorption cross section make NWs potential candidates as highly efficient and low-cost solar cells. Due to the effects of  elastic strain applied on the electronic band structure the strain can be used to achieve NW-based photovoltaic devices with new functionality. We have studied the effect of mechanical strain on the electrical resistance of nanowires. Electron energy loss spectroscopy was used to study the effect of strain on the electronic structure with emphasis on the low energy loss interval of 0 to 50 eV. Electron beam induced current measurements were also performed to study the effect of strain on the diffusion length of the charge carriers [1].

 

Heating of a transmission electron microscopy (TEM) specimen can be performed in several parallel modes and this talk will address three types of heating modes and show experimental results from nanostructured materials. One mode is by resistive heating of a ring shaped support in contact with the circumference of the entire TEM sample. An additional mode is by use of a heating wire patterned on the TEM sample where the wire is contacted by leads fed through the TEM sample holder. The third mode is by active Joule heating of the nanostructure of study, such as carbon nanotubes, graphene, or metal nanowires. The purpose of having several parallel modes of heating is to enable the separation of temperature dependence, effects of self-Joule heating, effects of radiative heating and thermal transport. It is also important to be able to extract the three dimensional information about the geometry of the investigated structures [2].

 

 

1.   L. Zeng, T.K. Nordkvist, P. Krogstrup, W. Jäger and E. Olsson, “Mechanical strain induced nonlinearity of the electrical transport properties of individual GaAs nanowires”, in manuscript.

2.   N. Voskanian and E. Olsson, “Heating holder for in situ three dimensional transmission electron microscopy studies”, in manuscript.


Eva OLSSON (Gothenburg, Sweden)
Invited
14:30 - 14:45 #6823 - IM02-OP075 In-Situ Hydration of MgO Nanocrystals to amorphous Mg(OH)2 using Liquid Cell Transmission Electron Microscopy.
In-Situ Hydration of MgO Nanocrystals to amorphous Mg(OH)2 using Liquid Cell Transmission Electron Microscopy.

The hydration reaction of MgO to amorphous Mg(OH)2 is a model hydration reaction and is important to diverse research fields, ranging from catalysis to Earth Sciences. Although the bulk thermodynamics and surface energies of these phases are well studied,[1,2] real time and real space analysis of the reaction at ambient pressure is lacking. In this study, the hydration of MgO nanocrystals is studied at the single particle level, both in real space and in diffraction space using in-situ Transmission Electron Microscopy (TEM) at near-ambient pressure and temperature. Upon exposure to water vapor and the electron beam, the MgO nanocrystals react with H2O and convert to amorphous Mg(OH)2.

 

Real-time recordings of the hydration reaction reveal that the reaction starts at the MgO nanocrystal surface and proceeds inwards at a constant rate while the Mg(OH)2 shell expands outwards. The growth rate is found to be constant throughout the reaction. Furthermore, as the applied dose rate is increased, the growth rate increases accordingly. Possible mechanisms for the beam-promoted transformation are discussed, including the role of defect formation and migration at the interior and at the surface of the MgO nanocrystals, H2O diffusion towards the MgO surface, and the possible influence of beam-generated H2O dissociation products. Assemblies of converting MgO/Mg(OH)2 nanocrystals exhibited a reorganization of the assembly framework due to the solid volume increase (~100%) of each individual nanocrystal.

References

[1] de Leeuw, N.H., Watson, G.W., and Parker, S.C., J. Phys. Chem., 1995, 99 (47), 17 219-17 225

[2] Geysermans, P., Finocchi, F., Goniakowski, J., Hacquart, R., und Jupille, J., Phys. Chem. Chem. Phys., 2009, 11 (13), 2228-2233


Wessel VLUG (Amsterdam, The Netherlands), Oliver PLÜMPER, Michael KANDIANIS, Alfons VAN BLAADEREN, Marijn VAN HUIS
14:45 - 15:00 #6563 - IM02-OP071 In-situ studies of the dendritic yttria precursor nanostructures growth dynamics at elevated temperatures using liquid-cell transmission electron microscopy.
In-situ studies of the dendritic yttria precursor nanostructures growth dynamics at elevated temperatures using liquid-cell transmission electron microscopy.

Yttria, a host for heavy rare earth elements, is an important up-conversion material, able to convert lower energy near-infrared light into higher energy visible light, opening the avenue for a wide spectrum of applications from laser technology, photovoltaics to theranostics [1,2]. The efficient use of yttria in the form of nanoparticles (NPs) is related to the understanding of the nucleation and early growth stage kinetics of yttria precursors, formed by the precipitation from the saturated solutions. In contrast to various analytical methods, where the kinetic data are deduced from large sampled volumes, in-situ transmission electron microscopy (TEM) combined with the specialized liquid cell offers the unique possibility to study the spatial and temporal evolution of NPs one-by-one, facilitating a complete reconstruction of early stage events that are vital for the formation of final products [3].

In-situ TEM experiments were performed by utilizing Jeol JEM 2100 LaB6 TEM operating at 200 kV and liquid cell TEM holder, Protochips Poseidon 300 with a heating capabilities up to 100 °C. The temperature controlled urea precipitation method was used for the synthesis of yttria precursors [4]. Namely, decomposition of urea at elevated temperatures releases precipitating agents (OH- and CO32-) homogeneously into the reaction system, avoiding localized distribution of the reactants, allowing precise control over the nucleation and growth of yttrium precursor, typically Y(OH)(CO3). The prepared solution was placed in a liquid sample enclosure contained in the liquid cell TEM holder. Water layer thickness during the observation was between 150 and 300 nm.

The initial solution was observed at a dose rate of 5000 e-/nm2/s, at room temperature (RT) for 30 minutes. Precipitation was not observed during that period, suggesting that additional chemical species that were created during the radiolysis of water by the incident electron beam did not have significant influence on the nucleation process at RT [5]. The formation rate of NPs increased drastically when the temperatures in the cell were raised above 90 °C. The resulting products were either faceted particles or dendritic nanostructures. While the faceted nanoparticles did not experience significant morphological changes during the observation, this was not true for dendritic nanostructures (Fig. 1). Dendrites first experience rapid growth by developing highly branched, hierarchical structure up to their final size of 50 nm in the first 45 s of the observation. In the second stage, in the period of about 45 s, dendrites undergo rapid fragmentation, resulting in the formation of several spherically shaped particles within the original dendrite volume that were dynamically changing either by the coalescence or Ostwald ripening. Finally, the spherical particles experience a complete dissolution within the observed area, accompanied by the appearance of faceted 150 nm sized NPs in the vicinity of the observation area.

We hypothesize that dendritic structure initially grew by the diffusion limited conditions to the stage when the depletion zone that developed around NPs hindered further growth, followed by coarsening as a result of surface area reduction. The dissolution and formation of NPs with faceted morphology is explained as a combined effect of water and urea decomposition at high temperatures, resulting in increase of [OH-] concentration, destabilizing initially formed particles and promoting a formation of more stable, plausibly Y(OH)3 hexagonal particles [6], as shown in Fig. 2.

 

References:

1 Feldmann, C., et al. (2003). Adv Funct Mater, 13 (7), 511-516.

2 Höppe, H. A. (2009). Angew Chem, Int. Ed., 48, 3572–3582.

3 Ross, F. M. (2015). Science, 350, 350 (6267), aaa9886-9.

4 Qin, H., et al. (2015). Ceramic International, 41, 11598-11604.

5 Schneider, N. M., et al. (2014). J Phys Chem, 118(38), 22373-22382.

6 Huang, S., et al. (2012). Mater. Chem., 22, 16136-16144.


Saso STURM (Ljubljana, Slovenia), Bojan AMBROZIC, Marjan BELE, Nina KOSTEVSEK, Kristina ZUZEK ROZMAN
15:00 - 15:15 #6492 - IM02-OP069 TEM compression of nano-particles in environmental mode and with atomic resolution observations.
TEM compression of nano-particles in environmental mode and with atomic resolution observations.

Characterization of nanomaterials or materials at the nanoscale has drastically increased during the last decades. This increase can be explained by (i) the necessity to obtain materials with nanometer-size grains, for instance nanocomposites, and by (ii) the use of nanoparticles in different fields, for instance lubrication applications. A challenge lies in the in situ microstructural characterization of such materials as it can give access to valuable pieces of information regarding the microstructural changes induced by their use.

 

The availability of dedicated TEM (Transmission Electron Microscopy) holders equipped with force and displacement sensors is of a very high interest to test, in situ, the size-dependent mechanical properties of nanometer-sized objects [1,2]. On crystalline nano-objects, Molecular Dynamics simulations have shown that dislocations nucleate at the surface [3,4]. Therefore, the surface state is of utmost importance in determining the nucleation stresses and types of dislocations. For materials which undergo surface reconstruction or changes in the surface chemistry under vacuum, it is necessary to perform experiments in a controlled environment (i.e. under gas pressure) which reproduces the real one.

 

Recently a Hysitron PI 95 Picoindenter has been installed on a Cs-corrected FEI TITAN ETEM (Environmental TEM) microscope. It opens the possibility of performing in situ compression under gas pressure, with high resolution imaging. We will present in situ tests of cubic CeO2, a multifunctional oxide widely used in catalysis. Nanocubes are compressed along either under vacuum or under air pressure. Introducing oxygen inside the chamber limits or avoids the reduction of CeO2 nanocubes induced more or less rapidly by the electron beam. A comparison of slopes of load-displacement curves obtained under vacuum at different electron doses and under air pressure (see Figure 1) strongly suggests that ceria reduced as Ce2O3 under the effect of an intense electron flux has a smaller Young modulus than unreduced or 'oxidized' ceria. Atomic resolution observations performed during the compression tests reveal the formation of dislocations and stacking faults (see Figure 2). Simulations are planned to further understand the deformation mechanisms as a function of the oxidation state (native, unreduced or oxidized states), as well as their reversibility [5].

 

References

[1] Q. Yu, M. Legros, A.M. Minor, MRS Bulletin 40, 62-70 (2015).

[2] E. Calvié, L. Joly-Pottuz, C. Esnouf, P. Clément, V. Garnier, J. Chevalier, Y. Jorand, A. Malchère, T. Epicier, K. Masenelli-Varlot, J. Eur. Ceram. Soc. 32 2067-2071 (2012).

[3] S. Lee, J. Im, Y. Yoo, E. Bitzek, D. Kiener, G. Richter, B. Kim, S.H. Oh, Nature Communications 5:3033 (2014).

[4] I. Issa, J. Amodéo, J. Réthoré, L. Joly-Pottuz, C. Esnouf, J. Morthomas, M. Perez, J. Chevalier, K. Masenelli-Varlot, Acta Materialia 86, 295-304 (2015).

[5] This work is performed within the framework of the LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). The authors thank the CLYM (Consortium Lyon-Saint Etienne de Microscopie, www.clym.fr) for the access to the microscope and A.K.P. Mann, Z. Wu and S.H. Overbury (ORNL, USA) for having provided the samples.


Thierry EPICIER, Lucile JOLY-POTTUZ (MATEIS / INSA, Lyon), Istvan JENEI, Douglas STAUFFER, Fabrice DASSENOY, Karine MASENELLI-VARLOT
15:15 - 15:30 #6125 - IM02-OP063 Microsecond time- and subnanometer spatial-scale in situ observations of crystallization process in amorphous antimony nanoparticles by the UHVEM newly developed at Osaka University.
Microsecond time- and subnanometer spatial-scale in situ observations of crystallization process in amorphous antimony nanoparticles by the UHVEM newly developed at Osaka University.

Fast in situ observation by TEM is one of useful techniques in researches on phase transitions of nanoparticles. In our previous study, it was evident that amorphous antimony nanoparticles can be crystallized with ease by stimulation from the outside. For example, when lead atoms are vapour-deposited onto amorphous antimony nanoparticles kept at room temperature, crystallization of the amorphous antimony nanoparticles is abruptly induced by an interfacial strain between an antimony nanoparticle and crystalline lead nanoparticles attached. On the other hand, knock-on displacements by high energy electron irradiation also become one of the stimulations for the crystallization of the amorphous nanoparticles. In the present study, electron-irradiation-induced crystallization processes of amorphous antimony nanoparticles have been studied by microsecond time- and subnanometer spatial-scale in situ observations by ultra-high voltage electron microscope developed with JEOL Ltd. at Osaka University recently.

Amorphous antimony nanoparticles supported on thin amorphous carbon substrates were prepared by a vapour-deposition method. Electron irradiation experiments and the simultaneous in situ observations were carried out by JEM-1000EES UHVEM operating at an accelerating voltage of 1 MV and the electron flux of the order of approximately 1024 e m-2 s-1, which was equipped with Gatan K2-IS electron direct detection CMOS camera. The time for one frame was 625 μs.

The figure 1 shows a typical example of migration of interface between an amorphous and crystalline phase during crystallization in an approximately 60 nm-sized nanoparticle as indicated by arrows. As indicated in fig. 1(a), the nucleation site of the crystalline phase is located on the particle surface. At the early stage of the crystallization, the interface has a small curvature as shown in (b) ~ (f), but at the steady state of (g) ~ (j), the interface becomes flat. The velocity of the interface migration is estimated to be approximately 10 μm s-1.

Atomic scale observations by HREM were carried out. The figure 2 shows the snapshots during crystal growth in about 20 nm sized nanoparticle. In fig, 2(a), 2 nm-sized crystalline nucleus appears on the surface of the particle, and the FFT pattern from the particle is in set. Week four spots are recognized as indicated by four arrows in the FFT pattern, and correspond to nucleation of the small crystal. In fig. 2(b), the nucleus grows up to approximately 5 nm in diameter, after that the amorphous nanoparticle is crystallized in the whole nanoparticle. In the FFT pattern, the week four spots change to an obvious net pattern, which is indexed as the [2-21] zone axis pattern of an antimony crystal. In this case of the 20 nm-sized nanoparticle, the velocity of interface migration is estimated to be approximately 20 μm s-1. The velocity of the interface migration depends on the particle size, and it was confirmed that the smaller the particle size is, the faster the velocity is. From the observation, the critical particle size for crystallization all over the nanoparticle is estimated to be approximately 5 nm. A strain on the interface between this crystalline nucleus and the amorphous nanoparticle may induce the crystallization all over the nanoparticle. A schematic illustration of crystallization mechanism in amorphous antimony nanoparticles is shown in the bottom of figure 2. The amorphous nanoparticle has to jump beyond the activation energy for the crystallization. At the early stage of the crystallization, small nucleus fluctuates between an appearance and a disappearance. However, when the size of the nucleus is larger than the critical size for crystallization, the strain energy of interface between this crystalline nucleus and the amorphous nanoparticle will be larger than the activation energy. It is suggested that the strain energy is a trigger for crystallization in amorphous antimony nanoparticles.


Hidehiro YASUDA (Osaka, Japan)
15:30 - 15:45 #6831 - IM02-OP076 Studying the Formation Dynamics of VLS Silicon Nanowire Devices using in situ TEM.
Studying the Formation Dynamics of VLS Silicon Nanowire Devices using in situ TEM.

Control of the electrical properties of Si nanowires, and in particular their connection to the macroscale environment, is important when developing nanowire applications. We therefore use in situ TEM to create suspended Si nanowire devices so that we can correlate the structure and transport properties of the nanowires and their contacts. In an ultra high vacuum TEM, we grow Si nanowires by the vapor-liquid-solid process using AuSi eutectic droplet catalysts and disilane gas. The nanowires grow from one microfabricated heater [1,2] across to a second heater 2-3 micrometers away (Figure 1). Temperature can be controlled in the VLS growth range 450-600oC, and we can control the voltage across the nanowire at the moment of contact, and perform IV measurements on the final nanowire device [3] at room temperature. We have shown that novel nanowire contact geometries such as necked or bulged contacts can be formed [4] by tuning the balance between the Si growth rate and the migration of Au from the contact region. This is achieved by controlling the growth conditions during contact formation.

Here we examine an additional parameter that is even more effective in controlling the contact geometry. This is electromigration, induced by flowing current through the nanowire during contact formation. In Figure 2 we show the effect of current flow (as well as disilane pressure) on the deposition of Si and the volume of AuSi. In Fig 2(a) a TEM image series shows the formation of a 10nm nano-gap by a nanowire (Si NW) connecting to a Si cantilever side wall with an AuSi droplet, and removing the AuSi by using electromigration. In Fig 2 (b, c), the AuSi and deposited Si volumes is plotted along with (b) disilane pressure and (c) current through the wire. In (b) Si is incorporated only at high disilane pressure; when pressure is reduced, the morphology becomes constant. In (c), once a current is flowed through the nanowire, the AuSi starts to shrink at 5400nm3/s due to Au electromigration; as Au moves away, the Si is deposited at 1400nm3/s. The net decrease in volume creates the 10nm gap. Hence the current flow can cause rapid loss of Au from the contact site, forcing a rapid segregation of Si from the AuSi droplet. This we show can controls the contact formation dynamics to create bulged, straight, necked or nanogap contacts [4].

Once contact has been established, the nanowire device can be electrically characterized and further modified, for example by oxidation of the Si surface. We find that nanowires can sustain tens of volts before disconnecting, and exhibit fairly consistent IV characteristics at room temperature, Figure 2(d).

The ability to control the contact structure, and measure its transport properties directly after formation, is helpful in understanding the behaviour of nanowires in processed devices. Electromigration appears to be a useful parameter that allows novel nanowire contact geometries to be created and hence greater flexibility in nanowire device design.

[1] C. Kallesøe et al., Small, vol. 6, 2010, pp. 2058–2064.

[2] K. Molhave et al., Small, vol. 4, Oct. 2008, pp. 1741–1746.

[3] C. Kallesøe et al., Nano Letters, vol. 12, Jun. 2012, pp. 2965–2970.

[4] S.B. Alam et al., Nano Letters, vol. 15, Oct. 2015, pp. 6535–6541.


Sardar B. ALAM, Federico PANCIERA, Ole HANSEN, Frances M ROSS, Kristian MØLHAVE (Lyngby, Denmark)
15:45 - 16:00 #5916 - IM02-OP060 Quantitative measurement of doping and surface charge in a ZnO nanowire using in-situ biasing and off-axis electron holography.
Quantitative measurement of doping and surface charge in a ZnO nanowire using in-situ biasing and off-axis electron holography.

Semiconducting nanowires (NWs) are widely studied because the properties that stem from their three-dimensional, nanoscale nature open new opportunities for device design. In particular ZnO NWs are widely studied for their interesting piezoelectronic properties. Though NWs can be readily grown today with increased carrier concentration due to doping, the measurement of the doping concentration at the nm scale remains challenging.

    We demonstrate that state-of-the-art off-axis electron holography in combination with electrical in-situ biasing can be used to detect active dopants and surface charges quantitatively in ZnO nanowires. The outline of the contacted NW is described in Fig. 1. We have acquired series of holograms and averaged the phase to increase signal to noise but avoid blurring due to specimen drift. The 0V bias images were used to remove contrast not related to the varying electrostatic potential and to verify the nanowire was not electrically modified during the experiment. We analyzed the depletion width in the nanowire due to an applied reverse bias to a Schottky contact on the nanowire, using a fit to the data.

    Comparison of the experimental data with 3D simulations that were similarly treated indicates an n-type doping level of 1x1018 at. cm-3 and a negative surface charge around -2.5x1012 charges cm-2. Fig. 2a shows the experimental vacuum corrected phase profiles converted to potential, and a fit to the data. The extracted depletion width is indicated with a cross. The inset shows the location of the phase profile in the NW core and two symmetrically defined phase profiles obtained in vacuum on either side of the NW. The average signal in vacuum was subtracted from the NW signal and the remaining phase signal was converted to potential using a thickness of 75 nm, much smaller than the 150 nm NW diameter. In Fig. 2b the experimental and simulated depletion length is compared for 3D simulations including varying doping and surface charge quantities. We expect that the real doping is between 1 and 2x1018 at. cm-3 by comparison of experiment and simulation. The surface charge results in a surface depletion to a depth of 36 nm. We found an active/undepleted core thickness of 70-75 nm, providing excellent agreement between the simulated thickness of the undepleted core and the active thickness observed in the experimental data.

    Off-axis electron holography thus offers unique capabilities for quantitative analysis of active dopant concentrations and surface charges in nanostructures with nanometer-scale spatial resolution.


Martien DEN HERTOG (Institut Neel CNRS, Grenoble), Fabrice DONATINI, Robert MCLEOD, Eva MONROY, Julien PERNOT

10:30-12:30
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IM1-I
IM1: Tomography and Multidimensional microscopy
SLOT I

IM1: Tomography and Multidimensional microscopy
SLOT I

Chairpersons: Sara BALS (Chairperson, Antwerpen, Belgium), Wolfgang LUDWIG (Chairperson, Lyon, France), Sergio MARCO (Chairperson, Paris, France)
10:30 - 11:00 #8328 - IM01-S31 Determining atomic coordinates in 3D by atomic electron tomography.
Determining atomic coordinates in 3D by atomic electron tomography.

At a basic level, materials properties depend on the three-dimensional arrangement of atoms, and it is necessary to determine their coordinates to make correlative measurements of structure and functionality from basic principles. Traditional 3D reconstruction techniques (such as X-ray crystallography and single-particle Cryo-Em) continue to provide critical insights into structure/property relationships but average over many identical structures. This will blur out the defects inherent to inhomogeneous nanoengineered materials important to their functionality. Aberration-corrected HR-TEM and HAADF-STEM are now indispensable techniques in materials science to examine the atomic structure of materials systems with sub-Å resolution and single atom sensitivity. Combining these new tools with powerful iterative 3D reconstruction and peak finding algorithms for electron tomography is opening a new field with the ability to determine atomic coordinates of all atoms in a structure without the assumption of crystallinity. This talk will cover recent develops and future directions of Atomic Electron Tomography (AET), which will be critical to our understanding of the atomic structure of complex materials systems.
HAADF-STEM and the equally sloped tomography method were recently used to determine the atomic coordinates of 3,769 atoms in 9 atomic layers at the apex of an etched tungsten needle (Figure 1) [1]. A tungsten point defect was unambiguously located in the material for the first time in three-dimensions. Comparing the experimental positions to the ideal bcc tungsten lattice produces the atomic displacement field with ±19 pm precision. Kernel density estimation applied to the differentiation of the displacement field was used to calculate the 6 components of the strain tensor with ~1 nm 3D spatial resolution indicating expansion along the [011] axis (x-axis) and compression along the [100] axis (y-axis). It was determined by experiments, DFT simulations and MD simulations that the strain in the lattice was due to a surface layer of tungsten carbide and sub-surface carbon. This result shows the capabilities of AET to measure atomic coordinates of inhomogeneous objects without the assumption of crystallinity providing and the capability of directly measuring materials properties.
Measurements of material structure in their native environment are now being accomplished using in-situ TEM, but has been limited by the thickness of the SiN windows and the contained liquid volume. A recent advance in this field was the introduction of the graphene liquid cell (GLC) to minimize the combined window/liquid thickness allowing observation of the growth and coalescence of colloidal Pt nanoparticles at atomic resolution [2]. It was discovered that stable NPs in the GLC were randomly rotating thus providing many orientations that could be reconstructed using methods developed in single-particle Cryo-Em. A direct electron detector and aberration-corrected HR-TEM were combined with a GLC in a technique called 3D SINGLE (3D Structure Identification of Nanoparticles by Graphene Liquid Cell EM) to determine the atomic-scale facets, lattice plane orientations and multi-twinned grain structure of a Pt nanoparticle in liquid with 2.10 Å resolution (Figure 2) [3]. The particle is constructed of three distinct regions: a central disk region of well-ordered {111} atomic planes with conical protrusions attached on each side connected by screw dislocations.
[1] Xu, R. et al., Nat Mater, 14, 1099–1103 (2015).
[2] Yuk, J. M. et al., Science, 336, 61–64 (2012).
[3] Park, J. et al. Science, 349, 290–295 (2015).


Peter ERCIUS (Berkeley, USA), Rui XU, Chien-Chun CHEN, Li WU, Mary SCOTT, Wolfgang THEIS, Colin OPHUS, Jungwon PARK, Hans ELMLUND, Alex ZETTL, A. Paul ALIVISATOS, Jianwei MIAO
Invited
11:00 - 11:15 #6438 - IM01-OP047 Environmental Transmission Electron Tomography: fast 3D analysis of nano-materials.
Environmental Transmission Electron Tomography: fast 3D analysis of nano-materials.

Modern environmental Transmission Electron Microscopes (ETEM) enables chemical reactions to be directly observed with new perspectives in the operando characterization of nano-materials. However, morphological features are essentially missing in 2-dimensional observations, thus nano-tomography under environmental conditions is a new promising challenge. Obviously, the essential condition to achieve this goal is to run fast tilt series acquisitions as compared to the kinetics of the reactions which are followed in situ in the microscope. This contribution will show that such experiments are possible by comparing the volumes respectively obtained from a classic or a fast tilt series acquisition in the bright field mode.

Firstly, simulations were performed on ghost models in order to appreciate the influence of the goniometer rotation speed during image acquisition on quality of images (sharpness and blurring effects). A typical micrograph of a nano-object, e.g. metallic nanoparticles encaged into mesoporous silicalites, was used to reconstruct a 2D model. The 1D projections were calculated according to different conditions intending to reproduce the effects of a continuous tilt during the acquisition. Figure 1 a-c) show the models at zero tilt projected perpendicularly to the tilt axis marked by a cross (the vertical direction is the projection direction); in a), a fixed image is shown as obtained at a given rotation; it is compared to images simulated by integrating a blur effect to a rotation of 3° during the acquisition, either whit a centered (b) or not centered rotation axis (c). To give an order of magnitude, a 120° rotation performed in 1 minute with acquisition of images every second without interrupting the rotation leads to an angular blur of only 2° in each image. From the 1D projection series (not shown here), 2D reconstructions were calculated using the simple Weighted-Back Projection (WBP) algorithm. Results from fig. 1 d-e) show that, at least in the case of nanoparticles with strong absorption contrast as presented here, the tomograms obtained from the blurred series are not significantly different from the constructed volume obtained from the conventional step-by-step acquisition scheme.

In a second step, we performed experimental nano-tomography experiments on Pd/Al2O3 samples deposited on holey carbon grids. Volume reconstructions shown in Fig.2 were obtained from the same object using two bright field tilt series acquired in a FEI Titan-ETEM microscope operated at 300 kV and equipped with a dedicated Fischione high-tilt sample holder. The first one was acquired through a classical step by step tilt series acquisition from 74° to + 66° with a step of 2° in mode Saxton within 45 minutes. The second one was recorded by 'fast tomography' in 150 seconds. From these data, a quantitative analysis of the Pd nanoparticles (NP) distribution and size was performed and reported in Fig. 3. Although differences obviously exist (especially, the fast tomography approach misses some of the NPs smaller than nominally 2 nm and tends to overestimate the size of the largest ones), it can be concluded that acquisitions of tilting series in very short times of the order of one minute, or even less, represent a promising way to provide 3D information on samples studied under dynamic gas and temperature conditions such as typically nano-catalysts studied in an Environmental TEM. This fast tomography approach can also be of a great interest for beam sensitive samples where the material is generally not able to bear a long exposure to the electron beam without any specific and sometimes hazardous pre-treatment or preparation.

Acknowledgements

Thanks are due to CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) for the access to the microscope funded by the Region Rhône-Alpes, the CNRS and the 'GrandLyon'.

Acknowledgments are also due to BQR project SEE3D granted by Insa-Lyon, ANR project 3DClean, Labex iMUST and IFP Energies Nouvelles for the financial support.


Siddardha KONETI, Lucian ROIBAN (MATEIS / INSA, Lyon), Voichita MAXIM, Thomas GRENIER, Priscilla AVENIER, Amandine CABIAC, Anne-Sophie GAY, Florent DALMAS, Thierry EPICIER
11:15 - 11:30 #5519 - IM01-OP038 Investigating lattice strain in Au nanodecahedrons.
Investigating lattice strain in Au nanodecahedrons.

The three dimensional (3D) structural characterization of nanoparticles is crucial in materials science since many properties heavily depend on size, surface to volume ratio and morphology.  In addition, the ability to investigate the crystal structure is just as essential because the presence of defects and surface relaxation will directly affect plasmonic or catalytic properties. A well-known example of strained nanoparticles are the so-called “nanodecahedra” or “pentagonal bipyramids”. Such particles consist of five segments bound by {111} twin boundaries, yielding a crystallographic forbidden morphology. Therefore, measuring strain fields in nanodecahedra by transmission electron microscopy (TEM) has been the topic of several studies. However, it is important to note that such studies are based on 2D projections, hereby neglecting the 3D nature of the lattice strain. [1,2] Here, we will quantify the lattice strain in 3D based on high resolution electron tomography reconstructions. [3]

 

Therefore, a continuous tilt series of 2D projection images was acquired using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and a dedicated alignment procedure was applied. These projection images are then used as an input for a model based tomography reconstruction algorithm. A major disadvantage of conventional reconstruction techniques is that a continuous volume is reconstructed hampering the extraction of atom coordinates without the use of dedicated post-processing methods. [4] We could overcome this limitation by assuming that the 3D atomic potential can be modelled by 3D Gaussian functions. This hypothesis significantly simplifies the reconstruction problem to a sparse inverse problem, yielding the coordinates of the individual atoms as a direct outcome of the reconstruction.

 

Visualizations of the final 3D reconstruction, obtained for a Au nanodecahedron containing more than 90,000 atoms, are presented in Figure 1.a-c along different viewing directions. Since the coordinates of the atoms are a direct outcome of the reconstruction, it becomes straightforward to calculate the 3D displacement map. We computed derivatives of the displacement map in such a manner that 3D volumes were obtained corresponding to εxx and εzz. Slices through the resulting εxx and εzz volumes are presented in Figure 1.d and Figure 1.f. Furthermore, the variation of the lattice parameters was investigated along x and z based on the same slices (Figure 1.e and Figure 1.g). Both along the x and z direction a systematic outward expansion of the lattice can be observed. The expansion along z is limited to a few of the outer atomic layers and shows an asymmetry (Figure 1.f-g) that is likely related to the fact that the decahedron is deposited on a carbon support.

 

[1] C.L. Johnson, et al., Nat. Mater. 7 (2007) 120-124

[2] M.J. Walsh, et al., Nano Letters 12 (2012) 2027-2031

[3] B. Goris, et al., Nano Letters 15 (2015) 6996-7001

[4] R. Xu, et al., Nat. Mater. 14 (2015) 1099–1103

[5] The authors gratefully acknowledge funding from the Research Foundation Flanders (project numbers G.0369.15, G.0374.13  and a post-doctoral grant to B.G. and A.D.B.). S.B. and D.Z. acknowledge the European Research Council, ERC grant N°335078 – Colouratom. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreements 312483 (ESTEEM2).  


Bart GORIS (Antwerp, Belgium), Jan DE BEENHOUWER, Annick DE BACKER, Daniele ZANAGA, Joost BATENBURG, Anna SANCHEZ-IGLESIAS, Luis LIZ-MARZAN, Sandra VAN AERT, Jan SIJBERS, Gustaaf VAN TENDELOO, Sara BALS
11:30 - 11:45 #6057 - IM01-OP042 Three dimensional confocal imaging using coherent elastically scattered electrons.
Three dimensional confocal imaging using coherent elastically scattered electrons.

To fully understand structure-property relationships in nanostructured materials, it is important to reveal the three dimensional (3D) structure at the nanometre scale.

Scanning electron confocal microscopy (SCEM) was introduced as an alternative approach to 3D imaging in 20031. The confocal method was originally developed in optical microscopy to image the 3D structure of biological samples2. The incident beam is focused at a certain depth in a thick sample, and the excited fluorescence signal is imaged onto the detector plane through the imaging system. A small pin hole before the detector only allows the signal from the confocal plane to reach the detector and blocks the out-of-focus signal. Critically, by using a fluorescent signal, the incident and outgoing waves lose their phase relationship and an incoherent 3D point spread function can be achieved.

The optical setup in SCEM is analogous to fluorescence confocal microscopy and also requires an incoherent signal to achieve the incoherent 3D point spread function. Several groups have developed different approaches to using inelastically scattered electrons to achieve an incoherent confocal condition in the TEM3,4,5, however, difficulties remain. Core-loss electrons have a suitably limited coherence length, however, the excitation probability is extremely low which leads to a poor signal to noise ratio (SNR). Low loss electrons give a much better SNR but still have a significant coherence length due to the collective nature of the  excitation.

In this work, we introduce a different approach to achieve 3D imaging in a confocal mode by using the elastically scattered, coherent electrons. This method exploits the depth sensitivity of electrons that have suffered a specific momentum change, rather than an intensity change.  According to Fourier optics, when a thin object is inserted at a distance z above the confocal plane, the new wave function at the confocal plane will be the original probe convoluted with the Fourier transform of the object function, together with a scale factor related to z (defined in fig.1). For crystalline specimens, the Fourier transform of the object function is a set of delta functions, so a diffraction-like pattern will be generated at the confocal plane. Importantly, the separation between the diffraction spots is proportional to the distance z (see fig.1), so that the resulting diffraction contrast is very sensitive to depth. This strong depth sensitivity is combined with the very strong SNR due to the use of the elastically scattered signal. Applications to the imaging of 3D engineered nanostructures are demonstrated (fig. 2 and 3).

1. S. P. Frigo, Z. H. Levine, and N. J. Zaluzec, Appl. Phys.Lett. 81, 2112 (2002 and N. J. Zaluzec, U.S. Patent No. 6,548,810 B2 (2003).

2. T. Wilson and C. Sheppard, Theory and practice of scanning optical microscopy (Academic Press, London ; Orlando, 1984)

3. Wang, P., Behan, G., Takeguchi, M., Hashimoto, A., Mitsuishi, K., Shimojo, M., & Nellist, P. D. (2010). Phys. Rev. Lett. 104(20), 200801.

4. Xin, H. L., Dwyer, C., Muller, D. A., Zheng, H., & Ercius, P. (2013). Microsc. Microanal., 19(04), 1036-1049.

5. C Zheng, Y Zhu, S Lazar, J Etheridge, Phys. Rev. Lett. (2014) 112 (16), 166101

Acknowledgement: The authors thanks staff at the Monash Centre for Electron Microscopy. The double-aberration Titan3 80-300 FEGTEM was funded by ARC grant LE0454166.


Changlin ZHENG (Melbourne, Australia), Ye ZHU, Sorin LAZAR, Joanne ETHERIDGE
11:45 - 12:00 #5183 - IM01-OP037 Multi-modal electron tomography for 3D spectroscopic analysis using limited projections.
Multi-modal electron tomography for 3D spectroscopic analysis using limited projections.

     Electron tomography applied to spectroscopic signals in the scanning transmission electron microscope (STEM) offers the possibility for quantitative determination of structure-chemistry relationships with nanometre spatial resolution. Electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS), however, often require long exposure times or high beam currents for sufficient data quality for spectral tomography. Many materials samples are not sufficiently stable under the electron beam for the prolonged irradiation times necessary for conventional tilt-series acquisition and back-projection tomographic reconstruction schemes using STEM spectrum imaging signals. Reduced dose acquisition strategies will, in general, require the use of fewer projections for tilt-series electron tomography because signals with sufficient signal-to-noise must be recorded on the respective detectors for quantitative chemical reconstructions, establishing a limit on the minimum acquisition time for individual spectrum images using current detector technologies. While methods such as compressive sensing electron tomography (CS-ET) [1] show promise for reducing the number of projections required for successful tomographic reconstructions, combining information from multiple simultaneous imaging modes in the STEM provides a complementary strategy for further reducing electron dose in spectral tomography. Simultaneously acquired signals that offer structural contrast information (e.g. ADF STEM, low-loss EELS, qualitative EDS tomography) in many cases enable the spectral tomography problem to be re-cast as a recovery problem with reduced dimensionality. The 3D reconstruction of spectral data can then be recovered quantitatively from substantially fewer spectrum images. In the case of surface plasmon modes of silver particles, ADF STEM tomography has already been applied in conjunction with EELS spectrum imaging to reconstruct the surface charge distributions [2], a two-dimensional reconstruction problem (on a surface) defined in three-dimensional spatial coordinates.

     This approach has been extended to the recovery of voxel spectra from the cloudy zone, a spinodal decomposition of Fe-Ni in the Tazewell meteorite (Figure 1). Due to minimal ADF STEM contrast, qualitative EDS tomography using the Ni K-alpha signal was analysed for structural segmentation of the sample volume. Re-projections of the extracted binarized volumes for each of the two phases were then used as a thickness-series to re-cast the recovery problem as an overdetermined system of linear equations, assuming homogeneous composition within each phase. The spectral intensity at each energy channel was decomposed according to the thickness data for each phase available at each pixel in the two-dimensional spectrum images, allowing relative spectral intensities to be attributed to the voxels assigned to each of the two phases. The resulting tomographically unmixed spectra enabled improved EDS quantification of the relative Fe-Ni ratios in each phase, giving results within 2% of quantification by atom probe tomography of similar material from the cloudy zone of the Tazewell meteorite.

     Applications to core-loss STEM-EELS analyses will be presented, further extending this family of methods to cases involving plural-scattering corrections implemented in conjunction with the linear thickness unmixing approach. Comparisons of signal unmixing determined from multi-modal structural and spectral tomography and blind-source separation methods (e.g. non-negative matrix factorization or independent component analysis) of two-dimensional spectrum image data will also be discussed.

 

References: [1] Saghi, Z.; Holland, D.J.; Leary, R.K.; Falqui, A.; Bertoni, G.; Sederman, A.J.; Gladden, L.F.; Midgley, P.A. Nano Lett., 2011, 11, 4666-4673. [2] Collins, S.M.; Ringe, E.; Duchamp, M.; Saghi, Z.; Dunin-Borkowski, R.E.; Midgley, P.A. ACS Photonics, 2015, 2, 1628-1635.

Acknowledgements: The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (No. FP7/2007-2013)/ERC Grant Agreement No. 291522-3DIMAGE and (No. FP7/2007-2013)/ERC Grant Agreement No. 320750-Nanopaleomagnetism as well as the European Union’s Seventh Framework Program under a contract for an Integrated Infrastructure Initiative (Reference No. 312483-ESTEEM2).


Sean COLLINS (Cambridge, United Kingdom), Joshua EINSLE, Zineb SAGHI, Robert BLUKIS, Richard HARRISON, Paul MIDGLEY
12:00 - 12:15 #6946 - IM01-OP053 Random Beam Scanning Transmission Electron Microscopy and Compressive Sensing as Tools for Drastic Electron Dose Reduction in Electron Tomography.
Random Beam Scanning Transmission Electron Microscopy and Compressive Sensing as Tools for Drastic Electron Dose Reduction in Electron Tomography.

Electron tomography is a fantastic tool for deciphering the structural information of complex 3D samples. During the last years, several tools have been developped to improve the 3D reconstruction quality of thick specimens. The direct detector cameras have incredibly increased the SNR and resolution of thin samples 2D projections, bringing electron microscopy resolution at the level of the one of X-ray diffraction studies. However, the study of thick biological samples in tomography still suffers from the too important electron dose one has to use in order to retrieve high quality images and reconstructions. It has been shown that STEM tomography can generate more accurate reconstructions than TEM tomography while preserving better the sample integrity. Previous uses of compressive sensing enabled the reduction of tilt-angles in tomography studies, unveiling electron dose reduction. Here, we push further the electron dose reduction thanks to a more effective compressive sensing method which uses incomplete images as incoherent data. The generation of incomplete images being performed at the microscope during the acquisition process where the beam randomly scans the surface of the sample.


Sylvain TREPOUT (Institut Curie / INSERM U1196), Masih NILCHIAN, Cédric MESSAOUDI, Laurène DONATI, Michael UNSER, Sergio MARCO
12:15 - 12:30 #6266 - IM01-154 Comparison of propagation-based phase contrast tomography and full-field optical coherence tomography on bone tissue.
IM01-154 Comparison of propagation-based phase contrast tomography and full-field optical coherence tomography on bone tissue.

The current huge development of new 3D microscopic techniques (synchrotron microtomography, optical coherence tomography, light sheet microscopy, …) opens a large variety of new perspectives for life sciences. The contrasts of these new microscopies are mostly well understood on samples of known material content such as those used in physics or instrumentation studies. The situation is different when it comes to the interpretation of the contrasts observed with complex heterogeneous media found in biology. Therefore determining which 3D microscopy technique is suited for which biological question is a topic of current interest (see [1,2] for instance in our group).

In this communication, we propose a comparison of the contrast observed with full-field optical coherence tomography (OCT) and propagation-based phase contrast tomography (PCT) on bone tissue at similar spatial resolution. A first comparison of OCT with standard absorption microtomography was given in [3] for bones and we extend this comparison to PCT which is known to provide enhanced contrast on bones at multiple scales [4]. The contrast of both these techniques are a priori interesting to be compared since they both rely on discontinuities of refraction index. This produces phase shift in PCT which operates in the X-ray domain with a monochromatic beam (generated by a synchrotron) while this generates direct intensity reflexion with OCT which only resorts to white light in the visible domain.

As visible in Figure 1, we specifically focussed our attention on the contrast observed in both techniques around the same bone structural unit, a so-called osteon, at a microscopic scale with images of same spatial resolution (voxel size 3.5µm). It happens that the osteons are visible in PCT while they are not perceptible with conventional absorption micro computed tomography. Also, concentric lamellae, corresponding to the so-called Harvers system, appear clearly visible in OCT while they are not perceptible with PCT at this spatial resolution. The contrast between the osteon and the surrounding bone tissue, is found in terms of homogeneous regions in PCT. However, this less spatially resolved contrast in PCT is constant throughout the sample while it is spatially variable in OCT where a continuous degradation of the contrast is observed along the direction Z of the propagation of light. We found, as given in Figure 2, that a certain spatial average of some 30 µm along Z was able to improve optimally the contrast across the concentric lamellae when inspected at the surface (up to 500 µm depth) of the sample with OCT. This contributes to establish quantitatively the complementarity of OCT and PCT for the characterization of bones at the microscopic scale.

 

References:

 

[1] Rousseau, D., Widiez, T., Tommaso, S., Rositi, H., Adrien, J., Maire, E., Langer, M., Olivier, C., Peyrin, F. Rogowsky, P. (2015). Fast virtual histology using X-ray in-line phase tomography: application to the 3D anatomy of maize developing seeds. Plant methods, 11(1), 1.

 

[2] Rositi, H., Frindel, C., Wiart, M., Langer, M., Olivier, C., Peyrin, F., Rousseau, D. (2014). Computer vision tools to optimize reconstruction parameters in x-ray in-line phase tomography. Physics in medicine and biology, 59(24), 7767.

 

[3] Kasseck, C., Kratz, M., Torcasio, A., Gerhardt, N. C., van Lenthe, G. H., Gambichler, T., . Hofmann, M. R. (2010). Comparison of optical coherence tomography, microcomputed tomography, and histology at a three-dimensionally imaged trabecular bone sample. Journal of biomedical optics, 15(4), 046019-046019.

 

[4] Peyrin, F., Dong, P., Pacureanu, A., & Langer, M. (2014). Micro-and Nano-CT for the Study of Bone Ultrastructure. Current osteoporosis reports, 12(4), 465-474.

 

Acknowledgement : This work was supported by the European Synchrotron Research Facility (ESRF, project LS-2290) through the allocation of beam time.


Sylvaine DI TOMMASO, Hugo ROSITI, Max LANGER, Carole FRINDEL, Cécile OLIVIER, Françoise PEYRIN, David ROUSSEAU (VILLEURBANNE CEDEX)

14:00-16:00
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MS3-I
MS3: Semiconductors and devices
SLOT I

MS3: Semiconductors and devices
SLOT I

Chairpersons: Catherine BOUGEROL (Chairperson, Grenoble, France), Vincenzo GRILLO (Chairperson, Modena, Italy)
14:00 - 14:30 #8413 - MS03-S72 Cathodoluminescence and EBIC study of widegap semiconductors and devices.
Cathodoluminescence and EBIC study of widegap semiconductors and devices.

Cathodoluminescence (CL) and electron-beam-induced current (EBIC) are versatile techniques to characterize semiconductor materials and devices. In this talk, we review our achievement on the study of widegap semiconductors, GaN and SiC.

The major defects in GaN are dislocations. GaN wafers on sapphire include dislocations of 109 to 107 cm-2 due to lattice mismatch. Fig. 1 shows the secondary electron (SE) and CL images of GaN wafer with different thickness. Such dislocations may agglomerate and form the hexagonal pits of micrometer size. These defects are detrimental for the device performance. Homoepitaxial GaN wafer take over the dislocations of 106 cm-2 from the seeds. We have performed CL study to distinguish dislocation characters and to clarify the effect of dislocations.

The major defects in SiC are threading screw dislocations (TSD) and stacking faults (SF). TSD act as the killer defects due to the surface roughness at the dislocation core region. SF may be generated and expanded due to e-beam irradiation. EBIC is very effective to characterize these defects in SiC. The e-beam enhanced defect generation of SF (Fig.2) will be reviewed.

At the end, we demonstrate 3D spectra imaging of CL, which is very promising to analyze the details of extended defects.

This work was supported from “GaN project”, Study of Future Semiconductors for Sustainable Society in MEXT, Japan.


Takashi SEKIGUCHI (, Japan)
Invited
14:30 - 14:45 #6432 - MS03-OP245 A novel way of measuring lifetime at the nanometer scale using specific fast electron-matter interactions.
A novel way of measuring lifetime at the nanometer scale using specific fast electron-matter interactions.

Charge carrier lifetime is a key parameter for understanding the physics of electronic or optical excitations. For example the excited state can unveil details of environmental influence, specifically the role of non-radiative transitions. From a practical point of view, lifetimes can largely determine the performances of devices, such as Light Emitting Devices (LEDs) or photovoltaic cells. These usually rely on nanometer scale structures for which small details, such as the presence of single point defects, have to be known with atomic precision. Despite the success of super resolution optical microscopies, they fail as general tools for lifetime measurement at the nanometer scale. In this presentation we will show how we can take advantage of the nanometer probe size formed in a Transmission Electron Microscope and a phenomenon that we recently discovered (referred hereafter as the bunching effect [1]), to study lifetimes of emitter at the nanometer scale without using a pulsed electron gun.

The effect takes its name from the fact that the autocorrelation function g(2)(τ) of the CL signal coming from quantum emitters (points defects or more generally single photon emitters –SPE-, quantum confined structures…) may exhibit a peak at zero delay – which is a fundamental difference with PL. To measure this effect, we use an intensity interferometry experiment that measures the CL g(2)(τ). Figure 1 shows that, at low incoming electron currents (I < 100 pA), the g(2)(τ) of the CL signal intensity I(t) displays a large nanosecond-range peak at zero delay (g(2)(0) > 35) (bunching), the amplitude of which depends on the incoming electron current. This behavior strongly departs from the PL g(2)(τ) function which is flat when multiple independent SPE are excited. In this presentation we will show that it occurs because an emitter, like a quantum well, will be excited multiple times by a single electron and will emit a bunch of photons on a time window close to its radiative lifetime. As it will be proved, by simply fitting the experimental curve of the g(2)(τ) function by an exponential we can retrieve the lifetime of the emitter.

Using this effect we were therefore able to measure very efficiently lifetimes of Gallium Nitride quantum wells (QWs) separated by less than 15 nm, together with their emission energy and atomic structure (Figure 2). Experiments on well separated individual quantum structures shows an excellent agreement with combined time-resolved μ-photoluminescence. We also demonstrate the possibility to measure the lifetimes of emitters of different kinds (defects, QWs, bulk) within a distance of a tenth of nanometers even for spectrally overlapping emissions. This technique is readily applicable to large ensembles of single photon sources and various emitters such as QWs, quantum dots, point defects and extended defects, such as stacking faults (SF).

 

[1] Meuret et al, PRL 114 197401 (2015)

[2] L. H. G. Tizei and M. Kociak, PRL 110  153604 (2013)


Sophie MEURET (CEMES, Toulouse), Luiz TIZEI, Thomas AUZELLE, Thibault CAZIMAJOU, Romain BOURRELLIER, Rudee SONGMUANG, Huan-Cheng CHANG, François TREUSSART, Bruno DAUDIN, Bruno GAYRAL, Mathieu KOCIAK
14:45 - 15:00 #6170 - MS03-OP242 Nanocathodoluminescence reveals the mitigation of the Stark shift in InGaN quantum wells by silicon doping.
Nanocathodoluminescence reveals the mitigation of the Stark shift in InGaN quantum wells by silicon doping.

InGaN quantum wells (QWs) show high internal quantum efficiencies over the ultraviolet to green spectrum and in white light emitting diodes (LEDs). However a persistent challenge to the development of higher efficiency devices is the strong polarisation field across the across the QWs along the polar axis. The polarisation induced internal electric fields cause the spatial separation of the electron and hole wavefunctions in the QWs, known as the quantum confined Stark effect (QCSE). It has been proposed that the internal electric field can be suppressed by silicon doping the quantum barriers (QBs) [1]. Moreover, Kim et al. have theoretically shown that the device efficiency may be improved by variations in the silicon dopant concentration through the QWs [2]. To confirm the simulated properties though, it is crucial to resolve the spectral properties of individual QWs.

In this study, nano-cathodoluminescence (nanoCL) reveals for the first time the spectral properties of individual InGaN QWs in high efficiency LEDs and the influence of silicon doping on the emission properties [3]. A silicon doped layer at 5×1018 cm-3 is included immediately prior to the growth of the 1st QW and the QBs between the QWs are subsequently doped to 1×1018 cm-3 (sample A). Two further multiple QW InGaN/GaN structures were also grown for reference with QB doping levels of 1×1018 cm-3 (sample B) and 1×1017 cm-3 or less (sample C). NanoCL reveals variations in the emission wavelength that directly correlate with individual QWs. With QB doping greater than 1×1018 cm-3, there is a continuous blue shift in the emission wavelength of each of the subsequently grown QWs. The inclusion of a higher doped layer immediately prior to the growth of the 1st QW in the LED structure leads to a blue shift unique to the 1st QW.

The experimental variations in the emission wavelengths were reproduced by Schrödinger-Poisson simulations. The blue shift in emission wavelength through the QWs due to QB doping is found to be caused by screening of the internal electric fields. The reduction in the emission wavelength of the first grown QW due to the higher doped layer is also found to be the result of screening of the internal electric field. The mitigation of the QCSE and consequently stronger overlap of the electron and hole wavefunction, thus should result in an increase in the radiative recombination. NanoCL thus may serve as an experimental approach to study and refine  the design of future optoelectronic nanostructures, including the effects from doping and lead to improvements in device efficiency and functionality.

[1] T. Deguchi, et al., Appl. Phys. Letts. 72, 3329 (1998)

[2] D. Y. Kim, et al., IEEE Photonics. 7, 1 (2015)

[3] J. T. Griffiths, et al., Nano Letts. 15, 7639 (2015)


James GRIFFITHS (Cambridge, United Kingdom), Siyuan ZHANG, Bertrand ROUET-LEDUC, Wai Yuen FU, Dandan ZHU, David WALLIS, Ashley HOWKINS, Ian BOYD, David STOWE, Colin HUMPHREYS, Rachel OLIVER
15:00 - 15:15 #5923 - MS03-OP240 Advanced characterization of colloidal semiconductor nanocrystals by 2D and 3D electron microscopy.
Advanced characterization of colloidal semiconductor nanocrystals by 2D and 3D electron microscopy.

Due to the specific size-dependent photoluminescence spectra of semiconductor nanocrystals (NCs), their use is promising as building blocks for new electronic and optical nanodevices such as light-emitting diodes, solar cells, lasers and biological sensors.1,2 In order to design these NCs with tailored properties for specific applications, a high level of control over their synthesis is of key importance. Therefore, it is of great importance to characterize both the shape as the composition of these systems. Here, a range of different colloidal semiconductor NCs are characterized using 2D and 3D electron microscopy techniques.

We will discuss, 2D semiconductor CdSe nanoplatelets (NPLs), both flat as helical shaped3, which are investigated using electron microscopy techniques. The aim is to retrieve structural information using high resolution imaging which enables us to study the growth mechanism of these NPLs. The flat NPLs have mainly {100} edges (Figure 1.A) and only a thickness of 4 to 5 atomic layers (Figure 1.B). The analysis of the helical NPLs shows that they are zinc blende and that the helices are folded uniquely around the ⟨110⟩ axis (Figure 1.D). In order to retrieve the helicity of the ultrathin helical shaped platelets, electron tomography is applied. The three-dimensional tomographic reconstructions confirm that the observed helices fully rotate over a diameter of ∼25 nm and that they are not preferentially left- or right-handed (Figure 1.C).

Furthermore, heteronanocrystals (HNCs) are studied as they improve the stability and, thereby, the surface passivation of the NCs when overgrown with a shell of a second semiconductor with a higher bandgap.  In this manner, the robustness of the system and the photoluminescence quantum yield of the core is increased.4  In order to understand the growth process of HNCs, both the 3D structure as the position of the core inside that structure is of key importance. We investigate two types of CdSe/CdS core/shell HNCs, with either a nanorod or bullet shape. High resolution HAADF-STEM microscopy enables us to investigate the crystal structure of the core-shell nanostructure (Figure 2.A,C). Advanced electron tomography based on novel reconstruction algorithms5 is used to investigate the 3D shape and to reveal the position of the CdSe core in the CdS shell (Figure 2.B,D). For the CdSe/CdS core/shell bullets, the presence of two types of morphologies was revealed (Figure 2.D). High resolution STEM imaging was used to characterize the surface facets of both morphologies, which enabled us to compare the surface energy of both morphologies. For the CdSe/CdS nanorods, a sequential topotactic cation exchange pathway that yields CuInSe2/CuInS2 nanorods with near-infrared luminescence is further investigated6.

[1]  Somers, R. C.; Bawendi, M. G.; Nocera, D. G. Chem. Soc. Rev. 2007, 36, 579–591.

[2]  Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389–458.

[3]  Hutter, E. M.; Bladt, E.; Goris, B.; Pietra, F.; van der Bok, J. C.; Boneschanscher, M. P.; de Mello Donegá, C.; Bals, S.; Vanmaekelbergh, D. Nano Lett. 2014, 14, 6257–6262.

[4]  Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463–9475.

[5]  Goris, B.; Van den Broek, W.; Batenburg, K. J.; Heidari Mezerji, H.; Bals, S. Ultramicroscopy 2012, 113, 120–130.

[6]  van der Stam, W.; Bladt, E.; Rabouw, F. T.; Bals, S.; de Mello Donega, C. ACS Nano 2015, 9, 11430–11438.

The authors acknowledge financial support from the Research Foundation - Flanders (FWO).


Eva BLADT (Antwerpen, Belgium), Bart GORIS, Eline HUTTER, Ward VAN DER STAM, Relinde MOES, Celso DE MELLO DONEGA, Daniël VANMAEKELBERGH, Sara BALS
15:15 - 15:30 #6244 - MS03-OP243 Picometre-precision atomic structure of inversion domain boundaries in GaN.
Picometre-precision atomic structure of inversion domain boundaries in GaN.

Here, we report on the precise analysis of the atomic structure of inversion domain boundaries (IDBs) in GaN by scanning transmission electron microscopy.  IDBs are a common defect in GaN that traps carriers and leads to a slightly modified luminescence wavelength [1,2].

Our analysis of IDBs in MOCVD grown nanowires confirms recent coherent Bragg imaging (CBI) results [3] stating that the atomic structure of this IDB is different or slightly different from the one determined in 1996 by first-principle calculations (IDB*) [4].  CBI experiments measured a 8 pm shift of the c-planes of the two domains [3], whereas  first-principle calculations predicted no shift. A previous study by STEM [5] found a shift of "ca. 0.6 Å" (60pm), corresponding roughly to the switch of the Ga and N positions without any additional shift. Here in addition to a shift along  c, we show that the interface configuration corresponds qualitatively to the IDB* model (cf. Fig. 1) and that there is a 10 pm dilatation perpendicular to the interface (shown in Fig. 3) in agreement with this model, while CBI did not find a dilatation.

To facilitate the measurement of atom positions across the IDB with picometre-precision, we use HAADF-STEM to avoid coherent effects leading to artefacts. Scanning and drift artefacts are being suppressed by acquiring series of rapid STEM images and aligning them using the newly developed Zorro code. This algorithm is based on calculating estimated drift positions by correlating every frame to multiple frames and minimizing the error of the overdetermined system to obtain a best estimate for the frame positions relative to each other. The sub-pixel aligned frames are then averaged and the peak positions are determined via TeMA (template-matching algorithm).

Our quantitative analysis of experimental and simulated STEM images shows that when atomic columns are very close to each other the measured distance can be slightly different from the real value.  For instance, when the distance between atomic columns becomes smaller than 0.1 nm, the difference between the measured and real values can account for several picometres. This effect can be well observed when an IDB kinks perpendicular to the observation direction leading to closely projected columns in the overlap region of the two domains as seen in Fig. 2. Electron scattering simulations show that the apparently wider distance between atoms is a channeling effect.

These results have provided elements to revisit previous theoretical models of IDBs in GaN.

  

[1] T. Auzelle et al., Appl. Phys. Lett. 107, 051904 (2015).

[2] R. Kirste et al., J. Appl. Phys., 110, 093503(2011).

[3] S. Labat et al., ACS Nano 9, 9210 (2015).

[4] J. E. Northrup et al., Phys. Rev. Lett. 77, 103 (1996).

[5] F. Liu et al., Adv. Mater. 20, 2162 (2008).


Benedikt HAAS (GRENOBLE CEDEX 9), Robert A. MCLEOD, Thomas AUZELLE, Bruno DAUDIN, Joël EYMERY, Frédéric LANÇON, Jian-Min ZUO, Jean-Luc ROUVIÈRE
15:30 - 15:45 #5980 - MS03-OP241 Si:B doping measurement by dark-field electron holography.
Si:B doping measurement by dark-field electron holography.

In modern MOS devices, sources and drains are of nanometric dimensions and highly doped (dopant concentration typically > 1020 at.cm-3). Measuring such dopant concentrations and visualizing their spatial extensions in silicon, although mandatory for the development of the technology, is elusive in practice. Several TEM techniques such as EELS and EDX seem suitable to map dopant concentrations with the required resolution but while they are accurate to measure impurities concentrations, they cannot assess whether these impurities are on interstitial or substitutional sites, what is essential to define doping levels. Moreover, the detection of boron suffers from other physical limitations. Finally, bright-field electron holography has been reported to be suited for such measurements but transforming the electrostatic fields which are measured into doping concentrations is far from straightforward.

In this work, we have explored the possibility to extract boron concentrations from the measurement of changes of the silicon lattice parameter induced by the substitution of boron atoms. For this we use dark-field electron holography (DFEH) on specifically designed samples.

In a first part, we will present the DFEH principle [1]. This is an interferometry technique able to map strain with a precision of the order of 10-4 and a few nanometers spatial resolution over micrometer fields of view. Two diffracted beams, one passing through an unstrained region of the lattice and acting as a reference, the other one passing through the region where strain has to be measured, are forced to interfere by using an electrostatic biprism and thus create an interference pattern (see figure 1). A phase map is extracted from the pattern by Fourier transform and converted into an atomic displacement field. By using two non-collinear diffraction vectors, all the components of the strain tensor in the observation plane can be obtained.

For our experiment, a sample consisting of five 50 nm-thick doped layers of increasing boron concentrations ranging from 3E18 at.cm-3 to 8.5E19 at.cm-3 was grown by RP-CVD, under conditions insuring both extremely low concentrations of impurities and the full activation of boron [2]. The sample was further checked by SIMS and ECVP measurements, demonstrating that 100 % of boron atoms are on substitutional sites in all the doped layers. DFEH was used to measure the deformation of the doped layers. We could thus deduce the silicon lattice expansion coefficient (β) resulting from the adding of boron atoms in the crystalline silicon network, from these measurements, as explained below.

The boron atoms being on substitutional sites, the Si:B doped layers can be seen as solid solutions as confirmed by the homogeneity of the deformations imaged by DFEH. These layers are pseudomorphic on the pure silicon lattice as confirmed by the mapping of the in-plane strain by DFEH. Thus, the change of the lattice parameter resulting from the incorporation of boron atoms is solely supported by the out-of-plane strain, through the Poisson’s reaction of the material (figure 2). From the modeling of this sample by FEM and taking into account the relaxation affecting the thin lamella used for DFEH, we are able to retrieve the values of the relaxed Si:B lattice parameter as a function of the substitutional boron concentration. As expected for a solid solution, we find a linear relation between these two parameters. Knowing the boron concentration and the Si:B lattice parameter profiles, we are able to deduce that β coefficient equals -6.5E-24 cm3 (figure 3). Figure 4 compares the results we have obtained with those found in the literature, often measured by XRD.

Finally, β, the coefficient relating the boron concentration to the lattice parameter, allows us to transform a strain map obtained by DFEH into a “substitutional boron concentration” map with a precision of 3E19 at.cm-3 and a spatial resolution of 5 nm. We will illustrate the DFEH effectiveness to measure and image dopant concentrations in “real samples” through few examples, and will discuss the complementarity of the information obtained by this method and by bright-field electron holography.

[1] M.J. Hÿtch, F. Houdellier, F. Hüe and E. Snoeck, Nature 453, pp. 1086-1090, 2008.

[2] F. Gonzatti, J.M. Hartmann, K. Yckache, ECS Transactions 16, pp. 485-493, 2008.


Victor BOUREAU (Toulouse), Daniel BENOIT, Jean-Michel HARTMANN, Martin HŸTCH, Alain CLAVERIE
15:45 - 16:00 #6633 - MS03-OP248 In situ tracking of the heat-induced replacement of GaAs by Au in nanowires.
In situ tracking of the heat-induced replacement of GaAs by Au in nanowires.

For devices, the junctions between the semiconductors and any metal contacts are crucial for the device performance. A heat treatment is commonly applied to improve the quality of the contacts. For GaAs nanowires with a Au-based contact, annealing can lead to a well-defined metal-GaAs junction within the nanowire [1]. Here, we report an in situ heating, high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) study on the formation and structural characteristics of such junctions. The nanowires are dispersed on a high-stability TEM heating chip, and local Au contacts are made by lithography before in situ heat treatment within the microscope (Figure 1) [2]. A replacement of GaAs by Au can take place and our study determined key aspects of the reaction mechanism and its kinetics such as the reaction rate and the activation energy. In general, the replacement proceeds one GaAs(111) bi-layer at a time, as demonstrated by lattice resolved HAADF STEM (Figure 2). Ga dissolves in Au and As desorbs, as was previously reported for planar GaAs-Au structures [3]. Using scanning precession electron diffraction (SPED) it was found that there is no fixed epitaxial relation between the newly formed 1D Au-phase and the original GaAs nanowire. The reaction rate and the activation energy for the exchange are accurately determined by tracking the interface between the two phases over relatively long (~0.5 μm) distances.

 

The morphology of the solid 1D Au-phase is the same as for the original GaAs nanowire. Within it, growth twins are observed. The morphology and growth twins do not alter upon cooling and reheating. For the case where the nanowire is attached to a relatively large Au contact (Au reservoir), the contact acts as a Ga diffusion sink, and only negligible amounts of Ga are found in the formed 1D Au-phase. The in situ STEM and electron diffraction results prove that the replacement reaction takes place in the solid state. For the case where the nanowire is attached to a limited Au supply, as would be the case for a small volume Au deposition onto the nanowire or a Au catalyst droplet used for the nanowire growth, the growing metal segment gradually becomes richer in Ga as the exchange reaction proceeds. The reaction rate is slowed down over time due to the Ga enrichment. Eventually and at sufficiently high temperatures, the Au-Ga segment becomes liquid. Upon cooling of such segments, different Au-Ga intermetallic phases form and the main phases could be identified using a combination of SPED and machine learning (Figure 3).

 

References:

[1]: M. Orrù, et al, Phys. Rev. Appl., 4, 044010, 2015. DOI: http://dx.doi.org/10.1103/PhysRevApplied.4.044010
[2]: V. T. Fauske, et al, submitted.
[3]: T. Sebestyen, Electronics Lett., 12, 96, 1976. DOI: 10.1049/el:19760075

 

The authors acknowledge: The Research Council of Norway for the support to the NorFab (197411/V30) and the NORTEM (197405) facilities, and the FRINATEK program (214235), NTNU for support of the initiative “Enabling Technologies” and the EU for support via ERC grant no. 259619 and grant no. 312483 ESTEEM2.


Vidar FAUSKE, Junghwan HUH, Giorgio DIVITINI, Mazid MUNSHI, Dasa Lakshmi DHEERAJ, Caterina DUCATI, Helge WEMAN, Bjørn-Ove FIMLAND, Antonius VAN HELVOORT (Trondheim, Norway)

10:30-12:30
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MS4-I
MS4: Complex materials and nanocomposites
SLOT I

MS4: Complex materials and nanocomposites
SLOT I

Chairpersons: Rick BRYDSON (Chairperson, Leeds, United Kingdom), Marc SCHMUTZ (Chairperson, CNRS-UNISTRA, Strasbourg, France)
10:30 - 11:00 Cryo MEB. Roger A. WEPF (Invited speaker, Zürich, Switzerland)
Invited
11:00 - 11:15 #5792 - MS04-OP249 Application of Cryogenic Focused Ion Beam Scanning Electron Microcopy to Hydrogel Characterisation.
Application of Cryogenic Focused Ion Beam Scanning Electron Microcopy to Hydrogel Characterisation.

Hydrogels are an important material as support matrices for cells to promote growth. These systems have been characterized by electron microscopy, through the application of fixation or sucrose embedding followed by ultramicrotomy. In this way the porosity of the gels can be assessed by transmission electron microscopy (TEM) and/or scanning electron microscopy (SEM). While this approach does give a guide to porosity of samples the absence of water in a hydrogel will have a detrimental effect and its absence distorts the dimensions of the remaining gel. Focused ion beam scanning electron microscopy (FIB-SEM) has been used on dried hydrogels [1], however, we propose a new method of gel porosity characterization by the use of cryogenic-FIB-SEM (cryo-FIB-SEM).  

Cryo-SEM is a long established technique to preserve the water content of a sample and more recently it has been demonstrated that cryo- FIB-SEM can be used for biological and soft matter materials [2]. In this work, the authors have used cryo-FIB-SEM to investigate the porosity and structure of gels whilst in the presence of water. Gels were plunge frozen in slush nitrogen or using a metal mirror freezer and transferred under liquid nitrogen to the sample shuttle of a Cryo-SEM system (Quorum PPT 2000, Quorum Technologies). In the prep-chamber, the sample was coated for 60 seconds using a Pt sputter target. The samples were then loaded into the FIB-SEM (FEI Quanta 3D, FEI). Once in the SEM chamber, the gels were prepared for FIB by deposition (3-4 seconds) of a platinum precursor from the gas injector (set to 27 °C) of the microscope.

The hydrogel samples were milled using an initial current of 1-3 nA to make a rough cut and then by further cuts at lower milling currents (0.3 nA-50 pA), to remove the common milling artefact known as curtaining. SEM micrographs of the visible milled face showed dark patches with largely white areas in between. It was initially postulated that the darker areas were the pores of the gel. In order to test this, the temperature in the SEM chamber was raised to -90 °C, leading to slow sublimation of the water at the FIB milled face (Figure 1). Over approximately 20 minutes images of the slowly subliming gel were acquired. The resulting images show a transition from the black features amongst the majority of lighter contrast through to images with inverse contrast. The final sublimed gel images are clearly interpretable as a porous gel where now the lighter contrast features are identified as the gel strands and the pores are now darker and devoid of water. With a better understanding of the location of these components the original non-sublimed images can be re-examined and the black contrast correlated directly to the polymer and the white to the water.

By inverting the contrast of the original milled face image, it is possible to give an image equivalent to the dehydrated image, but which has all the water bound and is therefore a truer representation of the gel’s morphology (Figure 2). The major advantage to this is that the sample does not undergo shrinkage, and that the process of imaging the milled face hydrated saves time. Additional slices of the freshly milled face may then be acquired to yield a series of slices suitable for 3D rendering (figure 3).

 

References:

[1] A Al-Abboodi et al., Biotech and Bioengineering, 110 (2013), p. 328.

[2] M Marko et al., J Microsc. 222 (2006), p. 42


Chris PARMENTER (Nottingham, United Kingdom), Abdulraman BAKI, Kevin SHAKESHEFF
11:15 - 11:30 #5948 - MS04-OP252 Low dose analysis of nanoparticles suspended in vitreous ice for near native state imaging.
MS04-OP252 Low dose analysis of nanoparticles suspended in vitreous ice for near native state imaging.

Most formulated fine chemical products are complex systems that contain multiple components, with nanoparticles and any incorporated surface coatings interacting with other particles and the dispersant (which can be a liquid or solid). Assessing nanoparticles when suspended in a liquid can be challenging as the particles may disperse individually, agglomerate, aggregate, sediment, chemically-alter or even dissolve and re-precipitate.  With the appropriate sample preparation however, TEM can be used to measure the dispersion and any transformation of nanoparticles suspended in, for example, biological or environmental media [1,2].

Conventional transmission electron microscopy (TEM), with samples prepared by simply drop-casting suspensions onto a thin carbon film, enables imaging and analysis of individual nanoparticles but, because of the drying process, does not capture the particle agglomeration in the dispersion or the surface chemistry when hydrated [3]. To overcome this problem we have prepared thin sections of nanoparticle suspensions for TEM by plunge-freezing a blotted grid into liquid ethane to ensure the aqueous phase vitrifies with no significant redistribution of suspended material. We have used this technique to quantify the dispersion of polymer coated quantum dots, silica and zinc oxide nanoparticles in water and biological cell culture media, identifying the true form in which these nanoparticles are taken up into cells in vitro and thereby providing mechanistic insight to the cellular response at these exposures [3,4,5].

Low dose electron microscopy of nanoparticles suspended in vitreous ice provides opportunity for the analysis of the structure and chemistry of the dispersion, both vital characteristics to understand before any successful biomedical exploitation of nanoparticles. Here, dextran coated iron oxide nanoparticles agglomerated in aqueous suspension and captured in vitreous ice were imaged by bright field TEM and analysed by energy dispersive X-ray (EDX) spectroscopy. Careful control of the illumination conditions (electron dose) permit near native state imaging and confirmation of composition before inducing significant damage to the surrounding ice matrix and subsequent movement of the particles (Figure 1). HAADF STEM imaging was conducted using a 1.3 Å probe and 60 pA probe current, with a resulting EDX map collected in just over one minute showing an iron signal appropriately localised to the nanoparticles (Figure 2).

Going forward, we will use the recently installed FEI Titan Cubed Themis 300 G2 S/TEM at the University of Leeds which is equipped with FEI SuperX EDX spectrometers, a Gatan Quantum ER imaging filter and Gatan OneView CCD to explore the limits of nanoparticle structural analysis (incorporating diffraction and lattice imaging), as well as use of STEM-EDX and electron energy loss spectroscopy for detailed elemental analysis when encased in vitreous ice. In addition to examining the dispersion state of nanoparticles in different suspensions, our goal is to identify and analyse the surface coatings on nanoparticles in the frozen hydrated state, thereby extending the capability of near native state imaging and analysis of nanoparticle suspensions by TEM.

 

 

 

1. N. Hondow, A. Brown and R. Brydson (2015) Frontiers of Nanoscience, 8, 183 – 216.

2. R. Brydson, A. Brown, C. Hodges, P. Abellan and N. Hondow (2015) J. Microscopy, 260, 238 – 247.

3.  N. Hondow, R. Brydson, P. Wang, M.D. Holton, M.R. Brown, P. Rees, H.D. Summers and A. Brown (2012), J. Nanopart. Res., 14, 977.

4. Q. Mu, N.S. Hondow, L. Krzeminski, A.P. Brown, L.J.C. Jeuken and M.N. Routledge (2012) Particle Fibre Toxicol. 9, 1.

5. R. Wallace, A.P. Brown, R. Brydson, S.J. Milne, N. Hondow, P. Wang (2012) J. Phys. Conf. Ser. 371, 012080.

 

Acknowledgment: We thank FEI for the data shown in the figures which were collected as part of a demonstration at the FEI Nanoport, Eindhoven, and Steve Evans (Swansea University) for the dextran coated iron oxide nanoparticles.


Nicole HONDOW (Leeds, United Kingdom), Michael WARD, Rik BRYDSON, Andy BROWN
11:30 - 11:45 #6121 - MS04-OP254 Scanning electron diffraction of polyethylene.
Scanning electron diffraction of polyethylene.

Microstructural investigation of light elements and highly beam sensitive polymer materials using electron microscopy is attractive for elucidating nanostructure but presents numerous challenges. In particular, heavy element staining, often used to obtain image contrast, may obscure or degrade the structure of interest and the acquisition of detailed and spatially resolved information must be balanced with damage of the specimen. Here, scanning electron diffraction (SED) has been used to analyse the crystalline microstructure of unstained polyethylene, overcoming these challenges. SED involves scanning the electron beam across the specimen and recording a diffraction pattern at each position [1] at a high frame rate to enable the data to be acquired before severe degradation of the structure has occurred. In this way, electron diffraction patterns were obtained from an unstained polyethylene sample in 5 nm steps, over areas of a few microns squared and with a 10 ms dwell time. Radiation damage was further minimised by using a high electron acceleration voltage (300kV) to minimise radiolysis and cooling the sample with liquid nitrogen [2]. The diffraction patterns, acquired at every position in the scan, were indexed and analysed by plotting the intensity of a particular reflection as a function of electron probe position to form ‘virtual’ dark field (VDF) images. VDF imaging is much more effective than conventional imaging for visualizing the microstructure of polyethylene. Clear contrast is obtained without staining and the versatile post-facto nature of VDF image formation enables multiple complementary images to to be produced from a single acquisition.
Two novel observations from our SED experiments on polyethylene are highlighted here. The sample of polyethylene was extruded from a melt so as to form ‘shish-kebab’ structures confirmed through BF images of stained microtomed sections. For our experiments, again the samples were microtomed but now unstained to avoid any influence of the stain on the diffraction patterns. The first experiment highlights a lamella-like fragment of polyethylene crystal (likely to be a part of the ‘kebab’ structure). Fig 1 shows a ‘virtual’ BF image and a sample of diffraction patterns that can only be indexed assuming the orthorhombic crystal structure of polyethylene and that the lamella is twisting about a single axis almost parallel to the vertical axis of the image. Moreover, forming consecutive VDF images made it possible to visualize each region of the crystal having a particular orientation in the twisted lamella (Fig 2). In the second experiment, we found that the sample had several micron sized islands of hexagonal polyethylene first seen as a high pressure phase [3]. However, here the patterns reveal a √3 superstructure with weak spots at the 1/3[110]* position (Fig 3). VDF images (Fig 3(b-d)) formed by these supercell reflections revealed domains within which ‘striped’ contrast can be seen; these stripes run at an orientation of approximately 120° to one another. This work demonstrates the applicability of the SED technique to highly beam sensitive materials like polyethylene and the potential for new microstructural insights to be made in this way.


[1] Moeck P. et al., Cryst. Res. Technol., 2011, 46, 586-606
[2] Egerton R. F. et al., Micron, 2004, 35, 399-409
[3] Bassett D. C., et al., Journal of Applied Physics, 1974, 4146-415

   

PAM and SJK would like to acknowledge funding under ERC Advanced Grant 291522-3DIMAGE. DNJ receives a Vice Chancellor’s award from the University of Cambridge. HJ and HT thank Ms. Makiko Ito for her help in microtoming the polyethylene samples. The authors would like to thanks Anton Jan Bons (ExxonMobil) for initiating this research and stimulating discussions.


Sungjin KANG (Cambridge, United Kingdom), Duncan JOHNSTONE, Hiroshi JINNAI, Takeshi HIGUCHI, Hiroki MURASE, Paul MIDGLEY
11:45 - 12:00 #6743 - MS04-OP258 Non-rigid image registration of low-dose image series of zeolite materials.
Non-rigid image registration of low-dose image series of zeolite materials.

Zeolites are an important group of materials with a wide range of application in the catalysis industry. Many structural studies of zeolites rely on high resolution electron microscope imaging [1]. However, due to their high sensitivity to electron irradiation, zeolites deteriorate quickly under exposure to the electron beam. Low-dose imaging techniques use a reduced electron flux to slow the crystal degradation process, which gives more time for adjustment of the microscope configuration and better control over the progression of damage. However the disadvantage of low-dose imaging is poor signal to noise ratio which is often alleviated by averaging multiple image frames in a time series for improved image quality. Traditional rigid cross-correlation function (XCF) image registration methods work well for aligning high-dose time series of radiation-robust materials which experience little or no deformation during image acquisition. However, the deformation in radiation-sensitive materials, often manifest by sample shrinkage, means that the single translational shift vector from rigid image registration may not be sufficient for aligning time series and hence a non-rigid registration scheme is needed.

 

In this work, a low-dose time series of ZSM-5 zeolite consisting 60 image frames were recorded using an aberration-corrected JEOL2200MCO TEM (Figure 1). Two registration methods, a rigid XCF registration and a new non-rigid registration, were used to align the series respectively. The non-rigid registration method [2] is assisted by an IQ factor criterion, which evaluates the quality of the averaged image of the series as the registration proceeds and selects the best averaged image as the reference for future registration iterations.

 

The results show that the new non-rigid registration is helpful for alignment of low-dose TEM image series of radiation-sensitive materials that experience deformation during imaging, especially when the number of frames is small and when the sample is already damaged (Figure 2). This implies that, for TEM image series, the non-rigid registration approach is more effective in noise suppression and in avoiding the image components of a damaged sample compromising the final averaged image.

 
For further comparison, a low-dose STEM time series of zeolite Y, was registered by both rigid and non-rigid methods. A comparative analysis of IQ factor was carried out on the averaged images and showed that the non-rigid registration consistently outperforms the rigid XCF registration (Figure 3). The reason for this advantage is attributed to the fact that the STEM images often suffer from additional scan noise due to the pixel-by-pixel acquisition in STEM imaging. 
 
 
References
[1] M. Pan and P. A. Crozier, Ultramicroscopy. 1993 48(3):332–340.
[2] B. Berkels, P. Binev, D. A. Blom, W. Dahmen, and R. C. Sharpley, Ultramicroscopy. 2014 138:46-56. 

Chen HUANG (Oxford, United Kingdom), Benjamin BERKELS, Angus KIRKLAND
12:00 - 12:15 #6980 - MS04-OP259 Three dimesional nano- and interfacial structures in the Si rich SiC systems analysed by spectroscopic electron tomography.
Three dimesional nano- and interfacial structures in the Si rich SiC systems analysed by spectroscopic electron tomography.

Silicon (Si) nanopartcles (NPs) embedded in the insulating or semiconducting matrices has attracted much interest for the third generation of photovoltaics, so called “all-Si” tandem solar cells. In this work, the amorphous Si rich silicon carbide (SRSC) absorber layers with 30% carbon content were deposited using plasma enhanced chemical vapour deposition (PECVD) on quartz substrate at 500 ˚C, and then the SRSC films were annealed at 1100 ˚C in nitrogen for 1 hour 1. The thermal treatment leads to the SRSC films spinodally decomposed into a Si-SiC nanocomposite. The nanostructures of the phase separated Si and SiC presented in the 15 minutes and 1 hour annealed SRSC films were investigated by two dimensional (2D) energy-filtered transmission electron microscopy (EFTEM). After the thermal treatment, the coexistence of crystalline Si and SiC nanoparticles (NPs) were observed from the high resolution TEM (HRTEM) images and verified by the selected area diffraction (SAD) patterns. After 1 hour annealing, neither Si nor SiC phases are complelely crystallized, the detailed morphologies of Si and SiC nanostructures were studied by electron tomography. For the first time, we make use of EFTEM spectra-imaging (SI) dataset to reveal the three dimensional distributions of Si, a-SiC and c-SiC in sub-volumes. In particular, to obtain more detailed and quantitative information, we have fitted the plasmon spectra with reference plasmon peaks. This enables us to not only to get a quantitative 3D image of all components involved in the materials system in the final tomogram, but also to obtain information about hitherto undetected phases in this system. In such energy resolved plasmon tomograms, the 3D shape of a thin amorphous SiC layer (1) was observed at interface between the crystalline Si network like structure and crystalline SiC NPs. The appearance of the a-SiC interfacial layer is expected from nucleation theory.

Acknowledgements

The authors ackonwledge the support from the EU funded FP7 Project SNAPSUN, the Knut and Alice Wallenberg Foundation and the Swedish Science Council.

References

1.        Perraud, S. et al. Silicon nanocrystals: Novel synthesis routes for photovoltaic applications. Phys. Status Solidi a-Applications Mater. Sci. 210, 649–657 (2013).


Ling XIE, Karol JAROLIMEK, Rene VAN SWAAIJ, Klaus LEIFER (Uppsala, Sweden)
12:15 - 12:30 #5927 - MS04-OP251 Understanding the complex structures in nanoglasses.
MS04-OP251 Understanding the complex structures in nanoglasses.

In recent years nanoglass materials have attracted a lot of interests due to their special physical properties, which differ significantly from traditional bulk amorphous materials of the same composition [1,2]. For example, a Sc75Fe25 nanoglass exhibited a remarkable plasticity whereas the corresponding ribbon glass was brittle [3]. It has been suggested that these special properties originate from the interfacial regions between amorphous nano domains (analogous to grain boundaries changing the properties of nanocrystalline materials), which have either a different atomic configuration or chemical composition, or both compared to the domain core. Due to the amorphous structure of the materials, the structural variations between the core of a glassy grain and the interfacial region are difficult to distinguish, especially when the composition is similar. By STEM-EDX/EELS spectrum imaging and by EFTEM imaging, such composition variations were confirmed for a number of nanoglass systems synthesized using various methods [2], for example inert gas condensation (IGC), magnetron sputtering and ultra-high vacuum (UHV) cluster deposition. On the other hand, radial distribution function (RDF), which can be extracted from electron diffraction, has proven very sensitive to the small difference in atomic configurations [4], e.g., interatomic distances and coordination numbers. The newly developed STEM-RDF mapping has been shown to be capable of resolving the different amorphous structures at nanoglass core and at the interface, respectively.

In this presentation, we investigated the Sc75Fe25 nanoglass primary particles synthesized by IGC as well as the pellet pressed at 6 GPa. The primary particles were directly collected by a carbon coated TEM grid and the pellet sample was processed by FEI strata focused ion beam (FIB) for TEM observation. From the STEM-EDX mapping on the primary particles (Fig. 1), a Sc-rich shell is clearly resolved. Quantification of the integrated spectra from the out part and from the core of the particle reveals the Sc:Fe atomic ratio being around 4:1 and 2:1, respectively. After consolidation under high pressure, the inhomogeneity in primary particles remains and leads to Sc-rich interface between the areas originated from the particle cores. Additionally, by performing STEM-nanobeam diffraction on the pressed Sc75Fe25 sample, RDF mapping was obtained and two different types of RDFs are distinguished, indicating that there exist two major components, one with higher Fe-Sc coordination number (red curve in Fig.2), and the other one with higher Sc-Sc coordination number (green curve in Fig. 2). By multiple linear least square (MLLS) fitting, corresponding component maps are constructed and shown in Fig.2. In the color-mix map, the green areas represent the interface between particle cores, where Sc-Sc bonding is dominating and the red areas represent the particle cores, where Fe-Sc coordination is considerably higher.

In nanoglass systems synthesized by other method, e.g. NiTi-Cu by magnetron sputtering [5] and NiP by electrochemical deposition, composition and structure fluctuations were also observed. The different amorphous structures constrained locally can be successfully revealed by (S)TEM spectroscopic and nanobeam diffraction methods, which is an important step towards understanding the unique structure in nanoglasses compared to the conventional glasses. The correlation between synthesis and structure of nanoglasses makes it possible to design the amorphous nanomaterials with desired functionalities.

References

1. H. Gleiter, Acta mater. 48 (2000) 1.

2. Gleiter, Beilstein J. Nanotechnol. 4 (2013) 517.

3. J.X. Fang, U. Vainio, W. Puff, R. Würschum, X.L. Wang, D. Wang, M. Ghafari, F. Jiang, J. Sun, H. Hahn, H. Gleiter, Nano Lett. 12 (2012) 5058.

4. X. Mu, S. Neelamraju, W. Sigle, C.T. Koch, N. Totò, J.C. Schön, A. Bach, D. Fischer, M. Jansen, P. van Aken, J. Appl. Cryst. 12 (2013) 1105.

5. Z.Śniadecki, D.Wang, Yu.Ivanisenko, V.S.K.Chakravadhanula, C.Kübel, H.Hahn, H.Gleiter, Materials characterization 113 (2016) 26.


Di WANG, Xiaoke MU (Eggenstein-Leopoldshafen, Germany), Chaomin WANG, Tao FENG, Aaron KOBLER, Christian KÜBEL, Horst HAHN, Herbert GLEITER

14:00-16:00
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IM5-II
IM5: Quantitative imaging and image processing
SLOT II

IM5: Quantitative imaging and image processing
SLOT II

Chairpersons: Joanne ETHERIDGE (Director) (Chairperson, Melbourne, Australia), Jean-Christophe OLIVO-MARIN (Chairperson, Paris, France)
14:00 - 14:30 #8366 - IM05-S45 Exposing New Atomic-scale Information about Materials by Improving the Quality and Quantifiability of Aberration-corrected STEM Data.
Exposing New Atomic-scale Information about Materials by Improving the Quality and Quantifiability of Aberration-corrected STEM Data.

Aberration-corrected scanning transmission electron microscopy (STEM) is providing previously unattainable views of materials at the atomic scale. The quality of STEM data is now often limited by environmental and experimental factors instead of instrument factors (e.g. electron optics). Some of these environmental limitations can be overcome by collecting and processing STEM data using new data science techniques. These techniques expose new atomic-scale materials information by improving our ability to measure atomic column positions, 3D structure, single point defects, and atomic-scale composition.

The precision in locating atomic column positions in STEM images is fundamentally limited by the image signal to noise ratio (SNR), but typically practical limits including sample and microscope instabilities that produce distortions in STEM images are encountered before reaching the SNR limit. We have developed a non-rigid registration (NRR) technique that corrects image distortion of all length scales and enables averaging to enhance the SNR.[1-2] Sub-pm precision images of single crystal materials have been achieved by NRR and averaging high angle annular dark field (HAADF) STEM images. NRR has allowed measurements of pm-scale bond length variations of Pt nanocatalyst atoms that may help explain their catalytic activity. Figure 1 shows NRR and averaged HAADF STEM data of a Pt nanocatalyst on an alumina support from a side-view that exhibits ~2 pm precision. Displacement measurements of each atomic column position reveal moderate Pt surface bond length contraction and strong but localized strain of Pt atoms near nanoparticle-support interface. Some of the interface strain is transferred up the twin boundary.

Determining the three dimensional atomic structure of materials from two dimensional S/TEM images is a major hurdle. The standardless atom counting technique is one promising route to measure local sample thickness by quantitatively comparing experimental and simulated HAADF STEM images and can be used to deduce 3D structure.[3] Unlike previous examples, NRR and averaging STEM images have allowed standardless atom counting with the uncertainty no longer dominated by Poisson noise [1]. This should allow the unique determination of the number of atoms in atomic columns, although this has not yet been demonstrated due to other sample limitations.

Point defects are critical to the properties of a wide range of materials, but imaging single defects is challenging. Quantitative STEM has allowed imaging single substitutional and interstitial dopant impurity atoms[4], but experimentally imaging single vacancies has remained elusive. We have used HAADF STEM frozen phonon multislice simulations to predict the detectability of La vacancies in LaMnO3 by the reduced atomic column intensity and atomic column distortions around the vacancy. NRR and averaging HAADF STEM images of LaMnO3 improves the SNR and the image precision sufficiently to potentially detect single La vacancies. Experimental images contain candidate single La vacancies that have local atomic column distortions and intensity variations which match simulated predictions.

Atomic-resolution composition maps can be created using STEM energy dispersive x-ray spectroscopy (EDS) spectrum imaging (SI). However, long total dwell times that may introduce spatial distortions are required because of low x-ray production and collection efficiency. The most common approach to minimize distortions is to sum multiple SIs using online drift-correction software that discards the individual SIs and HAADF images. The quality of EDS SIs of a Nd2/3TiO3 sample was improved by saving the simultaneously acquired raw HAADF and EDS SI series, and applying post acquisition NRR and averaging. The resulting elemental maps show less spatial distortions and more atomic localization of x-rays. In addition, a novel non-local principle component analysis further enhances the quality of EDS SIs compared to conventional denoising methods.[5]

[1] Yankovich et al Nature Communications, 5 4155 (2014)

[2] Yankovich et al Advanced Structural and Chemical Imaging 1:2 (2015)

[3] LeBeau et al Nano Letters, 10 4405 (2010)

[4] Voyles et al Nature, 416 826 (2002)

[5] Yankovich et al Nanotechnology, submitted (2016)


Andrew YANKOVICH (Göteborg, Sweden), Torben PINGEL, Jie FENG, Alex KVIT, Thomas SLATER, Sarah HAIGH, Dane MORGAN, Paul VOYLES, Eva OLSSON
Invited
14:30 - 14:45 #6658 - IM05-OP114 Nanoparticle Structure from Genetic Algorithm Refinement Against Quantitative STEM Data.
Nanoparticle Structure from Genetic Algorithm Refinement Against Quantitative STEM Data.

We have developed a structure refinement method based on genetic algorithm optimization to create structural models of individual nanostructures based on quantitative scanning transmission electron microscopy (STEM) data [1].  We defined a cost function C for a structural model s, as C(s) = E(s) + αχ2[I(s), Iexp], where E is the simulated potential energy of s, χ2 is goodness-of-fit between the experimental STEM data Iexp and the simulated STEM data I(s), and α is a weighting parameter.  A genetic algorithm (GA) is used to minimize C over structures s, resulting in a structure that is both at a local minimum in the (simulated) energy and in good agreement with experimental data.  The advantage of combining the energy and goodness-of-fit to experiments over optimization on just one or the other is the ability to refine structures that are not at a global energy minimum (like most nanoparticles) from experimental data that does not completely constrain the three-dimensional structure (like a STEM image in one orientation).

We have validated the approach and implementation using simulated experimental data from a metastable, 309-atom Au inodecahedron, as shown in Figure 1.  Figure 1(a) is the test structure, and Figure 1(b) is the simulated STEM image from that structure.  The energy is calculated using an embedded atom method empirical potential for Au.  Figure 1(d) shows the evolution of the two terms in the cost function and the total cost function over the course of the optimization.  Neither term decreases monotonically for the entire optimization, but the entire C(s) does.  Figure 1(c) shows the STEM image of the refined structure after 2200 generations, which is an essentially perfect match for the input image in (b).  Figure 1(e) shows that the 3D structures are also a perfect match, with a maximum difference in atomic positions of 0.02 Å.

As a first test, we have refined the structure of a ~6000 atom colloidal Au nanoparticle, as shown in Figure 2.  Figure 2(a) is the experimental STEM image of the particle [2].  Figure 2(c) shows the evolution of the cost function as it converges over 4000 generations to reach the final structure in Figure 2(b).  In this case, the optimization was allowed to change the number of atoms in the structure as well as their position.  The result faithful reproduces the image of the sample, including the outline and the twin boundary.  Figure 2(d) shows the displacement of matching atomic columns in the two images.  The large displacements near 0.3 Å arise from surface atoms which are not well-imaged in the experiment due to surface atom mobility under the electron beam, but which are recovered in the refined model.  Additional applications to Pt and Pt-Mo catalysts will be discussed.

1. “Integrated Computational and Experimental Structure Determination for Nanoparticles” Min Yu, Andrew B. Yankovich, Amy Kaczmarowski, Dane Morgan, Paul M. Voyles (submitted)

2. “High-precision scanning transmission electron microscopy at coarse pixel sampling” Andrew B. Yankovich, Benjamin Berkels, W. Dahmen, P. Binev, and Paul M. Voyles Advanced Chemical and Structural Imaging 1, 2 (2015).  DOI: 10.1186/s40679-015-0003-9


Paul VOYLES (Madison, USA), Zhewen SONG, Dan ZHOU, Zhongnan XU, Andrew YANKOVICH, Dane MORGAN
14:45 - 15:00 #6196 - IM05-OP108 Non-destructive nanoparticle characterisation using a minimum electron dose in quantitative ADF STEM: how low can one go?
Non-destructive nanoparticle characterisation using a minimum electron dose in quantitative ADF STEM: how low can one go?

Aberration-corrected STEM has become a powerful technique for materials characterisation of complex nanostructures. Recent progress in the development of quantitative methods allows us to extract reliable structural and chemical information from experimental images in 2D as well as in 3D. In quantitative STEM, images are treated as datasets from which structure parameters are determined by comparison with image simulations or by using parameter estimation-based methods [1]. So-called scattering cross-sections, measuring the total scattered intensity for each atomic column, are useful values for quantification [2, 3]. Their high sensitivity and robustness for imaging parameters in combination with a statistical analysis enables us to count atoms with single-atom sensitivity [4].  An example is shown in Figure 1, where atom-counting from a single image combined with an energy minimisation approach [5] is used to reconstruct the 3D atomic structure of a Au nanorod. The close match with the 3D tomography reconstruction resulting from images recorded along 3 viewing directions [6] demonstrates the accuracy of the method.

    

Reducing the number of images by avoiding tilt tomography will be of great help when studying beam-sensitive nanostructures. However, the tolerable electron dose is often still orders of magnitude lower than what is typically used for atomic resolution imaging. Therefore, the question arises: how to optimise the experiment design in order to reduce the electron dose? To investigate this, we have developed a statistical framework that allows us to study the effect of electron shot noise, scan noise, and radiation damage on the atom-counting precision. Figure 2 shows that even for low-dose image acquisitions, statistical parameter estimation theory is a powerful tool to refine structure parameters of an incoherent imaging model and to measure scattering cross-sections. However, the presence of electron shot noise in this dose regime is limiting the atom-counting precision. Severe overlap in the distributions of scattering cross-sections related to columns of different thicknesses hampers one to achieve single-atom sensitivity. We will show how the precision improves with increasing electron dose until scan noise, followed by radiation damage, become the main limiting factors. This analysis allows one to balance atom-counting reliability and structural damage as a function of electron dose.

 

Finally, it will be shown how quantitative ADF STEM may greatly benefit from statistical detection theory in order to optimise the detector settings [7]. This is illustrated in Figure 3, where the number of atoms of a beam-sensitive Pt particle is determined from a STEM image acquired under the computed optimal detector settings. In addition, use is made of a novel hybrid method to count the number of Pt atoms, in which the benefits of a statistics-based and image simulations-based method are efficiently combined in one framework. In conclusion, new developments in the field of quantitative STEM will be presented enabling one to quantify atomic structures in their native state with the highest possible precision.

     

[1] S. Van Aert et al., IUCrJ 3 (2016) 71-83.

[2] S. Van Aert et al., Ultramicroscopy 109 (2009) 1236-1244.

[3] H. E et al., Ultramicroscopy 133 (2013) 109-119.

[4] S. Van Aert et al., Physical Review B 87 (2013) 064107.

[5] L. Jones et al., Nano Letters 14 (2014) 6336-6341.

[6] B. Goris et al., Nature Materials 11 (2012) 930-935.

[7] A. De Backer et al., Ultramicroscopy 151 (2015) 46-55.

  

The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N) and postdoc grants to A.D.B. and B.G. S.B. and A.B. acknowledge funding from the European Research Council (Starting Grant No. COLOURATOMS 335078 and No. VORTEX 278510). The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).


Sandra VAN AERT (Antwerp, Belgium), Annick DE BACKER, Annelies DE WAEL, Lewys JONES, Gerardo T MARTINEZ, Bart GORIS, Thomas ALTANTZIS, Armand BÉCHÉ, Sara BALS, Peter D NELLIST
15:00 - 15:15 #5795 - IM05-OP105 Quantitative annular dark-field imaging at atomic resolution.
Quantitative annular dark-field imaging at atomic resolution.

Quantitative annular dark-filed (ADF) imaging in scanning transmission electron microscopy (STEM) enables us to identify the type and number of atoms of local crystal structures. A quantification procedure of ADF images was proposed by LeBeau and Stemmer, in which the signal at each pixel is placed on an absolute scale by normalizing the current reaching an ADF detector by the incident probe current [1]. Their procedure made possible a direct comparison between experimental and simulated ADF images without any arbitrary scaling parameters. In this study we acquired quantitative ADF images of a graphene and compared with simulated images to investigate how accurately the scattering intensities match between experiments and simulations.

We used a Titan3 microscope (FEI) equipped with spherical aberration correctors (DCOR and CETCOR, CEOS) operating at an acceleration voltage of 80 kV. An ADF detector (Model 3000, Fischione) and an analog-to-digital (A/D) converter (DigiScan II, Gatan) were used. We evaluated a nonlinear response of the ADF signal detection system, which had not been analyzed. Relationship between an ADF detector current (IADF [pA]) and an ADF image signal (SADF [count]) was measured as shown in Fig. 1. Quantitative contrasts QADF [%], i.e. IADF normalized by the incident probe current I0, were calculated from SADF using the nonlinear response. The quantification procedure was performed using an in-house DigitalMicrograph (Gatan) scripts.

The range of ADF detection angle was experimentally measured. The ADF inner angle (48.4 mrad) was measured by scanning an incident probe on the ADF detector. We found that the ADF outer angle (200 mrad) is limited by the aperture in the microscope column above the ADF detector, and the actual outer angle was measured by observing the shadow of the objective aperture [2]. The STEM image simulation was performed using a multislice program (xHREM with STEM Extension, HREM), in which defocus spread and residual aberrations (up to 5th order) were taken into account.

 

Figure 2 shows (a) a quantitative ADF image of graphene with 1–4 layers and (b) the histogram of the quantitative ADF image. The mean contrast, which was measured by averaging the value in areas including several unit cells, was 0.054% at a single-layer region. Since the mean value of a simulated image was 0.053%, the mean quantitative contrast exhibited excellent agreement between experimental and simulated images. We can instantly decide the number of graphene layers based on the quantitative ADF image.

Next we examined atomic-resolution ADF images of a single layer graphene, as shown in Fig. 3. To reproduce atomic ADF image profiles, an effective source distribution, which corresponds to a demagnified source image on the specimen, should be implemented in STEM simulation. Although a Gaussian function has been often utilized as the effective source distribution, we found that the linear combination between Gaussian and Lorentzian (G+L in Fig. 3c) well reproduces experimental results. We also found that there is a small systematic deviation, which is probably due to time-dependent aberrations (e.g., coma). Highly-stable microscope system and/or real-time aberration assessment are required for the advanced quantitative STEM imaging at atomic resolution.

References:

[1] J M LeBeau and S Stemmer, Ultramicroscopy 108 (2008), 1653.

[2] S Yamashita et al, Microscopy 64 (2015) 143 (doi: 10.1093/jmicro/dfu115).

[3] S Yamashita et al, Microscopy 64 (2015) 409 (doi: 10.1093/jmicro/dfv053).

This study was partly supported by the JST Research Acceleration Program and the Nano Platform Program of MEXT, Japan. The authors thank Dr. T. Nagai, Mr. K. Kurashima and Dr. J. Kikkawa for support in the STEM experiments.


Shunsuke YAMASHITA, Shogo KOSHIYA, Kazuo ISHIZUKA, Koji KIMOTO (Tsukuba, Japan)
15:15 - 15:30 #5050 - IM05-OP103 The atomic lensing model: extending HAADF STEM atom counting from homogeneous to heterogeneous nanostructures.
The atomic lensing model: extending HAADF STEM atom counting from homogeneous to heterogeneous nanostructures.

Counting the number of atoms in each atomic column from different viewing directions has proven to be a powerful technique to retrieve the 3D structure of homogeneous nanostructures [1]. In order to extend the atom counting technique to heterogeneous materials, this work presents a new atomic lensing model facilitating both atom counting and 3D compositional determination in such materials.

In the quantitative evaluation of high angle annular dark field scanning transmission electron microscopy (HAADF STEM) images the so-called scattering cross-section (SCS) has proven to be a successful performance measure [1-3]. Its monotonic increase with thickness can be used to count the number of atoms in homogeneous materials with single atom sensitivity [4]. However, for heterogeneous materials, small changes in atom ordering in the column can change the SCS (Fig. 1), significantly complicating atom counting. This depth dependency requires a quantitative method to predict SCSs of all possible 3D column configurations, already more than 2 million for a 20 atoms thick binary alloy. Image simulations can provide this information, but the amount of required simulations makes it an impossible task in terms of computing time. Therefore, a new atomic lensing model is developed based on the principles of the channelling theory [5], where each atom is considered to be an electrostatic lens resulting in an extra focussing effect on the probe. This model allows one to predict the SCSs of mixed columns based on the lensing factors of the individual atoms in monotonic atomic columns. As compared to a linear model neglecting channelling, this new approach leads to a significant improvement in the prediction of SCSs which is not restricted to the number of atom types (Fig. 2) and can be used for a wide range of detector angles (Fig. 3).

The power of the atomic lensing model to accurately predict SCSs enables one to extend the atom counting technique to heterogeneous materials. Here, simulated SCSs can be matched to the measured experimental SCSs. Next, the 3D structure can be determined by combining atom counts from different viewing directions. In this presentation, this technique will be demonstrated on experimentally recorded images of an Au@Ag nanocrystal. Another advantage of the atomic lensing model is its ability to accurately predict the atom ordering in the column from experimental SCSs (Fig. 1). Therefore, it opens up the possibility to extract 3D information from a single image. This ability will be presented on a simulated image of an Au@Ag nanorod.

In conclusion, a new atomic lensing model is developed which is of great importance for extending the atom counting technique from homogeneous to heterogeneous nanostructures.

 

References

[1] Van Aert et al., Nature 470 (2011), p. 374

[2] Van Aert et al., Ultramicroscopy 109 (2009), p. 1236

[3] E. et al., Ultramicroscopy 133 (2013), p. 109

[4] Van Aert et al., Physical Review B 87 (2013), 064107

[5] Van Dyck et al., Ultramicroscopy 64 (1996), p. 99

 

The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N) and research grants to K.H.W. van den Bos and A. De Backer. S. Bals and N. Winckelmans acknowledge funding from the European Research Council (Starting Grant No. COLOURATOMS 335078). The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).


Karel H W VAN DEN BOS (Antwerp, Belgium), Annick DE BACKER, Gerardo T MARTINEZ, Naomi WINCKELMANS, Sara BALS, Peter D NELLIST, Sandra VAN AERT
15:30 - 15:45 #6452 - IM05-OP111 Mapping 2D strain components from STEM moiré fringes.
Mapping 2D strain components from STEM moiré fringes.

Artificial moirés are created in a STEM by deliberately choosing a low magnification where the scan step is close to the crystalline periodicity (see Figure 1a) [1]. A moiré contrast then results from the interference between the scan and the crystal lattice. The technique has been developed to analyse strain [2], and has been applied to the study of strained-silicon devices [3].

 

In reciprocal space, STEM moiré fringes can be understood as the convolution of the lattice created by the scan, characterized by the scan-step, s in real-space, or s* in reciprocal space, and the reciprocal lattice of the crystalline lattice, characterized by d* (Figure 1b). Interference between neighbouring periodicities gives rise to moiré fringes of periodicity qM. In general, only a few moiré fringe periodicities will be present in the image as the scan reciprocal lattice is in reality multiplied by the MTF of the probe: the size of the probe limits the possible interference terms (indicated by the yellow area in Figure 1b). The moiré peridocity qM is related vectorially to s*, which is usually 1 or 2 pixel-1, and the d* (or g-vector) for a particular set of lattice fringes (Figure 1c).

 

Here we show how the strain information can be extracted using the concept of geometric phase, previously used for the analysis of high-resolution TEM images [4]. The advantage of this formulation is that the moiré fringes do not need to be aligned exactly with the crystalline lattice, thus freeing up the experimental work. The periodicity of the moiré fringes (Figure 2a) is identified from the power spectrum of the image and the corresponding geometric phase determined (Figure 2b). The strain is in turn calculated from the geometric phase (Figure 2c). It is not necessary to know the exact calibration of the original image providing a reference region of known crystal parameter is present, as the vectorial relationship (Figure 1c) provides a strong constraint.

 

We have also developed a procedure to determine maps of the 2D strain tensor from two differently oriented SMFs [5] (Figure 3). The results from two separate moiré fringe images need to be aligned (Figure 3b) and combined to determine the full 2D in-plane strain tensor. Whilst the examples here are simulated, we anticipate presenting experimental results from devices and piezo-electric materials.

 

[1] D. Su and Y. Zhu, Ultramicroscopy 110, 229–233 (2010).

[2] S. Kim, Y. Kondo, K. Lee, G. Byun, J. J. Kim, et al., APL 102, 161604 (2013).

[3] S. Kim, Y. Kondo, K. Lee, G. Byun, J. J. Kim, et al., APL 103, 033523 (2013).

[4] M. J. Hÿtch, E. Snoeck, R. Kilaas, Ultramicroscopy 74, 131–146 (1998).

[5] STEM Moiré Analysis (HREM Research Inc.) a plugin for DigitalMicrograph (Gatan)

 

Acknowledgments

The authors greatly acknowledge to Yukihito Kondo (JEOL) for valuable advice during the development of the DM plug-in. This work was funded through the European Metrology Research Programme (EMRP) Project IND54 Nanostrain. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. MJH and CG acknowledge the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.


Martin HYTCH (TOULOUSE CEDEX), Christophe GATEL, Akimitsu ISHIZUKA, Kazuo ISHIZUKA
15:45 - 16:00 #5993 - IM05-OP107 Precision and application of atom location in HAADF and ABF.
Precision and application of atom location in HAADF and ABF.

Precision and application of atom location in HAADF and ABF

Yi Wang1, Dan Zhou1*, Wilfried Sigle1, Y. E. Suyolcu1, Knut Müller-Caspary2, Florian F. Krause2, Andreas Rosenauer2, Peter A. van Aken1

1Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569, Stuttgart, Germany

*Current: Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue, Madison, WI 53706, USA

2Institut für Festkörperphysik, Universität Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany

 

The multifaceted magnetic, electrical, and structural functionalities of perovskite oxides are underpinned by the distortions of the crystal lattice [1]. These distortions include the displacement of cations, deformation of oxygen octahedra (BO6, where B is a transition metal atom), and collective tilts of the octahedral network. Controlling and engineering these distortions in the constituent oxides are crucial in designing and fabricating heterostructures with novel functional properties that are absent in the bulk form. Atomistic understanding of these distortions and elucidation of their influence on the final properties requires imaging and measuring of atomic positions of both cations and oxygen. With the application of spherical aberration (Cs) correctors, sub-Angstrom atomic resolution is nowadays regularly achievable in both TEM and STEM. The recent application of the annular bright-field (ABF) imaging technique in perovskite oxides has become increasingly popular, as it enables simultaneous imaging of heavy and light elements and allows for simultaneous acquisition of other signals [2, 3].

Here, we report the development of a software tool, written in Digital Micrograph scripting language [4], to extract quantitative information of the crystal lattice and of oxygen octahedron distortions of perovskite oxides from high-angle annular dark-field (HAADF) and ABF STEM images. Center-of-mass and two-dimensional (2D) Gaussian fitting methods are implemented to locate positions of individual atom columns. As shown in Fig.1, under daily reproducible working conditions, e.g. sample drift and contamination present, the precision is in the range of 3–4 pm. Applications of this tool will be presented.

The accuracy of atom location by ABF can be significantly influenced by atom-column tilts introduced by inadequate alignment by the operator or by strain near crystal defects. The influence of such tilts was quantitatively analyzed using image simulations. Figure 2 shows exemplarily simulated HAADF and ABF images for 0 and 10 mrad tilt [5].

 

References:

 

[1] R H Mitchell “Perovskites: Modern and Ancient”, (Almaz, Thunder Bay)

[2] S D Findlay et al., Appl. Phys. Lett. 95 (2009), p.191913.

[3] E Okunishi et al., Microsc.Microanal.164 (2009), p.15.

[4] D R G Mitchell, B Schaffer, Ultramicroscopy 103 (2005), p.319.

[5] The research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure Initiative I3).


Yi WANG, Dan ZHOU, Wilfried SIGLE (Stuttgart, Germany), Y. Eren SUYOLCU, Knut MÜLLER-CASPARY, Florian F KRAUSE, Andreas ROSENAUER, Peter VAN AKEN

10:30-12:30
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MS7-I
MS7: Materials for optics and nano-optics
SLOT I

MS7: Materials for optics and nano-optics
SLOT I

Chairpersons: David MCCOMB (Chairperson, Columbus, USA), Jérôme PLAIN (Chairperson, Troyes, France)
10:30 - 11:00 #8634 - MS07-S84 Characterizing Localized Surface Plasmons using Electron Energy-Loss Spectroscopy.
Characterizing Localized Surface Plasmons using Electron Energy-Loss Spectroscopy.

Localized surface plasmon resonances (LSPRs) are the coherent and collective oscillations of conduction band electrons at the surface of metallic nanoparticles. LSPRs are known to localize far-field light to a sub-diffraction-limited length scale, yielding an intense electric field at the particle surface. This effect has been harnessed to dramatically enhance light-matter interactions, leading to a variety of applications such as surface-enhanced Raman spectroscopy (SERS), photothermal cancer therapy and solar energy harvesting. Though a variety of near- and far-field optical methods are used to probe LSPRs, the spatial resolution of these methods is on the order of tens of nanometers, limiting their effectiveness. In contrast, electron energy loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) combines sub-nanometer resolving power with the capability to excite both optical-accessible and –inaccessible plasmon modes and therefore has emerged as one of the leading techniques (Figure 1). In this presentation, I will briefly introduce the STEM/EELS technique and demonstrate the power of STEM/EELS in the characterization of LSPRs. In addition to the traditional use of STEM/EELS for LSPR imaging, we have recently demonstrated that STEM/EELS can also be used to spatially map LSP-semiconductor energy transfer at the nanoscale. The future of STEM/EELS as a window into the nanoscopic world is especially promising, and we expect continued advances in the molecular, optical, materials, information, and energy sciences as a result.


Guoliang LI (Notre Dame, USA), Charles CHERQUI, Yueying WU, Philip RACK, David MASIELLO, Jon CAMDEN
Invited
11:00 - 11:30 #7882 - MS07-S85 Optics at the nanoscale with fast electron spectroscopies.
Optics at the nanoscale with fast electron spectroscopies.

Electron microscopy techniques have been used to probe the optical properties of materials in the subwavelength scale. In particular, it has been shown that using cathodoluminesncence (CL) and electron energy loss spectroscopy (EELS), along with a propitious choice of electron beam energy and target material, resolution below 10 nm is attainable.

In this seminar we will describe recent advances electron spectroscopies in a STEM to probe individual quantum structures. Different experiments attempting to probe their optical properties will be presented (including their lifetimes). Typically, a 1-nm-wide 60 keV electron beam was used to excite the sample. Fundamentally, we have measured spectroscopic signals (CL and EELS) and the second order correlation function (g2(t)) of the emitted light of different materials using an optical spectrometer, an EEL spectrometer and a Hanbury-Brown and Twiss interferometer.

We will start by showing how monochromated EELS and CL experiments are well adapted to probe excitonic excitations in different materials. We will show how EELS is specially adapted to probe excitons in 2-dimensional materials, using MoS2 and MoSe2 monolayers as example. For thicker materials, CL is able to more easily produce meaningful information from excitoninc excitations. As examples, we will discuss the luminescence of GaN quantum wells in AlN nanowires (Figure 1) and excitons in hBN flakes.

However, only spectroscopic information does not give a complete picture of the behavior of a specific system. One example are single photon emitters (SPE). To characterize such systems, light intensity interferometry is necessary (a typical experiment in quantum optics). We will demonstrate how a similar experiment using CL is possible, using as example the neutral Nitrogen-Vacancy center in diamond nanoparticles (Figure 2). Also, we will show how this setup allowed the detection of new SPE in hBN.

Finally, we will discuss how a CL setup with a HBT interferometer in a STEM microscope can be used to measure the lifetime of individual quantum emitters with a high spatial resolution (< 15 nm). We name this technique spatially resolved time-correlated cathodoluminescence, SRTC-CL. As an example, we will show that the lifetimes of 8 GaN quantum wells separated by 15 nm in an AlN nanowire can be measured by SRTC-CL.


Luiz TIZEI (ORSAY)
Invited
11:30 - 11:45 #4605 - MS07-OP297 Tomography of particle plasmon fields by electron energy-loss spectroscopy.
Tomography of particle plasmon fields by electron energy-loss spectroscopy.

Tailoring shape of metallic nanoparticles and alignment of nanoparticle assemblies allows controlling the properties of localized surface plasmon resonances, such as peak positions or near field-coupling and enhancement [1]. Electron beam lithography is a versatile tool for nanoparticle manufacturing, but the technique usually suffers from imperfections, surface roughness, and limited spatial resolution, which leads to particle shapes that deviate from design objectives. Similar limitations apply to chemical synthesis, which leads to nanoparticle assemblies with size dispersion and geometry variations. Therefore, to exploit the full potential of plasmonics, full 3D characterization and simulation taking into account the imperfections of real structures become mandatory. Here we present two different tomography-based approaches to understanding complex plasmonic nanoparticles created by electron beam lithography.

In our first approach the precise 3D geometry of a particle dimer fabricated by means of electron beam lithography was reconstructed through electron tomography. This full 3D morphological information was used as an input for simulations of energy-loss spectra and plasmon resonance maps (Figure 1). Here excellent agreement between measured EELS data and theory was found, bringing the comparison between EELS imaging and simulations to a quantitative and correlative level [2].

In our second approach we directly reconstruct particle plasmon fields from a tomographic tilt series of EELS spectrum images. While first approaches and demonstrations of plasmon field tomography were limited to very small particles [3–5] – small enough to neglect retardation – we lift this limitation with our approach making plasmon field tomography generally applicable to nanoparticles of all sizes [6]. Formulation EELS tomography as an inverse problem allows reconstructing the complete dyadic Green tensor for plasmonic particles, which is linked to the photonic local density of states (LDOS). Using this approach we are able to reconstruct the full 3D LDOS for a silver metallic nanoparticle (Figure 2).

This work overcomes the need for geometrical assumptions or symmetry restrictions of the sample in simulations and generalizes plasmon field tomography to particles of all sizes, paving the way for detailed investigations of realistic and complex plasmonic nanostructures.

 

[1]          S.A. Maier, Springer US, Boston, MA, 2007.

[2]          G. Haberfehlner et al., Nano Lett. 15: 7726–7730 (2015).

[3]          A. Hörl et al., Phys. Rev. Lett. 111:076801 (2013).

[4]          O. Nicoletti et al., Nature. 502: 80–84 (2013).

[5]          S.M. Collins et al., ACS Photonics. 2: 1628–1635 (2015).

[6]          A. Hörl et al., ACS Photonics. 2: 1429–1435 (2015).

 

We thank Joachim Krenn and Harald Ditlbacher for access to and support with electron beam lithography and helpful discussion. This research has received funding from the European Union within the 7th Framework Program [FP7/2007-2013] under Grant Agreement no. 312483 (ESTEEM2). We acknowledge support by the Austrian Science Fund FWF under project P27299-N27, the SFB F49 NextLite, and NAWI Graz


Georg HABERFEHLNER (Graz, Austria), Anton HÖRL, Franz P. SCHMIDT, Andreas TRÜGLER, Ulrich HOHENESTER, Gerald KOTHLEITNER
11:45 - 12:00 #5857 - MS07-OP300 Investigating Surface Plasmon-Enhanced Local Electric Fields by EELS with tunable <60meV Energy Resolution.
MS07-OP300 Investigating Surface Plasmon-Enhanced Local Electric Fields by EELS with tunable <60meV Energy Resolution.

Recent improvements in energy resolution, enabled by the use of electron energy monochromators, have the potential to turn EELS into a tool able to provide quantitative information of localized surface plasmons (LSPs), such as damping effects in single particles and electron kinetics of single plasmon modes.[1] Crucial to the prospect of quantitative analysis of LSPs is the requirement that the experimental energy resolution must be better than the natural line width of the plasmon resonances, all the while retaining high enough signal-to-noise ratio to enable an accurate determination of the properties of interest.[1] The energy resolution of EELS is customarily determined by the full-width at half-maximum (FWHM) of the zero-loss (ZL) peak. The plasmon resonances, lying in the low-loss regime, often overlap with the broad tail of the ZL peak, blurring many spectral signatures of interest. Until now, several processing techniques had to be applied to overcome these issues, relying for instance on deconvolution algorithms [2] which can introduce artifacts [3] A new generation of electron monochromators now allows for high signal-to-noise ratios while varying the energy resolution controllably, down to the 10meV regime [4].

Here we present recent results aimed at spatially and spectrally resolving the plasmon resonances of individual plasmonic nanostructures and of functional plasmonic devices using a Cs-corrected and monochromated Nion UltraSTEM 100MC (‘Hermes’) microscope with a nominal energy resolution of 10meV. Figure 1 shows spatially resolved LSP modes of two individual Ag particles with different shapes and different number of crystalline domains for the energy ranges indicated in the figure. Both shape and crystallinity appear to affect the plasmonic response. We show how the energy resolution, which also affects the attainable signal-to-noise ratio and dictates the required integration (exposure) time, can be conveniently set and tuned, depending on the inherent properties of the system of interest. Figure 2A shows as-recorded EEL spectra taken from the centre of a Ag nanowire (inset) and showing a narrow bulk plasmon resonance at 3.85eV. We note that the FWHM of the peak does not decrease with decreasing energy resolution from 40meV to 16meV, meaning that 40meV must be below the natural line width of the resonance. In this context, we will discuss the prospect of not only characterizing bare metallic nanostructures, but of also interrogating chemically functionalized plasmonic nanostructures using EELS. We note that the accessible energy ranges (sub-40 meV) also allows us to probe molecules adsorbed onto metal nanostructures. In the raw spectra in Fig. 2B taken from different locations within a sample, spectroscopic signatures of aromatic thiols chemisorbed onto multiple Ag nanoparticles are shown.[5]

 

References:

[1] M. Bosman et al., Scientific reports, 2013, 3.

[2] R. F. Egerton in “Electron Energy-Loss Spectroscopy in the Electron Microscope“, Springer (New York), 3rd Ed., 2011.

[3] E. P. Bellido et al., Microsc. Microanal., 2014, 20, 767; V. Keast and M. Bosman, Microscopy research and technique, 2007, 70, 211; S. Lazar et al., Ultramicroscopy, 2006, 106, 1091.

[4] O. L. Krivanek et al., Nature, 2014, 514, 209-212

[5] SuperSTEM is the UK EPSRC National Facility for Aberration-Corrected STEM, supported by the Engineering and Physical Science Research Council. PZE acknowledges support from the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory. WPH is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.


Patricia ABELLAN (Daresbury, United Kingdom), Patrick Z. EL-KHOURY, Fredrik S. HAGE, Josh COTTOM, Alan G. JOLY, Wayne P. HESS, Rik BRYDSON, Quentin M. RAMASSE
12:00 - 12:15 #5953 - MS07-OP302 How dark are dark plasmon modes - a correlative EELS and CL study on lithographed silver nanodisks.
How dark are dark plasmon modes - a correlative EELS and CL study on lithographed silver nanodisks.

Plasmonic nanostructures enable the concentration of light to the deep subwavelength regime and, thus, are the topic of intense fundamental and application oriented research. Nanoparticles acting as optical antennas are well known for their nanoscale mode volumes, due to the excitation of localized plasmon modes. In this context electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) became a powerful technique as it enables to map the full modal spectrum of plasmon eigenmodes with unprecedented high spatial resolution [1–3]. Beside EELS, Cathodoluminescence (CL) has also recently been used to gain information about the optical response taking advantage of the same high spatial resolution in a TEM [4]. While it is stated that EELS is linked to the full photonic local density of states (LDOS), the CL signal is related to the radiative LDOS [5].

In this work we present a combined EELS/CL study of plasmon eigenmodes on silver nanodisks, using fast electrons in a TEM. In particular we compare the differences of the EELS and CL response using experimental and simulated data. Precise variation of the disk size is achieved by means of electron beam lithography (Figure 1a), enabling a comprehensive study of plasmon excitations on silver nanodisks.

From theoretical considerations it is known, that for certain particle geometries (and therefore for specific surface charge distributions) there exist so called dark modes, which are “invisible” to photons but “visible” to electrons, and therefore can be measured with EELS but not with light [6]. Here we discuss how dark these dark modes are comparing EELS and CL (figure 1b+c). In particular, radial breathing modes (C in figure 1b+c) were predicted to be dark modes [6], although we will show how comparison between EELS and CL can mitigate this statement. Additionally, limitations for the theoretical predictions will be discussed, when the particle size is increased and therefore retardation effects become more important. In this case we show that dark modes are getting brighter. Furthermore symmetry breaking by the excitation source itself, a focused fast electron beam, will be discussed.

 

[1]          Nelayah et al., Nat. Phys. 2007, 3, 348−353.

[2]          Koh et al., Nano Lett. 2011, 11, 1323−1330.

[3]          Schmidt et al., Nat. Commun. 2014, 5, 4604.

[4]          Yamamoto et al., Phys. Rev. B 2001, 64, 205419.

[5]          Losquin et al., ACS Photonics 2015, 2, 1619–1627.

[6]          Schmidt et al., Nano Lett. 2012, 12, 5780−5783.

 

This research has received funding from the European Union within the 7th Framework Program [FP7/2007-2013] under Grant Agreement no. 312483 (ESTEEM2). We acknowledge support by the Austrian Science Fund FWF under project P21800-N20, the SFB F49 NextLite, and NAWI Graz.


Franz-Philipp SCHMIDT (Graz, Austria), Arthur LOSQUIN, Ferdinand HOFER, Joachim R. KRENN, Mathieu KOCIAK
12:15 - 12:30 #6630 - MS07-OP306 Systematic analysis of plasmon excitations and coupling by lithographic structure patterning and fast, monochromated STEM-EELS mapping.
MS07-OP306 Systematic analysis of plasmon excitations and coupling by lithographic structure patterning and fast, monochromated STEM-EELS mapping.

High energy resolution electron energy-loss spectroscopy (EELS) is now a recognised technique for mapping localized surface plasmon resonances and measuring their resonant energies with a nanometric spatial resolution [1–3]. In contrast to light-based techniques it can further excite and map high order harmonics that are optically forbidden. Here we move on from the study of isolated or tandem plasmonic structures randomly deposited on TEM grids, or more complex structures painstakingly patterned by FIB, to a fast, systematic EELS mapping of precisely patterned Au and Ag films on Si3N4 membranes. Following an electron lithography-based preparation, the spatial distributions and mode energies of plasmon resonances and coupling are studied in function of well-controlled variations in structure dimensions.

The measurements are made in STEM-EELS mode using a FEI Titan Themis 60-300 with Gatan Digiscan and GIF Quantum ERS spectrometer. The combination of the X-FEG gun and monochromator gives a sub-nm incident beam with 100–110 meV FWHM of the zero-loss peak and a current of up to 240–250 pA. A fundamental need of the work is that the plasmon excitation can be measured both in the pure Si3N4 membrane regions and in the metal film regions. To this end, a high tension of 300 kV is used because, by reducing the relative intensity of bulk plasmon scattering from the metal films, it improves surface plasmon excitation signal to noise. EELS data are normalized by the zero-loss intensity to give the true projected plasmon distribution, without “shadowing” by the metal film [4]. With the fast spectrum imaging mode and high beam current, dwell times are only 0.2–0.25 ms per pixel, allowing us to acquire maps with >105 pixels (e.g. 600 x 600 px) in < 10 minutes per map.

Applying this fast mapping with high spatial sampling to the lithographically-based structures of known layout and dimension gives a highly time-efficient method for studying plasmonic excitations in nanophotonic structures. Figure 1 shows example plasmon resonance low-loss EELS spectra and intensity maps. Parts (a–d) & (e–f) treat the well known plasmonic structures of a silver wire and nano-triangle. Owing to the energy resolution and good signal to noise ratio, there is no need to perform systematic deconvolution of the data to reveal plasmon excitations, even for modes at < 0.5 eV energy loss. High order multipoles are additionally well resolved for both non-penetrating and penetrating trajectories, such as the 2.8 eV breathing mode of the nano-triangle.

Figure 1 (g–h) shows data from more complex coupled gold heptamer apertures. We probe the effect of a nanoscale defect: a small 20 nm size gap in one of the heptamer arms. This feature induces the appearance of an additional low energy mode at 0.84 eV, and affects both the symmetry and intensity of higher order modes. This demonstrates how systematic study of varying geometries allows for in-depth analysis of plasmon resonances.

These experimental studies are combined with modeling of the excitations using a novel “in house” method which aids interpretation of multi-body interactions by simulating EELS spectra using the properties of plasmonic structures, and not the work done on electrons [5]. The simulations and experiments have a symbiotic relationship, with the modeling used to interpret experimental data, and the experimental data used to guide the modeling (e.g. for peak resonance values). Nevertheless, our experimental approach is significantly quicker than the modeling. While so far it is primarily applied to the systematic study of particles and apertures, in the future it will be used to explore the optical excitations of novel nanophotonic structures and materials.

 

References and Acknowledgements

[1] J. Nelayah et al., Nature Phys. 3 (2007) 348–353.

[2] M. Bosman et al., Nanotechnology 18 (2007) 165505.

[3] F.J. García de Abajo, Rev. Modern. Phys. 82 (2010) 209–275.

[4] N. Le Thomas et al., Phys. Rev. B 87 (2013) 155314.

[5] G.D. Bernasconi et al., J. Opt. Soc. Am. B 33 (2016) 768–779.

We thank the staff of the Center of Micro/Nanotechnology (CMI) of EPFL for support. This research was in part funded by the European Commission (FP7-ICT-2011-7, NANO-VISTA, under Grant Agreement No. 288263).


Valentin FLAURAUD, Gabriel BERNASCONI, Jérémy BUTET, Olivier MARTIN, Jürgen BRUGGER, Duncan ALEXANDER (Lausanne, Switzerland)

14:00-16:00
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MS7-II
MS7: Materials for optics and nano-optics
SLOT II

MS7: Materials for optics and nano-optics
SLOT II

Chairpersons: David MCCOMB (Chairperson, Columbus, USA), Jérôme PLAIN (Chairperson, Troyes, France)
14:00 - 14:30 #6254 - MS07-S86 Excitonic Properties of Inorganic-Organic Hybrid Perovskites and Nanophotonic Devices.
Excitonic Properties of Inorganic-Organic Hybrid Perovskites and Nanophotonic Devices.

Excitonic Properties of Inorganic-Organic Hybrid Perovskites and Nanophotonic Devices

Qihua Xiong

School of Physical and Mathematic Sciences & School of Electrical and Electronic Engineering

Nanyang Technological University, Singapore, 637371 

Abstract: In this talk, we present investigations of vapor phase synthesis of purely inorganic or organic-inorganic perovskites nanoplatelets by a van der Waals epitaxy mechanism and their excitonic properties. Those crystals exhibit 2D well-faceted triangular, hexagonal or square geometry with thickness range of tens to hundreds of nanometers. Optical spectroscopy investigations suggest that the crystals have large exciton binding energy, high external quantum efficiency and long diffusion lengths. The naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold optically pumped in-plane nanolasers ranging from ultraviolet and near-infrared, with an exceptionally high quality factor (>4000) in purely inorganic perovskite square-shaped crystals. Our findings open up a new class of wavelength tunable nanomaterials potentially suitable for on-chip integration and flexible optoelectronic devices. Progress in light-emitting diode and laser cooling will also be discussed.

 

References:

  1. S.T. Ha, C. Shen, J. Zhang and Q.H. Xiong*, “Laser Cooling of Organic-inorganic Lead Halide Perovskites”, Nature Photonics 10, 115-121 (2016)
  2. Q. Zhang, S.T. Ha, X.F. Liu, T.C. Sum* and Q.H. Xiong*, "Room-Temperature Near-Infrared High-Q Perovskite Whispering-Gallery Planar Nanolasers", Nano Lett. 14, 5995–6001 (2014)
  3. S.T. Ha, X.F. Liu, Q. Zhang, D. Giovanni, T. C. Sum and Q.H. Xiong*, "Synthesis of Organic–Inorganic Lead Halide Perovskite Nanoplatelets: Towards High-Performance Perovskite Solar Cells and Optoelectronic Devices”, Adv. Opt. Mater. 2, 838-844 (2014)
  4. J. Xing, X. F. Liu, Q. Zhang and Q. H. Xiong*, “Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers”, Nano Lett. 15, 4571 - 4577 (2015).

Qihua XIONG (singapore, Singapore)
Invited
14:30 - 14:45 #5150 - MS07-OP298 Angle-resolved cathodoluminescence of plasmonic crystal waveguide.
Angle-resolved cathodoluminescence of plasmonic crystal waveguide.

Slow-light manipulation in a photonic crystal (PhC) waveguide is expected to improve future optical information processing and communication technologies such as optical buffering and light compression [T. Baba, Nat. Photon. 2, 465-473 (2008)]. Waveguiding using bandgap of plasmonic crystal (PlC) has also been demonstrated [S. I. Bozhevolnyi et al. Phys. Rev. Lett. 86, 3008-3011 (2001)]. However, the dispersion characteristics of the guided modes, which are essential to control surface plasmon polariton (SPP) pulses, have not yet been understood. Electron beam spectroscopies at high spatial resolution are powerful characterization tools to observe electromagnetic modes nowadays. Momentum-resolved spectroscopy in electron microscopy is especially useful to investigate detailed optical properties of locally-modified structures introduced into a PhC [R. Sapienza et al. Nat. Mater. 11, 781-787 (2012)] and a PlC [H. Saito and N. Yamamoto, Nano Lett. 15, 5764-5769 (2015)]. We have studied the dispersion characteristics of SPPs in a PlC waveguide by angle-resolved chatodoluminescence performed in a STEM. The guided SPP modes were found to have two unique features : i) energy dependence of the phase shift at the wall, and ii) waveguide bandgap (WBG) due to the periodicity originating from PlC structure, which resulted in small group velocity of the guided SPP modes over a wide energy range.

The investigated PlC waveguide is composed of a silver dot array with a triangular lattice and silver plane surface as shown in Fig. 1a, which was structured by electron beam lithography and physical deposition. A full bandgap is formed from 1.8 eV to 2.3 eV in the present PlC. The SPPs with the energies in the full bandgap are confined in the flat waveguide area and guided parallel to the Γ-K direction as illustrated in Fig. 1a. Figure 1b shows the dispersion pattern measured in the PlC waveguide area with the waveguide width W of 650 nm, angle-scanned parallel to the direction of the waveguide. The details of the experimental setup for angle-resolved chatodoluminescence measurements are explained elsewhere [K. Takeuchi and N. Yamamoto, Opt. Express 19, 12365-12374 (2011)]. The guided SPP mode is observed (indicated by green ellipse). We also find the small gap about 0.01 nm-1 along the curve. The guided SPP mode can be approximately modelled as the guided wave between two interfaces with total internal reflections considering an energy-dependent phase shift. The details of this model will be explained in the congress. The theoretical curves relatively well fits the experimental curves for various waveguide widths except for the gaps. The measured dispersions indicate that the SPPs in the PlC waveguide become much slower than light in vacuum. The guided SPP in the waveguide with W = 520 nm is 7.5 times slower than light in vacuum. The velocity is even more slowed as the energy approaches the gap about 0.01 nm-1.

To understand the origin of the gap, photon map imaging was performed for W = 1040 nm. Interestingly, the interference fringes appear in the direction of the waveguide with the period of 300 nm, indicating the dot row facing the waveguide causes Bragg reflection, resulting in the WBG. The antinode positions for lower band-edge energy and upper band-edge energy are different from each other as illustrated in Figs. 1c and 1d. The antinodes of the lower band-edge mode are extended between the dots facing the flat waveguide area (Fig. 1c) while the upper band-edge mode is more tightly confined within the flat waveguide area (Fig. 1d). This difference in the effective waveguide width generates the energy difference between the band-edge modes, i.e. WBG.

The present results indicated that the PlC waveguide has potential advantages in manipulation of ultrashort pulses since it follows the linear dispersion with small group velocity over a wide energy range. Although the dispersion mainly inside the light cone was measured in this fundamental study, it could be shifted outside the light cone for a practical use. One of the possible solutions is a use of a hybrid waveguide composed of a dielectric strip on a metal surface [T. Liu et al. Opt. Express 22, 8219-8225 (2014)].

This work was supported by Kazato Research Foundation, the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) Nanotechnology Platform 12025014.


Hikaru SAITO (Fukuoka, Japan), Naoki YAMAMOTO, Takumi SANNOMIYA
14:45 - 15:00 #5764 - MS07-OP299 Sub-nanosecond electron beam blanking for deep-subwavelength lifetime probing of nanophotonical devices and nanoparticles.
Sub-nanosecond electron beam blanking for deep-subwavelength lifetime probing of nanophotonical devices and nanoparticles.

Optical techniques are used to probe carrier or excited energy dynamics down to femtosecond time-scales, but they lack the resolution to address nanoscopic length scales [1]. Higher spatial resolution can be obtained using pulsed beams of electrons, but this typically requires dedicated microscopes with laser-triggered electron sources [2]. Here, we use a standard scanning electron microscope equipped with an electrostatic beam blanker operated in conjugate mode (Fig. 1). We show the generation of sub-nanosecond electron pulses (Fig.2). This system is used to probe nanophotonical devices and nanoparticles using time-resolved cathodoluminescence (CL). In these devices, optical emitters couple with the nanoscale environment leading to a position-dependent excited state lifetime. The pulsed electron beam excites the emitters, giving rise to CL. The CL emission is detected using an integrated light microscope [3]. Photon arrival histograms are obtained using time-correlated single photon counting, synchronized with the input signal of the electron beam blanker, thus measuring spatially resolved CL lifetime.

 

We demonstrate the ability to identify nanoparticles based on their lifetime as well as their emission wavelength, which provides an additional source of information in nanoparticle-based biological imaging. Moreover, we conduct a nanoscopic version of the seminal work performed in the 1960s and 1970s by Drexhage, who showed that the lifetime of emitters depends on their distance to a metallic mirror [4]. We locally excite Ce3+ emitters in YAG, which is partially covered with a thin aluminum film (Fig. 3). We then measure the CL lifetime as a function of distance d to the metal, with deep subwavelength (<λ/10) resolution (Fig. 4). Our results firmly establish time-resolved electron spectroscopy of nanophotonical devices as a powerful characterization tool for nanophotonics.

 

 

 

[1]     MacDonald, K. F.; Sámson, Z. L.; Stockman, M. I.; Zheludev, N. I., Ultrafast active plasmonics. Nat. Photonics 2008, 3 (1), 55-58.

[2]     Yang, D. S.; Mohammed, O. F.; Zewail, A. H., Scanning ultrafast electron microscopy. Proc. Natl. Acad. Sci. U. S. A. 2010, 107 (34), 14993-8.

[3]     Zonnevylle, A. C.; Van Tol, R. F.; Liv, N.; Narváez, A. C.; Effting, A. P.; Kruit, P.; Hoogenboom, J. P., Integration of a high-NA light microscope in a scanning electron microscope. J Microsc 2013, 252 (1), 58-70.

[4]     Drexhage, K. H., Influence of a dielectric interface on fluorescence decay time. J. Lumin. 1970, 1–2 (0), 693-701.


Robert MOERLAND, Mathijs GARMING (Delft, The Netherlands), Gerward WEPPELMAN, Pieter KRUIT, Jacob HOOGENBOOM
15:00 - 15:15 #5869 - MS07-OP301 Optically Coupled Plasmonic Nanopores Observed by Cathodoluminescence Scanning Transmission Electronmicroscopy.
Optically Coupled Plasmonic Nanopores Observed by Cathodoluminescence Scanning Transmission Electronmicroscopy.

Control of the optical properties of nano-plasmonic structures is essential for next-generation optical circuits and high-throughput biosensing platforms. Realization of such nano-optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as “dark modes”, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well-studied, couplings of inverse structures, i.e., holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti-symmetric coupling modes has not been performed. Here, we present a systematic study of the symmetric and anti-symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy (CL-STEM) and electromagnetic simulation.

The nanopore samples were fabricated by colloidal lithography and film transfer by wet etching of the sacrificial layer. To achieve very close separation of nanopore pairs and to obtain high spatial resolution in STEM, we chose ultra-thin, free-standing film structures. For the measurement of single and coupled pairs of nanopores, 135 nm nanopores in an AlN(8 nm)/Au(16 nm)/AlN(8 nm) trilayer membrane were used (Figure 1b). With this sandwich layer structure, it is possible to obtain very thin and stable metal layers, even at high temperatures.

For CL-STEM measurement, 80 kV acceleration was used to avoid possible damage due to relatively high beam current. (5nA) Depending on the electron beam position it is possible to distinguish the symmetric and anti-symmetric dipolar coupling modes.(Fig. 1) The observed symmetric coupling mode, approximated as a pair of facing dipoles, appeared at a lower energy than that of the anti-symmetric coupling mode, indicating that the facing dipoles attract each other. The anti-symmetric coupling mode splits into the inner- and outer-edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as simple inverses of coupled disks. Electromagnetic simulation by finite difference time domain (FDTD) also showed consistent coupling behaviors. Models of FDTD simulation showed that the inner- and outer-edge anti-symmetric modes become fully localized with minimal influence of the opposite edges as the coupling distance decreases. Symmetric and anti-symmetric coupling modes are also observed in a short-range ordered pore array (Fig. 2), where one pore supports multiple local resonance modes, depending on the distance to the neighboring pores.


Takumi SANNOMIYA (Yokohama, Kanagawa, Japan), Hikaru SAITO, Junesch JULIANE, Naoki YAMAMOTO
15:15 - 15:30 #6028 - MS07-OP303 Excitation and probing of hyperbolic phonon polaritons in hexagonal boron nitride structures by fast electrons.
Excitation and probing of hyperbolic phonon polaritons in hexagonal boron nitride structures by fast electrons.

Hexagonal boron nitride (hBN) is a representative material of a wide class of two-dimensional systems in which individual atomic layers are only weakly coupled by van der Waals interaction, resulting, among others, in extreme optical anisotropy. The latter gives rise to hBN’s hyperbolic phonon polaritons (h-PhPs), i.e. coupled excitations of optical phonons and light that exhibit hyperbolic dispersion [1, 2] at mid-infrared (mid-IR) energies, specifically in the range of 90-200 meV. Hyperbolic polaritons might be a key to many emerging photonic technologies that rely on nanoscale light confinement and manipulation, such as nanoscale imaging or sensing [3]. Thus, efficient design and utilization of hBN (nano)structures require spectroscopic studies with adequate spatial resolution and energy range.

Electron energy loss spectroscopy (EELS) performed in scanning transmission electron microscope (STEM) is a versatile technique that employs fast electrons as an effective localized electromagnetic probe for spectroscopy with nanoscale spatial resolution. Successfully employed at visible and near-IR energies, this technique has limited capabilities for mid-IR spectroscopy primarily due to lack of monochromaticity of the primary electron beam which typically masks low energy excitations under ~200  meV with the zero loss peak (ZLP) originating from the elastic electron scattering and also limits the spectral resolution to ~100 meV.

Here we demonstrate that by an optimization of microscope data acquistion and signal processing it is possible to significantly reduce the ZLP width down to 50 meV (with corresponding resolution enhancement), placing mid-IR spectroscopy within the reach of standard TEM instruments.

To this end, we perform experimental mapping of the spectral signature at an hBN edge.  As summarized in Fig. a, we clearly observe the variation in spectral peak position as a function of the electron impact parameter (position of the electron beam with respect to the edge). As revealed by our theoretical analysis, this behavior is the manifestation of polaritonic nature of the induced excitations. Indeed, our developed analytical and numerical models of EELS in structured hyperbolic materials show the existence of multiple EEL peaks depending on the impact parameter (see Fig. b). These peaks are due to the reflection of propagating h-PhPs from the hBN edge, which proves that fast electrons can and do couple to the hyperbolic polaritons. After mimicking the experimental spectral resolution (via convolution of the calculated spectra with a Gaussian of proper experimental width), we obtain a good agreement with the experimental data (see Fig. c).

Our work provides first steps in understanding polaritonic excitations produced by fast electrons in hyperbolic materials and sets grounds for the rigorous analysis of the observed low-energy EELS. With the ongoing improvements of STEM-EELS instrumentation [4], we expect further enhancement of the spectral resolution and an extension of the applicable energy ranges in near future, thus enabling EELS in STEM as a versatile technique for infrared spectroscopy of polaritons.

 

 

References:

[1] Dai S. et al. Science 343 (2014), 1125.

[2] Yoxall, E. et al. Nat. Photonics 9 (2015), 674.

[3] Li, P. et al. Nat. Commun. 6 (2015), 7507.

[4] Krivanek, O. L. et al. Nature 514 (2014), 209.


Andrea KONEČNÁ (San Sebastian, Spain), Alexander GOVYADINOV, Andrey CHUVILIN, Irene DOLADO, Saül VÉLEZ, Javier AIZPURUA, Rainer HILLENBRAND
15:30 - 15:45 #6598 - MS07-OP304 Spatiotemporal imaging of few-cycle nanoplasmonic fields using photoemission electron microscopy.
MS07-OP304 Spatiotemporal imaging of few-cycle nanoplasmonic fields using photoemission electron microscopy.

Surface plasmons are capable of concentrating light on both a nanometre spatial and femtosecond temporal scale, thus serving as a basis for nanotechnology at optical frequencies. However, the simultaneously small and fast nature of surface plasmons leads to new challenges for spatiotemporal characterization of the electric fields. An especially successful method for this purpose is photoemission electron microscopy (PEEM) in combination with ultrashort laser pulses. This method uses the high spatial resolution offered by electron microscopy together with the temporal resolution offered by femtosecond laser technology. By combining PEEM with state-of-the-art sources of ultrashort bursts of light, we have contributed to two pathways towards the ultimate goal: the full spatiotemporal reconstruction of the surface electric field at arbitrary nanostructures.

The first approach is based on extending interferometric time-resolved PEEM (ITR-PEEM) [1] to the few light cycle regime by using two synchronized pulses from an ultra-broadband oscillator. Because the photon energy (1.2-2.0 eV) is well below the material work function, photoemission occurs through a multiphoton process. The measurement is performed by scanning the delay between two identical, sub-6 fs pulses and measuring the local photoemission intensity (Fig. 1a). We have applied this method to a variety of nanostructures, including rice-shaped silver particles, nanocubes, and gold bow-tie nanoantennas. As an example, results from the rice-shaped silver nanoparticles are shown in Fig. 1. We excited multipolar surface plasmons at grazing incidence, and imaged the photoelectrons emitted from the two ends of the nanoparticle (Fig. 1b). Upon scanning the delay between the two pulses, the interference fringes measured from the two ends of the nanoparticle are shifted with respect to each other (Fig. 1c). We show that these shifts correspond to locally different instantaneous frequencies of the near-field within the same nanoparticle, and that these differences occur due to a combination of retardation effects and the excitation of multiple surface plasmon modes [2].

The second approach is based on using high-order harmonic generation (HHG) to produce attosecond pulses in the extreme ultraviolet (XUV) region. Attosecond XUV pulses have been proposed to enable a direct spatiotemporal measurement of nanoplasmonic fields with a temporal resolution down to 100 as [3]. However, PEEM imaging using HHG light sources has turned out to be a major challenge due to numerous issues such as space charge effects, chromatic aberration, and poor image contrast [4-6]. To address these issues, we perform HHG using a new optical parametric chirped pulse amplification system delivering 7 fs pulses at 200 kHz repetition rate. We show how the XUV pulses generated by this system allow for PEEM imaging with both higher resolution and shorter acquisition times. For comparison, Fig. 2 shows PEEM images of silver nanowires on a gold substrate, imaged using high-order harmonics at 1 kHz repetition rate (Fig. 2a, acquisition time is 400 s) and at 200 kHz repetition rate (Fig. 2b, acquisition time is 30 s). The image quality is clearly improved (Fig. 2c). We also show how the higher repetition rate allows for PEEM imaging using only primary (“true”) photoelectrons, whereas previous studies have acquired images using secondary electrons [4-6].

[1] A. Kubo et al., Nano Lett. 5, 1123 (2005).

[2] E. Mårsell et al., Nano Lett. 15, 6601 (2015).

[3] M. I. Stockman et al., Nat. Photon. 1, 539 (2007).

[4] A. Mikkelsen et al., Rev. Sci. Instrum. 80, 123703 (2009).

[5] S. H. Chew et al., Appl. Phys. Lett. 100, 051904 (2012).

[6] E. Mårsell et al., Ann. Phys. (Berlin) 525, 162 (2013).


Erik MÅRSELL, Arthur LOSQUIN (Lund, Sweden), Chen GUO, Anne HARTH, Eleonora LOREK, Miguel MIRANDA, Cord ARNOLD, Hongxing XU, Johan MAURITSSON, Anne L'HUILLIER, Anders MIKKELSEN
15:45 - 16:00 #6624 - MS07-OP305 Toroidal dipole plasmon resonance modes in upright split ring resonators.
Toroidal dipole plasmon resonance modes in upright split ring resonators.

Nanoscale split ring resonators (SRRs) have been a popular topic of study due to their surface plasmon resonance (SPR) modes and their many interesting interactions with light. They can be used as components in metamaterials exhibiting, among other properties, a negative refractive index. The surface plasmon properties of these structures are strongly dependent on their size and spatial arrangement. Most studies so far have focussed on the horizontal SRR due to the ease of fabrication. However, there are some advantages to be gained in the design of materials using upright SRRs. We are studying a structure composed of four upright SRRs as shown in Figure 1. The coupling of these four upright SRRs produces a magnetic dipole moment and a toroidal dipole moment.

The toroidal dipole moment, when compared to electric and magnetic dipole moments, shows a higher quality factor and lower gain threshold for a nanoscale laser analogue, the spaser (surface plasmon amplification by stimulated emission of radiation) [1]. The presence of a strong toroidal dipole moment isolated from magnetic and electric dipole moments makes the structure under study a promising candidate for a spaser for use in on-chip telecommunications.

A similar structure was first realized experimentally in the microwave regime of the electromagnetic spectrum [2]. Scaling the geometry down to nanoscale dimensions has been shown by simulation to shift the toroidal dipole energies into the near infra-red regime [1]. In this work we demonstrate the experimental fabrication (Figure 2) and characterization of this structure using electron energy loss spectroscopy (EELS), with confirmation of the modes provided by finite element method (FEM) simulations.

We have fabricated this structure using a double patterning process in electron beam lithography, with precise alignment of the second lithography layer to the first. The structures are made from gold deposited on a 50 nm thick silicon nitride membrane. We probe the plasmon modes using EELS on a monochromated scanning transmission electron microscope, collecting spectrum images with nanometer spatial resolution and 60 meV energy resolution. We extract site-specific spectra (Figure 3a) and energy-resolved maps of the SPR modes (Figure 3b, c). We apply the Richardson-Lucy algorithm to further increase the effective energy resolution and identify the magnetic and toroidal dipole modes at energies of 0.52 eV and 0.72 eV, with SPR maps as shown in Figure 3b and 3c, respectively.

We are able to correlate our EELS results with COMSOL Multiphysics FEM simulations. The simulated SPR response is given in Figure 3a, d, and e, showing close agreement in the peaks with our experimental data. Simulations confirm the low energy magnetic dipole mode (0.56 eV) and reveal two closely spaced toroidal dipole modes (0.61 eV, 0.66 eV) which are not perfectly resolved in the EELS data. We are able to tune the energy and strength of the toroidal dipole moment through tuning of the fabrication parameters; with careful design this structure is a promising spaser design for a range of applications near telecommunications frequencies.

References

[1] Y.-W. Huang, et al., Sci. Rep., vol. 3, Feb. 2013.

[2] T. Kaelberer, et al., Science, vol. 330, no. 6010, pp. 1510–1512, Dec. 2010.

Acknowledgements: We gratefully acknowledge the financial support of NSERC and the province of Ontario.


Isobel BICKET (Hamilton, Canada), Edson BELLIDO, Ahmed ELSHARABASY, Mohamed BAKR, Gianluigi BOTTON

10:30-12:30
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IM5-I
IM5: Quantitative imaging and image processing
SLOT I

IM5: Quantitative imaging and image processing
SLOT I

Chairpersons: Joanne ETHERIDGE (Director) (Chairperson, Melbourne, Australia), Jean-Christophe OLIVO-MARIN (Chairperson, Paris, France)
10:30 - 11:00 #6670 - IM05-S44 Retrieving atomic structure from dynamical rocking curve measurements in both real and reciprocal space.
Retrieving atomic structure from dynamical rocking curve measurements in both real and reciprocal space.

Transmission electron microscopy (TEM) data, in particular high-resolution TEM and conventional electron diffraction has the reputation of being not easily interpretable in a quantitative manner in terms of the object being probed by the fast electrons. The reason for this lies in the fact that multiple scattering makes the detected signal a non-linear function of the scattering potential. In cases where the structure is approximately known, refinement of structure factors from convergent beam electron diffraction (CBED) data [1] or atom positions from HRTEM images [2] is possible. But the ab-initio inversion of multiple scattering to recover the structure of an unknown object has not yet been shown to work in a routine manner for experimental data. Structure determination approaches thus typically revert to techniques which collect tilt-averaged data, such as precession electron diffraction (PED) and scanning transmission electron microscopy (STEM) with a convergent probe. Integrating the signal over a range of different relative orientations between object and electron beam wave vector averages over different multiple scattering conditions – however, a large amount of structural information encoded in the multiple scattering signal gets lost, including valuable information about the object’s 3D structure [3].  

Fig. 1 demonstrates that in the case of experimental large-angle rocking-beam electron diffraction (LARBED) data [4], i.e. diffraction data for a large range of beam tilts (about 20 times larger than possible in conventional CBED of silicon), the projected potential can be recovered by straight forward gradient optimization from a starting guess in which all structure factors were initialized to the same value, i.e. initially, all Ug = (0.01 + 0.01i) Å-2 [5]. Although rocking curves of only 121 diffraction spots were measured, 456 structure factors could be determined from this data, since, all possible difference vectors between reflections extracted from the diffraction pattern were also included in the dynamical diffraction calculation.

Although CBED and HRTEM are very different modes of operation of the microscope, the multiple scattering contribution to the signal in tilt series of HRTEM images allows us to retrieve the 3D scattering potential [6], where the position and height of peaks allows direct interpretation as atom species and positions (see Fig. 2). This inversion of multiple scattering is based on an interpretation of the multislice algorithm as an artificial neural network that is taught by feeding it TEM data recorded under different experimental conditions. This can be HRTEM tilt series, ptychography data sets, or scanning confocal electron microscopy (SCEM) data [7].[8]

[1] J.M. Zuo, M. Kim, M. O'Keeffe, and J C.H. Spence, Nature 401 (1999) p. 49.
[2] G. Möbus, M. Rühle, Ultramicroscopy 56 (1994) 54-70
[3] R.S. Pennington, W. Van den Broek, C.T. Koch, Phys. Rev. B 89 (2014) 205409
[4] C.T. Koch, Ultramicroscopy 111 (2011) 828.
[5] F. Wang, R.S. Pennington, C.T. Koch (2016) submitted.
[6] W. Van den Broek and C.T. Koch, Phys. Rev. Lett. 109 (2012) p. 245502.
[7] W. Van den Broek and C.T. Koch, Phys. Rev. B 87 (2013) 184108
[8] The authors acknowledge funding from the German Research Foundation (DFG) as well as the Carl-Zeiss Foundation.


Christoph KOCH (Berlin, Germany), Wouter VAN DEN BROEK, Feng WANG, Robert PENNINGTON
Invited
11:00 - 11:15 #6511 - IM05-OP112 Efficient and quantitative phase imaging in two- and three-dimensions using electron ptychography in STEM.
Efficient and quantitative phase imaging in two- and three-dimensions using electron ptychography in STEM.

Historically, the scanning transmission electron microscope (STEM) has not been widely used for phase contrast imaging because the small bright-field detector required makes use of only a small fraction of the incident electrons and is therefore inefficient with respect to dose.  This limitation has hindered the efficient imaging of light elements in STEM.  Alternative modes also have limitations.  For example, annular dark-field (ADF) imaging of graphene only makes use of a few percent of the incident electrons, and annular bright-field imaging (ABF) requires lens aberrations to form an effective phase plate to get contrast from weakly scattering objects.

Electron ptychography in the STEM was first demonstrated more than 20 years ago in the context of improving image resolution [1].  At that time, the image field of view was restricted by the limitations of the camera technology and data handling technology.  Here we make use of the pnCCD (S)TEM camera, a direct electron pixelated detector from PNDetector, mounted on the JEOL ARM200-CF aberration corrected microscope. The detector has a grid of 264x264 pixels and operates at a speed of 1000 frames-per-second (fps). The detector can achieve a speed of up to 20,000 fps through binning/windowing. ADF images can be recorded simultaneously, as shown by the schematic in Fig. 1.

The resulting 4D data set is formed of a series of coherent convergent beam diffraction patterns recorded as a function of illuminating probe position.  Here we explore how the bright-field and dark-field regions of scattering can be used to enhance the capabilities of STEM.  We compare a range of methods that can be used to form the phase image from this data set, including single side-band [2,3], Wigner distribution deconvolution [4] (used to produce Fig. 2) and ePIE [5]. Phase imaging using ptychography has a relatively simple transfer function [3] and also provides an inherent filter of image noise without reducing the signal strength to form high quality phase images (Fig. 2).  Furthermore, the four-dimensional data set is highly redundant and it is possible to detect and correct for residual aberrations in the image. 

The ability to deconvolve lens aberrations can further be used to extract three-dimensional information from a single STEM image acquisition scan.   This is achieved by reconstructing the phase image at a specific depth in the sample, which can be performed even though the microscope may not have been focused at that depth (Fig. 3).  Finally, we explore the potential for using information outside the bright-field disc to enhance STEM imaging.

[1] P.D. Nellist, B.C. McCallum and J.M. Rodenburg, Nature 374 (1995) 630-632.

[2] T.J. Pennycook et al., Ultramicroscopy 151 (2015) 160-167.

[3] H. Yang et al., Ultramicroscopy 151 (2015) 232-239.

[4] J.M. Rodenburg and R.H.T. Bates, Philosophical Transactions of the Royal Society of London A, 339 (1992) 521-553.

[5] M.J. Humphry, B. Kraus, A.C. Hurst, A.M. Maiden, J.M. Rodenburg, Nat Commun, 3 (2012) 730.

[6] The authors acknowledge funding from the EPSRC through grant number EP/M010708/1.


Peter NELLIST (Oxford, United Kingdom), Hao YANG, Lewys JONES, Gerardo MARTINEZ, Reida RUTTE, Benjamin DAVIS, Timothy PENNYCOOK, Martin SIMSON, Martin HUTH, Heike SOLTAU, Lothar STRUEDER, Ryusuke SAGAWA, Yukihito KONDO, Martin HUMPHRY
11:15 - 11:30 #6213 - IM05-OP109 Atom-Resolved STEM Imaging Using a Segmented Detector.
Atom-Resolved STEM Imaging Using a Segmented Detector.

    In scanning transmission electron microscopy (STEM), differential phase contrast (DPC) imaging has been developed to visualize the local electromagnetic field distribution in materials at medium resolution [1, 2]. The electromagnetic field deflects the incident electron beam, and this deflection can be measured by taking the difference between signals detected in opposing detector segments. Recent rapid progress in high-sensitive segmented detectors has enabled DPC STEM imaging to be performed at atomic-resolution [3]. However, DPC STEM images are sensitive to thickness and defocus, because dynamical scattering strongly affects DPC imaging of crystals in zone axis orientations [4]. It thus remains a challenge to develop a practical imaging technique at atomic-resolution with the segmented detector.

    Fig. 1 shows images of SrTiO3 simultaneously obtained by different segments on a new segmented annular all field detector (SAAF2) installed in an aberration-corrected STEM (JEOL JEM-300F, 300kV). The relative orientation of the detector and the crystal structure is shown in Fig. 2. The images were at a defocus value of -3.7 nm relative to the defocus condition giving maximum contrast in annular dark field (ADF) imaging. These 512×512 pixel images were recorded with a dwell time of 38 µs per pixel, so the total imaging time is about 10 seconds. Each segment image can be qualitatively interpreted by electron beam deflection due to electric field from nuclei, including from the light oxygen atomic columns, though dynamical effects should be taken into account. According to image simulations, the DPC image appearance is largely unchanged with sample thickness if a defocus value is selected to obtain the highest contrast DPC image. This suggests that DPC STEM imaging at atomic resolution with a proper defocus value may be a new robust imaging mode that enables visualization of atomic column positions, including for light elements. Furthermore, this new imaging mode may contain information on charge redistribution due to charge transfer or orbital hybridization.

    In addition, we have found that the detector is sensitive enough to allow both segmented annular dark field imaging and DPC STEM imaging of single atoms to be performed. The details will be discussed in the presentation.

 

References

[1] J. N. Chapman et al., Ultramicroscopy 3, 203 (1978).

[2] M. Lohr et al., Ultramicroscopy 117, 7 (2012).

[3] N. Shibata et. al., Nat. Phys. 8, 611 (2012).

[4] R. Close et. al., Ultramicroscopy 159, 124 (2015).

[5] This work was supported by PRESTO and SENTAN, JST, and the JSPS KAKENHI Grant number 26289234. A part of this work was supported by Grant-in-Aid for Scientific Research on Innovative Areas (25106003). A part of this work was conducted in the Research Hub for Advanced Nano Characterization, The University of Tokyo, under the support of "Nanotechnology Platform" (Project No.12024046) by MEXT, Japan. This research was supported under the Discovery Projects funding scheme of the Australian Research Council (Project No. DP110101570).


Takehito SEKI (Tokyo, Japan), Gabriel SANCHEZ-SANTOLINO, Nathan LUGG, Ryo ISHIKAWA, Scott D. FINDLAY, Yuichi IKUHARA, Naoya SHIBATA
11:30 - 11:45 #5246 - IM05-OP104 ISTEM: A Realisation of Incoherent Imaging for Ultra-High Resolution TEM beyond the Classical Information Limit.
ISTEM: A Realisation of Incoherent Imaging for Ultra-High Resolution TEM beyond the Classical Information Limit.

The ISTEM (Imaging STEM) method [Phys. Rev Lett. 113, 096101(2014)] presented here constitutes a novel way for the realisation of TEM imaging with spatially incoherent illumination. It is well-known that such incoherent image formation allows for an increased resolution and higher robustness towards chromatic aberrations compared to coherent illumination as used in conventional TEM (CTEM). This has been realised in scanning TEM (STEM) via reciprocity, which however suffers from other resolution-limiting factors such as scan noise or the finite extent of the electron source.

The ISTEM mode circumvents these problems entirely. It combines STEM illumination with CTEM imaging as illustrated in Fig. 1: A camera is used to acquire images formed by the focused electron probe that is scanning over the specimen while the imaging system is in imaging mode. With an exposure time chosen equal to the STEM scan time, the resulting image corresponds to a sum over the images of all probe positions. Because different specimen positions are illuminated at different times, the corresponding intensities are summed up incoherently. ISTEM is therefore a spatially incoherent imaging mode and benefits from the associated improvement of resolution. Beyond this simple explanation, the equivalence of the ISTEM illumination and CTEM with an extended and incoherent electron source can be furthermore rigorously shown mathematically within the mutual intensity formalism. From this, the gain of resolution can be intuitively understood in the limit of total incoherence, in which case the transfer is given by the autocorrelation of the coherent transfer function. The theoretical considerations also show that neither scan noise nor source size have any influence on ISTEM-images. Aberrations and defocus of the condenser system cancel out completely as well. ISTEM imaging is therefore independent from probe correction.

In simulative studies the capability of ISTEM to extend the point resolution beyond the diffraction limit, its robustness towards temporal incoherence and the resulting possibility to overcome the information limit are demonstrated. These calculations are confirmed by experimental ISTEM-micrographs of GaN in [11-20] and [1-100] projection, as presented in Fig. 3, which are found in good agreement with simulations. For the [1-100] direction neighbouring gallium and nitrogen columns at a distance of only 63 pm are resolved despite an information limit of 83 pm of the image-corrected FEI TITAN 80/300 G1 microscope used for the acquisition. The classical information limit is thereby clearly overcome by 24%.

A further study was conducted that compared the results of strain-state analysis from simulated ISTEM images of a strained InGaAs-crystal with annular dark-field and bright-field STEM micrographs simulated for a probe-corrected microscope. The results are displayed in Fig. 2. They promise a significant increase in precision for ISTEM compared to STEM, due to the immunity to both scan noise and source size clearly recognisable by the smaller error bars. This was experimentally confirmed by an ISTEM study of PbTiO3 in which the heavy Pb and TiO atomic columns as well as the lighter oxygen columns are clearly resolved  and the evaluation based on a parametrically fitted model yields a significantly increased precision for position measurement compared to aberration corrected STEM images which were acquired from the same sample area.

With the help of the principle of reciprocity, ISTEM can finally be made equivalent to any STEM mode by appropriate choice of objective and condenser aperture, with the difference that ISTEM images will show no scan noise whatsoever. This will allow for the realisation of e.g. annular bright-field STEM, holding out the prospect of ultra-high resolution imaging of even lightest elements.


Florian F. KRAUSE (Bremen, Germany), Marco SCHOWALTER, Thorsten MEHRTENS, Knut MÜLLER-CASPARY, Armand BÉCHÉ, Karel W. H. VAN DEN BOS, Sandra VAN AERT, Johan VERBEECK, Andreas ROSENAUER
11:45 - 12:00 #6649 - IM05-OP113 Dark-Field Imaging with Electron Backscatter Diffraction Patterns.
Dark-Field Imaging with Electron Backscatter Diffraction Patterns.

Dark-field (DF) imaging can be performed by selecting a specific diffracted beam in the selected area diffraction pattern in conventional transmission electron microscope (CTEM) or in the convergent beam electron diffraction pattern in scanning transmission electron microscopy (STEM) mode [1]. The resultant micrograph provides high intensity of the objects in the probed volume that diffract in this particular direction. In contrast, dark-field micrographs can be obtained in STEM mode by capturing the signal from a specific range of scattering angles, with the most representative example being the high-angle annular dark-field imaging (HAADF) [2]. This leads to a contrast mostly based on atomic number differences between the different objects analysed [3].

These techniques were developed originally for CTEM and STEM. Because DF based on scattering angles is technically easy to obtain in a scanning electron microscope (SEM) by collecting the transmitted/diffracted signals with an electron detector below the thin specimen, it has been implemented in SEMs seriously since several years. This permitted taking advantage of the high contrast and low beam damage obtained at low accelerating voltages STEM in the SEM is now routinely achieved with a spatial resolution close to 1 nm in field-emission SEMs [4]. Despite these new possibilities, DF imaging only based on diffracted beams has not been achieved yet in a SEM.

The mostly used diffraction technique in the SEM has been, since the discovery of Venables [5], electron backscatter diffraction (EBSD) which has a spatial resolution of roughly 20-30 nm and which needs a limited bulk surface preparation compared to CTEM or STEM. EBSD is assumed to be related to the electron channeling pattern (ECP) diffraction technique by the reciprocity theorem [6], although its angular resolution is, at this time, limited by the pixel resolution of the acquisition equipment. Figure 1 is a comparison between an ECP and an EBSP acquired at 20 kV from a [001] (001) silicon wafer. In this work, pseudo-Kikuchi patterns (EBSP) recorded via EBSD were stored and reprocessed by reporting pixels or clusters of pixels intensities from a specific location in a reference EBSP to reconstruct the final image (EBSD map). A resulting micrograph (called EBSD-DF image) was produced with a direct link to the diffracted beams in the EBSP and hence, to the crystallography of the sample, i.e., a DF image. The origin of the contrast is then similar to that of electron channeling contrast image (ECCI) as shown in Figure 2, in which EBSD-DF micrographs of an indented compressed iron specimen with different reflections are displayed. However, the post-acquisition processing is an invaluable advantage over ECCI because it allows generating multiple micrographs at the same time with only one set of EBSPs recorded in a beam raster fashion. This opens new ways of extracting and using the information contained in each EBSP and the main applications, at this point, are understanding deformation behaviors and interpretation [7] of channeling contrast [8].

References:

[1] D.B. Williams and C.B. Carter, Transmission electron microscopy: a textbook for materials science. 2009: Springer.

[2] S. Pennycook, Ultramicroscopy, 30 (1989), pp. 58-69.

[3] O.L. Krivanek et al, Nature, 464 (2010), pp. 571-574.

[4] P.G.Merli et al, Microscopy and Microanalysis, 9 (2003), pp. 142-143.

[5] J. Venables and C. Harland, Philosophical Magazine, 27 (1973), pp. 1193-1200.

[6] O.C. Wells, Scanning, 21 (1999), pp. 368-371.

[7] N. Brodusch, H. Demers, and R. Gauvin, Ultramicroscopy, 148 (2015), pp. 123-131.

[8] S. Kaboli et al, Journal of Applied Crystallography, (2015), 48, pp. 776-785.


Raynald GAUVIN (Montreal, Canada), Hendrix DEMERS, Nicolas BRODUSCH
12:00 - 12:15 #6372 - IM05-OP110 Accurate and precise measurement of cluster sizes in localisation microscopy images using the Rényi divergence.
Accurate and precise measurement of cluster sizes in localisation microscopy images using the Rényi divergence.

Localisation microscopy is a super-resolution imaging technique based on detecting randomly activated single molecules in a sequence of images. A super-resolution image is then reconstructed as a collection of discrete points, using all of the localised single molecule positions. Clustering analysis of these points can provide quantitative information about sample structure, size of features and/or their number. The information obtained from clustering analysis allows characterisation of the functions or properties of biological systems, for example examining signalling pathways in T-cells antigen receptors. However, quantitative analysis of clusters in localisation microscopy images is challenging because the clusters are usually small and surrounded by relatively high noise.

The Rényi divergence quantifies differences between two distributions (in this case the observed data and a reference distribution). Its sensitivity to the degree to which one distribution differed from another can be tuned with a scaling parameter α, which allows us to adapt its robustness to noise. We approximated the data distribution by counting all points which were positioned closer to each other than a set threshold and the reference distribution as all the points concentrated in a single cluster. Ripley's K function, which is widely used for performing localisation microscopy analysis, is a special case of the Rényi divergence.

Here we present a comparison of the accuracy and precision of cluster size measurement performed using the Rényi divergence and Ripley's K function. Initial tests involved establishing noise adaptability of the two analysis methods using simulated data sets with characteristics similar to experimental images (with increasing noise levels), and optimising the tunable parameter of the Rényi divergence. We find that the adaptability of the Renyi divergence method is a particular advantage when dealing with localisation microscopy data, in which characteristics can vary a great deal between datasets. 


Adela STASZOWSKA (London, United Kingdom), Patrick FOX-ROBERTS, Susan COX
12:15 - 12:30 #5957 - IM05-OP106 STEM imaging of atom dynamics: novel methods for accurate particle tracking.
STEM imaging of atom dynamics: novel methods for accurate particle tracking.

Developments in scanning transmission electron microscopy (STEM) have opened up new possibilities for time-resolved imaging at the atomic scale. Recent examples include a study of the diffusion of dopant atoms in semiconductors [1] and, using environmental STEM, in situ studies of catalytic reactions [2]. Rapid imaging of single atom dynamics brings with it a new set of challenges. High frame rates and long total acquisition times mean novel methods are needed for handling and processing “big data” sets. Further, the need for short exposure times leads to severe problems with noise, but by exploiting the spatial and temporal correlations between frames, it is possible to considerably improve the signal-to-noise ratio using a method known as singular value thresholding [3]. Crucially, by employing robust procedures to automatically estimate the noise and motion characteristics, it is possible to optimize the process with little user input (Figure 1a,b). The identity and positions of individual atoms in the denoised data can then be determined using a newly-developed intensity-based classification algorithm. Building on the theme of automation, the classifier can be trained using simulated STEM images to robustly process long image sequences, where manual identification would be prohibitive.

As an example, we have applied these methods to investigate the diffusive behaviour of copper atoms on the (110) surface of silicon. The noise removal and atom identification steps are used along with particle tracking software [4] to extract a set of atomic trajectories from a series of annular dark-field STEM images (Figure 1c-e). The form of these trajectories can be related to the underlying silicon substrate, as in Figure 1f, which suggests the existence of preferred pinning sites for copper atoms. The interaction between adatoms and the substrate can be explored with unprecedented spatio-temporal resolution using rapid imaging, and interpreted by modelling with density functional theory (DFT) calculations (Figure 1g). This highlights the potential for combining time-resolved STEM with theory, forming a powerful approach to investigating and understanding the dynamic behaviour of materials at the atomic scale.

References

[1] Ishikawa R, et al. (2014). Phys. Rev. Lett. 113, 155501.

[2] Gai P, et al. (2014). Chemical Physics Letters. 592, 355-359.

[3] Furnival T, Leary R, Midgley PA. (2016). Manuscript submitted.

[4] Chenouard N, et al., (2013). IEEE Trans. Pattern Anal. Mach. Intell., 35, 2736-2750.

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 291522-3DIMAGE.


Tom FURNIVAL (Cambridge, United Kingdom), Eric SCHMIDT, Rowan LEARY, Daniel KNEZ, Ferdinand HOFER, Paul D BRISTOWE, Paul A MIDGLEY

14:00-16:00
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LS1-I
LS1: Macromolecular assemblies, supra molecular assemblies
SLOT I

LS1: Macromolecular assemblies, supra molecular assemblies
SLOT I

Chairpersons: Bettina BOETTCHER (Chairperson, Edinburgh, United Kingdom), Karen DAVIES (Staff Scientist) (Chairperson, Berkeley, USA), Guy SCHOEHN (Chairperson, Grenoble, France)
14:00 - 14:30 #8323 - LS01-S01 Native machinery of membrane-associated protein synthesis.
Native machinery of membrane-associated protein synthesis.

A large fraction of ribosomal synthesis occurs at organellar membranes. At the endoplasmic reticulum (ER), the inner mitochondrial, and the thylakoid membrane, nascent proteins are co-translationally inserted or transported into other compartments. Here, we structurally study membrane-bound ribosomes and their associated machineries in their native settings using cryo-electron tomography (cryo-ET) in combination with subtomogram analysis (Fig. 1).

Studies of ribosomes associated to isolated rough ER vesicles reveal the structure of the native ER translocon, as well as its compositional variability. Core components of the ER translocon are the protein-conducting channel Sec61, the translocon associated protein complex (TRAP), and the sub-stoichiometric oligosaccharyl transferase complex (OST), which all bind to the ribosome. Subnanometer resolution subtomogram averages indicate that the ribosome alone, even without a nascent chain, is sufficient for lateral opening of Sec61, contrary to recent mechanistic models. To elucidate the structures and functions of TRAP and OST in detail, we make use of mutations involved in congenital disorders, as well as their evolutionary diversity across different organisms. Analysis of cryo-tomograms from focused-ion-beam-milled whole cells allows studying the compositional variability of the ER-translocon and the relative arrangement of ER-associated ribosomes in vivo, which reveals a highly characteristic polysome organization.

Mitochondrial ribosomes specialize on the synthesis of few, very hydrophobic membrane proteins. Cryo-electron tomographic analysis of mitochondria isolated from Saccharomyces cerevisiae reveals the binding mode of mitoribosomes to the inner mitochondrial membrane, as well as their molecular organization into polysomes. The structures of mammalian mitoribosomes differ dramatically from their fungal counterparts and we study the consequences of these differences on membrane association and polysome organization. State-of-the-art phase plate imaging helps to overcome the contrast limitations set by the extremely dense and optically barely electron transparent mammalian mitochondria.

Chloroplast ribosomes constitute the third realm of eukaryotic ribosomes. We analyzed the in situ structure and intracellular distribution in green algae. The interaction mode of ribosomes with the thylakoid membrane appears to be much less defined than those of their cytoplasmic and mitochondrial counterparts.

In summary, in situ studies using cryoelectron tomography put atomic-level structural information of ribosomal complexes into context with their associated organellar membranes and their respective co-translational processing machineries, revealing high evolutionary diversity for organelles and organisms.


Stefan PFEFFER, Robert ENGLMEIER, Friedrich FOERSTER (martinsried, Germany)
Invited
14:30 - 15:00 #8743 - LS01-S02 Structure of hibernating ribosomal complexes from Gram-positive pathogenic bacteria Staphylococcus aureus, solved by single particle cryo-EM.
Structure of hibernating ribosomal complexes from Gram-positive pathogenic bacteria Staphylococcus aureus, solved by single particle cryo-EM.

Protein synthesis is a universally conserved process that is assured by a macromolecule called the ribosome (3.4 – 4.5 Mda). In spite of the conservation of the ribosome among all orders of life, its structure presents significant differences between eukaryotes and bacteria. Bacterial ribosome, smaller than its eukaryotic counterpart, presents specific particularities to which we owe the efficiency of numerous commonly used antibiotics that target the latter without hindering protein synthesis in the host. Such structural differences are found in all steps of protein translation.

Here, we attempt to explore by cryo-electron microscopy the ribosomal hibernation, one of the most mysterious regulation processes of protein translation in bacteria. Hibernation is a vital process that can be triggered as a response to stress and aims at shutting down translation in a reversible manner so that translation can recover quickly after the alleviation of stress. Here, we show several 3D reconstructions of hibernation ribosomal complexes from the Gram-positive pathogenic bacteria Staphylococcus aureus. Our structures display the formation of a disome (ribosomal dimer) mediated by a peculiar hibernation factor (HPF), thus setting up the disome in a unique fashion, different than other dimers of known bacterial species indicating the uniqueness of this process in S. aureus and perhaps in gram-positive bacteria more generally. In spite of the size of the imaged asymmetric complexes (~7 Mda, ~550Å diameter), our data-processing yielded several structures after particle sorting presenting an average resolution of ~3.7Å, thus enabling the modelling ab initio of S. aureus HPF, so far of unknown structure.

Our results represent a significant advance and pinpoint a unique process that can be targeted for designing drugs of improved specificity and efficiency against this dangerous pathogen and Gram-positive bacteria more generally.


Iskander KHUSAINOV, Quentin VICENS, Anthony BOCHLER, Alexander MYASINKOV, Srefano MARZI, Pascale ROMBY, Gulnara YUSUPOVA, Marat YUSUPOV, Yaser HASHEM (STRASBOURG CEDEX)
Invited
15:00 - 15:15 #5775 - LS01-OP001 Cryo-electron microscopy structure of La Crosse orthobunyavirus polymerase in presence or absence of viral RNA.
Cryo-electron microscopy structure of La Crosse orthobunyavirus polymerase in presence or absence of viral RNA.

Bunyaviridae is the largest family of segmented negative strand viruses (sNSV) which also include Orthomyxoviridae and Arenaviridae. Central to their viral cycle is the RNA-dependent RNA polymerase which replicates and transcribes the genome segments within circular ribonucleoprotein particles (RNPs). Here we describe a cryo-electron microscopy reconstruction of the full length La Crosse polymerase in complex with viral RNA (Figure 1), together with a reconstruction of its apo truncated form (Δ-Cterminal construct, Figure 2). Combined with the X-ray structure determined in the group, we provide a partial pseudo-atomic model of La Crosse polymerase (Figures 1 and 2). Identification of distinct template and product exit tunnels (Figure 3) and structural analysis of RNP (Figure 4) allows proposal of a detailed model for template-directed replication with minimal disruption to the circularised RNP. The similar overall architecture and vRNA binding of monomeric LACV to heterotrimeric influenza polymerase, despite high sequence divergence, suggests that all sNSV polymerases have a common evolutionary origin and mechanism of RNA synthesis.

Reference: Structural Insights into Bunyavirus Replication and Its Regulation by the vRNA Promoter. Gerlach P*, Malet H*, Cusack S¦, Reguera J¦. Cell. 2015 Jun 4;161:1267-79.


Helene MALET (Grenoble), Piotr GERLACH, Juan REGUERA, Stephen CUSACK
15:15 - 15:30 #6610 - LS01-OP006 Structure-function insights reveal the human ribosome as a cancer target for antibiotics.
Structure-function insights reveal the human ribosome as a cancer target for antibiotics.

Abstract

Many antibiotics in clinical use target the bacterial ribosome by interfering with several mechanistic steps of the protein synthesis machinery. However, targeting the human ribosome in the case of protein synthesis deregulations such as in highly proliferating cancer cells has not been much considered up to now. Here we report the first structure of the human 80S ribosome with a eukaryote-specific antibiotic and show its anti-proliferative effect on cancer cell lines. Structural sorting of the cryo electron microscopy data shows that the cycloheximide ligand induces an equilibrium shift of ribosome subpopulations in different states, revealing that the mechanism of action relies on an active release of the tRNA from the exit site. The structure provides unprecedented insights into the detailed interactions in a ligand-binding pocket of the human ribosome that are required for structure-assisted drug design. Furthermore, anti-proliferative dose response in leukemic cells and interference with synthesis of c-myc and mcl-1 short-lived protein markers reveals specificity of a series of antibiotics towards cytosolic rather than mitochondrial ribosomes, establishing the human ribosome as a promising cancer target. In addition, we present a protocol that primarily uses the crystallographic tools for atomic model building and refinement into cryo-EM maps, as exemplified by our recent human ribosome structure.


Alexander MYASNIKOV, Kundhavai NATCHIAR (Illkirch), Marielle NEBOUT, Isabelle ISABELLE HAZEMANN, Véronique IMBERT, Heena KHATTER, Jean-François PEYRON, Bruno KLAHOLZ
15:30 - 15:45 #6802 - LS01-OP007 Tripartite assembly of RND multidrug efflux pumps in nanodisc and amphipol.
Tripartite assembly of RND multidrug efflux pumps in nanodisc and amphipol.

 

*Two first authors equally contributed to this work

 

Tripartite multidrug efflux systems of Gram-negative bacteria export a large variety of antimicrobial compounds at the expense of ATP or the proton motive force, thereby conferring resistance to a wide variety of antibiotics. Pseudomonas aeruginosa MexAB-OprM and Escherichia coli AcrAB-TolC, are prototypic proton motive force-driven efflux systems from Resistance Nodulation and cell Division (RND) family. These efflux systems, composed of an inner membrane transporter, an outer membrane channel and a periplasmic adaptor protein, are assumed to form ducts inside the periplasm, facilitating drug exit across the outer membrane. Using lipid nanodisc system, we recently reported the reconstitution of native MexAB-OprM tripartite system. Single particle analysis by electron microscopy revealed the lipid nanodisc-embedded inner and outer membrane protein components linked together via the MexA periplasmic adaptor protein [1].

Recently the replacement of detergent by specially designed amphiphilic molecules such as amphipol or styrene–maleic acid copolymers is an alternative option to stabilize the membrane proteins in a free-lipid environment. To further investigate key parameters controlling the formation of the tripartite system, we focus our study on MexA that in its mature form is anchored to the inner membrane via its palmitoyl moiety. To assess whether the lipid anchor is required for the reconstitution, we report here on the reconstitution of OprM and MexB in amphipol and compare with our previous results obtained with nanodiscs.

 

 

[1] Daury L, Orange F, Taveau JC, Verchère A, Monlezun L, Gounou C, Marreddy RK, Picard M, Broutin I, Pos KM, Lambert O (2016). Tripartite assembly of RND multidrug efflux pumps. Nat Commun. 7:10731.


Dimitri SALVADOR, Marie GLAVIER, Cyril GARNIER, Martin PICARD, Isabelle BROUTIN, Jean-Christophe TAVEAU, Laetitia DAURY, Olivier LAMBERT (Bordeaux)
15:45 - 16:00 #6546 - LS01-OP004 The supramolecular packing of the gel-forming MUC5B and MUC2 mucins and its importance for cystic fibrosis.
The supramolecular packing of the gel-forming MUC5B and MUC2 mucins and its importance for cystic fibrosis.

Text:

Introduction: The genetically related gel forming mucins, MUC2 (intestine), MUC5B (airways), MUC5AC (airways, stomach) and MUC6 (stomach) have large sizes with heavily glycosylated mucin domains in the central part. The C-termini form intermolecular dimers. The N-terminal regions are evolutionarily similar with identical domain organization important for the oligomerization. MUC5B is vital for normal mucociliary clearance of the lungs whereas MUC2 in colon forms an inner dense and attached stratified layer impermeable to bacteria, and an outer loose and unattached layer habituating commensal bacteria. The MUC2 N-terminus (D1-D2-D′D3 domains) was shown to form concatenated polygone-structures under low pH- and high calcium conditions (1).
The Aim is to understand the cellular packing of the MUC5B and MUC2 mucins and how this influences their secretion.
Method: The N-terminal and D´D3 domains of MUC2 and MUC5B were expressed, purified and then analyzed by subsequent gel filtration, transmission electron microscopy and single particle image processing.
Results: MUC5B multimerizes by disulfide bonds between the D3-domains giving the MUC5B polymer a linear structure (2). Analysis of the MUC5B N-terminus at lower pH and higher calcium concentration revealed a tight dimer+dimer packing where the second dimer is turned upside down by 180 degrees and then slightly rotated. This way of packing the MUC5B in the granulae will allow a slow unwinding of a linear molecule.
Conclusion: The MUC5B and MUC2 mucins are packed in the mucin granulae in a way allowing the formation of linear strands or net-like structures, respectively.

References:
1. Ambort, D., Johansson, M. E. V., Gustafsson, J. K., Nilsson, H., Ermund, A., Johansson, B. R., Koeck, P. J. B., Hebert, H., and Hansson, G. C. (2012) Proc. Natl. Acad. Sci. U. S. A. 109, 5645-5650
2. Ridley, C., Kouvatsos, N., Raynal, B. D., Howard, M., Collins, R. F., Desseyn, J. L., Jowitt, T. A., Baldock, C., Davis, C. W., Hardingham, T. E., and Thornton, D. J. (2014) J. Biol. Chem. 289, 16409-16420

 


Harriet E. NILSSON (Huddinge, Sweden), Sergio MUYO TRILLO, Anna ERMUND, Malin BÄCKSTRÖM, Elisabeth THOMSSON, Daniel AMBORT, Philip J. B. KOECK, David J. THORNTON, Gunnar C. HANSSON, Hans HEBERT

10:30-12:30
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LS6-I
LS6: Interactions micro-organism-host
SLOT I

LS6: Interactions micro-organism-host
SLOT I

Chairpersons: Ilaria FERLENGHI (Structural Based Antigen Design) (Chairperson, Siena, Italy), Kay GRUNEWALD (Chairperson, Oxford, United Kingdom), Olivier LAMBERT (Chairperson, Bordeaux, France)
10:30 - 11:00 #8391 - LS06-S19 A huge but elusive macromolecular cage in enterobacterial stress response as seen by cryoEM and X-ray crystallography.
A huge but elusive macromolecular cage in enterobacterial stress response as seen by cryoEM and X-ray crystallography.

The E. coli inducible lysine decarboxylase LdcI is an important acid stress response enzyme, whereas the AAA+ ATPase RavA is involved in multiple stress response pathways. A complex between these two proteins is thought to counteract acid stress under starvation in E. coli. We solved the crystal structure of the RavA monomer and combined it with a negative stain EM reconstruction of the RavA-ADP hexamer (El Bakkouri et al., 2010). We also determined the crystal structure of the LdcI double pentamer (Kanjee et al., 2011). While these structures provided important insights into the functions of these individual proteins, the design principles of the LdcI-RavA complex appeared even more enigmatic because it was difficult to envision how a decamer can bind a hexamer.  CryoEM analysis allowed us to fit together the pieces of the jigsaw and revealed that the LdcI-RavA complex is a surprising macromolecular cage of the size of a ribosome formed by two LdcI decamers and five RavA hexamers (Malet et al., 2014). We identified molecular determinants of this interaction and specific elements essential for complex formation, as well as conformational rearrangements associated with the pH-dependent LdcI activation and with the RavA binding (Malet et al., 2014; Kandiah et al., 2016). Moreover, we uncovered differences between the LdcI and its close paralogue, the second E. coli lysine decarboxylase LdcC thought to play mainly a biosynthetic role, finally explaining why only the acid stress response enzyme is capable of binding RavA and forming the cage. Multiple sequence alignment coupled to a phylogenetic analysis reveals that certain enterobacteria exert evolutionary pressure on the lysine decarboxylase towards the cage-like assembly with RavA, implying that this complex may have an important function under particular stress conditions (Kandiah et al., 2016). Research on structure-functional relationships of this system by a combination of cryoEM, super-resolution fluorescence microscopy and other structural, biochemical, biophysical and cell biology techniques is on-going.

Kanjee, U. et al. EMBO J. 30, 931–944 (2011).

El Bakkouri, M. et al. Proc. Natl. Acad. Sci. U. S. A. 107, 22499–22504 (2010).

Malet, H. et al. eLife 3, e03653 (2014).

Kandiah, E. et al. Sci Rep. 6, 24601 (2016).

Acknowledgements

We thank Guy Schoehn for establishing and managing the cryo-electron microscopy platform and for providing training and support. We are grateful to all members of the Houry group and the Gutsche team who were or currently are involved in sample preparation and analysis for this study. For electron microscopy, this work used the platforms of the Grenoble Instruct center (ISBG; UMS 3518 CNRS-CEA-UJF-EMBL) with support from FRISBI (ANR-10-INSB-05-02) and GRAL (ANR-10-LABX-49-01) within the Grenoble Partnership for Structural Biology (PSB). The electron microscope facility (Polara electron microscope) is supported by the Rhône-Alpes Region (CIBLE and FEDER), the FRM, the CNRS, the University of Grenoble Alpes and the GIS-IBISA. DC is the recipient of the Grenoble Alliance for Integrated Structural and Cell Biology (GRAL) PhD fellowship. This work was supported by the ANR-12-JSV8-0002 and the ERC 647784 grants to IG, the CIHR MOP-130374 to WAH, the ANR-10-LABX-49-01 and the ANR-11-LABX-0003-01.


Eaazhisai KANDIAH, Hélène MALET, Diego CARRIEL, Julien PERARD, Maria BACIA, Walid A HOURY, Sandrine OLLAGNIER DE CHOUDENS, Sylvie ELSEN, Irina GUTSCHE (IBS, Grenoble)
Invited
11:00 - 11:30 #8370 - LS06-S20 Structure of the bacterial type 3 secretion system in action.
Structure of the bacterial type 3 secretion system in action.

Pathogenic bacteria commonly use the conserved type 3 secretion system (T3SS or Injectisome) to deliver virulence factors into the target eukaryotic cells in order to manipulate their functions. Injectisome is a multi-protein nanomachine assembled at the bacterial membranes containing the periplasmic basal body, cytoplasmic sorting platform and export apparatus1. A hollow needle extends from the periaplasmic part to the extracellular face serves as a channel for the secreted molecules.  The high resolution structure of the components of the injectisome is well understood: structure of the basal body at subnanometer resolution2 in combination with computational molecular analysis3 demonstrated the inter-subunit interaction interfaces and suggested a conserved oligomerization mechanism for the assembly. The cytoplasmic ATPase SctN (according to the general nomenclature) is structurally similar to the F- and V- type ATPases and is believed to be responsible for detachment of chaperones and unfolding the exported substrates4.

Recent fluorescent studies in Yersinia enterocolitica showed turnover of SctQ; the turnover was faster for secreting injectisomes therefore the injectisome was suggested to be a highly dynamic nanomachine5.  Additionally, upon activation of secretion newly formed injectisomes are established next to the existing ones6. Interestingly, trapping the exported substrate in the isolated needle complex (made of the basal body and the needle) results in only small conformational changes distant from the secretion path7. It is highly interesting to visualize the mechanics of substrate secretion happening inside the cell and understand the intermediate steps of this exciting process.

During my talk I will present the structural analysis of the injectisomes in Yersinia and Chlamydia performed by cryo electron tomography and subtomogram averaging.  Surprisingly, the basal bodies of injectsomes of Yersinia enterocolitica can vary in length up to 20% 8. This flexibility it attributed to the flexible domains of the proteins of the basal body. In Chlamydia the basal bodies of injectisomes strongly contract upon contact with the host cell membrane. This contraction is coupled to a structural stabilization of the cytoplasmic part of the injectisome9. This “pumping action” likely constitutes a general mechanism related to the injection of effectors. Higher resolution structural analysis in situ combined with mutagenesis will generate the clearer understanding of the mechanics of the T3SS action.

 

References: 

1.         Diepold, A. & Armitage, J. P. Type III secretion systems: the bacterial flagellum and the injectisome. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 370, (2015).

2.         Schraidt, O. & Marlovits, T. C. Three-dimensional model of Salmonella’s needle complex at subnanometer resolution. Science 331, 1192–1195 (2011).

3.         Bergeron, J. R. C. et al. A refined model of the prototypical Salmonella SPI-1 T3SS basal body reveals the molecular basis for its assembly. PLoS Pathog. 9, e1003307 (2013).

4.         Akeda, Y. & Galán, J. E. Chaperone release and unfolding of substrates in type III secretion. Nature 437, 911–915 (2005).

5.         Diepold, A., Kudryashev, M., Delalez, N. J., Berry, R. M. & Armitage, J. P. Composition, formation, and regulation of the cytosolic c-ring, a dynamic component of the type III secretion injectisome. PLoS Biol. 13, e1002039 (2015).

6.         Kudryashev, M. et al. Yersinia enterocolitica type III secretion injectisomes form regularly spaced clusters, which incorporate new machines upon activation. Mol. Microbiol. 95, 875–884 (2015).

7.         Radics, J., Königsmaier, L. & Marlovits, T. C. Structure of a pathogenic type 3 secretion system in action. Nat. Struct. Mol. Biol. 21, 82–87 (2014).

8.         Kudryashev, M. et al. In situ structural analysis of the Yersinia enterocolitica injectisome. eLife 2, e00792 (2013).

9.         Nans, A., Kudryashev, M., Saibil, H. R. & Hayward, R. D. Structure of a bacterial type III secretion system in contact with a host membrane in situ. Nat. Commun. 6, 10114 (2015).


Mikhail KUDRYASHEV (Frankfurt am Main, Germany)
Invited
11:30 - 11:45 #5273 - LS06-OP027 Characterisation of Trypanosoma spp. in Australian wildlife.
Characterisation of Trypanosoma spp. in Australian wildlife.

The genus Trypanosoma comprises numerous species of flagellated vector-borne protozoa that parasitise the blood and tissues of vertebrates. They are ubiquitous in terms of their geographical distribution and host range. In mammals, most species of Trypanosoma have been described from wildlife yet apart from their taxonomy, we know very little about the host parasite relationship, particularly those species in Australia.

Most of what is known about their host parasite relationships, life history, and developmental biology has been obtained from studies on the two species that invade cells and infect humans - T. brucei and T. cruzi - species endemic in Africa and South America respectively, and which are major causes of disease and death in humans and domestic animals. T. brucei is the cause of sleeping sickness as a result of its association with the nervous system, whereas T. cruzi is the cause of Chagas disease that results in cardiac, neurological, and digestive disorders.

Importantly, an Australian species - T. copemani G2 - has also been found to invade cells, thus demonstrating a pathogenic potential previously not associated with trypanosomes from Australia. Recently several new species of Trypanosoma in Australian marsupials (T. copemani (G1 & G2), T. vergrandis, and T. noyesi) have been characterised. These have varying affinities to T. cruzi, including surprisingly similar genetic relationships (e.g. close genetic link of T. noyesi and T. cruzi) and behavioural traits (e.g. cellular invasion by both T. copemani and T. cruzi). Such observations not only raise concerns about the impact of Australian trypanosomes on wildlife health and conservation but also in terms of biosecurity and human health given the potential for local transmission of imported cases of Chagas disease.

Here we present correlative data across a range of length scales demonstrating the ongoing characterisation of several Trypanosoma spp. from Australian wildlife. In particular we have used live cell imaging to show host cell-pathogen interactions (Figure 1), scanning (SEM) (Figure 2) and transmission (TEM) (Figure 3) electron microscopy for structural analysis, and Slice and ViewTM focussed ion beam-scanning electron microscopy (FIB-SEM)(Figure 4) to begin to image key structural features (e.g. kinetoplast, flagellum) at high resolution in 3-dimensions.

Together these data are i) providing a greater understanding of the pathogenic potential and host-parasite relationships of trypanosomes in Australian marsupials; ii) allowing for identification of biosecurity issues relating to potential local hosts and transmission of exotic species; and iii) generating information about the role of trypanosomes as a potential cause of disease in threatened and endangered Australian marsupials.

Acknowledgements: The authors acknowledge use of the facilities at the Centre for Microscopy, Characterisation & Analysis, UWA, which is funded by State and Commonwealth governments; and funding from the West Australian Government's State NRM Program.


Crystal COOPER, Andrew THOMPSON, Adriana BOTERO, Peta CLODE (Crawley, Australia)
11:45 - 12:00 #5210 - LS06-OP026 Flow cytometry, and Transmission electron microscopy; two valuable tools used in an innovative approach to isolate and describe new microorganisms.
Flow cytometry, and Transmission electron microscopy; two valuable tools used in an innovative approach to isolate and describe new microorganisms.

Flow cytometry and transmission electron microscopy are two reliable techniques used for decades in microbiology. In parallel, amoebal pathogens like giant viruses, and chlamydiae are of a big interest for the scientific community. Their ubiquity and diversity made by them a hot spot in environmental microbiology. The isolation of these pathogens of protists is the key to understand their evolution, their respective biotopes, and their potential or hidden pathogenicity. The co-culture is the routinely used tool to isolate these microorganisms. Here we report the association of flow cytometry and electron microscopy to the co-culture in a high throughput method capable of describing new isolates. Flow cytometry is used to detect and sort lytic or non-lytic amoebal pathogens and electron microscopy characterizes the developmental stages and phenotypic features of the new isolates. These improvements based on enrichment systems, targeted use of antibiotics and high-throughput methods associating FACS to TEM which are highly sensible regarding the old techniques, brought extreme benefits to the giant viruses and chlamydiae study and research fields. 


Jacques BOU KHALIL (MARSEILLE)
12:00 - 12:30 #6220 - LS06-S21 Soft X-Ray cryo-tomography reveals ultrastructural alterations of the host cell during Hepatitis C infection.
Soft X-Ray cryo-tomography reveals ultrastructural alterations of the host cell during Hepatitis C infection.

Chronic hepatitis C virus (HCV) infection causes severe liver disease in millions of humans worldwide. Pathogenesis of HCV infection is strongly driven by a deficient immune response of the host, although intersection of different aspects of the virus life cycle with cellular homeostasis is emerging as an important player in the pathogenesis and progression of the disease.

Cryo soft X-ray tomography (cryo-SXT) was performed to investigate the ultrastructural alterations induced by the interference of hepatitis C virus (HCV) replication with cellular homeostasis. Native, whole cell, three-dimensional maps were obtained in HCV replicon-harboring cells and in a surrogate model of HCV infection at 40nm resolution. Tomograms from HCV-replicating cells show blind-ended endoplasmic reticulum (ER) tubules with pseudo spherical extrusions and marked alterations of mitochondrial morphology that correlated topologically with the presence of ER alterations, suggesting a short-range influence of the viral machinery on mitochondrial homeostasis.

Both mitochondrial and ER alterations could be reverted by a combination of sofosbuvir/daclatasvir, which are clinically approved direct-acting antivirals (DAAs) for the treatment of chronic HCV infection. In addition to providing structural insight into cellular aspects of HCV pathogenesis our study illustrates how cryo-SXT is a powerful three-dimensional wide-field imaging tool for the assessment and understanding of complex cellular processes in a setting of near native whole hydrated cells. Our results also constitute a proof of concept for the use of cryo-SXT as a platform that enables determining the potential impact of candidate compounds on the ultrastructure of the cell that may  assist drug development at a preclinical level.


Ana Joaquina PEREZ-BERNA (Barcelona, Spain), Maria Jose RODRÍGUEZ, Andrea SORRENTINO, Francisco Javier CHICHON, Martina FRIESLAND, Jose Lopez CARRASCOSA, P GASTAMINZA, Eva PEREIRO
Invited

14:00-16:00
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LS7-I
LS7: Organism development and imaging
SLOT I

LS7: Organism development and imaging
SLOT I

Chairpersons: Grégoire MICHAUX (Chairperson, Rennes, France), Cristina PUJADES (co-chair) (Chairperson, Barcelona, Spain)
14:00 - 14:30 #8649 - LS07-S22 Eavesdropping on the molecular signatures of embryonic development.
Eavesdropping on the molecular signatures of embryonic development.

The study of embryonic development has been dramatically advanced by the wealth of high-throughput molecular studies that have defined the genes and proteins involved.  This wealth of data now presents the challenge of integrating a working knowledge of how these molecular components, often present at vanishingly small concentrations, generate reliable patterns of cell migration and cell differentiation.  In typical cell biology approaches, cultures of isolated cells have been used reveal mechanism.  What is needed to understand development is to carry out studies on cells in their normal context interacting with other cells and signals in the intact embryo.  Key events of embryonic development take place over dimensions of less than 500um in less than 5 hours, making it tractable for light imaging tools to be used to answer this challenge.   

 Imaging techniques are challenged by major tradeoffs between spatial resolution, temporal resolution, and the limited photon budget.  We are attempting to advance this tradeoff by constructing light sheet microscopes that maintain subcellular resolution in thick and scattering specimens.  Our two-photon light-sheet microscope, combines the deep penetration of two-photon microscopy and the speed of light sheet microscopy to generate images with more than ten-fold improved imaging speed and sensitivity.  As with other light sheet technologies, the collection of an entire 2-D optical section in parallel offers dramatically speeds acquisition rates.  By adopting two-photon SPIM is far less subject to light scattering, permitting subcellular resolution to be maintained far better than conventional light sheet microscopes.  The combination of attributes permits cell and molecular imaging with sufficient speed and resolution to generate unambiguous tracing of cells and signals in intact systems, which presents a major challenge in data management, processing and analysis. 

Multispectral imaging offers the chance of asking multiple questions of the same embodied cells.  Multiplex analyses permit the variance and the “noise” in a system to be exploited by asking about the analytes that co-vary with a selected gene product.  Transforming fluorescent spectra to a point on a 2D-plot with sinusoidal functions (phasors) provides a powerful tool for the cumbersome tasks of visualizing and analyzing hyper-spectral imaging data.  This technique offers excellent performance in the face of biological and instrumental uncertainties. Our results on live zebrafish embryos document the accuracy of hyper-spectral phasors and demonstrates their capability to distinguish multiple spectrally overlapping fluorophores in low signal-to-noise and fast analysis time.

Combined, these tools offer the multi-dimensional imaging required to follow key events in embryos as they take place, and allow us to use variance as an experimental tool rather than a limitation. 


Scott FRASER (Los Angeles, USA)
Invited
14:30 - 14:45 #4890 - LS07-OP028 Germ-line cysts organisation in clitellate annelids – from electron microscopy to live cell imaging.
Germ-line cysts organisation in clitellate annelids – from electron microscopy to live cell imaging.

 

At the onset of gametogenesis in animals, the germ-line cells usually form syncytial cysts (clusters). The germ cells in such cysts are interconnected by wide cytoplasmic channels known as intercellular (cytoplasmic) bridges that are modified contractile rings that do not close during late cytokinesis. These bridges ensure the cytoplasmic continuity between the interconnected cells and, as a result, the cytoplasm of the sister cells is common. It is believed that the interconnections of germ cells into syncytial clusters regulate and synchronise germ cell development. However, it seems that the actual role of the cell clustering is sex dependent.

                The germ-line cyst architecture (spatial organization) varies between taxa. Currently, we know several different modes of cysts organisation. The best characterised examples are: linear cysts in which the interconnected cells form chains (e.g. mammal spermatogenesis); branched cysts in which some cells may have more than two intercellular bridges and, as a result, “branchings” occur in these sites (e.g. male and female cysts in Drosophila); and cysts with germ cells that are clustered around a central anuclear mass of cytoplasm (e.g. in C. elegans).

              Clitellate annelids (Clitellata) is a monophyletic taxon of segmented worms, which comprises earthworms and their allies and leeches. At the early stages of spermatogenesis and oogenesis in Clitellata, germ-line cysts are also formed (with one known exception – female cells in Capilloventer australis). In both lines, male and female, the cysts have the same basal plan of organization – each germ cell is connected to the central and anuclear cytoplasmic mass, the cytophore via one cytoplasmic bridge (Fig. 1 and 2). However, the cyst morphology may differ substantially between taxa and even between sexes in the same specimen. These differences may be caused by: a different number of interconnected germ cells (from 16 in the pot worm Enchytraeus albidus (Fig. 2) to over 2 000 in the sludge worm Tubifex tubifex (Fig. 3)); a different shape and ratio of cytophore development (the cytophore in male cysts is usually spherical and well developed, while in female cysts the cytophore usually has the form of long cytoplasmic strands and is often poorly developed). Additionally, whereas male cysts are not associated with somatic cells, female clusters are usually enveloped by somatic cells and together form characteristic structures such as egg follicles or ovary cords.

              During our long-term studies, we have analysed the formation, architecture and functions in both male and female germ-line clusters in several representatives of clitellate annelids (e.g. the earthworm Dendrobaena veneta, the pot worm E. albidus, a sludge worm T. tubifex and the European medicinal leech Hirudo medicinalis). Using classical (light and fluorescent microscopy, electron microscopy) and modern tools (live cell imaging see Fig. 4, 3D reconstructions in scanning electron microscopy – SBEM method), we have revealed, among others, the great plasticity in cyst architecture and sex-dependent differences in their functions.

Acknowledgements This work was partially funded by National Science Centre, Poland. Contract grant number: DEC-2012/05/B/NZ4/02417.


Piotr ŚWIĄTEK, Anna Z. URBISZ, Karol MAŁOTA, Szymon GORGOŃ, Natalia JAROSZ, Piotr ŚWIĄTEK (Katowice, Poland)
14:45 - 15:15 #8757 - LS07-S23 Optical investigation of a novel sensory interface linking spinal fluid to motor circuits in vivo.
Optical investigation of a novel sensory interface linking spinal fluid to motor circuits in vivo.

The cerebrospinal fluid (CSF) is a complex solution circulating around the brain and spinal cord. Behavior has long been known to be influenced by the content and flow of the CSF, but the underlying mechanisms are completely unknown. CSF-contacting neurons by their location at the interface with the CSF are in ideal position to sense CSF cues and to relay information to the nervous system. By combining electrophysiology, optogenetics, bioluminescence monitoring with calcium imaging in vivo, we demonstrate that neurons contacting the CSF in the spinal cord detect local bending and in turn feed back GABAergic inhibition to multiple interneurons driving locomotion in the ventral spinal cord. Behaviour analysis of animals deprived of this mechano-sensory pathway reveals its contribution in modulating key parameters of locomotion. Altogether our approach developed in a transparent animal model shed light on a novel pathway enabling sensory motor integration between the CSF and motor circuits in the spinal cord.

 

References

Djenoune et al., Frontiers in Neuroanatomy 2014.

Fidelin et al., Current Biology 2015.

Bohm et al., Nature Communications 2016.

Hernandez et al., Nature Communications 2016.

Sternberg et al., Current Biology, in press.

Hubbard et al., Current Biology, in revision.

Knafo et al., submitted.


Claire WYART (Paris)
Invited
15:15 - 15:30 #6574 - LS07-OP029 The posterior neural plate in axolotl can form mesoderm or neuroectoderm.
The posterior neural plate in axolotl can form mesoderm or neuroectoderm.

Gastrulation is the developmental process where germ layers are formed. It was generally assumed that germ layer specification is finished by the end of gastrulation. However, previous studies in amphibians revealed that posterior neural plate and fold, although traditionally regarded as neural, have a mesodermal bias and give rise to tail and posterior trunk muscles whereas only more anterior regions give rise to spinal chord [1-4]. In addition, recent studies in zebrafish, chick, and mouse revealed bipotential stem cell populations at the posterior ends of these embryos. The stem cells can give rise to neural and mesodermal tissues depending on local signaling in the tail bud [5-7]. Here, we reinvestigated morphogenesis and fate of the posterior neural plate in an amphibian model, the axolotl (Ambystoma mexicanum), using GFP-labeled grafts of the posterior third of the neural plate (reg. 3; stage 15; Figure 1) for detailed lineage analysis. In situ hybridisation with riboprobes against mesodermal (brachyury, bra) and neuronal/stem cell (sox2) markers revealed an ambiguous determinative state of reg. 3 at the time of transplantation. While its central and posterior part is bra-positive, small left and right anterior regions expressed sox2. The more anterior plate regions 2 and 1 are sox2-positive and bra-negative. The border of the two markers is ill defined and shows some overlap. Histological analysis of reg. 3 grafts at early tailbud stages revealed that the cells of the most posterior part of reg. 3 start moving from dorsal to ventral forming the posterior wall. Then they turn anteriorly, become connected with paraxial presomitic mesoderm and form somites on either side of the embryo (Figure 1). As a result of this movement, the posterior half of reg. 3 gives rise to posterior trunk and anterior tail somites whereas the anterior half forms posterior tail somites and tail spinal cord. Only cells that conduct this anterior turn and pass the Wnt/b-catenin positive posterior wall will contribute to paraxial mesoderm. Cells that remain in the dorsal part of the tail-bud eventually form lateral and dorsal parts of the tail spinal chord. The floor plate of the posterior spinal chord and the axial mesoderm do not contain any reg. 3 cells which indicates that they may be formed from the chordoneural hinge, a connection between the posteriormost reg. 2 cells and the tip of the notochord [8]. Additional grafting experiments showed that the notochord is formed from axial mesoderm that was already involuted during gastrulation and underwent massive elongation, presumably by convergence and extension. Therefore, axial and paraxial mesoderm of the tail are formed by different mechanisms and are probably specified at different time points, i.e. during gastrulation and tail bud development.

Taken together, these data show that germ layer specification is not complete after gastrulation, but that part of the putative neural plate is specified during morphogenetic movements of tail bud stages in order to become paraxial mesoderm.

 

[1]        JH Bijtel, Roux Arch. EntwMech. Organ. 125 (1931) 448-486

[2]        AS Tucker and JMW Slack, Curr. Biol. 5 (1995) 807-813

[3]        CW Beck, WIREs Dev Biol 4 (2014) 33-44

[4]        Y Taniguchi, T Kurth, et al., Sci. Rep. 5 (2015) 11428

[5]        BL Martin and D Kimelman, Dev. Cell 22 (2012) 223-232

[6]        CM Bouldin, AJ Manning, et al., Development 142, 2499-2507

[7]        H Kondoh and T Takemoto, Curr. Opin. Genet. Dev. 22 (2012) 374-380

[8]        LK Gont, H Steinbeisser, et al., Development 119 (1993) 991-1004

[9]        Financial support by DFG (EP 8/11-1) is gratefully acknowledged.


Yuka TANIGUCHI, Thomas KURTH (Dresden, Germany), Verena KAPPERT, Saskia REICHELT, Susanne WEICHE, Akira TAZAKI, Cora RÖHLECKE, Hans-Henning EPPERLEIN
15:30 - 16:00 #7505 - LS07-S24 Tracking tumor metastasis in vivo at high-resolution.
Tracking tumor metastasis in vivo at high-resolution.

Three reasons explain why most of the critical events driving normal and pathological scenarios had been less investigated: they occur rarely in space and time, they are highly dynamic, they differ when studied in situ in an entire living organism. Metastasis is the primary cause for cancer-related mortality, but its mechanisms remain to be elucidated. Intravital imaging has opened the door to in vivo functional imaging in animal models of cancer, however it is limited in resolution. Ultrastructural analysis of tumor metastasis in vivo has so far been hindered by the limited field of view of the electron microscope, making it difficult to retrieve volumes of interest in complex tissues. We recently developed a multimodal correlative approach allowing us to rapidly and accurately combine functional in vivo imaging with high-resolution ultrastructural analysis of tumor cells in a relevant pathological context. The multimodal correlative approach that we propose here combines two-photon excitation microscopy (2PEM), microscopic X-ray computed tomography (microCT) and three-dimensional electron microscopy (3DEM). It enables a rapid and accurate correlation of functional imaging to high-resolution ultrastructural analysis of tumor cells in a relevant pathological context. As an example, we are now capable of providing high- and isotropic (8nm) resolution imaging of single metastasizing tumor cells previously imaged in the process of extravasation in the living mouse brain. This reliable and versatile workflow offers access to ultrastructural details of metastatic cells with an unprecedented throughput opening to crucial and unparalleled insights into the mechanisms of tumor invasion, extravasation and metastasis in vivo.


Jacky GOETZ (Strasbourg)
Invited

10:30-12:30
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LS4-I
LS4: Membrane Interaction
SLOT I

LS4: Membrane Interaction
SLOT I

Chairpersons: Hans HEBERT (Chairperson, Huddinge, Sweden), Aurélien ROUX (Chairperson, Genève, Switzerland)
10:30 - 11:00 #8753 - LS04-S13 New insights into plasma membrane organization from super-resolution STED microscopy.
New insights into plasma membrane organization from super-resolution STED microscopy.

Plasma membrane interactions such as the transient protein-protein or protein-lipid complexes, the formation of lipid nanodomains (often denoted “rafts”), or diffusional restrictions by the cortical cytoskeleton are considered to play a functional part in a whole range of membrane-associated processes. However, the direct and non-invasive observation of such structures in living cells is impeded by the resolution limit of >200nm of a conventional far-field optical microscope. Here we present the use of the combination of super-resolution STED microscopy with fluorescence correlation spectroscopy (FCS) for the disclosure of complex nanoscopic dynamical processes. By performing FCS measurements in focal spots tuned to a diameter of down to 30 nm, we have obtained new details of molecular membrane dynamics, such as of transient lipid-protein interactions and of diffusional restrictions by the cortical cytoskeleton. Further insights will be given for molecular dynamics in the plasma membrane of immune cells, specifically during T-cell activation.


Christian EGGELING (Oxford, United Kingdom)
Invited
11:00 - 11:30 #8742 - LS04-S14 Characterization of the plasma membrane dynamics during cell signaling processes investigated by spot variation fluorescence correlation spectroscopy (svFCS).
Characterization of the plasma membrane dynamics during cell signaling processes investigated by spot variation fluorescence correlation spectroscopy (svFCS).

The plasma membrane organization is highly heterogeneous as a result of the intrinsic molecular Brownian agitation and the vast diversity of membrane components. Selective interactions take place in the formation of local complex multicomponent assemblies of lipids and proteins on different time scales. Still, deciphering this lateral organization on living cells and on the appropriate length and temporal scales has been challenging but is crucial to advance our knowledge on the biological function of the plasma membrane.

Among the methodological developments made during the last decade, the spot variation FCS (svFCS), a fluorescent correlation spectroscopy (FCS)-based method, has allowed the significant progress in the characterization of cell membrane lateral organization at the sub-optical level, including to providing compelling evidence for the in vivo existence of lipid-dependent nanodomains (see for review )1.This method provides particular insight to identify possible molecular confinement occurring at the plasma membrane 1, 2. The svFCS is performed by changing the spot of illumination underfilling the back aperture of the objective 3. Theoretical models have been developed to predict how geometrical constraints such as the presence of adjacent or isolated domains affect the svFCS observations 4, 5.

We will illustrate how the investigations based on svFCS have provided compelling evidence for the in vivo existence of lipid-dependent nanodomains 2, 6, 7 and have allowed significant progress in the characterization of cell membrane lateral organization for different kind of receptors 8-10.

 

 

1. He, H.T. & Marguet, D. Annu Rev Phys Chem 62, 417-436 (2011).

2. Lenne, P.F. et al. EMBO J 25, 3245-3256 (2006).

3. Wawrezinieck, L., Lenne, P.F., Marguet, D. & Rigneault, H. P Soc Photo-Opt Inst 5462, 92-102 (2004).

4. Wawrezinieck, L., Rigneault, H., Marguet, D. & Lenne, P.F. Biophys J 89, 4029-4042 (2005).

5. Ruprecht, V., Wieser, S., Marguet, D. & Schutz, G.J. Biophys J 100, 2839-2845 (2011).

6. Lasserre, R. et al. Nat Chem Biol 4, 538-547 (2008).

7. Wenger, J. et al. Biophys J 92, 913-919 (2007).

8. Blouin, C.M. et al. Cell in press (2016).

9. Chakrabandhu, K. et al. EMBO J 26, 209-220 (2007).

10. Guia, S. et al. Sci Signal 4, ra21 (2011).

 


Sébastien MAILFERT, Yannick HAMON, Hai-Tao HE, Didier MARGUET (MARSEILLE CEDEX 9)
Invited
11:30 - 12:00 #7863 - LS04-S15 Electron microscopy approaches to studying lipid–protein interactions.
Electron microscopy approaches to studying lipid–protein interactions.

Membrane proteins play crucial roles in many cellular processes such as signaling, nutrient uptake and cell adhesion.  Although the lipid bilayer influences many aspects of membrane protein function, our understanding of lipid–protein interactions is limited.  In the first part of my talk, I will describe how electron crystallography of the water channel aquaporin-0 reconstituted with lipids into two-dimensional crystals can be used to address very basic questions in membrane biology, such as the driving forces that define lipid–protein interactions and the effects of hydrophobic mismatch.  In the second part, I will discuss how we use membrane proteins reconstituted into nanodiscs to make it possible to study lipid–protein interactions by single-particle cryo-electron microscopy.  In particular, I will discuss how the mechanosensitive channel MscS adapts to different lipid bilayers.


Thomas WALZ (New York, USA)
Invited
12:00 - 12:15 #4615 - LS04-OP021 How nanoparticles disturb the lipid bilayer vesicles.
How nanoparticles disturb the lipid bilayer vesicles.

 

Meriem Er-Rafik*,(1),Khalid Ferji (2), Olivier Sandre (2), Carlos M. Marques (1), Jean-Francois Le Meins (2), Marc Schmutz (1)

1. Institut Charles Sadron, Université de Strasbourg-CNRS UPR 22, 67034 Strasbourg, France

2. Laboratoire de Chimie des Polymères Organiques (LCPO), Université de Bordeaux, CNRS,  33607 Pessac, France

 

         Innumerous molecules and particles, from simply water to complex proteins or self-assembled small liposomal carriers, can frequently interact with the cell membrane with possible modification on it. Despite a constant increase of the variety of new particles or molecular assemblies, due to rapid progress in nanotechnology, the molecular features determining how is the membrane behaviour with respect to a given molecule are not yet elucidated.

            Here, we will present a new mechanism of the lipid bilayer of liposomes behaviour in presence of nanoparticles investigated by cryo-electron tomography. Cryo-electron microscopy is a relevant technique allowing not only to inspect the structure of the membrane, by resolving for instance the two leaflets of the bilayer, but reveals also geometric features of nanoparticles such as size and shape that play an important role for the potential interaction with lipid bilayer (fig. 1). Cryo-electron tomography resolve in 3D space the relative positions of particles and membranes, providing insight into the interplay between particle-lipid interactions and the ensuing bilayer transformations.

We wish to thank for the financial support the research association ANR (Agence Nationale de Recherche) of the project ANR-12-BS08-0018-01 and the French society of microscopy (SFµ).


Meriem ER-RAFIK (STRASBOURG)
12:15 - 12:30 #6815 - LS04-OP022 In vitro/ex vivo behavior of a new optical imaging agent targeting αVβ3 integrin.
In vitro/ex vivo behavior of a new optical imaging agent targeting αVβ3 integrin.

Introduction Integrin αvβ3 is usually expressed at low or undetectable levels in most adult epithelia, but it is highly upregulated in tumors and correlates with disease progression. Moreover, unlike in quiescent endothelium, αvβ3 is highly expressed in tumor-associated vessels.

RGD (Arg-Gly-Asp) peptides carry the minimal integrin-binding sequence and are well-known to bind preferentially to αvβ3. Thus, RGD-based strategies have been widely adopted to design targeted molecules for cancer therapy and/or diagnosis.

We synthesized a new cyclic RGD-based peptidomimetic conjugated with a NIR fluorophore intended for intrasurgical use during tumor resection, to allow proper identification of tumor margins and sparse metastases by optical imaging. We present here the in vitro/ex vivo characterization of this molecule, meant to define its specificity for the target receptor, its ability to enter the cells and its in vivo behavior after administration to mice. Cellular models were used to preliminarily characterize the effects and the fate of the molecule as it contacts the cells. Mouse tumor models were used to investigate the distribution of the molecule in tumors after in vivo administration.

Methods Fluorescence microscopy, flow cytometry and immunofluorescence assays were used to define the features of the interaction of our molecule with cells expressing different αvβ3 levels.

Results The in vitro characterization of the molecule in contact with adherent cells reveals that it is internalized into the endosomal compartment and that it interferes with αvβ3-mediated cell adhesion. The affinity to both αvβ3 and HSA of our fluorescent RGD-based probe, demonstrated by flow cytometry and microscopy, is suitable for its intended use.

The ex vivo analyses of probe distribution in tumor tissues after in vivo administration highlight its ability to accumulate into the tumor mass and its specificity to delineate tumor margins.

Conclusions Our fluorescent RGD-based molecule is a promising optical imaging probe for fluorescence-guided surgical resection of tumors characterized by variable expression of αvβ3 integrin.


Chiara BRIOSCHI (colleretto giacosa (TO), Italy), Federica CHIANALE, Alessia CORDARO, Giovanni VALBUSA, Federico MAISANO, Fabio TEDOLDI

14:00-16:00
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SCUR - I
The Skin Imaging Society meeting
SLOT I

The Skin Imaging Society meeting
SLOT I

14:00 - 14:10 Opening of the SCUR meeting.
14:10 - 15:10 Session 1. Oral communications.
15:10 - 16:00 Invited lecture: Recent advances in cryomethods for skin studies. Roger A. WEPF (Invited speaker, Zürich, Switzerland)

16:30-19:00
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SCUR - II
The Skin Imaging Society meeting
SLOT II

The Skin Imaging Society meeting
SLOT II

16:30 - 17:30 Session 2. Oral communications.
17:30 - 19:00 Annual General Assembly of SCUR.

08:00-18:15
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PSB
Poster Session B
Display poster from Sunday 28, 2:00 pm to Tuesday 30, 6:00 pm

Poster Session B
Display poster from Sunday 28, 2:00 pm to Tuesday 30, 6:00 pm

Poster sessions:
Monday 16.30 > 18.15
Tuesday 16.45 > 18.15
08:00 - 18:15 #5012 - IM01-126 STEM electron tomography of titanium oxide nanotubes surface functionalized by Pt nanoparticles.
IM01-126 STEM electron tomography of titanium oxide nanotubes surface functionalized by Pt nanoparticles.

Abstracts

The titanium oxide nanotubes have become a very attractive material with potential applications in biomedicine, photocatalysis, energy etc. Their properties depend mostly on morphology which is relatively easy to change by controlling the conditions of the anodization like the type of electrolyte used, the voltage and the time of anodization. There is a direct linear relation between the anodization voltage and the average diameter of the formed nanotubes, as the voltage increases the diameter of nanotubes also increases [1,2].  The possibility of the preparation of nanotubes with different size, shape and wall thickness leads to the control of their geometric surface area and specific surface area, which is an important parameter in the development of new substrates for example in heterogeneous catalysis. Nanotube surface functionalization with platinum nanoparticles is a way to fabricate active material for catalysis of oxidation reaction of methanol. Such nanostructured complex materials demand advanced methods for characterization and visualization of real structure, therefore application of TEM and electron tomography techniques is desired.

In this work titania nanotubes were obtained by electrochemical oxidation of pure titanium at voltage of 10 and 25 V, that resulted in creation of TiO2 nanotubes with 40 and 110 nm in diameter, respectively. After anodization the heat treatment was performed at 450oC for 1h to change amorphous structure of TiO2 nanotubes into crystalline anatase structure. A suitable amount of Pt - 0.2 mg/cm2 on the surface of the nanotubes was deposited using magnetron sputtering.  FIB prepared specimens were analyzed in a Hitachi HD-2700 dedicated STEM (Scanning Transmission Electron Microscopy).

Fig. 1 shows cross-section SEM images of TiO2 nanotubes with 0.2 mg/cm2 Pt deposit on the top. The platinum nanoparticles tend to choose the edges of TiO2 nanotubes and side walls. A high amount of Pt fills the interior of the nanotubes as shown in Fig. 1b. As nanotubes diameter increases from 40 to 110 nm the depth of deposition into nanotubes also increases (Fig. 1c). Morphology of prepared structures was characterized by scanning transmission electron microscopy tomography. This method provides three-dimensional structural information at nanoscale based on two dimensional projections acquired at different tilt angles. High angle annular dark field (HAADF) imaging was used for 2D projections. The results have shown distribution of platinum nanoparticles inside the nanotubes. The variations in platinum content introduced into different diameter nanotubes were also examined. Segmented volume of nanotubes was analyzed in terms of specific surface area and volume fraction.

 

References

[1] Roguska, A., Pisarek, M., Andrzejczuk, M., Dolata, M., Lewandowska, M., & Janik-Czachor, M. (2011) Materials Science and Engineering C, 31(5), 906-914

[2] M. Pisarek, A. Roguska, A. Kudelski, M. Andrzejczuk, M. Janik-Czachor, K.J. Kurzydłowski, Materials Chemistry and Physics, 139 (1), (2013) 55-65.

 

Acknowledgments

This work was supported by The National Science Centre through the research grant UMO-2014/13/D/ST8/03224


Mariusz ANDRZEJCZUK (Warsaw, Poland), Agata ROGUSKA, Marcin PISAREK, Małgorzata LEWANDOWSKA
08:00 - 18:15 #5209 - IM01-128 Compressed sensing tomography of inorganic and biological samples in the scanning electron microscope operated in the transmission mode.
IM01-128 Compressed sensing tomography of inorganic and biological samples in the scanning electron microscope operated in the transmission mode.

This paper summarizes the achievements in the 3D reconstruction of microscopic specimens through the tomographic algorithm applied to a set of projection\images obtained in the SEM. This approach is complementary to the serial-sectioning and the slice-and-view methods presently implemented in the SEM platform, and benefits from a compressed sensing approach to refine the reconstruction from a limited number of projections.

A Si-based electron detector has been specifically developed for the purpose of operating the microscope in the scanning-transmission imaging mode for the tomographic application, and the detection strategy has been tailored in order to maintain the projection requirement over the large tilt range, a requirement needed for the reconstruction workflow [1]. Either inorganic or biological samples have been investigated to demonstrate the adaptability of the compressed sensing refinement to the specimen characteristics: the former system is formed by cobalt particles within a carbon tube and the latter features collagen fibrils in dermal tissue.

Figure 1 shows a STEM image from the tilt series of Co nanoparticles inside a carbon tube. The contrast in the STEM image is determined by local specimen thickness and composition, the Co particles being visible with the highest contrast. The reconstruction has been obtained starting from 53 projections at 2°steps, and refined through compressive sensing with regularization parameters emphasizing sparsity in the gradient domain.

Figure 2 highlights the complex structure of the dermal tissue as revealed by the STEM imaging mode in the SEM. Cellular membranes and circular structures are mixed with bundles of collagen fibrils. The bundles were truncated by the fine sectioning and their disposition is clearly visible. A small bundle of collagen was selected as the region of interest for the tomographic reconstruction. Starting from 91 projections at 40.000× magnification and ranging between -50° to +40°. Compressed sensing was adapted to deal with the inherent complexity of biological images, and the final tomogram turned out to preserve the finest details of the fibrils. The known periodical striation (about 60 nm periodicity) of collagen was indeed recovered with adequate spatial resolution.

The proposed system exploits the capability of the STEM imaging mode, which can be applied for both biological and physical science for the 3D analysis of volumes below 100 mm3. The limit in resolution is posed by the probe size of the microscope, specimen composition and thickness, and the number of projections that can be acquired without significant beam damage of the sample. Compressed sensing is effective in improving the quality of the reconstruction. Owing to the flexibility of the SEM platform, cryo-preservation of the specimen as well as site-selective sample preparation could be pursued within the proposed approach for tomography in the SEM.

[1]  M Ferroni et al., Journal of Physics: Conference Series 644 (2015): 012012


Ferroni MATTEO (FERRARA, Italy), Alberto SIGNORONI, Andrea SANZOGNI, Andrea MIGLIORI, Luca ORTOLANI, Vittorio MORANDI
08:00 - 18:15 #5295 - IM01-130 Extending the Limits of Fast Acquisition in TEM Tomography and 4D-STEM.
IM01-130 Extending the Limits of Fast Acquisition in TEM Tomography and 4D-STEM.

Both transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) experiments profit from recording two-dimensional camera images at very high readout speeds. This includes, but is not limited to, tomography in TEM and ptychography in STEM. The pnCCD (S)TEM camera uses a direct detecting, radiation hard pnCCD with a minimum readout speed of 1 000 full frames per second (fps) with a size of 264x264 pixels [1]. It features binning and windowing modes, which allow to further increase the frame rate substantially. For example, 4-fold binning in one direction, i.e. 66x264 pixels, yields a readout speed of 4 000 fps. Up to 20 000 fps are possible in windowing modes. Further applications that benefit from the high readout speed range from imaging on the micro- and millisecond timescale to strain analysis or electric and magnetic field mapping.

Typical tomographic reconstructions use tilt series of fewer than 100 images which are recorded in 15 to 60 minutes with conventional cameras running at speeds below 40 fps. The series are recorded by stepwise rotation of the goniometer and taking a camera image after each rotation step. These long acquisition times restrict the acquisition of tomographic series for beam sensitive samples. We have recorded a tilt series containing 3 487 images of an inorganic nanotube in only 3.5 s with the pnCCD camera [2]. Due to the high readout speed it was possible to rotate the goniometer continuously over a tilt range of -70 ° to +30 ° in an FEI Titan 60-300, operated at 60 keV beam energy. The short acquisition time and the high sensitivity of the camera allowed to reduce the cumulative electron dose to about 8 electrons per Å2, i.e. about an order of magnitude lower than conventionally used for low dose tomography. A 3D reconstruction of the nanowire is shown in Figure 1. The acquisition time was not limited by the readout of the camera, but rather by the rotation speed of the goniometer.

Combining the high readout speed with the scanning mode makes 4D-STEM imaging feasible, a powerful imaging technique where a two-dimensional image is recorded for each probe position of a two-dimensional STEM diffraction pattern. With the pnCCD (S)TEM camera, a 4D data cube consisting of 256x256 (i.e. 65 536) probe positions with a 132x264 pixel detector image (using 2-fold binning) for each probe position can be recorded in about 35 s. Several measurements have been performed to prove the capability of the camera for 4D-STEM imaging, including strain analysis, magnetic domain mapping and electron ptychography. The latter is a 4D-STEM technique that was described theoretically already in 1993 [3] but was so far limited experimentally by the low readout speed of existing cameras. In electron ptychography, the intensity distribution in the bright field disk is recorded in 2D for each STEM probe position. In an electron wave-optical approach the phase and amplitude information is extracted from the recorded intensity images. The reconstructed phase image (Figure 2a) shows enhanced image contrast compared to the simultaneously acquired conventional annular dark field image (Figure 2b). Measurements with the pnCCD (S)TEM camera were carried out using a JEOL ARM200-CF to investigate different samples with the ptychographic phase reconstruction technique.

In conclusion, the pnCCD camera enables new techniques in TEM and STEM. Various fields of application benefit from recording two-dimensional detector images at high speeds. With its direct detection, high readout speed and radiation hardness the pnCCD (S)TEM camera permits the recording of  tomographic tilt series and large 4D-STEM data cubes in short times and thus paves the way for new science.

 

[1] H Ryll et al, Journal of Instrumentation (in press).

[2] V Migunov et al, Scientific Reports 5 (2015), 14516.

[3] JM Rodenburg, BC McCallum and PD Nellist, Ultramicroscopy 48 (1993), p.304–314.

[4] KJ Batenburg and J Sijbers, IEEE Trans. Image Process. 20 (2011), p.2542–2553.
[5] The authors acknowledge Xiaodong Zhuge, K. Joost Batenburg and Lothar Houben for their contributions to the tomography measurement.


Martin SIMSON (Munich, Germany), Rafal E. DUNIN-BORKOWSKI, Robert HARTMANN, Martin HUTH, Sebastian IHLE, Lewys JONES, Yukihito KONDO, Vadim MIGUNOV, Peter D. NELLIST, Robert RITZ, Henning RYLL, Ryusuke SAGAWA, Julia SCHMIDT, Heike SOLTAU, Lothar STRÜDER, Hao YANG
08:00 - 18:15 #5725 - IM01-132 Quantitative 3D analysis of huge nanoparticles assemblies.
IM01-132 Quantitative 3D analysis of huge nanoparticles assemblies.

Nanoparticle assemblies attract increasing interest because of the possibility of tuning their properties by adjusting the overall size and shape, the stacking of the individual nanoparticles, and the distances between them.[1]

Transmission electron microscopy is an important technique to characterize materials at the nanometer scale and below. However, it conventionally only allows for the acquisition of two-dimensional (2D) projections of three-dimensional (3D) objects, which is not sufficient for a quantitative characterization of complex 3D nanostructures. Electron tomography has therefore been developed to overcome this strict limitation, becoming a versatile and powerful tool, increasingly used in the field of materials science.[2]

For the characterization of nano-assemblies, electron tomography is nowadays a standard technique, yielding a 3D description of the morphology and inner structure.[3] Despite the valuable information that can be obtained, as the synthetized systems become more complex, an accurate characterization of the structure becomes more demanding. For example, 3D reconstructions based on classical algorithms, suffer from a number of restrictions that hamper an accurate characterization of closed-packed nanoparticles assemblies.

Here, we present a novel approach that enables us to determine the coordinates of each nanoparticle in an assembly, even when the assembly consists of up to 10,000 (spherical) particles.[4, 5] This technique will have a major impact as it enables a straightforward quantification of inter-particle distances and 3D symmetry of the stacking. Furthermore, the outcome of these measurements can be used as an input for modelling studies that predict the final 3D structure as a function of the parameters used during the synthesis.

 

[1]          N. A. Kotov, P. S. Weiss, ACS Nano 2014, 8, 3101.

[2]          P. Midgley, M. Weyland, Ultramicroscopy 2003, 96, 413.

[3]          A. Sánchez-Iglesias, M. Grzelczak, T. Altantzis, B. Goris, J. Perez-Juste, S. Bals, G. Van Tendeloo, S. H. Donaldson Jr, B. F. Chmelka, J. N. Israelachvili, ACS Nano 2012, 6, 11059.

[4]          B. de Nijs, S. Dussi, F. Smallenburg, J. D. Meeldijk, D. J. Groenendijk, L. Filion, A. Imhof, A. van Blaaderen, M. Dijkstra, Nature materials 2015, 14, 56.

[5]          D. Zanaga, F. Bleichrodt, T. Altantzis, N. N. Winckelmans, W. J. Palenstijn, J. Sijbers, B. van Nijs, M. van Huis, A. van Blaaderen, K. Joost Batenburg, Sara Bals, Gustaaf Van Tendeloo, Nanoscale 2015.

 

 

Acknowledgements

The authors acknowledge financial support from European Research Council (ERC Starting Grant # 335078-COLOURATOMS, ERC Advanced Grant # 291667 HierarSACol and ERC Advanced Grant 267867 – PLASMAQUO), the European Union under the FP7 (Integrated Infrastructure Initiative N. 262348 European Soft Matter Infrastructure, ESMI and N. 312483 ESTEEM2), and from the Netherlands Organisation for Scientific Research (NWO), project number 639.072.005 and NOW CW 700.57.026. Networking support was provided by COST Action MP1207.


Daniele ZANAGA (Antwerpen, Belgium), Folkert BLEICHRODT, Thomas ALTANTZIS, Naomi WINCKELMANSA, Willem Jan PALENSTIJN, Jan SIJBERS, Bart DE NIJS, Marijn A. VAN HUIS, Luis M. LIZ-MARZÁN, Alfons VAN BLAADEREN, K. Joost BATENBURG, Gustaaf VAN TENDELOO, Sara BALS
08:00 - 18:15 #5771 - IM01-134 Multi ADF detector tomography for 3D characterization of heterostructures.
IM01-134 Multi ADF detector tomography for 3D characterization of heterostructures.

Characterization of core-shell type nanoparticles in 3 dimensions (3D) by transmission electron microscopy (TEM) can be very challenging. Especially when both heavy and light elements co-exist within the same nanostructure, artefacts in the 3D reconstruction are often present. A representative example would be a particle comprising an anisotropic metallic (Au) nanoparticle coated with a (mesoporous) silica shell. To obtain a reliable 3D characterization of such an object, we collected high angle annular dark field scanning TEM (HAADF-STEM) and annular dark field tilt series (ADF-STEM) for tomography (Figure 1A and 1B respectively). Although the series acquired by ADF-STEM shows both the Au and the SiO2, artefacts are clearly present in the 3D reconstruction (Figure 2). Since the observed artefacts may cause loss of information or may lead to misinterpretation, it is extremely challenging to obtain reliable 3D results for core-shell hybrid materials using conventional electron tomography. When selecting an optimal value for the collection angle, a compromise is needed between optimal contrast, produced by the atomic number of coexisting elements, and the minimization of diffraction contrast.  

We here overcome this limitation by exploiting the flexibility of modern TEM instruments that enable one to collect multiple (HA)ADF-STEM series simultaneously, by using different (HA)ADF detectors at the same time. This multi-mode approach is very dose-efficient, as one is able to collect 2 images while keeping the necessary electron dose the same. Tilt series were simultaneously acquired using an ADF detector with inner and outer collection angles of 35 and 125 mrad and a HAADF-STEM detector using inner and outer collection angles of 150 and 220 mrad, respectively. To remove the artefacts that appear in the ADF-STEM tilt series, we removed the complete Au nanoparticle from the ADF-STEM projection images. Next, a technique known as inpainting was applied[1]. This approach replaces the absent information by a continuation of the texture of the surrounding area. This procedure was performed for each projection image of the tilt series separately (Figure 3A).  The processed tilt series was then used as an input for 3D reconstruction using the SIRT algorithm implemented in the ASTRA toolbox[2]. Finally, the 3D HAADF-STEM and ADF-STEM reconstructions are combined into one single visualization using the AMIRA software as illustrated in Figure 3B [3]. In this manner, we were able to reliably characterize the structure of mesoporous SiO2 Au nanoparticles. It must be noted that the methodology we propose here is generally applicable to a broad range of core shell hybrid nanostructures.

References

[1] G. Wang, D. Garcia, Y. Liu, R. de Jeu, A. J. Dolman, Environ. Modell. Softw. 2012 , 30 , 139.

[2] W. Van Aarle, W. J. Palenstijn, J. De Beenhouwer, T. Altantzis, S. Bals, K. J. Batenburg, J. Sijbers, Ultramicroscopy 2015 , 157 , 35

[3] D. Stalling, M. Westerhoff, H.-C. Hege, in The Visualization Handbook, (Eds: C. D. Hansen and C. R. Johnson), Academic Press, Elsevier, 2004 , pp. 749–767.


Kadir SENTOSUN (Antwerp, Belgium), Marta N. Sanz ORTIZ, K. Joost BATENBURG, Luis M. LIZ-MARZÁN, Sara BALS
08:00 - 18:15 #5868 - IM01-136 Effect on the SEM topography of different sample preparation methods for thin-film-composite membrane.
IM01-136 Effect on the SEM topography of different sample preparation methods for thin-film-composite membrane.

Thin-film-composite (TFC) membrane was prepared through interfacically polymerizing m-phenylene-diamine (MPD) and trimesoyl chloride (TMC) on top of a poly-sulfone substrate to form an ultrathin active polyamide layer, which is excellent in hydrophilicity, mechanical stability, thermal stability and hydrolytic stability. Scanning electron microscopy (SEM) is suitable for direct observation of TFC membrane structure, especially for superfine structure of ultrathin functional layer. The skin layer of TFC membrane is much denser than ultrafiltration membrane and its microstructure is more difficult to observe by SEM with regular way. At present, there are two common methods for fracturing membrane: brittle fracture of liquid nitrogen, embedded section. In addition, there are two less common methods with the help of precision instruments, like Focused Ion Beam cutting and Ion Milling technologies. We compare the advantages and disadvantages of four methods, and put forward a more convenient, practicable and effective method for SEM cross-sectional analysis.

(1)Embedded Section: This method will be finished after embedding in epoxy resin, solidifying in thermostat and slicing by microtome. It has strict technical requirements for the operator. The cross section is damaged badly in the Fig.1. 

(2)FIB cutting: We obtain the cross-sectional sample using Carl Zeiss AURIGA Cross Beam FIB/SEM. FIB probe is Ga ion beam. The operating voltage and current is 30kV and 50pA respectively. The ion beam intensity is too high to keep the integrity of film structure. The cortex is damaged in the Fig.2. So this method is not suitable for soft materials.

(3)Ion milling: The process is completed by IM4000 of HITACHI. We select the section grinding mode to reduce the damage of structure, with the help of liquid nitrogen cooling mode. Although this method is somewhat better than FIB cutting, we still can’t see the cortex distinctly.

(4)Brittle Fracture of Liquid Nitrogen: The cost of this method is very low. Although it’s easy to use without the aid of other equipment, the SEM microstructure is difficult to observe directly and clearly. Fig.4a shows the low and high magnification images. We can see the film cross section is distorted and the cortex sagged.

The above methods have many limitations, because the toughness of membrane is too big to fracture easily. We have developed an improved sample frozen and fractured technology: the TFC membrane has swollen in ethanol at room temperature for several minutes, and then we put the sample in liquid nitrogen and fracture it with a certain slope. The functional cortex can be observed directly and effectively in the Fig.4b below. 


Wenqing HUANG (Beijing, China), Xiaopei MIAO
08:00 - 18:15 #5940 - IM01-138 3D Elemental and interdependent reconstructions based on a novel compressed sensing algorithm in electron tomography.
IM01-138 3D Elemental and interdependent reconstructions based on a novel compressed sensing algorithm in electron tomography.

        Electron Tomography (ET) is a key technique to perform 3D characterization at the nanometer scale [1]. 2D projections at different tilt angles are first acquired in an electron microscope, then an inversion algorithm is used to reconstruct the 3D volume of the sample from the dataset. Classically, ET is performed in a HAADF STEM mode in materials science leading to 3D Z-contrast reconstructions. 3D elemental mapping based on EELS or EDS acquisitions is also possible in reconstruction theory [2]. Yet, reconstruction theory needs several hundreds of projections and 2D chemical mapping needs an important acquisition time and electron dose, therefore 3D elemental mapping is challenging. Nowadays, microscopes improvements limit the acquisition time of 2D chemical mapping. Moreover, powerful state-of-the-art reconstruction algorithms make possible the reconstruction from a limited dataset of a few dozens of projections only. As a consequence 3D elemental mapping is now possible with a reasonable acquisition time of a day or less.  

        New reconstruction algorithms add prior knowledge on the object to compensate for the lack of information due to limited number of available projections. The prior knowledge can be a limited number of possible grey levels in the reconstruction to perform discrete tomography [3]. This correspond to a limited number of known materials in the sample. In the case of Compressed Sensing (CS) algorithms [4], the prior knowledge is a sparsity of the object expressed in a particular basis. A special case of CS reconstruction is the use of the gradient sparsity of the object to perform Total Variation Minimization (TVM) algorithms [5]. In that case, objects constant by parts are preferably reconstructed.

        We propose a mixed approach suited for EDS acquisition. That mixed approach combines both projection denoising [6] and a TVM based algorithm that uses the reconstructions of each element all together. This new approach leads to higher reconstruction accuracy since a new kind of prior knowledge is used. Indeed, reconstructions of different elements should not be independent since a variation of an element is most of the time correlated to a variation of at least another element. The algorithm will be introduced. Simulations using projections with high Poisson noise and strong misalignment will be used to show the accuracy of our approach. Experimental results for a GaN - TiAl intermetallic sample in EDS tomography will also be presented.

        This work was supported by the French “Recherche Technologie de Base” (RTB) program. The authors acknowledge access to the nanocharacterization platform (PFNC) at the Minatec Campus in Grenoble. The authors thank Alphonse Torres from CEA Leti for providing the intermetallic specimen.

 

[1] P. A. Midgley and M. Weyland, “3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography.,” Ultramicroscopy, vol. 96, no. 3–4, pp. 413–31, Sep. 2003.

[2] G. Haberfehlner, A. Orthacker, M. Albu, J. Li, and G. Kothleitner, “Nanoscale voxel spectroscopy by simultaneous EELS and EDS tomography,” Nanoscale, vol. 6, no. 23, pp. 14563–14569, Oct. 2014.

[3] K. J. Batenburg and J. Sijbers, “DART: a practical reconstruction algorithm for discrete tomography.,” IEEE Trans. Image Process., vol. 20, no. 9, pp. 2542–53, 2011.

[4] R. Leary, Z. Saghi, P. A. Midgley, and D. Holland, “Compressed sensing electron tomography,” Ultramicroscopy, vol. 131, pp. 70–91, Aug. 2013.

[5] B. Goris, W. Van den Broek, K. J. Batenburg, H. Heidari Mezerji, and S. Bals, “Electron tomography based on a total variation minimization reconstruction technique,” Ultramicroscopy, vol. 113, pp. 120–130, Feb. 2012.

[6] T. Printemps, G. Mula, D. Sette, P. Bleuet, V. Delaye, N. Bernier, A. Grenier, G. Audoit, N. Gambacorti, and L. Hervé, “Self-adapting denoising, alignment and reconstruction in electron tomography in materials science,” Ultramicroscopy, vol. 160, pp. 23–34, 2016. 


Tony PRINTEMPS (GRENOBLE), Nicolas BERNIER, Eric ROBIN, Zineb SAGHI, Lionel HERVÉ
08:00 - 18:15 #5999 - IM01-140 Electron tomography: influence of defocus on the determination of reconstructed soot aggregates’ surface and volume.
IM01-140 Electron tomography: influence of defocus on the determination of reconstructed soot aggregates’ surface and volume.

Aerosols affect the climate system through various physical processes as they can scatter and absorb solar radiation, emit thermal radiation, or act as cloud condensation nuclei that modify the cloudiness coverage, changing then its albedo. Carbonaceous solid aerosols resulting either from anthropogenic processes or biomass burning are one of the most significant contributors to global climate change [1] with respect to their impact on radiative forcing. They are constituted by tiny primary spherules having a diameter typically ranging from a few nanometers to dozens nanometers. These spherules are aggregated to form particles of larger sizes (0.1 to 1 micrometre) showing a complex morphology (Figure 1) that plays an important role on their radiative and transport properties. These soot aggregates are commonly characterized from 2D transmission electron microscopy (TEM) images and their 3D shape is then deduced from TEM projections assuming geometrical models of spheres, cylinders or spheroids. In order to check the validity of the 2D to 3D transition of these geometrical models we investigated bright field (BF) TEM tomography and directly determine 3D soot morphological characteristics.

            To be suitable for tomographic reconstruction, images of the tilt series must fulfill the projection requirement and BF TEM images of amorphous specimen, which are dominated by mass-thickness contrast, can satisfy this assumption. We show in this work that attention must be paid on the influence of defocus, responsible of the appearance of Fresnel fringes, which do not answer the projection requirement and can lead to artefacts that do not ensure a realistic reconstruction of soot aggregates.

            To do so, we analyse the effect of defocus both on real and numerical soot particles. Thus the same real soot aggregate is reconstructed (with the SIRT algorithm) from different tilt series obtained at different defocus values. Its surface area and volume are determined by using the Amira software suite after a segmentation step based on the method of Adachi et al [2]. The same procedure (reconstruction, segmentation, surface area measurement) is then applied on an amorphous numerical particle, which is generated using a tight binding model [3] processed with the Nanofabric software developed by Y.Lebouar. Projection series are simulated with the EMS software based on the multislice method [4]. Both experimental and numerical approaches show that defocus drastically modifies the intensity profile of primary particles along their diameter and subsequently affects the shape of the reconstructed aggregate (Figure 2) and leads to overestimated values of their surface area and volume.

 

 

Acknowledgments:

 Y. LeBouar is greatly acknowledged for the generation of the amorphous numerical particle with the Nanofabric software he developed.

 

[1] U. Lohmann and J. Feichter, Atmos. Chem. Phys. 5, 715-737 (2005)

[2] K. Adachi, SH. Chung, H. Friedrich, and PR. Buseck, J. Geophys. Res. 112, D14202 (2007)

[3]  C. Ricolleau, Y. Le Bouar, H. Amara, O. Landon-Cardinal, and D. Alloyeau, J. Appl. Phys.114, 213504 (2013)

[4]  P.A. Stadelmann, Ultramicroscopy21, 131 (1987).


Martiane CABIÉ (Marseille), Marc GAILHANOU, Daniel FERRY
08:00 - 18:15 #6005 - IM01-142 How precise can atoms of a nanocluster be positioned in 3D from a tilt series of scanning transmission electron microscopy images?
IM01-142 How precise can atoms of a nanocluster be positioned in 3D from a tilt series of scanning transmission electron microscopy images?

Nanoclusters play key roles in a wide range of materials and devices because of their unique physical and chemical properties. These properties are determined by the specific three-dimensional (3D) morphology, structure and composition. It is well known that extremely small changes in their local structure may result in significant changes of their properties. Therefore, development of techniques to measure the atomic arrangement of individual atoms down to (sub)-picometer precision is important. This allows one to fully understand and greatly enhance the properties of the resulting materials, increasing the number of applications.

Electron tomography using aberration-corrected scanning transmission electron microscopy (STEM) is considered as one of the most promising techniques to achieve atomic resolution in 3D. Although this is not yet a standard possibility for all structures, significant progress has recently been achieved using different approaches [1,2]. Once the atoms can be resolved in 3D, the next challenge is to refine the atom positions in order to locate them as precisely as possible. However, the answer to the question how precise these measurements are, is still open. Here, we investigate the theoretical limits with which atoms of a nanocluster can be located in 3D based on the acquisition of a tilt series of annular dark field (ADF) STEM images.

A parametric model, describing the expectations of the intensities observed when recording a tilt series of ADF STEM images, is needed in order to derive an expression for the highest attainable precision [3,4]. Although the multislice method is more accurate to describe the electron-object interaction, it is very time-consuming, especially when simulating a tilt series of images. Therefore, a Gaussian approximation model has been used as well in order to perform fast, albeit approximate simulations that allow us to get insight into the precision that can be attained to locate atoms in 3D. The precision has been computed for locating the central atom of four gold nanoclusters of different sizes with a Mackay icosahedral morphology. A cross-section of such a nanoparticle is shown in Fig. 1(a) indicating the x-, y-, and z-axis.

In Fig. 1(b), the attainable precision is shown for the x-, y- and z-coordinate of the central atom computed taking  all the atoms into account, the atoms of the central plane (orange atoms and red atom in Fig. 1), or the central atom only (red atom in Fig. 1(a)) based on simulations using the Gaussian approximation model. From this figure, it can be seen that the precision is not significantly affected by neighbouring atoms, and therefore, it is allowed to use only the central atom to evaluate the attainable precision. In figure 2(a), 2(b) and 2(c) the attainable precision is illustrated as a function of the number of projections, the tilt range, and the incident electron dose. The precision increases with increasing number of projections, tilt range, and incident electron dose. Using optimal parameters for the number of projections, the tilt range and electron dose determined based on the calculation of the precision using the Gaussian approximation model, realistic STEM simulations have been performed using the multislice method. The precision has been evaluated for a dose of 8680 e-2 as a function of the inner detector radius of the annular STEM detector (Fig. 3(a)). The optimal inner angle equals the semi-convergence angle. Next, the precision to locate the central atom is determined for the different cluster sizes using all optimised settings (Fig. 3(b)). Here, it is shown that theoretically, a precision of a few picometers can be attained for locating atoms in 3D using a tilt series of ADF STEM images.

In conclusion, it is shown that the attainable precision for locating atoms in 3D can be optimized as a function of the number of projections, tilt range, electron dose, and inner radius of the STEM detector. It is demonstrated that a precision in the picometer range for positioning atoms in 3D is feasible.

 

References 

[1] S. Van Aert, et al., Nature 470, 374–377 (2011)

[2] B. Goris, et al., Nano Letters 15, 6996-7001 (2015)

[3] A. van den Bos, Parameter estimation for scientists and engineers, John Wiley & Sons, 2007.

[4] Van Aert, et al., Journal of Structural Biology 138, 21-33 (2002)

[5] The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N) and a post-doctoral grant to A. De Backer.


Marcos ALANIA (Antwerp, Belgium), Annick DE BACKER, Ivan LOBATO, Florian F. KRAUSE, Dirk VAN DYCK, Andreas ROSENAUER, Sandra VAN AERT
08:00 - 18:15 #6061 - IM01-144 New Approaches to Multi-Dimensional Experiments in S/TEM: Application of High Speed Cameras.
IM01-144 New Approaches to Multi-Dimensional Experiments in S/TEM: Application of High Speed Cameras.

Since the first experimental charge coupled device was reported in 1982 [1], there have been a series of major developments in digital imaging techniques for transmission electron microscopy (TEM). These include use of complementary metal-oxide semiconductor (CMOS) devices, which resulted in improvements in camera sensitivity, detective quantum efficiency (DQE) and speed. Here we will present how such developments can benefit some common TEM based experiments, such as electron tomography (ET) and four dimensional scanning TEM (4D-STEM) diffraction.

ET consists of acquisition of a series of images of the specimen in different viewing directions and is used for three dimensional (3D) studies of nanoscale materials in a TEM. The tilt range and tilt increment in an ET experiment directly affects the resolution of the 3D reconstruction.  In cases where the specimen is electron sensitive, the number of projections that can be recorded is typically limited as the sample is repeatedly exposed to the beam. Leveraging the advantages of a high speed camera can also benefit low dose ET and 3D time-resolved studies of dynamic processes in a TEM. Here high speed ET datasets will be presented that were collected using a high speed CMOS camera while the TEM goniometer was continuously tilting. Such an approach improves the resolution of 3D reconstruction for thicker specimens by reducing the tilt increment from several degrees to a small fraction of a degree, and reduces the data acquisition time from several tens of minutes to a few minutes, simultaneously improving angular resolution and potentially reducing beam damage to the specimen.

STEM diffraction imaging is a common analytical method to collect specimen structure, strain and texture. Here either a convergent or parallel electron beam is used to produce diffraction patterns, which can be used to characterize defects, interfaces and small nanostructures and allow accurate measurements of strain and crystal orientation. 4D-STEM diffraction is done by collecting a diffraction pattern pixel by pixel, as the electron beam is scanned on the specimen. Limited data collection speed (i.e., frame rate of the sensor) has been one of the main challenges of this technique. Conventional CCD cameras were limited to up to 30 frames per second (fps), which restricted the number of diffraction patterns collected in a given amount of time. This can be even more challenging in the cases of beam sensitive specimens, or when drift exists. We will present 4D-STEM datasets collected with high speed CMOS cameras and will show how these new systems with superior DQE and speed can benefit STEM diffraction imaging experiments.

Figure 1a below shows 2 images from a high speed tomography experiment on an array of Au nanoparticles. These two images are approximately 60 degrees apart and it was collected in 120 degree tilt range in 110 seconds. The reduction of such a data stream as a tomogram will be presented. And, Figure 1b shows CBED patterns from inside and outside of a vacancy dislocation loop in a Cu specimen.

Reference:

[1] PTE Roberts, JN Chapman and AM MacLeod, Ultramicroscopy 8 (1982), p. 385.


Anahita PAKZAD (Pleasanton, USA), Cory CZARNIK, Roy GEISS, David MASTRONARDE
08:00 - 18:15 #6137 - IM01-146 Atomic resolution HAADF STEM tomography using prior physical knowledge and simulated annealing.
IM01-146 Atomic resolution HAADF STEM tomography using prior physical knowledge and simulated annealing.

Atomic resolution electron tomography using HAADF STEM has become a key tool to get 3D atomic-scale structural information about the sample under study [1-3]. Different reconstruction algorithms exist including filtered back projection, simultaneous iterative reconstruction (SIRT), discrete tomography [4, 5] and total variation minimization [2]. However, most of these reconstruction techniques do not include prior knowledge concerning the atomistic building blocks of the specimen and the electron specimen interaction. A successful attempt to use atomistic prior knowledge of the specimen in the reconstruction was realized by Goris et al. [3] in which each atom is modeled by a 3D Gaussian function. However, the physical knowledge about the electron specimen interaction was not included. In order to partially overcome these problems, we modelled the specimen as a linear combination of spherical symmetric real functions, which are obtained from HAADF STEM simulations of a single atom. Furthermore, a distance constraint is included which guarantees that the distance between atoms is kept above a physical lower bound. The minimization of the cost function is performed using the simulated annealing technique [6]. The cost function is defined as the sum of the squared differences between the forward model and the projection images plus a Tikhonov regularization term. The advantage of using simulated annealing over other methods is that it statistically guarantees to find (an approximation of) the global optimum and that it allows one to process cost functions with a high degree of nonlinearity, arbitrary boundary conditions, and constraints imposed on the solution [7].

The proposed simulated annealing algorithm was demonstrated on a simulated tomography tilt series consisting of 9 projection images with a limited angular tilt range of 120 degrees of a Au nanoparticle consisting of 6525 atoms. Images were generated using the frozen lattice approach with the MULTEM software [8] with a numerical real space grid of 2048x2048 pixels and the following microscope settings: acceleration voltage (300keV), spherical aberration (0.001mm), defocus (14.03Å) and aperture objective radius (21mrad). The frozen atom simulation is performed by using the Einstein model with 20 configurations, slice thickness of 1Å and the three-dimensional rms displacements of all the atoms are set to 0.085Å. An ideal detector sensitivity is used with 40mrad and 95mrad for the inner and outer circular detector angles, respectively. An area which covers the whole nanoparticle was scanned with a pixel size of 15pm. This image was later convoluted with a Gaussian low pass filter (source broadening) with full width at half maximum of 0.8Å. Poisson noise was generated such that the signal-to-noise ratio (SNR) in the resulting images equals 7.  The SNR is defined as the ratio of the standard deviation of the image to the standard deviation of the noise. An example of such a simulated image is shown in Figure 1a.

The result of the simulated annealing based reconstruction method is shown in Figure 1b. The evolution of the cost function during the minimization process is shown in Fig. 2. When comparing position coordinates of all atoms in the reconstructed particle with the input parameters, it has been found that the average distance is less than 8 pm, demonstrating subpixel accuracy.

References

1. S. Van Aert et al., Nature 470 (2011) 374.

2. B. Goris et al., Nature Materials 11 (2012) 930.       

3. B. Goris et al., Nano Letters 15 (2015) 6996.

4. K. J. Batenburg et al. Ultramicroscopy 109 (2009) 730.

5. T. Roelandts et al. Ultramicroscopy 114 (2012) 96.

6. S. Kirkpatrick, C.D.Jr. Gelatt and M.P. Vecchi. Science 220 (1983) 671.

7. L. Ingber. Mathematical and Computer Modelling 18 (1993) 29.

8. I. Lobato and D. Van Dyck.  Ultramicroscopy 156 (2015) 9.

 Acknowledgement

The authors acknowledge financial support from the Research Foundation Flanders (FWO, Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N).The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).


Ivan LOBATO (Antwerpen, Belgium), Jan DE BEENHOUWER, Dirk VAN DYCK, Sandra VAN AERT, Jan SIJBERS
08:00 - 18:15 #6144 - IM01-148 FeCrMg composite and porous FeCr obtained by dealloying in metallic melt bath by Xray tomography and SEM.
IM01-148 FeCrMg composite and porous FeCr obtained by dealloying in metallic melt bath by Xray tomography and SEM.

Nanoporous metals have attracted considerable attention for their excellent functional properties [Snyder, 2010]. The most promising technique used to prepare such nanoporous metals is dealloying in aqueous solution. Nanoporous noble metals including Au have been prepared from binary alloy precursors [Forty, 1979]. The less noble metals, unstable in aqueous solution, are oxidized immediately when they contact water at a given potential so this process is only possible for noble metals. Porous structures with less noble metals such as Ti or Fe are highly desired for various applications including energy-harvesting devices [Sivula, 2010]. To overcome this limitation, a new dealloying method using a metallic melt instead of aqueous solution was developed [Wada, 2011]. Dealloying in the metallic melt is a selective dissolution phenomenon of a mono-phase alloy solid precursor: one component (referred as soluble component) being soluble in the metallic melt while the other (referred as targeted component) is not. When the solid precursor contacts the metallic melt, only atoms of the soluble component dissolve into the melt inducing a spontaneously organized bi-continuous structure (targeted+sacrificial phases), at a microstructure level. This sacrificial phase can finally be removed by chemical etching to obtain the final nanoporous materials. Because this is a water-free process, it has enabled the preparation of nanoporous structures in less noble metals such as Ti, Si, Fe, Nb, Co and Cr.

In this study, nanoporous FeCr samples were prepared using Ni as the soluble component, in a metallic melt bath of Mg. To introduce structural and mechanical anisotropy, some samples were cold-rolled before etching. The influence on the microstructure of the precursor composition, the dealloying conditions and the rolling process were investigated along the different steps by SEM-EBSD and Xray tomography to correlate the process with the microsctructure. Xray tomography (cf. Fig. 2 and 4)enables us to characterize qualitatively and quantitatively the volume while SEM (cf. Fig. 1 and 3) enables us to analyze larger areas with higher resolution 2D images. To confirm the validity of Xray tomography results, SEM-FIB analysis were also performed.

References :

[Snyder, 2010] J. Snyder, T. Fujita, M. Chen, J. Erlebacher. Nat. Mater., 9 (2010), p. 904

[Forty, 1979] A.J. Forty. Nature, 282 (1979), p. 597

[Sivula, 2010] K. Sivula, R. Zboril, F.L. Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Grätzel. J. Am. Chem. Soc., 137 (2010), p. 132

[Wada, 2011] T. Wada, K. Yubuta, A. Inoue, H. Kato. Mater. Lett., 65(2011), p. 1076


Morgane MOKHTARI (Villeurbanne), Eric MAIRE, Christophe LE BOURLOT, Takeshi WADA, Hidemi KATO, Anne BONNIN, Jannick DUCHET-RUMEAU
08:00 - 18:15 #6166 - IM01-150 Large volume 3D SEM for reconstruction of inner structure of soft materials.
IM01-150 Large volume 3D SEM for reconstruction of inner structure of soft materials.

Serial Block-Face Scanning Electron Microscopy (SBFSEM) is one the technique which provides true insight into the composition of the volume of different materials. The volume imaging method helps to understand not only the structure but the function as well. SBFSEM is based on the combination of an in situ ultramicrotomy and electron microscopy which can be turned into the powerful tool for high resolution imaging of large volumes. Tissue and cell biology represents the traditional domain of the method but its application within materials sciences is becoming more apparent. It has been successfully employed in the observation of polymers, composite materials, membranes, metals etc. [1].

Integration and automation of the complete process of the data acquisition; and subsequent data processing represent the challenging task. The method is destructive in its very nature and there are potentially many factors which can influence the cutting and imaging properties. Teneo VSTM is based on refined SBFSEM and designed for fully automatic data acquisition on stained resin embedded biological samples [2]. It combines hardware and software components into one integrated system. The in situ ultramicrotome, placed on the SEM stage, cuts the specimens into thin slices. The exposed block-face is scanned with the electron beam and the backscatter signal collected. Alternate slicing and imaging end up with a series of two dimensional images which come from different depth of the sample. Generally the depth resolution is limited by the thinnest slice thickness which can be cut by a diamond knife. To overcome this limitation virtual slicing was introduced [3]. A series of images at different accelerating voltages is acquired and processed. By their proper selection different depth emission profiles are created. Combination of both approaches allows shifting of the in-depth resolution towards nanometer level and to achieve isotropic voxel size. By using these methods in combination with thicker physical slices means more reliable section thickness and artefact control. 

Imaging of the stained, resin embedded samples is challenging. Both cutting properties and electrical conductivity have to be considered. To neutralize the charge built up on the sample surface low vacuum option is available including the dedicated backscatter detector. Nevertheless clever management of the signal can extend the applicability of the high vacuum mode to a broader range of samples. For Teneo VS such a unique dedicated extension is available to suppress the noise of a charging sample in image formation.

Fig. 1 shows the application of Teneo VS system to volume reconstruction of a polymer blend (isotactic polypropylene/ethylene propylene rubber particles) after a tensile test. Part of the fracture zone was visualized to trace the propagation of cracks through the bulk. Three dimensional field of micro-cracks and a distribution, structure of the filler particles can be studied at high resolution. The crack has been enhanced by RuO4 vapor staining.

As it can be seen here the advancements despite having been engineered for life sciences can be applied directly to other materials. It is hoped that the advances and enhancements which are enjoyed by life sciences with this technique can be fully realized by other sectors interested in volume microscopy.

 

Acknowledgement

We would like to thank to Dr. Armin Zankel (FELMI-ZFE Graz, Austria) who kindly provided us with samples; Technology Agency of the Czech Republic, project TE01020118 for funding.

 

References

[1] Zankel A. et al., Journal of Microscopy, vol. 233(1), pp. 140-148 (2009).

[2] Hovorka M. et al., MC 2015 – Microscopy Conference 2015, Göttingen, Germany.

[3] F. Boughorbel et al., SEM Imaging Method, Patent US 8,232,523 B2, 31st July 2012.


Milos HOVORKA (Brno, Czech Republic), Tomas JANOCKO, John MITCHELS, Libor STRAKOS, Tomas VYSTAVEL
08:00 - 18:15 #6233 - IM01-152 SAMFire – a smart adaptive fitting algorithm for multi-dimensional microscopy.
IM01-152 SAMFire – a smart adaptive fitting algorithm for multi-dimensional microscopy.

   The large amounts of high-quality “multi-dimensional” data generated by modern microscopes open new avenues for quantitative nano-characterization. Quantitative analysis of spectra and images often involve fitting a model to experimental data and, indeed, the literature is rich in applications; examples include atom counting [1], time resolved microscopy [2], electron energy loss [3] and cathodoluminescence [4] spectroscopy. However, using conventional methods to fit large datasets is challenging and when applied to multi-dimensional models, they may become ill-suited. The dominant and most common problem is that conventional methods struggle with any non-linearity in the model and often require an estimate of the starting parameters that are close to the true values. Here we present a Smart Adaptive Multi-dimensional Fitting algorithm (SAMFire) designed to ease the task of fitting such data by automatically generating best estimates for the parameters as the fitting progresses. SAMFire can fit multi-dimensional spectra, images and data of higher dimensionality and will be available in open-source software package HyperSpy v1.0.0 [5].

   SAMFire enables quantitative analysis of large multi-dimensional datasets that would be very challenging—if not impossible—to analyse by other means. It provides multiple fitting strategies that consist of pixel selection and parameter estimation, each tailored to different data structures. Example pixel selection orders are shown in Figure 1(a) for conventional fitting algorithms and in Figure 1(b) for one of the SAMFire strategies. The “raster” order is only viable for unusually stable and constrained models. In contrast, SAMFire follows the “path of least resistance”, learned from already fitted parts of the data and hence is applicable to a much broader range of problems.

   As an example of a complex electron microscopy data analysis problem that can be easily addressed with SAMFire, Figure 2 shows a single spectrum and the result of EELS elemental and bonding quantification by curve fitting from a tilt-series of spectrum-images of a mixed phase nanoparticle. The model consists of eleven components to accurately describe the five elements and a background. Due to the complexity of the model, the geometry of the particle and the low signal-to-noise-ratio, the outcome of fitting individual pixels was highly dependent on the starting parameters, making the analysis very challenging using conventional fitting routines. In contrast, SAMFire was able to fit the whole tilt-series with minimal user input.

   Since SAMFire enables highly sophisticated models to be fitted to large multi-dimensional datasets significantly faster and more easily than previous algorithms, we anticipate it will become standard analysis practice, especially when quantitative analysis is required. Examples that we are currently considering include tracking motion in a time series and quantification of both light and trace elements in multiple-domain structures.

   We acknowledge the support received from the European Union Seventh Framework Program under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3) and under Grant Agreement 291522-3DIMAGE. We thank Raul Arenal and Rowan Leary for providing the raw data shown in Figure 2.

   

[1] Van Aert, S., et al. Nature 470.7334 (2011): 374-377.

[2] Yurtsever, A., van der Veen, R. M., & Zewail, A. H. (2012). Science, 335(6064), 59-64.

[3] Verbeeck, J., and Van Aert, S. Ultramicroscopy 101.2 (2004): 207-224.

[4] Zagonel, L. F., et al. Nano Letters 11.2 (2010): 568-573.

[5] www.hyperspy.org


Tomas OSTAŠEVIČIUS (Cambridge, United Kingdom), Francisco DE LA PEÑA, Paul MIDGLEY
08:00 - 18:15 #6339 - IM01-156 Discrete STEM/EDX tomography for quantitative 3D reconstructions of chemical nanostructures.
IM01-156 Discrete STEM/EDX tomography for quantitative 3D reconstructions of chemical nanostructures.

We report here on the quantitative 3D reconstruction of core-shell nanostructures by STEM/EDX using two X-ray maps acquired at two different tilt angles perpendicular to each other (Rueda et al., 2016; fig. 1). The method is based on the modelling of the NW cross-section using a series of imbricated ellipses whose dimensions are defined by their major and minor diameters (fig. 2). The number of ellipse depends on the number of chemical phases which are identified from the concentration profiles. The position and orientation of each ellipse are determined by the coordinates of their respective centers and the overall tilt of the nanowire, respectively. More sophisticated models, using hexagons or rectangles instead of ellipses, have been developed in order to take into account the crystal structure of nanowires exhibiting facetted sidewalls. These models are based on the elliptical model, by constructing the tangents to an ellipse, and hence, are defined by the same parameters, which is useful when comparing models. Considering a system of a number of K ellipses with ξk,j the local concentration of element j for the kth ellipse (k=1 for the largest ellipse), then the average concentration Ci,j of element j for the ith pixel along the x-axis must satisfy the following equations (equations 1):

                                                       Ci,jt1,i = ξK,jtK,i  for K=1

                                                       Ci,jt1,i = ΣKk=2 ξk-1,j(tk-1,i - tk,i) + ξK,jtK,i  for K≥2

With t1,i and tk,i, the local thickness at pixel i of the first and kth ellipse, respectively. The local thickness of the first ellipse (= the total thickness of the cross-section) and the average concentration Ci,j of element j present along the beam axis is determined using the zeta-factor method (Watanabe and Williams, 2006):

                                                        t1,i = Σmj=1Ii,jζjAi,j/Ibρ                (equation 2)

                                                        Ci,j = Ii,jζjAi,jmj=1Ii,jζjAi,j                  (equation 3)

With: m the total number of element, Ib the beam current; ρ the sample density; ζj the zeta-factor of element j determined using reference samples of known composition and thickness (Lopez-Haro et al., 2014); Ii,j and Ai,j the net X-ray intensity and the absorption correction term for element j at pixel i, respectively. The absorption correction term is estimated from a simple model that takes into account the direction of the X-ray emission relative to the position of the detectors, knowing the thickness, density, and mass absorption coefficient of the material through which the radiation travels (Rueda et al., 2016).

The method for reconstructing the cross-section can be divided into three steps: 1) the appropriate cross-sectional model is selected by comparing the thickness profile calculated from equation [2] with the thickness profile simulated for elliptical, hexagonal, and rectangular cross-sections (figure 3); 2) the number of ellipses is determined, and their dimensions are evaluated, from the concentration profiles; 3) the local concentrations ξk,j are determined and the dimensions of the ellipses are adjusted by minimizing the compositional differences between profiles calculated from equation [3] and simulated by equation [1].

This method was applied for reconstructing core-shell nanostructures on (Mg, Mn, Cd, Zn)(Te,Se) and (Al, Cu)Ge nanowires and (Pt, Co) nanoparticles. Advantages and limitations of the method will be presented and discussed at the conference.

References: P. Rueda-Fonseca, E. Robin, E. Bellet-Amalric, M. Lopez-Haro, M. Den Hertog, Y. Genuist, R. Andre, A. Artioli, S. Tatarenko, D. Ferrand, and J. Cibert (2016) Quantitative Reconstructions of 3D Chemical Nanostructures in Nanowires, Nanoletters, DOI: 10.1021/acs.nanolett.5b04489.

M. Watanabe & D. B. Williams (2006) The quantitative analysis of thin specimens: a review of progress from the Cliff-Lorimer to the new ζ-factor methods, Journal of Microscopy 221, 89–109.

M. Lopez-Haro, P. Bayle-Guillemaud, N. Mollard, F. Saint-Antonin, C. Van Vilsteren, B. Freitag and E. Robin (2014) Obtaining an accurate quantification of light elements by EDX: K-factors vs. Zeta-factors, 18th International Microscopy Congress, Czechoslovak Microscopy Society: Prague.


Eric ROBIN (GRENOBLE), Miguel LOPEZ-HARO, Nicolas MOLLARD, Pamela RUEDA-FONSECA, Marta ORRU, Edith BELLET-AMALRIC, Yann GENUIST, Regis ANDRE, Alberto ARTIOLI, Serge TATARENKO, David FERRAND, Joel CIBERT, Khalil EL HAJRAOUI, Martien DEN HERTOG, Thibault CREMEL, Kuntheak KHENG, Laure GUETAZ
08:00 - 18:15 #4443 - IM02-158 Understanding the enhanced ductility of TiAl alloys using a hybrid study of in-situ TEM experiment and molecular dynamics.
IM02-158 Understanding the enhanced ductility of TiAl alloys using a hybrid study of in-situ TEM experiment and molecular dynamics.

An in-situ transmission electron microscopy study was conducted at room temperature in order to understand an underlying mechanism on room temperature ductility of TiAl alloys. Also, melecular dynamics simulation was conducted to calculate the stacking fault energy of TiAl alloys and to show which deformation mode is dominant. From in-situ straining transmission electron microscopy experiments, it was revealed that the crack path and deformation mode is different between the TiAl alloys with/without room temperature ductility. The crack in TiAl alloys having room temperature ductility interacted with lamellae by forming bridging ligaments between the two α2 lamellae and the γ lamellae. In contrast, the cracks in TiAl alloys without room temperature ductility propagated along grain (colony) boundaries showing brittle intergranular fracture. From the quantitative in-situ TEM experiements, it was found that the γ lamellar of TiAl alloys having room temperature ductility was deformed by slip (Fig. 1). However, the γ lamellar of TiAl alloys without room temperature ductility was deformed by deformation twin (Fig. 2). The difference in deformation mode was explained by stacking fault energy of the TiAl alloys which was calculated by molecular dynamics. The TiAl alloy with low stacking fault energy was deformed by deformation twin (Fig. 2) whereas the TiAl alloy with high stacking fault energy was deformed by dislocation slip (Fig. 1). Furthermore, the role of lamellar orientation of tensile direction on deformation behavior was examined using Schmid factor of each orientation.

Finally, we proposed the important microstructural factors to have room temperature ductility of TiAl alloys.


Seong-Woong KIM (Changwon, Korea), Seung-Hwa RYU, Young-Sang NA, Seung-Eon KIM
08:00 - 18:15 #5162 - IM02-160 Imaging of Electron Beam Triggered Phase Transformations and Chemical Reactions of Organic Molecules by Aberration-Corrected Low-Voltage Transmission Electron Microscopy.
IM02-160 Imaging of Electron Beam Triggered Phase Transformations and Chemical Reactions of Organic Molecules by Aberration-Corrected Low-Voltage Transmission Electron Microscopy.

Direct observation of organic single-molecules in their pristine state using transmission electron microscopy (TEM) is a challenging task because the electron irradiation during high-resolution imaging can modify the structure under investigation. However, recent advances in low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) allow atomic resolution even at accelerating voltages as low as 20kV [1] allowing atomic-resolution imaging even for light-element materials with knock-on damage thresholds below 80kV [3]. However knock-on damage is not the only damaging process. A reduction of the electron beam-induced charging and radiolysis effects can be obtained by dedicated sample preparation such as embedding the sensitive material into a chemically inert, conducting, and one-atom thick container such as a carbon nanotube [3] or in between two layers of graphene [4]. By combining dedicated low-voltage TEM instrumentation with sophisticated sample preparation, electron irradiation-induced damage mechanisms slow down or can even be completely turned off which even allows imaging of molecules containing hydrogen-carbon bonds [5].

In this work we apply low voltage AC-HRTEM not only to image but also to trigger a previously unknown chemical reaction – the polycondensation of perchlorocoronene (PCC), which leads to the formation of graphene nanoribbons, an exciting polymeric structure with significant potential for electronic applications. The multi-step mechanism of this reaction was determined by AC-HRTEM and is both complex and difficult to postulate a priori or from macroscopic observations.  However our time-series imaging at the single-molecule level reveals the nature of key intermediates and follows the pathways of their transformations, thus providing the most direct experience of chemical reactions and demonstrating the physical reality of the elusive steric factor in real space. Figure 1a shows a time series of PCC molecules stacked in single walled carbon nanotube and their electron beam induced changes over time as a product of the accumulated electron dose. A close examination of the experimental images (Figure 1b) indicates that intermolecular reactions are possible only when a PCC molecule can change its orientations with respect to the neighbouring molecules: two non-parallel molecules are able to join together to form angular species which gradually transform into planar species approximately twice the length of the original PCC molecules.

 

References

[1] Kaiser U, Biskupek J, Meyer JC, Leschner J, Lechner L, Rose H, Stöger-Pollach M,  Khlobystov AN, Hartel P, Müller H, Haider M, Eyhusen S, Benner G, Ultramicroscopy 111 (2011), 1239-1246

[2] Chamberlain TW, Zoberbier T, Biskupek J, Botos A, Kaiser U, Khlobystov AN, Chemical Science 3 (2012), 1919-1924

[3 ]Meyer JC,  Eder F, Kurasch S, Skakalova V, Kotakoski J, Park HJ, Roth S, Chuvilin A, Eyhusen S, Benner G, Krasheninnikov AV, Kaiser U, PRL 108 (2012), 196102

[4] Algara-Siller G, Kurasch D, Sedighi M, Lehtinen O, Kaiser U, Appl. Phys. Lett. 103 (2013) 203107

[5] Chamberlain TW, Biskupek J, Skowron ST, Bayliss PA, Bichoutskaia E, Kaiser U, Khlobystov AN, Small 11 (2015) 622-629

Acknowledgement

We acknowledge the financial support of the German Research Foundation (DFG) and the Ministry of Research, Science and the Arts of Baden-Württemberg within the SALVE project.


Johannes BISKUPEK (Ulm, Germany), Thomas CHAMBERLAIN, Stephen SKOWRON, Elena BESLEY, Andrei KHLOBYSTOV, Ute KAISER
08:00 - 18:15 #5163 - IM02-162 Time resolved HREM of Al crystal surface.
IM02-162 Time resolved HREM of Al crystal surface.

The crystallization of Ge:Al amorphous films was studied by time resolve TEM intensively in previous years and reported[1]. These studies included the geometry and the dynamics of Al-amorphous interface with time resolution of 40 milliseconds by conventional bright-field (BF) and dark-field (DF) TEM imaging of films of 50 nm thickness and by conventional high resolution (HR) TEM of films of 25nm thickness. The propagation of the Al interface is diffusion controlled, i.e. the velocity is temperature dependent [1]. The Al-amorphous interface was found to be rough with a fractal dimension of 1.2 for the projected image [2]. However, the quantitative analysis of the interface propagation indicates a long range interaction in the Al-amorphous phase interface [3]. These interactions were attributed to existence of ramified clusters of Al in the Ge:Al amorphous phase [4].

Here we will report on quantitative measurements that were obtained with 5 nm thick films heated locally by the electron beam resulting in the modification of the surface. The quantitative measurements will be based on observations that include aberration corrected HRTEM with low time resolution (1 sec) and by conventional HRTEM with high time resolution (few milliseconds). The later will be used also for evaluating the stability of the interface toward its 3D construction. 

 

References:

  1. Y. Lereah, E. Grunbaum and G. Deutscher, Physical Review A 44 8316 (1991)

  2. Y. Lereah, J.M. Penisson and A. Bourret, Applied Physics Letters 60 1682 (1992)

  3. Y. Lereah, A. Gladckikh, S. Buldyrev and H.E. Stanley, Physical Review Letters 83, 784 (1999)

  4. Y. Lereah, S. Buldyrev and H.E. Stanley, Materials Science Forum Vols. 294-296 (1999) p. 525-528

 


Yossi LEREAH (Tel aviv, Israel), Johannes BISKUPEK, Ute KAISER
08:00 - 18:15 #5241 - IM02-164 Dynamic oxidation and reduction of catalytic nickel nanoparticles using E(S)TEM.
IM02-164 Dynamic oxidation and reduction of catalytic nickel nanoparticles using E(S)TEM.

For many materials, nanoparticles show enhanced catalytic activity and selectivity compared to their bulk state [1]. While catalytic nanoparticles have many industrial, economic and environmental benefits, they also present new challenges, the most significant of which is to characterise the intermediate phases and structural changes to the active catalyst species under reaction conditions. TEM is uniquely suited to the study of solid-state heterogeneous catalysis as it allows us to directly characterise the catalyst with regard to its nanostructure on the atomic scale. Complementary spectroscopy techniques such as EDS and EELS can be used in tandem with imaging to add chemical data to the structural observations. The in-situ capabilities of the E(S)TEM allow for intermediate catalyst phases and transformations to be observed, some of which are only present under reaction conditions and would be lost using ex-situ methodologies [2].

Here we present a study of the dynamic nature of Ni based catalysts under redox conditions relevant to industrial processes [3, 4]. The increased activity of finely divided Ni means that it is important to directly observe the Ni nanoparticles rather than interpreting models based on bulk or ex-situ techniques. Many chemical processes that rely on Ni catalysts involve exposure to both oxidising and reducing gas in the reaction environment. Under reducing and oxidising conditions the structure of Ni nanoparticles is dynamic and involves transitions between single crystal Ni, core-shell Ni/NiO and hollowed Kirkendall-like structures [5, 6]. These changes are dependent on the material properties (size, shape and support interaction) and reaction parameters (temperature, gas type and pressure). Furthermore, these dynamic structure/shape changes influence the stability of the catalyst, which in turn is controlled by the reaction environment. As such, this area of research is crucial for understanding the conditions necessary for attaining and maintaining a particular catalytic species and designing the reaction routes required to reactivate spent Ni catalysts.

Environmental TEM/STEM has been used to probe the dynamic oxidation of Ni nanoparticles. The particle size, reaction time and temperature dependencies of the oxidation process have been investigated using both model and industrial Ni catalysts. Furthermore, we have applied the same in-situ techniques to reveal the conditions needed for complete reformation/reactivation of the original Ni species.

[1] B.R. Cuenya, Thin Solid Films, 518 (2010) 3127-3150.

[2] E.D. Boyes, M.R. Ward, L. Lari, P.L. Gai, Annalen der Physik, 525 (2013) 423-429.

[3] P.M. Mortensen, J.-D. Grunwaldt, P.A. Jensen, A.D. Jensen, Catalysis Today, 259 (2016) 277-284.

[4] S. Hu, M. Xue, H. Chen, Y. Sun, J. Shen, Chinese Journal of Catalysis, 32 (2011) 917-925.

[5] S. Chenna, P.A. Crozier, Micron, 43 (2012) 1188-1194.

[6] R. Nakamura, J.G. Lee, H. Mori, H. Nakajima, Philosophical Magazine, 88 (2008) 257-264.

Acknowledgements: The EPSRC (UK) is supporting the AC ESTEM development and continuing applications at York under strategic research grant EP/J018058/1.


David LLOYD (North Shields, United Kingdom), Alec LAGROW, Edward BOYES, Pratibha GAI
08:00 - 18:15 #5275 - IM02-166 A NanoWorkshop Toolkit for in situ Nanoassembly and Nanocharacterization.
IM02-166 A NanoWorkshop Toolkit for in situ Nanoassembly and Nanocharacterization.

Fitting a Focussed Ion Beam / Scanning Electron Microscope (FIB/SEM) with a micromanipulators as well as a range of plug-in tools transforms the microscope from a device purely used for observation to a workstation where materials can be manipulated, assembled, characterized, etc. - at the micron, sub-micron, and nano scale.

Typical applications include harvesting, arranging, and mechanically testing nanowires, nano tubes, and CNTs. It is also often of great interest to characterize nanowires, nanotubes, CNTs, etc. electrically.

Electrical and mechanical tests as well as structural investigations on MEMS devices are also commonly performed tasks.

This work will present a number of different experiments performed inside SEM or FIB/SEM tools. Among these are pick and place operations on sub-micron sized particles, mechanical testing of nanowires and CNTs as well as in situ thermal experiments.

One of the described experiments entails mounting a strand of CNTs to a force measurement cantilever inside an SEM and subsequently performing a tensile experiment on the strand of CNTs. The CNTs are mounted to the force measurement cantilever using a special vacuum compatible adhesive. The adhesive can be applied in situ using a fine tip on the end of a micromanipulator. The tip is dipped into a small droplet of the adhesive in order to wet it. Next, some adhesive is transferred to the force measurement cantilever. In an additional step, some adhesive is used to extract a strand of CNTs from a large bundle. The extracted CNTs are brought into contact to the wetted force measurement cantilever and the adhesive is cured using the electron beam. Finally, the a force measurement is performed revealing the CNTs tensile strength.


Andrew Jonathan SMITH, Andreas RUMMEL, Klaus SCHOCK, Stephan KLEINDIEK (Reutlingen, Germany)
08:00 - 18:15 #5286 - IM02-168 In situ and cryo (S)TEM imaging of internal microgel architectures.
IM02-168 In situ and cryo (S)TEM imaging of internal microgel architectures.

More than ever polymer science focuses on complex molecular structures and supramolecular assemblies. Microgels are responsive polymer materials and structures, which can be manipulated in e.g. charge or size  by external parameters like pH or temperature variation. The investigated microgels are soft particulate polymer networks that can be dispersed in an aqueos medium. They reveal unique features providing new opportunities to develop smart bio-inspired materials. In contrast to rigid colloidal particles, which lack the possibility to adapt their size and shape to enviromental requirements, microgels have switchable properties of form and function that make them very useful in a wide range of e.g. biological sciences and medical applications. They combine properties of dissolved macromolecules with those of colloidal particles.

The direct visualization of the internal structure of materials is very important to analyse the spatial distribution of different compartments and, thus, to design novel materials with tailored properties. Microgels can be prepared with various morphologies and functions in different compartments. Careful analysis of the correlation between architecture and function requires powerful methods to visualize inner structure and compartmentalization in the nanometer range.

Here, the direct visualization of different compartments within microgels using a combination of in situ and cryo transmission electron microscopy methods is shown. In particular, the challenge of determing the radial distribution of appropriately labeled compartments within single microgels and particles from 2D projections is adressed.

Microgels with core-shell architecture were obtained by precipitation polymerization. First a particle was synthesized and purified before a shell was synthesized on top by the seed and feed method. Core and shell have oppositely charged copolymers to create a two compartment amphoteric microgel system, that is alternately stained with gold and magnetite nanoparticles. [1], [2]

For in situ liquid cell experiments, a thin layer of liquid was embedded between two hermetically sealed, electron transparent Si3N4-windows. The used holder is an in situ-liquid cell holder manufactured by Hummingbird Company and the microscope is a Zeiss Libra 200FE with an acceleration voltage of 200 kV. The resolution is mainly limited by the thickness of the liquid.

Figure 1 shows a comparison between cryo TEM and in situ STEM. Due to the liquid layer thickness the resolution is limited in (b). Also the Brown emotion leads to a defocused and smudged image.

Figure 2 shows the comparison of the radial distribution of nanoparticles according to the two images above calculated by a MATLAB routine. For the cryo TEM image in (b) the relative particle density as a function of the relative particle radius is plotted. (c) shows a 3D reconstruction of the model.

References:

1. J, Dockendorff, M. Gauthier, A. Mourran, M. Moller, Macromolecules (Washington, DC, US), 41 (2008) 6621.

2. A. Pich, S. Bhattacharya, Y. Lu, V. Boyko, H.-J. P. Adler, Langmuir 20 (2004) 10706.

 

Acknowledgement:

The authors kindly acknowledge the financial support by the DFG through the SFB 985 "Functional Microgels and Microgel Systems".

 


Tobias CAUMANNS (Aachen, Germany), Arjan GELISSEN, Alexander OPPERMANN, Pascal HEBBEKER, Rahul TIWARI, Sarah TURNHOFF, Dominik WÖLL, Andreas WALTHER, Joachim MAYER, Walter RICHTERING
08:00 - 18:15 #5355 - IM02-170 In situ observation of heat-induced degradation of perovskite solar cells.
IM02-170 In situ observation of heat-induced degradation of perovskite solar cells.

The use of perovskite materials (particularly methylammonium lead iodide) in solar cells has become very attractive due to the fast increase in reported power conversion efficiencies over the last few years, leading to values above 20%. While this value is competitive with established photovoltaic technologies, the stability of perovskite-based solar cells is still insufficient for commercial applications. In particular, it is very well known that some components, including the perovskite layer and the hole transporter, can degrade when exposed to a combination of heat and moisture. In situ TEM is an ideal tool for investigating such degradation and understanding the phenomena underpinning it.

In this work [1,2] we prepare methylammonium lead iodide cells using different approaches from the literature (with the perovskite conversion carried out in single- and double-step in glovebox, in air or in vacuum); we prepare TEM cross-sectional samples using focused ion beam milling.

For each cell, we carry out scanning TEM imaging and EDX elemental mapping (shown in Figure 1) as they are heated in situ in the TEM. This is a procedure that requires careful control over the temperature and the electron dose. To that aim we exploit recent advances in TEM-related technology, such as Silicon Drift Detectors (SDD) for EDX, which collect energy-dispersed X-ray spectra with a good yield, and stable MEMS heaters, enabling the temperature to be cycled quickly and reproducibly. Moreover, we employ multivariate analysis (principal component analysis, PCA) to increase the signal-to-noise ratio of the spectral maps.

Cross-sectional views acquired after heating are reported in Figure 2. We do not observe changes in the morphology or the elemental distribution in the perovskite layer for heating up to 150°C for short times (employing a heating ramp with 30’ steps every 25°C). Since the ex-situ heating of the same samples above 90°C causes a significant decay in cell performance, we attribute such decay to the degradation of the charge transport properties of the hole transporter (spiro-OMeTAD in this case). Increasing the temperature further, different decomposition patterns emerge for the perovskite layer. In samples that had not been exposed to air, elemental migration of lead and iodine results in the formation of aggregates, which EDX suggests might be PbI2, clustering on the FTO electrode. In the sample exposed to air, a different phenomenon occurs – instead of forming aggregates, the elemental species diffuse from the perovskite into the hole transporter. This is visible both as an increased contrast in the high-angle annular dark field images (HAADF) and as features in the EDX spectra; we hypothesise that the trapped moisture within the cell might be hindering the formation of PbI2 and make elemental diffusion more favourable.

[1] Divitini, G. et al. – Nature Energy 201512 (2016)

[2] Matteocci, F. et al. – ACS Applied Materials & Interfaces 7, 26176 (2015)


Giorgio DIVITINI (Cambridge, United Kingdom), Stefania CACOVICH, Fabio MATTEOCCI, Lucio CINA', Aldo DI CARLO, Paul MIDGLEY, Caterina DUCATI
08:00 - 18:15 #5443 - IM02-172 In situ STEM observation of the impact of surface oxidation on the crystallization of GeTe Phase Change Material thin films.
IM02-172 In situ STEM observation of the impact of surface oxidation on the crystallization of GeTe Phase Change Material thin films.

Chalcogenide phase change materials (PCMs) such as Ge-Sb-Te and GeTe alloys exhibit outstanding properties, which has led to their successful use as non-volatile resistive memories in Phase Change Random Access Memories (PCRAM). PCRAM using PCMs can be switched reversibly between their crystalline and amorphous phases with different optical and electrical properties offering a unique set of features such as fast programming, good cyclability, high scalability, multi-level storage capability and good data retention. Controlling the crystallization is a challenge and numerous studies have been conducted to probe interface and size effects on the PCM crystallization. Surface engineering has a crucial role on the crystallization temperature and mechanisms[1,2]. Temperature resolved reflectometry experiments have shown that the crystallization temperature of GeTe films (in the thickness range 30-100 nm) change drastically depending on its surface state (Fig.2). For a better understanding of this phenomenon, we performed in situ STEM experiments to observe the complete crystallization mechanisms at a nanometer scale of GeTe films with various surface states.

Amorphous GeTe films were deposited by magnetron sputtering in an industrial cluster tool and were protected either by in situ deposition of a 10nm thick SiN capping layer or left uncapped before being exposed to air. For STEM analysis, a specifically adapted preparation method using focused ion beam (FIB) milling has been developed in order to perform in situ annealing and crystallization of the GeTe films directly in the microscope (Fig.1). In particular, a specific positioning of the FIB foil enables low energy cleaning despite the sample holder configuration. We will show that this new sample preparation method, combined with the precise temperature control and negligible spatial drift when using the Protochips Aduro sample holder, allows atomic resolution and quantitative analysis to be obtained during in situ annealing.

Results show that the uncapped (i.e. surface oxidized) GeTe film exhibits a two-step crystallization mechanism. First, the crystallization spreads across the sample over the top 20 nm of the initial amorphous layer. If the temperature ramp is allowed to continue, the nucleation-growth of the remaining amorphous part of the GeTe film is triggered at 50°C above the temperature corresponding to surface crystallization (Fig.3b,d,f).

We will give evidence that the GeTe film capped by a 10nm SiN layer prior to air exposure exhibits a very different crystallization temperature and mechanism. Indeed, in that sample a single-step crystallization occurs through a one-step nucleation- growth in the whole layer at a temperature corresponding to the second crystallization step of the uncapped GeTe film. By quenching before complete crystallization, crystalline nuclei were imaged at high resolution and we observed that crystallization occurred by volume nucleation within the amorphous layer (Fig.3a,c,e).

We will show that if protected from oxidation, the GeTe crystallization mechanism can be a pure nucleation-growth process happening about 50°C above previously reported values [2]. An interpretation of this crystallization mechanism will be proposed based on the elemental segregation obtained by EDS and live recording of the crystallization obtained using multiple STEM detectors. This information will be invaluable to improve reliability and data storage capability of GeTe based devices. By adapting our in situ procedure for electrical biasing, it will be possible to perform real time TEM observation of GeTe switching between ON and OFF states. Then by comparing both electrical and thermal induced crystallization, we will be able to obtain important information about GeTe switching at an atomic scale to provide better devices.

References

1.            R. Pandian, B.J Kooi,  J. De Hosson and A. Pauza, Journal of Applied Physics, 100, 123511 (2006).

2.            P. Noe et al, In press, Acta Materialia (2016).


Rémy BERTHIER (GRENOBLE), Nicolas BERNIER, David COOPER, Chiara SABBIONE, Francoise HIPPERT, Pierre NOÉ
08:00 - 18:15 #5568 - IM02-174 Microstructural and mechanical properties of hyper-deformed surfaces: In-situ micro-pillar compression and EBSD investigations in α-iron.
IM02-174 Microstructural and mechanical properties of hyper-deformed surfaces: In-situ micro-pillar compression and EBSD investigations in α-iron.

The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of impact-based treatments, a local microstructure refinement in the near surface is produced by a severe plastic deformation of the material, leading to a progressive reduction of the grain size over a few tens of microns, and consequently an increase of the hardness and mechanical properties. These zones are commonly known as Tribologically Transformed Surfaces (TTS). In this project, the micro-structural transformation in the near surface is produced on pure α-iron samples using a repetitive impact-based procedure: Micro-percussion treatment. In this technique, every impact is effectuated on the same position with a rigid conical indenter (tungsten carbide), controlling the number, angle and velocity of impacts. The resulting imprint (figure 1) is characterized by a significant grain size refinement and consequently a graded strengthening as a function of distance to the impacted surface. Moreover, several in-situ micro-pillar compression tests are carried out in the cross-section of the hyper-deformed surface (figure 2) in order to quantify this mechanical property gradient in-depth. However, the yield strength increment observed with this technique does not reveal the different micro-structural contributions (grain size effect, dislocation hardening, etc…) on the increase of mechanical properties. Indeed, the main purpose of this work is to correlate the mechanical properties gradient with the local microstructural evolution produced by the impact-based severe plastic deformation. For these purpose, EBSD mapping (figure 1) is used to determine the grain size distribution and the local “Kernel Average Misorientation” (KAM) in the cross section. A qualitative estimation of the geometry necessary dislocation density could be done from this latter estimation. With this analysis, the Hall-Petch and dislocation strengthening contributions could be correlated and compared with the experimental results from micro-pillar compressions (figure 2).


David TUMBAJOY-SPINEL, Sergio SAO JOAO (LYON CEDEX 7), Xavier MAEDER, Sylvie DESCARTES, Jean Michel BERGHEAU, Johann MICHLER, Guillaume KERMOUCHE
08:00 - 18:15 #5573 - IM02-176 Measurement of mechanical properties gradient on impact-based transformed surfaces: Nano-mechanical testing in graded micro-structured α-iron.
IM02-176 Measurement of mechanical properties gradient on impact-based transformed surfaces: Nano-mechanical testing in graded micro-structured α-iron.

In the industry, there are several techniques which improve the service lifetime of materials by increasing the local mechanical properties in the near-surface. In the case of mechanical surface treatments (such as impact-based), the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the surface, and then leading to a local refinement of the microstructure into the affected zone (Tribologically Transformed Surfaces - TTS). The microstructure’s transformation is characterized by a progressive increment of the grain size from the surface until the bulk material. Consequently, very interesting physical properties such as high hardness and better tribological properties are exhibit in these mechanically-induced transformed surfaces. Nowadays, it is well-known that the grain size gradient generated provokes an evolution on the mechanical properties in the impacted zone over a few tens of microns. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties due to the heterogeneity of the transformed surface. The main issue of this work is to assess and describe precisely the elastic-plastic behavior and the distribution of mechanical properties on deformed zones of a model material (pure α-iron). In our project, a characterization of the transformed microstructure, as well as a statistics measurement of the grain size distribution on the cross-section of the sample is presented firstly. Afterwards a methodology based on nano-indentation tests (Figure 1) and in-situ micro-pillars compression tests (Figure 2) is implemented to quantify the evolution of mechanical properties starting from the near-surface. A relation between the hardness gradient and the microstructure evolution is established, as well as a comparison between the properties measured by both techniques is discussed.


David TUMBAJOY-SPINEL (LYON CEDEX 7), Sylvie DESCARTES, Jean Michel BERGHEAU, Sergio SAO JOAO, Gaylord GUILLONNEAU, Johann MICHLER, Guillaume KERMOUCHE
08:00 - 18:15 #5579 - IM02-178 In-Situ ESTEM Observations of Asymmetric Oxidation and Reduction in Copper Nanoparticles.
IM02-178 In-Situ ESTEM Observations of Asymmetric Oxidation and Reduction in Copper Nanoparticles.

A fundamental understanding of the oxidation and corrosion mechanisms of metals is of critical importance to improving their performance in catalysis, and other industrial applications.1 For applications in nanocatalysis a metals oxidation pathway and subsequent reduction can lead to the rearrangement of catalytically active surface facets2 as well as deactivation through sintering and Ostwald ripening.3 In particular we are studying copper which can readily oxidize at room temperature and has two native oxides, cuprous oxide (Cu2O) and cupric oxide (CuO). The oxidation of copper has been previously reported to be dependent on its crystallography4 as well as the interaction between the copper and the substrate.5

In this talk we will discuss the use of environmental scanning transmission electron microscopy (ESTEM)6 to study the in-situ oxidation of copper. Environmental STEM was carried out in a modified JEOL 2200 which allowed for the introduction of gases into the microscope and using a DENSsolutions holder to control the reaction temperature. The copper is studied in the form of nanoparticles of 2 - 50 nm in size. With high angle annular dark field (HAADF) STEM we use conditions that are ideal to track the oxidation front as it progresses across the copper nanoparticles by following the changes in Z-contrast with time. In the case of copper, the oxidation occurred via the heterogeneous nucleation of the oxide phase (Cu2O) from the smallest point on the nanoparticle (Figure 1a and 1b). When the process is reversed, via reducing the particles with hydrogen, it was also observed that the reduction was initially nucleated from the smallest part of the nanoparticle and then spread across the particle. Preliminary analysis of the data suggests that once the oxidized or reduced phase is nucleated the reaction is mediated by the Cu/Cu2O interface.

References:

(1) Gattinoni, C.; Michaelides, A. Surf. Sci. Rep. 2015, 70, 424.

(2) Cabie, M.; Giorgio, S.; Henry, C. R.; Axet, M. R.; Philippot, K.; Chaudret, B. J. Phys. Chem. C 2010, 114, 2160.

(3) Martin, T. E.; Gai, P. L.; Boyes, E. D. ChemCatChem 2015, 7, 3705.

(4) Luo, L.; Kang, Y.; Yang, J. C.; Zhou, G. Surf. Sci. 2012, 606, 1790.

(5) Gai, P. L. and Boyes, E.D., Electron Microscopy in Heterogeneous Catalysis: IOPP (2003).
(6)  Boyes, E. D.  and Gai, P.L.,  C.R. Physique 2014, 15, 200.  

The ESPRC (UK) is supporting the AC ESTEM development and continuing applications at York under strategic research grant EP/J018058/1.


Alec LAGROW (York, United Kingdom), Michael WARD, David LLOYD, Edward BOYES, Pratibha GAI
08:00 - 18:15 #5585 - IM02-180 In situ TEM observation of electromigration in Ni nanobridges.
IM02-180 In situ TEM observation of electromigration in Ni nanobridges.

Using in situ scanning transmission electron microscopy (STEM) (FEI Titan microscope operating at 300 keV), a microelectromechanical system (MEMS) chip and a dedicated biasing and heating sample holders, built in-house, we investigated electrical and thermal properties of 15-nm-thick Ni nanobridges. These techniques allow to visualize nanobridge morphology transformations down to atomic scale while electrical current is passed. If thin metallic wire is subjected to high current density, the material transfer can start which results in the wire break. This phenomenon is called electromigration 1.

Ni nanobridges with a length of 500 – 1000 nm and a width 200 – 500 nm were produced by e-beam metal evaporation onto a 100-nm-thick freestanding silicon nitride membrane and patterned using electron beam lithography (Fig. 1a). Contacts to the nanobridges were made with a 100-nm-thick layer of Au and a 3-nm-thick adhesion layer of Cr. Initial resistance of the structures, including bridge, contact pads and leads, is in the range of 160 – 250 Ohm. More details of the sample preparation can be found elsewhere 2. Using electrical setup 3, I–V measurements were performed in bias-ramping mode. Voltage is gradually increased (with a speed of 15 mV/s) from 0 V to a predefined value of 500–600 mV, followed by a decrease back to 0 V, after which a new cycle with higher maximum voltage was performed (Fig. 1f).

Fig.1 shows STEM images of Ni nanobridge with 10-nm-thick Al2O3 oxidation-protective layer on top taken before electromigration and after each bias-ramping cycle with maximum voltages 500 mV, 520 mV, 540 mV and 580 mV. Fig. 2f shows corresponding I–V curves for four voltage cycles applied in a row. Sample temperature prior to voltage apply was 100 K. During electromigration experiments in Ni material transfer was shown to be voltage polarity dependent: Voids initially form near the cathode contact pad of the bridge, as in the majority of metals due to electron-wind force, but at the end bridge breaks near the anode side.

Also, we visualised morphological transformations in polycrystalline Ni film (deposited on top of the heater with 20-nm-thick windows in Si3N4 membrane) during substrate heating up to 400°C (Fig. 2) and estimated the bridge temperature achieved in electromigration experiments due to the Joule heating to be around the Curie point. In order to enhance the contrast between grains, annular dark-field STEM imaging was used 4.

Enriched with oxygen bubbles formation was found due to Ni nanobridges oxidation after a month of their storage at atmospheric pressure. In order to prevent samples oxidation, 10-nm-thick Al2O3 layer was used as a protective layer. The place of bridge break near the anode side was shown to be independent on the ambient pressure and substrate temperature.

Acknowledgement: The authors gratefully acknowledge STW UPON and ERC project 267922 for support.

 

1.         Ho, P. S.; Kwok, T. Rep Prog Phys 1989, 52, (3), 301-348.

2.         Kozlova, T.; Rudneva, M.; Zandbergen, H. Nanotechnology 2013, 24, 505708.

3.         Martin, C. A., et al. Rev Sci Instrum 2011, 82, 053907.

4.         Rudneva, M., Kozlova, T. & Zandbergen, H. Ultramicroscopy 2013, 134, 155-159. 


Tatiana KOZLOVA (Delft, The Netherlands), Henny W. ZANDBERGEN
08:00 - 18:15 #5629 - IM02-182 In situ SEM dynamic investigation of charging kinetics in insulating materials.
IM02-182 In situ SEM dynamic investigation of charging kinetics in insulating materials.

Dielectric breakdown constitute an important limitation in the use of insulating materials since it causes its damage. This catastrophic phenomenon (Figure 1) is obviously an important failure in the levels of equipment requiring some insulation safety or ensuring their proper functioning. This causes some technological problems associated with the manufacture and use of insulating materials in several industrial sectors like in microelectronics, high voltage electric energy transport and spacecraft. The choice of insulating material for those applications is related to the corresponding breakdown voltage value which limits their use. To improve the resistance to dielectric breakdown, it is imperative to understand and control the cause of this damage process reducing the reliability of some instrumentation. It is well known that breakdown is correlated with the presence of space charge within the insulators. Indeed, breakdown is related to a fast relaxation (detrapping) of trapped charge. Commonly, this space charge can be determined by the SEMME method (Scanning Electron Microscope Mirror Effect) which quantifies the final trapped charge amount. The purpose of this work is to develop a technique using a specific arrangement in the SEM chamber (Figure 2) in order to characterize the trapped charge dynamic by ICM (Induced Current Method).  This technique allows enhancing the understanding of trapping phenomenon, spreading and stability of trapped charges

The experiments were carried out in a FESEM (Field Emission Scanning Electron Microscope) Carl Zeiss SUPRA 55 VP using a specific configuration in the SEM sample holder (Figure 3). It permits to measure separately and simultaneously the influence current and the conduction current and tracing back to the trapped charge temporal evolution during (charging) and after (charge decay) electrons irradiation (Figure 4). Thereafter, the used technique of two injections separated by a pause time was a powerful method for monitoring and understanding the dynamics of the trapped and released charges in insulating materials. These results open the way for the establishment of a conventional characterization procedure, which will be useful in different contexts of use of insulating materials. The studied materials are α-alumina and Yttria Stabilized Zirconia (YSZ) polycrystalline ceramics.  Since the dielectric and electrical properties of an insulating material are highly dependent on its microstructure, the grain size effect and MgO doping effect are then studied and discussed. Via the developed technique, the microstructure - dielectric rigidity correlations could be well justified.


Sergio SAO JOAO (LYON CEDEX 7), Omar MEKNI, Dominique GOEURIOT, Gilles DAMAMME
08:00 - 18:15 #5702 - IM02-184 In situ electrical testing across nano-scale contact interfaces in the transmission electron microscope.
IM02-184 In situ electrical testing across nano-scale contact interfaces in the transmission electron microscope.

Understanding the electrical properties of nanoscale contacts is paramount in small-scale devices,  including probe-based microscopies [1], nanomanufacturing techniques [2] and  micro/nano-electromechanical systems (M/NEMS) [3]. In many cases, the electrical transport properties of the contact determines the device’s functionality, and yet the behavior of the contact conductance is multi-faceted and not easily characterized. There has been extensive characterization of the electrical properties of ultra-small contacts using mechanically controllable break junctions and scanning probe techniques [4]. However, in these techniques the shape, size, and atomic structure of the contacting bodies and the contact itself are typically unknown. Thus, confounding factors such as the presence of oxide films and contaminants; the evolving shapes of the bodies due to inelastic deformation; and inaccurate estimation of contact sizes causes uncertainty in experimental measurements based on contact properties. In situ transmission electron microscopy (TEM) measurements of electrical contacts can overcome these limitations. While investigations have been performed using in situ electrical measurements inside a TEM before – including on single-atom-width nanocontacts in gold [5] – these methods typically require specially prepared contacts and are limited to a range of materials and geometries. In this study we show initial results obtained with a new in situ TEM electrical characterization tool that contains a movable probe, which allows to make site-specific electrical contact measurements to study device-related nanoscale electrical contacts (Fig. 1). The flexibility of the present in situ tool rests in its unique removable sample cartridge that enables simple, repeatable and accurate probe positioning, high-resolution imaging, and accommodates a wide range of nanoscale contact samples.

Two contact configurations that are common to conductive scanning probe microscopy were recreated in situ in the TEM. Namely, a W substrate was contacted by a sharp nanoscale tip that is composed either of Pt/Ir or of doped Si. We demonstrate that current-voltage sweeps can be performed while real-time images of the nanoscale contact are acquired. As shown in Fig. 2(a), the metal/metal contact is ohmic (resistance 730 ohms). By contrast, the metal/semiconductor contact of Fig. 2(b) has a highly asymmetrical IV curve, displaying Schottky-type behavior – as commonly seen in conductive probe microscopy with doped-silicon tips [6].

As an example of the benefits of in situ imaging we compute the contact resistivity of the metal/metal contact. From the images of the contact we estimate a contact radius of 9.8 nm. The resistivity can be calculated using the classical (Maxwell), ballistic (Sharvin), or intermediate (Knudsen) limits [7]. The mean free path for W (estimated from the Fermi velocity and the bulk conductivity [8]) is close to 15 nm. Because this value is on the order of the contact radius, the intermediate resistivity limit is appropriate, leading to a value of rKnudsen = 620 mW-cm. By having a direct measure of the contact area – obviating the reliance on continuum contact models – we can compute the contact’s resistivity directly. It should be noted that this value is much larger than the bulk resistivity of W which is 4.82 mW-cm [9]. This is attributable to the presence of insulating surface films (such as oxide or contamination). 

References:

[1]JY Park et al, Materials today, 38 (2010), p. 38.

[2] C Cen et al, Nature Materials, 7 (2008), p. 298.

[3] OY Loh, HD Espinosa, Nature Nanotechnology, 7 (2012), p. 283.

[4] N Agrait, AL, Yeyati, JM van Ruitenbeek, Physics Reports, 377 (2003), p. 81.

[5] H Ohnishi, Y Kondo, K Takayanagi, Nature 395 (1998), p. 780.

[6] MA Lantz, SJ O’Shea, ME Welland, Review of scientific instruments 69 (1998), p. 1757.

[7] Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge U. Press (1994).

[8] Ashcroft & Mermin, Solid State Physics, Brooks Cole (1976).

[9] WM Haynes, ed. CRC handbook of chemistry and physics, CRC press (2014).

[10] The authors thank Julio A. Rodríguez-Manzo for his input and review of the abstract. T.D.B.J. acknowledges support from National Science Foundation under award

No. #CMMI-1536800. 


Daan Hein ALSEM (Lacey, USA), Siddharth SOOD, Norman SALMON, Tevis JACOBS
08:00 - 18:15 #5739 - IM02-186 The value of in situ transmission electron microscopy in studding ferroelectric materials.
IM02-186 The value of in situ transmission electron microscopy in studding ferroelectric materials.

Ferroelectrics play an important role in today’s modern life. A large variety of applications including piezoelectric actuators, sensors, dielectric capacitors, memory devices, etc. are based on these materials. Recently, scientific interest has been given to the Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) perovskite ferroelectric, that exhibits superior electrical and mechanical properties. It is known that domain morphology plays a significant role in electromechanical properties of ferroelectrics. As the external electric field induces domain wall motion or domain switching, it is important to perform direct observations of domain structure evolution under electric field.

In the present study in situ transmission electron microscopy (TEM) was employed to reveal the evolution of ferroelectric domains under electric field and temperature in BZT-xBCT. It is shown that in situ TEM is an extremely powerful tool in order to visualize the real-time microstructural evolution in these materials. During the in situ electric field TEM experiments, a multiple-domain state (A) → nanodomain state → single-domain state transformation occurred during the poling process. With further increase in the applied field a multiple-domain state (B) appeared. This state could be associated with strain incompatibility between neighbouring grains under the electric field. The displacement of the domain walls and changes in the domain configuration during electrical poling indicated a high extrinsic contribution to the piezoelectric response in lead-free BZT – xBCT. The temperature induced ferroelectric → paraelectric phase transition in the BZT – xBCT is also investigated. On heating and cooling a microstructure evolution in BZT – xBCT system was observed. Irregular domains with curved walls appeared in BZT – xBCT due to internal stresses associated with coexistence of rhombohedral and tetragonal domains within one grain.


Marina ZAKHOZHEVA (Delft, The Netherlands), Ljubomira Ana SCHMITT, Yevheniy PIVAK, Matias ACOSTA, Kun ZUO, Qiang XU, Hans-Joachim KLEEBE
08:00 - 18:15 #5746 - IM02-188 Gas TEM holder for in-situ biasing and heating experiments.
IM02-188 Gas TEM holder for in-situ biasing and heating experiments.

We present our custom designed Gas Transmission Electron Microscopy (TEM) holder for in-situ electrical, gas and heating experiments. The holder is currently compatible with FEI machines, but it can be easily redesigned for other transmission electron microscopes. The holder has a sliding cassette at the sample position (Figure 1, left side), which can be manually opened and closed through a rotating knob, located at the opposite end of the TEM holder. Closing the cassette will ensure complete gas sealing around the sample, up to a pressure difference of 1 atm (respect to the microscope vacuum). One gas line is used for both inlet and outlet.

The beam is blocked when the cassette is closed, meaning that it is not possible to acquire live imaging during the gas flow. However, this configuration is very practical from the point of view of sample loading and unloading, as it only takes a few minutes to replace the sample, without any O-rings or complicated sealing mechanisms. Moreover, the sample does not need to be exposed to air after the gas reaction, which gives a relevant advantage over ex-situ measurements.

The TEM holder also has 10 electrical feedthroughs, which can be used for in-situ electrical biasing experiments. Four of these electrical contacts can be used for in-situ heating, in combination with our custom made MEMS heaters [1] (Figure 1, right side), capable of reaching 1000 K in vacuum and 700 K in 1 atm Argon environment.

As a first application of our gas TEM holder, we exposed freestanding multilayer graphene to 1 atm of Hydrogen and simultaneously heated the sample at 550 K for a few minutes. As we can see from Figure 2, this high-pressure hydrogen annealing resulted in isotropic etching of graphene, with formation of round holes, approximately 80 nm in diameter. Further experiments will be performed to evaluate the etching rate and the isotropicity/anisotropicity as function of temperature, pressure and gas composition.

Acknowledgments: This work was supported by ERC funding,  project 267922 - NEMinTEM

References:

[1]  Sairam Malladi,   et al.   Chem. Commun., 2013,49, 10859-108


Leonardo VICARELLI (Delft, The Netherlands), Henny ZANDBERGEN
08:00 - 18:15 #5748 - IM02-190 In situ study of CeO2 microspheres sintering using HT-ESEM.
IM02-190 In situ study of CeO2 microspheres sintering using HT-ESEM.

Sintering could be defined as the transformation of a powdered compact into a cohesive material under heating at high temperature. It appears as a key-step in the preparation of ceramic materials such as UOx and MOx nuclear fuels. The sintering is usually described through three different stages. The initial stage involves the elaboration of necks between the grains and leads to the cohesion of material while the intermediate and final stages are dedicated to the elimination of porosity between the grains by the means of grain growth mechanisms [1]. Presently, only few experimental works are devoted to the kinetics of necks elaboration (i.e. first stage of sintering), and this stage is mainly described through numerical simulation of 2 to 4 spherical grains in contact [2]

 

In the present study, we report the first experimental observations of the initial stage of sintering of CeO2 microspheres using an Environmental Scanning Electron Microscopy at high temperature (HT-ESEM). Actually, the use of HT-ESEM allowed the in situ observation of the samples during long term heat treatments up to 1400°C under various atmospheres [3]. In a first step, CeO2 spherical grains were synthesized to investigate similar systems to those modeled. Then, the HT-ESEM was used to investigate the first stage of sintering. In this aim, three different systems (single grain, two and three grains in contact) were investigated between 900°C and 1200°C:

  • Monitoring of a single grain led to the evolution of the number of crystallites included in the sphere. From the micrographs series, the time necessary to reach a spherical single crystal through the growth of crystallites was determined, as well as the mechanisms involved and the associated activation energies. (Figure 1) [4]
  • The observation of the morphological modifications of two and three grains arrangements then led to assess the evolution of several parameters of interest such as neck size, dihedral angles between the spheres or distance between the grain centers. From the micrographs series, it was possible, for the first time, to identify experimentally the mechanisms of necks growth between the grains and to compare the behaviour of near ideal single-crystal systems with polycrystalline samples (Figure 2 and 3). [5]

 

The use of HT-ESEM observations appears of a great interest for the study of sintering phenomena. Image processing allows determining original and fundamental experimental data, such as the mechanisms of necks growth, characteristics of the processes occurring during the initial stage of sintering.

 

References:

 

[1] D. Bernache-Assolant, Chimie-physique du frottage, Hermes Eds, 348p.

[2] F. Wakai, Modeling and simulation of elementary process in ideal sintering. J. Am. Ceram. Soc., 89(5), 1471-1484 (2006).

[3] R. Podor, N. Clavier, J. Ravaux, L. Claparède, N. Dacheux and D. Bernache-Assollant, Dynamic aspects of cerium dioxide sintering: HT-ESEM study of grain growth and pore elimination, J. Eur. Ceram. Soc., 32, 353-362 (2012).

[4] G. Nkou Bouala, R. Podor, N. Clavier, J. Léchelle, A. Mesbah and N. Dacheux, In situ HT-ESEM study of CeO2 nano-ripening : toward a control of nanostructure. Ceram. Intern. (2015) 41 14703-14711

[5] G.I. Nkou Bouala, N. Clavier, J. Léchelle, S. Martin, N. Dacheux, J. Favrichon, H. P. Brau and R. Podor, From in situ HT-ESEM observations to simulation: how does polycristallinity affects the sintering of CeO2 microspheres? J. Phys. Chem. C (2016) 120 386-395


Galy Ingrid NKOU BOUALA, Renaud PODOR (ICSM, Marcoule), Jacques LECHELLE, Nicolas DACHEUX, Nicolas CLAVIER
08:00 - 18:15 #5766 - IM02-192 CelDi: Development of an advanced solid / fluid reaction stage for SEM.
IM02-192 CelDi: Development of an advanced solid / fluid reaction stage for SEM.

In numerous scientific fields such as life, materials and Earth sciences, or quality controls of industrial processes, there is a growing interest for the direct observation - at the submicroscopic scale - of processes occurring at solid / liquid and solid / gas interfaces. So far, only few experimental cells were designed to address this challenging issue. Most of them are devoted to a specific use in Transmission Electron Microscopy (TEM) and are not suitable for observation of large (or thick) samples and the other cells designed to be used in a Scanning Electron Microscope (SEM) chamber do not allow fluid flow.

To address this issue, a dedicated device was developed according to the following requirements: 1) The sample holder must be suitable for large samples. 2) The device must allow the renewal of the fluid through a continuous flow. 3) The device should be sufficiently efficient and secure to be used in any type of conventional SEM. 4) The device should be easy to implement and user friendly. A first prototype (Fig. 1) was recently tested  and patented [1].

 For the proof of concept, it was used to perform in situ experiments during which series of images was recorded with a SEM, using the back scattered electron detector,  high vacuum in the SEM chamber and e-beam acceleration voltage of 30kV. Fig. 2. presents several images of the growth of NaCl crystals obtained from a supersaturated solution. The image resolution is good enough to see details of a size of 50 nm. The liquid system that is inside the stage remained isolated from the SEM chamber during the complete experiment.

The CelDi project aims at the development of the second generation of this tool, integrating more safety protections, increasing the resolution of the images, finding solutions to achieve an easy and friendly use of the device. It will be possible to integrate a specific and fast BSE detector in combination with the stage, and work is under progress to develop automatized image processing through dedicated software.

In parallel, several tests will be carried out for different scientific applications in material science (corrosion) and life science (observation of live cells) to demonstrate the capabilities of the CelDi device.

[1]. R. Podor, S. Szenknect, H. P. Brau, J. Ravaux, J. Salacroup. Cellule de suivi de réaction solide-liquide ou solide-gaz pour microscope électronique à balayage. Patent n° FR 15 59465 (05/10/2015)


Johan SALACROUP, Gautier GONNET, Antoine CANDEIAS, Henri-Pierre BRAU, Stéphanie SZENKNECT, Paul IVALDI, Renaud PODOR (ICSM, Marcoule)
08:00 - 18:15 #5841 - IM02-194 In situ investigation of high temperature corrosion of Co-based alloys in the ESEM – the very first stages.
IM02-194 In situ investigation of high temperature corrosion of Co-based alloys in the ESEM – the very first stages.

In 2006 Sato et al. [1] discovered the existence of a ternary γ′-Co3 (Al, W) intermetallic phase with L12 structure in the system Co-Al-W, a structure similar to that of Ni-base superalloys. The high melting point of Co-alloys makes this class of materials a promising alternative for high-temperature applications. Nevertheless, their poor corrosion resistance remains a challenging and still open problem.

 

In the environmental scanning electron microscope the start of oxidation / corrosion processes and the progress of scale formation can be continuously monitored at high magnification. This enables a rather easy determination of the temperature where oxidation starts. Corrosion in hot steam was realized by use of water vapor as reaction gas. Besides the usage of different other gases, such as air, the temperature ramp can be varied as well. With respect to the diffusion velocities of the different elements in the alloy as a function of temperature the latter could be a critical factor. Nevertheless, the relative humidity / oxygen activity that can be used during in situ experiments cannot exceed certain limits, since otherwise the signal/noise ratio strongly decreases, which causes likewise a deterioration of the image quality.

 

For the investigations Co-based alloys with a nominal composition of Co-9Al-9W were used. To get a stress-free surface, mechanical polishing with diamond paste was followed by polishing with colloidal silica. Finally OIM (orientation imaging microscopy) maps were recorded to get information about the crystal orientation of the grains. The high-temperature oxidation experiments were performed by use of a heating stage mounted in the specimen chamber of an environmental scanning electron microscope ESEM Quanta 600 FEG (FEI, Eindhoven, The Netherlands). The experiments were carried out at a pressure of 133 Pa, which corresponds to a relative humidity of approximately 5% at room temperature (24 °C). Temperature ramps of 2 °C/min and 20 °C/min were used; the maximum temperature was around 800 °C.

 

 All results reveal that at the start of scale formation lattice diffusion and not grain boundary diffusion dominates. Fig. 1 shows that oxidation starts with the evolution of nodules scattered across the grains. Both, the onset temperatures of nodule growth, as well as the density of nodules per unit area are dependent on grain orientation. Fig. 1 also demonstrates that some of the grain boundaries are completely free of such corrosion structures (see arrow in the image), whereas others are nicely decorated by them. However, a clear correlation between scale formation and grain orientation or grain boundary structure could not be found. In case of rough surfaces an orientation dependence could no longer be observed, roughness dominated the oxidation behavior. It became also apparent, that oxidation started earlier at the slower temperature ramp.

 

To effectively slow down oxidation, the formation of a dense, protective oxide layer like Al2O3 would be necessary. The formation of such a dense looking alumina layer could be found occasionally, but only at individual grains [2]. Also the parameters governing the formation of such layers remain still unknown.

 

References

[1] Sato, J., Omori, T., Oikawa, K., Ohnuma, I., Kainuma, R., Ishida, K. (2006), Cobalt-base high-temperature alloys, Science 312, 90-91

[2] Weiser, M., Reichmann, A., Albu, M., Virtanen, S., Poelt P. (2015), In situ investigation of the oxidation of Cobalt-base superalloys in the environmental scanning electron microscope, Adv. Eng. Mat. 17 (8), 1158-1167


Angelika REICHMANN, Martin WEISER, Sannakaisa VIRTANEN, Peter POELT (Graz, Austria)
08:00 - 18:15 #5843 - IM02-196 In-situ thermal measurements with high spatial resolution in the TEM.
IM02-196 In-situ thermal measurements with high spatial resolution in the TEM.

        With the constant miniaturization of electronics, the thermal management issue is becoming the main limiting factor in dictating device performance [1]. Therefore, the study of thermodynamics is of practical interest as well as fundamental scientific interest. However, measuring temperature at these dimensions is difficult due to the increasing influence of the employed measurement tools and therefore new techniques need to be developed. One such technique, Electron Thermal Microscopy (EThM), has been previously used to study heat dissipation in nanowires and carbon nanotubes supported on SiN substrate [3, 4].

        EThM is an in-situ thermal imaging technique using the Transmission Electron Microscope (TEM) and relies on the observation of the solid to liquid phase transition of indium islands and provides a binary temperature map with 50nm spatial resolution[2]. The indium islands, thermally evaporated on the back of the substrate, as seen in the bright-field TEM image in Figure 1a, melt once heated to 429K. This solid to liquid phase transition is easily observed in the TEM, when operating in the appropriate dark-field conditions (Figure 1b), at which the molten islands appear bright compared to the solid ones. In addition, the presence of a high melting point thin oxide layer on the indium preserves the structure of the islands and allows the thermometry technique to be used repeatedly over a large experimental range.

        Two factors contribute to the observed temperature of the system; the heat source and the efficiency of the heat transfer mechanism to the lower temperature reservoir. Here we present the work on joule heated Pd nanowires supported on SiN substrate and use the EThM technique to evaluate the thermal properties of the device. The measured temperature and the efficiency of the heat transfer mechanism can be quantified in terms of the thermal conductivity of the different materials within the device and their thermal boundary resistances. Conventionally, the thermal transport in dielectrics, such as SiN, is phonon mediated. However, as the dimensions approach the mean free path of the phonons, new modasses of heat dissipation may dominate. Combining our experimental thermal measurements (Figure 1c) with simulations (Figure 1d), based on finite element analysis, we have explored different modes of thermal transport and show that the conventional phonon mediated thermal transport is not sufficient to explain the observed temperature gradient across the SiN, indicating that an additional mode is active.   

1.         Pop, E., Energy Dissipation and Transport in Nanoscale Devices. Nano Research, 2010. 3(3): p. 147-169.

2.         Brintlinger, T., et al., Electron thermal microscopy. Nano Letters, 2008. 8(2): p. 582-585.

3.         Baloch, K.H., et al., Remote Joule heating by a carbon nanotube. Nature Nanotechnology, 2012. 7(5): p. 315-318.

4.         Baloch, K.H., N. Voskanian, and J. Cumings, Controlling the thermal contact resistance of a carbon nanotube heat spreader. Applied Physics Letters, 2010. 97(6).


Norvik VOSKANIAN (Göteborg, Sweden), John CUMINGS, Eva OLSSON
08:00 - 18:15 #5846 - IM02-198 A simple shortcut for observing unroofed cells by either TEM or SEM.
IM02-198 A simple shortcut for observing unroofed cells by either TEM or SEM.

The “unroofing” technique has been successfully used to observe the cytoplasmic side of the plasma membrane (PM) using either light or electron microscopy. Combined to transmission electron microscopy (TEM), it is an invaluable method to reveal the composition of the PM and to directly observe macromolecular complexes including the cytoskeleton and endocytic membrane invaginations. This method has been optimized over decades to preserve membranes close to their native states by the combination of quick freezing of exposed membranes, followed by deep etching and rotary replication (the so-called “QF-DE-RR” technique). However, a serious setback in implementing unroofing combined with QF-DE-RR stems from the necessity to use complicated apparatus, such as quick freezing and freeze-fracturing devices, along with strong expertise to handle them. Moreover, the technical complexity renders these techniques time consuming and reduces the number of samples that can be processed simultaneously.

Here, we present a simple and straightforward protocol for observation of the cytoplasmic side of plasma membrane which only requires chemical treatment of samples prior to replication This method has been optimized towards sample preparation at room temperature, chemical fixation, dehydration, solvent drying and sequential metal coating. Moreover, this technique is easily amenable to higher throughput. We compared either TEM or high resolution SEM analysis of unroofed membranes from adherent cells and show the advantages and disadvantages of each technique towards visualization of the cytoskeleton and different endocytic structures such as clathrin coated pits and caveolae.


Agathe FRANCK, Jeanne LAINÉ, Marc BITOUN, Ghislaine FRÉBOURG, Michaël TRICHET (Paris), Stéphane VASSILOPOULOS
08:00 - 18:15 #5896 - IM02-202 In situ TEM nanocompression of MgO nanocubes and mechanical analysis.
IM02-202 In situ TEM nanocompression of MgO nanocubes and mechanical analysis.

In this study, we propose an innovative mechanical observation protocol of nanoparticles in the 100 nm size range. It consists of in situTEM nano-compression tests of isolated nanoparticles. Load–real displacements curves, obtained by Digital Image Correlation, TEM images (BF, DF and WBDF) are analyzed and these analyses are correlated with Molecular Dynamics simulations. Elementary process that governs the deformation mechanism of nanoparticles can be identified. A constitutive law with the mechanical parameters (Young modulus, Yield stress...) of the studied material at the nano-scale can be obtained.

In situ TEM nano-compression tests were performed on ceramic MgO nanocubes. Magnesium oxide is a model material and its plasticity is very well known at bulk. The MgO nanocubes show large plastic deformation, more than 50% of plastic strain without any fracture. Calculations of Schmid factors of possible slip systems in MgO under solicitation direction coupled with analysis of WBDF images, performed in situ in TEM nanocompression tests, contribute to full characterizations for dislocations in MgO nanocubes under uniaxial compression. Correlation of TEM images and stress-strain curves, obtained by DIC, allows the observation and description of dislocations activities and processes along the compression test. Coupling these analyses with MD simulations, the elementary process that governs the deformation mechanism of single crystal MgO nanocubes under uniaxial compression could be identified. In Figure 1, contrast appears in the cube when a change on the curve is observed. This contrast band may be attributed to a ½ dislocation that nucleate at surface and slip along {110} plan as obtained by MD calculations and by TEM analysis on possible dislocations in active slip systems near the diffraction condition in these TEM observations (as we are always near [001] zone axis) as shown in Figure 2.

Size-effect on dislocation processes could be obtained in MD simulations and in experiments. MD results show that in MgO nanocubes smaller than 8 nm, the deformation occurs through dislocation nucleation at surfaces and edges/corners and dislocation starvation process is observed simultaneously with stress drop, as shown in Figure 3 (snaps 1, 2 & 3). However larger nanocubes show dislocation interactions and junctions formation rather than dislocation starvation as shown in Figure 3 (snaps 4 & 5). Experimental results show that these two processes co-exist in MgO nanocubes in the size range [60-450] nm. However, TEM images and stress-strain curves show that there is predominance of dislocation starvation mechanism in smaller nanocubes (Figure 4 show a WBDF of a large nanocube after compression where persistent dislocations and dislocations networks assume that dislocation interactions process predominate in larger nanocubes rather dislocation starvation. 

 

The authors thank the Centre LYonnais de Microscopie (CLYM) for financial support and access to the JEOL 2010F microscope. Financial support from the Région Rhône-Alpes is also acknowledged.

 

Keywords: In situ TEM, plastic deformation, dislocations, ceramic nanoparticles, MgO nanocubes


Inas ISSA, Jonathan AMODEO, Lucile JOLY-POTTUZ (MATEIS / INSA, Lyon), Julien RÉTHORÉ, Claude ESNOUF, Vincent GARNIER, Julien MORTHOMAS, Karine MASENELLI-VARLOT
08:00 - 18:15 #5922 - IM02-204 In Situ TEM Characterization of Asphaltene Formation in Crude Oil.
IM02-204 In Situ TEM Characterization of Asphaltene Formation in Crude Oil.

Asphaltenes are aromatic hydrocarbons and defined as a solubility class as the n-heptane-insoluble, toluene-soluble fraction of a crude oil or carbonaceous material. They are always present in crude oils and influence the oil properties. Phase changes, viscosity, and interfacial properties of crude oils are strongly affected by asphaltenes. Problems arise when asphaltenes are exposed to changes in temperatures, pressure, or composition, and they become insoluble in the oil. When asphaltenes precipitate, they can deposit onto the walls of the pipe, inhibiting the flow of oil and can end up blocking the pipe entirely. Although, the negative impact of asphaltenes to the oil industries is well known, however, the exact mechanism by which asphaltene flocculation and aggregation occurs is still not fully understood.   

         Over the last decade methods have been developed to characterize and model the mechanisms of asphaltene flocculation, aggregation and precipitation. [1, and references listed therein].  To date, there have been TEM analyses of asphaltenes that have impacted petrochemical research activities [2].  However, the disadvantage is that the asphaltene sample may be altered as a consequence of sample preparation.  With the development of commercially available liquid cell holders for in situ TEM there is now the opportunity of direct observations of the oil emulsion system at the nm scale in their natural environment.

         Initial in situ TEM experiments of asphaltene formation and aggregation were conducted in a FEI Talos F200X TEM operated at 200 keV using the Protochips Poseidon P210 analytical liquid cell holder.  A light crude oil with a nominal asphaltene content of 3.7% was mixed with heptane to initiate flocculation of the asphaltenes in the liquid in situ TEM cell. Our first results indicate that the aggregation process is driven by the initial formation of 10-20 nm spherical colloids. These colloids cluster to flocculates in a range of several tens to hundreds of nanometers in the oil-heptane emulsion (Figure 1). The flocculation sequence is in good agreement with the proposed Yen model [1]. Further asphaltene flocculation experiments from different crude oils and their morphology evolution will be compared and discussed. In addition, opportunities and limitations for using in situ liquid cell holders for studying asphaltene flocculation in an analytical TEM will be described.

 

References:

[1] O.C. Mullins, Energy & Fuels, 24, (2010), p. 2179-2207.

[2] L. Goual et al, Langmuir, 30, (2014), p. 5394-5403.

 

Acknowledgement:

The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM), which made this research possible.


Arne JANSSEN (Manchester, United Kingdom), Nestor ZALUZEC, Matthew KULZICK, Greg MCMAHON, M.g. BURKE
08:00 - 18:15 #5932 - IM02-206 In Situ Study of Internal Structure of Spherical Polyelectrolyte Complex Capsules Using ESEM.
IM02-206 In Situ Study of Internal Structure of Spherical Polyelectrolyte Complex Capsules Using ESEM.

Polyelectrolyte complex (PEC) capsules/beads are very important for biotechnological applications such as drug delivery and bacterial whole-cell biocatalyst development. The very beam-sensitive bio-polymer capsules are laboratory produced as a uniform with a controlled shape, size, membrane thickness, permeability and mechanical resistance [1]. PEC capsules are very sensitive to any treatment and samples could be inspected in their fully native and functional state to prevent any misinterpretation. Characterization and study of PEC capsules properties is possible using thermodynamically stabile and fully wet state, precisely reached after very slow changing of conditions in the specimen chamber of ESEM. The morphological study using low current ESEM was already presented [2]. The internal structure can be in solvent, semisolid or solid state, depend on capsule type and manufacturing process [3], nevertheless it was not described in its native state yet. Study of inner part as well as surface morphology of PEC capsules using classical SEM or cryo-SEM can be misleading due to requirement of dry resp. freeze sample. The aim of this work is in-situ study of internal structure of PEC capsules in fully wet state and demonstration of state of matter of PEC capsules core.

PEC capsules has been produced by air-stripping nozzle via polyelectrolyte complexation (20 min) of sodium alginate and cellulose sulphate (CS) as polyanions, poly(methylene-co-guanidine) as a polycation, CaCl2 as a gelling agent and NaCl as an antigelling agent [1] without the use of a multiloop reactor. Due to the high beam sensitivity of samples and its relatively big size (800 μm in diameter), a combination of our published method [4] and special improvement of our ionization detector of SE were used. The gentle and slow sample chamber pumping procedure [4] and our ionization detector of SEs [2] (beam current up to 40 pA) enhanced for larger field of view (850 μm) were combined Samples were observed in conditions of vapor pressure 684 Pa, stage temperature 2°C, humidity 97%, acc. voltage 20 kV and probe current 35 pA.

Fully wet and well preserved PEC capsule with visible surface microstructure is presented in Fig. 1A. PEC capsules are very sensitive to beam impact which was used to in-situ disruption of outer shell. Afterwards the liquid core slowly rose by capillary action on the PEC capsules wall simultaneously with capsule collapsing due to its emptying, see Fig. 1B. Due to different temperatures between the sample and the Peltier cooling stage, the liquid core was dried and crystalized on the PEC capsule surface, see Fig. 1C. First results provide promising information leading to statement that the inner structure of this type of PEC capsules is viscous liquid.

 

[1] A Schenkmayerová et al., Applied Biochemistry and Biotechnology 174 (5) (2014), p. 1834.

[2] V Neděla, et al., Nuclear Instrumentation and Methodology A 645 (2011), p. 79.

[3] Q-X Wu et al., Mar. Drugs 12 (2014), p. 6236.

[4] E Tihlaříková, V Neděla and M Shiojiri, Microscopy and Microanalysis 19 (2013), p. 914.

This work was supported by the Grant Agency of the Czech Republic: grant No. GA 14-22777S and LO1212 together with the European Commission (ALISI No. CZ.1.05/2.1.00/01.0017).


Vilem NEDELA (Brno, Czech Republic), Marek BUCKO, Eva TIHLARIKOVA, Tomas KRAJCOVIC, Peter GEMEINER, Eva NAVRATILOVA, Jiri HUDEC
08:00 - 18:15 #5955 - IM02-208 Using the Deben Enhanced Coolstage for in-situ (E)SEM freeze-drying & high resolution imaging of polymer latices.
IM02-208 Using the Deben Enhanced Coolstage for in-situ (E)SEM freeze-drying & high resolution imaging of polymer latices.

A ‘simple’ methodology, combining the use of Environmental Scanning Electron Microscopy (ESEM) and the recently introduced DEBEN Enhanced Coolstage was successfully developed and not only used to study dynamic processes, e.g. different stages of latex film formation, but also for high resolution imaging of ‘freeze-dried’ structures. By using the extended temperature capability of the DEBEN Enhanced Coolstage (-50 to +160oC) it is possible to easily convert any (E)SEM chamber into what essentially can be described as a freeze-drying facility. By using this method it is also possible to preserve the structure and features of the studied system with minimum shrinkage and distortion and in the case of polymer latices at a desired stage of film formation. Moreover, specimens can then be readily imaged, without the need of conductive coatings and at much lower chamber gas pressures, thus minimising the beam skirting effects and allowing higher resolutions to be achieved. In this study this is clearly demonstrated (Figure 1 & 2) using a model poly-methyl methacrylate based latex dispersion; under ‘wet’ (partially dehydrating) conditions, whilst the individual particles can be seen it is difficult to distinguish them and any associated boundaries and/or arrangements, whether cubic or hexagonal; better images, as shown can be obtained from air-dried specimens, but this limits the time-frame of possible observations. However, subsequent freeze drying, as expected, resulted in the observation of a well-defined and more stable (in imaging terms) structure; it was also possible to image individual particles and their interactions at much higher resolutions. It is strongly believed that the methodology can be applied to other material systems, including biologicals and pharmaceuticals. 


Marzena TKACZYK (Oxford, United Kingdom), Kalin DRAGNEVSKI, Gary EDWARDS
08:00 - 18:15 #5962 - IM02-210 In-liquid TEM to visualize multimerization and self-assembly of DNA functionalized gold nanoparticles.
IM02-210 In-liquid TEM to visualize multimerization and self-assembly of DNA functionalized gold nanoparticles.

Base-pairing stability in DNA-gold nanoparticle (DNA-AuNP) multimers along with their dynamics under different electron beam intensities was investigated with in-liquid transmission electron microscopy (in-liquid TEM) using custom developed silicon nitride based liquid cells. Multimer formation was triggered by hybridization of DNA oligonucleotides to another DNA strand (Hyb-DNA) related to the concept of DNA origami. We analyzed the degree of multimer formation for a number of samples and a series of control samples to determine the specificity of the multimerization during the TEM imaging. DNA-AuNPs with Hyb-DNA showed an interactive motion and assembly into 1D structures once the electron beam intensity exceeds a threshold value. These findings indicate that DNA base pairing interactions are the driving force for in situ multimerization and DNA-metallic NP conjugates provide excellent models to understand structure-function correlation in biological systems with nanometer spatial resolution (Keskin et al., 2015, 10.1021/acs.jpclett.5b02075).

Acknowledgements:

This work was funded by the Max Planck Society and supported by the cluster of excellence “The Hamburg Centre of Ultrafast Imaging” (CUI). We thank, in particular, Josef Gonschior for the design of the liquid specimen holder. Furthermore, we thank the Centre for Applied Nanotechnologies (CAN), Hamburg, Germany (in particular, Katja Werner and Christian Supej) for generously providing the gold nanoparticles and technical assistance in coupling.


Sercan KESKIN (Hamburg, Germany), Stephanie BESZTEJAN, Guenther KASSIER, Stephanie MANZ, Robert BUECKER, Svenja RIEKEBERG, Hoc Khiem TRIEU, Andrea RENTMEISTER, Dwayne MILLER
08:00 - 18:15 #6007 - IM02-212 Structural evolution of strontium titanate nanocuboids under in-situ electron irradiation and heating.
IM02-212 Structural evolution of strontium titanate nanocuboids under in-situ electron irradiation and heating.

   Annealing thermal treatments are routinely used in the synthesis of nanoparticles to tailor their size and shape. To control particle growth at elevated temperatures, understanding the dynamics behind surface evolution is of primary importance. Time-resolved, in-situ, aberration-corrected high-resolution transmission electron microscopy (HRTEM) has been successfully used to image structural modifications of nanoparticles in response to thermal annealing, including, for example, surface faceting and sintering [1].

   This study reports the structural evolution of SrTiO3 nanocuboids [2] in response to thermal annealing at high temperature (≥ 500 °C) using HRTEM imaging. In-situ experiments were performed using a dedicated heating holder, in a JEOL 2200MCO microscope, operating at 200 keV, under low (4 x 106 e/nm2) and high (1010 e/nm2) electron dose conditions. Imaging at low electron doses reveals structural modifications to the nanoparticles that can be ascribed to heating only. At low dose, the effect of beam irradiation on the surface structure is negligible even for direct exposure times longer than 30 min. An example is illustrated in Figure 1, where a typical flat {001} facet remains unchanged after 2 min of direct beam exposure. By comparison, electron irradiation at high electron dose triggers the growth of TiO islands within a few seconds, consistent with previous observations by Lin and co-workers [3]. Figure 2 illustrates the formation of TiO islands after 3 s exposure at high electron dose (a), and the subsequent sputtering of surface atoms after 1 h 20 min of direct irradiation (b).

   Following in-situ thermal treatment at 800 C, surface faceting is observed at low dose (arrows in figure 3 (b)). The formation of the new facets is triggered by diffusion of the surface atoms, driven by the elevated temperature. Furthermore, atomic migration induces sintering of the particle (Ostwald ripening). For longer annealing times at higher temperatures, phase transformation of the facets is expected to take place, and TiOx islands will eventually start to grow [4]. In-situ thermal annealing of the particles at higher temperatures is currently under investigation, and the results will be also reported.  

 

References

[1] M. Chi et al., Nat. Comm. 6 (2015) 8925.

[2] Y. Lin et al., Phy. Rev. Lett. 111 (2013) 156101.

[3] Y. Lin et al., Micron 68 (2014) 152 - 157.

[4] S. Bo Lee et al., Ultramic. 104 (2005) 30 – 38.

[5] The authors acknowledge funding from the European Union Seventh Framework Programme under Grant agreement 312483-ESTEEM2, Prof. Laurence Marks, Prof. Kenneth Poeppelmeier and Dr. Yuyuan Lin for kindly providing the specimens. 


Emanuela LIBERTI (Oxford, United Kingdom), Judy KIM, Yuyuan LIN, Angus KIRKLAND
08:00 - 18:15 #6069 - IM02-214 Cathodoluminescence for in situ plasmonic sensing of beam effects.
IM02-214 Cathodoluminescence for in situ plasmonic sensing of beam effects.

In transmission electron microscopy (TEM), various in situ measurement in different environmental conditions, such as high temperature, gas atmosphere and aqueous solution, have become more popular. However, electron beam damage complicates the measurement. The true dynamics, which is to be observed, is no longer distinguishable from the continuously increasing damage caused by electron beam irradiation. To properly extract the real phenomenon, excluding the electron beam effect, it is important to know the electron beam damage quantitatively and consider the possible influences. Quantitative evaluation of the electron beam damage is also necessary to find the best measurement condition, such as beam dose and acceleration. Although the electron beam damage has been estimated either empirically or theoretically, experimental quantitative analysis has not been much performed due to the lack of local measurement methods in such a small scale as well as due to the limited accessibility in the TEM objective lens.

Here in this research, we propose to measure the electron beam damage effect using nanoplasmonic sensors. In plasmonic sensing the optical properties of metal nanostructures are utilized to sense, locally, changes occurring at the nanoscale either to the metal nanostructure itself or to its surrounding environment. We apply these nanosensors to monitor the electron beam induced environmental change and quantitatively evaluate the electron beam effect. We take advantage of cathodoluminescence technique to simultaneously measure the plasmonic response while the electron beam is irradiated on the sample.

One structure we introduce to measure the environmental change is a nanosized water container that we call the nanocuvette. The structure consists of a plasmonically active nanohole gold film caped by thin carbon films, see Figure 1. This structure allows for the inclusion of the system of interest into the nanoholes which are then sealed by the carbon layers. The system can successively be studied inside the TEM and, because of the plasmonically active gold film, it is also possible to detect changes happening to the specimen contained in the hole by following the plasmonic signal. Inside the TEM this would be achieved by cathodoluminescence. Accelerated electrons excite plasmons through transition radiation and the light radiation by the plasmon resonance can be simultaneously detected. In the work presented here we show the production of the proposed structure, and verify its plasmonic properties through ex situ measurements, combined with modelling. In particular we study the possibility to use the structures to optically detect temperature changes to the sample. This is, in this first step, done ex situ by heating a sensor structure inside a vacuum cell. The plasmonic response upon this heat treatment is studied by recording the optical transmission of the sample. We find that it is indeed possible to detect temperature changes to the sensor structure by studying its plasmon resonances. 

Another structure studied is a plasmonic nanoparticle. Compared to nanopore/hole structures, which are based on continuous metal films with high thermal conductivity, the temperature increase by electron beam irradiation should be more localized inside the particle. The cathodoluminescence signals of some gold particles are shown in Figure 2. As is evident from the figure, the signal vary greatly from particle to particle. It is therefore necessary to tune the particle size and structure in order to achieve enough sensitivity and signal to noise.


Carl WADELL (Yokohama, Japan), Satoshi INAGAKI, Hiroki OHNISHI, Takumi SANNOMIYA
08:00 - 18:15 #6180 - IM02-216 Stability and reactivity of anisotropic cobalt nanostructures under inert and reactive environments investigated by in-situ TEM.
IM02-216 Stability and reactivity of anisotropic cobalt nanostructures under inert and reactive environments investigated by in-situ TEM.

The use of Environmental TEM (ETEM) for investigating the materials evolution in terms of morphological, microstructural and chemical characteristics is of absolute need in catalysis. Owing to the high pressures and temperatures reached within the sealed environmental cells (E-cell), they are suitable to mimic reaction conditions similar to the ones encountered in practice. In addition, the set-up of a mass spectrometer at the cell exit would allow for evaluation of the reaction products and therefore the development of the “operando” methodology. This is crucial for understanding the relationship between the catalysts characteristics and their properties during its activation/operation and for accessing the mechanisms involved in its deactivation process.

The present work reports on the thermal stability and the reduction/oxidation behaviour of nanostructured metallic cobalt-based structures with “urchin-like” morphology  synthesized by reduction of a cobalt complex in the presence of ligands.1 These Co structures with high metallic surface area are foreseen as active phase for the Fischer-Tropsch synthesis, in which syngas, a mixture of CO and H2, is converted into hydrocarbons and water.2 The key-question of catalysts stability under reaction is addressed in this study by using the ETEM approach. The Co structures are submitted to different atmospheres and temperatures in an effort to gather a complete knowledge of the system stability under reaction. To this end, the system is exposed to thermal constraints under vacuum/inert atmosphere, pure hydrogen and oxygen followed by hydrogen atmosphere.

Under vacuum (Figure 1) and argon, the cobalt from the “urchin” branches migrates towards the center with increasing temperature in a direction dictated by the shape of the radial needle-like features. The Co migration is accompanied by the dissolution and subsequent rejection of the carbon atoms from the ligands on the metal surface, mechanism similar to the growth of carbon nanotubes (CNTs), but this time from an organic C-rich precursor. Moreover, in the high temperature range, i.e. 900°C, the ligands already converted in carbon become graphitic leading to the formation of tubular structures with graphitic walls radially disposed around the Co center. This richness of this finding relies not only on the thermal stability of Co-based urchin-like structures, but opens a new perspective for the synthesis of graphitic structures with well-defined tubular shapes.

Under pure hydrogen flow, the diffusion of metal atoms occurs up to 400°C as the ligands are converted into methane in this temperature range, reaction catalyzed by the Co. The “urchin”-like microstructure collapse during the gradual temperature increment from 280°C up to 400°C (Figure 2). This morphological instability can be seen as first evidence of such nanostructures inefficiency in reactions developed at more than 350°C.

In the presence of oxygen, the ligands decompose into CO and CO2 and the metal oxidizes according to the Kirkendall mechanism.3 As effect, part of the Co migrates from the branches centers to the rims, recombines with the oxygen and generates tubular structures connected to the Co-rich core and arranged in the initial configuration. The question arising is whether this structure stays stable under reactive conditions. In this sense, the oxidized system is submitted to an H2 flow (figure 3c-d) and progressive temperature increment. At 200°C, the Co oxide from the outer surface of “spines” reduces and migrates towards the tubes centers, leading after few hours to the formation of a metallic Co nanowire encapsulated surrounded by voids but encapsulated in a shell constituted in both Co and carbon. Obviously the mechanisms of Co oxide reduction into metallic Co upon H2 treatment are different from the initial system to the one submitted to prior oxidation, but the latter solution is suitable to conserve the morphology of such complex system up to 700°C. From a fundamental perspective, this investigation identifies the impact of specifically reactive environments on the thermally activated diffusion and stability of nanosystems with complex geometries. From a more general perspective, this investigation explores the potential of using the E-cell under a TEM to address complex dynamic thematic such as the mechanisms of metals reduction and oxidation under well-defined/controlled conditions.

1.             Liakakos, N. et al. J. Am. Chem. Soc. 134, 17922–17931 (2012).

2.             Andrei Y. Khodakov et al., Chem. Rev. 107, 1692−1744  (2007).

3.             Wang, W., Dahl, M. & Yin, Chem. Mater. 25, 1179–1189 (2013).


K. DEMBÉLÉ (Illkirch), S. MOLDOVAN, J. HARMEL, K. SOULANTICA, P. SERP, B. CHAUDRET, A-S GAY, S. MAURY, A. BERLIET, O. ERSEN
08:00 - 18:15 #6262 - IM02-218 Development of a novel straining holder for TEM compatible with electron tomography.
IM02-218 Development of a novel straining holder for TEM compatible with electron tomography.

   Electron tomography (ET) has been introduced in materials science in the past decade and it has opened a new prospect: the technique can retrieve three-dimensional (3D) structural information [1]. However, a major roadblock exists to combine the in-situ experiments with electron tomography, which is expected to reveal real time 3D structural changes [2]. Towards dynamic 3D (i.e., “4D-ET”) visualization of material’s microstructures under various straining conditions with a time scale of a few minutes or less, we designed and developed a new specimen holder compatible with tensile test and high-angle tilting, termed as “straining and tomography (SATO)” holder [3].

   Figure 1 shows a schematic illustration of a newly designed specimen holder (a) and a cartridge-type blade on which a specimen is glued (b). The area of gluing is marked by gray. The basic concept of this development is a single tilt-axis holder with a tensile mechanism and also being capable of electron tomography. To achieve straining and high-angle tilting simultaneously, we developed a novel mechanism as shown in Fig. 1(a). A linear motion actuator deforms a newly designed cartridge-type blade on which a specimen is glued. Deformation velocity of the blade is designed as 1/3 of that of the actuator. Figure 1(b) explains the motion of blade. The trajectory (dotted line) is an arc but the radius of curvature (R) is so large (3 mm) that the tensile axis is perpendicular to the holder for a nanometer- scale object whose center is located at O.

   Figure 2 shows (a) an appearance of the developed specimen holder and (b) a magnified photo of the cartridge-type blade. The holder motion is fully computer-controlled via graphical user interface developed for this system. We measured the deformation velocity of the blade and deduced the strain rate. The minimum and the maximum values obtained were 1.5×10-6 and 5.2×10-3 s-1. The blade as well as the holder is robust and multiple acquisitions raise no technical problem at all. This result demonstrates stability and reliability of the holder as a novel in-situ experimental instrument for 4D-ET. We also confirmed that the maximum tilt angle of the specimen holder reaches ±60o with a rectangular shape aluminum specimen.

   Figure 3 shows an example of in-situ tensile test using the newly developed holder. The material is an Al-Mg-Si alloy with a conventional 3 mm diameter disk shape prepared by electropolishing (Fig. 3(a)). When the actuator moved 9.87 μm from the initial position, slip bands were suddenly introduced (Fig. 3(b)). With increasing the tensile stress, slip bands were discontinuously but incrementally introduced in several parts of the field of view until a crack was introduced elsewhere. It should be mentioned that the drift of a field of view was negligible throughout the in-situ tensile experiment. The new specimen holder will have wide range potential applications in materials science.

 

References

[1] S. Hata, H. Miyazaki, S. Miyazaki et al., Ultramicrosc. 111, 1168 (2011).

[2] J. Kacher, G. S. Liu, and I. M. Robertson, Micron 43, 1099 (2012).

[3] K. Sato, H. Miyazaki, T. Gondo, S. Miyazaki, M. Murayama and S. Hata, Microscopy 64, 369 (2015).

[4] This study was supported by the Grant-in-Aid for Scientific Research on Innovative Area, "Bulk Nanostructured Metals" (Grant No. 25102703) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. K. S. and S. H. acknowledge the financial support from the Japan Science Technology Agency (JST) “Development of systems and technology for advanced measurement and analysis” program.


Kazuhisa SATO (Ibaraki, Japan), Hiroya MIYAZAKI, Takashi GONDO, Shinsuke MIYAZAKI, Mitsuhiro MURAYAMA, Satoshi HATA
08:00 - 18:15 #6300 - IM02-220 In situ mechanical quenching of nanoscale silica spheres in the TEM.
IM02-220 In situ mechanical quenching of nanoscale silica spheres in the TEM.

The physical and mechanical properties of glasses strongly depend on their bonding configuration and topology, which includes near, intermediate and long range order [1]. It is well-known that controlled application of mechanical load during cooling of glass melts can lead to topologically modified network structures [2,3]. Also uniaxial compression experiments can be used to introduce structural anisotropy into various glasses [4,5]. Moreover, moderate electron beam (e-beam) irradiation in the transmission electron microscope (TEM) [6,7] and scanning electron microscope (SEM) [8] can be exploited to induce enormous ductility in nanoscale silica spheres under mechanical load. However, still the question remains whether e-beam irradiation in combination with compression can lead to anisotropic glasses and how this affects their mechanical properties.

Here we present a novel approach to perform athermal mechanical quenching experiments in the TEM and evidence its impact on mechanical properties of nanoscale silica spheres [9]. Nanoscale silica spheres are compressed in the TEM under different e-beam conditions and loading scenarios by using the Hysitron PI95 TEM PicoindenterTM (Fig. 1). Prior to compression the silica spheres are irradiated with an e-beam current density of 0.09 A/cm2, leading to a shrinkage of 15-18% [7]. In experiment 1 the silica sphere is compressed at beam-off conditions and exhibits an elastic-plastic deformation behavior without fracture [7]. In experiment 2 the silica sphere is quenched under load. To achieve this the compression is started under e-beam irradiation (which we use to mimic temperature) and the e-beam is switched off during compression. The sudden absence of the e-beam quenches-in the modified silica network structure. Surprisingly, starting from the quenching point the slope of the force-displacement curve increases drastically, while a completely elastic loading-unloading behavior is obtained. In case of experiment 3 directly after the beam-on compression a holding segment is used, allowing for relaxation of stresses. During the following deformation at beam-off conditions the silica sphere shows a completely elastic loading-unloading behavior. Interestingly, complementary finite element method simulations reveal that the Young’s moduli (E) of silica spheres are altered: E values of 45 GPa, 38 GPa and 29 GPa are obtained for silica spheres from experiments 1, 2 and 3, respectively. As a direct reason for this observation structural anisotropy is proposed (Fig. 2) [9]. Quenching of silica spheres under load leads to a partially anisotropic silica network, while quenching after relaxation generates an even more anisotropic structure. During the relaxation period the silica sphere is in a compressed and confined state, during which structural re-organization is restricted along the compression direction [9]. This mechanism is further favored by residual tensile stresses acting perpendicular to the loading direction [10,11], which maintains the development of structural anisotropy reported here [9].

[1] L. Wondraczek et al., Adv. Mater. 2011, 23, 4578.
[2] T. Takamori, M. Tomozawa, In: Treatise Mater. Sci. Technol. 1977, 123-152, 152a, 153-155.
[3] R. Brückner, Glas. Berichte Glas. Sci. Technol. 1996, 69, 396.
[4] J. Wu et al., J. Chem. Phys. 2009, 131, 104504.
[5] A. Concustell et al., Scr. Mater. 2011, 64, 1091.
[6] K. Zheng et al., Nat. Commun. 2010, 1, 24.
[7] M. Mačković et al., Acta Mater. 2014, 79, 363.
[8] S. Romeis et al., Rev. Sci. Instrum. 2012, 83, 95105.
[9] M. Mačković et al., submitted.
[10] T.R. Simes et al., J. Strain Anal. 1984, 19, 135.
[11] J.G. Swadener et al., J. Mater. Res. 2001, 16, 2091.
Financial support by the DFG through the SPP1594 “Topological Engineering of Ultra-Strong Glasses”, Cluster of Excellence EXC 315 “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged. We thank R. Klupp-Taylor and M. Hanisch for providing the silica spheres and S. Romeis for valuable discussions.


Mirza MAČKOVIĆ (Erlangen, Germany), Florian NIEKIEL, Lothar WONDRACZEK, Erik BITZEK, Erdmann SPIECKER
08:00 - 18:15 #6309 - IM02-222 In situ compression experiments of fused silica pillars in the TEM and SEM.
IM02-222 In situ compression experiments of fused silica pillars in the TEM and SEM.

While fused silica is known for its brittleness on macroscopic scale [1], it exhibits an amount of plasticity on microscale [2]. Thermally-treated Stöber-Fink-Bohn (SFB)-type silica spheres are known to approach the structure of vitreous silica and show size-dependent mechanical properties [3,4]. Adequate electron-beam (e-beam) irradiation can be used to induce enormous ductility during compression of nanoscale silica spheres [5-7], and to alter their Young’s modulus (E) [7]. While a controlled introduction of structural anisotropy by cooling of glass melts under load [8-10] was shown to enhance the mechanical properties of glass fibers [11], we recently showed that e-beam-assisted quenching under load (turning off the e-beam during compression) inside the transmission electron microscope (TEM) may also lead to structural anisotropy and affects the mechanical properties of nanoscale silica spheres [12]. Here we prove the potential of e-beam-assisted quenching under load on fused silica pillars and further investigate their size-dependent mechanical behavior.

Fused silica pillars are prepared by two different methods from bulk fused silica, namely (1) reactive ion etching (RIE) and (2) focused ion beam (FIB) milling in combination with a charge neutralizer system (FEI Company). Mechanical testing was performed with the Hysitron PI95 TEM PicoindenterTM in the TEM and a custom-built indenter in the scanning electron microscope (SEM) [6]. Both, RIE and FIB milling lead to pillar structures with reproducible geometry and suitable for in situ mechanical experiments in TEM and SEM (Fig. 1). In situ compression of RIE pillars to high strains in the SEM eventually results in fracture with characteristic star-like fracture pattern (Fig. 2). In situ compression experiments at smaller strains carried out on FIB-prepared pillars in the TEM at beam-off conditions reveal a fully elastic deformation behavior, as exemplarily shown in Fig. 3. Thereby, an E = 78 GPa and compressive strength of ≥ 8 GPa are achieved. While E is slightly higher, the compressive strength clearly exceeds the one known for bulk fused silica [1], and the one of microscale fused silica pillars [2]. Further compression experiments on pillars in the TEM and SEM are planned, with the aim to explore their overall size-dependent mechanical behavior in direct relation to our work on nanoscale glass spheres [4,7]. Finally, we expand our recently reported e-beam-assisted quenching under load approach [12] also on fused silica pillars, with the aim to get a generalized picture of the mechanical properties of nanoscale glasses upon quenching under load in the TEM.

 

[1] R.F. Cook, G.M. Pharr, J. Am. Ceram. Soc. 1990, 73, 787.
[2] R. Lacroix et al., Acta Mater. 2012, 60, 5555.
[3] S. Romeis et al., Part. Part. Syst. Charact. 2014, 31, 664.
[4] J. Paul et al., Powder Techn. 2015, 270, 337.
[5] K. Zheng et al., Nat. Commun. 2010, 1, 24.
[6] S. Romeis et al., Rev. Sci. Instrum. 2012, 83, 95105.
[7] M. Mačković et al., Acta Mater. 2014, 79, 363.
[8] T. Takamori, M. Tomozawa, J. Am. Ceram. Soc. 1976, 59, 377.
[9] R. Brückner, Glas. Berichte Glas. Sci. Technol. 1996, 69, 396.
[10] J. Wu et al., J. Chem. Phys. 2009, 131, 104504.
[11] J. Endo et al., J. Am. Ceram. Soc. 2015, 1-5, 1.
[12] M. Mačković et al., submitted.
Financial support by the Deutsche Forschungsgemeinschaft (DFG) through the SPP1594 “Topological Engineering of Ultra-Strong Glasses”, Cluster of Excellence EXC 315 “Engineering of Advanced Materials” and GRK1896 “In situ microscopy with electrons, X-rays and scanning probes” is gratefully acknowledged.


Mirza MAČKOVIĆ (Erlangen, Germany), Thomas PRZYBILLA, Patrick HERRE, Stefan ROMEIS, Jonas PAUL, Etienne BARTHEL, Jeremie TEISSEIRE, Nadine SCHRENKER, Wolfgang PEUKERT, Erdmann SPIECKER
08:00 - 18:15 #8447 - IM02-223b In situ study of the electromechanical behaviour of Ti-Ag coated polymers for bio-sensing applications.
IM02-223b In situ study of the electromechanical behaviour of Ti-Ag coated polymers for bio-sensing applications.

Recent investigations in biosensors showed the advantages in the use of polymeric substrates, coated with conductive and biocompatible thin films such as Ti as bio-interfaces. Furthermore, Ag is widely known for its ductility and excellent electrical behavior. In addition, the bactericide effect of Ag allied to the Ti biocompatibility, have shown very interesting biological, electrical and mechanical properties [1].

In order to correlate the functional response of bio-sensors with the particular structure of Ti-Ag intermetallic phases, a fine TEM-scale investigation is performed. Moreover, the electric signal evolution as a function of the film’s deformation needs to be better understood. This microscale characterization is followed in realtime in situ using a tailored micro-tensile test machine implemented into a SEM chamber. For a local deformation quantification purpose, we proposed an original approach involving the Digital Image Correlation [3].

TiAgx thin films, with different amounts of Ag, were prepared by magnetron sputtering, using a titanium target with Ag pellets placed on its erosion zone. Submicrometer Ti-Ag thin films were deposited on specific 100µm-thick bone-shape polyethylene terephthalate (PET) substrates and on NaCl crystals for preparing the TEM thin foil. The electromechanical behavior of the coated polymers was evaluated under uniaxial stretching using a DEBEN machine.

HR-TEM examination clearly shows the nano-metric structure of films. Three different microstructures can be distinguished, explaining the three different functional response determined by electrical measurements. For different strain values depending on the film’s nature, a cracks network perpendicular to tensile direction appear. Initiation of cracks is strongly influenced by the growth microdefects.

 

[1] Lopes C, Gonçalves C, Pedrosa P, Macedo F, Alves E, Barradas NP, et al. TiAgx thin films for lower limb prosthesis pressure sensors: effect of composition and structural changes on the electrical and thermal response of the films. Appl Surf Sci 2013; 285: 10-8.

[2] Lopes C., Vieira M., Borges J., Fernandes J., Rodrigues M.S., E. Alves, N.P. Barradas, M. Apreutesei, P. Steyer, C.J. Tavares, L. Cunha, F. Vaz. Multifunctional TieMe (Me = Al, Cu) thin film systems for biomedical sensing devices. Vacuum 122 (2015) 353-359.

[3] Réthoré J, Morestin F, Lafarge L, Valverde P. 3D displacement measurements using a single camera. Optics and Lasers in Engineering, 57 (2014) 20-27


Aurelien ETIEMBLE, Claudia LOPES, Lucian ROIBAN, Beatriz FREITAS, Marco Sampaio RODRIGUES, Julien RETHORE, Filipe VAZ, Philippe STEYER (VILLEURBANNE CEDEX)
08:00 - 18:15 #4499 - IM03-224 THE NANOWORKBENCH: Automated Nanorobotic system inside of Scanning Electron or Focused Ion Beam Microscopes.
IM03-224 THE NANOWORKBENCH: Automated Nanorobotic system inside of Scanning Electron or Focused Ion Beam Microscopes.

The Nanoworkbench is the first system substituting eye-hand coordination effectively with nano-precision in a SEM/FIB-system. It can be imagined how technology could evolve, when tools within a SEM/FIB can be used as easily as tools used under optical microscopes.

Many every day developments would not exist today without preparation, handling and assembly of materials under optical microscopes. There would be no wristwatch, no in vitro fertilization, no mini-gearbox, just to mention a few. These products depend on using toolsets like tweezers, knives, hooks, probes and several different measurement tools in combination with optical microscopes. But material properties and functionalities also depend on structure dimensions that are smaller than the wavelength of light.
The operators of SEM, FIB or Dual Beam systems generally work without toolsets. One reason for this is the disconnected closed loop operation between human eyes and hands that enable complex operations under optical microscopes without even thinking about it

The two main aspects of the new Nanoworkbech by Klocke Nanotechnik GmbH, the development of its Nanorobotics technology and the applications enabled by it, are described in this paper.

Aspect 1, development of the technology: In general the success of in-SEM/FIB Nanorobotics depends on the co-operation of several important modules in one global system. The main developments include:

  • Nanomanipulators in automation, for movement of end-effectors and sample handling,
  • Different end-effectors for nano- probing, cutting, cleaning, force distance or wear measurements, gripping, sorting or material preparation and processing,
  • Automatic in-situ tip cleaning process
  • Automatic 3D position detection of all tools and SEM/FIB
  • A control of all tool and SEM/FIB sample stage positions in a common global coordinate system,
  • SEM picture assisted haptic interface by “Live Image Positioning”,
  • Modular design for fast configuration & teaching of nano-analytical or nano-handling processes.

With instantiating these technical demands the Nanoworkbench enables secure and easy usage of toolsets within SEM/FIB systems, for manual operation, for non-professional users and in high level of automation, e.g. for high throughput industrial processes, even as job-shop [1].

Aspect 2, development of a series of new applications in one system: Expanding the SEM/FIB to a material processing system and a nano-analytical workbench opens the door to many applications in all fields of research and development up to industrial production [5]. Several examples of these new interdisciplinary research and development fields will be described during the presentation.

A few examples of Nanoworkbench applications are highlighted in Figure 1. Although these examples may raise the impression of a review about different machines and their usage, this is not the case. Described is the development of the Nanoworkbench.

References

[1]   D. Morrant, EIEx Magazine of European Innovation Exchange, 1 (2009)

[2]   G. Schmid, M. Noyong, Colloid Polym Sci., (2008)

[3]   C.-H. Ke1, H.D. Espinosa, Journal of the Mechanics and Physics of solids, 53 (2005)

[4]   Seong Chu Lim, Keun Soo Kim, Kay Hyeok An, Dept. of Phys., Sungkyunkwan University, Korea (2002)

Supported by European Commission, IST and Ziel2.NRW


Ivo BURKART, Eva BURKART, Volker KLOCKE, David PETERS (Aachen, Germany)
08:00 - 18:15 #4770 - IM03-226 3D Fourier transform analysis and Diffractogram analysis to evaluate a high-performance TEM.
IM03-226 3D Fourier transform analysis and Diffractogram analysis to evaluate a high-performance TEM.

The resolution of HRTEM has been improved down to sub-angstrom by correcting the spherical aberration (Cs) of the objective lens, and the information limit is thus determined mainly by partial temporal coherence. Thus, a method to measure the partial temporal coherence becomes important more than ever. Since a traditional Young’s fringe test does not reveal the true information limit for an ultra-high resolution electron microscope, new methods to evaluate the focus spread, and thus temporal coherence have been proposed based on a tilted-beam diffractogram [1,2]. However, in order to observe literally an actual information transfer during the image formation down to a few ten pm, we need the strong scattering amorphous object, which will inevitably introduce pronounced non-linear contribution. Since the diffractogram analysis cannot be applied when the non-linear contribution becomes significant, we have proposed the method based on the three-dimensional (3D) Fourier transform (FT) of through-focus TEM images, and evaluated the performances of some Cs-corrected TEMs at lower-voltages [3,4]. In this report we generalize the 3D FT analysis and derive the 3D transmission cross coefficient (TCC). Then, we compare the 3D FT analysis with the tilted-beam diffractogram analysis (2D FT analysis), and clarify the necessity to use the 3D FT analysis to evaluate a high-performance TEM.

     The Fourier transform of the image intensity with a tilted-beam illumination may be written as Eq. (1) in Figure 1 with the (2D) TCC, where E/t and E/s are the temporal and spatial envelopes, respectively [5]. You may note that the 2D FT of the tilted-beam image intensity depends on z only through the 2D TCC. Thus, the 3D FT of the image stack may be written as Eq. (2) in Figure 1 with the 3D TCC, which is a Fourier transform of the 2D TCC along the z-direction, where E/Ewald is a normalized Gaussian, and may be called Ewald sphere envelope. Here, w/E represents the Ewald sphere, and w-w/E is a distance measured from the Ewald sphere along the w-axis to the spatial frequency g2 on the uv-plane.

     Figure 2 illustrates temporal damping of tilted-beam diffractogram, which is a power spectrum (an intensity of the 2D FT) of the image intensity. Here, we show the temporal damping for low-pass filtered diffractograms [1] and diffractogram envelopes [2]. We have to note that the diffractogram analysis cannot extract linear image information out from the image intensity. Furthermore, we have to make use of a weak scattering approximation, since the diffractogram cannot separate two linear image contributions. Moreover, the tilted-beam diffractogram becomes broad for the case of a small defocus spread as shown in Figure 2 (b). Thus, the diffractogram analysis may have difficulty for a Cc-corrected microscope or a microscope with a monocromator.

     Figure 3 shows an example of the Ewald sphere envelopes. Using the Ewald sphere envelopes we can extract linear image information from the image intensity. Thus, we can evaluate the temporal envelope on the sharp Ewald spheres, even when the temporal envelopes become broad for the case of a small defocus spread as shown in Fig. 2. Another profound difference of the 3D FT analysis from the diffractogram analysis is its capability to evaluate two linear image contributions separately on the Ewald sphere envelopes. Therefore, we can use a thick sample or a sample made from strong scattering elements, even when the dynamical/multiple scattering becomes significant. This is the necessary condition if we want to directly observe the linear image transfer down to a few ten pm. Furthermore, our method using 3D FT of the through-focus images gives a possibility to directly observe the distribution of the focus spread via a Fourier transform of the measured temporal envelope for a high-performance microscope.

References:

[1] J. Barthel, A. Thust, Physical Review Letters 101 (2008) p.200801. 

[2] M. Haider, et al, Micros. Microanal. 16 (2010) p.393. 

[3] K Kimoto, et al, Ultramicroscopy 121 (2012) p.31.  

[4] K Kimoto, et al, Ultramicroscopy 134 (2013) p.86. 

[5] K. Ishizuka, Ultramicroscopy 5 (1980) p.55.  

Acknowledgements

This study was partly supported by the JST Research Acceleration Program and the Nano Platform Program of MEXT, Japan.


Kazuo ISHIZUKA, Koji KIMOTO (Tsukuba, Japan)
08:00 - 18:15 #5026 - IM03-228 Backscattered-electron SEM contrast of SiO2 nanoparticles.
IM03-228 Backscattered-electron SEM contrast of SiO2 nanoparticles.

Scanning electron microscopy (SEM) is frequently used for the characterization of nanoparticles (NPs) and imaging with backscattered electrons (BSEs) is particularly interesting to reveal, e.g., contamination NPs in a NP-ensemble. However, the SEM contrast of samples with complex geometries, compared to flat bulk samples, cannot be quantitatively described by common theoretical models [1]. In this work we will show that a) the BSE SEM contrast of SiO2 NPs on a complex substrate strongly depends on the primary electron energy E0, working distance WD and the used substrate and b) that Monte Carlo (MC) simulations are well suited to model and optimize the NP-contrast.

For this purpose SiO2 NPs with diameters from 50 nm to 110 nm were deposited on two different substrates. The first substrate is interesting for correlative SEM and light microscopy imaging and consists of glass slides coated by electrically conducting indium-tin-oxide (ITO) with 180 nm thickness [2]. The second substrate type consists of amorphous (glassy) carbon, which is covered by only 20 nm ITO. A FEI Quanta 650 FEG equipped with an annular semiconductor BSE detector mounted below the objective pole piece was used. E0 between 3 and 17 keV and WDs between 4 and 12 mm were chosen. MC-simulations were performed with a modified version of NISTMonte program [3] employing screened Rutherford and Mott cross-sections (CSs) for comparison with the measured data. The baseline intensity Iblack was recorded with blanked electron beam. The NP-contrast was calculated by C=(INP-Isub) / (Isub-Iblack), where INP is the NP-intensity and Isub the substrate intensity.

Figs. 1a,b show 5 keV BSE SEM images of SiO2 NPs on the 180 nm ITO/glass substrate taken at WDs of 10 mm (Fig. 1a) and 4 mm (Fig. 1b). Although the same object is imaged, contrast inversion of SiO2 NPs is observed. Fig. 1c shows a 5 keV BSE SEM image (WD = 10 mm) of SiO2 NPs on the 20 nm ITO/carbon substrate where NP-contrast inversion can be observed compared to the 180 nm ITO/glass substrate (Fig. 1a). The images in Fig. 1 indicate that simple interpretation of BSE SEM images in terms of material contrast is not adequate for complex sample structures.

The experimental and simulated NP-contrast is in detail studied by systematically varying the WD for E0 = 5 keV (cf. Fig. 2). While the NP-contrast for the 20 nm ITO/carbon substrate approaches zero with increasing WD, there is a contrast inversion for the 180 nm ITO/glass substrate at WD ~ 6 mm. We attribute this contrast inversion to the anisotropic angular BSE scattering characteristics, whereby the scattering angle range of collected BSEs is controlled by the WD.

The dependence of the NP-contrast on E0 for a constant WD = 10 mm is presented in Fig. 3. Contrast reversal occurs at ~4.5 keV for SiO2 NPs on 20 nm ITO/carbon and at ~10 keV for NPs on 180 nm ITO/glass. The NP-contrast for larger E0 is in general higher on the ITO/carbon substrate due to the small ITO thickness and low BSE intensity from the carbon substrate below. Converging C-values for low E0 indicate a) that the primary electrons do not even penetrate through the 20 nm ITO-layer anymore and b) that contrast inversion for the different substrates is related to the ITO-thickness. Another contrast inversion stands out, if both substrates are compared directly as highlighted in Fig. 3 by a red arrow at 5 keV. Additional MC-simulations are included in Fig. 3 assuming hypothetical substrates with 20 nm ITO on glass (dashed light-blue line) and 180 nm ITO on carbon (dashed purple line). The additional simulations demonstrate that the contrast inversion is also ITO-thickness dependent and not substrate-material dependent, because contrast inversion does not occur for ITO-layers with the same thickness on different substrates. MC-simulations with screened Rutherford CSs describe the NP-contrast well while simulations with Mott CSs (not shown here) show larger deviations from the experimental data.
To summarize, two unexpected effects were observed for BSE SEM contrast of SiO2 NPs: a strong dependence on the used substrate, in our case especially the ITO layer thickness, and a “geometrical” contrast inversion which can be controlled by the WD. Optimum NP contrast is obtained for small E0 and WD-values.

 

References

[1] H. Niedrig, J. Appl. Phys., 53 (1982), pp. R15-R49.

[2] H. Pluk, et al., J. Microsc, 233 (2009), pp. 353–363.

[3] N.W.M Ritchie, Surf. Interface Anal., 37 (2005), pp. 1006–1011.


Thomas KOWOLL (Karlsruhe, Germany), Erich MUELLER, Dagmar GERTHSEN
08:00 - 18:15 #5205 - IM03-230 Site-specific 35-minute TEM-lamella preparation by FIB-SEM.
IM03-230 Site-specific 35-minute TEM-lamella preparation by FIB-SEM.

Sample preparation by DualBeam (FIB-SEM) allows site specific TEM lamella to be prepared and is the number one use case for such instruments worldwide. This technique is quite complicated and the variety of different materials and site specific orientation can complicate the process such that a great deal of knowledge and experience is usually required to achieve top results. Improvements in the robustness of various hardware components combined with technology advances are enabling automation of best known methodologies for lamella preparation such that novice users can obtain consistent results. This results in a shorter time to prepare a lamella and a more consistent quality result.

Automation for different materials presents challenges due to differences in milling rates, hardness, structural differences and intended orientation. With semiconductor materials the similarity of materials makes it possible for fully automated process development due to the consistency of sample types, though often the end-pointing on today’s small structures can be quite demanding. In such cases the final thinning is often still done manually. For materials science, FEI has chosen to segment three phases of the preparation process.  The distinct three routines allow the most flexibility for different sample types: chunk milling, lift-out and thinning to electron transparency. Any of these steps can be run manually if desired or required for a particular application.

Chunk milling is the process of defining the area of interest, laying protective layers of the correct thickness, and making fiducials to be used throughout the process for ion beam placement. Bulk milling is done at relatively high currents to make the process fast and then an undercut and cleanup produces a lift-out ready thick lamella.

Lift-out has been semi-automated with the user identifying the tip of the lift-out probe and the sample or grid edge and the software making the required moves. Attaching and releasing the sample/probe/grid is controlled by user placed patterns for milling or deposition when requested by the guided workflow. The EasyLift manipulator with rotation and high stability is essential. Scripts exist for the rotating EasyLift EX manipulator that allow 90 dedgree or 180 degree lamella attachment to further improve TEM lamella quality. Once attached to the grid, the final script for thinning can be used.

Currently final polishing scripts can be tweaked for hard or soft materials and finishing can be specified to the desired lamella thickness with 5kV surface cleaning. On the newer Helios systems the 0.5kV image looks as good as the 2kV from previous FIB sources and thus the lower currents can very effectively be used for automated final polishing.

The presentation will demonstrate an interactive guided TEM Sample preparation process on the Helios DualBeam. This method shortens the TEM lamella preparation process for expert users and enables novice users to obtain routine, high quality results. The method proposed can be used on almost any material to prepare lamellas from soft and hard materials and examples are shown in Figure 1. Guided TEM Sample preparation is available on the newest FEI DualBeams and can help meet TEM lamella preparation challenges in materials science.


Daniel PHIFER (Eindhoven, The Netherlands), Remco GEURTS
08:00 - 18:15 #5236 - IM03-232 Multicomponent garnet film scintillators for SEM electron detectors.
IM03-232 Multicomponent garnet film scintillators for SEM electron detectors.

With an Everhart-Thornley (ET) scintillation detector in SEM, an image is formed by signal electrons emerged after an interaction of focused scanning electron beam with the specimen surface. In such a case a scintillator plays an important role as a fast electron-photon signal conversion element. A selection of fast scintillation materials is very limited, because the only mechanism for scintillators applicable in SEM ET detectors consists in allowed 5d-4f transitions in lanthanide ions. Unfortunately, the widely used Czochralski grown single crystal YAG:Ce scintillators suffer from an afterglow, which deteriorate the ability to transfer high image contrast. The mentioned afterglow in the bulk single crystal is caused by inevitable structural defects, such as antisite defects. These trap states are responsible not only for delayed radiative recombination causing the afterglow, but also for a degradation of the light yield. The aim of this study is to introduce new multicomponent garnet film scintillators for SEM electron detectors that due to the substitution of Al by Ga in the Gd3Al5O12:Ce garnet extensively supress the shallow traps resulting in a significant increase of the cathodoluminescence (CL) efficiency and in improvement of the afterglow characteristics.

To avoid the defective bulk scintillators, isothermal dipping liquid phase epitaxy was chosen as a method for garnet single crystalline film preparation [1]. The high purity Ce activated GAGG (Gd3Al1.7Ga3.3O12:Ce) film of the thickness of 11 µm, grown on the single crystal YGG (Y3Ga5O12) substrate, was chosen to assess its applicability as the scintillator in the scintillation detector in SEM. Results of Monte Carlo (MC) simulation of electron interaction [2] in the GAGG:Ce film as well as in the YAG:Ce bulk scintillator in different depth of the scintillator are shown in Fig. 1. It is evident from the MC simulation that electron interaction active layers of the garnet scintillators are much thinner than 10 µm for standard signal electron energy. Furthermore, it is seen that the GAGG:Ce scintillators may be even thinner than the YAG:Ce ones. Comparison of optical absorption coefficients of the GAGG:Ce film,  YAG:Ce crystal and YGG substrate is in Fig. 2, and CL emission spectra of these scintillators obtained using the apparatus for the cathodoluminescence study [3] are shown in Fig. 3. The optical self-absorption together with the refractive index and the emission spectra of the scintillators are very important quantities for an assessment of the signal photon transport in the both examined scintillators. Although the GAGG:Ce film exhibits higher optical absorption, it has a higher collection efficiency of signal photons, since the path of photons in this film is much shorter than the path of photons in the bulk YAG:Ce scintillator, which was verified by MC simulation of a light transport [4] in the scintillation detector for SEM. As seen in Fig. 3, the CL efficiency of both scintillators is approximately the same. However, the GAGG:Ce film do not suffer from parasitic UV host emission. Regarding the scintillator-PMT matching, for both scintillators the photocathode S20 should be used. CL decay characteristics of both examined scintillators, measured using the CL apparatus [3], are shown in Fig. 4. The decay time as low as 22 ns and the afterglow of only 0.043 % at 0.5 µs after the end of excitation predetermines the GAGG:Ce film scintillators for extremely fast and efficient electron detectors in SEMs.

Acknowledgement
The research was supported by Czech Science Foundation (GA16-05631S and GA16-15569S), by Technology Agency of the Czech Republic (TE01020118), by Ministry of Education, Youth and Sports of the Czech Republic (LO1212), and by European Commission and Ministry of Education, Youth and Sports of the Czech Republic (CZ.1.05/2.1.00/01.0017).

References

[1] Bok, J.; Lalinský, O.; Hanuš, M.; Onderišinová, Z.; Kelar, J.; Kučera, M.: GAGG:Ce single crystalline films: New perspective scintillators for electron detection in SEM, Ultramicroscopy 163 (2016), 1‑5.

[2] Schauer, P.; Bok, J.: Study of spatial resolution of YAG:Ce cathodoluminescent imaging screens, Nucl. Instr. Meth. B 308 (2013), 68‑73.

[3] Bok, J.; Schauer, P.: LabVIEW-based control and data acquisition system for cathodoluminescence experiments, Rev. Sci. Instrum. 82 (2011), 113109.

[4] Schauer, P. Extended Algorithm for Simulation of Light Transport in Single Crystal Scintillation Detectors for S(T)EM, Scanning, 29 (2007), 249-253.


Petr SCHAUER (Brno, Czech Republic), Ondřej LALINSKÝ, Zuzana LUČENIČOVÁ, Miroslav KUČERA
08:00 - 18:15 #5318 - IM03-234 Spin polarisation with electron Bessel beams?
IM03-234 Spin polarisation with electron Bessel beams?

Despite the statement of Bohr and Pauli that Stern-Gerlach based spin separation for electrons cannot work [1], it has been argued that spin separation or filtering of electrons is possible in particular geometries [2,3]. The argument has been debated, see e.g. [4], and it seems that the effect exists but is too small to be exploited with present day technology. As of now, no Stern-Gerlach design of a spin polarizer for free electrons was successful. On the other hand, an unexpected intrinsic spin-orbit coupling (SOC) in relativistic vortex electrons was discovered, and it was proposed to use this effect to construct a spin filter for free electrons [5]. Recently, it has been shown [6] that crossed electric and magnetic quadrupole fields correspond to so-called q-plates which are used in laser optics for spin-to-orbital moment conversion (STOC).  In combination with electron vortex beams, this opens the possibility to couple the spin of free electrons to the spatial degree of freedom, and so design a spin filter [7]. However, the realisation of such devices is hampered by severe geometric constraints.

Here, we propose a different approach exploiting the magnetic fields created by the lenses already present in conventional TEMs. The vector potential of a round magnetic lens in the TEM has cylindrical symmetry over the propagation axis. This  is equivalent to an optical q-plate. Such a field can be used as a STOC device quite similar to the optics case because the total angular momentum J= L + S is a constant of motion. Thus, it seems that electron microscopes are intrinsic spin polarizers. Basic considerations show that a vortex beam of order one passing a standard magnetic round lens (the objective lens in the present case)  is intrinsically spin polarized. As shown in Fig. 1, the vortex in plane A can be seen as a continuous line of point sources (red dot) on the ring aperture, each of which results in a tilted plane wave in B. Classically, the momentum p of the particle in A is tilted by the Lorentz force to p’ at B (grey arrows). The spin vector (red arrows) performs a precession in the magnetic field when going from A to B. Conservation of the total angular momentum J=L+S creates small contributions of Bessel beams J0 or J2, depending on the original spin polarisation in plane A, which are superimposed onto the dominant J1 beam in plane B.  This spin-to-orbit coupling allows spin filtering because J0 and J have different radial profiles.

In the limit of infinitely small detectors on axis, the spin polarisation tends to 100 %. Increasing the detector size, the polarisation decreases rapidly, dropping below 10-5 for standard settings of medium voltage microscopes. For extremely low voltages, the figure of merit increases by two orders of magnitude, approaching that of existing Mott detectors (Fig. 2).

Our findings may lead to new desings of spin filters, an attractive option in view of its inherent combination with the electron microscope, especially at low voltage.

 

Acknowledgements: The financial support by the Austrian Science Fund (I543-N20 and J3732-N27) and by the European research council (ERC-StG-306447) are gratefully acknowledged.

 

[1] W. Pauli, Collected Scientific Papers,  2 (1964) 544.

[2] H. Batelaan et al., Physical Review Letters 79 (1997) 4517.

[3] B. Garraway, S. Stenholm, Physical Review A 60 (1999) 63.

[4] G. Rutherford, R. Grobe, Journal of Physics A 31 (1998) 9331.

[5] K.Y.Bliokh et al. Physical Review Letters 107 (2011) 174802.

[6] E. Karimi et al. Physical Review Letters 108 (2012) 044801.

[7] V. Grillo et al. New Journal of Physics 15 (2013) 093026.


Peter SCHATTSCHNEIDER (Wien, Austria), Vincenzo GRILLO, Thomas SCHACHINGER, Stefan LÖFFLER
08:00 - 18:15 #5428 - IM03-236 A Variable-Temperature Continuous-Flow Liquid-Helium Cryostat Inside a (Scanning) Transmission Electron Microscope.
IM03-236 A Variable-Temperature Continuous-Flow Liquid-Helium Cryostat Inside a (Scanning) Transmission Electron Microscope.

The progress in (scanning) transmission electron microscopy development had led to an unprecedented knowledge of the microscopic structure of functional materials at the atomic level. Additionally, although not widely used yet, electron holography is capable to map the electric and magnetic potential distributions at the sub-nanometer scale. This opens a route to investigate the phase structures of electronic and magnetic phenomena in condensed matter at a microscopic level. Many of the most interesting solid state phenomena occur at low temperatures only. Nevertheless, low temperature studies inside a (scanning) transmission electron microscope ((S)TEM) are extremely challenging because of the much restricted size and accessibility of the sample space. Up to date, there are no cryo-(S)TEMs or special sample holders that are capable to cool a sample controllably to any but its base temperature below room temperature.

Recently, we introduced a concept for a dedicated in-situ (S)TEM for flexible multi-stimuli experimental setups with the capabilities of holographic recording and scanning electron microscopy type imaging. A central part was a large sample chamber with multiple ports. With a prototype instrument, we demonstrated a maximum resolving power of about 1 nm in conventional imaging mode and substantially better than 5 nm in scanning mode while providing an effectively usable pole piece gap of 70 mm [1].

Here, we report about the state of the first major plug-in fitted into the prototype in-situ (S)TEM: A variable-temperature liquid-helium continuous-flow cryostat for nanometer resolved imaging and diffraction at controlled temperatures between 10 K and 300 K. Arbitrary temperatures in the offered range can be installed and held stable by a heating in the sample mount with the help of a PID controller. The cryostat has two operation modes, one with two cooled radiation shields for temperatures below 10 K and one without the shields for free sample access from outside the cryostat at temperatures down to 20 K. Sample drift due to negative thermal expansion is reduced by a circular cooled sample mount and a flexible copper strand to the cold finger. The design of a continuous flow cryostat with a low consumption rate offers a long working time at low temperature while sucking helium from a 100 l vessel. Additionally, the cryostat offers four cooled terminals for fixed electrical contacts and is prepared for a future incorporation of two mobile electrical probes.

Examples of experiments now possible with this new setup are the mapping of the phase structure of different electronic and magnetic phenomena, like charge density waves and Skyrmions.

Acknowledgements

The authors thank S. Leger for technical assistance.

[1] F Börrnert et al, Ultramicroscopy 151 (2015), p. 31.


Felix BÖRRNERT (Ulm, Germany), Alexander HORST, Michael A. KRZYZOWSKI, Bernd BÜCHNER
08:00 - 18:15 #5696 - IM03-238 Novel Linkage Technology of the Shared Alignment Sample Holder for Same Area Observations with Electron Microscopy and Scanning Probe Microscopy.
IM03-238 Novel Linkage Technology of the Shared Alignment Sample Holder for Same Area Observations with Electron Microscopy and Scanning Probe Microscopy.

    We developed an innovative air protection sample holder enabling a hermeneutically sealed sample transfer from Hitachi’s ion milling instrument to the Field Emission Scanning Electron Microscopes (FE-SEM) and the environment control high-vacuum Scanning Probe Microscope (SPM) AFM5300E for a correlative microscopy (Figure 1). In a previous study, we explained the advantages of this sample holder with regard to the analysis of cathode materials in a lithium-ion battery [1].

 

    Our novel SEM-SPM linkage system with the shared alignment sample holder enables a software-based alignment of the same measurement area for a comprehensive analysis of sample surfaces with Hitachi’s FE-SEM SU8200 Series and the new midsize-sample SPM AFM5500M, characterized by the automation of the cantilever exchange, laser alignment, feedback parameter tuning and data processing (Figure 2). As the XY-stage of both microscopes drives with high accuracy to the desired area by only registering three specified coordinates of the sample stage, this linkage system facilitates a correlative microscopy of samples that are difficult to align optically. In this study, we used this technology to analyse a multilayer graphene on a SiO2 substrate. For an observation of graphene, FE-SEM is one method to explain the relationship between SE contrasts and the thickness of graphene layers. Another method is the Kelvin force microscopy (KFM) explaining the quantitative relationship between surface potentials and topographic heights. Thus, a linkage of both observation methods enables a correlative analysis of SE contrasts, topography and surface potentials.

 

   Figure 3 shows the SE image obtained at a accelerating voltage of 0.5 kV, topography and the KFM image of a multilayer graphene on a SiO2 substrate measured with the linkage system. The grey island structure with two different contrasts and several lines in the SE image are well aligned with the topography and KFM images. Analysing the topography, we confirmed that SE contrast differences result from single graphene step heights. Furthermore, we have learned that the surface potential of a bilayer graphene is 15-20 mV higher than that of a monolayer graphene.

 

    In conclusion, the linkage system is a tool for a comprehensive analysis of a sample’s composition, structure, 3D topography, mechanic and electro-magnetic properties with the SEM and SPM instruments without any constraints in regard to their performances.

 

References:

[1]    T. Yamaoka, et al., The 34th Annual NANO Testing Symposium, 3 (2014), p.13-18.


Ulrich DIESTELHORST (Kawasaki-shi, Japan), Takehiro YAMAOKA, Kazunori ANDO, Yoichiro HASHIMOTO
08:00 - 18:15 #5721 - IM03-240 Electron beam lithography for the realization of electron beam vortices with large topological charge ( L=1000ħ).
IM03-240 Electron beam lithography for the realization of electron beam vortices with large topological charge ( L=1000ħ).

Electron vortex beams (EVBs) are an appealing topic, both in fundamental science and for practical applications in electron microscopy [1, 2]. Some of the most promising applications require beams that have large orbital angular momentum (OAM) [2, 3, 4]. Here, we demonstrate the largest (L=1000 ħ) high quality EVB by using electron beam lithography (EBL) to fabricate a phase hologram. EBL provides superior fabrication quality and a larger number of addressable points when compared with focused ion beam (FIB) milling. We measure the OAM of the generated EVB through propagation after a hard aperture cut [5]. Comparisons with simulations confirm an average OAM of (960±120)ħ , which is consistent  with the intended value.
A clear improvement when compared with a FIB-nanofabricated hologram is demonstrated in terms of 1) the maximum OAM that can be reached; 2) the minimum feature size (33 nm in the present study); 3) the improved uniformity of the frequency response; 4) the better suppression of higher order diffraction due to a nearly perfect rectangular groove profile.
We believe that EBL will be the fabrication technique of choice for most new diffractive optics with electrons in the future, permitting more complex holograms and new applications in material science.

[1] J. Verbeeck, H. Tian  P. Schattschneider Nature 467 (2010) 301
[2] B. J. McMorran,  A. Agrawal  et al. Science 331 (2011) 192
[3] V. Grillo et al .  Phys Rev Lett 114, 034801 (2015)
[4] I. P. Ivanov and D. V. Karlovets, Phys. Rev. A 88, 043840(2013).
[5] P. Schattschneider, T. Schachinger, et al. Nature Comm. 5, 4586 (2014).




Erfan MAFAKHERI, Amir TAVABI, Penghan LU, Roberto BALBONI, Federico VENTURI, Claudia MENOZZI, Gian Carlo GAZZADI, Stefano FRABBONI, Robert BOYD, Rafal DUNIN-BORKOWSKI, Ebrahim KARIMI, Vincenzo GRILLO (Modena, Italy)
08:00 - 18:15 #5737 - IM03-242 Transmission imaging of biological tissue with the Delft multi-beam SEM.
IM03-242 Transmission imaging of biological tissue with the Delft multi-beam SEM.

A major bottleneck for large-scale and volume EM is the imaging speed. The total acquisition time needed for a single sample can easily take days, or even weeks using standard single-beam SEM’s. Multi-beam microscopes have been developed to increase imaging speed[1,2], but it remains a challenge to achieve electron detection similar to a regular SEM in terms of signal type, contrast and resolution.

We have developed a SEM employing 196 electron beams using a standard column of a FEI Nova NanoSEM 200. The 196 electron beams are generated from a single high-brightness Schottky electron source, making use of a square aperture lens array grid of 14 by 14 holes. Modified source optics allows focusing of all beams in the sample plane, with the same probe current and probe size as in a single-beam SEM.  Both secondary and transmission electron signals can be detected in the system, of which an overview is shown in figure 1. For the detection of the transmitted electrons, the sample of interest is placed on a scintillating screen and the light generated by each beam is collected through an optical objective lens.  This light is focused on a CMOS camera placed outside the SEM chamber and the image is produced through online processing of the intensity of each beam. An example of rat pancreas tissue imaged by this method is shown in figure 2.  The secondary electrons are focused on a scintillating screen in the variable aperture plane, making use of a retarding lens and the electron optics used for the focusing of the primary beams.  This signal is again focused on a CMOS camera and the same process for imaging is performed as for the transmitted electrons.

We present proof-of-principle results showing that sub-10nm resolution can be obtained for transmission imaging of stained rat pancreas tissue. We will discuss our efforts towards improving the detection methods  and the data processing speed.  Furthermore, work will be shown on quantifying and comparing the signals obtained from secondary, transmitted and backscattered electrons on stained tissue sections, as imaged by a conventional SEM.

This work is part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organisation for Scientific Research (NWO).

 

[1] Mohammadi-Gheidari, A., C. W. Hagen, and P. Kruit. Journal of Vacuum Science & Technology B 28.6 (2010): C6G5-C6G10.

[2]  Lena Eberle, A., Schalek, R., Lichtman, J. W., Malloy, M., Thiel, B., & Zeidler, D. (2015). Multiple-Beam Scanning Electron Microscopy. Microscopy Today, 23(02), 12-19.

 

 

 

 

 

 

 

 

 


Wilco ZUIDEMA (Delft, The Netherlands), Yan REN, Jacob HOOGENBOOM, Cees HAGEN, Pieter KRUIT
08:00 - 18:15 #5761 - IM03-244 Ultrafast nano-fabrication and analysis using Xe plasma-FIB-SEM microscope and its applications for Cu milling using the Rocking-stage.
IM03-244 Ultrafast nano-fabrication and analysis using Xe plasma-FIB-SEM microscope and its applications for Cu milling using the Rocking-stage.

Conventional Ga FIB has a reasonable resolution (typically up to 2.5 nm). However, these instruments present some limitations, including Ga ion implantation and contamination, and slow sputtering rate. New liquid metal alloy ion sources (LMAIS) have been developed to overcome these limits[1]. However, none of the proposed LMAIS sources is suitable for rapid milling because they can only deliver probe current up to few tens of nA. Contrary, emerging Xe plasma FIB systems promise faster removal rates[2],[3].

Homogenous copper FIB milling arises from the need to perform various circuit edit operations below the dielectric layer following the copper layer.  If the layer beneath the dielectric is affected by inhomogeneous milling, it can lead to short-circuit and eventual device breakdown. Failure analysis on an integrated circuit was performed using rocking stage with 6-axes piezo movement capabilities together with the novel approach of the combined Xe-plasma ion source FIB and SEM system (XEIA). The new Xe plasma FIB offers sputtering speed up to 50 times faster than the most powerful Ga FIBs. Compared to conventional Ga ion sources, the Xe plasma ion source reduces dramatically the time for cross-sectioning from tens of hours or even days to a matter of hours[4],[5].

Site-specific milling of copper with different milling strategies were tested to optimize time and homogeneity of the milling across the target surface and to overcome the channeling effect posed by polycrystalline copper. Only during the last few nanometers of copper layer the water vapor is used to protect the dielectric layer. The complete removal of copper was followed with XeF2 assisted milling of the dielectric layer to observe the unharmed circuitry. Channeling effect was reduced by regulating the sputtering rates across different grains keeping the underlying dielectric layer safe. Ultra-high-resolution scanning electron microscopy (UHR-SEM) imaging was used for constant monitoring of the removed material to help modulate the process for highest throughput in the least possible amount of time[6].


[1] A. Benkouider et al, Thin Solid Films 543 (2013) 69-73

[2] T. Hrnčíř et al, 38th ISTFA Proceedings (2012) 26

[3] J. Jiruše et al, Microsc. and Microanal. 21 (2015) 1995

[4] A. Delobbe et al, Microsc. and Microanal. 20 (2014) 298

[5] T. Hrnčíř et al, 40th ISTFA Proceedings (2014) 136

[6] The authors would like to acknowledge that this work is performed within the European Commission Initial Training Network, STEEP (Grant no. 316560).


Abdelmalek BENKOUIDER (Brno, Czech Republic), Sharang SHARANG, Tomaš HRNČÍŘ, Jozef Vincenc OBOŇA, Jaroslav JIRUŠE, Edward PRINCIPE
08:00 - 18:15 #5763 - IM03-246 Quasi-Nanofluidic liquid cell for in situ liquid Trasmission Electron Microscopy.
IM03-246 Quasi-Nanofluidic liquid cell for in situ liquid Trasmission Electron Microscopy.

In this work we present a new microfabricated nanochannel device for in situ liquid TEM based on wafer bonding (Fig.1a). A schematic depiction of the chip cross section is presented in Fig.1b. The cell system was fabricated with a new direct bonding technique linking the adhesion properties of Silicon Rich Nitride (SiNx)/Stoichiometric Silicon Nitride (Si3N4) within atomic layer deposition (ALD) of Al2O3 tuned for this application to provide low temperature bonding. The liquid vessel is designed with multiple nanochannels on a suspended membrane area, with tunable liquid layer thickness ~100 nm and silicon nitride windows ~ 50 nm. The channel design of the system improves the control of the top and bottom membrane bulging compared to commercial liquid cell devices and is hence expected to improve the area with high spatial resolution achievable in liquid TEM imaging.  Our nanofluidic system together with a custom-made flow holder will give further control of liquid conditions dynamically varying experimental conditions.

 

The liquid cell was first tested by optical fluorescence microscopy using a solution of 10 nm quantum dots (QD) and as depicted in Fig.2. Flow and diffusive motion of the QDs could be followed. In a Tecnai TEM at 200 kV, a solution of 30 mM HAuCl4 was sealed in the channel with epoxy glue and upon TEM irradiation gold particles in average size between 5 and 15 nm were nucleated along the channels (Fig.3a). Sometimes rocking particle motion was observed, confirming their enclosure within the liquid layer (Fig.3b). In our design membranes are inner bending on each other and plastically deformed due to the extremely high (>12 bar) capillary force. Thus, a thin layer of water < 100 nm is trapped among the membranes. In contrast with other  recent liquid vessels [1], our nanofluidic system points toward higher resolution since liquid thickness  [2], biomineralization synthesis [3] , liquid phase displacement and in liquid holography.

 

References

  

1.                  Tanase, M. et al. Microsc. Microanal. 21, 1629–1638 (2015).

2.                  Nielsen, M. H. et al. Microsc. Microanal. 20, 425–436 (2014).

3.                  Smeets, P. J. M., Cho, K. R., Kempen, R. G. E., Sommerdijk, N. a J. M. & De Yoreo, J. J. Nat. Mater. 1–6 (2015). doi:10.1038/nmat4193

 


Simone LAGANA (Kgs. Lungby, Denmark), Esben KIRK MIKKELSEN, Hongyu SUN, Rodolphe MARIE, Kristian MØLHAVE
08:00 - 18:15 #5773 - IM03-248 Laser triggered microfabricated ultrafast electron beam blanker.
IM03-248 Laser triggered microfabricated ultrafast electron beam blanker.

Femtosecond electron pulses are typically created by illuminating a flat photocathode  with femtosecond laser pulses. [1] However, flat photocathodes have a low reduced brightness,  2 orders of magnitude lower than a Schottky electron source. A higher brightness can be achieved using a cold field emitter illuminated with femtosecond laser pulses. [2] Using a cold field emitter illuminated with UV pulses the group of Zewail has realized an ultrafast SEM. [3] However, such an USEM cannot easily be switched back to continuous beam operation. In addition, the pulse has to be accelerated from the tip onwards which leads to a broadened pulse at the sample.

 Here, we propose a beam blanker for use in regular EMs that allows switching between continuous-beam and ultrafast modes of operation. Previous approaches to ultrafast beam blanking were based on beam blankers using GHz magnetic or electric fields. [4,5] These GHz cavities are still relatively large and can’t be inserted directly in a standard commercial SEM.

 We use a miniaturized beam blanker controlled by a photoconductive switch, illuminated with femtosecond laser pulses, as schematically depicted in Figure 1. Hence, the blanker is locked jitter-free to the laser. We show that such a beam blanker needs to have micrometer scale dimensions for ultrafast operation. COMSOL simulation results, including the full 3D blanker design, are used to evaluate the time response of the system.

 We fabricated and integrated the deflector plates and the photoconductive switch in a one micrometer-scale device, see Figure 2. We will show fabrication results of the ultrafast blanker and its incorporation on an insert for a FEI Quanta FEG 200 SEM. We will also show alignment of both laser and electron beam on the ultrafast beam blanker. Also results will be presented showing laser triggered deflection of the electron beam.

 

 

References:

[1] A. H. Zewail, “Four-dimensional electron microscopy.” Science 328, 5975,  187–93 (2010).

[2] P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, and M. a. Kasevich, “Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses,” Phys. Rev. Lett., 96 (7), 077401 (2006).

[3] D.-S. Yang, O. F. Mohammed, and A. H. Zewail, “Scanning ultrafast electron microscopy.,” Proc. Natl. Acad. Sci. U. S. A., 107 (34), 14993–8, (2010).

[4] K. Ura, H. Fujioka, and T. Hosokawa, “Picosecond Pulse Stroboscopic Scanning Electron Microscope,” J. Electron Microsc., 27 (4), 247–252 (1978).

[5] A. Lassise, P. H. A. Mutsaers, and O. J. Luiten, “Compact, low power radio frequency cavity for femtosecond electron microscopy.,” Rev. Sci. Instrum., 83 (4), 043705 (2012).


Gerward WEPPELMAN, Robert MOERLAND, Ruud VAN TOL, Carel HEERKENS, Jacob HOOGENBOOM (Delft, The Netherlands), Pieter KRUIT
08:00 - 18:15 #5788 - IM03-250 Spin-multislice simulation of an electron inside the objective lens of a TEM.
IM03-250 Spin-multislice simulation of an electron inside the objective lens of a TEM.

Spin filtering of an unpolarized beam in a TEM is a fascinating field of research. Bohr conjectured that it is impossible to spin filter an electron beam or, using Bohr words, “to observe the spin of the electron, separated fully from its orbital momentum, by means of experiments based on the concept of classical particle trajectories”[1].
However, the principle seems to be violated by theoretical calculations [2,3]. One of the most convincing proposals for free electron polarization is a multipolar Wien filter. But the fields involved are typically very large [2] while multipolar Wien filters in microscopy are still rare. The device, together with the diffractive elements, is called as “q-filter” where q hints at the topologic charge of the field.
The objective lens of the microscope provides a very large field with the potentiality of introducing a spin-orbit coupling, we performed spin-multislice simulations [4], where a Bessel beam was propagated through the objective lens (modeled as a Glaser field) in order to quantify the degree of spin polarization.
We will discuss in particular that the spin-orbit conversion in the pre- and post-field can be understood in terms of the q-filter.
Fig 1 a shows on the left a scheme of the objective lens and of the electron wavefunction (blue) passing through it. A schematic “Bohmian” trajectory is indicated by a curve.  The image also features arrows indicating the classical spin orientation along the curve for an initial state with spin |↑> along the optic axis.
The fig 1b indicates the multislice calculated evolution of the wavefunction. While the expectation value of the spin operator S  has components < Sx >=< Sy > =0 we can track the expectation of the x,y vector P=(S.r,S.t) (with r being the in plane position versor and t its orthogonal in plane versor). The result is shown in fig 2. P represents a sort of local in plane projection of the spin operator. To a good degree of approximation here |P| is equal to the rate of conversion from |↑> to |↓>.

The results indicates a net, typically weak , increase of P as an effect of the objective lens .
The overall final wavefunction is described in Fig 3 as a non-separable  spin-orbital angular momentum state.
The multislice results are in quantitative agreement with ray tracing calculations, confirming the reliability of both methods in this case. However, the multislice approach enables us to use less classical states like Laguerre Gauss beams, to explore possible advantages and more quantum physical effects.
Acknowledgements: The financial support by the Austrian Science Fund (I543-N20) and by the European research council, project ERC-StG-306447 is gratefully acknowledged.

.
[1] Darrigol O 1984 Historical studies in the physical sciences 15 (1984) 39
[2] E. Karimi,  L. Marrucci et al. Physical Review Letters 108 (2012) 044801
[3] H. Batelaan et al. Physical Review Letters 79 (1997) 4517
[4] V. Grillo , L. Marrucci et al. New Journal Physics 15 (2013) 093026


Vincenzo GRILLO (Modena, Italy), Thomas SCHACHINGER, Ebrahim KARIMI, Peter SCHATTSCHNEIDER
08:00 - 18:15 #5830 - IM03-252 Development of low noise quantitative EBAC imaging in FEG SEM.
IM03-252 Development of low noise quantitative EBAC imaging in FEG SEM.

Electron Beam Absorbed Current (EBAC) is a specimen current imaging technique that has been established in the earliest stages of Scanning Electron Microscopy (SEM), but which has been somewhat overlooked for last few decades [1], with the exception of nanoprobing for failure analysis [2]. Whilst the technique has been noted for its uncomplicated electron collection geometry, it has not found use in routine microscopy because of the slow and noisy electronics of the time. This work revisits the design and application of EBAC to general SEM and demonstrates that modern low-noise and high-speed amplification entirely overcome the traditional limitation of the technique, whilst adding full quantification and unprecedented imaging flexibility.

Traditional limitations of EBAC amplification were linked to the very low signal intensity, as only a fraction of the primary electron current was passed outside the SEM chamber. In contrast with Everhart-Thornley or solid-state detectors, no amplification could be provided inside the chamber as traditional amplifiers could not be placed in situ. This is no longer the case with modern electronics, and a miniature pre-amplifier was designed and placed on the sample stage. A further amplifier was placed ex situ to control the gain further, and the signal was recorded with full quantification alongside the conventional Secondary Electron (SE) and In-Lens (IL) signals. A Tungsten wire test sample was loaded on a custom electrical holder for EBAC, and is used here to compare SE, IL and EBAC signals recorded simultaneously on a PE upgraded ZEISS DSM982 FEG SEM. EBAC electronics have sufficient bandwith for live monitoring, alignment and focus, and was used as the main signal throughout this work.

As illustrated in Figure 1, it is first found that resolution of the EBAC signal far exceeds that of SE at all accelerating voltages and working distances. Since at all points on the sample the sum of all electron currents must be constant, it follows that the higher resolution of IL signal must be present in EBAC signal. Indeed, the IL images (not shown in this abstract) and EBIC images are highly correlated. As reported by [1], it is found that EBAC imaging is largely independent from working distance, whilst the IL signal is limited to very short working distances in order to maintain good solid angle collection efficiency (not shown in this abstract).

Further differences arise from the direct nature of absorbed signal, which is not convoluted with information arising from the trajectories of emitted electrons as they leave the surface. This is observable in Figure 1 and explained more clearly with low magnification data of the W wire (Figure 2). SE signal presents very pronounced shadowing as the low energy electrons are attracted towards the detector, and thus the opposing side of the cylindrical wire appears darker. Such effects are less visible in the IL signal because of the collection geometry, whereas the EBAC signal is completely free of such shadowing.

Contrast of sub-micron grains is readily found in both IL and EBAC signals, albeit of different relative intensities (not shown in this abstract) and is attributed to orientation contrast (OC). As illustrated in Figures 1 and 3, grains with strong OC are presents in all images, but with the highest noise in SE and lowest noise in EBAC. The uncomplicated geometry and calibrated property of EBAC signal, presents the opportunity to quantify values of OC independent from imaging conditions (Figure 3). Whilst physical origin of OC in both IL and EBAC signals is thought to be the same, it is proposed that differences in relative intensities arise from the different collection geometries.

Further new observations are enabled by quantitative imaging, including the discovery that the EBAC signal can change polarity. It is found that for a range of conditions, the total sum of emitted electrons can exceed the sum of absorbed electrons. Examples include protruding nanoscale features (Fig. 1), grains of strong orientation contrast (Fig. 3), or locations of high electron beam incidence angle, as observed at the edges of the W wire (Fig. 2).

[1] Goldstein, J., Newbury, D.E., Joy, D.C., Lyman, C.E., Echlin, P., Lifshin, E., Sawyer, L., Michael, J.R., Scanning Electron Microscopy and X-Ray Microanalysis, Third Edition, 2003, Springer US.

[2] K. Dickson, G. Lange, K. Erington and J. Ybarra, Proceedings from the 36th International Symposium for Testing and Failure Analysis, November 14–18, 2010, Addison, Texas, USA.


Grigore MOLDOVAN (Halle (Saale), Germany), Uwe GRAUEL, Wolfgang JOACHIMI
08:00 - 18:15 #5854 - IM03-254 Crystallite orientation maps of starch granules from polarized Raman spectroscopy and synchrotron X-ray microdiffraction data.
IM03-254 Crystallite orientation maps of starch granules from polarized Raman spectroscopy and synchrotron X-ray microdiffraction data.

Polarized Raman spectroscopy (PRS) and synchrotron X-ray microdiffraction were used to determine the local orientation of crystalline regions in giant starch granules extracted from bulbs of tulip and the orchid Phajus grandifolius. Starch granules can be described as distorted spherulites composed of concentric growth rings in which molecules are radially oriented.

As previously validated on starch specimens [Galvis et al. J. Cereal Sci. 62 (2015), 73], the molecular orientation in the native granules was determined by measuring the anisotropic Raman response of certain chemical bonds at different polarization directions of the incident laser radiation. Wellner et al. [Starch-Stärke 63 (2011), 128] had shown that the response of the Raman band at 865 cm-1 assigned to the stretching of the glycosidic bonds C–O–C and ring breathing of glucose units exhibited a high spatial variation that could not only be explained by variations in the degree of crystallinity but also by the local molecular orientation in ordered structures. First, we have evaluated the response of the band at 865 cm-1 using model acicular "A-type" single crystals prepared from a fraction of short-chain amylose biosynthesized in vitro [Montesanti et al., Biomacromolecules 11 (2010), 3049]. The A-amylose crystals oriented "in plane" showed a maximal intensity when the polarization of the laser was along the chain axis of the crystal, i.e., parallel to the axis of the amylose double helices, and minimal when perpendicular. In addition, the Raman band at 1343 cm-1, assigned to C–O–H bending, showed only a small variation and was used as "internal standard" to calculate the intensity ratio of bands 865 / 1343. In parallel, hydrated single starch granules have been probed with 3-5 µm synchrotron X-ray beams and a raster step of 5 µm, at the ID13 microfocus beamline of ESRF. The collected fiber microdiffraction patterns were analyzed to deduce the local average orientation of the crystallites and produce maps over the whole granules.

PRS orientation maps of tulip (Figure 1a) and P. grandifolius starch granules revealed regions with an isotropic response close to the eccentred hilum (origin of the growth) and others with a high anisotropic response at the distal end (Figure 1b) [Galvis et al., in preparation]. The orientation maps of P. grandifolius granules were compared to those previously determined from synchrotron X-ray microdiffraction data [Chanzy et al., J. Struct. Biol. 154 (2006), 100] and those from tulip granules, to the data newly collected at ESRF (Figure 2a). Again, the diffraction patterns showed that the crystallite orientation was very high far from the hilum, in regions where the curvature of the growth rings is low (Figure 2b). Around the hilum, the crystallinity remained high and therefore, the lower orientation was likely due to the high curvature of the growth rings and the resulting 3D distribution of crystallites within the probed volume. The spatial resolution of the orientation maps is limited by the size/volume of the region over which the signal is collected and thus averaged, which, in particular, results in a lack of information along the incident laser (for PRS) or X-ray beam (for microdiffraction). However, both techniques are complementary and provide unique pictures of the local molecular organization in single objects.

Acknowledgement: We thank Laboratoire d'Ingénierie des Systèmes Biologiques des Procédés (Toulouse, France, P.-C. Escalier, G. Véronèse) where the amylose biosynthesized in vitro was prepared, the NanoBio-ICMG Electron Microscopy Platform (Grenoble, France) and ESRF (Grenoble, France).


Leonardo GALVIS, Carlo BERTINETTO, Britta WEINHAUSEN, Nicole MONTESANTI, Christine LANCELON-PIN, Tapani VUORINEN, Manfred BURGHAMMER, Jean-Luc PUTAUX (Grenoble Cedex 9)
08:00 - 18:15 #5862 - IM03-256 Development of new stage system for modern electron microscopes.
IM03-256 Development of new stage system for modern electron microscopes.

Analytical systems for high spatial resolution, such as transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM), are getting popular, since a target sample for modern science and industry is getting smaller. Thus, higher resolution and efficiency are required for modern microscope systems, along with further improved ease of use since a lot of functions are installed to a microscope and it makes its operations complicated. JEM-F200 has been developed as an easy-to-use electron microscope for high resolution imaging and analysis for the requirements of those mentioned above. Among the components of the microscope, specimen system is one of the most important hardware to be developed because the all users must use the system frequently, and all users need to be careful for treating a sample.  In this paper, we explain the features of a newly developed specimen system, which has three new features.

The first element of the new stage system is a redesigned specimen drive mechanism, that is called as "Pico Stage Drive". The specimen stage drive is fast and highly-precise. The new ultra-fast specimen drive enables the stage to move in approximately 7 seconds over a wide area of 2 mm diameter (highest speed: 0.3 mm / s). And the new ultrahigh-precision drive allows a specimen (on the stage) to move in steps of sub-nanometers (0.2 nm / step). The specimen stage can be driven with piezo device (0.05 nm / step) simultaneously.

The second element of the new stage system is an auto insertion/extraction mechanism for specimen holder, which is called as “SPECPORTER”. Insertion or extraction of a specimen holder has been considered to be an operation where human error might occur, especially for novice users. To avoid the error, a new automated loading/extracting system for specimen holders, which needs no human operations, has been developed. With the SPECPORTER, the operator sets a specimen holder at a designated position and activates the SPECPORTER by simply clicking a switch, and then the holder is automatically inserted or extracted safely as shown in Figure 1. The operations of evacuation and opening a valve for sample holder are programmed and installed. The sample maintains its attitude to be horizontal during the procedure. If the SPECPORTER were applied to cooling holders, no liq. N2 spilt is realized in the procedure of sample insertion. The system maintains the feature of JEOL double O-ring holder, and therefore users can insert a old sample  holder compatibly by manual loading and unloading. Furthermore, a conventionally-used specimen holder can be modified so that the holder is inserted or extracted automatically using the SPECPORTR.

The third element is a new clam shell, which covers a goniometer. The clam shell withstands pressure variation of the installation room to protect a sample. However, a few electrical feed through was prepared in old system. In the new system, more feed through are prepared for a variety of specimen holder (e.g. heating holder, see Figure 2).

In conclusion, the new specimen system provides the easy, safe and smooth operation of samples, which gives a high throughput to users. Especially, the ultra fine specimen drive system enables accurate positioning of the sample with large travel (2 mm), which is requested by all kinds of target functions such as high resolution imaging and high resolution analysis.


Kazuya YAMAZAKI (Akishima, Japan), Shuichi YUASA, Yuuta IKEDA, Masaaki KOBAYASHI, Kazunori SOMEHARA
08:00 - 18:15 #5865 - IM03-258 Benefits of angular and energy separation of slow signal electrons in SEM.
IM03-258 Benefits of angular and energy separation of slow signal electrons in SEM.

Recently developed scanning electron microscopes (SEM) are equipped by sophisticated detection systems, which offer very effective energy and angular separation of the signal electrons and extraordinary detection flexibility. The signal electrons can be collected by various types of detectors and character of the detected signal is possible to affect by many parameters (e. g. optical configuration of the column, detection geometry, presence of the specimen bias, etc.). Understanding of the detected signal origin and correct interpretation of the micrographs become very difficult, which hampers utilizing of full potential of modern SEMs.

Experiments have been performed with a novel Trinity detection system (Scios, FEI Comp.) consisting of three in-lens detectors:  the T1 and the T2 detectors located inside the final lens and the T3 detector situated inside the column just below the aperture strip (Fig. 1). The instrument is also equipped by a standard E-T detector (ETD) situated in conventional position. There is a possibility of simultaneous detection of all 4 images (i.e. T1, T2, T3 and ETD) and different type of information about the specimen can be achieved at the same time.

Oxide inclusions embedded in a conventional steel was used as an experimental material, which secures presence of the topographic, material and crystal orientation contrast in the micrographs. Moreover, the inclusions become charged by the electron beam irradiation and the influence of charging on the micrographs collected by the Trinity detectors can be observed.

There are many possibilities how to affect the detected signal origin. Fig. 2 demonstrates effect of the specimen bias on detected signal. The SEs are shared by the T3 and T2 detectors and are not detected by the ETD when the specimen bias of -4kV is applied. Strong collimation of the signal electrons towards the optical axis is evident. The high-angle BSEs are collimated towards the optical axis and the T1 detector shows topographical contrast.

Significant effect of a working distance (WD) on the signal collected by the Trinity detectors and the ETD is shown in Fig. 3. For a short WD, the T3 detector collects mainly the slow secondary electrons (SEs) and positive charging of the spinel inclusions is clearly visible. For a long WD, the electrons originally detected by the T3 detector are shifted towards the T2. The T1 detector collects the backscattered electrons (BSEs) and the channeling and topographical contrast are superimposed on the material (“Z”) contrast at short WD. Inversely, the material contrast intensifies with increasing WD. Obviously, increasing WD leads to less effective collimation of the slow signal electrons into the final lens (by the A-tube electrostatic field) and the ETD detection efficiency was improved.

Insight into an extraordinary detection flexibility of the Trinity system enables us more effective characterization of material microstructure. Accurate knowledge about the signal received at each detector and possibility of its modification can be successfully used for tuning of desired contrast or suppression of undesirable information.

The presentation is based on results obtained from pioneering project commissioned by the New Energy and industrial Technology Development Organization (NEDO).


Sarka MIKMEKOVA (Kawasaki, Japan), Haruo NAKAMICHI, Masayasu NAGOSHI
08:00 - 18:15 #5874 - IM03-260 Development of a new electrostatic Cs-corrector consisted of annular and circular electrodes.
IM03-260 Development of a new electrostatic Cs-corrector consisted of annular and circular electrodes.

For improving spatial resolution in electron microscopy, as is well known, the spherical aberration (Cs) has to be compensated. Currently, the Cs-correction devices consisted of multi-pole lenses have successfully realized sub-angstrom resolution in the scanning / transmission electron microscopes (S/TEMs) [1-3]. These correctors, however, require complex control of multiple optical components with high accuracy and stability. In addition, the microscope columns should be reconfigurated to insert additionally rather large corrector components, resulting in huge cost. In order to solve these problems, one of the coauthor Ikuta had newly proposed the very simple and compact Cs-corrector with axially-symmetric electrostatic-filed formed between annular and circular electrodes [4], as schematically shown in Fig. 1(a). We called it “ACE corrector”, meaning the Cs-corrector using Annular and Circular Electrodes. In the present paper, we report preliminary results of the ACE corrector installed in 200kV-STEM apparatus.

It can be simply explained how the ACE corrector compensates the Cs, as follows. In the electrostatic field formed around the circular electrode, the electrons going through the field are a little focused. In contrast, around the annular electrode, the electron trajectories are spread. They indicate that the field between the electrodes provides the compound lens effect of the convex and concave lenses arising from the circular and annular electrodes, respectively. Totally, as schematically shown in Fig. 1(b), the ACE corrector has the negative Cs value, while the effective area is restricted to be in the off-axis by the annular slit.

Fig. 2(a) is a cross-sectional illustration of the electrodes with typical sizes. The circular electrode can be easily obtained by the photolithography as well as the conventional apertures for the electron microscopes. Since the annular electrodes contain complicated structures, we have employed the focused ion beam (FIB) technique for their fabrication. Fig. 2(b) shows a SEM image of the annular slit corresponding to that in Fig. 2(a). This structure was processed at the center of the base tantalum plate having the size of 3mm in diameter and 10m in thickness. Two electrodes were assembled in the small device, as shown in Fig. 2(c), by sandwiching the insulator film between them. This device was installed in the STEM (Hitachi HD-2300S; 200kV) by attaching to the tip of the conventional aperture holder instrument, which were connected to the voltage supply. The constant negative voltage was applied to the circular electrode, and the annular electrode was grounded, via two lines attached to the device as in Fig. 2(c).

Figs. 3(a) show annular dark-field (ADF) images of CeO2 particles taken at different Cs conditions, i.e. the voltage applied to the ACE corrector varied from 0 V to 15 V. They indicate that the image obtained at 10 V show most clear contrast, which is consistent with the appropriate value predicted in advance by the simulation. In high-resolution condition, as shown in Figs. 3(b), a Cs corrected image taken at 10V can clearly exhibit atomic columns. These results demonstrate that our developed electrostatic device can effectively correct the intrinsic spherical-aberration of the objective lens.

References
[1] H. Rose, Optik 85 (1990) 19
[2] M. Haider, et al., Optik 99 (1995) 167    
[3] O. L. Krivanek, et al., Inst. Phys. Conf., 153 (1997) 35
[4] T. Kawasaki, et al., Proc. ALC, 27p-P-58, (2015)


Tadahiro KAWASAKI (Nagoya, Japan), Takafumi ISHIDA, Masahiro TOMITA, Tetsuji KODAMA, Takaomi MATSUTANI, Takashi IKUTA
08:00 - 18:15 #5875 - IM03-262 Development of New Generation Cryo TEM.
IM03-262 Development of New Generation Cryo TEM.

Cryo transmission electron microscopy (TEM) provides structural information of a specimen close to its natural state without any disturbance, due to the specimen preparation process, which exclude chemical reactions and physical stimulations. Recently, cryo TEM produces very exciting results of structural biology in combination with single particle analysis and electron tomography, since application field of the method expand to non crystalline samples or huge molecules.

We developed a new generation cryo TEM, which achieves high throughput and high usability. This microscope equips 200 kV field emission gun (FEG). Users can choose it from a Schottky-type (TFEG) or a cold FEG (CFEG). Since the energy spread of the emitted electrons from the CFEG is about 50% of TFEG and the size of the virtual source is less than 10 nm, the electron beam has a high coherences. With such beam, cryo TEM image has high contrast due to its high spatial coherence and is less affected by chromatic aberration due to its high temporal coherence. In low dose imaging, where the image resolution is mainly determined with dose density for the image. In the low dose density, S/N of image mostly determined by a statistical noise of electrons, since dose density in cryo TEM is typically several tens of electrons for angstrome square. Namely, the resolution is determined with the competition between the statiscal noise and image contrast. It means CFEG has posibility to have higher resolution for cryo TEM works.

Since this microscope also has dedicated cryo stage, cryo TEM observation can be performed at low temperature < 100 K  and with low grow ratio of ice contamination. In addition, this cryo stage is compatible for multi-specimen auto-loader, so users can exchange specimens automatically. It also has some automation functions, such as liquid nitrogen auto-refill system and auto acquisition software (JEOL Automated Data Acquisition System: JADAS). These automation functions will help users to perform high throughput works. On another front, the electron gun chamber and the TEM column are evacuated with sputter ion pump and turbo-molecular pump, because of this, a sample is kept in oil-free environment.

In addition, this microscope is compatible with omega-type energy filter and Zernike or hole-free phase plate. The cryo specimens exhibit low contrast in TEM images even using large defocus phase contrast imaging. The Cryo TEM is more advantageous when it is combined with these techniques of contrast enhancement.


Naoki HOSOGI (Tokyo, Japan), Takeshi KANEKO, Isamu ISHIKAWA, Syuuiti YUASA, Kimitaka HIYAMA, Naoki FUJIMOTO, Izuru CHIYO, Akihito KAMOSHITA, Yoshihiro OHKURA
08:00 - 18:15 #5942 - IM03-264 Precession-assisted Quasi-Parallel Illumination STEM on three condenser lenses TEMs.
IM03-264 Precession-assisted Quasi-Parallel Illumination STEM on three condenser lenses TEMs.

The analytical mapping applications for which the STEM illumination mode in TEM columns is mostly used imply a high convergence angle of the electron beam focused onto a nanometric probe on the specimen, so that high electron doses are obtained. This design then enables high lateral resolution for energy dispersive or electron energy loss spectrum maps [1]. In view of the recent fast growth of quantitative electron diffraction work [2] the natural extension of STEM illumination mode in “TEM/STEM columns” would be towards diffractive recording for either low dose work or electron diffraction tomography because of the stable condition of the projector system, which stays solely in diffraction mode. The main problem for these applications is that even when using small condenser apertures of 10 μm, the obtained diffraction patterns consist of large discs of 3-4 mrad spread instead of small spots of only 0.5-1 mrad, the latter values being required for structure determination from crystals with large unit cells, such as zeolites. Expanding the approach used in multifunctional dedicated “STEM columns” [3], we have developed a stable method for working under Precessed Quasi-Parallel illumination condition in TEM/STEM columns, using their internal scanning unit. The pre-requisites – usually found in modern microscopes - are that the column should be digitally controllable, equipped with a 3-lens condenser system and a condenser aperture of 10 μm, and have at least a set of deflecting coils past the objective lens, as well as the usual double set above the specimen plane. The key factor for enabling the Quasi-Parallel illumination lays in decreasing the excitation of precisely the condenser lens which controls beam convergence, the second one in the 3 lens condenser system. Naturally, a refocusing of the beam is needed and an accurate curve of beam spread versus convergence angle must be produced, but, once calibrated, the resulting desired configurations are conveniently stored in computer memory for later recall and use. In this work, we have furthermore added to such Quasi-Parallel Illumination STEM mode, the precession of the beam at 100 Hz, in order to obtain quasi-kinematical diffraction patterns. The challenge has been to adjust the Precessed Quasi-Parallel STEM HAADF image [4] [5] and a specific alignment method has proved to be specially suited, at least up to 0.6 degrees, providing almost non-distorted scanned images (see images bellow, which include Precessed Diffracion Patterns with and without the de-scanning below the sample, as well as Quasi-Parallell STEM HAADF image with and without precession).

Using this electron microscope configuration, we are able to obtain images of organic materials without an excessive degradation compared to the static NBD-TEM mode of the microscope. Moreover, the Precession-assisted Quasi-Parallel illumination STEM mode is suitable for electron diffraction tomography of both inorganic and organic structures, since sample drift and eucentricity at each tilting step may be controlled without changing the selected operative values in the projector system, usually corresponding to 12 cm for a 200KV high voltage. It also reduces the total time to obtain the whole data for the structure determination. Finally, the use of a precessed beam avoids the main dynamical effects on the diffraction patterns being able to solve structures with kinematical approximations.

Acknowledgements:

We acknowledge the financial support from NanoMEGAS. We also acknowledge the TEM facilities at the Scientific and Technological Center of the University of Barcelona (CCiT-UB).

References:

[1] - A. V. Crewe et al., (1969). Rev. Sci. Inst. 40 (2), 241-246.

[2] - L. Palatinus et al., (2013). Acta cryst. A69, 171-188.

[3] - H. Inada et al., (2009). Journal of Electron Microscopy 58(3), 111-122.

[4] - U. Kolb et al., (2007). Ultramicroscopy, 107, 507-513.

[5] - E. Mugnaioli et al., (2009). Ultramicroscopy, 109, 758-765.


Sergi PLANA (Barcelona, Spain), Joaquim PORTILLO, Sònia ESTRADÉ, Joan MENDOZA, Francesca PEIRÓ
08:00 - 18:15 #5973 - IM03-266 Optimisation of TEM preparation in metallic materials using low voltage ions.
IM03-266 Optimisation of TEM preparation in metallic materials using low voltage ions.

TEM samples of metallic materials can be prepared by mainly two ways: Electrothinning and ion thinning either by ion milling systems or Focused Ion Beam (FIB). In both cases, very thin lamellae can be obtained but residual artefacts are always present on their surfaces. Depending on which information is needed, those artefacts can limit and even prevent us from observing the samples properly. In the general case, electrothinning induces a residual oxide layer, mostly amorphous, that can evolve during TEM observations (Fig.1). On the other side, ions milling and FIB induce amorphous layers, irradiation defects and ions implantations although new FIB systems give the possibility to clean the specimens at low kV.

In this study, we show that ion polishing systems with low acceleration voltage can greatly improve the quality of electrothinned and FIB lamellae. Various cleaning conditions were tested using Precise Ion Polisher (PIPS I from GATAN) and with the new Precise Ion Polisher (PIPS II GATAN2012). Compared to PIPS I, PIPS II provides a better control of the ion beam at an acceleration voltage down to 100V. The sputtering kinetic was measured on various alloys (316L, Ni based alloy 600, oxidized and quenched Zr alloy). To do so, thickness maps were acquired with a FEI TECNAI OSIRIS equipped with an energy filtered imaging system (GIF Quantum from GATAN). Even though macroscopic dusts can be removed after Ar+ cleaning at 500V, thinning and decrease of amorphous layer is only slightly effective in PIPS I. To get a significant thinning rate in PIPS I, an accelerating voltage higher than 1kV has to be used but evidences of irradiation defects were seen on 316L. PIPS II experiments were conducted on various alloys with various ion thinning conditions, on both electrothinned and FIB lamellae. The kinetic rates measured are plotted on Fig.2 showing an effective thinning of lamellae even at 100V. When comparing the lamellae as-electrothinned and as-cut at 30kV by FIB to PIPS II cleaned lamella, a clear decrease of amorphous layer is observed and the quality of the lamellae is greatly improved. To be sure that no irradiation defects are induced by such thinning, we studied PIPS II cleaning on an Au+ pre-implanted 316L. Cross sectioned lamellae were prepared at 30kV in a Helios Nanolab Dual Beam from FEI. The effect of low voltage cleaning on  the lamellae  is showed on Fig.3. No irradiation defects were seen in non Au+ implanted area. However, a few nanometres thick layer of amorphous is still present on both surfaces. These results show that the thickness and the quality of metallic TEM samples (either prepared by electrothinning or by FIB) can be easily improved by using a complementary thinning/cleaning with low voltage Ar+ ions in PIPS II.

Experiments on 316L received a financial support from the French National Research Agency through the project CoIrrHeSim ANR-11-BS09-006.


Laurent LEGRAS (Moret sur loing), Marie Laure LESCOAT, Stephanie JUBLOT-LECLERC, Aurélie GENTILS
08:00 - 18:15 #6006 - IM03-268 Ultrafast transmission electron microscopy reveals electron dynamics and trajectories in a thermionic gun setup.
IM03-268 Ultrafast transmission electron microscopy reveals electron dynamics and trajectories in a thermionic gun setup.

Many efforts in the past decade have been made to improve the temporal resolution of in-situ TEM in order to reveal the dynamics of processes at the nanoscale. However, most processes occur at time scales in the micro- to femtosecond domain which is beyond the acquisition frequency of the TEM cameras (down to few milliseconds). Thus the salient details of sample dynamics such as defect formation, phase transformations, nucleation phenomena etc. are often inaccessible.

For time-resolved studies, a much higher temporal resolution is therefore required. This can be achieved by using short electron pulses in a pump-probe approach. Ultrafast TEM (UTEM) consists of a TEM combined with a pulsed laser (figure 1). A photo cathode in the electron gun is illuminated by a fs-laser to produce a photoelectron pulse with a duration of 2-10 ps. After laser excitation of the object (pump pulse), the photoelectron pulses serve as probes with a variable time delay after the excitation. Repeating this process at different pump-probe delays allows time-resolved studies.

In contrast to conventional TEM, the electron-electron interaction in one pulse is not negligible in UTEM and has to be studied in detail. The energy width (ΔE), temporal length (Δt) and different arrival times (t0) of the electron pulses on the specimen depend on many effects related to electron-electron interactions such as space charge limited current, Boersch effect, emission angles and trajectories, or filtering effects due to chromatic aberration of lenses. It is crucial to understand these as a function of the relevant experimental parameters (position and shape of the photocathode, laser power, Wehnelt bias) in order to optimize spatial and temporal resolution while preserving reasonable acquisition times.

Our experimental setup consists of a JEOL 2100 with a thermionic gun and Wehnelt electrode, combined with a femtosecond fiber laser. Measurements are based on the PINEM effect (photon-induced near-field electron microscopy), which occurs when pump and probe pulses are synchronized at the sample. It results in a change of the electron energy distribution due to inelastic electron scattering by the photonic near field around a sample that can be observed by EELS.

The ability of the pump-probe setup to precisely measure the arrival time of the electrons allows a deeper understanding of the emission pattern. A conical tantalum cathode with flattened tip positioned close to the opening of the Wehnelt shows two electron populations, i.e., an intense big halo and a central spot (figure 2). The arrival time of electrons from the outer halo is shifted with respect to the central spot; the time difference changes with the applied Wehnelt bias. These measurements enable us to decipher the emission areas and electron trajectories. The halo is attributed to shank emission from the side wall of the cone where electrons leave at larger angles. The central spot are electrons emitted from the flattened tip. Larger Wehnelt gaps cut shank emission so that only electrons from the tip reach the specimen. Here the emission resembles the one from a Ta disc where all electrons are emitted from the flat surface at any Wehnelt gap.

Furthermore, PINEM scans were measured at different pump-probe delays, giving the temporal evolution of the electron pulse. Repeating these scans at different experimental settings (UV intensity, Wehnelt bias) allows to extract Δt and ΔE (figure 3). For instance, an increasing UV power allows to shorten the acquisition time but increases space charge and Boersch effect. At high Wehnelt bias the energy width (ΔE) is narrow, allowing good spatial and energy but lower temporal resolution. A low bias gives the opposite: good temporal but lower energy resolution.

Such understanding of the electron dynamics allows us to define optimal settings for time-resolved experiments, which are always a compromise between temporal, spatial, and energy resolution as well as acquisition times. The detailed beam characteristics will be presented.


Kerstin BÜCKER (Strasbourg), Matthieu PICHER, Olivier CRÉGUT, Thomas LAGRANGE, Bryan REED, Sang Tae PARK, Dan MASIEL, Florian BANHART
08:00 - 18:15 #6009 - IM03-270 Manufacturing and application of a 2 µm dark field aperture in TEM.
IM03-270 Manufacturing and application of a 2 µm dark field aperture in TEM.

For an entire TEM characterization of many materials, it is necessary to achieve selected area electron diffraction (SAED) patterns of smallest regions with assigning the reflexions to their origins in the real image. In a previous work we showed that we were able to successfully reduce the field of view by a customized SAED aperture to a 15 nm range [1]. Though it gives us very local information about the samples structure, in daily work it is not always satisfying, since the real image is as important to understand the correlation between certain Bragg spots and the real structure, e.g. given by a series of dark field images. Especially for closely neighboured reflexions, commercial objective apertures are too large and do not allow the separated selection of these spots. Since our conventional TECNAI is equipped with the standard aperture-stripe, we are limited to a smallest size of 10 µm (12 µm in reality) which delivers a field of view of ca. 7.5 mrad inside the back focal plane. The smallest commercially available aperture has a diameter of 5 µm.

Figure 1a  displays a section of a polycrystalline fcc diffraction pattern. The marked large circle represents the standard 10 µm objective aperture, while the smaller one represents our custom made aperture with a diameter of 2 µm or 1.5 mrad inside the back focal plane. This example shows, with standard apertures it is impossible to select the (311) reflexions without overlap of their neighboured (220) or (222).

                Therefore, we reworked the present PtIr aperture-stripe by focused ion beam (FIB) in two steps [2]. At first an existing hole of the stripe - there are 2 rows of holes, one provides smaller and the other one larger diameters, which are seldom used - was closed by ion beam-induced Pt-deposition. As a second step, a centred opening was sputtered into that layer by using of circular masks up to 2-µm in diameter. To minimize a conical shape of the opening, at low ion beam current (280 pA) with a high aspect ratio is used and the hole is successively milled from both sides. If the Pt-deposition is too thin, there is a high risk that scattered electrons in the TEM will not be entirely blocked by the new aperture and create artefacts and distortions in the images. Therefore, it has a thickness of around 6.5 µm. First investigations with TEM proved that the deposited layer is not transparent for 200 kV electrons anymore and thermally stable as well.

                Figure 1b-d shows an application of the new objective aperture on a multi-twinned system of polycrystalline diamonds. Although the twinned areas are in the range of 5-10 nm it becomes possible to correlate the chosen diffraction spots with their origins in the real image. The adjustment of the new objective aperture has to be done very carefully, it can easily outshine the observation screen or the CCD camera, so one can easily lose the designated position, but the selection of certain diffraction spots requires a very accurate positioning. Other than at larger apertures where slight drifts are not critical because of the visibility of the selected area and therefore easier readjustments, slightest drifts must be avoided.

      In conclusion, the new 2-µm objective aperture can be very helpful for the understanding and structural characterization of samples according their crystallinity, their growth behaviour or even defect studies.

 

  1. S. Selve, D. Berger, Ch. Frey, L. Lachmann: Manufacturing and application of individually adapted SAD apertures for a conventional Tecnai G²20 TEM. In: Conference Proceedings MC2011 Kiel
  2. We kindly acknowledge EFRE founding of the project “Nano Werkbank” including a FEI Helios 600.
  3. We kindly acknowledge the Exzellenzcluster “UniCat” for the financial support of the TEM.

Sören SELVE (Berlin, Germany), Dirk BERGER
08:00 - 18:15 #6024 - IM03-272 An in-situ Low Energy Argon Ion Source for Local Surface Modification.
IM03-272 An in-situ Low Energy Argon Ion Source for Local Surface Modification.

A new in-situ low energy ion source for SEM and DualBeam has been designed. The static beam of low energy gaseous ions such as Ar+, O+ or Xe+ can be used for a local modification of the sample surface. Typical energies are in the range 5 - 500 V, covering the interaction types from chemical reaction to reactive ion etching and to ion milling, for energies above the milling threshold. The source is based on the following principle: electrons from the SEM’s electron beam partially convert an atomic or molecular gas flow into a beam of ions directed towards a biased sample. A schematic set up is shown in figure 1. A small nozzle delivers the gas and the electron beam enters this nozzle through a slotted hole. The beam is scanned in this slotted hole, penetrates the gas flow and generates thermal ions both by direct ionization and by ionization from beam interactions with the wall of the nozzle. The ions are pulled out of the nozzle by the protruding fields from the biased sample which is located at a short distance from the nozzle: the ions are accelerated in this electrostatic fieldand directed towards the sample. The slotted entry hole is roughly located at half the inner nozzle diameter from the edge.

 

The source produces a static beam of ions with selectable energy. The direction and width of the beam depend on the geometry and not on the applied bias voltage, because the electric fields define both the acceleration and the trajectories. With a typical SEM excitation condition of 2kV, 26nA and a nozzle to sample distance of 100 um, it is possible to generate a 100 eV Ar+ beam current of 5 nA and a full width half maximum (FWHM) of 8.2 um. This corresponds to a central average ion current density of 0.095 nA/um2, which is very similar to the current density at 500 V of a Ga+ beam produced by a regular FIB column. The FWHM is easily adjustable by changing the nozzle-to-sample distance, allowing for example a broader beam with a wider peak. The source is slightly focusing as shown in figure 2, so the beam diameter does not expend too rapidly with the sample to nozzle distance. In this way, the sample area that is affected by the low energy ions can be more or less defined.

 

Thanks to the low energy, the new source can be used for polishing the top surface of a sample such as the Ga doped layer after FIB operation (removal of Ga and reduction of damage layer thickness) or it can be used to clean a sample from residual hydrocarbons. The first application can be useful for improvement of the quality of a TEM lamella produced by FIB, or improvement of and EBSD surface prepared by FIB. An example of the interaction with the beam is shown in Figure 3, where a native oxide on Si has been removed in 6 seconds, using 200 V Ar+ ions.  Finally it should be noted that in case of using O2 gas, the source behavior (size, energy, field distribution) is the same, allowing cleaning of a sample chemically, below the milling threshold. Primary gas switching to other noble gases is straightforward, because the primary principle (ionization by electron impact) and ion trajectory formation and acceleration remain the same.  

 

 

 

 


Johannes MULDERS (Eindhoven, The Netherlands), Piet TROMPENAARS
08:00 - 18:15 #6026 - IM03-274 Pushing the limits of environmental scanning electron microscopy.
IM03-274 Pushing the limits of environmental scanning electron microscopy.

In environmental scanning electron microscopy (ESEM) electrical insulating, wet and biological samples can be investigated without additional sample preparation. The imaging gas inside the chamber suppresses charging and outgassing of the sample but it also decreases the signal to noise ratio (SNR) [1]. Especially applications in the kPa regime are limited by poor image quality (e.g. wetting experiments).

Recent publications on high pressure capabilities of state of the art microscopes have shown that they are working far away from physical limits and that there is plenty of room for improvements [2].

The key to high image quality at high pressures is to reduce scattering of the primary beam electrons inside the imaging gas as far as possible while maintaining ideal operation conditions for the SE-detector [3].

In the FEI Quanta 600 ESEM the gaseous environment in the sample chamber is separated by a differential pumping system and two pressure limiting apertures (PLA) from the high vacuum inside the electron column. Nevertheless, a lot of gas streams through the PLA upwards and a significant amount of scattering takes place even before the electron beam is entering the sample chamber [2].

Based on the insights of Monte Carlo and finite element simulations a new aperture holder was designed that significantly reduces this gas flow and therefore also the primary beam scattering. The PLAs are exchangeable and smaller diameters further increase the SNR at the expense of a smaller field of view.

In a conventional ESEM the secondary electron detector is a positively biased electrode which attracts and accelerates the secondary electrons. On their way to the detector the secondary electrons undergo collision ionization which amplifies the signal and generates positively biased gas ions. With increasing chamber pressure this SE signal amplification strongly decreases because the electron mean free path decreases and the SEs do not gain enough energy between collisions to ionize the imaging gas anymore.

By replacing the position and modifying the shape of the detector it can be optimized for high pressure applications. Nearby a needle detector with very small tip radius (R < 10 µm) the electric field is strong enough for SE amplification and by positioning the needle on the sample table it operates at ideal conditions regardless of pressure and working distance. The distance sample to PLA and sample to detector is no longer coupled. A by-product of this design is that the conventional position of the backscatter electron detector (BSE) at the end of the column is no longer blocked by the SE detector.

With this outstanding signal to noise ratio at high chamber pressures the limits of conventional ESEM technology can be crossed. Wetting experiments at low acceleration voltages and low dwell times are possible as well as imaging liquid samples without cooling (see figure 1,2). In figure 3 a BSE image of gold nanoparticle in oil at 10 kPa chamber pressure can be seen and the overall improvements are shown in figure 4.

1. Danilatos, G.D, Foundations of environmental scanning electron microscopy, Advances in Electronics and Electron Physics Vol. 71, 109-250, 1988

2. Danilatos, G.D., Rattenberger, J., Dracopoulos, V., Beam transfer characteristics of a commercial environmental SEM and a low vacuum SEM, Journal of Microscopy Vol. 242, 166-180, 2011

3. Fitzek H., Schroettner H., Wagner J., Hofer F., Rattenberger J., High-quality imaging in environmental scanning electron microscopy – optimizing the pressure limiting system and the secondary electron detection of a commercially available ESEM, Journal of Microscopy, Vol. 262, 85-91, 2015


Johannes RATTENBERGER, Harald FITZEK (Graz, Austria), Hartmuth SCHROETTNER, Julian WAGNER, Ferdinand HOFER
08:00 - 18:15 #6065 - IM03-276 Exploration of non-radioactive alternative stains to uranyl acetate.
IM03-276 Exploration of non-radioactive alternative stains to uranyl acetate.

Uranyl acetate (UAc) has been generally used as a useful and stable staining reagent for ultrathin sections of biological materials in transmission electron microscopy. In Japan, however, the use and purchase of radioactive UAc is regulated by the government due to the safety problem. Therefore it is urgently necessary to find some non-isotopic and non-hazardous alternative electron stains. Taking into account the staining mechanism of UAc at the binding sites of biological tissues (Hayat, 2000), from among the heavy metals belonging to the lanthanoid group (non-radioactive) we selected 3 reagents such as hafnium chloride (HfCl4), samarium chloride (SmCl3) and acetic acid gadolinium (GdAc) as the candidate alternative electron stains.

In order to evaluate precise stainability for these reagents, samples were prepared as follows: the resin blocks which embedded rat liver and rat small intestine mucosal were cut with ultramicrotome and a diamond knife. Serial ultrathin sections were mounted on copper grids coated with 0.5% Neoprene W in toluene. The sections were stained with 4% aqueous UAc and then with Sato’s Pb. As to the candidate alternative electron stains, preparation processing was carried out in the same way. These stained serial sections were observed with a transmission electron microscope (TEM). For determination of relative TEM contrast ratios of organelles in the specimens, each of photo images from sections was given a numerical value of illumination intensity (black-to-white ratio) of average of 30 points for each target structure by using Photoshop software. The relative contrast ratio was expressed as density value of the target structure against a ratio of the value for the plain resin alone.   

TEM observation showed that neither stain generated harmful effects such as uneven staining, contamination and cell disruption. The relative TEM contrast ratio analysis was indicated that HfCl4 have a good staining effect for each biomembranes (nuclear membrane, cell membrane, organelle membrane), nucleolus, mitochondria, rough endoplasmic reticulum, glycogen and heterochromatin at the same level as UAc. Superior stainability of ribosome and mitochondria was obtained on SmCl3 staining. GdAc exhibited good stainability for cytoskeletal filament such as actin filament in addition to organelles, biomembranes and cell components. From these results, GdAc could be excellent substitute for UAc for thin section staining. Furthermore, HfCl4 was helpful for carbohydrate staining, and SmCl3 was for protein substance, which indicated that these stains have different staining mechanism. Moreover, we examined the stain permeability because we frequently analysis semi-thin section samples with ultra-high voltage electron microscope. As the results, good stainability were obtained in the sections treated with HfCl4 and GdAc, respectively. To provide a stain reagent properly depending on the purpose, further development would be required from now on. Additionally, to assess general versatility, we should test the stainability with various samples such as plant, fungi and also material samples.     

 

Hayat MA. 2000a. Positive staining. In: Hayat MA, editor. Principles and techniques of electron microscopy (biological applications), 4th ed. Cambridge, UK: Cambridge University Press. pp. 242–366.  

 

This study was supported by ‘Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)’ of Ministry of Education, Culture, Sports, Science and Technology, Japan [No.: A-15-OS-0009].        


Kanako INOUE (Ibaraki, Japan), Yoshinori MURANAKA, Pyoyun PARK, Hidehiro YASUDA
08:00 - 18:15 #6072 - IM03-278 Grain size determination in nanocrystalline materials using the TKD technique.
IM03-278 Grain size determination in nanocrystalline materials using the TKD technique.

Nanocrystalline (nc) materials, i.e. polycrystalline structures with grain sizes below 100 nm exhibit extraordinary properties strength. As a first assumption, such property derives from the short paradigm “smaller is stronger”. For grain sizes below 50 nm deformation mechanisms usually involve a quasi-stationary balance between dislocation slip and grain boundary mediated mechanisms. But there is still an ongoing debate, which one of these mechanisms governs the deformation behavior of nc metals [1,2].

Therefore determination of grain size, analysis of the local texture and characterization of grain boundaries in nanocrystalline materials are crucial. Different techniques have been tested, such as automated crystal orientation mapping in Transmission Electron Microscopes (TEM) [3]. But they suffer a lack of accuracy due to the nanocrystalline nature of tested specimen. Grain overlapping, for instance, trends to observe smaller grains. By x-ray diffraction, it is also possible to determine grain size, but measurements provide size of coherent domains only that we consider equal to grains.

In this framework, a new technique based of Transmission Kikuchi Diffraction (TKD) has been been recently introduced as a Scanning Electron Microscope (SEM) based method capable of giving orientation maps as the EBSD method but with a spatial resolution improved by up to one order of magnitude [4]. The technique requires a specimen thin enough to be transparent to the electron beam. In the current configuration, it uses hardware and software developed for the EBSD technique. We proposed a new configuration of the TKD where the detector is disposed horizontally on the axis of the microscope instead of being vertically positioned as in the conventional configuration (see Figure). This achieves better spatial resolution and angular resolution than the ones of the current TKD configuration [5]. Moreover, acquisition times are shorter than in the conventional technique, because the intensity of the forward scattered electrons is much higher than that of the large angle scattered electrons.

Firstly, technological challenges such as technique design will be presented. Secondly first results on electrodeposited nanocrystalline nickel will be discussed and they clearly show the full potentialities of this original characterization set-up.

References

[1] Z. Sun, S. Van Petegem, A Cervellino, K. Durst, W. Blum and H. Van Swygenhoven Dynamic recovery in nanocrystalline Ni, Acta Materialia, 91, (2015) p. 91-100

[2] T. Shanmugasundaram, E. Bouzy, A.H. Chokshi Strengthening and weakening by repeated dynamic impact in microcrystals and nanocrystals Materials Science and Engineering A, 639, (2015), p. 97-102.

[3] J. Lohmiller, M. Grewer, C. Braun, A. Kobler, C. Kübel, K. Schüler, V. Honkimäki, H. Hahn, O. Kraft, R. Birringer and P. Gruber, Untangling dislocation and grain boundary mediated plasticity in nanocrystalline nickel, Acta Materialia, 65 (2014) p.295-307

[4] R.R. Keller, R.H. Geiss, Transmission EBSD from 10 nm domains in a scanning electron microscope J. Microsc. 245 (2012) 245-251.

[5] J.J. Fundenberger, E. Bouzy, D. Goran, J. Guyon, H. Yuan, A. Morawiec. Orientation mapping by transmission-SEM with an on-axis detector Ultramicroscopy 161 (2016) 17-22


Antoine GUITTON (METZ CEDEX 1), Julien GUYON, Yudong ZHANG, Emmanuel BOUZY
08:00 - 18:15 #6079 - IM03-280 Transmission Mode in the SEM: Direct measurement of charge load up, secondary electron yield and backscattering coefficient in dependence on the energy of the incident electrons.
IM03-280 Transmission Mode in the SEM: Direct measurement of charge load up, secondary electron yield and backscattering coefficient in dependence on the energy of the incident electrons.

One essential precondition for a successful characterization of material by using electron microscopy is a sufficient electronic conductivity. Insulating or non sufficient conductive specimens becomes negatively or positively charged beside the balance points E1,2,3 where the same amount of backscattered and secondary electrons leave the sample than primary electrons remain in the sample [1]. This charging effects are additionally an important factor for microscopy accessories like phase, monochromator plates etc [2]. We have implemented a modified transmission unit [3,4] in a SEM/FIB NVISION 40 SEM, which allows to measure the adsorbed and transmitted current simultaneously with the BioLogic SP-200 Potentiostat and the specimen current monitor of the SEM respectively. Four different free supporting films were investigated: formvar (C5H8O2), leaf gold, amorphous carbon and amorphous PCS (Pd77.5Cu6.0Si16.5) alloy. Fig. 1 shows the studied formvar film at a) 3 kV and b) 20 kV without and with charging effects. The measurement of the transmitted current delivers the electron range and the application of the formula of Fitting [5], the film thickness of the investigated samples. Fig 1 c) show the normalized transmitted currents It/Ip (It= transmitted current, Ip =probe current) in dependence on the accelerating voltage for two objective aperture sizes of 30 and 120 µm. The electron range is given as the point where the first time transmitted electrons can be detected. The results show the expected independence of the electron range on the aperture size and therefore on the beam current, except for formvar where the differences are caused by charging effects. The obtained thicknesses for gold (157±16 nm) and , Formvar (209±21nm ) agree well with Monte Carlo simulations and those obtained for  amorphous carbon (56±6 nm ) and   amorphous PCS (20±2 nm ) with the expected film thicknesses of 50 nm and 20 nm respectively [2]. With the knowledge of sample thickness, scan speed and adsorbed current, the implemented charge per volume N can be calculated. Fig 1 d) present N in dependence on the energy of the incident electrons exemplary for formvar and gold with the 30 µm aperture. The obtained E3 points are 2,0±0,3 keV and 4,3±0.3 keV. In a second step the adsorbed and transmitted current was measured with and without an additional potential of 50 eV on the sample for two different working distances. With the experimentally obtained currents,this enables to calculate the secondary electron yield δ and the backscattered coefficient η in dependence on the energy of the incident electrons. The results are shown for the amorphous carbon film in Fig.1 e) and f).

[1] Reimer L., Golla U., Böngeler R., Kässens M., Schindler B., Senkel R. Charging of bulk specimens, insulating layers and free-supporting films in scanning electron microscopy. (1992) Optik 92 14-22

[2] Dries, M., Janzen, R., Schulze, T.,  Schundelmeier, J.,  Hettler, S., Golla-Schindler,U.,  Jaud, B.,  Kaiser, U., Gerthsen, D.: The Role of Secondary Electron Emission in the Charging of Thin-Film Phase Plates (2016) Conference Proceeding Microscopy and Microanalysis Ohio; submitted

 [3] Inventor of the Patent of Carl Zeiss SMT GmbH, Entwicklung und Prototypenbau eines STEM Detektors für ein Rasterelektronenmikroskop: Patentnr.: DE       10211977, EP 1347490,US 2003/0230713 A1

[4] Golla U., Schindler B. and Reimer L. Contrast in the transmission mode of a low-voltage scanning electron microscope. (1994)  Journal of Microscopy 173 219-    225

 [5] Fitting, H.-J.: Transmission, energy distribution, and SE excitation of fast electrons in thin solid films. In: physica status solidi (a) 26 (1974), Nr. 2, 525–535.

[6] We kindly acknowledge M. Dries and R. Janzen of the KIT Karlsruhe for the amorphous carbon and PCS sample, the BMBF “Li-EcoSafe” joint research project  (FKZ: 03X4636A) and the Ministry of Science, Research and the Arts (MWK) of Baden-Wuerttemberg in the frame of the SALVE (Sub Angstrom Low-Voltage Electron microscopy and spectroscopy project) for financial support.


Bianca JAUD, Jörg BERNHARD, Ute KAISER, Ute GOLLA-SCHINDLER (Ulm, Germany)
08:00 - 18:15 #6106 - IM03-282 Improvement of a 40-120 kV analytical TEM system for electron beam irradiation sensitive nano materials.
IM03-282 Improvement of a 40-120 kV analytical TEM system for electron beam irradiation sensitive nano materials.

The performance of advanced nanomaterials such as the electrode catalysts of fuel cells is closely related to their composition, morphology and crystal structure. At the nanoscale, high resolution TEM is essential to understand the relationship between structure and electrochemical properties. In case of TEM characterization of chemically synthesized nanomaterials, highest attention should be paid to electron irradiation damage. If the observation conditions, especially accelerating voltage and electron beam density, are not optimized, an initial structure will be changed. For example, different from general inorganic materials, some materials such as chemically synthesized amorphous metal nano particles are quickly crystallized by the electron irradiation and it often mislead the characterization. Therefore, careful control of illumination conditions and irradiation time are essential for the analysis of those composites. The selection of an optimal accelerating voltage based on composition is one option to reduce irradiation damage.  In addition, sample observation with lower accelerating voltages is advantageous to generate higher image contrast.

Since standard 120 kV TEMs are optimized for applications at lower magnification with higher contrast requirements, we have developed an ultra high resolution objective lens (UHRLENS) to extend the application in nanomaterial field of a 40-120 kV HT7700 analytical TEM [1,2]. It mounted with the UHRLENS which provides lattice resolution of 0.2 nm with on-axis illumination at 120 kV and accommodates a high solid angle silicon drift detector of an energy dispersive X-ray (EDX) analyzer. Figure 1 shows an example of high resolution and high contrast observation of the fuel cell electrode catalyst by the newly developed TEM. Lattice images of platinum nanoparticles (lattice spacing: 0.23 nm) and the carbon support (lattice spacing: 0.34 nm) were observed clearly at an acceleration voltage of 120 kV.  The binder is clearly observed with sufficient contrast.

To analyze the crystal structure of nanomaterials, we have used a selected-area electron beam diffraction (SAED) technique with a micro-fabricated SA aperture hole rather than nano-probe diffraction [3]. Because the damage caused by the electron beam irradiation was much less than that of the nano-probe electron diffraction technique. An FIB fabricated apertures with the diameter of 1mm are equipped for structural analysis of individual nanomaterial. The smallest selected area diameter on the specimen is calculated to be 18nm. In case of characterization of  nano materials, the spatial error in SAED pattern due to a spherical aberration is not serious because;

1) No high-order diffraction spots available from nano-materials such as nano particles

2) High order diffraction  spots are not used in practice

To improve operability for acquirements of the SAED, a new automatic operation function, called “nano analysis function”, is produced. This function enables automatic acquirements of SAED at plural analysis positions pre-designated by a user. The analysis position of the SAED is precisely controlled by an image shift coil mounted just below the objective lens. The minimum diameter of SA aperture is 1 um, which corresponds to the diameter of the selected area of 18 nm on the specimen. Figure 2 shows a TEM image (a) of an asbestos specimen with the corresponding SAED patterns (b) acquired by the nano analysis function. The selected area of each analysis position is displayed by a circle on the TEM image.  The diffraction patterns of the acquired SAED images can be analyzed with an optional function of Hitachi EMIP software called “diffraction analysis function”. This function enables automatic measurement of diffraction spot intervals and assumption of elements contained in the selected area of the specimen from the database.

The conventional 40-120kV analytical TEM has been improved for characterization of electron beam sensitive nanomaterial by high-resolution TEM with high contrast. The improved functions made it possible to identify the crystal structure by selected nano area electron diffraction technique.

References:

[1] Kubo, T., et al, 2013.  Microsc.Microanal. 19 (Suppl 2), 1328.

[2] Yaguchi, T., et al, 2015.  Microsc.Microanal. 21 (Suppl 3), 1817.

[3] Kamino T., et al, Proc.of IMC 18, Prague, Czech Republic (2014) IT-6-P-1552.


Toshie YAGUCHI (Hitachinaka-shi, Japan), Keiji TAMURA, Takashi KUBO, Masaki KONDO, Hiromi MISE, Hiroaki MATSUMOTO
08:00 - 18:15 #6127 - IM03-284 An absolute sample position referencing solution for convenient cross-platform observations; application to the assessment of microscope stability and translation stage reproducibility.
IM03-284 An absolute sample position referencing solution for convenient cross-platform observations; application to the assessment of microscope stability and translation stage reproducibility.

We present the prototype of an instrument add-on that tracks the absolute position and orientation of a sample, and that can be implemented on several microscopes in order to provide virtual coupling between the platforms. It is aimed at avoiding wasting observation time at finding the same regions of interest indentified in former observations. It is also believed to improve the throughput of characterization platforms by providing an easy way for researchers to resume observations initiated at different availability slots of a microscope.

While fiducial markers and shuttle systems offer some solution for localizing regions of interests, they come along with a number of limitations, including requirement for tedious calibration, inadequate accuracy, and limitation to proprietary hardware. Another approach consists in attaching to the sample a plane with coded markers [1], and deduce the sample position from the observation of these markers. We have improved this concept by introducing the possibility to perform real-time determination of the sample position [2] during normal microscopy observations. The system consists in a coded plane substrate holder, an add-on to the microscope that allows the observation of the coded plane, a software module that deciphers the coded substrate, and a user interface.

The precision of the position and angular orientation measurements have been investigated. It is found that the standard deviation is less than 1nm in position, and less than 10µrad in orientation. Such precision values obtained with an image-recognition based approach establish that our position recognition system works in a superresolution regime, corresponding to localization microscopy. While this exceeds the usual requirements of optical microscopy, in practice, it is useful to assess the mechanical drifts of the microscope that degrade the precision, and check that theses drifts are acceptable, as discussed further. Fig. 1 gives an example of a sample map that has been produced with a succession of observations performed on two different microscopes and with different magnifications. The red rectangle outlines the real-time observation region on the sample map. Such cross-plateform absolute referencing system is believed to make it easier for communities who have some definite microscopy modalities as gold standard, to add the benefits of other modalities such as Raman microscopy.

Another interesting application of our approach is the assessment of the drift of microscopy apparatus and the reproducibility of moving stages. In this “stage patroller” configuration, the coded plane is observed through the main camera of the microscope and the obtained images are interpreted into position and orientation information. The drift of the platform is measured by recording the position as a function of time, with no intentional movement. Fig. 2 shows the drift on an Olympus BX41 equipped with a Marzhauser stage.

The stage patroller configuration has also been used in order to assess the accuracy of a moving stage. This is of particular importance for a number of microscopy modalities. While long distance accuracy of moving stages is generally not so crucial in microscopy, the capability to return at a same exact position is very important. Any departure from positioning reproducibility may result in misleading images due to distortion or shifts, when microscopy information is obtained by sweeping the stage around the position of interest. Our “stage patroller” configuration provides an easy way to observe the reproducibility accuracy of a moving stage. In an experiment, we instructed the moving stage to move repeatedly between 4 corners of a 500µm square. Figure 3 shows a close view around one of the square corner of the measured trajectory map. It can be observed that there is a significant scattering of the corner position, which may blur even normal resolution optical microscopy images. As a consequence, real-time absolute sample position referencing add-on may prove very useful to avoid such hard-to-detect blurs and shifts. The stage patroller may also be used to improve the reproducibility of a moving stage, by adjusting some rather cryptic user-defined stage settings, through a trial and error approach.

[1] P. Sandozn R. Zeggari, L. Froehly, J. Prétet, C. Mougin, J. Microsc., 225, 293–303 (2007)

[2] O. Acher, A. Podzorov, PCT Pat. Appl. WO2014016526.


Olivier ACHER (PALAISEAU), Alexander PODZOROV, Alexandre GRIGORIEV
08:00 - 18:15 #6143 - IM03-286 ImageEval. A software for the processing, evaluation and acquisition of (S)TEM images.
IM03-286 ImageEval. A software for the processing, evaluation and acquisition of (S)TEM images.

Quantitative scanning or conventional TEM, as well as electron diffraction techniques usually require the processing of raw data, the comparison with simulated counterparts and a meaningful visualisation of results. Here we present ImageEval, a Matlab-based software which contains established techniques in a modularised manner with a graphical user interface. This enables the efficient, reproducible application of established techniques and assures both the access to and the transparent exchange of know-how among different groups after a pioneer methodical development. In the following we summarise central ImageEval features.

The concept. ImageEval currently hosts 13 evaluation methods as separate modules. All modules can be used independently of each other but share a common data structure, so that results of one module can be further evaluated in another. Intermediate results, simulated reference or structural data are stored as meta data that is available for comparison or evaluation in all modules. The program reads Gatan (dm3) and FEI (emi/ser) file formats as well as common image formats. Large image series, e.g., 4-dimensional STEM data which often contain ~106 images and more, can be handled by pointers to files so as to load images into workspace only when to be processed. ImageEval is available as a standalone executable Version for Windows/Unix or as the Matlab source code directly, the latter option enabling the adaptation of evaluations for individual tasks.

STEM Z-contrast. This module allows for the quantitative composition and/or thickness mapping based on (High-resolution, HR-) STEM images either on the atomic lattice or on a regular user-defined grid. To this end, STEM intensities extracted from a Voronoi segmentation are compared with simulations to be loaded as reference data [Ultram. 109, 1171 (2009)]. For complex unit cells, the comparison can be distinguished with respect to an arbitrary number of sublattices for the different atomic sites, and intensities extracted from different images (e.g. bright & dark field acquired simultaneously) may be used simultaneously.

High-resolution strain state analysis. HR(S)TEM images can be evaluated by detecting atomic column positions (intensity maxima or minima) or lattice fringes with subpixel accuracy, e.g., by parabolian or Gaussian fits. Various filtering options are available, namely a Wiener noise filter which preserves the contrast of the high-resolution image. The vector field of the displacement from a regular lattice, its projection along a given direction and the local strain can be calculated [Optik 102, 63 (1996)].

Strain analysis by nano-beam electron diffraction (SANBED). Series of parallel- or convergent-beam electron diffraction patterns (CBED) are evaluated as to the positions of the reflections using centre of gravity computation (spot pattern) as well as radial gradient maximisation, edge detection or cross correlation methods [Micr. Microanal. 18, 995 (2012)]. Typically these are 3D or 4D data sets, corresponding to the acquisition of diffraction patterns on a STEM scan line or area.

Differential Phase Contrast (DPC). This module calculates the centre of gravity (average momentum [Nat. Comm. 5, 5653 (2014)]), the signals of a segmented quadrant detector, bright- and annular bright field signals from a 3D or 4D data set, i.e. ronchigrams recorded at each position of the STEM probe. Tools for angle calibration and calculating the charge density are also available.

COM interface. ImageEval can communicate with the common object model (COM) of the Microscope and the acquisition software. This enables the efficient implementation of individual experimental procedures such as acquiring a STEM image series while automically changing imaging parameters subsequently (e.g. acceptance angles of the ADF detector).

Basic Tools. Routinely needed tools such as cross-correlation of images to correct for relative shifts, (inverse) Fourier transform, line profiling, calculation of rotational average/sum, applying circular, annular or polygon masks, binning and image rotation are collected here.

Further methods include the analysis of angle-resolved STEM data, composition evaluation by lattice fringe analysis (CELFA, [Ultram. 72, 121 (1998)]) exploiting chemically sensitive imaging, geometric phase analysis (GPA (Ultram. 74, 131 (1998)]), data reduction in spot diffraction patterns, parametric fits ("atom counting" [Nature 470, 374 (2011)]), and processing of simulation results from the STEMsim software [Spr. Proc. Phys. 120, 169 (2007)] .

[1] K. M-C. was supported by the Deutsche Forschungsgemeinschaft (DFG) under contract No MU 3660/1-1.


Knut MÜLLER-CASPARY (Bremen, Germany), Thorsten MEHRTENS, Marco SCHOWALTER, Tim GRIEB, Andreas ROSENAUER, Florian F. KRAUSE, Christoph MAHR, Pavel POTAPOV
08:00 - 18:15 #6167 - IM03-288 ‘Slice & View’ nano-tomography of porous media using FIB-SEM.
IM03-288 ‘Slice & View’ nano-tomography of porous media using FIB-SEM.

Research on porous media is of major importance to many fields varying from catalysis to Earth science. Characterizing such media in 3D is expected to provide information on surface and volume rendering, which is required for understanding fluid transport. In the present study, electron tomography performed in the FIB-SEM (Focused Ion Beam - Scanning Electron Microscope) [1] is applied to a highly porous Diatomaceous Earth material. Representative volumes (Figure 1) have been obtained and treated in Fiji and Avizo software, the pore volume and pore connections are studied and quantified to generate the pore network [2-5]. Next, the data is used for fluid simulation through Computational Fluid Dynamics (CFD) methods, in order to study and analyze flow and diffusion properties in the obtained pore network (Figure 2). The geometry of pores is also evaluated to study how it influences transport properties in the sample. Furthermore, the effects of image correction and segmentation on the simulation are also examined. 

 

 

[1] L. Holzer et al, Journal of Microscopy, 216 (2004), 84–95.

[2] P.S. Jørgensen et al, Utramicroscopy, 110 (2010), 216-228.

[3] T. Prill et al, Journal of Microscopy, 250 (2013), 77–87.

[4] M. Salzer et al, Journal of Microscopy, 257 (2015), 23–30.

[5] K.R. Mangipudi et al, Utramicroscopy, 163(2016), 38-47. 


Yang LIU (Utrecht, The Netherlands), Pierre LE FUR, Marijn VAN HUIS, Oliver PLÜMPER
08:00 - 18:15 #6175 - IM03-290 3D surface reconstruction with segmented BSE detector: New improvements and application for fracture analysis in SEM.
IM03-290 3D surface reconstruction with segmented BSE detector: New improvements and application for fracture analysis in SEM.

Using the signals of four backscattered electron (BSE) detectors with different detection angles in the scanning electron microscope (SEM) the three-dimensional surface topography of various samples, i.e. catalysts, fractured surfaces and micro-devices can be derived and analyzed [1], [2]. An efficient shape from shading reconstruction algorithm is applied to these signals to extract high resolution height and texture information. The surface reconstruction is very fast and needs no sample tilting since the surface topography is calculated from the four simultaneous recorded backscattered electron images.

While the 3D reconstruction of the surface topography works very well, the calculation of quantitative height differences depends on different imaging and geometric parameters and requires a calibration of the used BSE detectors. This includes the adjustment of gain and offset of the signals as well as the checking of the geometrical properties of the detector, i.e. detector radius, height, detection area and the horizontal angle to the scan rotation. Especially the height determination, which also depends on the adjustment of the working distance, is hard to determine. Therefore, a spatial calibration is applied with the help of 3D calibration standards [3]. As a result, not only lateral scaling factors, but also z-scale and shearing effects are estimated. Furthermore, nonlinear deviations are calculated and allow an evaluation of the local and overall accuracy of topographic data, which is achieved applying 3D reconstruction using a calibrated 4Q-BSE detector.

For better accuracy, the reconstruction algorithm was improved by applying refined geometry for the primary and the backscattered electron beam. At low magnifications, the electron beam is not perpendicular to a horizontal specimen surface and the distribution of the backscattered electrons is not isotropic. In addition, the distance between the specimen and the detector is not constant for all image points. Without consideration and correction, this yields to spherical distorted surfaces. The advanced 3D reconstruction algorithm includes geometrical improvements, allowing a distortion-free mapping of the surface topography over a large magnification range.

As an example for application of 3D reconstruction in material sciences, fracture surfaces of a copper base alloy were analyzed. The applied SEM (Hitachi S-520) is equipped with a complete digital imaging system and in particular with a 4-quadrant BSE detector (point electronic GmbH) and was geometrically calibrated using a 3D calibration standard. Thus, the spatial scale factors were determined for a magnification of 1000x to cx = 1.013, cy = 1.024, cz = 1.198. While the remaining maximum geometrical deviations after application of these calibration parameters were evaluated with dx = 60 nm, dy = 41 nm and dz = 57 nm the spatial mean deviations for the whole measurement volume is 16 nm.

The figures present some results of the investigation. The upper row of pictures shows the dimpled surface microstructure of a forced fracture where the material mostly cracks in a ductile trans-crystalline manner. The pictures of the bottom row show some crystallographically oriented facets of a fatigue fracture of the same material. On the left for both cases the secondary electron (SE) images are shown, on the right side pictures of the 3D reconstructions of the BSE images are presented. Height differences can be visualised immediately whereas more complex data will be derived from the 3D datasets.

The improved 3D reconstruction algorithm is available as standalone software version as well as integrated solution for SEM. Integration into a SEM system allows not only on-line 3D visualisation, but also easier calibration and especially more reliable application, because full control over all relevant physical and imaging parameters is guaranteed. The integrated topographic 3D reconstruction was developed in cooperation with point electronic GmbH and is now available within their SEM control hard- and software DISS. Therefore we like to thank point electronic GmbH for the fruitful cooperation.

[1] Beil, W., Carlsen, I. C.: Surface Reconstruction from Stereoscopy and “Shape from Shading” in SEM images. Machine Vision and Applications (1991) 4:271-285.

[2] Paluszynski, J., Slowko, W.: Surface reconstruction with the photometric method in SEM. Vacuum 78 (2005) 533-537.

[3] Berger, D., Ritter, M., Hemmleb, M., Dai, G., Dziomba, T.: A new quantitative height standard for the routine calibration of a 4-quadrant-large-angles BSE-detector. EMC (2009) 533-534.


Matthias HEMMLEB, Dirk BETTGE, Iryna DRIEHORST, Dirk BERGER (Berlin, Germany)
08:00 - 18:15 #4960 - IM05-292 Characterization of the splitting ratio of bi-component spunlaced fabric via pixel classification.
IM05-292 Characterization of the splitting ratio of bi-component spunlaced fabric via pixel classification.

As a consequence of air pollution deterioration worldwide, the global filtration industry is constantly searching for cost-effective solutions with extremely high separation efficiency and remarkably high air flow at low energy consumption for various applications. To answer these market needs, SABIC is developing new nonwoven bi-component spunlaced filer media technology. Studies indicate that an increase in splitting ratio of bi-component fibers enhances the absorption and filtration efficiency, and the mechanical properties of the nonwoven fabric [1]. Currently, fiber splitting ratio is characterized by visual inspection of the Scanning Electron Microscopy (SEM) images, or determined indirectly by measuring filtration properties. There is little information available in literature for quantifying the splitting ratio via image analysis [2].

A novel method has been developed to quantitatively characterize the splitting ratio of bi-component fibers based on SEM images. The workflow for the proposed method is given in Figure 1. As there is no intensity differences between the split and un-split fibers, the thresholding methods for image segmentation are not applicable for this purpose. Alternatively, texture information offers a description of spatially extended patterns of intensity distributions within a neighborhood. The split fibers have smaller diameters, which makes the frequency of edges high. Besides, the changes in orientation are also very frequent because of the entanglement of the twisted fibers. Both texture patterns can be captured by Log-Gabor filter bank [3]. The extracted Gabor texture features are then treated as input to Expectation-Maximization clustering (EM-clustering) to discriminate the two different texture patterns from the two types of fibers. Originally, EM-Clustering is unsupervised machine learning method, which assumes there is a mixture of a definite number of Gaussians within a set of unlabeled data [3]. However, fibers entanglement and fibers depth-penetration make the variations in texture patterns too wide to converge the un-supervised EM-clustering. As such, semi-supervised EM-clustering is implemented instead, which includes a training step on a small set of labeled data to gain some prior knowledge on the targeted Gaussian Mixtures Model. For each image, around 10% of the pixels are randomly selected for training. Initiated with the trained model, pixels belonging to split and un-split fibers are classified further with EM-clustering on extracted Gabor texture features [3]. Post morphology processing follows to finalize the image segmentation. Based on the final image segmentation result, the fiber splitting ratio is then characterized in terms of area percentage of split fiber within the imaged web. Examples of stepwise image analysis results are illustrated in Figure 2. Furthermore, a correlation is established between the derived splitting rate and the permeability of the products. The results on the fiber splitting ratio give a proof of concept in understanding the correlation between filtration efficiency and splitting ratio. This method can also work with SEM images of the sample cross sections to gain an insight of the splitting ratio through the thickness of the sample.

[1]. Baker, B., Bicomponent fibers: A personal perspective, International Fiber Journal, 1998, 13(3), 26-35.

[2]. L. B. Suragani Venu, E. Shim, N. Anantharamaiah and B. Pourdeyhimi, 3D Structural Characterization of Nonwoven Fabrics. Microscopy and Microanalysis, 2012, 8, 1368-1379.

[3]. F. van der Heijden, R. Duin, D. de Ridder, D. M. J. Tax, Classification, Parameter Estimation and State Estimation: An Engineering Approach Using MATLAB, Wiley, 2004.


Chanjuan LIU (Bergen op Zoom, The Netherlands), Sebastien PIERRAT, François COURTECUISSE, Richard PETERS, Richard LUCAS
08:00 - 18:15 #5006 - IM05-294 Experimental determination of electron-beam broadening in low-energy STEM.
IM05-294 Experimental determination of electron-beam broadening in low-energy STEM.

The interest in scanning transmission electron microscopy (STEM) at low primary electron energies E ≤ 30 keV has steadily grown in the past. The benefits of low-keV STEM are high material contrast for low atomic number materials and prevention of knock-on damage. Moreover, low-keV STEM can be easily performed in a standard scanning electron microscope. However, the mean free path values decrease with lower E which substantially increases the number of scattering events and leads to plural and multiple scattering even at small specimen thicknesses. As a consequence, the beam is broadened within the sample which worsens the lateral resolution of the technique. Since single scattering events and the resulting beam broadening cannot be measured directly, we will present an approach to estimate beam broadening by using an amorphous carbon (aC) thin-film.

The aC-film was deposited by electron-beam evaporation onto a cleaved mica substrate. The film thickness of 10 nm was determined by TEM of a cross-section TEM-sample from a simultaneously coated Si-substrate. The aC-film is floated off the mica substrate on a distilled water surface and deposited on a copper grid. In this process, cracks are formed in the film which lead to multiple folding of the film at the crack edges. Such a crack edge is imaged with the bright-field (BF) detector in Figure 1. The film is folded in a way that regions are created where a discrete number of aC-layers are stacked. Some of these regions are marked with the number of stacked layers. Averaging the 16-bit gray-scale values in these areas gives intensity values for several discrete film thicknesses. The red dots in Figure 2 show the resulting intensities for the areas in Figure 1 after subtracting the black value (intensity with blanked electron beam) IB and normalization with the intensity of the incident electrons I0.

The used STEM detector is a semiconductor detector which is composed of a circular BF segment, four separately controllable annular dark-field (DF) segments and a large high-angle annular dark-field (HAADF) segment. Figure 3b shows a scheme of the detector rings and the corresponding outer detection angles for the used working distance of 6.3 mm. The region close to the crack edge (cf. Figure 1) was imaged with all detector segments. The accumulated normalized intensity values up to the indicated detector at E = 20 keV are plotted in Figure 2.

The radial intensity of an electron probe can be defined by the integration of a Gaussian intensity distribution [1]. We refer to the beam width b as the diameter of the circle that contains 68 % (1 σ of a Gaussian) of the total probe current which is marked by the dotted black line in Figure 2. The crossings of the dashed vertical lines in Figure 2 with the dotted line indicate the film thicknesses at which the intensity falls below 68 % for the different detector segments. At these thicknesses the beam is broadened to the outer detection angle of the indicated detector. Assuming the mean scattering position at half thickness of the film, the beam width at the bottom of the film can be calculated by b=t tanφ [2] (cf. Figure 3a). The red curve in Figure 4 shows the calculated beam widths as a function of the sample thickness for E = 20 keV derived from Figure 2. The same procedure was repeated for various electron energies from 10 to 30 keV and the resulting beam widths are plotted in Figure 4.

For all energies the beam width rises with increasing thickness. This behavior is expected due to the increasing scattering probability. Less pronounced broadening for higher E is expected for the same reason as well. The absolute values show that even for small thicknesses up to 90 nm, like in our experiment, and a low-density material like amorphous carbon, the beam width increases to a multiple of the original beam diameter. This points out that thin samples for high-resolution low-keV STEM are required.

 

References

[1]    L. Reimer, Scanning electron microscopy: Physics of image formation and microanalysis, 2nd ed., Springer, Berlin, London, 1998.

[2]    J.I. Goldstein, J.L. Costley, G.W. Lorimer, S.J. Reed, Scanning Electron Microscopy 1 (1977) 24.

[3]    We acknowledge funding by the German Research Foundation (DFG).


Holger DREES (Karlsruhe, Germany), Erich MÜLLER, Dagmar GERTHSEN
08:00 - 18:15 #5075 - IM05-296 Non-local averaging in EM: decreasing the required electron dose in crystal image reconstruction without losing spatial resolution.
IM05-296 Non-local averaging in EM: decreasing the required electron dose in crystal image reconstruction without losing spatial resolution.

Todays electron microscopes enable imaging of materials at atomic resolution. However, in many relevant applications, the resolution is not limited by the microscope's physical properties, but by the beam sensitivity of the material. The beam sensitivity limits the applicable electron dose before beam damage corrupts the measurement, which leads to low signal-to-noise ratios (SNR). The SNR in turn limits the effective resolution, or, in other words, the precision of any analysis performed on the measurement.

Typically, the SNR is improved either by local averaging or by averaging multiple aligned images of the same specimen. Unfortunately, the former reduces the effective spatial resolution of the image, while the latter increases the applied electron dose. We propose to circumvent these issues by combining the benefits of both approaches in a single method. The key observation is that most EM measurements (e.g. (S)TEM, EELS, EDX) of atomic resolution contain many self-similar regions: due to the crystal structure, any unit cell is typically pictured more than once. This is an ideal setting for non-local averaging methods, which have become very popular over the past few years due to their ability of substantially reducing noise without blurring the image.

The gold standard amongst modern non-local averaging algorithms in general digital photography is the Block-matching and 3D filtering algorithm (BM3D) [1]. However, due to its generality, BM3D does not make full use of the aforementioned rich self-similarity typically found in atomic scale micrographs. Thus, we propose a denoising strategy based on BM3D, but specially tailored to atomic scale electron micrographs [2]. The key feature is a new method for the automated analysis of the projected specimen geometry from the image, i.e. the detection of regions with different crystal structure [3] as well as their primitive unit cell dimensions [4]. This information allows us to predict the position of similar image parts, thus maximizing the potential of BM3D.

Most single image averaging techniques, including BM3D, are designed to remove Gaussian noise. However, aside from approximately Gaussian noise generated by the sensor itself, EM measurements typically contain contributions of Poisson noise as well due to the electron counting statistics. To address this issue, we developed an automated procedure for estimating the parameters of a mixed Poisson-Gaussian noise model directly from raw EM data, which allows the use of averaging techniques designed for Gaussian noise removal.

Compared to the commonly used local averaging techniques, our proposed method achieves significantly higher effective resolution. Furthermore, our method can be easily combined with the aforementioned alignment and averaging techniques for multiple images of the same specimen, thus significantly reducing the electron dose required for a useful reconstruction.

Figure 1 demonstrates the proposed method on an exemplary HAADF-STEM image with three crystal regions (SrTiO3, BaTiO3 and SrRuO3) [5]. Figure 1a shows the electron micrograph and indicates the detected region boundaries (green), as well as the lattice vectors of a primitive unit cell within each of the three crystals (red). The result of non-local averaging with the proposed method is shown in Figure 1b. A zoom into a region around the bottom crystal inferface is given (yellow) that simplifies the assessment of the increase in SNR. While in the original STEM image, the distinction between the SrTiO3 and BaTiO3 regions is hardly possible with the naked eye, the increased contrast of the denoised image makes it clearly visible.

In Figure 2a we show a few selected spectra of an EELS dataset with a low SNR especially in the last components of the spectrum [6]. Figure 2b shows the corresponding spectra after denoising with the proposed method. The result demonstrates that the proposed method also applies very well to hyper-spectral data and excels at significantly increasing the SNR of the spectra without introducing artifacts or blurring fine details.


The authors would like to thank Daesung Park for providing us with the experimental HAADF-STEM images, as well as Martial Duchamp for providing the EELS dataset.

[1] Dabov, K., Foi, A., Katkovnik, V. et al., IEEE Transactions on Image Processing 16, 2080-2095 (2007).
[2] Mevenkamp, N., Binev, P., Dahmen, W. et al., Advanced Structural and Chemical Imaging 1 (2015).
[3] Mevenkamp, N., Berkels, B., WACV Proceedings (2016).
[4] Mevenkamp, N., Berkels, B., GCPR Proceedings, pp. 105–116. (2015).
[5] Park, D., Herpers, A., Menke, T. et al., Microscopy and Microanalysis 20, 740-747 (2014).
[6] Duchamp, M., Lachmann, M., Boothroydet, C.B. et al., Applied Physics Letters 102.13, 133902 (2013).


Niklas MEVENKAMP (Aachen, Germany), Benjamin BERKELS
08:00 - 18:15 #5117 - IM05-298 Determination of the three particle structure factor from experimental images.
IM05-298 Determination of the three particle structure factor from experimental images.

The structure of amorphous materials can be described by their correlation functions gn(r1,...,rn) where gn(r1,...,rn) d3r1···d3rn gives the probability of finding particle i in the volume d3ri and so on [1]. The pair correlation function g2(r1, r2) is well known experimentally from scattering experiments. However, it describes only the distribution of atomic distances. Information about bond angles is lost. This would be accessible if the triple correlation function g3 (r1,r2,r3) could be measured.
The Fourier transform of the correlation functions are the so-called structure factors. These can be directly obtained from experimental images [2]. Previous attempts to implement this approach, however, have failed [3, 4].
A few years ago Huang et al. [5] made incredibly well defined images of a layer of amorphous silica where the resolution was high enough to resolve atomic spacings. As the atomic positions are directly visible in these images, we used them to obtain the three particle structure factor.

In a first approximation we assumed ideal imaging and determined the structure factor S(1) simply as the Fourier transform of the measured intensity. The two particle structure factor S(2) is then calculated as the square of its absolute value and finally the three particle structure factor is obtained as S(3) (q1,q2) = S(1)(q1) S(1)(q2) S(1)(−q1−q2) where q1 , q2are spatial frequencies in two dimensions [2].

Figure 1 shows the two particle structure factor of amorphous silica. There are two peaks. The first one at q ≈ 0.3 Å-1 is broader and we expect it to represent the atomic distances in the specimen. The second peak at q ≈ 0.5 Å-1 is sharper and due to the graphene substrate. Since amorphous matter is expected to be isotropic we average over one spatial angle and consider S(3) as a function of only three variables |q1|, |q2| and the angle φ between qand q2. To reduce the number of degrees of freedom even more, we took q1 = q2 =: q and chose q to be at the first peak in S(2)(q) and only varied φ. A first result is shown in figure 2.
In a crystal one has well-defined binding angles and thus expects S(3)(φ) to have sharp peaks at those angles. In our case we find peaks around 60° and 120° stemming from the approximate 6-fold symmetry of silica but compared to the case of crystals they are smeared out. For the first time we have been able to determine the three particle structure factor from TEM images. We are now undertaking a systematic study to obtain further insights into the amorphous structure of two-dimensional glasses.



Acknowledgements: We are grateful to Prof. Dr. Ute Kaiser (University of Ulm) for providing her image data to us.

                  

[1] J.M. Ziman. Models of Disorder: The Theoretical Physics of Homogeneously Disordered Systems. Cambridge University Press, 1979.
[2] M Hammel and H Kohl. Determination of the triple correlation-function of amorphous specimens from em micrographs. In Inst. Phys. Conf. Ser., number 93, pages 209–210, 1988.

[3] Michael Hammel. Bestimmung der abbildungsparameter und der korrelationsfunktion dritter ordnung amorpher objekte aus elektronenmikroskopischen phasenkontrastaufnahmen. Master’s thesis, TH Darmstadt, 1988.
[4] Ansgar Haking. Bestimmung des drei-teilchen-strukturfaktors amorpher stoffe aus hochaufgelösten elektronenmikroskopischen aufnahmen. Master’s thesis, WWU Münster, 1995.
[5] Pinshane Y. Huang, Simon Kurasch, Anchal Srivastava, Viera Skakalova, Jani Kotakoski, Arkady V. Krasheninnikov, Robert Hovden, Qingyun Mao, Jannik C. Meyer, Jurgen Smet, David A. Muller, and Ute Kaiser. Direct imaging of a two-dimensional silica glass on graphene. Nano Letters, 12(2):1081–1086, 2012.


Semir VRANA (Münster, Germany), Helmut KOHL
08:00 - 18:15 #5152 - IM05-300 Composition quantification of thin samples by backscattered electron imaging in scanning electron microscopy.
IM05-300 Composition quantification of thin samples by backscattered electron imaging in scanning electron microscopy.

The contrast of backscattered electron (BSE) images in scanning electron microscopy can be exploited for atomic number or material density determination [1]. However, BSE images suffer from limited spatial resolution for bulk specimens due to the large interaction volume of the primary electrons. This limitation can be overcome by using electron transparent samples as is demonstrated in this work.

Comparison of experimental BSE intensities with calculations is required for the quantification of material contrast. We apply here the electron diffusion model of Werner et al. [2] which considers single electron scattering and electron diffusion. To verify and adapt the diffusion model, the calculated results are compared with Monte-Carlo (MC) simulations [3]. The limited detection angle range of the used annular semiconductor detector from 2.3 rad to 2.77 rad must be taken into account in the calculations. It is also important to take into account the threshold energy of the semiconductor detector, because BSE below 2 keV are not detected by our BSE detector. Hence, the calculation of the energy loss of the BSE is necessary and was accomplished on the basis of an expression for the electron energy dissipation given in [4]. The validity of our procedure for composition analysis is verified by analyzing a sample with known composition and geometry.

The investigated sample contains four InxGa1-xAs layers of 25 nm thickness with In-concentrations of x = 0.1, 0.2, 0.3 and 0.4 which are embedded in GaAs-barrier layers with 35 nm thickness. Details on the growth and verification of the composition of the analyzed sample by alternative techniques are outlined by Volkenandt et al. [5]. Cross-section samples with wedge-shaped thickness profiles are prepared perpendicular to the layer system by focused-ion-beam (FIB) techniques. A FEI Quanta ESEM equipped with an annular BSE semiconductor detector is used for the measurements.

Fig. 1a shows a BSE cross-section image of a wedge sample with the brighter InxGa1-xAs layers separated by GaAs with lower intensity. A Pt-layer was deposited prior to FIB milling to protect the sample. The thickness of the wedge sample is determined in a region with known composition (here GaAs). For this purpose, an intensity line scan along the wedge with increasing thickness is performed in the GaAs substrate (green arrow in Fig. 1a) at different primary electron energies (Fig. 1b). The thickness-dependent BSE intensity is normalized with respect to intensity in the thickest part of the wedge, which corresponds in a good approximation to the bulk BSE intensity. By comparison with calculations of the thickness-dependent backscattering-coefficient ratio η(t)/η(bulk) (black lines) the offset thickness at the thin edge of the wedge and the local thickness along the line scan can be determined.

Subsequently a line scan perpendicular to the layer system (red arrow in Fig. 1a) is performed at a constant thickness of 200 nm. BSE intensity ratios of the InxGa1-xAs quantum wells with respect to the GaAs barrier layers are shown in Fig. 2a. Lines with different colors denote calculations for η(t)InGaAs/η(t)GaAs for different E0 and thicknesses of (200 ±  20) nm. The calculated intensity ratios agree well with the measurements. The accuracy of the technique improves for higher E0 values because the gradient of the intensity ratios increases. This allows to distinguish In-concentration differences of 10%.

Fig. 2b shows η(t)InGaAs/η(t)GaAs for 20 keV as a function of the sample thickness. Only a weak dependence on the local specimen thickness is observed between 50 and 250 nm giving the optimal range for composition quantifications. At lower thicknesses the BSE intensity is low, while at higher thicknesses the contrast blurs due to the electron beam broadening.

It is shown that contrast quantification of BSE images is possible with a high lateral resolution. The sample thickness and the material composition were determined within one single image. Quantifications are successfully performed by comparison of the experimental with calculated data from an analytical model.

References

[1] S. A. Reid, A. Boyde, J. Bone Miner. Res. 2, 13 (1987)

[2] U. Werner et al., Ultramicroscopy, 8, 417 (1982)

[3] N.W.M. Ritchie, Surf. Interface Anal., 37, 1006 (2005)

[4] L. Reimer, Scanning Electron Microscopy, Heidelberg, Germany: Springer, 105 (1998)

[5] T. Volkenandt et al., Microsc. Microanal. 16, 604 (2010)


Erich MÜLLER (Karlsruhe, Germany), Dagmar GERTHSEN
08:00 - 18:15 #5190 - IM05-302 Principle component analysis applied to high resolution cross sectional STEM imaging: Quantitative analysis of 2D heterostructures.
IM05-302 Principle component analysis applied to high resolution cross sectional STEM imaging: Quantitative analysis of 2D heterostructures.

Monolayers of 2D transition metal dichalcogenides (TMDCs) provide excellent semiconducting
counterparts to insulating hexagonal boron nitride (hBN) and conductive graphene.[1] Combining all
three materials in a Van der Waals vertical heterostructure allows the electronic, photovoltaic and
electroluminescent properties of the TMDCs to be studied.[2] Whilst transport and optoelectronic
measurements can probe the properties of exotic charge carriers, and ARPES can map the
bandstructure such systems, direct high resolution imaging of the buried interfaces is only possible
via high resolution cross sectional (S)TEM imaging.[3] The nature of these van der Waals interfaces
not only determines the carrier injection between components[4], but also affects the bandstructure
of the device and its ultimate functionality[5].


Here we present a novel strategy to denoise high resolution HAADF STEM images of cross sections
using principal component analysis (PCA).[6] Cross sections are fabricated in a dual-beam FIB-SEM
instrument using the in situ lift-out method and polished with low energy ions to achieve electron
transparency.[7] Cross sections were imaged in high resolution HAADF STEM using a probe-side
aberration corrected FEI Titan G2 80-200 kV with an X-FEG electron source. By treating each line
profile perpendicular to the fringes as a signal, components associated with noise and scattering can
be separated by their variance. Removing the noise components allows us to accurately determine
the separation between dissimilar crystals at these unique interfaces. More widely, the approach
developed here also has application in a variety of layered material systems.


[1] X. Duan, C. Wang, A. Pan, R. Yu, and X. Duan, ‘Two-dimensional transition metal dichalcogenides
as atomically thin semiconductors: opportunities and challenges’, Chem. Soc. Rev., vol. 44, no.
24, pp. 8859–8876, Nov. 2015.
[2] F. Withers, O. Del Pozo-Zamudio, A. Mishchenko, A. P. Rooney, A. Gholinia, K. Watanabe, T.
Taniguchi, S. J. Haigh, A. K. Geim, A. I. Tartakovskii, and K. S. Novoselov, ‘Light-emitting diodes by
band-structure engineering in van der Waals heterostructures’, Nat Mater, vol. 14, no. 3, pp.
301–306, 2015.
[3] S. J. Haigh, A. Gholinia, R. Jalil, S. Romani, L. Britnell, D. C. Elias, K. S. Novoselov, L. A.
Ponomarenko, A. K. Geim, and R. Gorbachev, ‘Cross-sectional imaging of individual layers and
buried interfaces of graphene-based heterostructures and superlattices’, Nat Mater, vol. 11, no.
9, pp. 764–767, 2012.
[4] L. Britnell, R. V. Gorbachev, R. Jalil, B. D. Belle, F. Schedin, M. I. Katsnelson, L. Eaves, S. V.
Morozov, A. S. Mayorov, N. M. R. Peres, A. H. Castro Neto, J. Leist, A. K. Geim, L. A.Ponomarenko, and K. S. Novoselov, ‘Electron Tunneling through Ultrathin Boron Nitride
Crystalline Barriers’, Nano Lett., vol. 12, no. 3, pp. 1707–1710, Mar. 2012.
[5] A. M. van der Zande, J. Kunstmann, A. Chernikov, D. A. Chenet, Y. You, X. Zhang, P. Y. Huang, T.
C. Berkelbach, L. Wang, F. Zhang, M. S. Hybertsen, D. A. Muller, D. R. Reichman, T. F. Heinz, and
J. C. Hone, ‘Tailoring the Electronic Structure in Bilayer Molybdenum Disulfide via Interlayer
Twist’, Nano Lett., vol. 14, no. 7, pp. 3869–3875, Jul. 2014.
[6] D. Rossouw, P. Burdet, F. de la Peña, C. Ducati, B. R. Knappett, A. E. H. Wheatley, and P. A.
Midgley, ‘Multicomponent Signal Unmixing from Nanoheterostructures: Overcoming the
Traditional Challenges of Nanoscale X-ray Analysis via Machine Learning’, Nano Lett., vol. 15, no.
4, pp. 2716–2720, Apr. 2015.
[7] M. Schaffer, B. Schaffer, and Q. Ramasse, ‘Sample preparation for atomic-resolution STEM at
low voltages by FIB’, Ultramicroscopy, vol. 114, no. 0, pp. 62–71, 2012.


Aidan ROONEY (Manchester, United Kingdom), Aleksey KOZIKOV, Eric PRESTAT, Freddie WITHERS, Andre GEIM, Konstantin NOVOSELOV, Sarah HAIGH
08:00 - 18:15 #5198 - IM05-304 Application of moiré pseudo atomic column elemental mapping to electron beam-sensitive crystal of mineral.
IM05-304 Application of moiré pseudo atomic column elemental mapping to electron beam-sensitive crystal of mineral.

Atomic column elemental mapping is getting popular, since elemental species and positions of atomic sites are determined simultaneously [1]. In this method, the chemical information is generally detected by electron energy loss spectrometry (EELS) and/or energy dispersive X-ray spectrometry (EDS). Allowable electron dosage for a sample limits the usage of the method, since more dosage is required for the chemical analyses than that for imaging, due to small ionization cross section of atoms. The sensitivity of EDS rises rapidly, since a silicon drift detector (SDD), which is a new type of EDS detector, has design flexibility of its shape and has quick processing time, resulting in predominantly use in recent years. And total solid angle of X-ray detection rises rapidly to be 1.5-2.0 sr by detection systems with multiple detectors.

The allowable electron dosage still limits the application of the method to battery, carbon and organic materials, which are strongly requested to be analyzed by the industries. Therefore, it is required to reduce the dosage or to increase critical dosage of these materials. Trials to increase the critical dosage have been succeeded by finding an appropriate accelerating voltage and sample cooling. On the other hand, not so many trials to reduce the dose density onto a sample have been done.  We first succeeded in showing a pseudo atomic column elemental map with a lower average dose density ( < 1 % of that used in the conventional atomic column elemental mapping), utilizing a two dimensional (2D) moiré pattern [2].  In this paper, we applied this method to a beam sensitive sample.

The sample for our experiment was selected to be beryl (Be3Al2Si6O18: Fe2+) (known as aquamarine, having hexagonal structure with a = b = 0.922 nm, c = 0.920 nm see Fig. 1), which is one of cyclosilicates and has channels along the c axis. An [001] oriented sample was made by Ar ion thinning. Carbon was evaporated on the sample surface to avoid sample charging. We used for the experiment an aberration corrected 300 kV microscope (JEOL, JEM-ARM300F) equipped with a cold FEG and dual SDD X-ray detector system (total solid angle = 1.63 sr). All image observations and analyses in this paper were obtained under conditions: acc. Volt. = 300 kV, probe current = 24 pA. It is noteworthy that no direct atomic column elemental mapping was succeeded due to the sample damage. A high resolution STEM image of the sample is shown in Fig. 2(b).

In the experiments, the number of pixels (n x n) for the maps was selected to be 64 x 64, and pixel intervals in the x and y directions( drx and dry ) were set to be acrx and bcry nm, where rx and ry are numbers of unit cells in a pixel intervals in x and y directions.  The widths of the unit cell (ac and bc) are 0.922 and 1.60 nm, since bc / ac = 31/2 (see 2D Cartesian unit cell in Fig. 1(a)). The Cartesian unit cell is required for a common STEM, because the pixel positions (the electron irradiation points)  of a scanning image are on the Cartesian grid. The moiré magnification (M = dmoire / dlattice) is determined from the following relations: Mx = 1 / | 1 – rx / N |, My = 1 / | 1 – ry / N |, where N is the closest integral number to rx and ry (see Fig. 1(b)).  Figures 2 (c) and (d-f) show a moiré HAADF image and moiré pseudo atomic column elemental maps by 102 cyclic acquisitions. The image width in the x direction (ndrx) and Mxac are measured to be 239.1 and 69.1 nm from the image, and the Mx is derived to be 78.2 by the relation mentioned above with N = 4, rx = drx / ac = 4.053. The image height (ndry) is calculated to be 412 nm with the derived relation ( ndry = 0.5nNbc(1 + [1 + 4/(Nnbc)]1/2), where nbc is number of pixels for Mybc in image). The ry is determined to be 4.029. The drx and dry is derived to be 3.73 and 6.43 nm. The total scanned area is estimated to be 9.82 x 104 nm2. The dose density on the sample was estimated to be 1.30 x 1010 electrons / nm2 with the total analysis time = 2089 sec and the probe current = 24 pA. The equivalent dose density, if the analysis were performed by the conventional direct method, is estimated to be 1.23 x 1014 electrons / nm2. Therefore, we could reduce the dose density to be < 10-4 in this experiment. The results in Fig. 2 (d-f) show the clear atomic column elemental maps for Al, Si and O due to the low dose acquisition. We proposed and demonstrate a method to reduce dose density for the analysis of a real fragile sample. The reduced rate was a one-ten-thousandth of one by the conventional method.  

 

References

 

[1] E Okunishi et al, Microsc. Microanal. 12(S2) (2006), p.1150.

[2] Y Kondo and E Okunishi, Microscopy. 63 (5) (2014), p. 391.


Yukihito KONDO (Tokyo, Japan), Keiichi FUKUNAGA, Eiji OKUNISHI, Ichiro ONISHI
08:00 - 18:15 #5211 - IM05-306 An artificial neural network based algorithm for three-dimensional quantitative imaging in optical microscopy.
IM05-306 An artificial neural network based algorithm for three-dimensional quantitative imaging in optical microscopy.

In [1] an algorithm was proposed, inverse dynamical photon scattering (IDPS), which uses as a forward model the propagation of the optical wave through the sample and the objective lens with the multislice method. By recasting the forward model as an artificial neural network (ANN), an error metric can be chosen, and the derivatives of this metric with respect to the unknown values of the discretized object become available at the low computational cost of one extra pass through the ANN. These gradients are deployed in a derivative-based optimization scheme to retrieve a three-dimensional reconstruction of the specimen; Polak-Ribière conjugate gradients (PRCG) and alternate directions augmented Lagrangian (ADAL) are opted for. IDPS is implemented on the graphics processing unit.

IDPS is verified using the open source data from [2,3], where two stacked 1995 US Air Force resolution test charts, 110 mm apart, were adopted as specimen. This dataset is acquired with a microscope with a numerical aperture (NA) of 0.1. An LED array is placed sufficiently far away to consider each individual LED to illuminate the specimen with a spatially coherent plane wave. Nine bright-field and 284 dark-field images are recorded with LEDs up to 0.44 illumination NA, providing an effective NA of 0.54. Since the central wavelength of the LEDs is 643 nm, the 0.54 NA corresponds to a lateral resolution of 1.20 µm. The resolution of the reconstructions is evaluated as the center-to-center distance of the bars in the smallest element in the resolution target that is still discernible: 2.76 µm for element 4 of group 8, and 1.38 µm for element 4 of group 9.

Given that results are obtained from an initial guess of zero, no preprocessing steps such as light field refocusing are necessary. The free choice of error metric is important as the standard choice of sum of squared differences (SSD) leads to a high amount of resolution-limiting noise, see Fig. 1, and to erratic behavior in the R-factor. The alternative sum of normalized absolute differences (SNAD) yields a better-behaved and lower R-factor, as well as a better resolution (see Fig. 2). This poses a problem for Gerchberg-Saxton-type algorithms since they can be thought of as implicitly minimizing SSD [1].

Estimation of certain nuisance parameters—the focal value and illumination angle in this case—together with the specimen itself, improves the result by decreasing the R-factor and diminishing cross-talk between layers. Combined with a total variation regularization of the object, achieved by the ADAL approach, a reconstruction with the same resolution and considerably smoother and strongly diminished spurious oscillations is obtained, as is shown in Fig. 3. [4]

[1] X. Jiang et al., “Inverse dynamical photon scattering (IDPS): an artificial neural network based algorithm for three-dimensional quantitative imaging in optical microscopy,” Optics Express (2016), accepted.

[2] L. Tian, “3D FPM on LED array microscope,” (2015). https://sites.google.com/site/leitianoptics/open-source.

[3] L. Tian and L.Waller, “3D intensity and phase imaging from light field measurements in an LED array microscope,” Optica 2, 104–111 (2015).

[4] The Carl Zeiss Foundation is gratefully acknowledged by all authors. C.T. Koch also acknowledges the DFG (KO 2911/7-1). The authors acknowledge the helpful discussion and the open source data provided by L. Waller and L. Tian, University of California, Berkeley.


Wouter VAN DEN BROEK (Berlin, Germany), Xiaoming JIANG, Christoph T. KOCH
08:00 - 18:15 #5215 - IM05-308 Unravelling the structural and property changes in graphite under high dose electron beam irradiation.
IM05-308 Unravelling the structural and property changes in graphite under high dose electron beam irradiation.

Neutron radiation damage is a significant problem for graphite moderated nuclear reactors where graphite serves as both the neutron moderator and a key structural component. Exposure to neutron radiation introduces a variety of chemical and physical property changes, such as a reduction in the thermal conductivity, increase in Young’s modulus and dimensional change, creating cracks [1]. Understanding the damage processes experienced by irradiated nuclear graphite over a range of length scales is essential in predicting the lifetime of the material, which influences the overall lifetime of the reactor.

In this study, electron irradiation is used as a surrogate for neutron irradiation. A multi-faceted approach has been applied to transmission electron microscopy (TEM)/ electron energy loss spectroscopy (EELS) data obtained from nuclear grade graphite exposed to the electron beam for differing time periods.  This involved deriving initial 3D structural models from a series of 2D TEM images at differing stages of damage, these models were then used to derive theoretical TEM and EELS data which were then in-turn compared to back to the experimental data from the same sample; finally these models were used to predict mechanical and transport properties relevant to reactor operation. 

TEM, selected area electron diffraction (SAED) patterns and EEL spectra were collected periodically (along the graphite [100] orientation) during electron beam exposure at 200 kV for 13 minutes (total fluence = 2 × 108 e-nm-2). Electron micrographs were subject to 2D image analysis to measure the change in (002) fringe length and tortuosity, following the method outlined in [2]. The EELS series were analysed to extract information about bond length and ratio of non-planar to planar sp2 bonded carbon, following the method outlined in [3]. In recent years, a reconstruction procedure has been developed, called image guided atomistic reconstruction (IGAR), that allows large and realistic representations of disordered, yet anisotropic, graphite-based carbons to be built [4–6], from data inferred from their TEM images (Figure 1). The IGAR procedure was applied to the experimental TEM image series to produce 13 separate models at differing stages of electron beam induced damage.  These models provide information about the different atomic environments of the carbon atoms within the structure, e.g. whether they are 2, 3 or 4-fold, sp2 bonded or defective. They were used to produce simulated TEM lattice images, which were analysed in the same way as experimental TEM images; the analysis results were then compared.  EEL spectra were also calculated from the reconstructed models using the plane wave density functional theory [7,8] (DFT) code CASTEP [9]. These were analysed in the same way as the experimental spectra (Figure 2). Experimental and theoretical data were compared and showed a reasonable correlation (Figures 3 and 4): the proportion of non-planar to planar sp2 bonded carbon was observed to increase following a fluence of 2 × 108 e-nm-2 and the C-C bond length was also observed to increase.

These reconstructed models bridge the gap between the primary damage obtained in former molecular dynamics studies which can only cope with short timescales, and the severe damage observed in TEM images and EEL spectra after prolonged exposure to neutrons or electrons.

[1]        D.E. Baker, Nucl. Eng. Des. 14 (1970) 413–444.

[2]        P.I. Raynal et al., Carbon Conf., 2010, pp. 1–2.

[3]        B.E. Mironov et al., Carbon 83 (2015) 106–117.

[4]        J.-M. Leyssale et al., Appl. Phys. Lett. 95 (2009) 231912.

[5]        J.-M. Leyssale et al., Carbon 50 (2012) 4388–4400.

[6]        B. Farbos et al., Carbon 80 (2014) 472–489.

[7]        P. Hohenberg, W. Kohn, Phys. Rev 136 (1964).

[8]        W. Kohn, L.J. Sham, Phys. Rev 140 (1965).

[9]        S.J. Clark et al., Zeitschrift Für Krist. 220 (2005) 567–570. 


Helen FREEMAN (Leeds, United Kingdom), Trevor HARDCASTLE, Baptiste FARBOS, Jean-Pierre DA COSTA, Rik BRYDSON, Andrew SCOTT, Christian GERMAIN, Gérard VIGNOLES, Jean-Marc LEYSSALE
08:00 - 18:15 #5268 - IM05-310 Effect of electron dose density on silicon nitride compared between two different atomic column elemental maps by 2D moiré and conventional methods.
IM05-310 Effect of electron dose density on silicon nitride compared between two different atomic column elemental maps by 2D moiré and conventional methods.

A challenge to expand a variety of samples in atomic column elemental mapping [1] is to overcome the sample damage caused by electron beam irradiation, since the signal of this method is smaller than that of imaging, due to small probability to generate the signals for elemental identification, such as energy loss electrons or X-ray emission.On the other hands, scanning transmission microscopy (STEM) moiré method is useful for strain analysis in semiconductor industry [2], since the method allows us to observe larger field view than conventional geometrical phase analysis utilizing real lattice fringes. Concurrently, the electron dose density is greatly reduced in STEM moiré method, since a pixel interval to form the pseudo lattice fringe is sparser than that to form a direct lattice.

We have tried to apply the 2D STEM moiré method to obtain a pseudo atomic column map [3]. In the 2D STEM moiré method, the electron density is reduced to be < 1 % of one used for the conventional method. With a flat area of a sample, the pseudo image map show identical intensity distribution and chemical information compare to the conventional elemental map, because the electron probe profiles  identical positions from each atomic sites in both methods, The pixel are intervals drmoire = dlattice + Δmoire for the moiré method and drdirect = Δdirect for the conventional method when Δmoiré is similar to Δdirect, where drmoire, drdirect and dlattice are a pixel interval in moiré method, a pixel interval in conventional method and the width of unit cell of a sample. It is noted that Δmoire is identical to Δdirect in the two methods. Figure 1 show the scheme of the relation. In this paper, we compare the effect of average electron dose density on atomic elemental column maps obtained by the two methods.

For the experiment, we used an aberration corrected microscope (JEOL, JEM-ARM200F) equipped with a cold field emission gun and dual SDD X-ray detection system, whose total solid angle is 1.73 sr. The results in this paper were obtained under the conditions: acc. Volt. = 120 kV, probe current = 38 pA, and probe size = 0.2 nm.  The sample we selected to be Si3N4, which is more of a robust sample for electron beam irradiation, since the sample was more likely to survive under beam irradiation. The crystallographic parameters for the sample are listed as a = b = 0.7617 nm and c = 0.291 nm. The sample thickness was estimated to be 45 nm judging from the X-ray emission rate.

Figure 2 shows the results taken by the 2D moiré and conventional methods. The results were accumulated over 508 cyclic acquisitions. The dwell time for a pixel in both methods was 1 ms.  The peaks in the Si map coincident with the atomic sites of the sample crystal structure in both methods.  However, the peaks in the N map do not show the atomic site configuration of the crystal structure in the map by direct method, especially on the N-Si hexagonal ring. Figure 3(a) and (b) show intensity profiles of Si, N and HAADF image signals on the maps by the two methods. While the peaks in the profile on the map by the 2D moiré method shown in Fig. 3(a) correlate well with atomic sites of N and Si. However, those in profile on the map by conventional method do not correlate with atomic sites of N. The difference between these elemental maps comes from the dose density. The dose densities for maps by moiré method is derived to be 4.79 x 1011 electrons/nm2 from the measured na and nb, where na and nb are numbers of pixels inside the mignified unit cell in the 2D moiré HAADF image. The dose density for the map by the direct method is simply calculated to be 1.62 x 1015 electrons /nm2 from the experimental conditions. The reduced rate of the dose density by 2D moiré method was one three-thousandth to the direct method.

In conclusion, the results clearly show the effect of dose density on sample in atomic resolution elemental mapping with reduced dose density realized by the 2D moiré method. The method can be used for analysis of beam-sensitive samples, , which does not need high resolution but need to see the atomic sites in a unit cell such as ions in battery materials. And the method can be used to check the critical dose density for unanalyzed samples.

 

References

 

[1] E Okunishi et al, Microsc. Microanal. 12(S2) (2006), p.1150.

[2] N Endo and Y Kondo, Proc. 32th LSI Testing Symposium (Osaka) (2012), p. 73.

[3] Y Kondo and E Okunishi, Microscopy 63(5) (2014), p. 391.


Yukihito KONDO, Eiji OKUNISHI (Tokyo, Japan)
08:00 - 18:15 #5281 - IM05-312 Precise atomic column position measurements using ISTEM.
IM05-312 Precise atomic column position measurements using ISTEM.

Nowadays, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is one of the most popular materials’ characterisation techniques because of its ability to provide direct structural images at the atomic resolution [1]. Recently, Rosenauer et al. proposed a new imaging technique called imaging STEM (ISTEM) combining the conventional TEM imaging with STEM illumination [2]. This new, spatially incoherent imaging mode is particularly interesting as it provides direct structural images and visualisation of light elements, while it is robust towards chromatic aberrations. In this work, the ISTEM and STEM precision with which atomic column positions can be measured in a quantitative manner are compared.

In HAADF STEM imaging, statistical parameter estimation theory is an excellent tool to quantitatively extract structure parameters [3]. Here, an empirical model is fitted to an experimental image by optimising a criterion of goodness of fit. In this model, the shape of an atomic column is described by a Gaussian peak. The parameters of this empirical model can be linked to the unknown structure parameters of the material under study. The precision with which those parameters can be measured is mainly determined by the presence of shot noise and scan noise in the images. While post-processing techniques reduce both effects [4,5], scan noise errors and probe instabilities have no influence in the ISTEM imaging mode as images for all probe positions are integrated. Therefore, it is expected that without the use of post-processing techniques, atomic column positions can be measured more precisely for ISTEM imaging as compared to STEM imaging.

This assumption is tested on experimental images of PbTiO3. The ISTEM image in Fig. 1a clearly resolves the light oxygen columns, while in the HAADF STEM image, Fig. 2a, these columns cannot be resolved. In order to extract the atomic column positions, Gaussian models are fitted to both images, shown in Figs. 1b and 2b. The precision with which individual column positions can be estimated is determined by calculating the standard deviation on the distance between the columns (Fig. 3). These results confirm that atomic column positions are measured more precise for ISTEM as compared to ADF STEM imaging. Furthermore, it demonstrates that the position of individual light atomic columns, like oxygen columns in the present example, can be estimated with a precision in the picometer range.

In conclusion, it is shown that ISTEM is a very promising technique for precise measurements of atomic column positions.

 

References

[1] Pennycook and Jesson, Physical Review Letters 64 (1990), p. 938

[2] Rosenauer et al., Physical Review Letters 113 (2014), 096101

[3] Van Aert et al., Ultramicroscopy 109 (2009), p. 1236

[4] Sang and LeBeau, Ultramicroscopy 138 (2014), p. 28

[5] Jones et al., Advanced Structural and Chemical Imaging 1 (2015), 8

 

The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N) and a PhD grant to K.H.W. van den Bos. The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).


Karel H W VAN DEN BOS (Antwerp, Belgium), Florian F KRAUSE, Armand BÉCHÉ, Johan VERBEECK, Andreas ROSENAUER, Sandra VAN AERT
08:00 - 18:15 #5339 - IM05-314 Measurement of Diffraction Pattern Distortions for Quantitative STEM.
IM05-314 Measurement of Diffraction Pattern Distortions for Quantitative STEM.

The shape and sensitivity distribution of the detector used for the acquisition of STEM micrographs is of the utmost importance for the accuracy of quantitative evaluation. This is especially the case for HAADF-STEM, which is frequently used for thickness measurement and chemical composition determination using Z-contrast. The roundness and uniformity of different available ring detectors and the influence of the exact centering on the optical axis has consequently been a central item of investigation in numerous publications [Ultramicroscopy 108, 1653 (2008); Ultramicroscopy 124, 50 (2015); J. Phys. Conf. Ser. 522, 012018 (2014)]. In those, the detector sensitivity is commonly characterised by scanning the focused electron probe over the detector without a specimen working in imaging mode to get a so-called detector scan. While this allows for a precise measurement of the shape and sensitivity of the physical detector itself, using the imaging mode for this detector scan completely neglects any effects that will occur exclusively in diffraction mode, in which the actual STEM acquisitions are done.

However, looking at a diffraction pattern in a microscope with an image aberration corrector quickly reveals that the diffraction pattern is indeed strongly distorted with a symmetry corresponding to the corrector architecture and also is limited by a visible cutoff at higher spatial frequencies, which can be seen in the diffraction pattern in Fig. 1. The distortion field responsible for the deformation can strongly change the shape of the effective detector sensitivity experienced by the scattered electrons: Electrons leaving the specimen under a certain angle may be deflected into a completely different area of the detector than expected without the distortions or may not even reach the detection plane at all due to the cutoff. Therefore even a perfectly round and uniform detector can result in an unfavourable anisotropic sensitivity.

To take all these effects of the diffraction pattern into account, an alternative detector scan procedure that is operated in diffraction mode is presented [Ultramicroscopy 161, 146 (2016)]. It allows direct determination of the effective sensitivity of the detector, which includes both the cutoff and all occurring distortions. This "tilt-based" detector scan method is easy to implement and automatable.

The effective detector sensitivity of a Fischione Model 3000 HAADF detector in an image corrected FEI Titan 80-300 microscope was characterised with the tilt-bases detector scan for various camera lengths and clear and significant differences to the conventionally determined sensitivities were found. The results are shown in Fig. 2: The effective sensitivity is not nearly as round as the almost perfectly annular form of the physical detector, which partially results in large deviations in the radial sensitivity curves. The quantitative influence of these deviations on the results of thickness and composition measurements was investigated.

In a further study, the tilt-based detector scan method was combined with an aperture in front of the detector plane to measure not only the effective sensitivity but also the radial position where specific scattering angles are detected in the detection plane. From this the radial distortion field can be extracted.


Florian F. KRAUSE (Bremen, Germany), Marco SCHOWALTER, Tim GRIEB, Knut MÜLLER-CASPARY, Thorsten MEHRTENS, Andreas ROSENAUER
08:00 - 18:15 #5448 - IM05-316 Engineering the Contrast Transfer through the Cc/Cs Corrected 20−80 kV SALVE Microscope.
IM05-316 Engineering the Contrast Transfer through the Cc/Cs Corrected 20−80 kV SALVE Microscope.

Modern aberration-corrected transmission electron microscopy (AC-TEM) is able to routinely image sample structures with true atomic resolution with a resolution better than 1 Å. Unfortunately, the energy of the electrons used for imaging accelerated with voltages of 200–300 kV damages the samples under observation. Especially, light elements suffer severe damage via the knock-on mechanism [1–3]. To avoid this kind of sample damage, imaging at lower acceleration voltages of 60–80 kV became popular recently to undercut the threshold for knock-on damage and thanks to geometrical aberration correction, an image resolution of below 2 Å can be maintained. One interest when performing atomic resolution microscopy of thin and/or light-element materials lies in interfaces and defects and there, the aforementioned damage threshold is significantly lowered calling for even lower imaging electron energies [4,5].

In TEM at acceleration voltages of 30 kV and lower, in addition to the spherical aberration, the chromatic aberration coefficient of the imaging lens limits the resolution. The chromatic aberration can be corrected by using a Cc/Cs corrector [5] or it can be minimized by employing a monochromator for the primary electrons [6].

In this contribution, we discuss the optimum imaging parameters for the contrast transfer through the 20−80 kV Cc/Cs corrected SALVE (sub angstrom low voltage electron microscope) instrument. This includes the influence of a partially adjustable positive fifth order spherical aberration. Positive and negative phase contrast transfer are compared with the help of graphene images taken at different electron acceleration voltages; in the example in Figure 1 imaging has been performed at 30 kV at positive and negative atom contrast; as can be seen from the Fourier transform inserted, frequencies up to (107 pm)-1 have been transferred [7].


Acknowledgements

Fruitful cooperation within the SALVE project and financial support by the DFG (German Research Foundation) and by the Ministry of Science, Research, and the Arts (MWK) of Baden-Württemberg are gratefully acknowledged.


[1] F Banhart, Reports on Progress in Physics 62 (1999), p. 1181.
[2] R F Egerton, P Li, and M Malac, Micron 35 (2004), p. 399.
[3] J C Meyer et al, Physical Review Letters 108 (2012), p. 196102.
[4] T Sasaki et al, Journal of Electron Microscopy 59 (2010), p. S7.
[5] U Kaiser et al, Ultramicroscopy 111 (2011), p. 1239.
[6] S Morishita et al, Applied Physics Letters 108 (2016), p. 013107.


Felix BÖRRNERT (Ulm, Germany), Johannes BISKUPEK, Zhongbo LEE, Martin LINCK, Peter HARTEL, Heiko MÜLLER, Maximilian HAIDER, Ute KAISER
08:00 - 18:15 #5701 - IM05-318 Aberration corrected HRTEM imaging of zeolitic nanocavities.
IM05-318 Aberration corrected HRTEM imaging of zeolitic nanocavities.

Zeolites composed of alumino-silicate frameworks have great potentials in various fields. Their covalent frameworks built from TO4-tetrahedra (T: Si or Al) produce various kinds of nano-cavities inside the crystals. These characteristic nanospaces play important roles in the various applications of zeolite. Non-framework species such as counter cations and adsorbed molecules are located within nanocavities. Structural information of such non-framework species is essential for understanding specific behavior of zeolites. High resolution imaging by an aberration corrected (scanning) transmission electron microscopy (AC-(S)TEM) has been a powerful technique for structural analyses of various materials. However, structural parameters of non-framework species within zeolitic nanocavities are too complex to analyze quantitatively with AC-(S)TEM observations. As the first step of qualitative analysis for non-framework species, it could be important to examine whether the nano-cavities are empty, in order that it is necessary to evaluate the image contrast of the empty cavities. In this study, we consider the AC-HRTEM image contrast of zeolitic nano-cavities by experimental observations and multi-slice image simulation depending on optical conditions and specimen thickness.

Commercially available high silica type MFI and FAU type zeolites with Si/Al ratios of 3760 and 1540 respectively were used in this study. In order to ignore the contribution of counter cations to AC-HRTEM image, counter cations of these materials had been substituted originally to H+. Crystalline samples were crushed in an agate mortal with ethanol and collected on a gold coated TEM microgrid. The observations were performed on a JEOL JEM2200FS (accelerating voltage of 200kV) attached with a CS corrector (CEOS CETCOR). In order to remove molecules adsorbed in zeolite, the specimens were held in a vacuum of microscopic column overnight prior to observation. The CS values were controlled to be +15 µm or –15 µm. All HRTEM images were taken by a slow scan CCD camera (14 µm × 14 µm of 2048 × 2048 pixels, Gatan UltraScan 1000). The images were magnified by ~ 200 thousand times on the camera. Simulation of AC-HRTEM images were executed on Total Resolution LLC MacTempas X based on a multi-slice method. Comparing an experimental AC-HRTEM image and a simulated through focus image series for various thickness of thin crystal, we find out an optimum focused area from one image. Pure silicalite composition was applied as a structural model for image simulations. So no counter cations were included in structural models for simulations.

Figure 1(a) shows structural models projected along [100] direction of the monoclinic MFI-type zeolite with the unit cell dimensions of a = 1.9879 nm, b = 2.0107 nm, c = 1.3369 nm and β = 90.67°. The straight cavities are consisted of 10 membered ring surrounded by eight 5 membered rings and two 6 membered rings. As shown in Fig. 1(b), the amplitudes of exit wave function (EXWF) were increased at atomic column positions by electron channeling, in exchanged for reduction at neighboring area. Especially, amplitudes of EXWF at small channels go out of unity due to the adjacent atomic columns. The amplitude and phase of EXWF are intricately converted into the phase and amplitude on image plane, according to Lichte’s diagram. For quantitative imaging of nanocavities as vacuum, it is necessary to make samples thinner to the utmost. Fig. 2 shows experimental AC-HRTEM images taken under negative CS imaging (NCSI) and positive CS imaging (PCSI) conditions. Even with high resolution imaging in atomic scale, the contrast modulation of the area surrounding atomic columns is not so small. So the image contrasts of small zeolitic cavities swerve from the intensity of vacuum region.

 

Acknowledgements

A part of this work was supported by CREST, JST. A part of this work was supported by Kyoto University Nano Technology Hub in "Nanotechnology Platform Project" sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.


Kaname YOSHIDA (Nagoya, Japan), Yukichi SASAKI, Hiroki KURATA
08:00 - 18:15 #5740 - IM05-320 Microscopy Image Browser: an open-source platform for segmentation and analysis of multidimensional datasets.
IM05-320 Microscopy Image Browser: an open-source platform for segmentation and analysis of multidimensional datasets.

Understanding the structure – function relationship of cells and cell organelles in their natural context requires multidimensional imaging. Recent technical advances especially in multimodal 3D-5D imaging techniques have enabled a new insight into the morphology of tissues, cells and organelles. As the performance and access to such techniques are improving, the amounts of collected data are growing exponentially posing a question about effective processing, visualization, and analysis of these large datasets. Quite often the detailed analysis of multidimensional data is impossible without segmentation of objects of interest out of the volume (creating of a model). Usually the segmentation is the most time consuming and challenging part of the image analysis routine. For example, it may take up to a month to properly segment a single electron tomogram. The slowness of the process is caused by two main factors: limited number of good software tools (even commercial ones) and segmentation algorithms that can be applied to facilitate the modeling. As a result, the real potential of the collected data is not completely realized.

Here, we present a free user-friendly software package (Microscopy Image Browser, MIB) for effective segmentation and image processing of multidimensional datasets that improves and facilitates the full utilization of acquired data and enables quantitative analysis of morphological features. MIB is written in Matlab language which is familiar to many researchers and available for main common operating systems (Windows, Mac and Linux); alternatively MIB is also distributed as a standalone package for both Windows and Mac OS. The access to the code and the open-source environment enables fine tuning and possibility of adding new plug-ins to customize MIB for specific needs of any research project.

Even though the focus of the program is 3D segmentation of electron microscopy datasets, MIB is rather universal and can be used to perform segmentation, analysis and visualization of 2D-4D datasets obtained by light microscopy.

We already utilized MIB in more than 10 scientific research projects, where it allowed us to facilitating the segmentation part of the image processing workflow significantly. Here we demonstrate its successful application by presenting results of segmentation and quantification of cells and different cellular organelles imaged using various electron and light microscopy techniques.


Ilya BELEVICH (Helsinki, Finland), Merja JOENSUU, Darshan KUMAR, Helena VIHINEN, Eija JOKITALO
08:00 - 18:15 #5757 - IM05-322 A morphological approach for texture detection, application to SEM stereo reconstruction.
IM05-322 A morphological approach for texture detection, application to SEM stereo reconstruction.

3D surface reconstruction from multiple SEM images has been widely addressed in the scientific literature. Different approaches exist, most of them use variants of Digital Image Correlation (DIC) [1][2].

 

DIC methods perform well on textured areas of images but provide limited results on uniform or flat zones. Approaches using segmentation techniques, that match discernible regions of images, provide a solution to solve this problem on low-textured regions [3]. However, these methods are more computationally expensive and are less accurate than DIC on textured areas. Our objective is therefore to combine these two methods. One preliminary step is to differentiate textured areas, where DIC can be used, from low-textured areas, where segmentation techniques are needed. This step will be addressed in this paper, and we propose a new approach using morphological operators to do that.

 

Our approach uses the watershed operator with morphological gradient and markers obtained with h-minima [4]. On each region, average gradient value is computed. Automatic thresholding and post processing using opening by criteria [5] is then performed to differentiate textured areas from low-textured ones.

 

Comparison of our approach with more standard methods of texture detection such as local entropy of local variance calculations [6] will be presented.

 

A major benefit of our approach is that transition between textured areas and low-textured ones strongly follow real contours of flat zones, limiting interpolation problems between DIC and methods using segmentation techniques.

 

We will illustrate our texture detection algorithm by applying it on 3D surface reconstruction with SEM images of crystals of zeolites and catalyst with alumina supports.

 

[1] J.V. Sharp (et al), 1965. Automatic Map Compilation Using Digital Techniques. Photogrammetric Engineering. Vol. 31, No 2, pp. 223-239.
[2] Accurate 3D Shape and Displacement Measurement using a Scanning Electron Microscope. PhD Thesis, University of South Carolina, Institut National des Sciences Appliquées de Toulouse, Juin 2005.
[3] Jean-Charles Bricola, Michel Bilodeau, and Serge Beucher. A top-down methodology to depth map estimation controlled by morphological segmentation.
[4] Serge Beucher. Segmentation d’images et Morphologie Mathématique. PhD thesis, Ecole Nationale Supérieure des Mines de Paris, Juin 1990.
[5] Thomas Walter. Application de la Morphologie Mathématique au diagnostic de la Rétinopathie Diabétique à partir d’images couleur. PhD thesis, Ecole Nationale Supérieure des Mines de Paris, 2003.
[6] K. Itoh, A. Hayashi, and Y. Ichioka, “Digitized optical microscopy with extended depth of field,” Appl. Opt., vol. 28, no. 15, pp. 3487–3493, 1989.

is that transition between textured areas and low-textured ones strongly follow


Sébastien DROUYER (Fontainebleau), Serge BEUCHER, Michel BILODEAU, Maxime MOREAUD, Loïc SORBIER
08:00 - 18:15 #5831 - IM05-324 MASDET2 – software for quantitative STEM imaging.
IM05-324 MASDET2 – software for quantitative STEM imaging.

The idea of mass determination using a TEM was first proposed by E. Zeitler and G. F. Bahr [1]. The principle based on the single electron scattering approach (approximately linear relationship between the fraction of incident electrons scattered by a thin proteinaceous specimen and its molecular mass) was realized experimentally later by dedicated scanning transmission electron microscopes (STEM) equipped by a field emission gun and an annular dark-field (ADF) detector (for actual review see [2]). Besides the aforementioned technical realization, several mass determination software packages for the data analysis were developed independently – IMPSYS [3] and PCMass [4]. This idea for mass determination was later applied to so-called low voltage STEM by extending a commercial SEM by a proper ADF detector [5] and dedicated software MASDET [6] that worked on the linear approximation like IMPSYS. Here, we report on extensions of MASDET program that uses nonlinear relations and significantly extends applicable range of thickness.

Typically, programs using the single electron scattering approach give only exact results for samples not thicker than a few percent of the mean-free-path-length (MFPL) of the impinging electrons. One improvement was introduced in MASDET, using a so-called linearization, where the applicable range of thickness was extended to approximately 2x MFPL [6]. Here, we present MASDET version 2 which avoids the limitations of the linear approximation by determining the nonlinear relation between signal and mass with the help of Monte Carlo simulation software packages like MONCA [7]. With this improvement we may reach thicknesses approximately 7x MFPL (note that this depends on the detection geometry of the STEM detector).

As the previous version, MASDET2 has a user-friendly graphical interface allowing almost all operations to be done by mouse. It relies on MATLAB (MathWorks), and therefore should run under all systems suitable for MATLAB. MASDET2 allows the mass of specified regions of interest (ROI) to be calculated conveniently and in a highly automated manner from the digital ADF micrographs and recording parameters. Three procedures are available allowing the calculation of the mass-per-area (MPA) of sheet-like structures, the mass-per-length (MPL) of filaments, and the mass-per-particle (MPP) of globular particles.

Note that all calculations are done as absolute measurements without any mass calibration standards. Figure 1 shows the proof-of-principle measurement. Here, Tobacco Mosaic Virus (TMV) was investigated. For the mass calculations the relation of the ADF signal vs thickness was simulated using MONCA (for comparison a relation for lipid is shown on Figure 1b; the used SEM is S-5000 (Hitachi) equipped by a home-made ADF detector, operating at 30 keV). With this relation, the recorded signal can be converted into a mass value which can be finally added up in each ROI. Figure 1c shows the final result of the MPL determination which is 126±4 kDa/nm. After correction of the beam-induced mass loss, see figure 1d, which can also be done within the new version of MASDET2 software, the MPL value of TMV was corrected to 131±6 kDa/nm which represents the theoretical MPL value of TMV, see also [8].

References:

1. Zeitler, E. and G.F. Bahr, Journal of Applied Physics 33 (1962), 847-853.

2. Hawkes, P.W., ed. Advances in Imaging and Electron Physics 159 (2009).

3. Müller, S.A., et al., Ultramicroscopy 46 (1992), 317-334.

4. Wall, J.S. and M.N. Simon, Methods in Molecular Biology 148 (2001), 589-601.

5. Krzyzanek, V., et al., Microscopy and Microanalysis 19(S2) (2013), 130-131.

6. Krzyzanek, V., et al., Journal of Structural Biology 165 (2009), 78-87.

7. Krzyzanek, V. and R. Reichelt, Microscopy and Microanalysis 9(S3) (2003), 110-111.

8. Tacke, S., et al., Biophysical Journal 110 (2016), 758-765.

9. This research was supported by the grants 14-20012S (GACR) and RE 782/11 (DFG).


Vladislav KRZYZANEK (Brno, Czech Republic), Sebastian TACKE
08:00 - 18:15 #5833 - IM05-326 Effects of instrument imperfections on quantitative scanning transmission electron microscopy.
IM05-326 Effects of instrument imperfections on quantitative scanning transmission electron microscopy.

Quantitative scanning transmission electron microscopy is widely used for structural and chemical analysis nowadays. Experimental data can be directly compared with simulations by normalizing raw intensities J with respect to the incoming beam intensity J1-Jo [1]. J1 is usually determined by scanning the electron beam in image mode over the detector. From the detector region the intensity J1 and from regions beside the detector an offset intensity J0det is determined that has to be subtracted from J and J1. The normalized intensity I can be derived by I=(J-J0det)/(J1-J0det). However, typical detector scans exhibit a non-circular shape and non-uniform sensitivity. These effects are usually taken into account by a radial sensitivity curve in simulations [1,2].

In this contribution we present several further instrumental imperfections and characterize how they affect quantitative STEM using the example of measurement of specimen thickness:

1. The effect of centering of the diffration pattern and the anisotropy of the ADF detector was studied as follows: A 2-D sensitivity map was generated from a detector scan and then used to derive the average ADF intensity in a unit cell of GaAs. The sensitivity map was rotated or shifted with respect to simulated diffraction patterns. Then the relative intensity to the perfectly centered case was calculated. The error in the ADF intensity caused by different rotations was found to be only 0.5 %, whereas a decentering of only 7 mrad causes an error of about 6 %.

2. A cut-off and distortions of the diffraction pattern due to an image corrector have been found (Fig. 1a). These effects can be taken into account by a detector scan procedure, where the beam is scanned over the detector in diffraction mode by tilting the incoming electron beam. The advantage of this procedure is that cut-off and distortions are directly embedded in an effective sensitivity (Fig. 1b). We found the most severe deviation of the sensitivity map for a camera length of 102 mm. The respective radial sensitivity is compared in Fig. 1c with the conventional one. At this camera length an error of about 15 nm specimen thickness for 50 nm GaAs would be found, if cut-off and distortions would be neglected by using a conventional detector scan procedure.

3. Intense illumination, as e.g. a zero beam positioned accidentally on the detector, might cause a local sensitivity enhancement as shown in the inset of Fig. 2a. This results in a modification of the sensitivity curve (Fig. 2a). The modification of the sensitivity may cause an error of about 1 nm for thickness measurement of 50 nm GaAs.

4. We found that the measured incoming beam intensity J1 depends on the dwell time and is overestimated for long dwell times (Fig. 2b), independently on the previous history of the detector. This effect can lead to a thickness underestimation of about 4 nm for 50 nm thick GaAs.

5. An afterglow of the detector with a typical decay time of about 260 μs was found (Fig. 3a). This afterglow results in an overestimation of the bias of the amplifier from a detector scan, if J0det is determined by averaging over a region beside the detector. This overestimation leads to negative normalized intensities in an image from a vacuum region. Histograms of detector scans (Fig. 3c) reveal a sharp peak at an intensity J0amp that is attributed to the bias level of the amplifier, suggesting the usage of J0amp.

6. The usage of J0amp results in a positive normalized intensity in vacuum regions. Histograms of such regions revealed that accidental electrons hit the detector (Fig. 3b) [3,4] and therefore the mean value of the intensity in a vacuum image J0vac must be used for J0det. Effects 5 and 6 together may result in an error of about 2.3 nm for a 5 nm thick Si specimen, which is a significant error in an atom counting experiment.

The reported instrumental imperfection have been investigated using our FEI Titan80-300 ST microscope equipped with an corrector for the aberrations of the imaging lens and a Fischione HAADF detector. 

[1] J.M. LeBeau, S. Stemmer, Ultramicroscopy 108 (2008) 1653.

[2 ] A. Rosenauer, et al., Ultramicroscopy 111 (2011), 1316.

[3] R. Ishikawa, et al., Microsc. Microanal. 20 (2014) 99.

[4] F.F. Krause, M. Schowalter et al., Ultramicroscopy 161 (2016), 146.


Marco SCHOWALTER (Bremen, Germany), Florian Fritz KRAUSE, Tim GRIEB, Knut MÜLLER-CASPARY, Thorsten MEHRTENS, Andreas ROSENAUER
08:00 - 18:15 #5994 - IM05-330 Denoising and compensation of the missing wedge in cryo electron tomography.
IM05-330 Denoising and compensation of the missing wedge in cryo electron tomography.

In this study, we have addressed two important issues in cryo electron tomography images: the low signal-to-noise ratio and the presence of a missing wedge (MW) of information in the spectral domain. Indeed, according to the Fourier slice theorem, limited angle tomography results into an incomplete sampling of the Fourier domain. Therefore, the Fourier domain is separated into two regions: the known spectrum (KS) and the unknown spectrum, the latter having the shape of a missing wedge (see Figure). The proposed method tackles both issues jointly, by iteratively applying a denoising algorithm in order to fill up the MW, and proceeds as follows: 

[1] Excitation step: Add noise into the MW

[2] Denoising step: Apply a patch-based denoising algorithm

[3] Repeat steps 1 and 2, by keeping KS constant through the iterations

The excitation step is used to randomly initialize the coefficients of the MW, whereas the denoising step acts as a spatial regularization. The employed denoising algorithm, which exploits the self-similarity of the image, filters out coefficient values which are dissimilar to KS, thereby keeping similar ones. By iterating these steps, we are able to diffuse the information contained in KS into the MW.

An application example on experimental data can be seen on the Figure, which shows the data in both spectral and spatial domain. The data contains a spherical gold particle, deformed by MW induced artifacts: elongation of the object, side- and ray-artifacts. From the residue image it can be seen that noise and MW artifacts have been reduced, while preserving the details of the image. Experiments are being performed to verify if particle detection and alignment are enhanced by using the method as a pre-processing step. 


Emmanuel MOEBEL (RENNES), Charles KERVRANN
08:00 - 18:15 #5995 - IM05-332 Plasma cleaning effect on the stability of the Epon resin sections.
IM05-332 Plasma cleaning effect on the stability of the Epon resin sections.

Low voltage TEM and STEM (transmission and scanning transmission electron microscope) can be regarded as the method of choice for many structural studies of very thin biological samples like ultrathin sections, viruses etc. [1]. Unfortunately, the specimen contamination increases with electron flux and therefore a specimen cleanliness is an important factor in obtaining of valuable data especially in STEM [2]. An important parameter for imaging of those samples is a sensitivity of the sample to degradation by electron beam. The mass loss indicates a degree of the radiation damage. We investigated the mass loss of embedding medium (Epon resin of middle hardness) in combination of different thickness of the sections (60 nm and 150 nm) with using or not-using of plasma cleaning which is often used to removing of contamination from the sample.

The repeated imaging was performed by SEM Magellan 400L (FEI) at the acceleration voltage of 30 kV and the lowest possible probe current of 1.6 pA in the bright field (BF) using the commercial STEM3 detector (FEI). An electron dose for each scan was about 60 e-/nm2. The measurement was performed at the eighth day after their preparation by the ultramicrotome Leica Ultracut UCT. Some slices were cleaned by Plasma Cleaner (FEI) installed at the Magellan SEM for 20 seconds just before imaging, so they did not leave high vacuum. For each experiment, three different places on each sample were imaged for downgrading space dependency of the measurement. The mass loss was obtained from the normalized BF signals using the Monte Carlo simulation of electron scattering MONCA [3] using the methodology of mass measurements by STEM [4]. All data processing was programmed in MATLAB (Mathworks).

As seen on Fig. 1a, the normalized BF signal (showing the fraction of scattered electron to the BF detector) has similar shape in both thicknesses of the slices without plasma cleaning; the curves are shifted only by the different thickness. However, slices treated with the plasma cleaner show higher sensitivity to incident electron beam with rapidly increasing BF signal; Fig. 1b) shows the remaining mass. At the total irradiation dose of 3000 e-/nm2 there still remains 89% of initial mass for the 150 nm slice and 70% for the 60 nm slice, respectively. However, the slices cleaned by plasma cleaning are much more sensitive to the electron beam, with remaining 55% for the 150 nm slice and local destruction for the 60 nm slice. We observed higher sensitivity of cleaned Epon thin sections under incident electron beam. This limits the usable dose for imaging by the low voltage STEM (for 30 kV) because cleaned sections are more susceptible to burn-out than non-cleaned ones.

References:

[1] H. Schatten, J. W. Jan, A. Litwin, Scanning Electron Microscopy for the Life Sciences, New York: Cambridge University Press (2013) ISBN 9780521195997.

[2] D. R. G. Mitchell, Micron, 73 (1993), 36-46.

[3] V. Krzyzanek, R. Reichelt, Microscopy and Microanalysis, 9 (2003), 110-111.

[4] V. Krzyzanek, S. A. Muller, A. Engel, Journal of Structural Biology, 165 (2009), 78-87.

[5] The research was supported by the Czech Science Foundation (GA14-20012S), Technology Agency of the Czech Republic (TE01020118), Ministry of Education, Youth and Sports of the Czech Republic (LO1212). The research infrastructure was funded by Ministry of Education, Youth and Sports of the Czech Republic and European Commission (CZ.1.05/2.1.00/01.0017).


Radim SKOUPÝ (Brno, Czech Republic), Vladislav KRZYZANEK, Jana NEBESAROVA
08:00 - 18:15 #6010 - IM05-334 Atom-counting in a non-probe corrected STEM.
IM05-334 Atom-counting in a non-probe corrected STEM.

In recent years scanning transmission electron microscopy (STEM) has attracted great attention due to its high sensitivity with respect to atomic number and specimen thickness. The great advantage of this technique is to determine the number of atoms in single atomic columns in the specimen [1,2]. For instance, the reconstruction of the atomic structure of Ag nanoparticles was realized by acquiring a small series of high resolution STEM images in different zone axis orientations. Such a reconstruction enables to study facets of nanoparticles.

Basically two different atom counting techniques exist: (1) In simulation-based techniques typically the mean intensity in a well-known material within a certain region around each atom column position, such as a Voronoi cell [3] is measured and compared with appropriate simulations. (2) Statistics-based techniques, as introduced by Van Aert et al. [2] fit a parametric model consisting of Gaussians at the positions of the atom columns to the image intensities. Then volumes below the Gaussians are computed. Gaussian mixture models are fitted to the distribution of the volumes using the expectation maximization algorithm (see e.g. Fig. 1a) as a function of the number of Gaussian components. For each fit an order selection criterion is computed and plotted as shown in Fig. 1b. The minimum of the order selection criterion then determines the number of components in the applicable mixture model [2,4]. Finally, the number of atoms (Fig. 1c) in each column is determined by maximizing the probability that a column's intensity volume belongs to a certain component of the selected mixture model. The component with smallest volume is assumed to belong to a column with one atom.

Usually, atom counting is performed using probe corrected STEM. We performed a simulation test, whether atom counting is possible in a non-probe corrected STEM on a hypothetical Au wedge used by De Backer et al. [4]. In the respective model the number of atoms increases from 1 to 7 and then decreases again from 7 to one. From Fig. 1 it becomes clear that atoms were correctly counted from the simulated image.

In general it can be expected that the column with mimimum number of atoms in an image does not necessarily contain only one atom. We studied the effect of an offset in the number of atoms on the counting result by adding additional layers of Au on top of the model. In order to account for the offset in the statistics-based measurement positions of the Gaussian components were linearly fitted and the respective offset was derived from the parameters of the fit. We found that for small offsets the method worked reasonable, however for an offset of 7 atoms the number of atoms was overestimated by one atom, due to small deviations from the linear behaviour of the volumes. For a larger offset of 13 atoms severe errors were observed and a combination with simulations is needed to retrieve the offset value [5].

In a further test we added an amorphous carbon layer to an InAs cleavage wedge (Fig. 2a). Fig. 2b shows that the statistics-based method well identifies the number of atoms despite the amorphous carbon layer. Errors can be observed for simulation-based atom counting due to the additional intensity arising in the amorphous carbon layers.

Fig. 3a shows an experimental HRSTEM image of a twinned Pt nanoparticle taken in our non-probe corrected Titan 80-300ST. Atoms have been counted in one of the twins using the simulation-based atom counting technique (Fig 3b). For a statistics-based atom counting evaluation the same limits have been found. For the evaluations instrumental imperfections such as reported in ref. [6] were taken care.

[1] J. M. LeBeau, S. D. Findlay, L.J. Allen and S. Stemmer, Nano Letters 10 (2010), 4405.

[2] S. Van Aert, K.J. Batenburg, M.D. Rossell, R. Erni and G. Van Tendeloo, Nature. 470 (2011) 374.

[3] A. Rosenauer et al. Ultramicroscopy 109 (2009), 1171.

[4] A. De Backer, G.T. Martinez, A. Rosenauer and S. Van Aert, Ultramicroscopy 134 (2013) 23.

[5] S. Van Aert, et al. Phys. Rev. B 87 (2013), 064107

[6] F.F. Krause, M. Schowalter et al., Ultramicroscopy 161 (2016), 146.


Marco SCHOWALTER, Beeke GERKEN (28359 Bremen, Germany), Florian Fritz KRAUSE, Tim GRIEB, Knut MÜLLER-CASPARY, Christoph MAHR, Thorsten MEHRTENS, Mehtap ÖZASLAN, Annick DE BACKER, Sandra VAN AERT, Andreas ROSENAUER
08:00 - 18:15 #6056 - IM05-336 Focal series reconstruction of bismuth telluride using a conventional transmission electron microscopy.
IM05-336 Focal series reconstruction of bismuth telluride using a conventional transmission electron microscopy.

High Resolution Transmission Electron Microscopy allows the determination of the crystalline structure of materials through various methods of electron diffraction or direct imaging. However, the interpretation and quantification of the high-resolution images are complex, because of the strong interaction between the electron beam and the material [1]. Besides, the material’s exit wave function is modified by the components of the microscope [2]. As the high-resolution microscopy images are a convolution between the exit wave function of the sample and the function of the microscope, the interference fringes can be modified changing the condition of defocus [3]. The aim of the research is to understand the focal series reconstruction routines and use them in the study of bismuth telluride and its alloys, in order to identify their crystalline structure.

Through focal series reconstruction is a technique consisting in obtaining high-resolution image series with different values of defocus, generating different conditions of constructive and destructive interference fringes. At the same time, high-resolution images are simulated from a theoretical model of the crystal convoluted with the transfer function of the microscope, considering several types of aberration of the microscope, where the spherical aberration of the objective lens and the chromatic aberration are dominant. The simulated images are compared with the experimental images and through the correlation between them the theoretical model is optimized. There are several software packages which can be used to simulate the structure and/or generate a focal series: JEMS, True Image, FTSR, IWFR and REW. The present work shows the first results on focal series reconstruction routine using REW [4].

Bismuth telluride, Bi2Te3, is a thermoelectric material with high coefficients at room temperature [5] and has recently been identified as a topological insulator [6], with rhombohedral crystal structure and space group R(-3)m with five atoms per unit cell. When aligned in the [2,-1,-1,0] direction is possible to visualize the quintuple layer structure of Te-Bi-Te-Bi-Te. Studies related to the doping of Bi2Te3, in which some elements are intercalated among their quintuple layers [7], have been conducted in order to determine the variations of their basic properties. The focal series reconstruction will be used to verify the quality of the intercalation experiment.

A through focal series of 20 high resolution images was taken of a Bi2Te3 sample prepared via ultramicrotomy, approximately 20-30 nm thick, using a Tecnai G-20 LaB6 S-Twin (Cs = 1.2 mm) at 200 keV. A magnification of 490kx was used over a range of focus of -180.09 nm to 9.09 nm, with a focus variation of approximately of 10 nm. The HRTEM image at the Scherzer focus is shown in Figure 1, and the image of amplitude and phase resulting from the exit wave reconstruction of the sample using the software REW are shown in Figure 2 and Figure 3.

It is possible to identify the atomic position of bismuth and tellurium in the quintuple layers of the phase image obtained from the focal series reconstruction, as shown in Figure 4, using a conventional transmission electron microscope.

References

[1] L. Reimer, H. Kohl, Transmission Electron Microscopy: The Physics of Image Formation, Springer, 5ª ed.

[2] L.J. Allen, et. al., Ultramicroscopy 100 (2004) 91.

[3] A. Thust, Microscopy and Microanalysis 11 (2005) Suppl 2.

[4] Lin, F; Chen, Q.; Peng, L.M. Journal of Applied Crystallography 40 (2007) 614.

[5] H.J. Goldsmid, Materials 7 (2014) 2577.

[6] C. Kane, J. Moore, Physics World, February (2011) 32.

[7] Zhitinskaya, M.K.; Nemov, S.A.; Svechnikova, T.E. Materials Science in Semiconductor Processing 6 (2003) 449.

[8] Acknowledgments: We are grateful for the financial support of FAPEMIG, CAPES and the graduate program of the Physics Department of UFMG. Experiments and analysis involving electron microscopy were performed at the Center of Microscopy of UFMG (http://www.microscopia.ufmg.br).


Thais MILAGRES DE OLIVEIRA, Karla BALZUWEIT (Belo Horizonte, Brazil), Von Braun NASCIMENTO, Luís Orlando LADEIRA, Edmar AVELLAR SOARES, Vagner EUSTÁQUIO DE CARVALHO
08:00 - 18:15 #6128 - IM05-338 Understanding the use of scattering cross-sections in quantitative ADF STEM.
IM05-338 Understanding the use of scattering cross-sections in quantitative ADF STEM.

Quantitative scanning transmission electron microscopy (STEM) using an annular dark field (ADF) detector has become a widely used technique for the characterization of materials at the atomic level. The quantification process involves the comparison of experimental data with image simulations, the use of statistical tools in a parameter estimation framework or a combination of both [1]. These methods have been developed using different measures for comparison, like peak intensities at the atom column position [2], image contrast variations [3] or so-called scattering cross-sections [4, 5]. The latter correspond to the total scattered intensity integrated over the atom column area. They have been shown to be very sensitive to the number of atoms in a column and its composition [1, 4, 6, 7]. Figure 1a shows the increase in peak intensity (green axis) and cross-section (black axis) versus increase in number of atoms for a Pt column in [110] zone axis. As it can be observed, the peak intensity saturates after around 8 atoms meanwhile the cross-section monotonically increases. In this work, we perform an analysis of how the electron wave propagates inside the crystal for the probe positions that conform the scattering cross-section. With this, we analyse how the signal is generated for different detector collection angle regimes. Then, the analysis allows to identify the origin of the scattered signal and why scattering cross-sections are more sensitive for composition and number of atoms as compared to peak intensities. In Figure 1b, we show a simulated image of a unit cell of a Pt crystal in [110] zone axis with a color-edited version indicating the labels of the probe positions that form the scattering cross-section and their respective distance to the atom column position. Figure 2 shows the probability amplitude of the electron wave as it propagates through the crystal for probe position r0 (a), which corresponds to the peak intensity, and for the sum of all the probe positions that conform the scattering cross-section (b). From this, we observe that the atom column is excited deeper in the column when analyzing the cross-sections. The off-column probe positions carry more rich information about the scattering process for different thickness and collection regimes, which explains the increased sensitivity of this measure to the number of atoms and its composition. We then discuss the contribution to the scattered intensity for different detector collection angle regimes, such as LAADF, MAADF and HAADF.

[1] S. Van Aert, et al., Physical Review B 87 (2013) 064107.

[2] S. D. Findlay and J. M. LeBeau, Ultramicroscopy 124 (2013) 52 - 60.

[3] D. O. Klenov and S. Stemmer, Ultramicroscopy 106 (2006) 889 - 901.

[4] S. Van Aert, et al., Ultramicroscopy 109 (2009) 1236 - 1244.

[5] H. E, et al., Ultramicroscopy 133 (2013) 109 - 119.

[6] A. De Backer, et al., Ultramicroscopy 134 (2013) 23 - 33.

[7] G. T. Martinez, et al., Ultramicroscopy 137 (2014) 12 - 19.

Acknowledgement

The research leading to these results has received funding from Research Foundation Flanders (FWO, Belgium) through projects G.0374.13N, G.0368.15N, G.0369.15N and a PhD grant to K.H.W. van den Bos and from the European Union 7th Framework Programme under Grant Agreement 312483 - ESTEEM2. The authors are grateful to A. Rosenauer for providing the StemSim program.


Gerardo T MARTINEZ (Oxford, United Kingdom), Karel H.w. VAN DEN BOS, Marcos ALANIA, Peter D NELLIST, Sandra VAN AERT
08:00 - 18:15 #5125 - IM06-340 A new method to orient samples by STEM in a scanning electron microscope.
IM06-340 A new method to orient samples by STEM in a scanning electron microscope.

 Transmission electron microscopy is a widely used technique for dislocation characterization. To determine, e.g. dislocation Burgers vectors, the specimen must be oriented in a two-beam condition where only one Bragg reflection is strongly excited [1]. Similar diffraction information can be in principle obtained by scanning transmission electron microscopy (STEM) in a scanning electron microscope at low electron energies E0 up to 30 keV. Low-energy STEM has been shown to be a promising technique [2, 3], which is particularly interesting for the investigation of radiation-sensitive materials. However, diffraction patterns cannot be taken in a scanning electron microscope without additional instrumental attachments. Some groups already succeeded in obtaining electron channeling patterns by rocking the electron beam on a small sample area while recording backscattered electrons [4]. Here we will present an orientation technique by using the STEM detector in a scanning electron microscope. We will describe the underlying principle and present the orientation procedure for a 100 nmspecimen area by using the six-segment high-angle annular dark-field (HAADF) and bright-field (BF) STEM detectors (cf. Fig. 2). All studies were performed with E0 = 30 keV. An electron transparent cross-section specimen of a 500 nm InN layer on a Si-substrate is shown in Fig. 1, which was used as a test object.

  Diffraction information at low E0 can be obtained even at moderate sample thickness and high scattering angles as demonstrated in previous work [5]. The procedure described in the following is suited to orient the electron beam precisely along a zone-axis orientation. The single crystalline sample was prepared roughly along the [210] InN zone-axis and the main Kikuchi bands are indicated in Fig. 2 by green, black and grey lines. By tilting the sample, the Kikuchi pattern of the inspected sample region moves across the HAADF detector leading to image intensity changes. The intensity of segments A-C and D-F is normalized by the intensity of the incident electron beam on the respective HAADF segments to take into account different possible amplification characteristics. The difference of the normalized intensities between segments A-C and D-F is plotted in Fig. 3. After a 2.5o tilt the intensity difference between A-C and D-F is zero and the green Kikuchi band is oriented along the dashed red line “a” in Fig. 2. To reach a [210] zone-axis orientation the sample was then tilted around the perpendicular tilt axis. By recording a tilt series around the second tilt axis, the intensity differences in the corresponding segments are compared. In analogy to the first tilt series, the intensity difference for opposite HAADF segments becomes zero for an 8° tilt. The black Kikuchi band in Fig. 2 is then aligned along the dashed red line “b”, and the zone-axis is reached. A final check can be made by comparing the intensity differences between all opposite HAADF segments, which should be zero in all cases.

  By tilting the sample around one axis, we can scan the intensity across one Kikuchi line by using the BF detector. Fig. 4 shows the intensity of BF images as a function of the tilt angle. Since the width of the Kikuchi band yields information about the Bragg angle, it can be used to determine the lattice parameter. With the half width of the intensity curve in Fig. 4, the Bragg angle of the green Kikuchi band in Fig. 2 was determined to be ~ 1.25° and 0.6° for the black Kikuchi band. Both values agree well with Bragg angles of 1.14° and 0.704° for the (002) and (1-20) planes. Based on the analysis of the intensity curves, two-beam conditions for the inspected area can be obtained if the sample is tilted at angles, which are marked by dashed red lines in Fig. 4. This means that the technique can be applied in low-energy STEM to characterize dislocations in the future.

1. D.B. Williams, C.B. Carter, Transmission electron microscopy, Springer, 2nd edition, 2009, p.441-479.

2. T. Klein, E. Buhr, Frase, C.G. Frase, Advances in Imaging and Electron Physics. 171, 297 (2012).

3. T. Volkenandt, E. Müller, D. Gerthsen, Microsc. Microanal. 20, 111 (2014).

4. D.C. Joy et al., J. Appl. Phys. 53, R81 (1982).

5. T. Volkenandt et al., High-angle transmission electron diffraction in a scanning electron microscope, 15th European Microscopy Congress (EMC2012), Manchester, 2012.

6. This work is funded by the German Research Foundation (DFG).


Cheng SUN (Karlsruhe, Germany), Erich MÜLLER, Dagmar GERTHSEN
08:00 - 18:15 #5224 - IM06-342 Scanning electron diffraction using the pnCCD (S)TEM Camera.
IM06-342 Scanning electron diffraction using the pnCCD (S)TEM Camera.

Scanning electron diffraction (SED), performed in a (S)TEM, is a powerful technique combining information in reciprocal space and real space to achieve nanoscale crystal cartography of materials structure. SED involves scanning a focused electron beam across a specimen and recording an electron diffraction pattern at each position to yield a 4D dataset comprising a 2D diffraction pattern at every position in the 2D scan region. Obtaining high quality data depends on fast acquisition, large dynamic range, and accurate recording of the location and intensity of diffraction spots. Here, we present SED measurements using the pnCCD (S)TEM camera taking a Ti-Fe-Mo alloy for demonstration. The large number of pixels and high readout speed of this camera enables the recording of high quality diffraction patterns in a short acquisition time. Further, using the various camera operation modes, position and intensity of diffraction spots can be determined precisely.

 

The pnCCD (S)TEM camera provides fast acquisition of 2D camera images using a direct detecting, radiation hard pnCCD with 264x264 pixels [1]. Routinely, the readout speed is 1000 frames per second (fps) and can be further increased by binning and windowing. For example, with the pnCCD (S)TEM camera, a 256x256 STEM dataset -- where a camera image is recorded at each of the 65 536 probe positions -- can be recorded in less than 70 s. The camera properties can be changed by modifying the voltages applied to the pnCCD and thus adjusted to the experimental needs [2]. Considering scanning electron diffraction experiments, which are performed at high electron beam intensities, the combination of data recorded in two different camera operation modes allows a comprehensive diffraction pattern analysis with quantitative and spatial information. In the high-charge-handling-capacity (HCHC) mode, up to 16 000 incident electrons per pixel per second can be processed for a primary electron energy of 80 keV and a readout speed of 1000 fps. In the case of higher electron rates where the amount of signal exceeds the charge handling capacity of the affected detector pixels, signal spills over into neighboring pixels. Although diffraction spots broaden, the quantitative information is preserved. In the anti-blooming (AB) mode, the amount of signal exceeding the charge handling capacity is drained from the detector preventing an overflowing of pixels. Thus, the spatial information is preserved. The data can be analysed in a number of ways [3], most simply by plotting the intensity of a subset of pixels as a function of probe position in flexible post-experiment schemes to obtain ‘virtual diffraction images’ or to perform differential phase contrast analysis.

 

Results are shown (Figure 1) from a Ti(40 at.%)-Fe(20 at.%)-Mo(40 at.%) alloy from which SED data was acquired in an FEI Titan G2 80-200 ChemiSTEM microscope, operated at 200 keV. A diffraction pattern was recorded for each of the 512x512 probe positions using both HCHC and AB modes of the pnCCD (S)TEM camera at a readout speed of 1000 fps. Each dataset was thus acquired with a total acquisition time of less than 5 minutes per STEM dataset. Virtual diffraction images using the AB-mode data were then formed to discriminate the two phases existing in an ultra-fine lamellar microstructure [4] in this Ti-Fe-Mo alloy.

 

[1] H. Ryll et al, Journal of Instrumentation, in press.

[2] J. Schmidt et al, Journal of Instrumentation 11  (2016), p. P01012

[3] P. Meock et al, Crystal Research and Technology 46 (2011), p.589-606

[4] A.J. Knowles et al, Ti-2015, proceedings of the 13th World Conference on Titanium, in press.

 

DNJ, RKL & PAM acknowledge: ERC grant 291522-3DIMAGE and EU grant 312483-ESTEEM2.


Robert RITZ, Martin HUTH (Muenchen, Germany), Sebastian IHLE, Julia SCHMIDT, Martin SIMSON, Heike SOLTAU, Duncan N. JOHNSTONE, Rowan K. LEARY, Paul A. MIDGLEY, Martial DUCHAMP, Vadim MIGUNOV, Rafal E. DUNIN-BORKOWSKI, Henning RYLL, Lothar STRÜDER
08:00 - 18:15 #5231 - IM06-344 Atomic-level elastic strain measurement of amorphous materials by quantification of local selected area electron diffraction patterns.
IM06-344 Atomic-level elastic strain measurement of amorphous materials by quantification of local selected area electron diffraction patterns.

Metallic glasses have been promising materials for application as structural materials. Therefore, intense research has been carried out to understand their mechanical properties and the underlying physical phenomena [1]. An important aspect of this research is measuring the response of metallic glasses to external and/or internal stresses and the resulting atomic displacements. These atomic level strains can be measured by quantification of the peak shifts in synchrotron diffraction experiments as demonstrated by Poulsen et. al. [2].

 

Here, we present a novel TEM method for measuring the atomic level strains on a local scale by means of electron diffraction. A series of selected area electron diffraction (SAD) patterns of amorphous TiAl tensile test samples are recorded in a CM200 electron microscope equipped with a Gatan Orion CCD camera. External stress is applied in-situ and the evolution of the 2D strain tensor is calculated from the distortion of the characteristic amorphous diffraction halo. The full evaluation is carried out automatically by a plugin written for the Digital MicrographTM platform: The peak maxima positions are extracted with sub-pixel accuracy from azimuthal integrated sectors of 1° (cf. Fig.1(a)). This is achieved by a non-linear least squares fit using a pseudo-Voigt model function (cf. Fig.1(b)). By fitting an ellipse to the maxima positions also the center is determined with sub-pixel accuracy. By iteration the data is refined and the polar form of the maxima positions is obtained. Using an unstrained SAD pattern as reference, the 2D strain tensor can be calculated from the difference of the peak maxima positions. By fitting the polar form of the strain tensor finally the principal strain magnitude and direction can be obtained relative to the SAD patterns coordinate system (cf. Fig. 2).

 

Simulated diffraction patterns with known parameters and different levels of noise are used to check the strain accuracy of the method. The relative error is calculated with respect to the known input parameters. The method has an accuracy of about 1x10-4 in determination of the parameters, the relative error in principal stress is below 3% even at principal strain below 0.5% (cf. Fig. 3).

 

In addition to measure the atomic-level strain response to an applied external stress the method allows also to map the strains on a local scale, limited in principle only by selected aperture size and intensity fluctuations of the diffraction data for small sampling volumes. An example for such mapping capabilities is given in Fig. 4, where the strain distribution over the width of a strained specimen is given. In this case the strain distribution is non-uniform and varies between 1.3% at center and 1.35% at edge regions.

 

[1] A.L. Greer, Metallic glasses... on the threshold, Mater. Today. 12 (2009)14-22. doi:10.1016/S1369-7021(09)77037-9.

[2] H.F. Poulsen, J. a. Wert, J. Neuefeind, V. Honkimäki, M. Daymond, Measuring strain distributions in amorphous materials, Nat. Mater. 4 (2005) 33–36. doi:10.1038/nmat1266.

 

C. E. and C. R. acknowledge financial support by the Austrian Science Fund FWF: [I1309]. R. S. and J. R. acknowledge funding from the National Science Foundation (NSF) grants CMMI 1400505 and DMR 1454109.


Christian EBNER (Vienna, Austria), Rohit SARKAR, Jagannathan RAJAGOPALAN, Christian RENTENBERGER
08:00 - 18:15 #5300 - IM06-346 Structural Verification of Magnetite Nanocrystals in PVDF-fibers by Scanning Electron Nano Diffraction (SEND).
IM06-346 Structural Verification of Magnetite Nanocrystals in PVDF-fibers by Scanning Electron Nano Diffraction (SEND).

Incorporation of inorganic nanocrystals in polymer fibers is always a challenging process with two major requirements, i.e. the particles should be homogenously distributed in the polymer matrix and the nanomaterial properties must be kept during the processing of the fibers. These criteria become even more demanding if the final product is intended for later medical application, where the material must comply with the high safety standards. However, the incorporated total nanoparticle concentration in polymer fibers is usually very low, which implies that certification of the unaltered crystal structure of the nanomaterial cannot be achieved by conventional XRD methods. Within the present study we demonstrate that Scanning Electron Nano Diffraction (SEND) [1] in a STEM is a suitable alternative technique to XRD for handling this issue. Thus the SEND method was applied on magnetite (Fe3O4) nanocrystals incorporated in melt spinned polyvinylidene fluoride (PVDF) fibers for verifying the inverse spinel structure of the iron oxide particles. The diffraction experiments were carried out in a FEI Titan S/TEM @ 300 kV using ultrathin cross section samples received from ultramicrotomy on resin embedded nano magnetite loaded PVDF-fibers. Within one measuring campaign a total number of 100 diffraction patterns were recorded from sample regions ranging from 21x21 nm2 to 34x41 nm2 in size. Interplanar spacings and angles between the diffraction spots were determined from 12 unambiguous indexed zone axis patterns and used for verifying the structure of magnetite and for calculation of the cubic lattice parameter. Hence evaluation of 90 unique indexed interplanar spacings in the range between 4.8 Å and 1.0 Å yielded a lattice parameter of a = 8.36(9) Å for the cubic unit cell with Fd-3m symmetry. 

 

 

References:

[1] J.M. Zou and J. Tao, Scanning Electron Nanodiffraction and Diffraction Imaging in Scanning Transmission Electron Microscopy – Imaging and Analysis, Springer Science+Buisness Media, pp. 393–427, 2011.


Nelly WIRCH (Aachen, Germany), Ioana SLABU, Ralf THEISSMANN, Maria KRÜGER, Thomas E. WEIRICH
08:00 - 18:15 #5404 - IM06-348 Interpretation of phase structures using the program "e;DiffraCalc"e; and EDX-spectrometry.
IM06-348 Interpretation of phase structures using the program "e;DiffraCalc"e; and EDX-spectrometry.

The micro-diffraction method in transmission electron microscopy allows to determine the crystalline types of phase components in alloys, that is necessary to calculate the effect on mechanical properties of the materials.

The analysis of chemical composition of the selection has to be done to determine the crystalline type of phase inclusion within the micro-diffraction method. The method of EDX-spectrometry is usually used in case of steel researches, because this method is more sensitive to heavy elements which are defining in the formation of phase components in iron-based alloys. When the elemental composition of the inclusion obtained from containing sample site is taken into account, decoding of the diffraction pattern can be made. This procedure helps to determine the crystallographic structure of the considered phase.

This paper presents a modified program for indexing electron diffraction patterns «DiffraCalc», which allows to decode the diffraction patterns obtained from the phase inclusions, considering the chemical composition. This work is based on a well-known method of diffraction patterns indexing - a method of paired reflexes [1]. Diffraction patterns indexing manually using this method has several disadvantages. Usage of this modified program has solved most of the indexing problems and has led to significant work acceleration and has improved the quality of indexing results.

The program includes: automatic indexing algorithm, the connection to the extensive database of crystalline substances, compatibility with standard CIF (Crystallografic information file) [2] files, the availability to choose three reflexes pairs at the same time and the ability to work simultaneously with several phases within one-time setting scale. Indexing mode in the modified program allows to provide automatic calculations that are held simultaneously in several prospective phases, and moreover to analyze the phase matching for diffraction pattern without user intervention. The developed program has also implemented simulation mode of the diffraction pattern of a single crystal. This mode is intended to check the indexing results.

Verification of the diffraction patterns simulation mode was carried out under the program Single Crystall [3] for a number of substances (Fe, Fe7C3, Fe3C, etc.) with all types of crystal systems and has shown good compatibility between the calculated diffraction patterns of the Single Crystall results and the one obtained in the modified program «DiffraCalc». The examples of diffraction patterns simulation from a variety of substances is shown on Figure 1.

Analysis with the proposed program does not require multiple downloads of the experimental image of diffraction pattern, has the advantage of automatic conformity assessment estimated the experimental diffraction pattern based on an assessment of conformity of interplanar distances and angles between the reciprocal lattice vectors. Thus, the program «DiffraCalc» has broad capabilities to simulate the calculated diffraction patterns and indexing of experimental one.

Also, the paper contains investigations of low-alloyed reactor pressure vessel (RPV) steels phase composition. Studies of the chemical composition of phase components were carried out by EDX-method, the crystal structure of the phase components investigations were carried out by the micro-diffraction method with subsequent analysis of the experimental diffraction patterns using the «DiffraCalc» program. An example of the automatic indexing of one of the electron diffraction patterns is shown on Figure 2, 3. This phase inclusion into the RPV steel is carbide of type Cr21Fe2C6. Experimental reflexes positions have shown good compatibility of reflexes positions, calculated in the program, with considering small deviation of the zone axis [111] from the direction of the electron beam.

 

References

 

  1. Williams D., Carter A. Transmission electron microscopy. New York.: Springer. 2009. 760 р.
  2. Hall, S. R., Allen, F. H. and Brown, I. D. (1991). "The Crystallographic Information File (CIF): A New Standard Archive File for Crystallography", Acta Cryst., A47, 655-685.
  3. CrystalMaker Software Limited. SingleCrystal 2. http://www.crystalmaker.com/singlecrystal/index.html

Evgenia KULESHOVA, Alexey FROLOV, Ekaterina KRIKUN (Moscow, Russia)
08:00 - 18:15 #5605 - IM06-350 Angle-resolved Scanning Transmission Electron Microscopy (ARSTEM) for materials analysis.
IM06-350 Angle-resolved Scanning Transmission Electron Microscopy (ARSTEM) for materials analysis.

Many solid-state properties leave characteristic fingerprints in the angular dependence of electron scattering. STEM is dedicated to probe scattered intensity at atomic resolution, but it drastically lacks angular resolution due to detectors integrating over broad solid angles. By developing a setup which is capable of recording STEM images for dedicated acceptance angles of annular detectors, we firstly report the simultaneous measurement of specimen thickness, chemical composition and strain in a GaNxAs1-x/GaAs layer at atomic resolution. Our analysis exploits two angle ranges, namely A: 42-66 and B: 82-141mrad which exhibit different dependencies on nitrogen content x and specimen thickness as shown by the simulation in Fig.1a. To acquire images for dedicated angle settings, we developed a motorised, software-controlled iris aperture (Fig.2a) mounted above the Fischione 3000 ring detector in a Titan 80/300 (S)TEM, which is used to control the outer acceptance angle of the detector as depicted by the detector scans in the inset. Consequently, a STEM image formed by electrons scattered to the angular interval [α,β] is obtained by the difference I(β)-I(α) between 2 images taken at iris radii α and β.

For GaNAs, we recorded four 2Kx2K high-resolution STEM (HRSTEM) images with iris radii 42, 66, 82 and 141 mrad, performed a Voronoi segmentation with respect to atomic columns and averaged the intensity within the Voronoi cells. By mapping the Voronoi intensity with respect to their cell index and correlating the images, specimen drift was compensated for. Subtracting respective images yields the data in Fig.1b,c for the desired angular ranges. The GaNAs layer is imaged with high contrast for the angular range of 42-66 mrad whereas it is invisible for a detector acceptance of 82-141 mrad. By simultaneously comparing Fig.1b,c with simulations [1], the local nitrogen content and specimen thickness were obtained as shown in Fig.1d,e. Hence our method overcomes the common problem to interpolate thickness from regions with known composition. Finally, profiles across the GaNAs layer (not shown) reveal an average N content of x=2.5% and a mean thickness of 186nm. The nitrogen content is verified by X-ray studies and strain state analysis in one of the HRSTEM images. The total acquisition took 5:40min here whereas a 1kHz camera would have needed 70min to obtain this data.

Secondly, we imaged a GexSi1-x/Si field effect transistor using a dense sampling of the scattering angles between 16 and 255 mrad at two specimen thicknesses of 50 and 150 nm. Two different camera lengths with each 16 outer acceptance angles were used as shown in the radial sensitivity curves in Fig.2b, obtained by scanning the beam over the detector with iris aperture. Fig.2c exemplarily shows several images recorded at the larger camera length. The first image for the range [16,22 mrad] completely lacks chemical contrast in favour of strain-dominated intensity modulations in the vicinity of the Ge-containing source S and drain D stressors. Towards image 4 for [16,34 mrad] strain and the onset of Z-contrast determine the image contrast comparably, the latter caused by the 2 times larger atomic number of Ge compared to Si. In subsequent images Z-contrast dominates the signal revealing the two composition regimes of Ge with x=22% and x=37% which we determined using EDX with the chemiSTEM system.

By subsequently subtracting the images we obtain the explicit angular dependence of scattered intensity for pure Si and the two Ge regimes in Fig.3a-c. Obviously the scattered intensity increases with both thickness and Ge content over the broad angle range covered here. Particular attention is to be drawn at the theoretical models included for comparison in Fig.3: δ denotes the exponent obtained for assuming the Rutherford model where intensity is proportional to Zδ. The variety of δ values shows that there is no consistent trend as to a composition, thickness and angle-dependence, so that the Rutherford theory is inapplicable to our STEM data. Moreover, frozen-lattice multislice simulations for strain-relaxed (alloy) supercells are shown by the dashed lines. Although perfect agreement is found for angles above 35mrad, significant deviations are observed at smaller angles. This mismatch of contemporary simulations, which are fully elastic except for phonon scattering, is discussed in detail with respect to further inelastic scattering on the basis of the angular dependence measured from energy-filtered diffraction patterns.

[1] Rosenauer and Schowalter: Springer Proceedings in Physics, Springer, 2007, 120, 169-172.

[2] K. M.-C. is supported by the DFG under contract MU3660/1-1.


Knut MÜLLER-CASPARY (Bremen, Germany), Oliver OPPERMANN, Tim GRIEB, Andreas ROSENAUER, Marco SCHOWALTER, Florian F. KRAUSE, Thorsten MEHRTENS, Pavel POTAPOV, Andreas BEYER, Kerstin VOLZ
08:00 - 18:15 #5920 - IM06-352 Measurement of strain in nanoporous gold using nano-beam electron diffraction.
IM06-352 Measurement of strain in nanoporous gold using nano-beam electron diffraction.

Nanoporous gold (npAu) has attracted a lot of attention during the last decade as it has interesting applications particularly in the field of catalysis [1]. It is prepared by corrosion of a suitable gold master alloy, e.g. gold/silver. The remaining material forms a sponge-like structure built of ligaments and pores still preserving a crystalline structure over several tens of nanometres. A high surface to volume ratio, open porosity making it permeable for gases and liquids and a strongly curved ligament surface providing surface atoms of different coordination are only some properties which qualify npAu as catalytic material with well adjustable and reproducible structure.

One important structural property that is expected to have strong influence on the catalytic activity is lattice strain, because strain affects the electronic states [2]. Here we present measurements of strain in npAu. A measurement of lattice strain by means of high-resolution TEM for example by the analysis of lattice plane distances is possible only in a very small field of view for npAu. As the positions of intensity maxima in the images depend on several experimental parameters, such as defocus, orientation, composition, lens aberrations and especially specimen thickness, maxima detection in TEM micrographs succeeds only in small parts of the sample, because thickness is varying strongly. This can be seen in Fig. 1. Close to the surface intensity maxima can be detected and thus strain can be measured, in this case tensile strain up to 5% has been found. But as the ligament gets thicker and defects become visible, contrast changes and strain analysis fails.

A method that overcomes these problems is strain analysis using nano-beam electron diffraction (NBED) [3]. Here a focussed electron probe is scanned across the sample and at each position of the scanning beam the corresponding diffraction pattern is recorded. As distances between the non-overlapping diffraction discs depend basically on the local lattice parameter according to Bragg’s law, strain can be measured by comparing distances between diffraction discs at different positions of the scanning beam. By an analysis of distances between diffraction discs in two linearly independent directions strain as well as shear-strain and rotation can be measured.

A further advantage of strain analysis by NBED is its large field of view, because at first sight it is limited only by the size of the scanned part of the sample. Hence strain and rotation of neighbouring ligaments in npAu can be measured. On the other hand a large field of view requires the acquisition of a large number of diffraction patterns in a short time to avoid effects of sample drift, beam induced sample damage and contamination. Here we present strain and rotation maps of npAu measured using a delay-line detector for the acquisition of the diffraction patterns. With this detector strain maps at a scanning raster of e.g. 100x100 pixels can be recorded, allowing measurements in a field of view of several hundreds of nanometers (Fig. 2) still preserving a sampling limited spatial resolution of about 1.6 nm [4].

Furthermore we show by evaluation of simulations that two important aspects concerning the precision of the measurement have to be taken into account. As the precision of the measurement suffers from noise in the diffraction pattern, the precision degrades for shorter image integration times. On the other hand the precision can be increased using a precessing [5, 6] electron beam, as the diffraction discs are illuminated more homogeneously and hence their positions can be detected more precisely. In this way a compromise between precision and speed / size of the measurement has to be found.

 

[1] A. Wittstock et al., Science 327 (2010), p.319.

[2] M. Mavrikakis et al., Physical Review Letters 81 (1998), p.2819.

[3] K. Müller, A. Rosenauer et al., Microscopy and Microanalysis 18 (2012), p.995.

[4] K. Müller-Caspary et al., Applied Physics Letters 107 (2015), p.072110.

[5] J.-L. Rouviere et al., Applied Physics Letters 103 (2013), p.241913.

[6] C. Mahr et al., Ultramicroscopy 158 (2015), p.38.

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under contracts no. RO 2057/12-1, RO2057/11-1 and MU3660/1-1


Christoph MAHR (Bremen, Germany), Knut MÜLLER-CASPARY, Tim GRIEB, Florian F. KRAUSE, Marco SCHOWALTER, Anastasia LACKMANN, Arne WITTSTOCK, Andreas ROSENAUER
08:00 - 18:15 #6012 - IM06-354 Large area orientation mapping on nanoscale materials using SEM.
IM06-354 Large area orientation mapping on nanoscale materials using SEM.

The increasing interest in nanostructured materials has raised the need for high spatial resolution orientation mapping and large-scale quantitative characterisation of such microstructures. Because Electron Back Scatter Diffraction (EBSD) does not achieve such high spatial resolution on bulk samples, these kind of studies are often done using a Transmission Electron Microscope (TEM). However, TEM-based orientation mapping techniques suffer from small field of view. As a result, Transmission Kikuchi Diffraction (TKD) in Scanning Electron Microscope (SEM) was developed as a technique capable of delivering the same type of results as EBSD but with a spatial resolution improved by up to one order of magnitude [1,2]. TKD analysis is conducted on an electron transparent sample using the same hardware and software as for EBSD system.  But when using conventional EBSD geometry, the transmitted patterns (TKP) are captured by a vertical phosphor screen with a considerable loss of signal and with strong distortions induced by gnomonic projection. Also, with standard TKD detector configuration, most of the transmitted signal does not reach the phosphor screen and results in lower quality patterns which can have negative effect in the measurement quality.

The limitations of such non-optimal sample-detector geometry can be overcome by an on-axis detection system. With a horizontal phosphor screen placed underneath the sample, the transmitted signal is captured where it is the strongest and TKPs will have minimal distortions. Using low probe currents, the spatial resolution can be increased and the beam-induced specimen drift reduced as compared to standard TKD detector configuration [3]. The improved stability and high spatial resolution allow the user to conduct large-area TKD orientation mapping.

Using a partially recrystallized ultrafine stainless steel sample, we will demonstrate that statistical data can be obtained for the quantitative characterisation of nanostructured materials in the SEM (figure 1).

 

 

References:

[1] R.R. Keller and R.H. Geiss, Journal of Microscopy, Vol. 245, Pt. 3, pp. 245-251, 2012.

[2] P. W. Trimby, Ultramicroscopy, 120, 16-24, 2012.

[3] M. Abbasi et al., ACS Nano, vol.9, no.11, 10991- 1002, 2015.


Laurie PALASSE (Berlin, Germany), Daniel GORAN
08:00 - 18:15 #6039 - IM06-356 Investigation of structural changes of ZnO:Ti thin films prepared by RF sputtering.
IM06-356 Investigation of structural changes of ZnO:Ti thin films prepared by RF sputtering.

ZnO is a wide used ferroelectric material with a variety of applications. This study investigates ZnO thin films doped by Ti as showing structure change from ZnO wurtzite up to ZnTiO3 perovskite.

ZnO:Ti thin films were prepared by RF reactive magnetron co-sputtering in Ar/O2 atmosphere from pure Zinc and Titanium targets (99.99%). Changes of amount of Ti in films were documented by EDS and EELS measurements, structure changes were measured by electron diffraction from thin film cross section (XTEM ED) and two geometries of XRD together with measurement of material optical properties.

Pure ZnO wurtzite structure show with increasing amount of Ti texture change from 8 degrees for 2.4 at % Ti to the 39 degrees for 8.7 at % Ti as it can be seen on electron diffraction patterns on figure 1a together with figure 1b. Structure measured by XRD appear to be more amorphous with Ti increasing as intensity of diffraction lines decreases and interatomic distances change from wurtzite to perovskite ZnTiO3 structure (fig. 2).

Diffraction measurement is by the Voight function method also capable to measure from single diffraction line size and lattice distortions of the diffracted particles. In our case results for 2.4 at % Ti ZnO line 002 is 52 nm and for 7.8 at % Ti ZnTiO3 line 104 is 19 nm, both with dualism indicates two regions of low and highly distorted part of particles. This correspond to relaxed core and distorted surface of the grains as it can be seen on figure 1c and d for Ti content 2.4 at % Ti or 8.7 at % Ti respectively.

Electron diffraction pattern from cross section is also capable to show and compare information from XRD measurements in two different geometries; situation is illustrated on figure 3. Circle integration of ED by ProcessDiffraction program [1] is capable to compare JCPDS diffraction database with electron diffraction pattern, however, this procedure do not take into consideration texture of investigated materials. Different geometries of XRD measurement takes in case of textured material different places indicated on XTEM electron diffraction pattern on Fig.3. Comparing of both results have to be carefully considered.

Direct comparation of measured interplanar distances is illustrated in Fig.4.

[1]       J.L. Lábár: Consistent indexing of a (set of) SAED pattern(s) with the Process Diffraction program, Ultramicroscopy, 103 (2005) 237-249.


Rostislav MEDLÍN (Pilsen, Czech Republic), Pavol SUTTA, Marie NETRVALOVA, Petr NOVAK
08:00 - 18:15 #6091 - IM06-358 Indexation of diffraction patterns for overlapping crystals in TEM thin foils – Application to orientation mappings.
IM06-358 Indexation of diffraction patterns for overlapping crystals in TEM thin foils – Application to orientation mappings.

The indexing of Precession Electron Diffraction (PED) patterns in TEMs for crystals orientation and phase determination as operated by the ACOM-TEM technique [1] tends to become a standard procedure. However, analyzing transmitted signals requires to deal with significant effects related to the lamella thickness. Indexing limitations emerge as soon as grain size is smaller than the sample thickness. In contrast with TKD patterns [2], information in PED patterns comes from all of the overlapping grains crossed by the electron beam. Analyzing such a mixture of Bragg reflections prevents the safe recognition of the orientations and phases and leads to two specific outcomes. First, the grains appearing in the resulting maps are not necessarily located on a same exact layer of the sample thickness. Second, the probability of mis-indexing is increased as reflections from every crystal may be taken for template matching. While it remains unclear which of the overlapping crystals is selected by template matching, it is necessary to understand if and in what means the Bragg spots number and related intensity of each crystal can differ from each other in the acquired patterns. In the present work, the influence of volume fraction and arrangement of grains with respect to the illumination direction are examined.

A sample composed of two overlaid copper plates was considered for the present purpose. ACOM-TEM characterizations were realized on a planar section of the stacked plates for different lamella orientations: at zero tilt, zero tilt after a 180° flip of the grid in the sample holder, and a 15° tilt. Seven cross sections were then cut to determine the respective microstructures and thicknesses of the superimposed plates. Using TEM images of the cross views, the overall thickness of the stacked plates was found to evolve from 330 nm to 470 nm from one side to the other, with one plate being 1.4 to 2 times thicker than the other. The crystallographic orientations were determined using the NanoMEGAS ASTARTM system implemented on a FEI Tecnai G2 F20 S-Twin FEG (S)TEM operating at 200 keV. A precession angle of 0.5° was systematically applied to a quasi-parallel probe of 4 nm at HMFW and 0.4 mrad semi-angle of convergence. TEM lamellae were prepared using a FEI HELIOS FIB.

No significant differences are observed when the orientation maps related to the non-tilted sample and its 180° flip are compared (Fig. 1c-d). In other terms, the intensity distribution of Bragg reflections in diffraction patterns is not governed by the illumination direction. The comparison between the planar orientation maps and the cross cuts (Fig. 2) shows that the detected grains are mostly related to the thickest plate. At the light of this, it seems reasonable to expect the Bragg reflections related to the thickest plate to be the most intense and, consequently, the ones mainly detected in the acquired diffraction patterns. Nevertheless, a few grains related to the thinnest plate are indexed in the planar cuts. This means that the pattern selection is not solely sensitive to the volume fraction of the diffracting crystals. The last finding is confirmed with the sample tilted at 15°. With such tilt, the number of Bragg spots and their related intensities vary as different crystal planes are excited by the electron beam. It can be seen in Fig. 1b that some variations are detected in the grains orientation. The main conclusion of this study is that, although volume fraction of each plate is here the dominant factor that determines the template matching orientation selection, the correlation index appears to be also dependent on crystals orientation and potentially related dynamical effects. More details on the effects of volume fraction with respect to crystal favorable orientations will be discussed.

 

1.         E. F. Rauch and M. Véron, Mater. Charact. 98 (2014), p. 1.

2.         R. R. Keller and  R.H. Geiss, J. Microsc. 245 (2011), p. 245.


Alexia VALERY (Grenoble), Frederic LORUT, Laurent CLÉMENT, Edgar RAUCH
08:00 - 18:15 #4667 - IM07-360 The role of secondary electron emission in the charging of thin-film phase plates.
IM07-360 The role of secondary electron emission in the charging of thin-film phase plates.

In the past few years, physical phase plates (PP) emerged as an interesting tool to achieve phase contrast of weak-phase objects in transmission electron microscopy (TEM). Research has focused on thin-film PPs, which are typically fabricated from amorphous carbon (aC)-films [1]. However, the illumination with high-energy electrons initiates an irreversible degeneration of the aC-film, which causes electrostatic charging and affects the phase-shifting properties.

Electrostatic charging is a limiting factor in the application of thin-film PPs. However, the mechanisms of charging are not well-understood. This work shows that charging predominantly occurs due to secondary electron (SE) emission. For this purpose, Hilbert PPs were fabricated from different materials to study their charging behavior under electron beam illumination. Besides aC, thin films of the metallic glass alloy Pd77.5Cu6.0Si16.5 (PCS) were used for PP fabrication. The amorphous PCS-alloy is characterized by a high electrical conductivity and a strong resistance towards oxidation [2], which at first sight suggests minor charging of PCS-films. For use as a Hilbert PP, the film thickness was adjusted to a phase shift of π. At 200 keV electron energy, this corresponds to a film thickness of 49 nm for aC and 19 nm for the PCS-alloy.

Fig. 1 shows phase-contrast TEM images and corresponding power spectra of an aC test-object, which were acquired using a Hilbert PP fabricated from an aC- (Fig. 1a) or a PCS-film (Fig. 1b). Despite its excellent electrical properties, charging occurs for the PCS Hilbert PP as can be deduced from the strong distortion of the Thon-ring system in Fig. 1b. However, charging is significantly reduced if the PCS Hilbert PP is coated on both sides with a thin aC-layer of 6 nm thickness as shown in Fig. 1c. It is noted, that for smaller cut-on frequencies charging also occurs for the aC (Fig. 1a) and the aC/PCS (Fig. 1c) Hilbert PP. In contrast to pure aC-films, the carbon content is drastically reduced for the aC/PCS Hilbert PP, which improves its stability under electron beam illumination.

The reduced amount of charging in Figs. 1a,c is attributed to the low SE emission of aC compared to the PCS-alloy. The emission of secondary and backscattered electrons was investigated in a Zeiss NVision 40 scanning electron microscope. The Hilbert PPs were mounted on a specific device, where the absorbed and transmitted current can be measured separately. The absorbed current to ground I_S was measured for different primary electron energies E from 0.5 keV to 20 keV. Fig. 2 shows the absorbed current normalized with respect to the current of the incident electron beam I_PE for aC and the PCS-alloy. Between 1 keV and 3 keV aC is negatively charged. Above 3 keV the aC-film only shows a small tendency for charging. Although the blue curve is close to zero, I_S/I_PE takes finite values below the measurement accuracy. The PCS-alloy has a negative charge below 5 keV, but is positively charged for electron energies up to 20 keV. The low tendency for charging of aC and the positive charging of the PCS-alloy are consistent with the observations made in Fig. 1. It is noted, that electron energies are much higher in TEM. However, the curves in Fig. 2 show trends, which might continue towards higher energies.

Electrostatic charging implies poor electrical conductivity. Otherwise, any positive (negative) charge balance would be immediately compensated by an electrical current from (to) ground. We assume that our Hilbert PPs are properly grounded and that aC as well as the PCS-alloy have a sufficient electrical conductivity for the small beam currents used in TEM. A possible reason for poor electrical conductivity is beam-induced contamination. Low energy SEs, which get trapped in the highly insulating contamination layer, cause the formation of an electrical dipole layer at the interface between the contamination and the PP film.

 

References

[1] R. Danev and K. Nagayama, J. Phys. Soc. Jpn. 73 (2004), p. 2718.
[2] B. Chelluri and R. Kirchheim, J. Non-Cryst. Solids 54 (1983), p. 107.

 

Acknowledgement

Financial support by the Deutsche Forschungsgemeinschaft (DFG).


Manuel DRIES (Karlsruhe, Germany), Roland JANZEN, Tina SCHULZE, Jonas SCHUNDELMEIER, Simon HETTLER, Ute GOLLA-SCHINDLER, Bianca JAUD, Ute KAISER, Dagmar GERTHSEN
08:00 - 18:15 #4914 - IM07-362 Generation with phase-and-amplitude electron holograms of Laguerre-Gauss beams with orbital angular momentum up to 200ħ.
IM07-362 Generation with phase-and-amplitude electron holograms of Laguerre-Gauss beams with orbital angular momentum up to 200ħ.

Phase-and-amplitude electron holograms provide a flexible way to encode an arbitrary wavefunction by modulating only the hologram phase [1] [2]. This is an innovative step in the direction of novel experiments with structured electron waves [3]. The most interesting application example is the generation of Laguerre-Gaussian (LG) beams as they can be used to match exactly a single Landau state of electrons inside the objective lens of a transmission electron microscope (TEM). LG beams are a solution of the paraxial Schrödinger equation. They are mainly characterised by two indexes: l, the azimuthal index, representing the orbital angular momentum (OAM), and p, the radial index, where p+1 is the number of intensity radial nodes. They will be referred to with the contract notation LG(l,p). Landau states, on the other hand, are the quantized eigenstates of a charged particle with OAM in a magnetic field. Remarkably, LG beams have, at a given plane, the same form of quantized Landau states [4]. They only differ in the z evolution: while in a magnetic field Landau states are non-diffractive, LG beams in vacuum expand with defocus but maintain the same intensity shape.

We fabricated the holograms with Focused Ion Beam (FIB) on Si3N4 membranes. As a first check of fabrication accuracy, the Energy Filtered-TEM thickness map is taken. Thickness maps can be considered as a measure of the phase of the electron wavefunction after the hologram. In the first row of figure 1 thickness maps of the holograms LG(0,10), LG(10,0) and LG(10,10) are shown. The second row shows on top the experimentally acquired intensity at the Fraunhofer plane, and at the bottom the intensity and phase (represented by the colour hue) calculated with the software STEM_CELL starting from the thickness maps. The beams with l=10 show indeed the well-known azimuthal phase ramp of vortex beams while the beams with p=10 show 11 intensity nodes in radial direction with alternating phase as prescribed for LG beams. This allows us to say that we were able to produce LG beams with arbitrary l, p indexes. If the LG beam was generated inside the magnetic field of the objective lens of a TEM, this would permit the visualization of exact Landau states.

A more direct test of the LG character of these beams is obtained by visualising their shape invariance after propagation. To this aim, we investigated the propagation behaviour of the beam LG(10,0) with simulations, with w0 is the beam waist and zR = π W02/λ is the Rayleigh range for electrons with wavelength λ. Simulations are shown in the first row of figure 2, reporting the beam intensity shape propagated over Δz distance. When propagating, the beam width increases but the circular intensity shape remains the same. This is not true in general for all vortex beams. In order to make a comparison, we generated a vortex beam with 10ħ OAM (named L=10). The second row of figure 2 shows the experimental intensity of the beam L=10, acquired at the Fraunhofer plane (Δz = 0), and with different defocuses. The external intensity ripples are due to the abrupt intensity profile of the hologram (which in this case was not made with the phase-and-amplitude technique) and have the same character at all defocuses. The shape of the vortex, which is the circle with maximum intensity, with increasing defocus develops some internal ripples: this beam is therefore not shape invariant after propagation. As a further instance of the improvements due to the phase-and-amplitude scheme with respect to the previous ones, in figure 3 a phase-and-amplitude LG(200,0) beam (a) and its radial profile (b) is compared with an ordinary hypergeometric-Gauss beam with L=200ħ (c) and its profile (d). The striking feature is the transverse confinement of the intensity in the LG beam, with respect to L=200 that shows many external ripples.

Generating beams with phase-and-amplitude holograms shows clear advantages, from the suppression of the unwanted beam intensity ripples to the control of radial and azimuthal degrees of freedom of LG beams, and is of great importance in order to generate shape-invariant LG beams that can match exactly a single Landau state.

[1] E. Bolduc, N. Bent, E. Santamato, E. Karimi, and R. W. Boyd, Optics Letters 38 (2013)
[2] V. Grillo, E. Karimi et al. Microscopy and Microanalysis 21(S3) (2015)
[3] J. Harris, V. Grillo, E. Mafakheri, G. C. Gazzadi, S. Frabboni, R. W. Boyd, and E. Karimi, Nat. Phys. 11 (2015)
[4] K. Y. Bliokh, P. Schattschneider, J. Verbeeck, and Franco Nori, Phys. Rev. X 2 (2012)


Federico VENTURI (Modena, Italy), Roberto BALBONI, Gian Carlo GAZZADI, Marco CAMPANINI, Ebrahim KARIMI, Vincenzo GRILLO, Stefano FRABBONI, Robert W BOYD
08:00 - 18:15 #5140 - IM07-364 Double crystal interference experiments.
IM07-364 Double crystal interference experiments.

In 1978, Rackham and co-workers observed remarkable and unusual diffraction patterns from an object that consisted of two perfectly aligned, simultaneously reflecting crystals that were separated by a gap [1]. They reported that they could obtain such double crystals routinely by ion bombardment. However, their specimen preparation method did not allow the the gap between the crystals to be controlled and the maximum gap that they achieved was on the order of 1-2 μm. A subsequent realization of a double crystal interferometer (DCI) was achieved using voids in spinel [2], again with a crystal spacing of below 1 μm. In 1995, Zhou and co-workers [3] presented new results by combining a Si double-crystal interferometer with convergent beam electron diffraction (CBED), taking advantage of a special structure formed at the broken edge of a Si [111] crystal. The gap was still on the order of 1 μm or below.

.

Here, we use focused ion beam (FIB) milling to build DCIs that have gaps of up to 8 μm and to provide better control over results that were previously obtained by chance. Figure 1 shows a top view scanning electron micrograph of such an interferometer. The gap separation is 800 nm. Both single crystal and double crystal areas have been patterned. Superimposed on the image is a sketch of the ray path of a convergent beam that illuminates the upper crystal, generating a transmitted beam and a diffracted beam. These beams, in turn, impinge on the second crystal, generating further transmitted and diffracted beams that overlap in the diffraction plane, resulting in the formation of interference fringes.

.

Figure 2 shows a comparison of diffraction patterns recorded from a single crystal (left) and two overlapped crystals (right). The spacing of the interference fringes depends on the electron wavelength, the excited Bragg reflection and the camera length. More impressive results are obtained when the orientation of the crystal is close to a zone axis. Figure 3 shows a comparison of a standard CBED pattern (left) with a complicated system of interference fringes arising from overlap of many diffracted beams (right). The interference phenomena in these patterns encode information about the crystal structure. The fringe spacing is inversely propotional to the gap width. Therefore, for an 8 μm gap, ten times more fringes are present in the overlapped discs and the interferogram can be considered as a hologram, as showsn in Fig. 4.

.

As suggested by the first experimenters [1], accurate lattice parameter measurements can be made using a DCI when one crystal is the specimen of interest. If, instead, a specimen in inserted between the crystals or deposited onto the lower crystal, then it will be possible to obtain an off-axis Fresnel hologram with a reduced exposure time that is not affected by Fresnel

diffraction from the edges of a biprism wire, as is the case when an electron biprism is used as an interferometric device. Moreover, the reduced exposure time due to amplitude division beam splitting could open the way to dynamic recording and processing of holograms.

.

[1] G.M. Rackham, J.E. Loveluck and J.W. Steeds. Journal of Physics: Conference Series, 41 (1977) 435.

[2] C.B. DeCooman and C.B. Carter. Ultramicroscopy, 13 (1984) 233.

[3] F. Zhou, E. Plies and G. Möllenstedt. Optik,98 (1995) 95.

.

We acknowledge financial support from the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative (Reference 312483 ESTEEM2) and the European Research Council for an Advanced Grant (Reference 320832 IMAGINE).


Amir H. TAVABI, Martial DUCHAMP (Jülich, Germany), Rafal E. DUNIN-BORKOWSKI, Giulio POZZI
08:00 - 18:15 #5812 - IM07-366 Diffraction holography for the phase retrieval of vortex beams.
IM07-366 Diffraction holography for the phase retrieval of vortex beams.

The problem of phase retrieval in electron microscopy is generally related to the characterization of electric and magnetic fields in materials, to the retrieval of crystallographic structure or to the imaging of very weakly scattering objects. Recently, increasing attention is payed to the phase retrieval of electron vortex beams (EVB) i.e. beams carrying orbital angular momentum (OAM) [1] [2] [3]. The difficulty in this case arises due to the presence of an inherent phase singularity. There exist various kinds of phase retrieval schemes, and they mainly divide into off- and on-axis, whether the electron beam is displaced from the electro-optical axis or not. Furthermore, they can be computational (iterative or deterministic) or interferential. In this work, we use interferometry with synthetic beam shaping [4] to retrieve the phase of an EVB. In particular, we use an off-axis interferential method, where a reference beam interferes with a vortex beam in the diffraction plane. From the interference pattern it is then possible to retrieve the phase with Fourier methods. This method overcomes the difficulties on in-line methods and can be applied to the diffraction of many nanometer-sized features.

 

For this experiment, two holograms have been closely spaced and imprinted with focused ion beam (FIB) on a Si3N4 membrane [5]. The two holograms are fabricated close to each other in the same membrane window, and their diffraction patterns superimpose in the diffraction plane. The first produces the aimed EVB in the form of a Laguerre Gauss with topological charge 10, and the other one is a hologram with a parabolic modulation that produces the reference wave.

 

A scanning electron microscope (SEM) image of the two holograms can be observed in figure 1a (the parabolic hologram is on top and the LG hologram is at the bottom). Their separate diffractions are shown in figure 1b and 1c.The EVB shows the expected circular symmetry and the dark region in the central region. Conversely, the parabolic beam is characterized by a fully circular diffraction. The visible set of fringes here come from the interference with the 0th order background. An image of superposition diffraction pattern is shown in figure 2. The parabolic wave hologram has the effect of adding a uniform phase ramp to the phase of the LG hologram, resulting in a pitchfork pattern typical of the superposition of beams with an azimuthal and linear phase ramp.

 

The phase reconstruction then proceeds as in conventional holography: the interference (figure 3a) is Fourier transformed (figure 3b), a sideband is isolated and back Fourier transformed to obtain the phase shift as in figure 3c. Here the spiralling phase is visible, winding up by 10 x 2π in a cycle as expected for this EVB. The parabolic phase effect can be easily removed but does not alter the topologic charge consideration.

 

This case study opens the way to a reliable solution of the phase problem in low angle electron diffraction.

References

[1] M. Uchida, A. Tonomura, Nature 464, 737-739 (2010)

[2] J. Verbeeck, H. Tian, and P. Schattschneider, Nature 467, 301 (2010)

[3] J. McMorran, A. Agrawal, I. M. Anderson, A. A. Herzing, H. J. Lezec, J. J. McClelland, and J. Unguris, Science 331, 192 (2011)

[4] J. Harris, V. Grillo, E. Mafakheri, G. C. Gazzadi, S. Frabboni, R. W. Boyd, E. Karimi, Nature Phys. 11, 629-633 (2015)

[5] V. Grillo, G. C. Gazzadi, E. Karimi, E. Mafakheri, R. W. Boyd, and S. Frabboni, Appl. Phys. Lett. 104, 043109 (2014)


Federico VENTURI (Modena, Italy), Vincenzo GRILLO, Ebrahim KARIMI, Roberto BALBONI, Gian Carlo GAZZADI, Marco CAMPANINI, Stefano FRABBONI, Robert W BOYD
08:00 - 18:15 #5909 - IM07-368 STEM phase retrieval method for thick specimen by using quasi-Bessel beam.
IM07-368 STEM phase retrieval method for thick specimen by using quasi-Bessel beam.

Segmented detectors in scanning transmission electron microscopy (STEM) have been rapidly progressed, and recently have reached to the pixel array detectors (PADs) enabling to obtain whole information in reciprocal space during probe scanning [1]. One of the coauthor Ikuta also had developed a squarely arranged PAD for STEM, which consists of 8x8 detection portions [2]. As an application using the PAD in STEM is phase retrieval such as the ptychographic microscopy [3]. This iterative technique is capable to represent phase images, however, in which specimen thickness has to be enough thin to be as the weak phase objects. In order to overcome this restriction, we have proposed the novel STEM phase retrieval technique using annularly-shaped pixel array detector (A-PAD) combined with quasi-Bessel beam [4]. In this paper, we demonstrate applicability of our method, called Phase Retrieval for Thick Specimens (PRETS), by using a SrTiO3 [100] single crystal.

Figure 1 shows a schematic illustration of the proposed phase retrieval method in STEM. By using an annular aperture to form hollow-cone-shaped probe, its intensity distribution is elongated along the optical axis [4], as well as in Bessel beam [5], resulting in applicability to thick specimens. In this case, however, the depth of focus (DOF) is limited finitely due to the finite width of the annular slit. This means that such probe does not correspond to the Bessel beam providing infinite DOF but should be called as the quasi-Bessel beam. The annular aperture has the other effects to restrict the distribution of the electrons going forward to the detection plane, as shown in Fig. 1. This leads to that, for bright-field (BF) imaging, the pixelated detectors are permitted to be located just at the annular-shaped region where the direct beam illuminated. This is the reason why we have adopted the annularly arrayed pixel detectors. Each detectors in the A-PAD yields BF-STEM images containing different information of specimen. From these components, the phase information can be extracted and reconstructed by a dedicated Fourier filter having an acentric annular shape and a summation of the filtered components.

Figure 2 shows the developed A-PAD apparatus. As shown in Fig. 2(a), this system contains 31 detectors in which 24 channels (#0-23) are used for the phase retrieval and the others for normal BF imaging. Each channel consists of a bare optical fiber, one end of which is coated directly by fluorescent P-47 powders (Fig. 2(b)). The other ends of the fibers are connected to a multianode photomultiplier tube (PMT). These fibers transfer the photons converted from electrons on the detectors. Eventually, output signals from the PMT are fed into an image processing computer. The detector is mounted on the x-y position adjustable stage, as shown in Fig. 2(d), which was assembled in the STEM column.

Figure 3 shows phase maps of a SrTiO3 [100] single crystal, for comparison in terms of thickness effect, obtained by means of the multi-slice simulations. The phase in the TEM corresponding to imaginary part of the exit wave is strongly affected by increment of specimen thickness, as shown in Fig. 3(b). In contrast, phase retrieved by the proposed method can represent directly atomic structures without the contrast reversal even at 15 nm in thickness. This clearly proves the effectiveness of our novel technique PRETS.

References
[1] H. Yang, et al., Ultramicrosc. 151 (2015) 232  
[2] M. Taya, et al., Rev. Sci. Inst. 78 (2007) 083705
[3] M. J. Jumphry, et al., Nat. Comm., 3 (2012) 730
[4] T. Kawasaki, et al., Ultramicrosc. 110 (2010) 1332
[5] V. Grillo, et al., Phys. Rev. X, 4 (2014) 011013


Tadahiro KAWASAKI (Nagoya, Japan), Takafumi ISHIDA, Tetsuji KODAMA, Takayoshi TANJI, Takashi IKUTA
08:00 - 18:15 #5917 - IM07-370 Preparation of high fidelity holographic vortex masks using advanced FIB milling strategies.
IM07-370 Preparation of high fidelity holographic vortex masks using advanced FIB milling strategies.

Holographic masks (HMs) with dislocation gratings placed in the condenser system of a TEM have been proven to be a reliable and robust method to impart quantized orbital angular momentum (OAM), as well as quantized magnetic moment onto the imaging electrons [1]. These so-called electron vortex beams (EVBs) gathered a lot of attention due to some unusual properties like topological protection [2], peculiar rotation dynamics in magnetic fields [3] and intrinsic chirality. It has been suggested to use a holographic vortex mask as a vorticity filter after the specimen, in the selected area aperture holder, in order to detect spin polarized or other chiral transitions. This would bring up the unique chance to study magnetic properties of amorphous or nanocrystalline materials because the specimen’s role as a crystal beam splitter – necessary in the standard energy-loss magnetic chiral dichroism (EMCD) geometry - is obsolete in this setup.

High fidelity HMs are needed for such experiments. Also, in order to achieve high vortex order separation, the grating periodicity should be very fine. To improve the signal-to-noise ratio of the EMCD measurements, the dimensions of the HMs need to be large. FIB milling proved to be a robust and reliable technique to produce HMs, but with ever-increasing demands on structure size and fidelity, the ordinary milling strategy using raster- or serpentine scanning showed limited success. Therefore, we developed a new threefold milling strategy. The first step is to employ a so called “vector scan” technique, where “stream”-files provide the possibility to fully control the position and dwell time of the ion beam to generate spiral milling paths for every hole in the HM structure (see Fig. 1). The next step is to reverse the milling order and -direction after each pass [4]. Inspired by [5], the last part consists of a position-dependent dwell time reduction in the proximity of the hole edges to enhance the HM bar edge fidelity. Fig. 2 shows an exemplary 30 µm vortex mask with a grating periodicity of 500 nm and a thickness of roughly 700 nm.

One challenge encountered with this new strategy is limited digital-to-analog-converter resolution as well as memory issues for large “stream”-files. Using a state of the art FIB, it was possible to cut 50 µm HMs and to compare the ordinary raster scanning technique to the one proposed here, see Figs. 3 and 4. These results indicate that our new threefold scanning ansatz enhances the edge quality. Howerver, issues like sample- and beam drift as well as the crystallinity of the mask material have to be addressed in order to further improve the fidelity of the HMs’ edges.

 

Acknowledgements: The authors are indebted to Tina Sturm for the production of HMs. The financial support by the Austrian Science Fund (I543-N20, J3732-N27) and the European research council (ERC-StG-306447) is gratefully acknowledged.

 

References:

[1] J. Verbeeck et al., Nature 467 (2010): 301-304

[2] A. Lubk et al., Physical Review A 87 (2013): 033834

[3] T. Schachinger et al., Ultramicroscopy 158 (2015): 17–25

[4] T. R. Harvey et al., New Journal of Physics 16 (2014): 093039

[5] R. Winkler et al., ACS Applied Materials and Interfaces 7, 5 (2015): 3289–3297


Thomas SCHACHINGER (Vienna, Austria), Andreas STEIGER-THIRSFELD, Stefan LÖFFLER, Michael STÖGER-POLLACH, Sebastian SCHNEIDER, Darius POHL, Bernd RELLINGHAUS, Peter SCHATTSCHNEIDER
08:00 - 18:15 #5939 - IM07-372 Analysis of GaAs compound semiconductors and the semiconductor laser diode using electron holography, Lorentz microscopy, electron diffraction microscopy and differential phase contrast STEM.
IM07-372 Analysis of GaAs compound semiconductors and the semiconductor laser diode using electron holography, Lorentz microscopy, electron diffraction microscopy and differential phase contrast STEM.

 In order to develop and manufacture semiconductor devices which are key components of the optical telecommunication products, such as the semiconductor laser diode, it is essential to confirm whether it is manufactured as designed. Electric potential distributions of the semiconductor devices are designed in nanoscale, so two dimensional methods to evaluate the electrical potential in the semiconductors with a high spatial resolution are necessary for product management. The observation of the gallium arsenide (GaAs) model specimen was carried out by using the electron holography and Lorentz microscopy [1].

 Lorentz images and intensity profiles are shown in FIG 1 (a)-(f). The p-n junctions are clearly seen in both the 0.6 mm under-focused and over-focused images, but hardly any interfaces of different dopant concentration can be observed in the images. FIG 2 shows the electron holographic reconstructed phase image. The p- and n-type regions are clearly seen as areas of dark and bright contrast, and some differences in the changing dopant concentrations can also be seen.

 A phase image of semiconductor laser diode by the electron holography is shown in FIG 3 [2]. In FIG 3(a), the interface region is approximately 5 μm. And the spacing between interface fringes is approximately 30 nm. Next, in order to observe the pn junction near the active layer in a high spatial resolution, the photograph was taken by changing the interference fringes conditions. The expanded phase image of a part of FIG 3(a), surrounded by a dotted line, is shown in FIG 3(b). Since the interference region is approximately 1.5 μm, and the interference fringe spacing is 5 nm, the spatial resolution is approximately 15 nm. As can be recognized from the phase image, we can understand that more detailed structure can be observed in the higher spatial resolution in comparison with the phase image in FIG 3(a). Here, the designed location of the pn junction was positioned at the dotted line, but it was found from the electron holography observation results that the pn junction did not exist in the original position. This semiconductor laser diode could not have the expected output characteristics. The structural defect of the pn junction, found out in this observation, is considered to be the cause.

 For other semiconductor electric voltage evaluation methods by TEM, electron diffraction microscopy [3] which is one method of phase reconstruction method, differential phase contrast [4] (DPC) which is one method of STEM are also effective and possible to be utilized complementarily with the electron holography. We will discuss about these methods applied for semiconductor in this presentation.

 

References

[1] H. Sasaki, et al., Microscopy, 63(2014) 235.

[2] H. Sasaki, et al., Furukawa Review, 46 (2015) 19.

[3] J. Yamasaki, et al., Appl. Phys. Lett. 101 (2012) 234105.

[4] N. Shibata, et al., Scientific Reports, 5 (2015), 10040.


Hirokazu SASAKI (Yokohama, Japan), Shinya OTOMO, Ryuichiro MINATO, Kazuo YAMAMOTO, Tsukasa HIRAYAMA, Jun YAMASAKI, Naoya SHIBATA
08:00 - 18:15 #5972 - IM07-374 Charge transfer sensitivity and dose efficiency with pixilated detectors and ptychographic phase contrast imaging in STEM.
IM07-374 Charge transfer sensitivity and dose efficiency with pixilated detectors and ptychographic phase contrast imaging in STEM.

 Ptychography provides a sophisticated means of retrieving the complex object function via coherent diffractive imaging. It has become successfully established in the x-ray and visible light communities as a means of lensless imaging and for its super-resolution capability. Super-resolution was also the original use of the method in electron microscopy [1]. However the technique did not become popular in the high resolution electron microscopy community due to the difficulty of acquiring and processing the four dimensional datasets required. Recent advances in detector technology however have resulted in a resurgence of interest in the method. As aberration correction now provides atomic resolution in hardware without the need for super-resolution techniques, interest in ptychography in scanning transmission electron microscopy (STEM) has shifted towards achieving efficient phase contrast imaging.

 STEM provides sensitivity to atomic number via Z-contrast annular dark field (ADF) imaging. The approximately quadratic variation of the intensity in ADF images with atomic number provides relatively facile compositional interpretability as compared to phase contrast imaging. However a relatively small proportion of the beam current is scattered out to the high angles sampled by ADF detectors, particularly for thin samples composed of light elements. Most of the transmitted electrons are contained within the bright field (BF) disk. Ptychography has recently been shown to be more efficient than other phase contrast imaging methods used in STEM, including conventional BF, annular bright field (ABF), and differential phase contrast (DPC) [2,3]. It has also proven superior to these modes at revealing the positions of light elements hidden by the scattering of heavy elements in the ADF signal [4]. Furthermore, ptychographic phase imaging requires no aberrations to achieve contrast, meaning the electron probe can be tuned to maximum capability of the aberration corrector.

 Here we investigate the sensitivity of STEM ptychography for two different applications. The first makes use of the sensitivity of phase contrast imaging to electromagnetic fields to detect charge transfer. Such charge transfer sensitivity was demonstrated in conventional TEM by Meyer et. al. by making use of lens aberrations to reveal contrast changes in N-doped graphene and hexagonal boron nitride (hBN) that only matched with simulations based on potentials including the effects of charge transfer produced by density functional theory (DFT) and not the neutral atom potentials. We will present the results of testing charge transfer sensitivity in STEM with ptychography and various low dimensional materials. Figure 1 compares the projected potentials of (hBN) simulated with and without charge transfer. Figure 2 shows an example of simultaneously acquired ADF and ptychographic phase images of a region of single layer hBN surrounded by a double layer taken with the microscope fully tuned with the aberration corrector. As residual aberrations can affect phase images, we will also investigate the use of post acquisition aberration quantification and correction applied to ptychographic datasets of samples with the relatively subtle contrast effects of charge transfer.

 The second application of the sensitivity of STEM ptychography is its use for beam sensitive samples. We will assess the dose effectiveness of the method through simulations of varies samples, including biological samples frozen in amorphous ice, and compare to conventional TEM imaging. Consideration will be made of the pixelated detector technologies currently available, as the sensitivity and speed of the detector directly influence the dose effectiveness of the ptychographic phase images. 

[1] P.D. Nellist, B.C. McCallum and J.M. Rodenburg, Nature 374 (1995) 630-632.

[2] T.J. Pennycook et al., Ultramicroscopy 151 (2015) 160-167.

[3] H. Yang et al., Ultramicroscopy 151 (2015) 232-239.

[4] H. Yang et al., J. Phys.: Conf. Ser. 644 (2015) 012032.

[5] J.C. Meyer et al., Nature Materials 10 (2011) 209-215.

[6] The authors acknowledge support from the Austrian Science Fund (FWF) under grant number P25721-N20 and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655760 – DIGIPHASE.


Timothy PENNYCOOK (Vienna, Austria), Hao YANG, Clemens MANGLER, Stefen HUMMEL, Bernhard BAYER, Jani KOTAKOSKI, Peter NELLIST, Jannik MEYER
08:00 - 18:15 #6055 - IM07-376 Efficient generation of electron Bessel beams using generic magnetic vortex structures.
IM07-376 Efficient generation of electron Bessel beams using generic magnetic vortex structures.

As espoused by Montgomery1 and later by Durnin et al.2, the Helmholtz equation admits solutions which are invariant with respect to free space propagation, known as non-diffractive waves. Perhaps the simplest such solution is a scalar wave function where the amplitude of the wave function is in the form of a Bessel function of the first kind and is independent of the propagation distance z. Thus the intensity of the beam will not disperse laterally during propagation. In addition to this propagation-invariant property, Bessel beams can also reform after scattering from an object; the so-called “self-healing” property.  With such unique features, electron Bessel beams has potential applications in transmission electron microscopy (TEM). For example, in electron tomography, which requires electron probe intensity distributions to be invariant with sample depth; or in particle trapping, in analogy with optical tweezers.

How can one efficiently generate an electron Bessel beam for TEM? A simple method is to use a thin annular aperture to obtain a Bessel beam in the far-field diffraction plane3. This is based on the fact that the Hankel transform of a one dimensional delta function is a Bessel function. However, this method has extremely low efficiency since most of the beam is blocked by the aperture. A different approach has been taken by Grillo et al.4, who used a nanoscale-manufactured kinoform as a binary electron phase grating to create substantial depth of focus, utilising the intrinsic diffraction free property to demonstrate the suitability for electron tomography.

In this work, we present a simple, alternative approach to generate an electron Bessel beam, which is analogous to the axicon lens used in light optics.

In light optics, an axicon lens has circular symmetry and a thickness which varies linearly along the radial direction, so that a linear phase shift is imparted to the incident light along the radial direction (see fig.1), (in contrast to the usual quadratic phase shift from a thin lens). The incident beam is tilted due to the linear phase ramp, thereby forming a cone structure which generates the Bessel beam in the near field.

Can an axicon lens be fabricated for electrons? One could potentially use a conical nanostructure. The electron phase shift would be linearly modified by the varied thickness for a homogenous material of constant mean inner potential, for a sufficiently thin nanostructure. Alternatively, to reduce the size, the phase plate could be modulated with periodically varying thickness but this would then be even more challenging to fabricate.

In the present work, we introduce a natural and generic approach to efficiently create electron Bessel beams using magnetic vortex structures. For these ubiquitous dipole moment configurations, the magnetic vector potential imparts a linear phase shift upon incident electron waves, for specimens of constant thickness, such as thin films, resulting in a conical wavefront deformation centred about the vortex core. Thus magnetic vortex structures naturally behave as effective axicon lenses in the absence of electrostatic potential variations.

We prove this experimentally here, in a TEM using thin films of FeCo based nanocrystalline alloys (Fig 2). An electron Bessel beam was observed in the near field using Lorentz microscopy. The propagation-invariant property was verified using a through focal series. The coherent exit wave was recorded using off-axis electron holography, and the maximal non-diffractive distance was measured.  Utilising reciprocity to provide a further cross-check, a narrow annulus donut beam was also observed in the far field diffraction plane.

1. W. D. Montgomery, J. Opt. Soc. Am. 58(8), 1112–1124 (1968).

2. J. Durnin, J. J. Miceli, Jr., and J. H. Eberly, Phys. Rev. Lett. 58, 1499 (1987).

3. K. Saitoh, K. Hirakawa, H. Nambu, N. Tanaka, and M. Uchida, J. Phys. Soc. Jpn. 85, 043501 (2016).

4. V. Grillo, E. Karimi, G.C. Gazzadi, S. Frabboni, M.R. Dennis and R.W. Boyd, Phys. Rev X, 4(1), p.011013 (2014).

 


Changlin ZHENG (Melbourne, Australia), Timothy C PETERSEN, Holm KIRMSE, Wolfgang NEUMANN, Joanne ETHERIDGE
08:00 - 18:15 #6092 - IM07-378 Optical quantitative phase imaging by focal series reconstruction with partially coherent illumination.
IM07-378 Optical quantitative phase imaging by focal series reconstruction with partially coherent illumination.

Optical phase imaging has many applications, it allows to image transparent objects like cells without staining, which is a huge benefit in biology, and also allows to measure the phase from reflecting objects to determine their topography. Furthermore, there are many applications in life science, material science, healthcare and industry for optical phase imaging.

There exist different methods to image phases, phase contrast microscopy converts the phase information into amplitude contrast, but is difficult to interpret quantitatively, quantitative phase microscopy (QPM) on the other hand tries to determine the phase information quantitatively. Several full-field QPM methods exist, they can be divided in off-axis approaches and inline approaches. Off-axis approaches need a special microscope due to the reference beam and suffer from the high coherence of the illumination resulting in phase noise. Inline approaches could be divided in phase-shifting methods, common-path methods, digital inline holography, ptychography, Shack–Hartmann wave front sensors and focal series reconstruction, e.g. from the transport of intensity equation (TIE). Some of these approaches need special equipment, like laser illumination, gratings or spatial light modulators, which makes them expensive or introduces artefacts from the experimental setup or from the reconstruction algorithm.

Focal series reconstruction has the advantage, that it only needs a standard optical microscope plus a computer and no additional equipment, this makes quantitative phase imaging from focal series reconstruction both an easy and a low-cost method.

Figs. 1 & 2 present results obtained by our current approach to optical quantitative phase imaging, where we use focal series reconstruction with partially coherent illumination (a green LED source). The images were aquired using a Zeiss Axiovert 200M microscope and a pco edge sCMOS camera by varying the object height (z = 0µm is the focal plane). We apply our flux-preserving iterative reconstruction algorithm [1] and combine it with a TIE-like approach [2]. It aligns the images during reconstruction and applies gradient-flipping regularization [3]. This full-resolution inline holography (FRIH) algorithm [2] was originally developed for electron microscopy, but here we apply it to optical microscopy. We will discuss the validity of the reconstruction, especially with respect to artefacts and present the algorithm and the results in more detail.

 

[1] C.T. Koch, A flux-preserving non-linear inline holography reconstruction algorithm for partially coherent electrons. Ultramicroscopy 108 (2008),141–150. DOI: 10.1016/j.ultramic.2007.03.007

[2] C.T. Koch, Towards full-resolution inline electron holography. Micron 63 (2014) 69-75. DOI: 10.1016/j.micron.2013.10.009

[3] A. Parvizi, W. Van den Broek, C.T. Koch, Recovering low spatial frequencies in wave front sensing based on intensity measurements. Advanced Structural and Chemical Imaging (2016) in press


Johannes MUELLER (Berlin, Germany), Katharina BLESSING, Christoph KOCH
08:00 - 18:15 #6161 - IM07-380 Can quantum wave filters outperform image processing?
IM07-380 Can quantum wave filters outperform image processing?

As electron microscopists, we are often limited by both low contrast, and high-noise levels for
beam-sensitive materials. The contrast is a function of the imaging technique, and the sample under
study. The noise level of the image is a fundamental property of the finite electron dose, which must
remain limited, to avoid damaging the sample.


As a result of these, one route to improving microscopy images lies in developments of novel
imaging techniques. Image post processing techniques are commonly used for example, to remove
noise in images or increase the contrast of specific features of interest. These methods perform well
in specific cases, but one can wonder whether this post processing approach is the most optimal in
terms of electron dose efficiency.


The primary source of noise in electron micrographs is Poisson noise due to the electron counting
process occurring at the point of detection.
However, if we can manipulate the coherent electron wave prior to its detection, implementing a
specific operator acting on the wave, noise will occur on the detected processed image rather than
prior to the processing.


Such a setup can be obtained by using phase plates in the diffraction plane. Ideally, these phase
plates affect the phase of the passing electron wave without invoking a detection process,
effectively acting as a quantum filter.


We discuss and compare three primary examples of such quantum wave filtering, and the results
they have on the noise behaviour of the resulting images: vortex filtering as an edge enhancement
filter [1], wave background removal [2], and tuneable wave background reduction. We find each
method improves the image signal-to-noise ratio compared to image post processing implementing
a similar filter. We show that each quantum wave filter has different advantages which may be used
to enhance certain image features of interest.


Removal of the background in the wave decreases noise specifically in image regions of low
intensity, reducing the variance of the noise in the image, allowing more precise measurements.
Reduction rather than removal of the background however, enables a noise decrease, while avoiding
contrast reversals due to sign changes in the wave, improving direct interpretability. The vortex-
filtering method provides robust directional, or isotropic edge detection, with high contrast possible.


We demonstrate each of these options on a selection of different model samples, and discuss their
noise properties, required dose levels and their possible implementation.

References

[1] Blackburn, A. M., and J. C. Loudon, Ultramicroscopy, 136, (2014), 127-143.
[2] Zhang, Chao, et al., Ultramicroscopy, 134 (2013): 200-206.


Acknowledgements
LC and JV acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC Starting Grant No. 278510-VORTEX. JV acknowledges financial support from the European Union under the 7th Framework Program (FP7) under a contract for an Integrated Infrastructure Initiative (Reference No. 312483 ESTEEM2).


Laura CLARK (Antwerp, Belgium), Jo VERBEECK
08:00 - 18:15 #6205 - IM07-382 Concepts for an electrostatic phase shifting device.
IM07-382 Concepts for an electrostatic phase shifting device.

The advantage of providing amplitude and phase information of an object exit-wave makes off-axis electron holography a powerful tool for analyzing field and potential distributions up to an atomic scale. The ability to selectively manipulate the phase of the incoming electron wave additionally opens doors to new possibilities and  microscopic methods, such as the direct interferometric measurement of the coherence length of an electron wave packet [1, 2] or energy-loss magnetic chiral dichroism (EMCD) based investigations [3].

 

In principle, a selective phase shifting device is realized by directing one of two coherent electron beams through an electrostatic potential surrounded by a grounded electrode to shield stray fields, while the other beam propagates undisturbed in the vacuum. We present two different concepts to create such a device.

 

The first one is a variation of an experiment G. Moellenstedt suggested in 1980 [1] and was first realized in 1985 by H. Schmid [2]. Instead of using two concentric tubes, we realized a setup with two separated and electrically isolated 1 mm long micro tubes on an especially developed modular carrier chip (M1) to electrify them (figure 1).

 

The second concept includes two perforated metallic plates (20 µm X 20 µm X 3 µm) in a distance of 3 µm to each other. The micro device shown in figure 2 was produced with focussed ion beam (FIB) allowing both a selective phase shift as well as the investigation of influences of stray fields.

 

We performed first in-situ biasing TEM experiments with the phase shifting micro tubes. A big and necessary achievement was the enlargement of the hologram width from 2 µm to more than 25 µm. This was realized by using solely the TL22 lens of the image Cs corrector as objective lens of the FEI Titan 80-300 Berlin Holography Special TEM. However, the micrographs in figure 3 show strong artefacts which we attribute to beam tilting due to charging effects of the tube surfaces. These artefacts manifest in a bending-like projection of a biprism beneath the micro tubes.

 

 

 

 

 

 

 

1.    G. Möllenstedt, G. Wohland, Seventh European Congress on Electron Microscopy, the Hague (1980).

2.    H. Schmid, Dissertation (1985).

3.    P. Schattschneider  et al., Nature 441, 486–488 (2006).

4.   The authors kindly acknowledge the support from the Backend and Packaging Group of the HHI during the development of the M1.


Tolga WAGNER (Berlin, Germany), Tore NIERMANN, Dirk BERGER, Michael LEHMANN
08:00 - 18:15 #6221 - IM07-384 Generation of super-oscillatory electron beams beyond the diffraction limit.
IM07-384 Generation of super-oscillatory electron beams beyond the diffraction limit.

In 1873, Ernst Abbe discovered that the imaging resolution of conventional lenses is fundamentally limited by diffraction, which, since then, has been overcome using a variety of different approaches in optical microscopy. In electron microscopy, thanks to remarkable developments in aberration corrected electron optics, the resolution of transmission electron microscopes (TEMs) and scanning TEMs (STEMs) has reached the sub-Ångström regime. However, it is still limited by instrumental stability, residual higher-order aberrations and the diffraction limit of the electron-optical system. Recently, a concept termed super-oscillation, which is analogous to the idea of super-directive antennas in the microwave community [1], was proposed [2, 3] and applied in light optics for far field imaging of sub-wavelength, barely-resolved objects beyond the diffraction limit [4]. A super-oscillating function is a band-limited function that is able to oscillate faster locally than its highest Fourier component and thereby produce an arbitrarily small spot in the far field.

Here, we demonstrate experimentally for the first time a super-oscillatory electron beam whose characteristic probe size is much smaller than the Abbe diffraction limit. Figure 1(a) shows scanning electron microscopy (SEM) images of a conventional grating mask (left) and a super-oscillation off-axis hologram (right) that have the same outer diameters (10 µm). The masks were fabricated by focused ion beam milling 200-nm-thick SiN membranes coated with 150 nm Au. The masks were inserted into the C2 aperture plane of a probe-corrected  FEI Titan 80-300 (S)TEM. Owing to the probe aberration corrector and relatively small numerical aperture (convergence semi-angle), diffraction-limited spots could be easily obtained from the conventional grating (Fig. 1, left), while a super-oscillatory electron probe, which was generated at the first diffraction order (Fig. 1, right), produced a much smaller hot-spot in the center. The size of the super oscillation hot-spot is approximately one third of that of the diffraction-limited spot. It could theoretically be decreased further, even below the de-Broglie wavelength of the electrons, by varying the ratio between the inner and outer radii.

Further applications of such super-oscillatory electron wave functions, e.g. enhanced STEM imaging, will be presented.

 

References

1.   G. Di Francia, Super-gain antennas and optical resolving power, Nuovo Cim. 9, 426 (1952).

2.   Y. Aharonov, J. Anandan, S. Popescu and L. Vaidman, Superpositions of time evolutions of a quantum system and a quantum time-translation machine, Phys. Rev. Lett. 64, 2965-2968 (1990).

3.   M. Berry, Faster than Fourier, in Quantum Coherence and Reality, Celebration of the 60th Birthday of Yakir Aharonov, J. S. Anandan and J. L. Safko, eds. (World Scientific, 1994), pp. 55-65.

4.   E. T. F. Rogers, J. Lindberg, T. Roy, S. Savo, J. E. Chad, M. R. Dennis and N. I. Zheludev, A super-oscillatory lens optical microscope for subwavelength imaging, Nat. Mater. 11, 432-435 (2012).

 

Acknowledgments

This work was supported by the German-Israeli Project Cooperation (DIP) program from the German Research Foundation (DFG) and the Israel Science Foundation, Grant No. 1310/13. RDB thanks the European Research Council for an Advanced Grant.


Roei REMEZ, Yuval TSUR, Peng-Han LU (Jülich, Germany), Amir H. TAVABI, Rafal E. DUNIN-BORKOWSKI, Ady ARIE
08:00 - 18:15 #6235 - IM07-386 Calculation of phase contrast in Cc/Cs-corrected STEM.
IM07-386 Calculation of phase contrast in Cc/Cs-corrected STEM.

For STEM imaging, the pattern on the detector is a result of interference between elastically scattered wave and the incident wave. By recording the constructive interference and destructive interference with separate detectors, as well as by subtracting the two parts of intensity from each other, one can remove the background intensity as well as nonlinear information. As result, the remaining signal is enhanced phase contrast [1]. A recent  experimental demonstration of differential contrast in STEM achieved by matching the detector geometry and the physical phase plates in STEM mode has been reported in [2].

On an aberration-corrected STEM, an optimized differential phase contrast can be obtained by well designing the PCTF of the objective lens, and detector geometry accordant with the designed phase plate. The integrated spatial frequencies corresponding to positive contrast transfer is equal to those corresponding to negative contrast transfer. This ensures that nonlinear information cancels when the two parts of detector signals subtract each other.

Our calculation in Fig. 1 for Cc/Cs-corrected STEM without damping factors illustrates the advantage of differential contrast compared with the conventional bright-field imaging in STEM. The differential contrasts of single-layer graphene are dominantly stronger than the contrasts achieved in STEM-BF mode equipped with the same corrector. The differential contrast can reach 12, 18 and 26 times of the bright-field contrast at 20kV, 50kV and 80kV, respectively! This demonstrates that in STEM mode equipped with an aberration corrector and a detector, matching the geometry of the phase plate, phase objects can be imaged with descent contrast.  

With further improvement of the corrected state of the microscope, our calculation show that phase contrast imaging in STEM offers even more exciting possibilities. When the 5th-order spherical aberration is corrected, the illumination angle dependent on the largest usable aperture, also increases, resulting in a shallow depth of field. This allows accurate focus at certain thickness of the sample, as shown in Fig. 2. At the 8th layer of a total 16-layer (28nm) thick sample of Si, one silicon atom is replaced by a Germanium atom. A focal series through the sample shows that the layer marked by a substituted Ge atom is in focus at ∆f=-12Å. 

As a summary, the method of obtaining differential contrast in an aberration-corrected STEM can be powerful for investigating weak scattering objects. Under a further improved corrected state of the Cc/Cs corrector, the differential contrast realized with this technique is extremely thickness-sensitive, and focussing through atomic planes may come into reach. 

References
[1] H. Rose. Ultramicroscopy 2(1977), p. 251-267.
[2] C. Ophus, J. Ciston, J. Pierce et al. Nature Communications 7(2016), p. 10719.
[3] The authors greatly acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany in the frame of the SALVE project.


Zhongbo LEE (Ulm, Germany), Ute KAISER, Harald ROSE
08:00 - 18:15 #6246 - IM07-388 Developing new electron interferometry configurations in I2TEM thanks to electron optics simulations.
IM07-388 Developing new electron interferometry configurations in I2TEM thanks to electron optics simulations.

The In situ Interferometry Transmission Electron Microscope (I2TEM), is a microscope designed to easily performed new electron interferometry experiment [1,2,3]. The microscope is equipped with a 300kV cold field emission source, one electrostatic Möllenstedt biprism (BP) installed before the three condenser lenses, two goniometer stages situated respectively above and inside the objective lens pole piece (namely high resolution stage and Lorentz stage) and finally three BPs placed between the intermediate lenses. The strength of this microscope lies in its wide optical flexibility, which can be used to develop new configurations very difficult to obtain using standard TEM. On the other hand, this flexibility increases the optical complexity of the operation and implies being able to simulate the electrons trajectories as a function of the various optical conditions (lenses excitations, High voltage, gun lens ratio, …) to predict  “a priori” the hologram properties which will be obtained (width, interference fringes, …).

To perform such simulations, all lenses strengths have been calculated using finite element modelling software COMSOL multiphysics (see figure 1) [4], as a function of their ampere-turns excitations. These results are then used in SIMION software (see figure 2) [5] to compute the complete electron trajectories starting from the FE tip through the biprism to finish in the detector plane.

The simulations are finally compared to experimental data in order to calibrate the model.

 

[1]  T Denneulin et al, Ultramicroscopy  160 (2016), 98–109.

[2]  F Röder et al, Ultramicroscopy 161 (2016), 23–40.

[3]  F Houdellier et al, Ultramicroscopy 159, Part 1 (2015), 59–66.

[4] https://www.comsol.fr

[5] http://simion.com


Yudai KUBO (Toulouse), Christophe GATEL, Yoshifumi TANIGUCHI, Etienne SNOECK, Florent HOUDELLIER
08:00 - 18:15 #6252 - IM07-390 Quantitative measurement of the charge distribution along a tungsten nanotip using transmission electron holography.
IM07-390 Quantitative measurement of the charge distribution along a tungsten nanotip using transmission electron holography.

Off-axis electron holography can be used to measure the electron-optical phase shift associated with a charge density distribution in the transmission electron microscope (TEM). The charge density can then be recovered either by integrating the Laplacian of the reconstructed phase1 or, equivalently, by applying a loop integral2.  Whichever approach is used, the perturbed reference wave3 does not affect the measurement of the projected charge density inside the specimen so long as it does not itself contain any charges. Here, we study a W nanotip, in which the charge density distribution is of interest for applications in field emission and atom probe tomography. We assess artefacts and noise in the measurements.

Figure 1(a) shows an off-axis electron hologram of a W nanotip recorded at 300 kV using an FEI Titan 60-300 TEM.  The interference fringe spacing is 0.318 nm, the nominal magnification is 140 000 and the voltage applied to the electrostatic biprism is 90 V. The apex of the nanotip has a diameter of approximately 5 nm and is covered with a layer of tungsten oxide.  A voltage of 50 V was applied between the nanotip and a flat electrode positioned approximately 3 µm away from it. In order to remove the contribution to the phase shift from the mean inner potential, two holograms with and without a voltage applied to the nanotip were recorded. The difference between the two phase images was then evaluated after sub-pixel alignment. Figures 1(b) and (c) show the resulting unwrapped phase before and after adding phase contours of spacing 2π/3 radians. Figure 1(d) shows the charge distribution calculated by applying a Laplacian operator to a median-filtered version of the phase image. Figure 1(e) shows cumulative charge profiles along the nanotip determined both using a loop integral and by applying a Laplacian operator to either an unwrapped phase image or the original complex image wave. The integration region is marked by a green dashed rectangle in Fig. 1 (b). The measured charge profile is consistent between the three approaches. Figure 1(f) shows an evaluation of noise in the measurement obtained by performing a similar integration in a region of vacuum indicated by the red dashed rectangle in Fig. 1(b). Results such as those shown in Figs. 1(d) and (e) can be used to infer the electric field and electrostatic potential around the tip. Future work will involve comparing the present approaches with using a model-based technique for determining the charge density from a recorded phase image.

Acknowledgements: We acknowledge the European Union for funding through the Marie Curie Initial Training Network SIMDALEE2 and the European Research council for an Advanced Grant.

References:

1.         Gatel, C., Lubk, A., Pozzi, G., Snoeck, E. & Hÿtch, M. Counting Elementary Charges on Nanoparticles by Electron Holography. Phys. Rev. Lett. 111, (2013).

2.         Beleggia, M., Kasama, T., Dunin-Borkowski, R. E., Hofmann, S. & Pozzi, G. Direct measurement of the charge distribution along a biased carbon nanotube bundle using electron holography. Appl. Phys. Lett. 98, 243101 (2011).

3.         Matteucci, G., Missiroli, G. F., Muccini, M. & Pozzi, G. Electron holography in the study of the electrostatic fields: the case of charged microtips. Ultramicroscopy 45, 77–83 (1992).


Fengshan ZHENG (Juelich, Germany), Vadim MIGUNOV, Urs RAMSPERGER, Danilo PESCIA, Rafal E.DUNIN-BORKOWSKI
08:00 - 18:15 #4513 - IM08-392 Electron microscopy characterization of yttrium-doped barium zirconate electrolytes prepared with Ni additive: Influence of hydrogen treatment.
IM08-392 Electron microscopy characterization of yttrium-doped barium zirconate electrolytes prepared with Ni additive: Influence of hydrogen treatment.

Proton-conducting Fuel or Electrolysis Cells (PCFCs) are thought to be a promising alternative to Solid Oxide Fuel/Electrolysis Cells [1-2]. The most interesting proton conducting materials for electrolytes include doped barium zirconate, doped barium cerate or their solid solution. Recently, an innovative approach was proposed which allowed obtaining dense ceramics with grain size of typically 2-4 µm. This new process is based on the reactive sintering of all oxide precursors with the use of NiO as sintering aid [2-4]. Besides being dense, the samples obtained, such as yttrium-doped barium zirconate (BZY), show a very high proton conduction with little influence of Ni-species on conduction properties.

We tested the mechanical properties of this BZY material for application as PCFCs electrolyte and we showed that materials made from this process present a fast degradation of mechanical properties when put in hydrogen-rich conditions.

The objective of the present work was thus to understand the atomic-scale origin of this fast degradation. For this purpose, different electron microscopy techniques have used such as Scanning Transmission Electron Microscopy (STEM-HAADF), Energy dispersive spectroscopy (EDS), Electron Energy Loss spectroscopy (EELS). Structural and chemical microscopy analysis show that the sample failure is due to the reduction of NiO nanoparticles at grain boundaries (Figure 1, 2).

 

Acknowledgments : The microscopy work was carried out within the MATMECA consortium supported by the ANR under contract number ANR-10-EQUIPEX-37.

 

*paul.haghi-ashtiani@centralesupelec.fr, *Guilhem.dezanneau@centralesupelec.fr

 

References:

1    F. Lefebvre-Joud, G. Gauthier, J. Mougin, J. Applied Electrochem., 2009, 39(4), 535

2    E. Ruiz-Trejo, J.T.S. Irvine, Solid State Ionics, 2012, 216, 36–40

3    S. Nikodemski, J. Tong, R. O'Hayre, Solid State Ionics, 2013, 253, 201

4    J.H. Tong, D. Clarck, L. Bernau, M Sanders, R. O'Hayre, J. Materials Chem., 2010, 20(30), 6333


Paul HAGHI-ASHTIANI (CHATENAY-MALABRY), Mohamed BEN HASSINE, Desiree CIRIA, Manuel JIMENEZ-MELENDO, Veronique AUBIN, Guilhem DEZANNEAU
08:00 - 18:15 #4540 - IM08-394 Quantifying transition radiation by employing CL and EELS.
IM08-394 Quantifying transition radiation by employing CL and EELS.

The excitation probability per incoming electron for transition radiation (TR) is measured by employing electron energy losses spectrometry (EELS) and cathodoluminescence (CL) in a transmission electron microscope (TEM) using beam energies varying from 20 – 200 keV. We further demonstrate that TR is excited only at the sample surfaces, because the emitted intensity and the respective energy loss are independent of the sample thickness. As specimen we use an aluminum single crystal, because TR is known to be strongest on metals.

For the CL experiments we use a GATAN VULCAN system with upper and lower mirror. The recorded light spectra are corrected for the system response of the spectrometer and glass fiber cables. Then the spectra were integrated over the whole visible range (400-900 nm) and finally the conversion coefficient of counts to emission probability was done for the 20 keV spectra as described in [1]. Once the conversion coefficient was found, this unique value (depending on the collection efficiency of the optical mirrors in the VULCAN system) was applied to all other spectra thus giving a beam energy dependent emission probability (Fig 1 right). The behavior is perfectly in agreement with the Ginzburg-Frank theory [2]. The experiment was performed also with varying sample thicknesses in order to demonstrate that TR excitation is a surface effect only (Fig 1 left).

Using EELS we perform three different experiments: (i) an angle resolved measurement where we see the angular distribution of the transition radiation losses (TRLs) having an angular resolution of 7.5 µrad (see Fig 2), (ii) a thickness dependent experiment using 40 keV electrons showing the surface loss character of the TRLs (Fig 3 left), and (iii) an energy dependent experiment for determining the TR emission probability (Fig 3 right). The TR emission probability is the ratio between the TRL intensity and the total electron intensity.

For the angular resolved experiment we are raising the specimen out of the eucentric position by 278 µm and are choosing the largest possible magnification. Thus we achieve an effective camera length in the diffraction pattern of 288 m. The angular resolution of the electron beam is measured to be 7.5 µrad. Subsequently we move the beam 400 µrad across the spectrometer entrance aperture while recording a set of 200 single spectra, all of them 2 µrad apart (Fig 2 center).

For the experiments determining the emission probability per incoming electron we simply divide the integrated intensity of the TRLs after subtraction of the zero loss peak and the intraband transition intensities by the total spectral intensity. At the respective experimental conditions (collection semi-angle of 0.6 mrad) only appr. 70% of the angular distribution of the EELS spectrum falls into the spectrometer entrance aperture, whereas all of the TRL intensity is collected. This is because the angular distribution is extremely narrow (see Fig 2). Consequently we have to correct for the not collected intensity, too. Finally we find that the integrated emission probabilities measured with CL and EELS are consistent to each other. The results are given in Fig 3 (right hand side).

As expected from theory and from the CL experiment, a thickness dependence cannot be observed in EELS, too.

Summing up it is the first time that within a TEM transition radiation of aluminum was studied by means of EELS and CL. We proved experimentally that the angular dispersion of the TRLs is within the light cone, that TR is emitted from the surfaces only, and we determined the photon emission probability with respect to the beam energy. Due to the fact, that the probability for the emission of TR is very low, even for aluminum, it is even smaller in the case of semiconductors, where low loss EELS is used for determining the local dielectric behavior.

[1]       BJM Brenny, T Coenen and A Polman, J. Appl. Phys. 115 (2014) 244307
[2]       VL Ginzburg and IM Frank, JETP 16 (1946) 1 - 15


Michael STÖGER-POLLACH (Vienna, Austria)
08:00 - 18:15 #4568 - IM08-396 Atomic plane resolution EMCD measurement by STEM-EELS under 3-beam diffraction condition.
IM08-396 Atomic plane resolution EMCD measurement by STEM-EELS under 3-beam diffraction condition.

    Electron magnetic circular dichroism (EMCD) in the transmission electron microscope (TEM) is a relatively immature experimental technique, though it has already demonstrated quantitative results with a spatial resolution superior to what can be achieved with X-rays [1]. The main obstacles for widely sharing the method for routine use are (i) low SNR because of off-axis EELS detection, (ii) necessity of a very thin sample (a few nanometers) for quantitative analysis and (iii) unstable pre- and post-edge backgrounds by subsequent measurements in changing the aperture positions. There have been a number of attempts to overcome those difficulties, such as collecting a large quantity of data and applying statistical/information data processing methods [2,3]. In the present study we found a novel experimental condition that allowed us to solve almost all these issues and even applicable to atomic resolution EMCD measurements.

     Consider the symmetrical 3-beam diffraction condition with an appropriate convergence angle, where the diffraction discs are partially overlapped, as shown in Fig. 1. In this configuration the magnetic signals included in a core-loss spectrum (e.g.,  Fe-L2,3 white-line) is expressed in a simplified form of Eq. (1), where TG is the transmittance of the Bragg disc of G, the argument of the sine function is the phase difference between the electron wavefunctions at a point of the disc overlapping regions, and S(q, q - G, E) is the mixed dynamical form factor [4]. The first term stands for the classical EMCD signal. The second term is a new ‘phase-dependent EMCD’, which can be further simplified within the dipole approximation as in Eq. (2), where Mz is the net magnetization in the direction of the optic axis and Δx is the spatial coordinate in real space, measured perpendicular to the lattice planes with respect to the arbitrary atomic plane. Eq. (2) implies that a quite large fraction of chiral ± magnetic signals appear in an alternating manner in scanning the sub-nanometer electron probe on the sample perpendicular to the lattice fringes with the EELS entrance aperture covering an extended area over the either side of the diffraction discs with respect to the x axis in Fig. 1.

     The above proposed scheme was tested on the symmetrical 3-beam [110] diffraction condition in a 20 nm thick Fe film for STEM mode with the EELS aperture placed slightly off-axis on either side of the systematic row of the reflections. A focused electron probe with the convergence semi-angle of 10 mrad is scanned across the sample surface to find 1-dimensional lattice fringes as the ADF-STEM image, as shown in Figs. 2(a) and (b). The chiral ± EMCD signals should appear at the 1/4 of the lattice interval on both sides from the exact atomic column fringes, with the EMCD signal of the opposite sign appearing on the opposite sides of a lattice fringe, as shown in Fig. 2(c). Finally we applied a set of statistical treatments [5] to efficiently extract the EMCD signal. The extracted EMCD signal and its spatial localization map are shown in Fig. 3. Although the spatial distribution of the magnetic signal was quite noisy, the experimental intensity profiles of the non-magnetic and magnetic (EMCD) signals averaged over the direction parallel to the lattice planes showed the expected localization patterns, as shown in Fig. 4.

     This method requires only a single scan to obtain the chiral ± EMCD signals and also exhibits stable signal fractions with the change in sample thickness. The present scheme solves most of the existing experimental difficulties and is a novel breakthrough for quantitative atomic scale EMCD measurements.

 

References

[1] Hebert and Schattschneider, Ultramicrosc. 96 (2003) 463-468.

[2] Muto, Tatsumi and Rusz, Ultramicrosc. 135 (2013) 89-96.

[3] Muto et al, Nature Commun. 5 (2014) 3138-1-5.

[4] J. Rusz, J. C. Idrobo, and S. Bhowmick, Phys. Rev. Lett. 113, 145501 (2014).

[5] C. Andersson, R. Bro, Chem. Int. Lab. Sys. 52, 1 (2000).

[6]  This work was in part supported by Grant-in-Aid for Scientific Research of JSPS and Japan-Sweden Joint Research Program by JSPS and STINT.


Jan RUSZ, Shunsuke MUTO (Nagoya, Japan), Jakob SPIEGELBERG, Roman ADAM, Daniel E. BÜRGLER, Claus M. SCHNEIDER
08:00 - 18:15 #5116 - IM08-398 Transition potentials for inelastic scattering of relativistic electrons.
IM08-398 Transition potentials for inelastic scattering of relativistic electrons.

For acceleration voltages routinely used in the Transmission Electron Microscope (TEM), the kinetic energies of the beam electrons encompass a significant portion of their rest energies. Consequently relativistic corrections can become important for image calculation, as seen for example in relativistic calculations of electron energy loss images [1,2]. Therefore one has to describe the scattering process between a beam electron and an electron in the specimen using a completely relativistic formalism.
However, since the electrons in the specimen are usually non-relativistic, the question arises which relativistic corrections are important for a non-relativistic specimen electron excited by a relativistic beam electron. To examine this problem, one can describe the excitation of a specimen electron using a transition potential, as is often done in multislice simulations of inelastic scattering [3,4]. We derive a relativistic generalization of the transition potential to elucidate which relativistic corrections are important for the specimen electron.


In first order perturbation theory, an excitation event can be described by eq. (1), where He is the Hamiltonian of the beam electron and Hi is the Hamiltonian of the interaction between beam electron Ψ and specimen atom χ. The last term in eq. (1) can be used to define the transition potential, which in this case describes the transition of the specimen atom from the ground state 0 to the excited state m≠0.
Inserting non-relativistic quantities for He, Hi, χ and Ψ yields the well known Yoshioka equations [5]. Using relativistic quantities in eq. (1), we obtain the Dirac equation (2). Here, Aμ is the electromagnetic four-potential of the specimen, while {Am0}μ is the relativistic transition potential and γμ=(γ0, γ1, γ2, γ3) are the Dirac matrices. Since the relativistic transition potential has to satisfy an inhomogeneous wave equation, one way to obtain it is to solve eq. (3), for example with the help of the Green's formalism. This leads to eq. (4) (see also [6]), where φ is the four-spinor of an electron in the specimen and ΔEm0 is the energy difference between the states 0 and m divided by hc.


To calculate the matrix element Iμ(q) (see eq. (6)), the four-spinors of the excited specimen electron before and after the excitation by the beam electron are required. We describe the specimen electrons using Darwin wavefunctions [7]. These wavefunctions, shown in eq. (5) for the spin up and the spin down case, are an approximation of the four-component spinor in the non-relativistic limit. They relate the (relativistic) four-spinor φ to the (non-relativistic) Schrödinger wavefunction Φ.
One can now insert, for example, the spin up Darwin wavefunction into Iμ(q), obtaining eq. (6). Here, I0 is a matrix element also occurring in the non-relativistic calculation, while the other terms are relativistic corrections. To assess which correction terms are important, we will evaluate the integrals I0 ,Ix , Iy ,IZ and IΔ. The resulting transition potentials can be used for relativistic multislice simulations.



[1] R. Knippelmeyer et al., J. Microsc. 194 (1999) 30
[2] C. Dwyer, Phys. Rev. B 72 (2005) 144102
[3] C. Dwyer, Ultramicroscopy 104 (2005) 141
[4] L. J. Allen et al., Ultramicroscopy 151 (2015) 11
[5] H. Yoshioka, J. Phys. Soc. Jpn. 12 (1957) 618
[6] A. Lubk, PhD thesis, TU Dresden (2010)
[7] C. G. Darwin, Proc. Roy. Soc. A 118 (1928) 654


Stephan MAJERT (Münster, Germany), Helmut KOHL
08:00 - 18:15 #5124 - IM08-400 Real-space mapping of electronic orbitals.
IM08-400 Real-space mapping of electronic orbitals.

The world as we know it is shaped by electronic states. Be it optical, electrical, or magnetic properties, thermal conductivity, or chemical bonding: almost all macroscopic properties can be traced back to the electronic states on the nanoscale. It is all the more surprising that they remained mostly elusive from an experimental perspective so far.

In this work, we show that the mapping of transitions between electronic states in real space with Ångström resolution is indeed possible using state-of-the-art TEM and EELS [1]. As a model system, we used a 20 nm thick rutile sample oriented in [0 0 1] direction. In this system, the Ti L2,3 edge splits into contributions from states with eg and t2g symmetry, respectively. Fig. 1 shows the experimental L2-eg map extracted from the dataset acquired on a double Cs-corrected FEI Titan cubed microscope operated at 80 keV after drift-correction and averaging over 12 unit cells. An asymmetry that is rotated by 90° for nearest neighbors is clearly visible that is caused by the peculiar shape of the eg states as shown in the charge density distribution. Furthermore, simulations using the multislice [2] and mixed dynamic form factor [3] approaches were performed. As is evident from fig. 1, the simulations are in excellent agreement with the experimental data.

One crucial prerequisite for such asymmetries to appear lies in the local environment of the atom that is being probed [4]. If the atomic site is invariant under a high symmetry point group, many states will be degenerate and their contributions to the scattered intensity will add up to a circularly symmetric map according to Unsöld's theorem [5]. A prototypical example of this for p-states is shown in fig. 2. Only if the point group symmetry is low enough, the degeneracy is lifted and transitions to individual states can be mapped by selecting a suitable energy window.

This work shows that the mapping of individual electronic states is possible with widely used tools such as TEM and EELS. Thus, it paves the way for exciting new applications such as probing defect states at surfaces and interfaces that could revolutionize material science, as well as our experimental grasp on electronic properties and bonds on the atomic scale.

 

Acknowledgements: Support by the Austrian Science Fund (FWF), grant nrs. I543-N20, SFB F45 FOXSI, J3732-N27, the ERC under the EU's 7th Framework, grant nr. 306447, and the Natural Sciences and Engineering Research Council of Canada (NSERC) is acknowledged.

 

[1] Löffler et al., submitted
[2] Kirkland, “Electron Energy-Loss Spectroscopy in the Electron Microscope”, Plenum Press 1996
[3] Löffler et al., Ultramicroscopy 131 (2013) 39
[4] Löffler et al., in preparation
[5] Tinkham, “Group Theory and Quantum Mechanics”, Dover Publications 2003


Stefan LÖFFLER (Wien, Austria), Matthieu BUGNET, Nicolas GAUQUELIN, Sorin LAZAR, Elias ASSMANN, Karsten HELD, Gianluigi A. BOTTON, Peter SCHATTSCHNEIDER
08:00 - 18:15 #5357 - IM08-402 Minimising the Change of Collection Angle with Energy Loss in STEM-EELS.
IM08-402 Minimising the Change of Collection Angle with Energy Loss in STEM-EELS.

STEM-EELS is a powerful tool for investigating materials systems. With aberration corrected columns, the probe is large, requiring a large collection angle and hence a small camera length. The probe angle and the collection angles are normally measured using electrons that have lost no energy. For the electrons that lose energy in the specimen, each lens after the specimen increases in strength. Thus the camera length, the pattern distortion and the position of the final cross-over change.

With the four projector lenses in a modern TEM, many lens configurations can give a specific camera length. Some configurations lead to zero 3rd order distortions, and Craven and Buggy (1981) showed some configurations give no chromatic change of camera length. A lens can either create a real image of the probe after the lens (α mode) or a virtual image before it (β mode). These have opposite chromatic effects on the camera length and so can be used to null the change. If the four lenses are used in a  β, α, β, α arrangement, the camera length and the final axial cross-over position (the object for the spectrometer) can be set while maintaining no chromatic change of camera length.

Lens calculations were used to identify a series of configurations with the desired camera length and crossover position. These were then investigated experimentally and the two giving the best performance were identified for further electron optical investigation. This paper compares one, (S-9), with the original best configuration (S-1), which was based on a β, α, α, α arrangement.

Figure 1 shows the post-spectrometer camera output when using a Si specimen in a JEOL ARM 200F equipped with a Gatan QUANTUM DualEELS system, operated at 200kV.   The width corresponds to an energy loss range of 2048eV. The upper four parts are for S-1 and have offsets of 0, 1000, 2000 and 3000eV. The lower four parts are the equivalent results for the S-9. The narrowing of the intensity band with energy loss seen with S-1 is no longer present with S-9.

The upper part of Figure 2 shows the Si K-edge recorded with the two configurations while the lower part shows the peak from stray scattering within the gun. In both cases, the features are sharper with S-9.

To investigate the chromatic change of camera length, the microscope EHT was reduced in steps of 500V while leaving the lens excitations unchanged.   The STEM image was refocused after each change using the condenser mini-lens.   In this way, the electrons in the lower column behave as though they had lost the equivalent energy.   Diffraction patterns from a low index pole of Si were recorded at each offset using the pre-spectrometer camera. The chromatic change of camera length was measured using the width of the Kikuchi bands. The first order Laue zone was used to determine the 3rd order radial distortion.   The diameter of the pattern cut-off was also measured.

The upper part of Figure 3 shows the corresponding change in the acceptance angle of the 2.5mm spectrometer aperture. This confirms the reduced chromatic change of camera length for S-9. Quantification based on the 0V acceptance angle would give significant errors when using S-1, particularly for higher losses.

S-1 had zero 3rd order radial distortion at 0V offset but it became steadily more negative with increasing loss.   S-9 had approximately constant positive 3rd order radial distortion.

The lower part of Figure 3 shows the cut-off diameters for the two configurations in camera pixels.   Both shrink steadily with energy loss but the S-9 diameters are much bigger.   At 3000V, the cut-off of S-1 is close to the 5mm spectrometer aperture diameter.   To determine the scattering angles corresponding to these cut-offs needs a more detailed understanding of the higher order aberrations which cause them.

In conclusion, careful choice of the configuration of the projector lenses can minimise the chromatic effects on the collection angle in EELS and so avoid errors in quantification, particularly for higher energy losses. 

REFERENCES:  Craven AJ and Buggy TW Ultramicroscopy 7 21 (1981)

ACKNOWLEDGEMENTS:  The authors would like to thank SUPA and the University of Glasgow for funding.


Alan J. CRAVEN, Ian MACLAREN (Glasgow, United Kingdom), Sam MCFADZEAN, Hidetaka SAWADA
08:00 - 18:15 #5487 - IM08-404 Raman-spectroscopic imaging of intracellular bacteria.
IM08-404 Raman-spectroscopic imaging of intracellular bacteria.

Several bacteria are adapted to colonize and live together with multicellular organisms. There are some pathogenic bacteria that are even able to invade eukaryotic cells for different purposes, usually to escape the immune response or antibiotic treatment, but also to take advantage of growth in the nutrient rich host cell environment. Intracellular bacteria commonly are associated with chronic infections and difficult to treat. Studying intracellular bacteria in order to gain more knowledge about their pathogenesis requires sophisticated methods that deliver specific information from the bacteria while residing in the host cell environment. In this regard imaging methods are superior to follow both, the bacteria and host cell, at the same time without the necessity of extracting the bacteria.

Confocal Raman micro spectroscopy is a powerful tool that allows to make hyperspectral false-color images after scanning of a biological sample in a non-destructive and label-free way. We demonstrate that it is possible to obtain such images of endothelial cells containing Staphylococcus aureus (1), a prominent hospital-related pathogen that is assumed to use intracellular lifestyle to persist in the body. Raman spectroscopic scans using a 532 nm laser and a low step size of 0.25 µm for scanning were generated and bacteria could be specifically detected using the spectral unmixing algorithm N-FINDR (2). Additionally, it was possible to visualize host cell organelles and particles such as lipid droplets, the nucleus of the host cell or peri-nuclear region. Host cell organelles that appear similar to bacteria through their size and shape could be well discriminated from the bacteria through the specific Raman spectrum. False-color images revealed the morphology and different location of the bacteria in the host cell even in three dimensions by confocal scanning in Z. The detection was verified through immunofluorescence labelling and fluorescence microscopy afterwards.

The Raman spectra extracted from the images further could be used to analyze growth changes based on the chemical information delivered by the spectra such as a relative change in the nucleic acid and protein amount. This information helps to follow the intracellular bacteria in their growth behavior and detect changes due to changing host cell conditions.

Additionally, it is shown that Raman-based imaging can be extended from single host cells in vitro to in situ tissue samples as we demonstrate for symbiotic bacteria detected in special pockets near to the hindgut of larvae of the forest cockchafer. Through this approach production of poly-3-hydroxybutyrate, an important bacterial storage compound, could be detected and located to different areas and bacterial populations in the Raman image.

We conclude that confocal Raman spectroscopy has the potential to become a valuable imaging tool for studying pathogenic bacteria in host cells and tissues because of the label-free and specific detection capacity that allows to analyse even living samples. Further, both bacteria and host cells can be studied at the same time. A combination with other imaging methods such as fluorescence microscopy allows to study the bacteria in more detail, i.g. the role of specific proteins in overall changes of the bacteria growth.

Acknowledgements go to the financial support by the BMBF (FKZ 01EO1002 and FKZ 01EO1502) and the EU within the Framework Program 7 (P4L, Grant agreement no.: 224014), and C. Beleites for help with “R”.

The authors declare no conflict of interests.

References:

(1)    Große, C., Bergner N., Dellith, J., Heller, R., Bauer, M., Mellmann, A., Popp, J., Neugebauer, U. Anal. Chem. 2015, 87, 2137-2142.

(2)    Winter, M. E. Proc. SPIE 1999, 3753, 266


Christina GROSSE (Jena, Germany), Pol ALONSO, Erika ARIAS CORDERO, Jan DELLITH, Regine HELLER, Wilhelm BOLAND, Alexander MELLMANN, Michael BAUER, Jürgen POPP, Ute NEUGEBAUER
08:00 - 18:15 #5762 - IM08-406 X-ray absorption in pillar shaped TEM specimens.
IM08-406 X-ray absorption in pillar shaped TEM specimens.

In thin TEM specimens quantification of X-ray analysis is generally done with the Cliff-Lorimer method neglecting X-ray absorption in the specimen.  This assumption is valid for X-ray transitions with similar energies but can, even for specimens thinner than 100 nm, lead to appreciable error when low and high energy peaks are combined for the quantification.  These considerations become more important for 360º X-ray tomography for which pillar shaped specimens are used with larger diameter than the thickness of normal plan-parallel TEM specimens.

In this work the absorption effects are compared for pillar and 2º wedge shaped specimens prepared for a Si75Ge25 layer on Si and capped for the TEM specimen preparation with a 150 nm SiO2 layer and ion deposited Pt in the FIB.  The EDS analysis is made in a Titan3 60-300 instrument with SuperX EDS detector at 120 kV.  Quantification is done with linescans extracted from EDS maps using the Bruker Esprit software. The considered X-ray peaks are SiK at 1.739 keV, GeL at 1.188 keV and GeK at 9.885 keV. The orientation of the specimens relative to the 4 detectors is shown in Fig. 1. 

The quantitative linescan across the radius of the pillar is shown on Fig. 2a vs specimen thickness as calculated based on the diameter of the pillar.  The outer layer is oxidized due to air exposure of the specimen after the FIB preparation.  Next an amorphous SiGe layer is present due to the 30kV FIB preparation.  The Ge concentration calculated with GeK & SiK and GeL & SiK differs slightly which can be attributed to absorption of the low energy X-rays.  But in both cases the Ge concentrations are nearly constant through the full diameter of the pillar (except in the oxide which is Si-rich), i.e. it can be concluded that even for transitions with large difference of energy, the absorption is not dependent on the specimen thickness.  This is a result of the shape of the pillar as is illustrated on Fig. 3a which shows a section through the pillar in the plane across the electron beam direction and 2 opposite detectors.  In the center of the pillar, i.e. for maximum specimen thickness, the path lengths of the X-rays through the specimen vary with depth along the beam axis.  On average the depth dependence can be approximated by the lengths at half thickness, i.e. in total for the 4 detectors it is 4*lc. At the thinnest edge of the pillar the X-rays travel a longer distance le through the pillar, i.e. in total for all detectors 2*le.  Both total lengths are nearly equal and this holds also for positions between center and edge, i.e. for intermediate thickness. Therefore, as observed on Fig. 2a, the strength of the absorption is independent of thickness. 

In a plan-parallel specimen with thickness equal to the diameter of the pillar the path lengths of the X-rays are much longer (Fig. 3b).  Taking half depth as the average length, the total length is 4*lp, which is more than 2 times larger than the total lengths in the pillar case.  Due to the wedge shape of the specimen the length to the detectors on left and right side slightly differ (not shown on the figure).  Moreover, for the plan parallel specimen the total absorption path length is directly proportional to the specimen thickness.  Therefore the absorption increases with specimen thickness and the Ge concentration increases with thickness when using transitions with large energy difference (GeK & SiK) and decreases in the case of GeL combined with SiK (Fig. 2b).  Extrapolated to zero thickness the concentrations coincide, i.e. the difference of the calculated concentrations with GeK and GeL is due to absorption.  The calculated concentrations in the pillar with diameter 320 nm are similar to the concentrations in the wedge specimen near 150 nm thickness as can be expected based on the ratio of the absorption path lengths in the two cases.

Exact calculation of the path lengths in the pillars requires an integration over the thickness along the electron beam direction.  The strength of the absorption in the direction of the different detectors also depends on the different layers that are crossed, i.e. the capping stack above the SiGe or the Si substrate below the SiGe.  Therefore a full quantitative estimate of the absorption is not easily possible. 

It can be concluded that absorption effects are constant throughout the diameter of a pillar specimen and are about a factor of 2 weaker than for plan-parallel specimens with thickness equal to the diameter of the pillar.

1. Y. Qiu, V.H. Nguyen, A. Dobbie, M. Myronov, T. Walther, J. Phys. Conf. Ser. 471, 012031 (2013).


Hugo BENDER (Leuven, Belgium), Paola FAVIA, Yang QIU, Olivier RICHARD
08:00 - 18:15 #5838 - IM08-408 Chemical state analysis using Soft X-ray Emission Spectrometry in low voltage FE-SEM.
IM08-408 Chemical state analysis using Soft X-ray Emission Spectrometry in low voltage FE-SEM.

The development of low voltage (LV) FE-SEMs has been in progress, and spatial resolution for observation of less than 1.0 nm can now be achieved even at 1 kV. On the other hand, recent advanced nano materials are getting more complex and micronized. Therefore to understand details of nano materials precisely, it is necessary to utilize high spatial resolution images. However, chemical state analysis in FE-SEM is also important as well as the high spatial resolution imaging. Recently, it becomes possible to analyse both elemental and chemical states in FE-SEM by using newly developed Soft X-ray Emission Spectrometer (SXES). The advantages of SEM-SXES are high energy resolution spectra, analytical spatial resolution and applicability to bulk samples.  In this report, we focus on high resolution imaging and chemical state analysis using SXES in low voltage FE-SEM.

                                     

The first commercial type SXES has been developed by Prof. M.Terauchi group of Tohoku University in collaboration with JEOL1). The SXES is possible to analyse chemical bonding states because it has high energy resolution at X-ray energy of about 200 eV or even below such as 0.3 eV at the Al Fermi edge. In addition, this technique can detect X-rays in an energy range from 50 to 210 eV, of which characteristic X-ray energy range includes the emission spectra based on the valance band transitions in many elements2).

 

The SXES is installed in the low voltage (LV) FE-SEM JSM-7800F Prime. This LVFE-SEM can achieve large probe current even under low voltage condition such as 20 nA at 2 kV with a 30 μm diameter objective aperture, which is due to new design of high brightness Schottky emitter electron gun so called In lens Schottky plus. In addition, there is the decelerating method in the JSM-7800F prime, called Gentle Beam Super High resolution (GBSH) mode. A negative voltage is applied to a sample in the GBSH mode. Therefore the incident voltage is reduced at low voltage to keep smaller probe size. Combination of the in-lens Schottky plus electron gun and the GBSH mode is suitable for the SXES analysis.

 

Usually the SXES is used at low kV to reduce background of continuous X-rays. Figure 1 shows SEM images and spectra taken by SXES for the comparison between the incident electron energies of 2 keV and 15 keV. Sample is Prussian blue (PB, FeIII4[FeII(CN)6]3) and these images were acquired by Everhart-Thornley detector. The results show that the spectrum taken at 2 keV has less background than at 15 keV due to small electron interaction volume in the sample at 2 keV.

 

Figure 2 shows SXES spectra of PB and Prussian white (PW: K4FeII4[FeII(CN)6]3) , in which the PW sample was reduced chemically from PB sample. The incident electron energy was 2 keV and the probe current was 22 nA. The PW is well known in that the CN bonding state is different from PB. The result shows that the peak energy and spectral shape are different between the N Kα spectra of PW and PB because of different chemical bonding states.

 

References:

[1] M. Terauchi, H. Yamamoto and M. Tanaka, Journal of Electron Microscopy, 50, 101, (2001).

[2] M. Takakura, T. Murano, and H. Takahashi, JEOL News volume50, July (2015)

[3] S. Asahina, M. Suga, H. Takahashi, H. Y. Jeong, C. Galeano, F. Schüth, and O. Terasaki,. APL Materials 2, 113317; doi: 10.1063/1.4902435, (2014)


Yusuke SAKUDA (Tokyo, Japan), Manabu ISHIZAKI, Takanari TOGASHI, Shunsuke ASAHINA, Masaru TAKAKURA, Hideyuki TAKAHASHI, Masato KURIHARA
08:00 - 18:15 #5844 - IM08-410 Using EELS analysis in STEM to investigate the helium content in irradiated materials.
IM08-410 Using EELS analysis in STEM to investigate the helium content in irradiated materials.

Helium very often is forming inside the reactor structural materials during neutron irradiation due to different nuclear reactions. Because of the helium-vacancy interaction effect the later may be stabilized by this elements. The temperature vacancy mobility leads to formation of the pores inside the materials that take part in the swelling effect. It is very important for steels under high fluence fast neutron applications, internal parts of pressure vessel nuclear reactors and also for fusion reactors. Nowadays researchers are performing experiments at accelerators to simulate under ion irradiation the materials atomic damage production and understand the possible radiation stability of the new designed materials. He ions irradiation can be used also to simulate above mentioned helium production.

Pores inside irradiated steels were visible for many years because of the typical uderfocus/overfocus TEM contrast changing. Many pores were characterized by the faceting shape that means the vacancy nature of these objects, but the presence of helium inside can be proved using analytical energy loss technique only. In this work, we show the implementation of EELS technique in STEM mode to find out the helium content inside the pores in the 40 keV He ion beam irradiated up to 5*1020m-2 at 650oC Eurofer ODS steel. Because of the low value of projected range of these ions the best way was to make cross-section samples using FIB technique.

Figure 1 shows the STEM dark field image of the steel cross-section sample under irradiation at the ions projected range region. The thickness of the sample at interesting area was about 0.7*λ. We can see that pores with different sizes and shapes were formed under high temperature irradiation. Some of pores were faceting type that indicates the vacancy nature of their structure.

To understand the helium content inside the pore we took low energy EELS spectra from the center of the pore (probe position was indicated at figure 1) in STEM and the corresponding spectra from matrix close to the pore position. After background subtraction we normalized the intensities of two spectra by fitting the intensities of the second peak (after plasmon peak) at 57.7 eV position (see figure 2). As was shown at figure 2, the difference between these normalized spectra appeared to identify the He core-loss line with maximum position at 22.8 eV. Experimental measuring of the He peak intensity (I) together with the measuring of elastic peak intensity (Io) allowed us to calculate the amount of helium atoms in the pore [1]:

N=I/(Io*σ*d), where σ is the He cross section and d is the pore diameter.

It is known [1], that the value of energy shift between the positions of the He core-edge in the pore inside the solids and for free molecular gas (21.218 eV) is correlated with the pressure of the gas inside the pore and its He density. We have got densities of ~19 at.He/nm3 for pores with 20nm diameter and ~70 at.He/nm3 for 3.4nm pores respectively that is close to the literature data [2]. Thus, ELLS technique is very useful for irradiated materials helium content analysis.

Literature

[1] C.A. Walsh, J. Yuan, L.M. Brown, Philos. Mag. A 80 (2000) 1507.

[2] S. Fréchard, M. Walls, M. Kociak, J.P. Chevalier, J. Henry, D. Gorse, Journal of Nuclear Materials 393 (2009) 102–107


Kirill PRIKHODKO (Moscow, Russia), Olga EMELYANOVA
08:00 - 18:15 #5853 - IM08-412 EELS Probing of lithium based 2-D battery compounds processed by liquid phase exfoliation.
IM08-412 EELS Probing of lithium based 2-D battery compounds processed by liquid phase exfoliation.

Two-dimensional lithiated nanosheets usually show excellent electrochemical performance due to an increase in surface area and shorter diffusion paths. However, processing techniques, such as shear mixing or liquid phase exfoliation could induce phase changes or knock out some of the structural lithium (Li) ions, what in turn might result in poor electrochemical performance. Here different lithiated layered compounds mainly LiCoO2, LiMn2O4, and Li5Ti4O12 were chemically exfoliated and investigated using electron energy loss spectroscopy (EELS) for their Li-K edge. Further analyses were carried out, looking at the oxygen (O) K edge with their respective transition metal core loss peak (Mn, Co and Ti) which revealed changes in the Energy loss near edge structures (ELNES) when compared to the unlithiated compounds. STEM-EELS analyses confirmed uneven distribution of lithium within the lithiated layered materials. In this work, EELS was used for the first time to detect and to probe the chemical environment of the lithium in liquid phase exfoliated material. 


Anuj POKLE (Dublin, Ireland), João COELHO, Eva MACGUIRE, Clive DOWNING, Patrick CASEY, Cormac MCGUINNESS, Valeria NICOLOSI
08:00 - 18:15 #5855 - IM08-414 HRTEM and EELS investigations in superconductive NbN films modified under ion beam irradiation.
IM08-414 HRTEM and EELS investigations in superconductive NbN films modified under ion beam irradiation.

HRTEM and EELS investigations in superconductive NbN films modified under ion beam irradiation.

M.M. Dementyeva1, K.E. Prikhodko1

NATIONAL RESEARCH CENTRE «KURCHATOV INSTITUTE»,

Dementyeva_mm@nrcki.ru

 In this work we investigated superconductive NbN thin films. Interest in the latter is caused by demand for high quality new devices. Cryoelectronic devices is nanoscale functional elements, for instance SSPD, SQUID, THz HEB, based on ultra-thin films with high limiting characteristics. One of the techniques of creating such devices was developed in Russia in the NRC "Kurchatov Institute". The developed technique was named radiation-induced method. This method allows to realize a control manner selectively changes in the atomic structure of thin-film materials and modifications of the physical properties under irradiation with low-energy beams with different composition [1, 2].

 Samples containing niobium and nitrogen (NbN) were deposited on a single crystal silicon substrate coated with a 0,15 microns layer of amorphous oxide SiO2. The thickness of the initial film was 5 nm.

 The NbN ultrathin films were irradiated by ion beams extracted from a high-frequency discharge plasma. Ion beams consist of protons and OH ions with energies (0,1-1) keV in a dosage range (1,6-4) d. p. a. for nitrogen.

 To study the chemical composition of the origin and irradiated samples were applied electron energy-loss spectroscopy (EELS). Spectra were recorded with a Titan 80-300 electron microscope equipped with a GIF-2001 energy loss spectrometer. Data were transferred into TEM Imaging & Analysis Software (TIA). EELS spectra were collected in the STEM mode at 200 keV beam energy. The collection semi-angle at the sample, as defined by the objective aperture and camera length, was 5,6 mrad. Typically acquisition times were ~200 s. Cross sections samples NbN/SiO2/Si were prepared by FIB Helios Nanolab 650 at 30 keV accelerating voltage of ion gun and 2,5 nA current and the final thinning was done at 2-5 keV and 0,12 nA current.

 Quantitative analysis was carried out with equation (1): NA/NB=[IA(β,Δ)*σB(β,Δ)]/[IB(β,Δ)*σA(β,Δ)], where IA, IB - integrated intensities of the peaks after background subtracting, and σA and σB - ionization cross section [3]. Determination of the phase composition in the initial and irradiated samples was performed by the Fourier - transform diffraction pattern obtained from the corresponding HRTEM image.

 Diffraction analysis showed that grains of non-irradiated material correspond to NbN cubic crystal system (Fm-3m) with lattice parameter a=0,4394 nm. Phase composition changes were observed when the irradiation dose was 2 d. p. a. for nitrogen. Figure 1 presents the diffraction pattern analysis from individual grains. The formation NbN0,64O1,36 monoclinic system phase (P21/c(14)) with cell parameters a=0.49808 nm, b=0.50250 nm, c=0.52097 nm, α=γ=90о, β=100оwas defined. These data were confirmed by the electron energy loss spectroscopy (figure 2).

 Quantitative analysis demonstrated that films irradiated at different doses consist of two regions: region with modified atomic composition and region of the initial NbN film. According to the results of the profile spectra, asymptotic functions were calculated for nitrogen, niobium and oxygen (fig.3, 4). Using asymptotic function, we concluded following. First, with increasing ion irradiated dose, nitrogen atomic concentration decreased to zero in the region close to the surface and the ratio of atomic concentrations for oxygen and niobium correspond to niobium oxide. Secondly, the thickness of the oxidized upper layer increased with increasing irradiation dose, due to the volume changes accompanying the niobium oxide formation from the niobium nitride phase. Thirdly, the nitrogen atomic concentration decreased in the region adjoined to the substrate, but wasn’t decreased to zero, and oxygen atomic concentration was increased, i.e. there was a partial replacement of the nitrogen atoms to the oxygen atoms as a result of the selective displacement of nitrogen atoms by oxygen atoms.

[1] B.A. Gurovich, K.E. Prikhodko, E.A. Kuleshova, et all. JETP, 2013, 143, 1062-1076.

[2] B.A. Gurovich, M.A. Tarkhov, K.E. Prikhodko, ,et all. Nanotechnologies in Russia, 2014,9,7–8,16-20.

[3] D. Williams, A. Carter. Electron microscopy. New York: Springer, 2009.


Maria DEMENTYEVA (Moscow, Russia), Kirill PRIKHODKO
08:00 - 18:15 #5859 - IM08-416 Elastic delocalization in EELS.
IM08-416 Elastic delocalization in EELS.

Inelastic delocalization – caused by the long-ranged Coulomb interaction – is a well-known phenomenon in EELS that limits the achievable spatial resolution [1]. For low-loss EELS, it can lead to a spatial resolution of worse than several nanometers. For core-loss EELS with energy transfers > ~100 eV, on the other hand, it is generally of the order of 1 Ångström and, therefore, generally does not prevent the acquisition of atomically resolved elemental maps.

Another aspect that is often overlooked, however, is the elastic delocalization caused by the extent and the elastic scattering of the electron beam itself inside the crystal [2]. With the ever-improving aberration correctors and, consequently, ever-increasing convergence angles, this becomes more and more of an issue. Especially when dealing with samples that are not ideal single crystals, e.g., due to inhomogeneities, embedded nanoparticles, or interfaces, the elastic delocalization can become a severe challenge for atomic-resolution EELS.

In fig. 1, the case of a NdGaO3/LaMnO3 interface is shown for different convergence angles. The propagation was calculated using the multislice approach [3] for an incident beam energy of 300 keV and no spherical aberration. It is clear that even when the beam is nominally positioned well inside one material, parts of it still extend across the interface into the other material [4]. Therefore, in this situation, one will pick up EELS intensity coming from both sides of the interface (assuming a sufficiently large collection angle; for small collection angles, the situation will be complicated further by the elastic scattering of the beam after the inelastic excitation, which may lead to scattering outside the aperture). In addition, elastic scattering complicates the z sensitivity.

There are several ways to circumvent the problem of elastic delocalization. On the one hand, it is possible to use very thin samples for which elastic scattering and beam broadening are less severe. For large convergence angles, however, this limits the thickness to below ~10 nm which, in turn, decreases the total EELS signal due to the reduced number of atoms. On the other hand, as is evident from fig. 1, the influence of the elastic delocalization can also be decreased by decreasing the convergence angle. While this may seem counter-intuitive at first, it can significantly decrease the beam broadening, thus reducing spurious signals coming from adjacent columns. In addition, electron-vortex beams [5] are also a promising candidate for reducing elastic delocalization due to topological protection [6,7,8]. In addition, the vorticity causes their intensity to vanish in the center, giving them their typical donut shape. While this can make ADF images more difficult to interpret as the elastic scattering likelihood has its maximum when the beam is not actually centered on an atomic column, it does not pose a problem for core-loss EELS for which the probe beam scatters off the sample electrons: as the electron cloud surrounds the nuclei, the inelastic scattering likelihood has its maximum when the donut-shaped beam is on the atomic column.

Elastic delocalization is unavoidable. This work shows possible ways to mitigate its detrimental effects on core-loss EELS and thereby paves the way for a better interpretation and quantification of atomic-resolution mapping, especially in the practically relevant cases of non-homogeneous samples and interfaces.

 

Acknowledgements: Financial support by the Austrian Sciences Fund (FWF) under grant nr. J3732-N27 is gratefully acknowledged.

 

[1] Egerton, “Electron Energy-Loss Spectroscopy in the Electron Microscope”, Plenum Press 1996
[2] Dwyer & Etheridge, Ultramicroscopy 96 (2003) 343
[3] Kirkland, “Advanced computing in electron microscopy Plenum Press”, 1998
[4] Löffler, in preparation
[5] Verbeeck et al., Nature 467 (2010) 301
[6] Lubk et al., Phys. Rev. A. 87 (2013) 033834
[7] Löffler & Schattschneider, Acta. Cryst. A. 68 (2012) 443
[8] Xin & Zheng, Microsc. Microanal. 18 (2012) 711


Stefan LÖFFLER (Wien, Austria)
08:00 - 18:15 #5864 - IM08-418 Application of statistical beam-rocking TEM-EDX analysis to quantitative occupation site determination of Zn substituted for multiple Fe sites in W-type hexagonal ferrite.
IM08-418 Application of statistical beam-rocking TEM-EDX analysis to quantitative occupation site determination of Zn substituted for multiple Fe sites in W-type hexagonal ferrite.

    W-type strontium hexagonal ferrite, SrMe2+2Fe3+16O27 (Me2+: divalent cation), is a hard magnetic material that exhibits strong magneto-crystalline anisotropy (MCA), showing saturation magnetization, Ms, approximately 10% higher than that of M-type ferrite, one of the current mainstream materials. It is known that partial substitution of a divalent magnetic, nonmagnetic cations or a combination of both for Me2+ occupying appropriate Fe sites improves MCA and Ms, and it is thus important to investigate which Fe site and how much fraction in each site the dopant atoms are actually substituted for in order to find a guiding principle for further improvements of MCA and Ms.

    In this study, we have quantitatively determined the occupation sites of Zn2+ in Zn-doped W-type ferrite, SrZn2Fe16O27, using a suite of beam-rocking transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), theoretical dynamical elastic/inelastic electron scattering simulation and statistical analysis, an extended version of high-angular resolution electron channeling X-ray spectroscopy (HARECXS) [1]. The sintered specimen includes small amounts of hetero-phases which hamper the application of more macroscopic methods, such as neutron diffraction and synchrotron radiation. In this statistical beam-rocking TEM-EDX analysis, the intensity variations of characteristic X-ray peaks (incoherent channeling patterns (ICPs)) as functions of the incident beam direction with respect to a crystalline specimen reflect the occupation sites of the elements of interest, applicable to a small crystal grain, and the multivariate linear regression between the ICPs from the trace dopants and host elements allows us to quantitatively evaluate the dopant concentrations and their occupancies on different crystallographic sites [2]. The present material, however, has seven crystallographically inequivalent sites (the 2c as the hexagonal site, the 4e and 4fIV as the tetrahedral sites, and the 4f, 4fVI, 6g and 12k as the octahedral sites in the Wyckoff notation) for Fe, and accordingly the site-specific ICPs of the seven sites cannot be obtained separately. We have developed a scheme combining theoretical prediction of the site-specific ICPs, based on dynamical electron diffraction theory and the statistical atom-location by channeling-enhanced microanalysis (ALCHEMI) method [2] to overcome this difficulty, which was successfully applied to Co-doped M-type ferrite [3]. We have thus applied the scheme to present analysis.

    Figure 1 shows X-ray ICPs of the Sr-L, Fe-K, O-K and Zn-K lines obtained around the [1-20] zone axis, in which seven Fe sites are definitely separated in the projected atomic structure. The Zn-K ICP looks apparently different from some of the Fe-K ICPs, which qualitatively implies that Zn preferentially occupies some of the specific Fe sites, rather than all of them uniformly. Figure 2(a) shows the theoretical Fe-K ICP of the non-doped SrFe18O27, corresponding to Fig. 1(b), showing sufficiently good agreement with each other. The experimental site-specific Fe-K ICPs for the 12k, 6g, 4fVI, 4fIV, 4f, 4e and 2c sites can be thus obtained by the proportional distribution scheme of the experimental Fe-K ICP, as shown in Figs. 2(b-h), then followed by applying the statistical ALCHEMI method to Zn-K ICP (Fig. 1(d)) and site-specific Fe-K ICPs. Table 1 shows the derived parameters. This quantified result suggests that Zn mainly occupies 4e and 4fIV sites. The estimated Zn concentration of 3.1 atom% is slightly lower than the stoichiometric concentration, which is attributable to a grain-to-grain variation of impurity concentration. Ms arises due to a spontaneous molecular magnetic moment M0, the vector sum of the magnetic moments of Fe3+, Fe2+ and Zn2+ cations of 5μB, 4μB and 0μB, respectively, with the spin orientation considered. M0 was estimated to be 36.0μB and 37.6μB for the cases assuming the estimated Zn concentration and the stoichiometric Zn concentration, respectively. These values are close to the magnetization saturations of 35.0μB [4] and 38.2μB [5] previously reported for a similar W-type ferrite BaZn2Fe16O27.

 

References

[1] Yasuda et al., Nucl. Instr. Meth., B250, 238-244 (2006). [2] Rossouw et al., Philos. Mag. Lett., 60, 225-232 (1989). [3] Ohtsuka et al., Microscopy, doi:10.1093/jmicro/dfv356 (2015). [4] Albanese et al., Appl. Phys., 11, 81-88 (1976). [5] Lotgering et al., J. Phys. Chem. Solids, 41, 481-487 (1980).


Masahiro OHTSUKA (Aichi-ken, Japan), Shunsuke MUTO, Yoshihiro ANAN, Yoshinori KOBAYASHI
08:00 - 18:15 #5884 - IM08-420 Spatially Mapping the Plasmon Resonances of Hollow 1D Nanostructures: Hybrid AuAg Nanotubes.
IM08-420 Spatially Mapping the Plasmon Resonances of Hollow 1D Nanostructures: Hybrid AuAg Nanotubes.

Morphological control at the nanoscale paves the way to fabricate nanostructures with desired plasmonic properties. We present the nanoengineering of plasmon resonances in 1D hollow nanostructures of two different AuAg nanotubes; completely hollow nanotubes (Figure 1) and hybrid nanotubes comprising the sequential formation of solid Ag parts and hollow AuAg parts (Figure 2). Spatially resolved plasmon mapping by electron energy loss spectroscopy (EELS) revealed the presence of high order resonator-like modes and localized surface plasmon resonance (LSPR) modes in both nanotubes. Experimental findings are accurately correlated with the boundary element method (BEM) simulations, where both experiments and simulations revealed that the plasmon resonances are intensely present inside the nanotubes. Based on the experimental and simulated results obtained in the present study, we show that the novel hybrid AuAg nanotubes possess two significant features: (i) LSPRs have been generated distinctively from the hollow and solid parts of the hybrid AuAg nanotubes which opens the way to control a broad range of plasmon resonances with one single nanostructure and (ii) the periodicity of the high order modes are disrupted due to the interaction of solid and hollow parts. 

KEYWORDS: metal nanotubes, electron energy-loss spectroscopy, AuAg, localized surface plasmon resonances, boundary element method

 

We acknowledge the funding from Generalitat de Catalunya 2014 SGR 1638, 2014 SGR 797 and MINECO coordinated projects between IREC and ICN2 TNT-FUELS and e-TNT (MAT2014-59961-C2-2-R). J.P., N.G.B. and V.P. acknowledge financial support from the Generalitat de Catalunya 2014-SGR-612, Spanish MICINN (MAT2012-33330) and European Community (EU-FP7) through the FutureNanoNeeds project. N.G.B. thanks the Spanish MICINN for the financial support through the Juan de la Cierva program and European Commission for the Career Integration Grant (CIG)-Marie Curie Action. Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative – I3).


Aziz GENÇ (Bartin, Turkey), Javier PATARROYO, Jordi SANCHO-PARRAMON, Raul ARENAL, Neus G. BASTUS, Victor PUNTES, Jordi ARBIOL
08:00 - 18:15 #5893 - IM08-422 Towards EMCD with an electron vortex filter.
IM08-422 Towards EMCD with an electron vortex filter.

The electrons’ wavefront can be arbitrarily shaped by placing holographic masks (HMs) in the condenser system of a TEM. Using HMs with dislocation gratings, it is possible to impart quantized orbital angular momentum (OAM), as well as quantized magnetic moment onto the imaging electrons [1]. Due to their OAM, some peculiar effects can be observed for these so-called vortex electrons or electron vortex beams (EVBs), e.g. topological protection [2], peculiar rotation dynamics in magnetic fields [3] and intrinsic chirality. Owing to the latter of these properties EVBs have become a promising candidate for atomic scale energy-loss magnetic chiral dichroism (EMCD) measurements. However, it soon became clear that atom-sized EVBs are needed to achieve this goal [4,5].

In magnetic materials, the outgoing inelastically scattered probe electrons carry OAM, so they are EVBs. This fact can be utilised to detect spin polarized transitions in an alternative manner by placing a HM in the selected-area-aperture (SAA) holder and using it as a vorticity filter after the specimen, see Fig. 1. This approach does not rely on the standard EMCD geometry and the specimen’s role as a beam splitter and thus would not need a precise alignment of the crystal. The scattering geometry is chosen such that the SAA HM is in the far-field of the scattering centres, which is realized by lifting the specimen in the z-direction. Additionally, the electron probe is focused onto the lifted specimen in order to reduce the effective source size the SAA HM eventually “sees”. Nevertheless, the incident electron wave has a flat phase surface (i.e., behaves similar to a plane wave) all over the illuminated area, provided that the Rayleigh range of the probe beam is much larger than the sample thickness. Therefore, all the scattering ”light cones” point in the same direction towards the vortex filter HM. As the scattered probe electrons are of atomic-size their focused image could not be resolved, thus the imaging plane is defocused by 4 µm to observe broader vortices. A proof-of-principle experiment is shown in Fig. 2a. The azimuthally averaged radial intensity profiles of the upper and lower vortex orders (red and green full dots in Fig. 2b) are in good agreement with the simulation (blue and orange full lines in Fig. 2b). Curiously, the experimental radial profiles show stronger differences in the central region than is expected from the simulation, compare the experimental EMCD signal (magenta open circles) to the theoretical one (green dot-dashed curve) in Fig. 2b. This is probably due to skew optic axes giving rise to slight differences in apparent defocus for the positive and negative vortex orders. Also, artefacts from the mask production and OAM impurities could deteriorate the signal.

The experiment shows that the RMS error (magenta shaded region in Fig.2b) is still too high, such that the faint EMCD signal cannot be discerned under present experimental conditions. To improve the SNR we propose to incorporate larger SAA HMs, e.g. at least 30 to 50 µm in diameter, as the collected signal scales with the mask area, lowering the acquisition times. Also, increasing the coherence of the probe while still keeping the probe current high, which is possible in state-of-the-art aberration corrected microscopes, would enhance the EMCD signal strength by an order of magnitude. If successful, this technique could be applied to study magnetic properties of amorphous or nanocrystalline materials.

 

Acknowledgements: The financial support by the Austrian Science Fund (I543-N20, J3732-N27) and the European research council (ERC-StG-306447) is gratefully acknowledged.

 

References:

[1] J. Verbeeck et al., Nature 467 (2010): 301-304

[2] A. Lubk et al., Physical Review A 87 (2013): 033834

[3] T. Schachinger et al., Ultramicroscopy 158 (2015): 17–25

[4] J. Rusz and S. Bhowmick, Physical Review Letters 111 (2013): 105504

[5] P. Schattschneider et al., Ultramicroscopy 136 (2014): 81–85


Thomas SCHACHINGER (Vienna, Austria), Andreas STEIGER-THIRSFELD, Stefan LÖFFLER, Michael STÖGER-POLLACH, Sebastian SCHNEIDER, Darius POHL, Bernd RELLINGHAUS, Peter SCHATTSCHNEIDER
08:00 - 18:15 #5900 - IM08-424 High energy resolution EELS of copper phthalocyanine crystals.
IM08-424 High energy resolution EELS of copper phthalocyanine crystals.

Recent developments of a monochromator for transmission electron microscopy provide a new tool for chemical analysis in local area of materials. Owing to the improved energy resolution, the energy range probed by electron energy-loss spectroscopy (EELS) has been extended to visible or infrared region, allowing vibrational spectroscopy in the electron microscope [1, 2]. Moreover, one can also measure the fine energy-loss near-edge structure (ELNES) by exciting electrons in relative shallow inner-shells like a carbon K-edge, being an advantage in the analysis of organic compounds. In the present work, we demonstrate the high energy resolution EELS study of copper phthalocyanine (Fig. 1) crystals performed by a JEM-ARM200F equipped with a double Wien filter [3] and spherical aberration correctors.

Figure 2 shows the carbon K-edge ELNES measured from the thin film crystals of copper phthalocyanine (CuPc) and chlorinated one (Cl16CuPc). Since the radiation damage is serious for the CuPc crystal compared to the Cl16CuPc one, its ELNES is rather noisy due to the limited electron dosage (0.5 C/cm2). However, the chlorination effects are clearly observed in the change of ELNES, which is attributed to the chemical shift of 1s level of carbon atoms because the molecule has four independent carbon atoms with a different 1s binding energy. From the orientation dependence of ELNES intensity, the peaks (a) to (d) can be assigned to the transitions from 1s to π* unoccupied molecular orbitals. The peak (a) is related to the excitations of the carbon atoms forming a benzene ring in CuPc, while the peak (c) in the spectrum of Cl16CuPc is assigned to the excitations of carbon atoms bonding to chlorine atoms having a large electronegativity. The intensity of peak (a) in the spectrum of CuPc rapidly decreased with the increase of electron dosage as shown in Fig. 3. This means that the primary damage process is C-H bond scission. Figure 4 shows the low-loss spectra of CuPc extended to the infrared region, in which the C-H stretch excitation is observed at 376 meV as a shoulder peak. This peak also disappeared when an electron dosage increased. The low-loss spectra obtained from these crystals will also be presented and compared to the optical measurements.

 

References

1. O. L. Krivanek, T. C. Lovejoy, N. Dellby, T. Aoki, R. W. Carpenter, P. Rez, E. Soignard, P. E. Batson, M. J. Lagos, R. F. Egerton and P. A. Crozier, Nature 514, 209-212 (2014).

2. T. Miyata, M. Fukuyama, A. Hibata, E. Okunishi, M. Mukai and T. Mizoguchi, Microscopy 63, 377-382 (2014).

3. M. Mukai, E. Okunishi, M. Ashino, K. Omoto, T. Fukuda, A. Ikeda, K. Somehara, T. Kaneyama, T. Saitoh, T. Hirayama and Y. Ikuhara, Microscopy 64, 151-158 (2015).


Hiroki KURATA (Uji, Japan), Yoshifumi FUJIYOSHI, Yuriko TOMISAKI, Takashi NEMOTO, Mitsutaka HARUTA
08:00 - 18:15 #5903 - IM08-426 Charging kinetics features of different modifications of the Al2O3-dielectrics (sapphire, polycrystalline and ceramics) under electron-beam irradiation.
IM08-426 Charging kinetics features of different modifications of the Al2O3-dielectrics (sapphire, polycrystalline and ceramics) under electron-beam irradiation.

The phenomenon of charging dielectrics via the electron-beam irradiation has been studied very intensively. However, published in recent years the results of experimental and theoretical studies of dielectrics charged under electron irradiation gives contradictory description of this phenomenon [1-6]. These contradictions arise mainly due to the differences in the approaches and fragmentation of experimental studies and differences in used physical models of charging mechanisms.

In this report on the basis of extensive experimental data of simultaneous measurement of key charging parameters we are given a balanced assessment of the multiple factors influencing on the charging process. We present the new features of this process and given its interpretation.

Based on comprehensive methodology [7, 8], we measure simultaneously the main charging parameters of different modification Al2O3, such as surface potential VS, electron-emission current Iσ, displacement current ID, and accumulated charge Q versus time of electron irradiation. The experimental setup that allows simultaneously measure the basic charging parameters published in the [9].

The experimental results of complex research of the key charging parameters for sapphire and ceramics Al2O3 are presented in Fig. 1, Fig. 2, Fig. 3 and Fig 4. The main conclusion of these investigations is that although charging processes of dielectric targets are self-consistent and mutually dependent, the accumulation of the negative charge and, correspondingly, an increase in the charging potential, is the dominant (leading) effect, while the variation in the secondary electron emission coefficient, which depends on the surface potential, is a driven adjusting process. For this reason, a delay occurs in the time for attaining the equilibrium of two fundamental charging parameters, viz., the emission current (a short-time process) and charging potential (a long-time process). In other words, for all modifications Al2O3 coefficient of secondary electron emission σ reaches an equilibrium value for used doses of electron irradiation significantly faster than the surface potential.

This work was funded thanks to the support of RFBR (grant 15-02-01557).

 

References

1. L. Reimer, U. Golla, R. Bongeler, M. Kassens, B. Schindler, R. Senkel, Optik, 90 (1992) 14-22.

2. A. Melchinger, S. Hofmann, J. Appl. Phys., 78 (1995) 6224-6232.

3. J. Cazaux, J. Electr. Spectrosc. Rel. Phenom., 176 (2010) 58-79.

4. M. Belhaj, T. Tondu, V. Inguimbert, J. Phys. D. Appl. Phys., 42 (2009) 145306.

5. T. Thome, D. Braga, G. Blaise, J. Appl. Phys., 95 (2004) 2619-2624.

6. M. Touzin, D. Goeuriot, C. Guerret-Piecourt, D. Juve, D. Treheux, H.-J. Fitting, J. Appl. Phys., 99 (2006) 114110.

7. S. Fakhfakh, O. Jbara, M. Belhaj, Z. Fakhfakh, A. Kallel, E.I. Rau, Nucl. Instr. Methods B., 197 (2002) 114-127.

8. E. N. Evstaf’eva, E. I. Rau, A. A. Tatarintsev. MoscowUniversity Physics Bulletin. 68(2) (2013) 34-37.

9. E.I. Rau, A.A. Tatarintsev, V.V. Khvostov, V.E. Yurasova. Vacuum. (2016) http://dx.doi.org/10.1016/j.vacuum.2016.02.002


Eduard RAU (Moscow, Russia), Andrey TATARINTSEV, A KHAIDAROV, Stepan KUPREENKO
08:00 - 18:15 #5914 - IM08-428 HRTEM, HREELS analyses and modelling of nanometric oxide layers formed on 316L in simulated Pressurized Water Reactor (PWR) conditions.
IM08-428 HRTEM, HREELS analyses and modelling of nanometric oxide layers formed on 316L in simulated Pressurized Water Reactor (PWR) conditions.

The development of a passivation layer on stainless steels in Pressurized Water Reactor (PWR) environment is a key phenomenon in Stress Corrosion Cracking (SCC) behavior. Although several works pointed out the presence of a duplex oxide layers, the contribution of the oxide layers themselves is still poorly explored and appears as a crucial factor in the understanding of SCC in PWR medium. The present work studies the oxides growth during short time corrosion tests (0h up to 24h) in a specific corrosion loop running in PWR conditions. Post mortem investigations are carried out using various characterization techniques ranging from macroscopic to atomic scale1. HRTEM images, acquired on an objective lens aberrations corrected TITAN at Marseille IM2NP2, are analyzed using ASTAR software3 in order to investigate the crystallographic structure of oxides and the evolutions as a function of oxidation time (Fig 1&2). The outer oxide crystallites present a spinel crystal structure. Their size increases with oxidation time. The inner oxide layer is amorphous during the first stages of growth and becomes fully crystalline after 10 mn leading to mono-crystalline domains on every metal grains. These mono crystals have a FCC spinel structure in epitaxy with the underlying metal grains. EELS analyses showed that the Cr/Fe ratio in the outer layer decreases and becomes close to pure magnetite after 10mn whereas the Cr/Fe ratio in the inner layer is increasing up to 1 showing a Cr enrichment. Depending on the ratio of Cr/Fe, the structure of the monocrystals in the inner oxide layer might be either normal or inversed spinel4. In order to investigate this question, the evolution of the Fe-L2,3 Near Edge Structures (NES) were followed using condensor lens aberrations corrected TITAN with a mono-chromated gun (∆E<200mV) equipped with Gatan Quantum HR (EDF R&D). The experimental Fe edges were reconstructed using linear combination method5 of spectra obtained on pure FeO, Fe2O3 and Fe3O4 (Fig3) as standards. Whereas a good match is obtained for the outer oxide layer, the reconstruction method fails for the inner layer. Thus, ab initio simulations were performed in order to understand the influence of the Fe substitution by Cr on Fe3O4 crystal structure and electronic properties. A hybrid functional, combining PBE exchange-correlation functional with Hartree-Fock exchange, was parameterized in order to reproduce the various magnetic phases of this spinel, first of all the spinel inverse phase of Fe3O4 which is the ground state of this oxide. Then, the crystal structure and electronic properties of Fe(3-x)CrxO4 were determined (Fig 4). For each ratio, the spin repartition was determined in order to minimize the total energy of the system. A semi-metal to insulating phase transition was found for 0 < x < 0.05. All the obtained data are discussed in terms of lattice parameters, band structures, DOS and mulliken charges evolutions.


Laurent LEGRAS (Moret sur loing), Jean-Louis MANSOT, Philippe BARANEK, Romain SOULAS
08:00 - 18:15 #5918 - IM08-430 Quantifying magnetism on the nanometer scale: EMCD on individual FePt nanoparticles.
IM08-430 Quantifying magnetism on the nanometer scale: EMCD on individual FePt nanoparticles.

Electron energy-loss magnetic chiral dichroism (EMCD), which is the electron wave analogue of X-ray magnetic circular dichroism (XMCD), offers the possibility to study magnetic properties at the nanoscale in a TEM. The relatively young method of EMCD [1] was already refined to such an extent that it is possible to probe magnetic moments of thin films of a variety of ferromagnets [2, 3, 4]. By now, these measurements already surpass the resolution of XMCD experiments. However, quantitative EMCD measurements are so far only reported on thin films rather than on nanoparticles, which are expected to reveal distinct magnetic properties due to their reduced dimensions and enhanced surface to volume ratio.

We report on EMCD measurements on a single FePt nanoparticle (cf. Figure 1) and compare our experimental findings with simulations. L10 ordered FePt is a particularly interesting material since it offers the highest magneto-crystalline anisotropy among the oxidation-resistant hard magnets [5]. It is therefore a promising material for future high density magnetic data storage media. The L10 ordered FePt nanoparticles on a STO substrate were prepared by sputtering. Prior to the spectroscopic measurements, samples in plan view geometry were subjected to mechanical thinning and grazing incidence Ar+ ion milling in order to obtain 10 nm thick substrate-free nanoparticles.

The experiments were performed on a FEI Titan3 80-300 microscope equipped with an image CS corrector. The sample was oriented in three beam condition with the [001] easy axis of L10 FePt oriented (close to) parallel to the electron beam. Binned-gain acquisition of the EEL spectra was used to optimize the S/N ratio [6]. Particular attention was paid to the analysis of the EEL spectra. A measurement route is presented that allows for the extraction of a dichroic signal from spectra that still suffer from non-optimal S/N ratio. Our experiments are supported by simulations of EEL spectra utilizing the WIEN2k program package [7] in combination with Bloch-wave (BW) methods. These simulations are used to (pre-)determine optimal experimental parameters, that provide for the highest EMCD signals [8, 9]. The experiments reveal a small but reproducible dichroic signal (cf. Figure 2) that agrees well with the results of the theoretical calculations. From these experimental spectra, a ratio of angular to spin magnetic moment ml/ms = 0.08 ± 0.08 is for the first time quantitatively derived for individual FePt nanoparticles [10], which agrees well with the XMCD result ml/ms = 0.09 for a large ensemble of L10 ordered FePt nanoparticles [11].

 

[1] P. Schattschneider et al, Nature 441 (2006), p. 486.

[2] T. Thersleff et al, Scientific Reports 5 (2015), 13012.

[3] X. Fu et al, Appl. Phys. Lett. 107 (2015), 062402.

[4] D. Song et al, Appl. Phys. Lett. 107 (2015), 042401.

[5] J. Lyubina et al, Handbook of Magnetic Materials 19 (2011), p. 291.

[6] M. Bosman and V. J. Keast, Ultramicroscopy 108 (2008), p. 837.

[7] K. Schwarz and P. Blaha, Computational Materials Science 28 (2003), p. 259.

[8] S. Löffler and P. Schattschneider, Ultramicroscopy 110 (2010), p. 831.

[9] J. Rusz, S. Muto and K. Tatsumi, Ultramicroscopy 125 (2013), p. 81.

[10] S.Schneider et al, in preparation.

[11] C. Antoniak et al, Phys. Rev. Lett. 97 (2006), 117201.

P. Schattschneider  acknowledges financial support by the Austrian Science Fund (FWF) under grant nr. I543-N20. S. Löffler acknowledges financial support by the Austrian Science Fund (FWF) under grant nr. J3732-N27.


Sebastian SCHNEIDER (Dresden, Germany), Darius POHL, Stefan LÖFFLER, Deepa KASINATHAN, Jan RUSZ, Peter SCHATTSCHNEIDER, Ludwig SCHULTZ, Bernd RELLINGHAUS
08:00 - 18:15 #5931 - IM08-432 The method of the ELNES theoretical calculation with van der Waals interaction.
IM08-432 The method of the ELNES theoretical calculation with van der Waals interaction.

In recent year, the investigation of the local electronic and atomic structure of materials has been attracting attention due to their importance on the macroscopic properties. The electron energy-loss near edge structure (ELNES) in the electron energy-loss spectrum (EELS) reflects partial electronic state of the material. Therefore, ELNES becomes one of the most powerful method to analyze the local electronic and atomic structure because it is observed with a transmission electron microscopy (TEM) and scanning TEM (STEM), which accomplishes an atomic resolution [1]. The TEM/STEM-ELNES method can determine detailed electronic and atomic structures and detect their small but important changes in the interpretation of the ELNES spectra.

In order to interpret the experimental ELNES and acquire the information of electronic and atomic structures, such as the bonding state, coordination environment and valence state, a theoretical calculation of the ELNES is essential. The ELNES simulation with the first-principles calculation is almost established and can reproduce the experimental spectrum with high accuracy. Although, there is still room for improvement, for example, the effect of the van der Waals (vdW) interaction. In this study, we focused on that of vdW interaction on the ELNES calculation. The power of the vdW interaction was much smaller interaction than that of the ionic and covalent bonding. However, this interaction is acted on the all materials, and the ELNES provides a great benefit to investigate the vdW interaction at a local region.

Therefore, in this study, we investigated the effects of vdW interaction in the theoretical calculation of ELNES. The vdW effect differs depending on the state, the solid, liquid and gaseous states were systematically investigated to influence of the vdW interaction on the ELNES calculation.

We composed the solid, liquid and gaseous models, and calculated their ELNES spectra. These structural models are shown in FIG.1 (1-6). The ELNES spectra of these structures were calculated using the first-principles plane-wave basis pseudopotential method. To introduce the vdW interaction into the ELNES calculation, we applied the semi-empirical vdW-TS approach of Tkatchenko and Scheffler [2]. This method is reported to be able to calculate vdW interaction accurately.

In the case of solid and liquid, due to the influence of the vdW interaction, the transition energies decreased by approximately 0.1 eV in FIG.2 (1-2). The energy shift implies that the vdW interaction exerts more influence on the excited state than the ground state. It is for this reason that the excited state has more electrons at the delocalized unoccupied bands. Furthermore, the relationships between the vdW interaction and local electronic structure and molecular configuration were discussed in Fig. 3. In contrast to the case of the solid and liquid structures, the gaseous models are little dependent of the vdW interaction owing to the large intermolecular distance in FIG.2 (3) [3]. We are going to talk about detailed results in my presentation.

References

[1] M. Varela et al., Phys. Rev. Lett. 92 (2004) 095502.

[2]A. Tkatchenko et al., Phys. Rev. Lett. 102 (2009) 073005.

[3] H. Katsukura et al., submitted.


Hirotaka KATSUKURA (Tokyo, Japan), Tomohiro MIYATA, Teruyasu MIZOGUCHI
08:00 - 18:15 #5950 - IM08-434 Bandgap measurement of high refractive index materials by EELS.
IM08-434 Bandgap measurement of high refractive index materials by EELS.

The determination of bandgaps and optical properties using valence electron energy loss spectroscopy (VEELS) has gained attracted interest since monochromated transmission electron microscopes (TEMs) with excellent spatial resolution and high energy resolution­ have become available. However, to measure and interpret the bandgap of many semi-conductors has turned out to be more challenging than expected – in particular the precise measurement of bandgaps of high refractive index materials, such as Si and GaAs, has proven especially difficult. The reasons for these difficulties are not related to any energy resolution constraints, but due to that relativistic losses, surface plasmons and waveguide modes dominate the spectrum signal in the low energy loss region of the bandgap [1-3]. For such high refractive index materials the Cerenkov limit, i.e. the acceleration voltage one needs to stay below in order to avoid relativistic losses, is below the available voltage range of most TEMs.

In this work, we present a set-up in scanning transmission electron microscopy (STEM) mode where we do not allow Cerenkov losses, surface plasmons or waveguide modes to come onto the EEL spectrometer. We exploit that the relativistic losses, surface modes and bulk waveguide modes all are extremely forward scattered and exist only inside a narrow solid angle that extends out to a few tens of µradians. We have a set-up where the semi-convergence and –collection angles both are in the range below 200 µrad. This set-up gives a nearly parallel electron beam which combined with off-axis/dark field EELS allows for detection of an energy loss signal from a region of reciprocal space that is still very close to the center of the first Brillouin zone. The off-axis conditions are such that we are outside the narrow angular range where the unwanted signals from relativistic losses etc. compromise the EEL spectra, but still close enough to the center of the first Brillouin zone to detect bandgap excitations of almost direct bandgap transitions. Furthermore, such a set-up is not restricted to low acceleration voltages, but can be used over a very broad range of voltages.

Our experiments were performed with a double corrected, monochromated Titan microscope, operated at 80 kV. In Fig. 1 we show two EEL spectra: One with an on-axis set-up and where the semi-convergence and –collection angles are both in the milliradian range. From this spectrum it is more or less impossible to extract any reliable bandgap information. The other spectrum was acquired with semi-convergence and collection angles of 200 and 50 µrad, respectively. Even though the energy resolution is "only" 230 meV, a bandgap of 1.42 eV± 0.02 is measured directly from the raw, un-subtracted spectrum. This value is very close to the direct bandgap of 1.42 eV for GaAs. We will further show that the present off-axis set-up can be used to determine the bandgap of several GaAs-based compositions in a sample with multiple layers of III-V materials.

 

References

 

[1] M Horak and M.S Pollach, Ultramicroscopy, 157(2015) 73-78

[2] M.S Pollach and P. Schattschneider, Ultramicroscopy, 107(2007) 1178-1185

[3] R Erni and N D. Browning, Ultramicroscopy, 108 (2008) 84-99

 

Acknowledgments

The authors would like to acknowledge ESTEEM2 for financially supporting this work. The Norwegian Research Council is acknowledged for funding the HighQ-IB project under contract no. 10415201.


Maryam VATANPARAST (Trondheim, Norway), Per Erik VULLUM, Johan VERBEECK, Ricardo EGOAVIL, Turid W. REENAAS, Randi HOLMESTAD
08:00 - 18:15 #5984 - IM08-436 An approach towards solving 3D information by combining quantitative ADF STEM and EDX.
IM08-436 An approach towards solving 3D information by combining quantitative ADF STEM and EDX.

Annular dark-field scanning transmission electron microscopy (ADF STEM) is an invaluable tool for materials characterisation, providing atomic scale information of materials structure and defects. ADF STEM in particular produces images where the intensity contains both composition and thickness information. In fact, through careful quantification of the intensity itself it becomes possible to extract quantitative information and even atom counting. Typically this is either done through comparison with careful standards, comparison with simulations or via statistical methods. For single element nanoparticles it becomes possible to estimate their 3D structure from atom counting and then applying an energy minimisation [1].

  

Silicon drift detectors (SDDs) have opened up a new era in energy dispersive x-ray (EDX) microanalysis at the nanometre and even sub-nanometre scales. The improved detector design allows for larger devices and therefore increased solid angles for x-ray collection. This provides a significant improvement in x-ray count rates such that atomic resolution maps and x-ray tomography are both now possible. In addition, aberration correction in the STEM allows the use of larger probe-forming apertures to produce increased beam currents in much smaller probes. The combined result provides huge improvements in x-ray counts from small volumes leading to the potential for improved quantitative analysis at the nanometre scale and below.

In the same way that the scattering cross section, σ, can be calculated from ADF image intensity [2] and for ionisation edges in EELS [3], it is possible to calculate an EDX partial cross section using an approach that demonstrates similarities with the ζ-factor method [4]. This quantification method was applied to PtCo alloy nanoparticles that have been acid-leached to provide platinum enrichment (or rather cobalt depletion) at the particle surface. It is possible to quantify the levels of cobalt depletion in the first few atomic layers of the particle, showing that the leaching produces a localised surface depletion that can only be determined by this high resolution EDX quantification [5], [6].

Both of these methods of quantification can be used to yield a range of information about the structure of our material when used individual. Here we look at the potential information which can be achieved through combining the information from each technique. In particular we look at extracting 3D information of bimetallic fuel cell catalysts.

[1]         L. Jones, K. E. MacArthur et al. Nano. Lett 14 (2014) 6336

[2]         H. E, K. E. MacArthur et al. Ultramicroscopy 133 (2013) 109-119

[3]         R. F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope New York 2011

[4]         M. Watanabe, Z. Horita, M. Nemoto, Ultramicroscopy 65 (1996) 187;

[5]         K. E. MacArthur, T. J. A. Slater et al. Microsc. & Microanal.  22 (1) (2016) 71-81

[6]         K. E. MacArthur, T. J. A. Slater et al. Mater. Sci. Technol. In press


Katherine MACARTHUR (Juelich, Germany), Dogan OZKAYA, Sergio LOZANO-PEREZ, Peter NELLIST
08:00 - 18:15 #6078 - IM08-438 EELS analysis of the interaction betweeen frozen acqueous samples and incident electrons in TEM.
IM08-438 EELS analysis of the interaction betweeen frozen acqueous samples and incident electrons in TEM.

Radiation damage is a major limiting factor in obtaining high resolution images in cryo-electron microscopy. Quantifying and understanding those radiation damages are considerable challenges. While it seems commonly admitted that the major part of the damages comes from radiolysis[1], local charging within the ice or the sample has been mentioned several time as a source of damage[2].

Due to the isolator behavior of frozen H2O, samples undergo electrostatic charging. While it has never been directly measured, this charging effect is often observable in cryo-electron microscopy in the form of beam deflection or Coulomb-explosion (as shown in Figure 1).

Aranova et al. showed that Electron Energy Loss Spectroscopy (EELS) could be used to measure the production of H2 and O2 associated to “bubbles” often observed in cryo-EM when damages start to occurs [3]. A very interesting finding in their report is that no bubbles were observed in pure frozen water (without sample in it). One possible explanation is that radicals recombine more quickly in H2O than in frozen hydrated specimens in a Transmission Electron Microsocope.

During this presentation, we will study with mono-chromated EELS the effects of the incident electron beam on frozen H2O as well as on frozen aqueous samples. Both the radiolysis and the electrostatic aspects of the beam/matter interaction can be measured with EELS using Kramer-Kronig formula (through dielectric formalism) and Valence EELS (to look at chemical structure changes and bubbles production). We will show that depending on the type of electron illumination (STEM or TEM), the nature and intensity of those damages can vary. We will, then, look at several possible solutions to reduce such damages.

References:

[1] C. Laffon, S. Lacombe, F. Bournel, P. Parent., 2006. Radiation effects in water ice:a near-edge X-ray absorption fine structure study. Journal of Chemical Physics 125 (20), 204714.

[2] Egerton R.F. 2014. Choice of operating voltage for a transmission electron microscope. Ultramicroscopy 145, 85-93.

[3]M.A. Aronova, A.A. Sousa, R.D. Leapman 2011. EELS characterization of radiolytic products in frozen samples. Micron 42, 252-256


Alan MAIGNE (Tancha, Japan), Matthias WOLF
08:00 - 18:15 #6098 - IM08-440 Focused Ion Beam fabrication of defined scalable roughness structures.
IM08-440 Focused Ion Beam fabrication of defined scalable roughness structures.

Focused ion beam (FIB) technology is a reliable tool for the defined local surface modification on the nanoscale and therefore a promising technique to “write” a predefined texture on a point-by-point basis. FIB applications allow flexible scaling of surface patterns both laterally as well as in their height and have therefore the potential to become a versatile tool for purpose-tailored roughness standards.

For surface texture creation by FIB, both real measurement data of actual surfaces and artificially defined surface models can be used. Before FIB is applied to create the desired surface roughness, the input topography data needs to be converted into control commands of the FIB instrument. For this purpose, an automated procedure has been developed. This includes the conversion of the resolution of the given surface topography data into the resolution of the FIB patterning engine and the vertical segmentation of this surface topography into equidistant height layers that are later written by FIB milling (subtractive) or deposition (additive). In addition, this software allows the projection of the data on parametric surfaces (i.e. cylindrical surfaces or spheres). Finally, all data are integrated together with supporting tasks, like orientation marks, finder grid and identifier into a patterning script, which allows the automated “writing” of the structure by FIB. While FIB deposition is not material-dependent, the milling rate strongly depends on the specimen material. The correct milling dose has therefore to be determined experimentally before the creation of the roughness fields. In addition, the pixel size of the FIB patterning layers has to be chosen with respect to the diameter of the used focused ion beam.

In the initial tests, the actual roughness data used as input data for replication by FIB were real AFM (Atomic Force Microscopy) measurements with a nearly symmetric distribution of height values; Sq (root-mean-square roughness) values around 100 nm and Sz (peak-to-peak) values around 1000 nm. The FIB reproductions were then investigated by AFM in the same way, thus allowing best possible high-resolution comparison of input and result. Tests were successfully performed both with a higher-frequency and a lower-frequency roughness to check the FIB limits. While in the beginning, both deposition and milling were tested with success, milling was chosen in the following, as it can be applied faster and thereby reduces machine hours and thus costs.

The figures show a polished Si-specimen with a FIB-milled roughness area of about 180 µm x 180 µm. The similarity of the FIB-created roughness and the model data is apparent, while a closer look reveals that the highest frequency components are not reproduced by FIB. As these frequencies of rather small amplitudes are only on top of the dominating surface texture, their absence does not alter the key roughness values significantly, that agree with those of the model data within 10 % in this case.

The AFM investigations showed that FIB structuring allows the reproduction of a given surface texture on different substrates, resulting in a homogeneous, isotropic roughness. Tests with a more precise milling depth calibration proved that amplitudes are reproduced with only 1 % to 3 % deviation from the chosen model data.

Furthermore, these works prove that FIB is a unique tool to rescale a given roughness in all three directions, opening a wide range of applications, both to the smaller and to the larger sizes, over a rather broad range of dimensions: Towards lower vertical scales, tests with amplitudes downscaled down to 1/30 (i.e. Sq ~ 3 nm) were performed successfully. In the opposite direction, lateral scales were enlarged by a factor of 5 so the roughness is composed of spatial frequencies that can well be measured by most optical surface measurement techniques. Larger roughness fields require the application of advanced stitching techniques; roughness structures with a size up to 290 µm x 290 µm were already created successfully. For many practical applications, the performance of topography measurement techniques on curved surface needs to be characterized. In order to address this issue, defined roughness fields again of 290 µm x 290 µm were transferred successfully onto cylinders and spheres (both with a diameter of about 1 mm) by FIB and applied for characterization of a broad scope of instruments, including AFM, WLI (White Light Interferometry) and optical 3D microscopes.

The authors like to thank André Felgner and Peter Krebs (both PTB) for extensive AFM and CLSM measurements. This work is partly supported by the EMRP JRP IND59 “Microparts” jointly funded by the EMRP participating countries within EURAMET and the European Union.


Matthias HEMMLEB, Dirk BERGER (Berlin, Germany), Thorsten DZIOMBA
08:00 - 18:15 #6116 - IM08-442 3-D reconstruction of surface topography in SEM by means of energy filtered SE and BSE.
IM08-442 3-D reconstruction of surface topography in SEM by means of energy filtered SE and BSE.

The reconstruction of surface topography based on the detection of backscattered or secondary electrons in SEM has found increasing interest during long time [1-5].

For more accurate reconstruction one should know angular dependecies of secondary electron emission (SE) coefficient δ and backscattered electron (BSE) coefficient η, which, in general, are quite well investigated and defined [6]. But for increasing sensitivity in determination of local surface slopes it is offered to detect BSE and SE not integrally, as in previous works, but differentially by energy, i. e. by value of signal detected in narrow energy range of emitted electrons. As it was shown in our experiments, sensitivity increases in times in such formulation of experiment. Experiments were carried out in LEO-1455VP (Zeiss) equipped with a two-channel toroidal electron spectrometer [7]. We measured energy spectra of SE and BSE for various angles of incidence α (Fig. 1). SE spectra are slightly deformed because of applying negative bias Vs=-33 V to stage (electrons leave surface nonisotropically). Comparison of integral and differential coefficients is depicted on Fig. 2. Values of integral coefficients δ and η were calculated by formulas from [8] (by substituting R to Rcosα [9]) and [10] respectively. Values of integral coefficient at fixed angle of detection θ were obtained by simple integration of measured spectra. LineScans of Ti-ball, obtained as difference signal between two symetrically mounted detectors at fixed energy of electrons, are depicted on Fig. 3.

Obtained results allow us to conclude that secondary electrons are best suited for proplem of surface relief reconstruction than backscattered electrons.

This work was funded thanks to the support of RFBR (grant 15-02-01557)

References

1. L. Reimer, R. Böngeler, V. Desai. Scanning microscopy, 1987, 1, 3, pp. 963-973

2. T. Czepkowski, W. Slowko. Scanning, 1996, 18, pp. 433-446

3. D. Kaczmarek. Optica Applicata, 1997, XXVII, 3

4. T. Suganuma. Journal of Electron Microscopy, 1985, 34, 4, 328-337

5. J. Lebiedzik. Scanning, 1979, 2, pp. 230-237

6. L. Reimer. Scanning Electron Microscopy (Physics of Image Formation and Microanalysis), Springer-Verlag Berlin Heidelberg, 1998, pp. 135-169

7. A.V. Gostev, N.A. Orlikovskii, E.I. Rau, A.A. Trubitsyn. Technical Physics, 2013, 58(3), pp. 447-454

8. Y. Lin, D.C. Joy. Surf. Interface Anal., 2005, 37, pp. 895-900

9. N. Bundaleski, M. Belhaj, T. Gineste, O.M.N.D. Teodoro. Vacuum, 2015, 122, pp. 255-259

10. P.-F. Staub. J. Phys. D: Appl. Phys., 1994, 27, pp. 1533-1537


Stepan KUPREENKO, Eduard RAU (Moscow, Russia), Andrey TATARINTSEV, Sergey ZAYTSEV
08:00 - 18:15 #6129 - IM08-444 Characterization of intergranular corrosion defects in a 2024 T351 aluminium alloy.
IM08-444 Characterization of intergranular corrosion defects in a 2024 T351 aluminium alloy.

In the 2xxx series alloys, intergranular corrosion is generally related to the strong reactivity of copper-rich intergranular precipitates leading to a copper enrichment of these particles. While the nature of the oxides formed inside the intergranular corrosion defects was assumed to strongly influence the intergranular corrosion propagation rate, it was not clearly identified due to the thickness of the oxide layer formed which required to use high resolution analytical techniques. The present work aims to characterize the intergranular corrosion defects formed for a 2024-T351 aluminium alloy after a 24 hours continuous immersion in a 1 M NaCl solution and compares the results to literature data concerning the oxide layers formed on copper-rich model alloys. An intergranular defect obtain after cyclic immersion (8 hours continuous immersion, 16 hours emersion) was also observed and characterised. In order to obtain a thin sample in a localized region, i.e. in an intergranular corrosion defect, a Focused Ion Beam (FIB) / Scanning Electron Microscope (SEM) FEI HELIOS 600i equipped with a field emission gun (FEG) was used. The thin sample preparation was done using conventional lift out procedure; it is summarized in Fig. 1. Location of interest (intergranular corrosion defect) was chosen (Fig. 1a) and a platinum coating was deposited using electron beam prior to using ion beam to protect the area beneath from being contaminated by the Gallium (Ga ions) (Fig. 1b). Using a large beam current for fast ion milling, two tranches were milled on either side of the Pt coating. The sample of size (10x10x7 µm3), so prepared, was then mounted on a TEM sample holder. A cross-section view of the intergranular corrosion defect was therefore obtained (Fig. 1c). It was then polished using successive lower beam current. Finally, the sample was thinned to 100 nm or less using 1 keV ion beam to minimize the artefacts from sample preparation (Fig. 1d). A transparent section was obtained. Some intermetallic precipitates were visible inside and all around the intergranular corrosion defect.

Then, a combination of transmission electron microscopy (TEM) observations (Fig. 2) and electron energy loss spectroscopy (EELS) analyses was used to accurately characterize both the morphology and chemical composition of the intergranular corrosion defects. Results evidenced the dissolution of intergranular copper-rich particles, the formation of a 10-200 nm-thin metallic copper-rich layer at the oxide/metal interface and the incorporation of copper inside the amorphous alumina oxide film leading to the formation of structural defects of the oxide film.

Acknowledgment

This work is supported by ANR-14-CE07-0027-01 – M-SCOT: Multi Scale COrrosion Testing.


Marie-Laetitia DE BONFILS-LAHOVARY, Lydia LAFFONT (Toulouse), Christine BLANC
08:00 - 18:15 #6222 - IM08-446 Charge, strain and polarization profiling in Ferroelectric/Ferromagnetic epitaxial heterostructures.
IM08-446 Charge, strain and polarization profiling in Ferroelectric/Ferromagnetic epitaxial heterostructures.

The heterostructures consisting of different perovskite ABO3 oxides provide a remarkable rich platform for creating new physical state and functionalities, as it can tailor the degree of the coupling between lattices, charge, orbital and spins. The multiferroic heterostructures showing a strong coupling between electric and magnetic orders draw extensive interest because of their promising applications in the modern spintronic devices. The magnetic order is coupled with the ferroelectric order at the interface, thereby permitting reversible electric field tuning of local spin and transport properties. Current understandings of this interfacial coupling effects are still limited, pioneering works found experimental evidences for different interface-mediated magnetization models, such as charge-transfer screening effect and local strain fluctuations. [1-3]. To address current disputes, the main challenge is to map directly the local change of the ferroelectric polarization, the interfacial electronic (charge, orbital polarization) and magnetic behavior at atomic scale. In principle, all these aspects might be map using combination of STEM and EELS technique.  

Our preliminary work has been conducted on a model multiferroic system consisting of epitaxial La1-xSrxMnO3/Pb(Zr,Ti)O3 (LSMO/PZT) heterostructures grown onto STO (100) substrates (Fig.1 (a)).  The LSMO oxide shows a strong interplay between transport and magnetic properties, which can be tuned via ferroelectric polarization reversal of the PZT layer. Two focused-ion-beam (FIB) TEM lamella samples at [100] zone axis has been prepared with the two opposite polarization states of the PZT, i.e. pointing toward or away from the LSMO layer, respectively. Microstructure and charge analyses were done by using atomic resolved high angle annular dark field (HAADF) imaging, annular bright field (ABF) imaging and high energy resolved electron energy loss spectroscopy (EELS) in a Cs-corrected scanning transmission electron microscope (NION USTEM200). The recent developed ABF technique allows the simultaneous visualization of both light and heavy elements, which is ideal to precisely determine the oxygen positions and therefore the BO6 octahedra distortions. In Fig.1(b) and (c), two ABF images of the PZT are obtained respectively from the two differently polarized lamellas, and their ferroelectric polarization were determined by the relative displacement between the position of B-site Zr/Ti cation and the center of oxygen octahedra. Two opposite polarization directions are found which confirm the well preservation of PZT polar state during the FIB preparation. The EELS spectrum extracted from the atomic planes in the vicinity of the LSMO/PZT interface are shown in Fig.2, focusing on the fine structure of the O-K and Mn-L2,3 edges, which are sensitive to the local bonding environment. We found that in the LSMO layer, even in the middle of the layer which is 5 u.c. far away from the interface, the pre-peak of O-K edge shows differences for the two polar states, indicating a relative hole doping when the polarization is pointing toward LSMO and a relative electron doping when it is pointing away. It is also coincident with the chemical shift of Mn edges where a higher valence is found in the hole doping configuration, and vice versa. Moreover, when approaching toward the LSMO/PZT interface, an apparent reduction of the Mn valence is found in the polar down state starting from the LSMO layer 2 u.c. away from the interface and continually into the diffusion region at PZT side. At the meantime, the Mn valence in the polar up state is well maintained until the interface where a slightly reduction appears, indicating a stronger resistance for the interfacial charge transfer.

                Our preliminary results suggest a clear link between the ferroelectric polarization direction and the change of charge configuration in the interface and the LSMO layer, indicating an efficient tuning of carrier injection by the ferroelectric field, which may play an important role in the magnetization modulation. Further study in the quantitative analysis on the amount of charge transfer and local interfacial magnetization change will be carried out for a thorough understanding of the magnetoelectric coupling.

[1] Spurgeon, S.R., et al., Nat Commun, 2015. 6: p. 6735.

[2] Spurgeon, S.R., et al., ACSNano, 2014. 8(1): p. 894-903.

[3] Lu, H., et al., 2012. 100(23): p. 232904.


Xiaoyan LI (Orsay), Alexandre GLOTER, Daniele PREZIOSI
08:00 - 18:15 #6229 - IM08-448 Lateral resolution of quantitative element analysis of low-Z elements.
IM08-448 Lateral resolution of quantitative element analysis of low-Z elements.

This work is a continuation of the investigation of the lateral resolution for quantitative analysis in a field emission electron probe microanalyser (FE-EPMA) [1]. Now, these studies are extended from sputtered gold layer to low-Z elements like aluminium. The aim is to determine the minimum thickness of an aluminium layer, for which a precise element quantification is still possible at a given accelerating voltage. For this purpose, Al-layers with different thicknesses are deposited on silicon substrates (Fig. 1) by evaporation of thin pure aluminium rods from a tungsten coil. Cross-section preparation is made by cleaving and subsequent surface polishing by use of a focused Ga-ion beam (FIB, FEI Helios NanoLab 600). The material of the substrate and the FIB-protection layer are chosen for minimum fluorescence by Al-Kα. The X-ray emitting volume is estimated by Monte Carlo (MC) simulations (Casino v2.48) of the electron scattering in Al for electron energies between the primary energy (15, 8, 7, 5, 4 and 3 keV respectively) and the critical ionization energy of Al-Kα at 1.559 keV. The density of Al used for the simulation is measured to be (2.68 +/- 0.02) g/cm3, which is confirmed by thin film measurements evaluated by the software package STRATAGEM. For determination of the lateral resolution, 99% of the simulated electron trajectories were taken into account. Fig. 2 shows the MC simulation for 7 keV and 4 keV primary electron energy indicating a lateral resolution of 790 nm and 285 nm, respectively. For the quantitative WDS analysis, a 3820 nm Al-layer is used as reference standard measured with a TAP-crystal and a measurement time of 20 s for the peak and 10 s for the background.

Table 1 summarized the results of the quantitative measurements of the cross-sectional aluminium layers, so far, and the lateral resolutions appraised from the MC simulation. A quantification result of (100.0 +/-0.5) wt% Al and a content of silicon and platinum equal to 0 wt% indicates that the source volume of the emitted X-rays is completely inside the Al-layer. So far, a layer thickness of about 465 nm can be resolved quantitatively using an accelerating voltage of 4 kV. At 5 keV we found a discrepancy between measurement and simulation, since the latter predicts a better resolution. All measurements with different layer thicknesses will be presented on the poster.

 

 

[1] Berger D and Nissen J  2016  IOP Conf. Ser.: Mater. Sci. Engng. 109 012001.


Jörg NISSEN (Berlin, Germany), Dirk BERGER
08:00 - 18:15 #6232 - IM08-450 Investigation of Plasmonic Modes of Gold Tapers by EELS.
IM08-450 Investigation of Plasmonic Modes of Gold Tapers by EELS.

Plasmonic tapers have been studied intensively due to the ability of adiabatically coupling the propagating surface plasmon polaritons along their shaft to the nanolocalized plasmons at their apex. Therefore, they can find applications in the fields of sub-diffraction-limit nanofocusing, ultrafast photoemission, and near-field optical microscopy.

We investigate the plasmonic modes of three-dimensional single crystalline gold tapers by means of EELS (electron energy loss spectroscopy) and FDTD (finite-difference time-domain) numerical calculation. We observe a broadband excitation in the proximity of the apex and discrete higher-order azimuthal plasmonic modes along the taper shaft in the gold taper with an opening angle of ~45°. Interestingly, the energy dispersions of these higher-order plasmonic modes are roughly proportional to the inverse local radius. The importance of phase-matching between electron field and radiative taper modes in mesoscopic structure is demonstrated [1]. The role of an alternative mechanism suggested by Schröder et al. [2] was analyzed by systematically studying changes in the dispersion of higher-order plasmonic modes of gold tapers for different opening angles.

[1] N. Talebi, W. Sigle, R. Vogelgesang, M. Esmann, S. F. Becker, C. Lienau, P. A. van Aken. ACS Nano, 2015, 9 (7), 7641–7648.

[2] B. Schröder, T. Weber, S. V. Yalunin, T. Kiel, C. Matyssek, M. Sivis, S. Schäfer, F. v. Cube, S. Irsen, K. Busch, C. Ropers, S. Linden, Phys. Rev. B 2015, 92, 085411.


Surong GUO (Stuttgart, Germany), Nahid TALEBI, Wilfried SIGLE, Ralf VOGELGESANG, Martin ESMANN, Simon F. BECKER, Christoph LIENAU, Peter VAN AKEN
08:00 - 18:15 #6237 - IM08-452 Influence of background subtraction and deconvolution on calculation of EELS core-loss intensities.
IM08-452 Influence of background subtraction and deconvolution on calculation of EELS core-loss intensities.

Quantitative analysis of electron energy-loss spectra (EELS) can be highly influenced by plural scattering for large thicknesses (t/λ>0.5) and background modelling. For quantification by integration [1], plural scattering can be accounted for by choosing large integration ranges or by deconvolving with the low-loss function. Richardson-Lucy (maximum likelihood) or Fourier-Ratio deconvolution are state-of-the-art techniques. Fourier-Ratio deconvolution enhances noise which can be partially compensated by re-convolving with a Gaussian kernel. The height of the edge onset of the deconvolved core-loss will usually be lower than the actual onset, see figure 1. Richardson-Lucy (RL) deconvolution is frequently used to improve astronomical observations, de-blur images and resolve near edge structures from monochromated EELS in transmission electron microscopy (TEM) [2]. The iterative RL method produces ringing artefacts which are studied in figure 1. The amplitude and position of the artefacts changes depending on the number of iterations as shown in figure 2 for a simulated hydrogenic line without background. The deconvolved core-loss is sharper and the onset is almost at the precise location it should be. The other effect that influences quantification is background subtraction. Background subtraction is usually done by fitting an inverse power-law (AE-r) function to the pre-edge region. The errors associated with background fit and extrapolation have been discussed by Egerton in terms of so-called h-parameters. Other methods such as multiple linear least-squares fits have been implemented in software packages such as Hyperspy [3], EELSMODEL [4] and Digital Micrograph [5]. In background subtraction, there is always a trade-off between systematic and statistical errors in quantification of core-losses. In some cases, due to noise, near edge or extended fine structures in preceding edges, the extrapolated background can cross the spectrum, which leads to large systematic under-estimate of the net core-loss intensity. Background subtraction techniques with exponential fitting can be explored more systematically and a new approach on how quantification can be improved by choosing different functions to fit in pre-edge regions will be discussed. In particular, modelled pre-edge backgrounds can be forced to not cross the spectrum by introducing a linear offset function, thereby minimizing the under-estimate of the core-loss. Modelling the background can also be explored more extensively by fitting an inverse power-law or exponential fit to the post-ionisation range and shifting the fitted curve downwards to pass though the edge onset. This leads to an overestimate of the core-loss intensity. The possible best background fit and its reliability can be calculated from the error bars associated with the under and over-estimated intensities as described in figure 3. The histograms in figure 4 show that the over- and under-estimate of the As-L edge intensity influences the quantification while optimal fitting provides quantification in better agreement with Ga/As ratio of unity for GaAs.

 

[1] R. F. Egerton. (2011). EELS in the Electron Microscope, 3rd ed, Springer, New York.

[2] A. Gloter et al. (2003). Ultramicroscopy 69, 385–400.

[3] F. de la Peña et al. (2016). HyperSpy 0.8.4.  http://hyperspy.org/.

[4] J. Verbeeck. (2015). EELSModel 4.1. http://www.eelsmodel.ua.ac.be/.

[5] Gatan. (2015). http://www.gatan.com/products/tem-analysis/gatan-microscopy-suite-software.

[6] V. C. Angadi, C. Abhayaratne, T. Walther. (2016), J.Microscopy, in print, doi:10.1111/jmi.12397


Veerendra C ANGADI (Sheffield, United Kingdom), Thomas WALTHER
08:00 - 18:15 #6238 - IM08-454 Joint plasmon and core-loss fitting for electron energy loss spectroscopy of InGaN.
IM08-454 Joint plasmon and core-loss fitting for electron energy loss spectroscopy of InGaN.

We demonstrate a method to fit electron energy-loss spectra (EELS) of InGaN thin film samples by fitting both plasmon and core losses over the energy range of 13-30eV. The main plasmon peak is relatively strong and broad. In our previous research, we have suppressed noise by using Lorentz fitting for the plasmon peaks of InGaN. Pure core-loss spectra of Ga (Ga 3d transitions yield M4,5 peaks at 23.8 and 28.5eV) and In (In 4d transitions yield N4,5 peaks at 20.0 and 25.9eV) can be artificially constructed from EELS of binary compounds by subtracting the GaN plasmon peak (19.35eV) or the InN plasmon peak (15.5eV), see figure 1. Then the Ga 3d and In 4d reference spectra can be obtained by further smoothing the core-loss spectra. Multiple linear regression is applied to experimental GaN and InN spectra of different relative thicknesses (t/λ), and the fitting quality is defined by adjusted R2, which lies higher than 0.998. In order to fit spectra from InGaN with different indium concentrations, InGaN core-loss reference spectra were constructed by using artificial InN and GaN core-loss spectra with linear compositional weightings, simultaneously, the chemical shift and broadening of the plasmon loss is considered in the MLLS fitting. For plasmon peak shifts we have applied our previous research results on the relationship between indium concentration and plasmon peak position [1]. The core-loss chemical shift was assumed to follow the plasmon loss chemical shift, as depicted in figure 2. Finally, the MLLS regression can be performed to fit experimental spectra from InGaN as weighted superpositions of reference plasmon and core-loss spectra corresponding to GaN, InN and a specific ternary InGaN alloy.

InGaN samples grown at high temperatures as typically applied in metal organic chemical vapour phase deposition are prone to phase separation, which was first predicted by Ho and Stringfellow [2]. The experimental spectra were recorded in TEM mode with a conventional Schottky field-emission transmission electron microscope (FEG-TEM). By using the joint plasmon and core-loss spectra fitting of GaN, InN and InGaN, we have studied InGaN spectra with nominal indium concentrations of x=0.54 and x=0.62, where energy dispersive x-ray spectroscopy suggests the indium concentrations are close to x=0.59 and x=0.68 respectively. The fitted spectrum in figure 3 indicates a strong evidence of phase separation in the nominal x=0.62 InGaN sample. 

References:

[1]. X. Wang, M.P. Chauvat, P. Ruterana and T. Walther, Semicond.Sci.Technol.30(11), 114011 (2015).

[2]. I. Ho and G.B. Stringfellow , Appl. Phys. Lett. 69, 2701 (1996)


Xiaoyi WANG (sheffield, United Kingdom), Thomas WALTHER
08:00 - 18:15 #6250 - IM08-456 HAADF-STEM and EDS tilt-series simulations of 25x25x25 nm semiconductors.
IM08-456 HAADF-STEM and EDS tilt-series simulations of 25x25x25 nm semiconductors.

In recent years significant progress has been made in moving towards a more quantitative analysis
of electron microscope images. Computational simulations have played a large part in this
evolution, allowing insights into the image formation process and informing the optimization of
experimental acquisitions [1-3].
Recent innovations in X-ray detector technology have prompted a renewal of interest in EDS
mapping by substantially improving signal to noise ratios and offering commensurate reductions in
acquisition times [4]. To continue the trend in quantitative analysis, it is vital for computational
simulations to incorporate this resurging modality.
Here, a new simulation program is introduced which extends the well-founded multislice protocol
to include the ionization of atoms by the electron beam, and the subsequent generation of
characteristic X-rays. In addition to describing the formulation of the multislice simulation program,
this presentation will describe how the program is being used on the Dutch national supercomputer
to address challenges faced by the semiconductor fabrication industry.
The ongoing miniaturisation of computer chips is putting an increasing strain on fabrication
techniques, resulting in greater occurrences of manufacturing defects. In order to increase yields, a
technique is needed which is capable of identifying and characterizing these defects. Typically, the
defects have dimensions on the order of a nanometre, making HAADF-STEM one of the few viable
options. Whilst the atomic number sensitivity of HAADF-STEM can be used to differentiate some
elements, modern semiconductor devices make use of electronically disparate elements with similar
atomic numbers that cannot be readily distinguished by HAADF-STEM alone. For example, the
chemical sensitivity of EDS is required in cases such as the substitution of the high dielectric
constant element hafnium (Z=72), with a typical gate metal, tantalum (Z=73).
Modern finFET transistors have complex 3-dimensional structures, so defect detection must be
performed in the framework of tilt-series tomography. To meet the needs of industry, the multi-
modal reconstruction and analysis procedure must be accurate, robust, and fast. The starting point is
to develop an algorithm that accurately reconstructs the non-linear images produced using HAADF-
STEM+EDS. To achieve this, a tomographic dataset is required for which the true nature of the
specimen is well defined. This can only be achieved through simulation, and requires substantial
computational cost.
This presentation will describe how two such datasets have been produced, in which tilt series simulations of a 30x30x30 nm region of
a finFET device have been calculated. The finFET device consists of a crystalline silicon fin with a
thin oxide layer at the surface, coated with a 20 nm thick amorphous hafnium dioxide layer. On top of
this is a gate metal layer of 20 nm of amorphous tantalum. The remaining volume is filled by
polycrystalline titanium aluminium nitride. The first tilt-series features an ideal device, whilst the
second includes roughening at interfaces, pinhole defects in the dielectric layer, and a 7 nm carbon
nanoparticle contaminant. Each dataset consists of 179 projections in 2 degree increments (no
missing wedge) with 8 elemental maps and a number of annular detector geometries. This large
calculation was made possible through the use of both multiple CPUs and multiple GPUs. The
construction of the model shown in figure 1 will be described as will the computational techniques
that were employed to simulate the tomographic projections. An example of a HAADF-STEM
image and EDS maps from one projection can be seen in figures 2-4.

REFERENCES
[1] Lebeau, J. M., Findlay, S. D., Allen, L. J., & Stemmer, S. (2010). Nano Letters, 10(4405), 4405–
4408.
[2] Aveyard, R., Ferrando, R., Johnston, R. L., & Yuan, J. (2014). Physical Review Letters, 113(7),
075501.
[3] De Backer, A., De Wael, A., Gonnissen, J., & Van Aert, S. (2014). Ultramicroscopy, 151, 46–55.
[4] Pantel, R. (2011). Ultramicroscopy, 111(11), 1607-1618.


Richard AVEYARD (Delft, The Netherlands), Bernd RIEGER
08:00 - 18:15 #6267 - IM08-458 Characterisation of ordering in the A-site deficient perovskite Ca1-xLa2x/3TiO3 using STEM / EELS.
IM08-458 Characterisation of ordering in the A-site deficient perovskite Ca1-xLa2x/3TiO3 using STEM / EELS.

Perovskite structures based on the formulation Ca1-xLa2x/3TiO3 have been extensively studied across a wide range of possible applications, such as anodes for solid oxide fuel cells [1], dielectric resonators [2], high density memory storage devices [4], and as host matrices for inert matrix nuclear fuels and as containment media for high-level nuclear waste forms [5–6]. Understanding the crystallographic ordering at the atomic scale and the nature of present defects is essential in order to successfully utilize this class of perovskites across the multitude of applications.

 

We have studied the vacancy ordering behaviour of the A-site deficient perovskite system, Ca1-xLa2x/3TiO3, using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), this system adopts Pbnm symmetry, with no indication of long-range ordering. Atomic resolution high-angle annular dark-field (HAADF) STEM image, acquired along [010]p pseudo-cubic zone axis, Figure 1(a), shows varying intensities indicating changes in La3+ / Ca2+ ratio across the field of view. Elemental intensity maps from characteristic core-loss edges, shown in Figure 1(b), demonstrate anti-correlated Ca versus La intensities. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of polarisation maps from A-site cations in the HAADF signal, are shown to be tilt boundaries.

 

At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. One such ordered region is imaged in Figure 2(a) along the [100]p zone axis, in conjunction with EELS elemental maps shown in panel (b), showing the alternating La-rich and La-poor atomic planes. These highly-ordered domains can be found isolated within a random distribution of vacancies / La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, e.g. the area imaged in Figure 3, these highly-ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries, as shown in panels (a) and (b), respectively. The occurrence and characteristics of these ordered structures will be discussed and compared with similar perovskite systems.

 

Acknowledgements

Funding is acknowledged from the UK’s Engineering and Physical Sciences Research Council (EPSRC) under grants EP/K029770/1 and EP/L005581/1. SuperSTEM is the UK National Facility for Aberration-Corrected STEM, supported by EPSRC.

References

[1] V Vashook et al., J. Alloys Compd. 2003, 354 (1-2), 13–23.

[2] I-S Kim et al., Mater. Res. Bull. 1995, 30 (3), 307–316.

[3] EKH Salje et al., ChemPhysChem 2010, 11 (5), 940–950.

[4] E Salje et al., Phase Transit. 2009, 82 (6), 452–469.

[5] Z Zhang et al., J. Solid State Chem. 2007, 180 (3), 1083–1092.

[6] AE Ringwood et al., Nature 1979, 278 (5701), 219–223.


Mohsen DANAIE (Oxford, United Kingdom), Demie KEPAPTSOGLOU, Quentin RAMASSE, Colin OPHUS, Karl WHITTLE, Sebastian LAWSON, Stella PEDRAZZINI, Neil YOUNG, Paul BAGOT, Philip EDMONDSON
08:00 - 18:15 #6272 - IM08-460 Quantitative use of EELS Mo-M2,3 edges for the study of molybdenum oxides: elemental quantification and determination of Mo valence state.
IM08-460 Quantitative use of EELS Mo-M2,3 edges for the study of molybdenum oxides: elemental quantification and determination of Mo valence state.

Intro There is currently a strong revival in the study of molybdenum oxides triggered by the recent developments following their nanostructuration.1,2,3 This should open the way for an emerging field of research aiming at the characterization and optimization of Mo-based nanodevices. EELS performed in a TEM is an unrivaled tool for such analyses even though EELS analyses of Mo oxides can be  tricky. Since Mo-L2,3 white lines are situated around 2500 eV, they cannot be used with confidence, such high-energies implying excessively long dwell times and therefore unavoidable irradiation beam damages. Furthermore, these lines are located too far away from the O-K edge to allow Mo valence determination and Mo/O elemental quantification from the same spectra. On the other hand, Mo-M2,3 white lines are located at lower energies and are closer to the O-K edge. The main issue in using these edges is however the delayed maxima of the Mo-M4,5 edges (Fig. 1a) that hinders the background subtraction with the usual inverse power low function.

         In this contribution, we use a combination of EELS experiments, multiplet and density functional theory (DFT) calculations to establish that elemental quantification and Mo valence states can indeed be reliably derived from Mo-M2,3 edges.

 

Material & Methods EELS spectra were acquired on commercial MoO3 (MoVI) and MoO2 (MoIV) powders using a Hitachi HF2000 TEM (100 kV) equipped with a cold FEG and a modified Gatan PEELS 666 spectrometer. The energy resolution was 1.5 eV and the energy dispersion 0.20 eV/pixel. EELS spectra were acquired at magic angle condition for the Mo-M2,3 edges to avoid anisotropy effects playing a role in the M2,3 intensity ratio determination. Experiments were performed at liquid nitrogen temperature to minimize carbon contamination and irradiation beam damage. Background subtraction for the M2,3 edges is based on the determination of post-edge parameter (Fig.1b) to avoid the detrimental effect of the Mo-M4,5 edges on the background subtraction. After removal of the multiple-inelastic scattering effects, M3/M2 intensity ratios were determined by subtracting a two steps function followed by area integration. Theoretical intensity ratios were also derived from multiplet calculations by using the CTM4XAS program,4 the crystal field splitting parameter being determined from DFT calculations with the Wien2K code.5

 

Results To determine the feasibility of elemental quantification, the k-factors (Fig. 2a) and the corresponding standard errors (Fig. 2b) were determined as a function of the width of the energy window used for the integration. The standard errors reach a minimum close to 2% for energy windows of 15 and 20 eV for MoO2 and MoO3 respectively. The relative difference to the mean value presents also strong variations depending on the energy window and the best accuracy (2%) is found for a width of 10 eV. The precision and the accuracy of these results validate the method we used to subtract the background. In addition, theoretical M3/M2 ratios were also determined from multiplet calculated spectra (Fig. 3a) and compared to experimental ratios (Fig. 3b). The agreement between experiences and calculations is excellent and strengthens our experimental methodology.

         All these results will be detailed together with the possibility to discriminate the two oxides thanks to chemical shifts and energy-loss near-edge structures. This work provides thus a complete picture on the ability to obtain a wealth of precise and accurate chemical information on Mo oxides from the conjugated analyzes of O-K and Mo-M2,3 edges. It will also open interesting opportunities for the EELS studies of a large variety of materials as it is directly transposable to the whole family of 4d transition metal oxides.6

 

1. Le Xin Song et al., CrystEngComm 14 (8):2675–2682, (2012)

2. L. Lajaunie et al., Phys. Rev. B 88 (11):115141 (2013)

3. M.M.Y.A. Alsaif et al., Advanced Functional Materials 26 (1), 91-100 (2016)

4. E.Stavitski and F.M.F. de Groot, Micron 41 (7), 687–694, (2010)

5. Wien2k,  P. Blaha et al., Techn. Universitat Wien, Austria (2001)

6. L. Lajaunie et al., Ultramicroscopy 149, 1–8 (2015)


Luc LAJAUNIE (Zaragoza, Spain), Florent BOUCHER, Rémi DESSAPT, Philippe MOREAU
08:00 - 18:15 #6280 - IM08-462 Dynamic spectro-microscopy of nanoparticle growth and corrosion.
IM08-462 Dynamic spectro-microscopy of nanoparticle growth and corrosion.

X-ray spectro-microscopy provides quantitative chemical information similar to electron energy-loss spectroscopy (EELS), but at different spatial and spectral resolutions and penetration length into the sample. The relaxed thickness constraint of X-ray microscopy also offers exciting opportunities for spectro-microscopy of samples in liquids. Here we present X-ray microscopy (XM) studies of in situ nanoparticle growth and corrosion. Dynamic X-ray experiments are correlated with electron microscopy analysis of ex situ samples to provide structural and/or chemical information at higher spatial resolution.

Figure 1 shows a custom-built electrochemical cell which has been developed to allow real-time imaging of the growth of ZnO nanostructures [1]. This method allows us to directly observe transient events which occur during electrodeposition such as instantaneous versus delayed nucleation, providing insights into the growth mechanisms of electrodeposited ZnO. The X-ray microscope provides information with ~30 nm spatial resolution; this data is then correlated to subsequent ex situ morphological analysis in the scanning electron microscope.

These techniques have then been applied to the corrosion of nanoscale wear debris from cobalt-chromium-molybdenum metal-on-metal (MOM) hip prostheses. The wear debris has been implicated in the eventual failure of MOM hips. EELS and XM analysis of explanted tissue from patients with failed hip prostheses reveals debris that is rich in octahedrally coordinated Cr3+ and Co-deficient. However, the mechanism for cobalt loss is not understood. Using adapted electrochemical cells to simulate oxidative biological environments, the response of CoCrMo nanoparticles is investigated by in situ XM, revealing the conditions under which Co is leached from the wear debris.

[1] SER Tay et al, Nanoscale 8 (2016)  p1849


Angela GOODE (London, United Kingdom), Mohamed KORONFEL, Johanna NELSON WEKER, Stephen TAY, Amy CRUICKSHANK, Sandrine HEUTZ, Alister HART, Alexandra PORTER, Michael TONEY, Mary RYAN
08:00 - 18:15 #8313 - LS02-042b The effect of tau hyperphosphorylation on Pin1 expression in primary cortical neurons: in okadaic acid induced AD model.
LS02-042b The effect of tau hyperphosphorylation on Pin1 expression in primary cortical neurons: in okadaic acid induced AD model.

Hyperphosphorylation of tau leading to neurofibrillary tangles (NFT) is a key pathological hallmarks in neurodegenerative disorders such as Alzheimer’s disease (AD) (1). Peptidyl-prolyl cis-trans isomerase (Pin1) regulates the phosphorylation of Ser/Thr sites of tau protein, and promote microtubule assembly (2). In this study, we aimed to determine the interaction between Pin1 expression and tau hyperphosphorylation in primary cortical neurons using okadaic acid (OKA) model (3) utilized to study AD.

Cortical neurons were obtained from embryonic day 16(E16) Sprague Dawley rat embryos. The neurons were treated with 25 nM OKA on day 7 of culture. Then at 4, 8 and 24 hours after treatment OKA, tau phosphorylation was analyzed by western blot using anti-tau antibodies including Thr231 and Tau-1. Immunocytochemistry was used for Pin1 protein expression and localization.  Pin1 mRNA expression was determined by qRT-PCR at 4, 8 and 24 hours. For cytotoxicity LDH analysis was performed by ELISA. At 8 hours with OKA, tau phosphorylation at Thr231 was increased and non-phosphorylated Tau-1 was decreased compared with the untreated control. Pin1 mRNA expression levels at both 4 and 8 hours post-OKA treatment were lower than the control group.  No significant differences at Pin1 mRNA and protein expression levels were observed between the OKA-treated group and the untreated control group at 24 hours of treatment. Pin1 was mainly localized in the nucleus of control groups, whereas was found in cytoplasm of OKA-treated group. Apoptotic nuclear morphology in OKA-treated group was detected more than the control neurons. In OKA-treated group the LDH release was not significantly different than the other groups at 4 and 8 hours, whereas it significantly increased at 24 hours.

Our study indicates that OKA induces the tau-hyperphosphorylation, affects Pin-1 expression, and causes to translocation of Pin-1 proteins into cytoplasm from nucleus. This study will provide a new approach for AD molecular pathophysiology of in OKA- induced AD model.

References:

1-Iqbal K et al:  Curr Alzheimer Res 2010,7(8):656–664

2-Lu KP, Zhou XZ: Nat Rev Mol Cell Biol 2007,8(11):904–916

3-Martin L, Page G, Terro F: Neurochem Int 2011,59(2):235-250

 


Derya METIN, Duygu GEZEN AK, Erdinç DURSUN, İrem ATASOY, Selma YILMAZER, Arzu KARABAY KORKMAZ, Melek OZTURK (Istanbul, Turkey)
08:00 - 18:15 #4464 - MS00-464 PH and concentration effect on the optical absorption properties of porphyrin nanorods functionalized graphene oxide.
MS00-464 PH and concentration effect on the optical absorption properties of porphyrin nanorods functionalized graphene oxide.

Graphene oxide (FGO) decorated with  nanostructured porphyrin (PN) was synthesized and the interfacial interaction between these two components were investigated by using Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Raman scattering, Fourier transform infrared (FT-IR), X-ray, and Uv-visible.  SEM and TEM showed good dispersion of GO and PN. FT- IR and Raman results revealed a π-π intermolecular interaction confirming the energy/charge transfer. Moreover, X-ray diffraction study confirmed the intercalation of PN in GO and their disaggregation. The Uv-visible results showed an important interaction between GO and porphyrin nanorods under pH values and different concentrations resulting an effect on the optical absorption of GO-PN. The findings presented here are important to achieve the functionalization of graphene oxide with PN for various optoelectronic applications.


Omar BAJJOU (Taounate, Morocco), Malik MAAZA, Mohammed KHENFOUCH, B MOTHUDI, M LEKALA, Mimouna BAITOUL
08:00 - 18:15 #4470 - MS00-466 Synthesis and characterization of dandelion-like ZnS with high antibacterial activity.
MS00-466 Synthesis and characterization of dandelion-like ZnS with high antibacterial activity.

    Nanostructure materials have been the subject of widespread research over the past couple of decades. Recent experiments on nanostructure materials have revealed a host of novel physical and chemical properties, which are significantly different from that of the conventional materials. Many workers are devoted to developing new synthesis methods to fabricate materials with novel nanostructures. ZnS, as a vital wide-gap semiconductor, has been extensively investigated due to its outstanding photoelectric effect, high catalytic activity and wide applications. Recently, ZnS nanomaterials with various geometrical shapes such as 1D wire, rod, or 2D sheet, belt and so on, have been prepared using variety of physical or chemical methods [1-2]. Dandelion-like ZnS materials assembled by 2D nanosheets or 1D nanowires are of great interest as they provide extremely large specific surface areas and unique porous microstructure [3]. However, research into the 3D nanostructure ZnS assembled by 1D ZnS nanowires is still less dealt with. What’s more, majority researchers were devoted to photoluminescence and photocatalytic, few of them pay enough attention to the antibacterial activity of ZnS.

    Microbial contamination has become increasing difficult to control owing to the resistance offered by microbes against conventional antimicrobial agents. It is well-know that inorganic nanomaterials, such as TiO2, AgPO3, ZnO, reveal high antibacterial activities [4-5]. To date, only scant information about antibacterial ability of the ZnS has been recorded. In this work, dandelion-like ZnS has been prepared via the method of facile one-pot hydrothermal synthesis. The dandelion-like ZnS was characterized by transmission electron microscope, scanning electron microscope, energy dispersive spectrometer and X-ray diffraction. The results reveal that the surface topographies of the 3D dandelion-like ZnS particles are actually assembled by plenty of interlaced 1D ZnS nanowires. The influence of reaction time, reaction temperature, Zn/S mole ratio and different zinc and sulfur sources to the dandelion-like structure were investigated. The dandelion-like ZnS exhibits superior ability in inhibiting the growth of Escherichia coli, which makes it promising candidate for biological materials. The large specific surface area, porous surface morphology and the releasing of the Zn2+ ions are considered probable causes for the high antibiotic activity of the dandelion-like ZnS.

References

1. W. Bai, L. Cai, C. Wu, X. Xiao, X. Fan, K. Chen and J. Lin: Alcohothermal synthesis of flower-like ZnS nano-microstructures with high visible light photocatalytic activity. Mater. Lett. 124, 177 (2014).

2. X.H. Guan, L. Yang, X. Guan and G.-S. Wang: Synthesis of a flower-like CuS/ZnS nanocomposite decorated on reduced graphene oxide and its photocatalytic performance. RSC Adv. 5(46), 36185 (2015).

3. B.D. Liu, B. Yang, B. Dierre, T. Sekiguchi and X. Jiang: Local defect-induced red-shift of cathodoluminescence in individual ZnS nanobelts. Nanoscale 6(21), 12414 (2014).

4. M. Li, L. Zhu and D. Lin: Toxicity of ZnO nanoparticles to Escherichia coli: mechanism and the influence of medium components. Environ. Sci. Technol. 45(5), 1977 (2011).

5. K.R. Raghupathi, R.T. Koodali and A.C. Manna: Size-dependent bacterial growth inhibition and mechanism of antibacterial activity of zinc oxide nanoparticles. Langmuir 27(7), 4020 (2011).


Fangwang MING, Zhoucheng WANG (Xiamen, China)
08:00 - 18:15 #4486 - MS00-468 Image processing tools for morphological analysis of nanoscale objects.
MS00-468 Image processing tools for morphological analysis of nanoscale objects.

In this work, we have developed a fast, reliable and unbias algorithm to analyze size distributions and morphological properties of nanoscale images taken by Transmission Electron Microscopy. As the physicochemical properties of nanostructures strongly depend on their size, shape and surface characteristics, it is of great importance to have access to a set of reliable tools to quantify them. Our image processing process is subdivided into three major subroutines: image preprocessing, image analysis and results interpretation. Several algorithms are used to adjust the image contrast as the adapthisteq function based on the contrast-limited adaptive histogram equalization (CLAHE) method.  A segmentation procedure, based on watershed transformation, has been implemented and tested. Introducing a simple geometrical criteria the software is able to distinguish between spherical, triangular and hexagonal shapes. The sphericity, roundness and roughness are also quantified. Figure 1 and 2 show the application of the software for different examples. We demonstrate that it is possible to distinguish between alive and dead bacteria by looking at the roundness, surface roughness and the ratio between the 2D projection area of the bacteria and the bounding box area enclosing the bacteria. For the case of nanoparticles, using the fractal dimension, we can predict the reactivity of iron nanoparticles used for environmental remediation.


Carlos ARROYO, Alexis DEBUT (Sangolquí, Ecuador), Andrea VACA, Brajesh KUMAR, Luis CUMBAL
08:00 - 18:15 #4489 - MS00-470 Diamond Shape Formation by Spontaneous Aggregation of Silver Clusters in Gels.
MS00-470 Diamond Shape Formation by Spontaneous Aggregation of Silver Clusters in Gels.

Particles aggregations in liquid are of great interest because the random collisions of particles are sometimes accompanied with directional assembly, leading to the formation of highly ordered structures. In this work, we find a easy method to synthesize diamond-shaped silver nano particles in gels solution. The time dependent structure characterization indicates that these diamond shape particles were assembled by the spontaneous aggregation of tiny primary Ag clusters. Since the diffusion of Ag clusters are limited in viscous gels solution, Each cluster approaching to the aggregates has more sufficient time and space to optimize its location. The minimization of surface free energy leads to a selective binding of two approaching nuclei. The high energy facets of each nuclei have priority to be the receiving surface for aggregation, while the low energy facets are largely exposed in the aggregation, which led to a oriented self-assembling of Ag clusters. The findings in this paper indicate that the diffusion of building blocks plays an important role in the shape development of materials.


Lin QIANG, Li JINBING, Han YONGSHENG, Lin WEI, Lin QIANG (Beijing, China)
08:00 - 18:15 #4497 - MS00-472 Structural and electrical characterization of SnO2/CeO2 composite films.
MS00-472 Structural and electrical characterization of SnO2/CeO2 composite films.

 Pure SnO2 films and  CeO2/SnO2 composite films were prepared via sol-gel technique.The deposited films have film thicknesses range from 90 to 450 nm. the CeO2 added to SnO 2 with the amounts range from 2 mol% to 10 mol%. Polycrystalline structures of the prepared composites powder and films were characterized by x-ray diffraction analysis. The microstructure of the prepared films was investigated by SEM and TEM respectively. The electrical properties such as the electrical conductivity has been studied. The electrical conductivity was measured as a function of temperature at film thickness 450 nm in case of air and vacuum. The results showed that the SnO2 films are absorptive in case of air. The sensing properties for the prepared films CeO2/SnO2 were  characterized and compared with pure SnO2 towards the H2S, CO and LPG gases at room temperature. The sensitivity measured at different film thickness, showed that the film thickness and the film microstructure had noticeable effect on the value of sensitivity. The sensitivity of CeO 2/SnO2 films to CO gas is 0.990 % at a response time for the sample 6 mol% while is 0.995 at 50 sec for sample 6mol%. Through all the previous studies, it was found that there is a capability to improve the gas sensing properties of pure SnO2 by controlling the amount of additive and the good choice of the proper catalytic.


Gehan EL KOMY, Gehan EL KOMY (Cairo, Egypt), Zainab EL MANDOUH
08:00 - 18:15 #5127 - MS00-474 HAADF-STEM observation of twinned structure formed in gold nanorods by near-infrared pulsed laser irradiation.
MS00-474 HAADF-STEM observation of twinned structure formed in gold nanorods by near-infrared pulsed laser irradiation.

     Gold nanorods have been drawing much interests widely in various science and engineering fields because of their characteristic optical properties [1]. Due to their anisotropic rod shape, gold nanorods generally absorb visible and near-infrared light by localized surface plasmon resonances. Absorbed light more or less heats up the nanorods through electron-phonon coupling. This heating due to photo-thermal conversion sometimes causes deformation of gold nanorods under light illumination. Actually, it has been reported that gold nanorods suspended in an aqueous solution changes their shape into sphere, singular Φ-shape or elongated rod when irradiated with pulsed laser light [2]. Recently our group set up a pulsed laser light illumination system attached to a high voltage electron microscope (HVEM) and performed in-situ observation of deformation process under pulsed laser light illumination [3]. In the present study, we carried out HAADF-STEM characterization of atomic structural change in a gold nanorod due to pulsed laser illumination.

     Gold nanorods used in the present study were produced in CTAB micelle solution by a photochemical method and synthesized to be about 50 nm in length and 10 nm in diameter (products of Dai Nihon Toryo Co. Ltd in Japan). The Au nanorods show two optical absorption peaks around 520 and 980 nm in wavelength. QuantifoilTM carbon films were used for sample supporting mesh. Laser illumination to the samples was performed in a JEM-1300NEF HVEM equipped with an optical guide path of laser pulses into its specimen chamber [3]. The wavelength of laser pulses was 1064 nm, and the pulse duration was 6 to 8 ns. The averaged intensity was 7.3×103 J/m2pulse. HAADF-STEM atomic-resolution observation was carried out with a JEM-ARM200CF operated at an acceleration voltage of 120 kV. In order to suppress the influence due to sample drift during STEM operation, the observation was performed on a drift compensating operation, where rapidly scanned plural images of an interesting area were overlaid with autocorrelation. 

     Figure A shows an atomic resolution image of an original gold nanorod before irradiation with pulsed laser light. Here the incident electrons were illuminated along the [110] zone axis. We are convinced that the drift compensation operation is quite powerful to obtain an atomic structure HAADF image of the whole of a nanorod without any serious distortion. It is clearly shown that the virgin nanorod is a single crystal oriented to [001] along its longer axis. One may confirm faceting tendencies of surfaces; {100} for top and bottom ends and {111} in the tip sides. The main side surfaces are quite smooth and flat {110}.

     Figure B gives a HAADF image of the same nanorod but after experiencing one shot illumination of a laser pulse. The outer shape has been significantly deformed to be nearly spherical. The particle interior also has been complexly changed in atom configuration, and has been divided into tiny blocks in different crystal orientations. The surface is surrounded with mixture of {111} and {100} facets, and {110} surface has disappeared. A rectangle region in Fig. B is further magnified in Fig. C. One may clearly recognize in the close-up view that the particle interior consists of blocks with twinned orientation relationships. The five orientations rotating on a common [110] axis are classified with different colors in Fig. C. The blocks are separated by single layer twin boundaries with {111} mirror symmetry and double layered twin or stacking faults. One can find multiple twin junctions in squared areas in Fig. C. As the rotating angle between two twined orientations is 70.53 degree, five-fold decagonal junction of twins results in a solid-angle deficiency of 7.35 degree [4]. The angle deficiency due to five-fold junction in the left squared region is mostly accommodated with insertion of double layered stacking faults in the right-hand side blocks. At junctions of four blocks recognized in the right square, on the other hand, blue and yellow ones are not in twined relationship any longer, and are separated by a wide angle grain boundary. One may notice that atom columns close to the junction in the yellow block are significantly displaced from their regular positions.

 

This study was partly supported by JSPS Grant-in-Aid for Scientific Research B (# 25289221).

 

[1]  X. Huang, et al., Adv. Mater., 21, 4880 (2009).

[2]  S. Link, Z. L. Wang, and M. A. El-Sayed, J. Phys. Chem. B 104, 7867 (2000).

[3]  N. Sumimoto, et al, Microscopy 63, 261 (2014).

[4]  C. L. Johnson, et al., Nat. Mater. 7, 120 (2008).


Kohei ASO (Fukuoka, Japan), Koji SHIGEMATSU, Tomokazu YAMAMOTO, Syo MATSUMURA
08:00 - 18:15 #5223 - MS00-476 Comparison of sample preparations for TEM observations of lipid nanoparticles (Lipidots®).
MS00-476 Comparison of sample preparations for TEM observations of lipid nanoparticles (Lipidots®).

            Lipid nanoparticles (lipidots or liposomes for example) can be used in medicine as drug nanocarriers. Their core-shell structure allows the functionalization of these nanoparticles using antibodies, peptides or proteins. The main aim of this targeting is to reduce the drug toxicity occurring during treatment. Such nanoparticles are currently used to treat breast cancer [1]. Further applications as fluorescent agent carriers for in-vitro and in-vivo diagnostics are also considered [2]. To have a better understanding of the effectiveness of these nanoparticles, characterization of these objects is required. Classical imaging in TEM mode leads to many issues as dehydrating, agglomeration and lack of contrast. Thus, optimal sample preparation is mandatory.

            In this study, several preparation methods have been tested to determine the most appropriate one for TEM observations and 3D electron tomography acquisition. F80-Lipidots (expected diameter 80nm) have been synthetized at CEA-Grenoble using ultrasound or high pressure homogenization technologies [3]. TEM micrographs were taken using a FEI Tecnai Osiris having a cryo operating mode and working at 200kV. It is also equipped with four EDX detectors and a GIF for chemical analysis.

            These particles were firstly observed by classical imaging (droplet drying) with and without negative staining. It appeared that the staining remarkably improved the contrast of the nano-objects which appeared to have a circular shape. Nevertheless, Lipidot size determination was difficult due to aggregation phenomena during drying procedure. This method needed to be complemented by cryo-microscopy.

            Cryo-techniques were used to ensure the preservation of the samples in their native state. Rapid freezing in ethane on C-Flat grids after blotting was performed thanks to a FEI Vitrobot. Its settings (blot force and blot time) were adjusted to obtain vitrified films with a controlled thickness.Ice thickness was firstly determined thanks to EELS and EFTEM [4]. It showed that the ice is thinner in the center of the grid holes (Fig. 1). This could be a problem if this thickness is thinner than the particle size. Indeed, in that case, the particles tend to concentrate next to the carbon film of the grid (Fig. 2) and make it difficult to record tomography images. To avoid these artifacts, high pressure freezing (EM PACT2, LEICA) was also performed. It was then followed by cryo-substitution, polymer resin embedding (EM AFS2, LEICA) and ultramicrotomy; or cryo-ultramicrotomy (EM-FC7, LEICA) directly after freezing (CEMOVIS). The constant thickness of the slices is thus an asset for tomogram acquisition. The main difficulty lies in maintaining the “cold chain” from the preparation to the observation.

            The pros and cons of different sample methods (simplicity, reproducibility and reliability) will be discussed and results in what concerns the shape and the size of the particles will be compared. These results will be correlated with other techniques for size measurement at the nanometric scale such as DLS.

REFERENCES

[1] Ranson, M. R. et al. (1997). Journal of Clinical Oncology, 15(10), 3185-3191.

[2] Gravier, J. et al. (2011). Journal of biomedical optics, 16(9), 096013-096013.

[3] Delmas, T. et al. (2011). Journal of colloid and interface science, 360(2), 471-481.

[4] Malis, T., Cheng, S. C., & Egerton, R. F. (1988). Journal of electron microscopy technique, 8(2), 193-200.


Amandine ARNOULD (GRENOBLE), Maria BACIA, Fanny CAPUTO, Anne-Claude COUFFIN, Benoit GALLET, Constantin MATEI, Romain SOULAS, Jean-François DAMLENCOURT
08:00 - 18:15 #5277 - MS00-478 Primary particle size distribution measurement of aggregated nanoparticles.
MS00-478 Primary particle size distribution measurement of aggregated nanoparticles.

     Industrial applications of nanomaterials have recently been reported in many fields. The European Union (EU) announced their definition of nanomaterial in 2012. According to the EU definition, nanomaterial means a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as agglomerate and where, for 50 % or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm - 100 nm. They also announced that the particle size of the primary particles in agglomerates or aggregates should be considered. For regulatory purposes, it is necessary to measure the size distribution of nanoparticles based on the particle number concentration. The European Food Safety Authority (EFSA) recommended using at least two different analytical methods to identify nanomaterials for the EU regulation, one of which should be electron microscopy [1]. Transmission electron microscope (TEM) is a most useful technique that can provide precise information on the shape and size of the primary nanoparticles. Standardization on particle size measurement is performed Technical Committee (TC) 229 of International Organization for Standardization (ISO). Scope of ISO/TC229 is standardization in the field of nanotechnologies. We have performed interlobratry comparison (ILC) test of particle size distribution measurement of titania (TiO2) nanoparticles. In this study, details and results of this ILC is introduced.   

     Participants of this ILC were 6 national research institutes including 4 national metrology instututes, and 3 companies producing nanomaterials. TEM sample preparation is very important because nanoparticles are easy to aggregate in preparation. In this ILC test, chairperson (KY) prepared TEM specimens and distributed to participants of ILC. TiO2 nanoparticle was dispersed in the 1mg/ml of sodium hexametaphosphate (NaPO3)6) aqueous solution. TiO2 content was 25mg/ml. Ultrasonic irradiation to TiO2 nanoparticle dispersion was performed for 1 hour, and stable TiO2 nanoparticle dispersion was obtained. TEM image and aggregate size by laser diffraction are shown in Figure 1(a)-(b). Average size of aggregate was 200nm, and TiO2 nanoparticles was well dispersed. Copper metal TEM grid with amorphous carbon support membrane was used. The support membrane surface of a TEM grid was made hydrophilic using a hydrophilic treatment device. Filter paper was placed on a hot plate that has been warmed at 100℃, and the TEM grid with the hydrophilized support membrane was placed on the top of this. 15μL of the TiO2 dispersion liquid was collected using a micropipette, and dripped onto the support membrane on the hot plate. The TEM grid with support membrane was dried on a hot plate.

     Protocol of ILC was basis on the manual tracing of primary particle shapes in aggregate, which has clear contrast and distinguishable. At least 500 particles should be counted with the image resolution more better than 0.5 nm/pixel. Max./min. Feret and  area-equivalent circle diameters (ECD) of each particle should be measured using image software. One example data of ECD distribution obtained in ILC test is shown in Figure 2. In this data, 1033 particles were counted. Median diameter (D50) is 37.5nm, and standard deviation is 12.1nm. Cumulative distribution data of ECD reported from all participants are shown in Figure 3. 5 data in 9 data agree well. We examined the measurement conditions of TEM, image pixel size, and scale calibration methods of TEM. For TEM measurements, the focusing condition or the z-position of specimen influences strongly to size measurement. We summarize how to measure particle size correctly are; 1) the same z-position of specimen in measurement and in calibration, 2) the same focusing condition in measurement and in calibration, and 3) using the same sized calibration standard to measured particles.  

References

[1] E.A.J. Bleeker et al, RIVM Letter report 601358001(2012).


Kazuhiro YAMAMOTO (Tsukuba, Japan), Toshiyuki FUJIMOTO, Eric A. GRULKE
08:00 - 18:15 #5316 - MS00-480 Optical and structural characterization of Copper Indium Sulphide Quantum Dots.
MS00-480 Optical and structural characterization of Copper Indium Sulphide Quantum Dots.

Copper indium sulfide (CIS) quantum dots (QDs) have been synthesized according to a simple heat up method starting from In(OAc)3 and three different copper precursors: CuI, CuBr, or CuCl. The obtained nanoparticles (NPS) were  characterized for their optical properties, the CIS QDs prepared using different copper(I) salts show different behaviors depending on the nature of the chosen precursor salt. The absorption spectra show a shift of the absorption edge towards longer wavelengths with increasing reaction time. This is in accordance with the quantum confinement effect: bigger nanoparticles display a wider band-gap. The absorption spectra have been analyzed according to the Tauc interpretation in order to obtain information about the dimension of the nanosized semiconductors. The band gap energy (Eg) decreases with increasing reaction time. From the absorption analyses we observed a difference in the behavior of the nanoparticles synthesized in the presence of different anions. According to the Hofmeister interpretation, the three halogen ions possess different interacting properties with hydrophobic molecules. The charge distribution of the electronic cloud is rather diffuse in heavier halogen ions, the polarizability decreases according to the series I->Br->Cl-. The latter ion, having a high surface charge, interact weakly with hydrophobic surfaces. We tried to give an interpretation of the different behavior observed in the presence of different anions in the reaction environment. We can observe that when CuI is used the QDs obtained after 5 min of reaction have a smaller band gap than those obtained with CuBr or CuCl. Prolonging the reaction time the growth of the NPs continues and the absorption can be extended up above 800 nm. When copper salts with smaller counterions are used, this growth seems somehow inhibited. The same trend is confirmed by the photoluminescence analyses. The emission peak shifts upon increasing the reaction time: although not being excitonic in nature, the emission in this kind of QDs is known to be size-related. The actual emission mechanism is not fully understood, but defect states (mainly copper vacancies) and bound band states have been demonstrated to play a role. For this reason, as the diameter of the quantum dots increases, the emission experiences a bathochromic shift accordingly. We also noted that the emission intensity is dependent upon the copper source used in the synthesis. The sample synthesized from CuI has better optical properties than the ones of the samples synthesized from CuBr or CuCl. The structural characterization has been done by TEM & XPS: for  the analyses the NPs were washed using chloroform:methanol (1:1) to precipitate the QDs out from hexanes. From TEM observations, the NPs appear to have a good crystallinity after 2 hours of reaction. The morphology is polygonal, with a crystalline habitus typical of a tetragonal structure, supporting the attribution of the phase to the chalcopyrite polymorph. It is hard to determine the actual size of the NPs in each sample due to the rather irregular shape, from the FFT analysis the sample made starting from CuI shows smaller NPs. The EDX analysis returns the expected elements in the samples, with Cu, In, and S being the most abundant ones. Beside them, iodine and bromine lines can be observed in the spectra of the samples synthesized starting from CuI and CuBr, while no chlorine is detectable in the last sample. These results are in accordance with those coming from XPS analyses. The QDs are copper deficient, which is a characteristic beneficial from the point of view of the optical properties, since it has been demonstrated that the copper vacancies acts as defect levels involved in the electronic transitions responsible for the emission in this type of QDs. Sulfur vacancies are also supposed to give rise to intraband levels related to the emission mechanism. Our samples have a sulfur content slightly below the expected value. We observed the presence of halogen ions in the samples. It is also to be noted that the anion fraction in the sample increases in the order I->Br->Cl-. This trend is in accordance with the EDX analyses performed during the TEM observations. One of the mechanism affecting the properties of the QDs is the fact that the anions can be incorporated in the lattice, thus perturbing the chemical surrounding of the ions. The incorporation of higher amount of impurities should be also beneficial in terms of the optical quality of the NPs, according to the defect-related nature of the emission of these QDs. The identification of the process through which the anions influence the synthesis and the properties of the NPs remains challenging. It is undoubtful that the use of different precursors leads to QDs with diverse properties, and that the trend follows that of the polarizability (Hofmeister series) of the copper counterions.


Riccardo MARIN, Fiorenzo VETRONE, Tamie LOH, Daniel CHUA, Patrizia CANTON (Venezia-Mestre, Italy)
08:00 - 18:15 #5361 - MS00-482 TEM analysis of deformation induced dynamic nanocrystallization in an amorphous CoTi alloy.
MS00-482 TEM analysis of deformation induced dynamic nanocrystallization in an amorphous CoTi alloy.

Nano-sized crystals in an amorphous matrix are considered to change the mechanical properties of an amorphous alloy. Therefore, it is of special interest to manipulate and control both the size and structure of nanocrystals. In many cases, nanocrystals are formed by special heat treatments. Here, we show that nanocrystals can emerge out of the amorphous phase during severe plastic deformation. This can be revealed by studying the composition and the atomic structure of the crystals using different transmission electron microscopy (TEM) methods.

In our work pure components are used to make a Co3Ti alloy. Homogenisation at 950°C for 100 hours leads to a L12 long range ordered single phase alloy. The samples were deformed by high pressure torsion (HPT) using 4 GPa pressure and 80 rotations. After deformation TEM imaging with a Philips CM200 yields both crystalline and amorphous regions present in the samples. The situation is similar to that of Zr3Al, a L12 alloy, that can be made amorphous by severe plastic deformation [1]. The striking result of the present study is that in the amorphous regions nanocrystals of about 2-20 nm in size are embedded. Their average size is about 12 ± 0.5 nm and they exhibit a volume fraction of about 2 ± 1% (cf. Fig.1). From the analysis of bright field (BF) images taken from different sample sections it can be concluded that the nanocrystals have a spherical shape.

The chemical composition of the nanocrystals is analysed in a FEI Titan microscope by electron energy loss spectroscopy. The Ti atomic concentration for individual nanocrystals is 18 % higher than those of the surrounding amorphous matrix. This indicates that the nanocrystals are of the Laves phase Co2.1Ti0.9. Therefore we conclude that the nanocrystals are not retained crystalline material but rather formed during deformation by dynamic crystallisation.

For structural information high resolution transmission electron (HRTEM) images of the nanocrystals are acquired. The HRTEM images show lattice planes according to the Kagome layers of Laves phases (cf. Fig.2). Nevertheless, the analysis of the stacking sequence of the Kagome layers A, B and C does not reveal unambiguously the corresponding Laves phase due to a high density of faults. The structure can be described either by a faulted Co2Ti (stacking sequence ABC) or a faulted Co2.1Ti0.9 (stacking sequence ABAC). In order to have reference images of an unfaulted Laves phase the as-cast alloy containing the Co2.1Ti0.9 Laves phase (C36) was studied. Therefore HRTEM images are acquired with a CM30 microscope. By using the Kikuchi patterns the sample was tilted to a [100] pole of Co2.1Ti0.9. Fig.3 shows the corresponding HRTEM image of the Kagome layers with an unfaulted ABAC stacking order.

 

[1] D. Geist, S. Ii, K. Tsuchiya, H.P. Karnthaler, G. Stefanov, C. Rentenberger, Nanocrystalline Zr3Al made through amorphization by repeated cold rolling and followed by crystallization, J. Alloys Compd. 509 (2011) 1815–1818. doi:10.1016/j.jallcom.2010.10.050.

 

Acknowledgements

We kindly acknowledge financial support by the Austrian Science Fund (FWF):[I1309, P22440, J3397].


Stefan NOISTERNIG (Vienna, Austria), Christian EBNER, Christoph GAMMER, Christian RENTENBERGER, Christian GSPAN, Hans-Peter KARNTHALER
08:00 - 18:15 #5364 - MS00-484 Developments in unconventional dark field TEM for characterising nanocatalyst systems.
MS00-484 Developments in unconventional dark field TEM for characterising nanocatalyst systems.

Recent methods of dark field TEM are being explored to extent the information and frequency of data capture in dynamic in-situ experiments on nanocatalyst systems under reaction conditions (1).    The broad need is for a faster frame rate than can conveniently be achieved with HAADF STEM imaging, starting from the similarly expressed goals for tomography of Bals et al (2) and gentle high resolution by Zhang et al (3).   Beam stop spiders described in (2,3) and other geometries including displaced aperture arcs stopping the central beam have been explored.    FIB fabricated custom devices have been introduced into a custom mechanism using regular 3mm apertures for easier customisation.   This is primarily inserted into the high contrast (lower) objective aperture position on our modified JEOL 2200FS with aberration correctors for both TEM image and STEM probe.   The preliminary results with the new applications are encouraging with 0.2nm lattice images recorded from larger (~10nm) particles (Fig.1) and sensitivity down to a few atoms (Fig.2) recorded using 1 second exposures, rather than requiring 10x that for direct STEM methods.    The aim is to be able to track individual migrating atoms and nanoparticles with sufficient frequency to have confidence for each one in their source and endpoint to better inform our understanding of key coarsening mechanisms (4) which lead to catalyst inefficiencies, including in environmental emission controls (5).    This requires analysis at sufficient frequency that the mean atom/nanoparticle movement is significantly less than the spacing between sites.

References
1. E D Boyes, M R Ward, L Lari and P L Gai, Ann Phys (Berlin), 525 (2013) 423

2. S Bals, G V Tendeloo and C Kisielowski, Adv Mater, 18 (2006) 892

3. C Zhang, Q Xua, P J Peters and H Zandbergen, Ultramicroscopy, 134 (2013) 200

4. P Wynblatt and N A Gjostein, Prog Solid State Ch, 9 (1975) 21

5. M R Ward, T Hyde, E D Boyes and P L Gai, Chem Cat Chem, 4 (2012) 1622

Acknowledgement

The AC ESTEM catalysis project at York is supported primarily by EPSRC (UK) strategic grant EP/J018058/1


Gnanavel THIRUNAVUKKARASU (York, United Kingdom), Michael R WARD, Pratibha L GAI, Edward D BOYES
08:00 - 18:15 #5735 - MS00-486 Characterizing the protein corona of polystyrene nanoparticles.
MS00-486 Characterizing the protein corona of polystyrene nanoparticles.

Nanoparticles are of great interest for their use in biomedical applications like imaging and drug delivery. They are used as delivery vesicles to carry drugs to cells, while protecting them from degradation and allowing a targeted delivery. It is important to understand the interaction of nanoparticles with the biological systems, in order to make these processes more efficient. Upon injection of nanoparticles inside the blood, there is a competition of different biological molecules to adsorb on the surface of the nanoparticles. Numerous proteins are present in the plasma at high concentrations and interact with the nanomaterial surface forming a cloud of proteins known as the “protein corona”. The protein corona alters the size and composition of a nanomaterial, giving it an identity which is not the same as its synthetic identity [1]. The physical parameters of nanoparticles could affect the composition of protein corona, which is dynamic as the proteins are exchanging and the composition is evolving. This interface can be divided into the “hard” and the “soft” corona, depending on the binding strength and exchange rates of the proteins associated with the nanomaterial surface. The proteins with high binding affinities that are tightly bound to the nanoparticles form the “hard” protein corona and the ones that are loosely bound, have high exchange rates and can be easily replaced, form the “soft” corona. The cell or organ sees the nanoparticle-protein complexes when interacting with a nanoparticle dispersed in a biological medium. That means that the corona defines the biological identity of nanoparticles and influences the cytotoxicity and endocytosis. Due to the complexity of the protein corona, it has been challenging to characterize. Approaching the project with microscopy techniques will answer critical questions about the structure of nanoparticles and their interactions with the biological systems, which can be used to make biomedical applications more efficient.


[1]: Chem. Soc. Rev., 2012, 41, 2780-2799


Maria KOKKINOPOULOU (Mainz, Germany), Johanna SIMON, Volker MAILAENDER, Ingo LIEBERWIRTH, Katharina LANDFESTER
08:00 - 18:15 #5744 - MS00-488 In situ TEM study of nanoalloys in solution.
MS00-488 In situ TEM study of nanoalloys in solution.

1. Introduction

Different Pt-Pd nanoalloys were prepared from organic precursors in solution. Their nucleation and growth in the liquid was in situ studied in a graphene oxide liquid cell by the direct reduction in the electron beam [1-3].

Then, the morphological evolution of the nanoalloys under gas pressure was in situ studied in an environmental sample holder [4] by standard TEM.

2.Pt@Pd core shell NPs

Increasing amounts of Pt could be deposited on Pd nanocubes cubes by sequential reduction, with a resulting concave shape as seen in fig. 1.

Fig.2 is a set of images during the Pt growth around the same Pd cube, during 20 mn. The operating conditions (magnification 800 K, electron density 3. 105 A/m2) corresponds to an increasing electron dose.  During this time, the variation of liquid quantity in the drop was not visible. From the set of images, it seems that the Pt layer has an homogeneous thickness during the growth, so that the final concave shape results from other steps in the preparation process.

Then, the morphology of Pt@Pd nanocubes was in situ observed during oxido reduction cycles in a few mbar of pure O2 and H2.

Fig. 3 clearly shows that the Pd@Pt nanocubes have concave shapes in pure H2. In pure O2, they are much rounded at the corners and the (110) facets are extended compared to the same samples in H2, as the anisotropy ratio between the surface free energies of  (001) and (110) faces,  increases from 0.7 in H2 to 0.8 in O2.

3. Properties

A maximal reactivity in gas has been found for an equivalent thickness of 0.4 atomic Pt layers on the Pd nanocubes. For this thickness the core-shell particles are more active than pure Pd cubes or similar Pt cubes reported in literature. This behavior is explained by a decrease of the adsorption energy of molecules, due a compressive strain, induced by the misfit between the two metal bulk lattices and by a ligand effect due to the modification of the electronic structure of Pt atoms in contact with Pd atoms. A similar qualitative evolution as a function of thickness of the Pt layer was already observed in electrocatalysis and was also explained by the decrease of the strength of adsorbed species.

 

References

[1]  Yuk M. et al.,Science , 336,  61  (2012)

[2]  De Clercq A. et al., J. Phys Chem. Letters, 5,  2126-2130  (2014)

[3]  Alloyeau D. et al. Nanoletters (2015)

[4]  Giorgio S. et al., Ultramicroscopy 106 -6, 503 (2006)  


Astrid DE CLERCQ, Olivier MARGEAT, Claude R HENRY, Suzanne GIORGIO (Marseille)
08:00 - 18:15 #5753 - MS00-490 In-situ atomic scale studies of ammonia synthesis over ruthenium nanocatalysts.
MS00-490 In-situ atomic scale studies of ammonia synthesis over ruthenium nanocatalysts.

Ammonia is an important chemical for the production of fertilisers and in chemical synthesis. Catalyst materials are employed to improve the rate of formation of ammonia from diatomic gas precursors. Historically, the industrial catalyst most widely used for this process is based on the magnetite phase of iron oxide, often promoted by alumina and other additives.1 More recently, graphitic carbon supported ruthenium based catalysts have been developed, which can operate more efficiently and at lower pressures than iron catalysts.2, 3 In this work, the sintering of Ru/C catalysts under ammonia synthesis conditions was studied by Environmental Scanning Transmission Electron Microscopy (ESTEM).4

Samples were prepared by incipient wetness impregnation of Ru(NO)(NO3)3 on graphitic carbon. After drying at 150 °C, powdered samples were deposited onto 5 nm amorphous carbon coated MEMS chips supplied by DENS solutions. Additionally, model Ru samples were prepared by direct deposition of Ru precursor onto amorphous carbon MEMS chips. Ru nanoparticles were formed in the microscope by in-situ reduction in H2 gas. Suitable regions were identified and treated at 300 – 450 °C in H2, N2, or a mixed H2/N2 gas atmosphere. Images were taken before and after treatment to study nanoparticle sintering. To limit beam exposure, the beam was blanked between images and during heating steps.

Typical images of the Ru/C samples are shown in Figure 1, showing Ru particles around 1 – 5 nm in size in addition to smaller clusters. After initial reduction, electron diffraction patterns from particles on both supports can be assigned to hexagonal phase Ru metal, space group P63/mmc. An example heating series for Ru on amorphous carbon is shown in Figure 2. Particle migration and coalescence is observed following treatments at both 300 °C for 2 h and 450 °C for 1 h. For this image series the particle size distribution increased from 1.5 ± 0.38 nm before H2/N2 treatment, to 1.57 ± 0.33 nm and 1.64 ± 0.4 nm after 300 °C/450 °C treatments respectively. The sintering behaviour of Ru nanoparticles will be investigated as a function of support, gas atmosphere and temperature.

The authors would like to thank Mr Ian Wright and Dr Leonardo Lari for technical support, and the EPSRC (UK) for the strategic research grant EP/J018058/1.

References:

1.            L. Lloyd, Handbook of Industrial Catalysts, Springer, New York, 2011

2.            D. E. Brown, T. Edmonds, R. W. Joyner, J. J. McCarroll and S. R. Tennison, Catal. Lett., 2014, 144, 545

3.            Z. Kowalczyk, S. Jodzis, W. Raróg, J. Zieliński, J. Pielaszek and A. Presz, Applied Catalysis A: General, 1999, 184, 95

4.            E. D. Boyes, M. R. Ward, L. Lari and P. L. Gai, Ann. Phys., 2013, 525, 423


Robert MITCHELL (York, United Kingdom), Edward BOYES, Pratibha GAI
08:00 - 18:15 #5770 - MS00-492 Cost effective implementation of nanoparticle size measurement for regulation purposes.
MS00-492 Cost effective implementation of nanoparticle size measurement for regulation purposes.

While nano-scaled intermediate and consumer products are omnipresent in many industries, a huge challenge consists in the development of methods that reliably identify, characterize and quantify nanomaterials both as a substance and in various matrices. For product registration purposes, the European Commission proposed a definition of nanomaterial [1] which requires a quantitative size determination of the primary particles of a sample down to sizes of 1 nm. According to [1] a material is defined as nano if 50% of the primary particles are observed to comprise a smallest dimension <100 nm.  The NanoDefine project [5] was set up to develop and validate a robust, readily implementable and cost-effective measurement approach to obtain a quantitative particle size distribution and to distinguish between nano and non-nano materials according to the definition [1].

In the present study the mean particle size derived from Brunauer, Emmet, Teller (BET) surface measurement by gas adsorption is systematically compared with the particle size derived from TEM images by manual and automated image evaluation. A correlation between mean particle diameters of 10 different organic pigments is given in figure 1. Automated and manual image evaluation lead to consistent results for the D50 particle size (red and blue bars). Automated TEM image evaluation was accomplished by a software package developed within the NanoDefine project which will soon be available as public ImageJ-Plugin [2,3]. Samples were carefully selected to represent typical organic pigment particles of different grinding degrees. Figure 2 shows TEM images of such a series of pigment grades. The red ellipses mark the result of the automated particle detection using direct ellipse fitting. The length of the minor axis of each ellipsis was taken as approximation for the particles minimum feret diameter.

In the present study a tiered approach for particle classification is proposed. BET measurements can be used for a coarse classification of the material including a categorization into nano/non-nano [4]. The presented data help to define the thresholds for this surface-based classification. Automated evaluation of TEM images improves this classification with reliable results for the number-based particle distribution within some remaining constraints. The according limitations are elaborated in the present study by comparison with the D50 size values determined by manually evaluation of TEM images.

 

References:

[1] European Commission, Commission Recommendation of 18 October 2011 on the definition of nanomaterial, Official Journal of the European Union. 2011/696/EU (2011) p.38.

[2] Schneider, C. A.; Rasband, W. S. & Eliceiri, K. W. (2012) Nature methods 9(7): p. 671.

[3] Wagner, T., Wiemann M., Lipinski H.-G., Kaegi, R., (2015), Symposium on Frontier Researches in Sustainable Humanosphere 2015, Kyoto, Japan

[4] NanoDefine Public Deliverable D3.5; peer reviewed publication in preparation

[5] The research leading to these results has received funding from the European Union’s Seventh

Framework Programme (FP7/2007-2013) under grant agreement n° 604347 – NanoDefine (www.nanodefine.eu).


Philipp MÜLLER (Ludwigshafen am Rhein, Germany), Wendel WOHLLEBEN, Thorsten WIECZOREK, Thorsten WAGNER
08:00 - 18:15 #5772 - MS00-494 Behaviour of platinum nanoparticles under reducing and oxidising conditions using ESTEM.
MS00-494 Behaviour of platinum nanoparticles under reducing and oxidising conditions using ESTEM.

Platinum nanoparticles are well-known to be catalytically active in a wide variety of important chemical processes including hydrogen fuel cells 1 and diesel oxidation catalysts 2. Platinum, as well as other nanoparticle catalysts are known to reduce their efficiency over time under operating conditions due to several mechanisms. The first method is via particle migration and coalescence where nanoparticles become mobile, collide and eventually form larger structures. Alternatively, the nanoparticles may undergo Ostwald ripening where large nanoparticles grow at the expense of smaller ones due to diffusion of less stable atoms across the support or in a gaseous phase. In either case, valuable catalytically active sites such as edge, corner and adatom sites may be lost as larger more stable facets take their place. As nanoparticles coalesce, more of the precious metal is locked away beneath the surface of the nanoparticles resulting in loss of active surface area and ultimately wasted metal.

These processes have been studied with the help of atomic resolution transmission electron microscopy where heat combined with gas can be used to replicate real world operating conditions inside the microscope 3,4. HAADF-STEM with its Z contrast imaging capability is ideal for studying these nanoparticle growth mechanisms, particularly Ostwald ripening where small clusters/atoms are generally not visible in TEM. To further this important field of research, we investigate the differences in nanoparticle dispersion on two different supports before and after being exposed to a range of gases at different temperatures. We utilised the York JEOL 2200FS featuring double aberration correction and  environmental TEM/STEM capability which has previously demonstrated single atom resolution in gas 5. A MEMs chip holder from DENSsolutions was used for heating.

To produce a simple model system to infer nanoparticle aging mechanisms on more complex industrial catalysts, we deposited platinum via magnetron sputtering onto a SiNX MEMs chip upon which graphite had previously been deposited (from ethanol suspension of graphite powder). Figure 1 shows an example of a graphite sheet loaded onto the SiNx MEMs chip. Figure 2 shows an example of the differences in dispersion before and after exposure to O2 at elevated temperature for 6 hours. The image in Figure 2 is taken near a graphite sheet edge. The nanoparticles on the graphite are slightly larger than those on the SiNx on the freshly deposited sample but after the heat treatment in O2, the nanoparticles on the graphite are much larger. These results and environmental (S)TEM in general open new avenues into nanoparticle research using electron microscopy and have wide applications in chemical production, exhaust catalyst and future fuel cell design.

Acknowledgements

We acknowledge the EPSRC (UK) for the critical mass research grant EP/J018058/1 for funding and Ian Wright of the York Nanocentre for assistance.

References

1. Wang, Y.J., et al., Carbon-Supported Pt-Based Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity. Chemical Reviews, 2015. 115(9): p. 3433-3467.

2. Russell, A. and W.S. Epling, Diesel Oxidation Catalysts. Catalysis Reviews-Science and Engineering, 2011. 53(4): p. 337-423.

3. Gai, P.L. and E.D. Boyes, Advances in atomic resolution in situ environmental transmission electron microscopy and 1Å aberration corrected in situ electron microscopy. Microscopy Research and Technique, 2009. 72(3): p. 153-164.

4. Simonsen, S.B., et al., Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM. Journal of the American Chemical Society, 2010. 132(23): p. 7968-7975.

5. Boyes, E.D., et al., ESTEM imaging of single atoms under controlled temperature and gas environment conditions in catalyst reaction studies. Annalen Der Physik, 2013. 525(6): p. 423-429.


Michael WARD (York, United Kingdom), Ed BOYES, Pratibha GAI
08:00 - 18:15 #5774 - MS00-496 Element partitioning and atom location of alloying elements in Co‐base superalloys.
MS00-496 Element partitioning and atom location of alloying elements in Co‐base superalloys.

    The discovery of an ordered L12 precipitate (Co3(Al, W)) phase in the ternary Co-Al-W system in 2006 which is stable up to temperatures of 950 °C attracts significant research on microstructure design for a new class of load-bearing Co-base high-temperature alloys. The low mismatch between the fcc γ-Co solid-solution phase and the L12 γ’-Co3(Al, W) phase helps to establish a microstructure with coherent cuboidal γ’ precipitates embedded in a continuous γ matrix, analogous to Ni-base superalloys. These new Co-base superalloys have the potential to exhibit excellent high-temperature mechanical properties and are considered to be ideal turbine blade materials.

    In the ternary Co-Al-W system, it is reported that the L12 phase is in equilibrium with the B2-CoAl and D019-Co3W phases and transforms into those after extended annealing[1]. In order to increase the temperature capability and stabilize the γ’-L12 phase, additional alloying elements are added to the Co-Al-W system. With additions of Cr, Mo, Ni, Re, Ta and V, the solidus and liquidus temperatures of Co-Al-W alloys are 100-150 °C higher than those of advanced Ni-base single-crystal alloys strengthened by the L12 phase[2]. It is speculated in literature that Mo, V, Nb, Ta and Ti increase the γ’ solvus temperature of Co-Al-W superalloys because they occupy B-sites in the A3B ordered γ’ phase and thereby increase its volume fraction, while Fe, Mn and Cr tend to distribute to the γ phase and decrease the amount of γ’ phase[3]. Because the distribution of additional elements will influence the morphology and amount of the γ’ phase, it is important to investigate their partitioning behavior and atomic site occupation in the Co-Al-W system. Therefore the high-temperature strength and thermal stability of Co-base superalloys are optimized.

    In this study, alloy samples with nominal compositions Co-9Al-9W-2X (X=Ti, Nb, V, Ta, Cr and Mo, at.%) were produced. The partitioning behavior of the alloying elements between γ and γ’ phases were investigated by energy-dispersive X-ray spectroscopy (EDS) in the transmission electron microscope (TEM). In order to analyze the element occupation at atomic sites, atom location by channeling enhanced microanalysis (ALCHEMI) technique was applied.

    The details about the partitioning behavior and the atomic site occupation of alloying elements will be discussed in the presentation.

References

[1] S. Kobayashi, Tsukamoto, Y., Takasugi, T., Chinen, H., Omori, T., Ishida, K., Zaefferer, S., Intermetallics, 17 (2009) 1085-1089.

[2] T.M. Pollock, Dibbern, J., Tsunekane, M., Zhu, J., Suzuki, A., JOM, 62 (2010) 58-63.

[3] S. Meher, Yan, H. Y., Nag, S., Dye, D., Banerjee, R., Scripta Mater, 67 (2012) 850-853.


Li WANG (Geesthacht, Germany), Michael OEHRING, Uwe LORENZ, Florian PYCZAK
08:00 - 18:15 #5848 - MS00-498 Mapping the plasmonic modes of silver nanoparticle aggregates.
MS00-498 Mapping the plasmonic modes of silver nanoparticle aggregates.

The optical properties of the noble metal nanoparticles (NPs) are dominated by localized surface plasmon resonances (LSPR) [1]. A spherical NP suspended in vacuum would present a LSPR mode that can be modelled as a dipole, hence called dipolar mode. When NPs are close enough to each other, they couple splitting the plasmonic modes of the same order and creating two new modes, the bonding dipolar plasmonic mode (BDP), and the antibonding dipolar plasmonic mode (ADP) [2]. The BDP is a low-energy mode while the ADP resonates at a value slightly higher than the dipolar mode of a sphere. The exact energy value for both modes depends on the inter-particle distance, being smaller as they are closer to each other [3]. It also depends on the aspect ratio of the group with lower energy values as the aspect ratio gets larger [4]. The third conditioning factor is the geometric shape of the cluster. In the same way that triangular NPs have plasmonic modes at lower energies than a sphere [5], a triangular or rhomboidal shaped group of NPs shows plasmonic modes at smaller energies than a spherical one [6].

In this work, silver NPs were created and were forced to cluster. Samples were taken at different stages of the aggregation process. They were analyzed at a large scale by UV-Vis spectroscopy (UV-Vis) and at nanometre scale by energy-filtering transmission electron microscopy (EFTEM). The individual, dipolar mode was clearly identified for isolated NPs corresponding to the early stages of the clustering process. As bigger clusters are created, the collective modes become more apparent.

This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-1-R, 2-R and CONSOLIDER INGENIO 2010 CSD2009-00013), Generalitat Valenciana (PROMETEOII/2014/059) and Junta de Andalucía (PAI research group TEP-946).  The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007/2013] under Grant Agreement No. 312483 (ESTEEM2) and H2020 Program (PROMIS ITN European network).

References

1.             Maier, S.A., Plasmonics: Fundamentals and Applications. 1st ed. 2007: Springer: New York.

2.             Halas, N.J., et al., Plasmons in Strongly Coupled Metallic Nanostructures. Chemical Reviews, 2011. 111(6): p. 3913-3961.

3.             Duan, H.G., et al., Nanoplasmonics: Classical down to the Nanometer Scale. Nano Letters, 2012. 12(3): p. 1683-1689.

4.             Barrow, S.J., et al., Surface Plasmon Resonances in Strongly Coupled Gold Nanosphere Chains from Monomer to Hexamer. Nano Letters, 2011. 11(10): p. 4180-4187.

5.             Koh, A.L., et al., High-Resolution Mapping of Electron-Beam-Excited Plasmon Modes in Lithographically Defined Gold Nanostructures. Nano Letters, 2011. 11(3): p. 1323-1330.

6.             Diaz-Egea, C., et al., High spatial resolution mapping of surface plasmon resonance modes in single and aggregated gold nanoparticles assembled on DNA strands. Nanoscale Research Letters, 2013. 8(1): p. 337.


Carlos DIAZ-EGEA (Puerto Real, Cadiz, Spain), Rafael ABARGUES, Juan P MARTÍNEZ-PASTOR, Wilfried SIGLE, Peter A. VAN AKEN, Sergio I MOLINA
08:00 - 18:15 #5863 - MS00-500 In-situ HREM observation and adsorbed gas imaging of ceramics supported fine metal particle catalysts under reaction gas atmosphere.
MS00-500 In-situ HREM observation and adsorbed gas imaging of ceramics supported fine metal particle catalysts under reaction gas atmosphere.

   Three-way catalysts used for purifying automotive exhaust gas usually take a form of noble metal (platinum group metals) fine particles supported on a heat-resistant metal oxide, which converts NOx, CO, HC to N2, CO2, H2O. Incessant efforts have been made to improve the catalytic performance because the governmental regulations for exhausted gas emissions has been continuously updated in a more strict direction, while the price of those rare metals are unstable, because of the uneven distribution of the resources. Technical issues of compatibility between the high catalytic activity and material costs by reducing rare metals or by development of alternative catalysts without them have not yet been fully solved. It is thus important to design an appropriate catalyst without relying on a rule of thumb by unraveling the mechanism of catalytic activities. In the present study we observed in situ catalytic reactions of fine metal particles under gas atmosphere at atomic level resolution and also tried to visualize the catalytic active site by imaging gas molecules adsorbed on the particle surface, using electron energy-loss spectroscopy (EELS).
   We selected Rh/ZrO2, Pt/ZrO2 system, where the particle-supporting ceramics interaction is suppose2d to be relatively weak, to examine the structural changes and adsorption behaviors of reactive gas molecules. The sample powders were dispersed in an organic solvent and a drop of the solution was painted by a brush on a tungsten wire of the single-tilt specimen heating holder. The observation was done between RT and 600 degrees Celsius under vacuum, under O2, NO or CO atmosphere (diluted by Ne), using the Reaction Science Ultra-High Voltage S/TEM, JEM1000K RS of Nagoya University, operated at 1000 kV, equipped with a GIF quantum equivalent post-column EELS and with a differential pumping environmental cell which allows us to introduce a gas to the specimen chamber up to the pressure of 10,000 Pa [1]. Dynamical structural changes were observed at the HRTEM mode and energy-filtered TEM spectral imaging (EFTEM-SI) was applied to the plasmon-loss region. A multivariate analysis method was applied to the obtained datacubes to isolate the spectral features specific to volume and surface plasmon of the metal particles and gas molecules [2].
   The reference plasmon spectra of Rh2O3, ZrO2 and L2,3 ELNES of the gas molecules to identify the resolved spectra from the SI datacubes are shown in Fig. 1. Before observing catalytic reactions, the sample was heated up to remove the organic solvent, though it should be noted that fast heating brought about surface precipitates covering Rh particles under O2 atmosphere, which was found to come from ZrO2 support. This phenomenon was already reported as the strong metal support interaction (SMSI) [3], as shown in Fig.2. Under a mixture gas atmosphere of NO and CO at 300 degrees Celsius carbon layers covering the surfaces of ZrO2 support and partly Rh particles were observed, presumably due to the reduction of CO, though it is unclear whether it is caused by a catalytic reaction. On the other hand, it was observed in case of the Pt/ZrO2 system that reversible solution/precipitation of Pt particles into/from the ZrO2 support by heating/cooling between RT and 300 degrees Celsius, which could be also SMSI.
   Considering that the strong core-loss spectra of the gas molecules appear in the low-loss region, we attempted to detect and image adsorbed gas molecules on metallic particles, using the EFTEM-SI technique and multivariate analysis. Unfortunately no gas molecule-related spectrum was found at Rh/ZrO2 under O2 atmosphere because of SMSI, whereas under a mixture gas atmosphere of NO and CO the spectral component having a peak around 12 eV (Component 3 in Fig. 3), different from that of ZrO2, Rh, or Rh2O3 was separated, which is presumably originated from NO and/or CO, as shown in Fig. 3. We have also conducted the EFTEM-SI experiments under the conditions where SMSI is carefully avoided. Detailed results and argument are presented in the poster.

References

[1] N. Tanaka et al, Microscopy, 62 (2013) 205-215. [2] S. Muto, T. Yoshida and K. Tatsumi, Mater. Trans. 50 (2009) 964-969. [3] S. Bernal et al, Catalysis Letters, 76 (2001) 131-137. [4] A part of this works was supported by Nagoya University microstructural characterization platform as a program of "Nanotechnology Platform" of the Ministry oEducation,Culture,Sports,Science and Technology (MEXT), Japan.


Yuuki ARAKAWA (Tajimi, Japan), Hiromochi TANAKA, Shunsuke MUTO, Keisuke KISHITA, Yasumori SAKURABAYASHI, Hirohito HIRATA
08:00 - 18:15 #5930 - MS00-502 Structural and chemical characterization and 3D modelling of metal oxide core-shell nanoparticles with complex morphology.
MS00-502 Structural and chemical characterization and 3D modelling of metal oxide core-shell nanoparticles with complex morphology.

A wide variety of metal oxide manganite and ferrite core-shell nanoparticles (NPs) with adjustable composition were characterized by means of Transmission Electron Microscopy related tools in order to precisely understand their composition, the arrangement of the different chemical species and cation valence state variation along the nanoparticles.

 

The nanoparticles were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn or Fe by Co or Ni in preformed Mn3O4 or Fe3O4 NPs, allowing the formation of a core shell structure. These nanoparticle systems present a complex 3D structure, which has been modelled with Rhodius software [1] and simulated with the STEM-CELL packages [2], giving rise to a fine determination of the studied systems.

Special attention has been paid on manganite – cobalt systems. By selecting the proper cobalt precursor, CoO crystallites could be simultaneously nucleated on the NP surface to form Mn3O4@CoMn2O4–CoO (Figure 1). In this latest case,  heterostructured NPs exhibited improved performance and durability as bifunctional catalysts for the oxygen reduction and evolution reactions (ORR, OER) over commercial Pt and IrO2-based catalysts and over previously reported spinel electrocatalysts in alkaline solution.

 

Acknowledgements:

SM acknowledges funding from "Programa Internacional de Becas "la Caixa"-Severo Ochoa”, from Generalitat de Catalunya 2014 SGR 1638, the Spanish MINECO MAT2014-51480-ERC (e-ATOM), Severo Ochoa Excellence Program and coordinated projects between IREC and ICN2, TNT-FUELS and e-TNT (MAT2014-59961-C2-2-R). Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program.

 

[1] S. Bernal, et al., Ultramicroscopy 72, 135−164 (1998)

[2] V. Grillo, E. Rotunno, Ultramicroscopy 125, 97–111 (2013)


Sara MARTÍ-SÁNCHEZ (Bellaterra, Spain), Aziz GENÇ, Zhishan LUO, Maria IBÁÑEZ, María DE LA MATA, Andreu CABOT, Jordi ARBIOL
08:00 - 18:15 #5963 - MS00-504 In situ study of Au-Rh nanoalloys.
MS00-504 In situ study of Au-Rh nanoalloys.

1. Introduction

Au-Rh nanoalloys were prepared from colloidal solution [1]. The growth mechanism was studied in situ by TEM in a graphene liquid cell [2-3]. Then, in an environmental sample holder [4],the influence of oxygen or hydrogen adsorption on the structure of AuRh/TiO2 catalysts was observed at a pressure of a few mbar and at room temperature.

2. Growth mechanism

The colloidal solution was encapsulated in a graphene oxide liquid cell and directly observed in a standard electron microscope. The particle growth was initiated under the electron beam.

The same area was observed during about 80 s and images were recorded every 5 s. The average particle size increases until 4 nm. The density number plotted in fig. 1, indicates a maximal density after 25 s, then a drop after 50 s, corresponding to a growth mechanism by direct adsorption of the monomers. This process is followed by coalescence of the  particles, as directly seen in fig. 2.

3. Segregation in hydrogen

Au-Rh nanoparticles with an homogeneous structure, and supported on rutile TiO2 nanorods powders, were observed during oxidation- reduction cycles in an environmental sample holder. In the largest NPs, core- shell formation was clearly observed during hydrogen adsorption, as seen in figure 3. On the other hand, the observation of Au-Rh NPs annealed ex situ in H2 at 400 °C, also shows the core- shell contrast which was not visible before H2 treatment.

The strong interaction between hydrogen and Rh is certainly responsible for the surface segregation of Rh.

Acknowledgments:

We thank the ANR DINAMIC  -11-BS10-009 for financial support and Région PACA for a grant given for the PhD thesis of A. De Clercq.

References

  1. Konuspayeva et al., PCCP 17 (2015) 28122
  2. J.M. Yuk et al., Science, 336 (2012) 61
  3. De Clercq, et al., The Journal of Physical Chemistry Letters, 5 (2014) 2126-2130
  4. S. Giorgio et al., Ultramicroscopy. 106 (2006) 503     

Astrid DE CLERCQ, Laurent PICCOLO, Suzanne GIORGIO (Marseille)
08:00 - 18:15 #5986 - MS00-506 Investigating the Nucleation and Growth of Quaternary Cu2ZnSnS4 Nanocrystals.
MS00-506 Investigating the Nucleation and Growth of Quaternary Cu2ZnSnS4 Nanocrystals.

Among inorganic semiconductors, ternary and quaternary chalcogenides have attracted interest as light absorbers in photovoltaic applications. Cu2ZnSnS4 (CZTS) has drown considerable attention as it has band-gap suitable for solar-harvesting applications, it shows p-type conductivity and a high absorption coefficient. Moreover it only consists of inexpensive, non-toxic and earth-abundant materials. Synthesis by wet-chemical methods are promising alternatives to physical deposition processes, as more easily implemented and cheaper. One of the challenges in the synthesis of colloidal CZTS nanocrystals is the control of internal structure and composition, which influence significantly their optoelectronic properties [1]. In this presentation we show the evidence of cation ordering in CZTS structure thanks to STEM HAADF imaging and we analyze nanocrystals homogeneity and composition by STEM EDX.

CZTS nanocrystals were syntesized following an heating-up method [2]. The first stage of the synthesis consists in a 30 minutes pre-heating at 110°C of the organometallic precursors mixed in oleylamine. Then, CZTS nCs are obtained by increasing the mixing temperature up to 280°C and keeping it constant for one hour. The presence of long-chained organic ligands passivating the surface of nanocrystals is fundamental for avoiding agglomeration in solution phase, it allows a slow and controlled growth; nevertheless it is detrimental for application in devices and for electron microscopy studies, in particular in spectroscopy (where contamination is critical). By drop-casting the sample on graphene membranes, we could test the influence of several purification strategies. Thanks to the low-contrast support we could image the unwanted parasitic residuals. In particular we proved the efficiency of solvent/antisolvent chloroform/aceton + acetic acid dispersion cycles [3]. HRTEM characterization was performed ex-situ. HRTEM and STEM-HAADF images were used to measure size dispersion of the nanocrystals. HRSTEM-HAADF is sensible to chemical contrast, the signal being dependent on the atomic number Z; it is then possible to observe the sites occupied by the heavier atoms (Sn) in the structure, and distinguish then between kesterite (space group I-4) or stannite (space group I-42m) and pre-mixed Cu-Au (PMCA, space group P-42m) structures, which show different characteristic “bright” motifs. The latter (PCMA) structure was the one found when nanocrystals were showing the good direction for phase identification (111). HRSTEM-HAADF experimental images were compared with simulated ones obtained by multislice method and thermal diffusion scattering approximation [4]. STEM-EDX was carried out on a dedicated FEI Themis with SuperX detector, in order to ensure chemical homogeneity between nanocrystals and inside a single crystal. Spectra were analyzed and quantified using Bruker Esprit 1.9 software.An overview of the nucleation and growing process was obtained by in-situ Wide-Angle X-ray Scattering (WAXS) and Small-Angle X-ray Scattering (SAXS), performed on the ID01 beamline at the European Synchrotron Radiation Facility.

 

[1] Chen, Physical Review B 79, 1665211 (2009)

[2] Steinhagen, JACS 131, 12554 (2009)

[3] Akdas, J. Colloid Interface Sci. 445, 337 (2015)

[4] Kirkland, “Advanced Computing in Electron Microscopy”, ed. Springer, (New York)


Fabio AGNESE (Grenoble), Aurelie LEFRANÇOIS, Stephanie POUGET, Louis VAURE, Ourania MAKRYGENNI, Fréderic CHANDEZON, Pascale BAYLE-GUILLEMAUD, Hanako OKUNO, Peter REISS, Jean-Luc ROUVIERE
08:00 - 18:15 #5998 - MS00-508 Analytical electron microscopy of barium titanate and barium-strontium titanate nanoparticles for second-harmonic biomarkers.
MS00-508 Analytical electron microscopy of barium titanate and barium-strontium titanate nanoparticles for second-harmonic biomarkers.

Nanoparticles with non-centrosymmetric crystal structures exhibit second harmonic generation (SHG) of light when illuminated by a femtosecond pulsed laser. Such nanoparticles can be used as optical biomarkers to circumvent the drawbacks associated with fluorescent proteins and semiconductor quantum dots, such as photobleaching and fluorescent intermittency (blinking). Bulk barium titanate has a tetragonal crystal structure at room temperature however, reduction in particle sizes generally correlates with an increasing phase fraction of cubic material which does not exhibit SHG [1].

 

In this study we have produced barium titanate (BaTiO3) and barium-strontium titanate (Ba1-xSrxTiO3) nanoparticles by the hydrothermal method. These nanoparticles appear predominantly cubic by laboratory-XRD but Rietveld refinement on synchrotron X-Ray powder diffraction data suggests a mixture of tetragonal and cubic phases. Transmission electron microscopy (TEM) analysis techniques such as electron energy loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) spectroscopy have been used to determine the inter- and intra-particle phase and composition of BaTiO3 and Ba1-xSrxTiO3 nanoparticles. Prior STEM-EELS work, suggests an intra-particle phase distribution of cubic and tetragonal phases [2]. STEM-EEL linescans by aberration corrected scanning transmission electron microscopy (SuperSTEM) confirm that these hydrothermal samples exhibit intra-particle phase distribution of a tetragonal core and a cubic shell (Figure 1 & 2).

 

Multi-photon microscopy correlated with SEM demonstrates the SHG signals from the BaTiO3 and Ba1-xSrxTiO3 nanoparticles [3]. The cellular uptake and biocompatibility of the BaTiO3 and Ba1-xSrxTiO3 nanoparticles have been determined by cell viability (MTT) and genotoxicity (Comet) assays. Uptake was confirmed by backscattered Z-contrast imaging by SEM and EDX (Figure 3), along with bright field TEM and HAADF-STEM of resin embedded cell sections. Direct correlation between electron microscopy (SEM & TEM) and multi-photon microscopy will be used to determine SHG characteristics at the individual particle level when taken up by cells.


    

[1]        E. Kim, A. Steinbrück, M. T. Buscaglia, V. Buscaglia, T. Pertsch, R. Grange, et al., Second-Harmonic Generation of Single BaTiO3 Nanoparticles down to 22 nm Diameter, ACS Nano. 7 (2013) 5343–5349.

[2]      S M Moon, X Wang, N.H. Cho, Identification of Local Phase of Nanoscale BaTiO3 Powders by High-Resolution Electron Energy Loss Spectroscopy, Microsc. Microanal. 19 (2013) 123.

[3]      O. Matar, O.M. Posada, N.S. Hondow, C. Wälti, M. Saunders, C.A. Murray, et al., Barium Titanate Nanoparticles for Biomarker Applications, J. Phys. Conf. Ser. 644 (2015). 012037


Omar MATAR (Cockermouth, United Kingdom), Nicole HONDOW, Olga POSADA, Michael ROUTLEDGE, David HERNANDEZ-MALDONADO, Christoph WÄLTI, Claire MURRAY, Rik BRYDSON, Steve MILNE, Andy BROWN
08:00 - 18:15 #6014 - MS00-510 Morphology and composition tailoring of cobalt ferrite nanoparticles.
MS00-510 Morphology and composition tailoring of cobalt ferrite nanoparticles.

Cobalt ferrite nanoparticles have scientific and technological interest due to their magnetic properties, good chemical stability and low cost, combined with catalytic properties. These factors allow their use in various applications, such as ferrofluid technology [1], catalysts [2] and gas sensors [3].  One way to improve the catalytic properties of the cobalt ferrite is to control the size and the morphology of the nanoparticles. Studies show that crystallites which expose only a particular family of crystallographic planes have enhanced catalytic activity [4].We already obtained nanooctahedron exposing only {111} facets, 20 nm in size, by  a solvothermal method [5]. Another way to enhance catalytic properties is to control the composition of cobalt ferrites; previous studies showed that high amount of cobalt favors the catalytic conversion of methane [2]. The aim of this study is to synthesize cobalt ferrite nanoparticles with different compositions (CoFe2O4 and Co1.8Fe1.2O4) and morphologies, in order to control and optimize the catalytic properties.

The powders were obtained by solvothermal synthesis using different solvents and precursors. The control of the shape was realised using different surfactants. For cobalt ferrite Co1.8Fe1.2O4 cobalt nitrate and iron nitrate were used as precursors,water and ethylene glycol as solvents. The experiments showed that the solvent has a significant influence on the powder composition.  When water or a mixture of water and ethylene glycol were used as solvents, two phases were identified in the final product: Co(OH)2 and Co1.5Fe1.5O4 (fig. 1). When only ethylene glycol was used, a pure phase with homogeneous composition was obtained: Co1.8Fe1.2O4 with the spinel structure and size around 8 nm (fig. 2).

In order to obtain different morphologies of CoFe2O4, cobalt acetylacetonate and iron acetylacetonate with benzyl alcohol as solvent were used with different amount of oleic acid and oleylamine as surfactants. The cobalt ferrite powder produced by both oleic acid and oleylamine has a heterogeneous composition containing some crystals with cubic shape (fig. 3). Using only acid oleic as surfactant led to a CoFe2O4 powder with homogeneous composition. The shapes of the particles are nearly cubic or octahedral (fig.4). These preliminary results indicate that shape of the nanoparticles is controled by the amount and nature of surfactant.

 

[1]      J. Li, D. Dai, X. Liu, Y. Lin, Y. Huang, L. Bai, J. Mater. Res. 22 (2007) 886-892.

[2]      L. Ajroudi, S. Villain, V. Madigou, N. Mliki, Ch. Leroux, J. Cryst. Growth 312 (2010) 2465–2471.

[3]      C. Xiangfeng, J. Dongli, G. Yu, Z. Chenmou, Sensors Actuators B Chem. 120 (2006) 177–181.

[4]      N. Ballarini, F. Cavani, S. Passeri, L. Pesaresi, A.F. Lee, K. Wilson, 366 (2009) 184–192.

[5]      A.L. Lopes-Moriyama, V. Madigou, C.P. de Souza, Ch. Leroux, Powder Technol. 256 (2014) 482–489.

 

Acknowledgments

This work was done in the general framework of the CAPES COFECUB Ph-C 777-13 french – brazilian cooperation project.


Indira Aritana FERNANDES DE MEDEIROS (La Garde), André Luís LOPES-MORIYAMA, Véronique MADIGOU, Carlson PEREIRA DE SOUZA, Christine LEROUX
08:00 - 18:15 #6021 - MS00-512 Preparation and structural characterization of Au nanoparticles supported on metal oxide nanoplatelets for catalysis by a new two-step method.
MS00-512 Preparation and structural characterization of Au nanoparticles supported on metal oxide nanoplatelets for catalysis by a new two-step method.

While gold has long been regarded as a poorly active catalyst, it is now widely investigated in the field of catalysis and gas sensing. Indeed, it exhibits surprisingly high catalytic activity when deposited as nanoparticles (NPs) on base metal oxides, carbon materials or organic polymers. Especially, gold nanoparticles catalysts with 2 to 10 nm diameters are active for many reactions, such as CO oxidation at a temperature as low as - 70°C. The catalytic performance of supported gold NPs depends on the kind of support materials, the size of gold NPs, and the gold/metal oxide interface structure[1].

 

A variety of preparation methods (more than 10) have been developed to obtain gold NPs with homogeneous dispersions on supports. Classical chemical methods need a calcination step in order to (i) reduce Au3+ ions deposited from a precursor (HAuCl4 is the most popular one) by means of impregnation or deposition–precipitation techniques; (ii) remove organic ligands such as polyvinyl pyrrolidone or polyvinyl alcohol, which prevent the aggregation of gold NPs in the sol-immobilisation method;(iii) crystallize the metal oxide support in coprecipitation method. During the calcination step, the deposited gold particles generally grow to larger ones, so a precise control of the NPs size is difficult with these methods. On the other side, physical methods (PVD, Cathodic Arc Plasma Deposition) are cleaner and allow precise size distribution of the gold NPs on supporting materials, but they needs specific and expansive devices.

 

In this work, an original chemical method to prepare gold NPs deposited on a metal oxide is presented. No calcination is required. The gold precursor is HAuCl4. The oxide used as supporting material must have a lamellar structure. In this work, we choose an Aurivillius phase (Bi3.25La0.75Ti3O12).The preparation consists in two steps. First, lithium is intercalated in the oxide structure, using n-butyl-lithium. After washing and drying at room temperature, a stable lithiated compound is obtained, in which some metallic cations have been reduced at a lower oxidation state [2]. Then, the lithiated powder is mixed with a gold precursor in aqueous solution. Gold ions are directly reduced near the support surfaces, without any other reducing agent. and the nanoparticles are formed only atthe oxide surface and they are well dispersed (figure 1). In addition, the NPs formation is accompanied by a partial delamination of the oxide grains which are separated in nanoplatelets (figures 2 and 3).

 

The materials have been characterized by electronic microscopies (HRSEM, TEM) at each step of the preparation process. The effect of the gold concentration has been investigated and the kinetics of the NPs deposition has been studied by UV-VIS spectroscopy (figure 4), using the gold NPs localized plasmon surface resonance property.

 

[1] Takei, T. et al; Heterogeneous Catalysis by Gold, in Advances In Catalysis, 55, 1-126, 2012, doi: 10.1016/B978-0-12-385516-9.00001-6

[2] Chevallier, V. et al., Exfoliated nanoplatelets of an Aurivillius phase, Bi3.25La0.75Ti3O12: Characterisation by X-ray diffraction and by high-resolution electron microscopy, J. of Solid State Chemistry, 181, 439–449, 2008   doi:10.1016/j.jssc.2007.12.012


Virginie CHEVALLIER, Véronique MADIGOU (LA GARDE)
08:00 - 18:15 #6035 - MS00-514 Synthesis and Structural Control of Bimetallic Pt-Ni nanoparticles.
MS00-514 Synthesis and Structural Control of Bimetallic Pt-Ni nanoparticles.

Noble metals nanoparticles (e.g. platinum, palladium) are commonly used as catalysts in alcohol oxidation reactions [1]. Catalytic efficiency of the nanoparticles may be greatly increased by a partial replacement of one of the precious metals with  d-block transition metals (e.g. iron, cobalt, nickel) [2]. One of many examples of such combinations are bimetallic Pt-Ni nanoparticles, which have a higher catalytic performance in the oxidation of methanol than pure platinum nanoparticles [3]. However, the design and synthesis of nanoparticles, which will have the appropriate size, shape and composition, is challenging. Due to the rapid course of the synthesis reaction and the possibility of changing a variety of parameters such as temperature, reactant concentration, reaction time or even the rapidity of reactants addition, it is possible obtaining nanoparticles significantly different from each other in shape and size. Appropriate selection of the reaction conditions allows to obtain nanoparticles of various shapes, ranging from simple shapes (e.g. circular, cubic, polyhedral) [4] to more complicated 3D structures (e.g. stars [2] or dendritic structures [5]). Changing the reaction conditions affects on the structure of the nanoparticles, thus it is possible to achieve bimetallic nanoalloys, in which the atoms of two metals are randomly mixed. It is also possible to obtain an ordered structure of core-shell type, in which the atoms of one metal form the core of nanoparticle and the atoms of the second surround this core [6].

The aim of this study was to synthesize 3D nanoparticles having a rhombic dodecahedron shape, composed of a Pt frame around a Ni core. For this purpose, a number of syntheses was performed in order to investigate the influence of various reaction parameters on the obtained bimetallic nanoparticles. The following parameters were changed: concentration of metal precursors, temperature in which the metal precursors were added to the solution and the duration of the reaction. The obtained nanoparticles were characterized using transmission electron microscopy (TEM) technique. The morphology of the nanoparticles and their size distribution was imaged by HAADF STEM. Energy-dispersive X-ray spectroscopy (EDX) was used to examine the distribution of chemical elements in the sample.

The HAADF STEM structural analysis showed that in all syntheses bimetallic nanoparticles in different shape were obtained. Dendritic rhombic dodecahedron shapes, regular rhombic dodecahedron and approximately spherical shapes were observed, HAADF images in Fig. 1. All samples had a crystalline structure, which was confirmed by HRTEM images. The size of the nanoparticles varied from 20 to 50 nm, depending on the synthesis method. EDS analysis of all samples confirmed the presence of platinum in the frame and nickel in the core of the nanoparticles (Fig. 1). The obtained results allow to conclude that even a small change of a single parameter during the synthesis procedure, leads to a different structure of the 3D nanoparticles.

 

[1] B. Corain, G. Schmid, N. Toshima, Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control, Elsevier B.V. (2008).

[2] L. Han, P. Cui, H. He, H. Liu, Z. Peng, J. Yang, Journal of Power Sources 286 (2015).

[3] Y. Wu, D. Wang, Z. Niu, P. Chen, . Zhou, Y. Li, Angew. Chem. Int. Ed. 51 (2012).

[4] B. Lim, M. Jiang, J. Tao, P. Camargo, Y. Zhu, Y. Xia, Adv. Funct. Mater. 19 (2009).

[5] S. Wang, N. Kristian, S. Jiang, X. Wang, Nanotechnology 20 (2009).

[6] A. Mendez-Vilas, Materials and processes for energy: communicating current research and technological development, Formatex Research Center, 2013.

 

ACKNOWLEDGMENTS

We thank the Center for Innovation and Transfer of Natural Sciences and Engineering Knowledge of the University of Rzeszow, Poland for using the TEM instrument. Financial support from the Polish National Science Centre (NCN), grant UMO-2014/13/B/ST5/04497 is acknowledged


Grzegorz GRUZEL (Kraków, Poland), Andrzej KOWAL, Magdalena PARLINSKA-WOJTAN
08:00 - 18:15 #6089 - MS00-516 Gas sensing properties of cobalt ferrite nanooctahedra and nanocubes.
MS00-516 Gas sensing properties of cobalt ferrite nanooctahedra and nanocubes.

            The detection function of a sensing material is dependant of a high surface to volume ratio, but also to the exposed crystallographic facets. It should then be possible to tailor the reactivity and sensitivity of the sensing materials by controlling their shape and size, for a given composition [1]. We already showed that the composition of the cobalt ferrite CoxFe3-xO4 influences their catalytic properties [2]. In order to understand and control the gas sensing properties as well as the catalytic properties, we synthesized cobalt ferrites as nanoparticles with various shapes and sizes. Nanoparticles with specific shapes allow to study the influence of the cristallographic facets, hence the cation distribution at the surface, in the gas interaction with the particles.

            By solvothermal methods, we synthesised CoFe2O4 nanooctahedra (Fig. 1) and nanocubes (Fig. 2). Conventional TEM coupled with EDS, high resolution TEM, environmental TEM, were carried out in order to understand the mechanisms involved in the growth of the grains and their reaction under gas. Octahedron-like nanoparticles of CoFe2O4 were submitted to H2 -O2 cycles, at ambient temperature, under 1mbar gas pressure in an TEM 300 kV. The {100} facets extended which led to truncated octahedra and the {111} facets became more rounded under oxygen. The phenomenon was reversible and rounded particles under O2 became facetted under H2 (Figure 3). These CoFe2O4 nanooctahedra exhibit a high sensibility to oxidative gases like NO2  at low gas concentration, and the shape effect on sensibility was clearly demonstrated.The study of nanocubes under oxydo reduction cycles is under progress.

[1] C. Wang, L. Yin , L. Zhang, D. Xiang and R. Gao, Sensors 2010, 10, 2088-2106

[2] L. Ajroudi,S. Villain,V. Madigou,N. Mliki,Ch. Leroux, J. Cryst. Growth 312 (2010) 2465–2471.

[3] A. L. Lopes-Moriyama, V. Madigou, C. Pereira de Souza, Ch. Leroux Powder Tech., 256,482-489 , 2014

 

Acknowledgments :

This work was done in the general framework of the CAPES COFECUB Ph-C 777-13 french – brazilian cooperation project.


Andre-Luis LOPES-MORIYAMA, Indira Aritana FERNANDES DE MEDEIROS, Veronique MADIGOU, Madjid ARAB, Carlson PEREIRA DE SOUZA, Suzanne GIORGIO, Christine LEROUX (IM2NP, Toulon)
08:00 - 18:15 #6099 - MS00-518 Atomic structures of interfacial complexions between gold nanoparticles and nominally stable spinel-substrate.
MS00-518 Atomic structures of interfacial complexions between gold nanoparticles and nominally stable spinel-substrate.

The interfacial complexions, with distinct structures, are considered as quasi-two-dimensional phases, which undergo the structural and chemical changes associated with thermodynamic parameters [1-2]. We recently discovered the formation of gold-spinel interfacial complexions, with well-defined atomic structures, and the related growth of nominally stable spinel lattice underneath the gold nanoparticles after annealing [3-5]. As shown in Figure 1, the necking structure, which is tens of nanometers high, is detected under dewetted gold nanoparticles. Such necking structure has the same contrast with spinel substrate, maintains the spinel composition confirmed by the energy dispersive X-ray spectroscopy, and keeps an ideal epitaxial relationship with the substrate. In associated with the substrate growth, an interfacial bilayer with distinct crystalline structure forms between gold nanoparticles and the substrate. Further studies reveal that the formation and migration of the interfacial complexions are related to the defects at the interfaces, such as, the intersections of gold twinned planes and the interface (see details in Figure 2). In spite of their importance in synthesizing nanostructures, the atomic structures of interfacial complexions have not been fully elucidated yet.

 

Herein, in this paper, we investigated the detailed atomic models of such interfacial complexions in combination of atomic-resolution experimental images and first-principle computations. Experimentally, we synthesized a series of Au-MgAl2O4 samples within different annealing profiles and proposed the initial atomic models based on atomic-resolution scanning transmission electron microscopy (STEM) - high-angle annular dark-field (HAADF) images in Figure 3. Experimental images with two orthogonal crystallographic directions were selected to provide the three-dimensional structural information. A few possible atomic models, with different oxygen vacancies, were built through our MATLAB codes, and inputted into the density functional theory (DFT) computation. The relaxed atomic models, carried out with generalized gradient approximation (GGA) and Perdew-Burker-Ernzerhof (PBE) exchange-correlation density functional [6], were applied to simulate the HAADF images [7] in order to further verify the validity of the structures. Our results provide a new clue to the field of interfacial complexions, particularly to the structural origins of the abnormal phenomenon of self-assembled growth of oxide substrates underneath dewetted gold nanoparticles.

 

References:

[1] S. J. Dillon, et al, JOM 61(2009), p. 38-44.

[2] P. R. Cantwell, et al, Acta Materialia 62(2014), p. 1-48.

[3] G.-z. Zhu, et al, Applied Physics Letters 105(2014), p. 231607.

[4] F. Liu, et al, Materials Characterization 113(2016), p. 67-70.

[5] T. Majdi, et al, Applied Physics Letters 107(2015), p. 241601.

[6] J. P. Perdew, et al, Physical Review Letters 77(1996), p. 3865.

[7] E. J. Kirkland, Advanced Computing in Electron Microscopy, second ed. Springer Scieence & Business Media, 2010.

 

Acknowledgments

We acknowledge the financial support from the National Science Foundation of China (No. 51401124). 


Fang LIU (Shanghai, China), Dong Yue XIE, Yong-Sheng FU, Guo-Zhen ZHU
08:00 - 18:15 #6104 - MS00-520 Synthesis and characterization of nanocatalysts for ethanol oxidation.
MS00-520 Synthesis and characterization of nanocatalysts for ethanol oxidation.

Nowadays limited resources of fossil fuels and environmental concerns increase interest in alternative sources of energy [1]. Recently, fuel cells became very popular and interesting as a good solution for this problem. However, it should be remembered that the oxidation reaction between the catalyst and the fuel (ethanol) occurring in fuel cells is complex and generates a lot of by-products. This whole process does not promote a better efficiency of the cell, on the contrary, it leads to poisoning of the catalyst, decreasing the efficiency of the device. Therefore the key challenge for this branch of science is primarily the development of the appropriate type of catalysts [1]. Recently promising technology seem to be ternary nanocatalysts containing platinum, rhodium, and tin oxide (IV) [2].

The motivation for our work is a better understanding of the synergistic effect between these three components in nanocatalysts, replacing the rhodium by rhenium and determining their selectivity for total oxidation of ethanol to CO2.In the present study we used three methods of synthesis: polyol [3], citrate [4] and microwave assisted [5].

The obtained nanoparticles were characterized by Photon Correlation Spectroscopy (PCS), Transmission Electron Microscopy (TEM) and Fourier Transform Infrared Spectroscopy (FTIR).

The HAADF STEM structural analysis showed that the nanoparticles obtained by all three methods have similar dimensions - about 2 nm. In the case of the citrate and polyol methods the nanoparticles were strongly agglomerated, which was visible not only in the TEM images, but also confirmed by the results obtained by the PCS. On the other hand, nanoparticles obtained by the microwave assisted synthesis did not show such a strong agglomeration as those obtained by the two other methods. All SnO2 samples had a crystalline structure, which was confirmed by HRSTEM images (Fig. 1). Additionally fourier transform infrared spectroscopy (FTIR) was applied to determined the structure of tin oxide obtained in the two differences synthesis (microwave and polyol assisted). It was found, that in the infrared spectrum of Sn oxide synthesized by polyol methods, a stretching modes of Sn-O from Sn(OH)4 was not observed. Moreover, in this samples, more stretching modes of O-Sn-O (Sn4+) was noticed, whereas the samples synthesized by microwave methods, characterized by larger amounts of Sn-O (Sn2+) stretching modes (Fig. 2). The size of the nanoparticles varied from 2 to 12 nm, depending on the synthesis parameters.

The next step is the synthesis of PtRh and PtRe nanoparticles on the obtained SnO2 supports.

Our research confirmed that the crystalline structure, particle size and shape, and surface properties are highly dependent on the chosen method of synthesis.

 

[1] M. Li, W.-P. Zhou, N. S. Marinkovic, K. Sasaki and R. R. Adzic, Electrochimica Acta 104 (2013)

[2] A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel and R.R. Adzic, Nat Mater. 8(2009)

[3] L. Jiang, G. Sun, Z. Zhou, S. Sun, Q. Wang, S. Yan, H. Li, J. Tian, J. Guo, B. Zhou and Q Xin, J. Phys. Chem. B 109 (2005)

[4] L.M. Sikhwivhilu, S.K. Pillai and T.K. Hillie, J Nanosci Nanotechnol 11(2011)

[5] V. Subramanian,W.W. Burke, Z. Hongwei and W. Bingqing, J. Phys. Chem. C 112 (2008)

ACKNOWLEDGMENTS

We thank the Institute of Engineering Materials and Biomaterials of the Technical University of Gliwice, Poland for using the TEM instrument and the Department of Materials Science and Ceramics of the AGH University of Science and Technology of Cracow for using PCS instrument. We also thank the Center for Innovation and Transfer of Natural Sciences and Engineering Knowledge of the University of Rzeszow, Poland for using FTIR instrument.  Financial support from the Polish National Science Centre (NCN), grant UMO-2014/13/B/ST5/04497 is acknowledged.


Elzbieta ROGA (Kraków, Poland), Grzegorz GRUZEL, Joanna DEPCIUCH, Andrzej KOWAL, Magdalena PARLINSKA-WOJTAN
08:00 - 18:15 #6114 - MS00-522 In situ UHVEM observation of atomic ordering in magnetic nanoparticles using a direct detection camera.
MS00-522 In situ UHVEM observation of atomic ordering in magnetic nanoparticles using a direct detection camera.

   Recent demands for ultrahigh density magnetic storage technology require the development of recording media with higher magnetocrystalline anisotropy energy in order to ensure thermal stability of magnetization as well as ultrahigh recording density. For such a purpose, equiatomic CoPt alloy nanoparticles (NPs) are one of the candidate materials [1]. The hard magnetic property of this alloy is attributed to the tetragonal L10-type ordered structure; the anisotropy energy is dependent on the degree of order of the ordered phase. Therefore, formation of the L10 ordered phase is the key issue for practical applications. In this study, we hence intend to observe atomic ordering of CoPt NPs by ultra-high voltage electron microscopy (UHVEM) equipped with a direct detection camera.

   Thin films of disordered CoPt alloy NPs were synthesized by co-deposition of Co and Pt targets using rf-magnetron sputtering onto NaCl(001) substrates kept at 620 K. After the deposition of Co and Pt, surface of the NPs were coated by an amorphous carbon (a-C) thin film. The specimen films were removed from the NaCl substrate by immersing the substrate in distilled water, and then floating films were mounted onto microgrids for electron transparency. Structure and composition of the NPs were characterized using a JEOL JEM-ARM200F TEM (200 kV). Electron irradiation experiments and the simultaneous in situ heating observation were carried out using a JEOL JEM-1000EES UHVEM (1 MV) equipped with a Gatan K2-IS electron direct detection CMOS camera newly installed at the Research Center for UHVEM, Osaka University [2].

   Figure 1 shows a selected area electron diffraction (SAED) pattern and a TEM image of as-sputtered CoPt alloy NPs with disordered fcc structure. As seen, (100)-oriented CoPt alloy NPs with sizes of ~15 nm are dispersed. Average alloy composition was Co-45at%Pt as determined by EDX analysis (Fig. 1(c)).

   Figure 2 shows two successive TEM images of a CoPt NP acquired in situ with a frame rate of 1/400 s. The observation was made at 583 K under electron irradiation at 1 MV with a dose rate of 1.5 × 1025 e/m2s. In Fig. 2(a), disordered phase with the fcc structure is seen. After 2.5 ms, the ordered fringes suddenly appeared as shown in the middle of the NP surrounded by the square (Fig. 2(b)). The atomic ordering can also be confirmed by appearance of weak 001 superlattice reflection in the attached Fast Fourier Transform (FFT) pattern. The observed kinetic ordering temperature of 583 K for binary CoPt NPs is lower than that reported in the previous study (> 800 K) [3]. The low temperature atomic ordering can be attributed to the enhancement of atom migration by high-energy electron irradiation [4].

 

References

[1] P. Andreazza V. Pierron-Bohnes, F. Tournus et al., Surf. Sci. Rep. 70, 188 (2015).

[2] H. Yasuda, Microscopy 64 (S1), i27 (2015).

[3] K. Sato, T. Kosaka, and T. J. Konno, J. Ceramic Soc. Jpn. 122, 317 (2014).

[4] S. Banerjee and K. Urban, Phys. Stat. Sol. (a) 81, 145 (1984).

[5] This study was partially supported by the Grant-in-Aid for Scientific Research (B) (Grant No.26286021) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.


Kazuhisa SATO (Ibaraki, Japan), Hidehiro YASUDA
08:00 - 18:15 #6120 - MS00-524 Recognition and measurements of nanoparticles in TEM/STEM images by fitting the model grayscale distribution to the real one: new approach for automated statistical analysis.
MS00-524 Recognition and measurements of nanoparticles in TEM/STEM images by fitting the model grayscale distribution to the real one: new approach for automated statistical analysis.

The main challenge to manufacture nanoparticles for applications in catalysis, medicine and pharmaceuticals is a mass production of stable nanoparticles with a narrow size distribution to target and control specific effects. Therefore reliable and fast statistical analysis of (nano)particles is of great interest especially for the particle size less than 10 nm due to strong chemical and biological activity associated with high penetrating capabilities through cell membranes. We would like to see these particles, to know their structure and composition, and to measure sizes. The intelligent, fast and reliable program can be a very useful tool for image analysis of “small” nanoparticles in TEM/STEM images.

In our work, we show that fitting of the calculated grayscale distribution to the real distribution in (S)TEM images is able to provide the maximum accuracy in measurements of the particle diameters in opposite to algorithms based on image binarization.

We apply such fitting to the truth in the vicinity of a nanoparticle image revealing the mass-thickness, diffraction, and Z-contrast. In order to describe the dependence of grayscale from thickness of nanoparticles the polynomial g(t)=g0+g1t+g2t2+... with sufficiently high power (≥2) and uncertain coefficients was chosen. The high-degree polynomial is required to take into account the possible non-monotonic dependence of the grayscale from particle thickness due to the presence of diffraction contrast (in opposite to pure mass-thickness contrast). Monotonic dependence of the grayscales from specimen thickness is the characteristic of mass-thickness contrast of particles (amorphous or crystalline particles positioned out of Bragg conditions) in TEM images and Z-contrast in STEM images. The transfer function of CCD cameras determined the grayscale in the given point of the micrograph through the intensity of the incident wave has also monotonic character. Only the presence of diffraction contrast in the images breaks the monotonic dependence. Thickness of the spherical nanoparticle in a point having (x,y)-coordinates can be expressed as t(x,y)=((d/2)2-(x-xc)2-(y-yc)2)^(1/2). During fitting, the uncertain coefficients gi, coordinates of the particle center (xc,yc), and the particle diameter d are computing. Our algorithm for particle recognition and measuring sizes is proposed and realized in the program ANN (Automatic Nanoparticle Numerator).

Our algorithm for particle recognition and measuring sizes out of thresholding approach is proposed and realized in the program ANN (Automatic Nanoparticle Numerator). The comparative study of distributions of silver nanoparticle synthesized in different polymer-water solutions determined manually (about 1000 particles), using ImageJ and ANN was performed (Fig.1 and Fig.2). It shoved a good agreement between results obtained manually and with ANN.


Dmitry SHVEDCHENKO (Moscow, Russia), Elena SUVOROVA
08:00 - 18:15 #6155 - MS00-528 In-situ and cryogenic electron microscopic study of genesis and dynamics of cobalt nanoparticle formation.
MS00-528 In-situ and cryogenic electron microscopic study of genesis and dynamics of cobalt nanoparticle formation.

Cobalt nanoparticles have a high potential as catalysts for the Fischer-Tropsch synthesis, i.e. to convert hydrogen and carbon monoxide, which can be derived from a variety of renewable feedstocks, into industrially useful hydrocarbons. The cobalt nanoparticles are commonly created via a deposition precipitation (DP) process In the DP process, catalyst supports such as silica and titania are suspended in solutions of cobalt precursor salts, which upon increasing/decreasing the pH precipitated a cobalt intermediates e.g. cobalt hydroxides and cobalt carbonates on the liquid-solid interface.  At present how and where the precursor salts in solution are nucleated and how they crystallize as cobalt intermediates is still unknown. In this contribution  the morphological and structural development of the cobalt intermediates with and without support materials during the DP process has been investigated using advanced cryo-EM and in situ liquid cell TEM.

First, cryo-TEM was employed to study Co nanoparticle formation during an decreasing pH induced by out-gassing ammonia from the synthesis solution. At the early stage of the nucleation and growth, particles with a diameter of 1 to 2 nm were found. It is difficult to image these 1~ 2 nm particles through a 100 to 200 nm thick vitrified ice layer due to significant scattering of electrons in the embedding medium. Hence, we vitrified our sample on graphene oxide (GOx) supports which cover a normal TEM holy grid (figure 1a). Because of the very low background and high hydrophilicity of GOx, we generate ultra-thin aqueous layers for high contrast  cryo-TEM imaging (figure 1b). In addition, the presence of GOx makes it possible to focus more accurately to acquire high contrast and resolution cryo images at only a few hundred nanometers of defocus.

Second, liquid cell TEM was employed to study in-situ particle formation by exposing a solution of Co2+ ions to the vapor -NH3 and CO2. Here Co(NO3)2 solution is flown through the liquid cell to fill the system, after which wet N2 is then flown in to remove the Co(NO3)2 solution in the tube (fig 2a, 2b). Because the chips are cleaned by O2 plasma before mounting, the surface of two chips is highly hydrophilic. So that liquid between the two chips is not removed by the wet N2. This way a thin liquid layer containing Co2+ ions is generated between two chips (fig 2b). Subsequently, a syringe containing (NH4)2COpowder is connected to the other port and NH3 and CO2 vapor is released from the decomposition of solid (NH4)2CO3 (figure 2c). In this way, we could generate a thin liquid layer with a thickness of 250 ~ 600 nm in the center in a repeatable manner (figure 3), making the edge of viewing area to be good position for (S)TEM imaging of particle nucleation and growth.

 

Acknowledgements: the authors would like to thank Shell Global Solutions, Netherlands Organization for Scientific Research (NWO) and Eindhoven University of Technology for financial support.


Hao SU (Eindhoven, The Netherlands), Paul BOMANS, Heiner FRIEDRICH, Nico SOMMERDIJK
08:00 - 18:15 #6171 - MS00-530 Self-assembled Supraparticles by Spherical Confinement.
MS00-530 Self-assembled Supraparticles by Spherical Confinement.

Colloidal supraparticles [1,2] which are assembled from size- and morphology- controlled nanoparticles, combine multi scale properties of the single particles such as quantum confinement and localized plasmon resonances with collective effects resulting from being arranged in near proximity on a 3D lattice. In addition, properties on longer length scales, e.g. photonic, are affected by the supraparticle size and its effective refractive index [3] and additionally can be controlled by a subsequent self-assembly step. One way to realize such colloidal supraparticles is by suspending nanocrystals in emulsion droplets of a low boiling point solvent in water and slowly evaporating the solvent [3,4] (or vice versa for particles dispersible in a polar solvent). In this way the nanocrystals are forced to self-assemble into supraballs and surprises already were found in the structure of hard spherical particles crystallized in a spherical confinement where icosahedral crystals were found for supraparticles containing several thousands of spheres or less [4]. By tuning the concentrations and types of nanocrystals, supraballs and differently shaped supraparticles with different structures and sizes can be obtained.

The goal of this research is to extend the spherical confinement method to binary particles systems and anisotropic particles systems. For instance, spherical 8 nm sized (including stabilizer) PbSe nanocrystals and 10 nm PbSe nanocrystals were used to synthesize binary crystalline supraballs with a bulk AB2 binary colloidal crystal phase [5]. We further found 17.5 nm sized Ag/10.7 nm sized Fe3O4 and 8 nm PbSe/11 nm Au binary systems will self-assemble into Janus/patchy shape binary supraballs (where segregation into the two pure components was found) or core-shell binary supraballs where one of the components ends exclusively in a shell on the outside, respectively. From computer simulations between hard particles we infer that the core-shell morphology is most likely caused by the presence of attractions, but this preliminary conclusion needs further study. EuF3 round platelets nanocrystals were also found to self-assemble into highly ordered supraballs with a liquid-crystal-like interior structure. In addition, crystalline supraparticles were successfully assembled by using rounded edged FeO/CoFe2O4 nanocubes as building blocks. After freeze drying, the structure of the supraballs was studied with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography and secondary electron scanning transmission electron microscopy (SE-STEM). By coating the supraballs with a thin (~50 nm) layer of (meso)porous or non-porous silica, the particles become more robust and do not deform by drying on a substrate. To study how the structure of the more complex supraballs is affected by the spherical confinement, work is in progress by advanced electron microscopy [6,7] and 3D particle fitting techniques [8,9]

[1] T. Wang et al. Chemical Society Reviews 42, 2804-2823 (2013).

[2] Z. Lu et al. Chemical Society Reviews 41, 6874-6887 (2012).

[3] D. Vanmaekelbergh et al. ACS Nano 9, 3942-3950 (2015).

[4] B. de Nijs et al. Nature Materials 14, 56-60 (2015).

[5] A. Dong et al. Nature 466, 474-477 (2010).

[6] H. Friedrich et al. Nano Letters 9, 2719-2724 (2009).

[7] S. Bals et al. Angewandte Chemie Int. Ed. 53, 10600-10610 (2014).

[8] D. Zanaga et al. Nanoscale 8, 292-299 (2015).

[9] T. H. Besseling et al. Journal of Physics: Condensed Matter 27, 194109 (2015).


Da WANG, Bart DE NIJS, Nick TASIOS, Simone DUSSI, Frank SMALLENBURG, Laura FILION, Yang LIU (Utrecht, The Netherlands), Thomas ALTANTZIS, Daniele ZANAGA, Yaoting WU, Stan NAJMR, E. Ashley GAULDING, Johannes D. MEELDIJK, Dirk GROENENDIJK, Christopher B. MURRAY, Sara BALS, Marijn A. VAN HUIS, Arnout IMHOF, Marjolein DIJKSTRA, Alfons VAN BLAADEREN
08:00 - 18:15 #4556 - MS01-532 HREM observation and elucidation of edge-bonding MoS2 on {101}-facet exposing surface of anatase TiO2 support.
MS01-532 HREM observation and elucidation of edge-bonding MoS2 on {101}-facet exposing surface of anatase TiO2 support.

An urgent demand for clean fraction oil makes a great pressure on the hydrotreating (HDT) technology due to progressively stringent environmental regulations.  Conventional HDT catalysts supported g-Al2O3 has been used for near 100 years. An alternative of taking anatase-type TiO2 (A-TiO2) as support other than g-Al2O3 could greatly improve the performance of the catalyst. The superior performance of Mo/TiO2 originates from the increasing of type II active sites numbers because of the presence of new MoS2-TiO2 interaction in an edge-bondingway3. However, the interaction was never reported from specific crystallography. The sulfidation process of Mo/TiO2 catalyst could be partly regarded as epitaxial growth of MoS2 on TiO2 support. And thus, the crystallographic orientation relationship (OR) between MoS2 and TiO2 is very important for understanding the active sites of the catalyst.

The TiO2 support was obtained by pressing anatase TiO2 powder, synthesized by hydrothermal method, into disc under 5 MPa pressure and breaking up into 20~40 mesh grains. Mo/TiO2 (10wt%) was synthesized by impregnating ammonium molybdate solution onto TiO2 support followed by dryness and calcination. Sulfiding experiments were carried out on a fixed-bed micro-reactor at 350°C under 10%H2S/H2 atmosphere. The catalysts were observed by HREM on a JEOL 2200FS microscope operated at 200kV.

HREM image in Figure 1A shows the interface relationship between MoS2 and TiO2 in sulfided Mo/TiO2 catalyst. MoS2 slab is anchored on (101) facet-exposing surface of A-TiO2 in edge-bonding way. The angle between (001)MoS2 and (101) A-TiO2 is 66°. The interface relationship could be elucidated by Coincidence Reciprocal Lattice Points (CRLP)4 theory, expressed by a scheme in Figure 2. The intersecting volume function of reciprocal lattice points of two crystals (hexagonal MoS2 and tetrahedral TiO2 shown in Figure 3A) could be computed from home-made program. The OR of two crystals at the initial orientation is (001)MoS2//(001)A-TiO2 and [100]MoS2//[100] A-TiO2 (Figure 3B).The intersecting volume function V(a,b) is computed and plotted versus a and b angle. Figure 3C shows the corresponding 3D drawing. The same peak value presents at (0,15), (0,45) and (0,75) points, which reflects optimum OR presents at the orientations. At initial orientation, the deduced angle between (001)MoS2 and (101) A-TiO2 is 68 degree. At (0,15), (0,45) and (0,75) orientation, the values are 83°, 66°and 36°, respectively. The 66° angle between (001)MoS2 and (101) A-TiO2 at (0, 45) is highly consistent with that value observed by HREM. As a result, the conclusion could be drawn that CRLP theory could well predict interface relationship in MoS2/TiO2 HDT catalyst.

Acknowledgement
We thank the financial support of SINOPEC Project (115048)

References

1.M.Signorile, A. Damin, A. Budnyk, et al. J. cata. 2015, 328:225-235.

2. Edisson Morgado Jr,Jose′L. Zotin,Marco A.S. de Abreu,et al. Appl. Catal: A Gen. 2009,357 : 142–149.

3. SakashitaY, Araki Y,Honna k,et al. Appl. Catal, 1993, 105:69-75.

4. Susanne Stemmer, Pirouz Pirouz, Yuichi Ikuhara et al.Phy. Rev. Lett.,1996,77(9):1797-1800.

 


Changyou GUO (Fushun, China), Zhiqi SHEN, Shaojun WANG, Fengxiang LING
08:00 - 18:15 #4557 - MS01-534 Influence of Hydrogenation on Microstructure of Ti3Al Alloy.
MS01-534 Influence of Hydrogenation on Microstructure of Ti3Al Alloy.

The hydrogenation technology with different parameters of Ti3Al alloy was studied in this paper, and the effect of hydrogen on microstructure of Ti3Al alloy was analyzied. The results showed that the higher the hydrogenation temperature and the larger specific surface area of sample, the hydrogen content of Ti3Al alloy increased. With hydrogen content increasing, the proportion of α2 phase diminishes greatly and disappears finally, B2 phase laths become narrow and the O phase laths become wider. When the hydrogen content reached 0.3wt%, the plate-like hydrides, ε phase, precipitated from O phase, and with increasing hydrogen content the plate-like hydrides grew and the dislocation density inreased. The orientation relationship between ε phase and O phase possibly exists as following : [111][212]Oε,(101)(223)Oε. The results of hot compression test showed that the reduction of peak stress of Ti3Al alloy could be attributed to the hydrogen. In addition, the hydrogen may accelerate the dynamic recrystallization and dynamic recovery process of Ti3Al alloy during the deformation.


Qing WANG (HARBIN, China), Qiongyang ZHAO, Yueqi SUN, Dongli SUN
08:00 - 18:15 #4590 - MS01-536 The study on interface structure of SiC/Ti joint diffusion bonded under pulse electric field.
MS01-536 The study on interface structure of SiC/Ti joint diffusion bonded under pulse electric field.

The pulse electric field applied during the diffusion bonding process can impact the diffusion of the atoms and defects in materials and improve the structure and properties. The diffusion bonding process, interface structure and mechanical properties of SiC/Ti joint bonded under pulse electric field have been studied. The results show that the interface reaction layer thickness increases with increasing the pulse amplitude and the duty ratio, whereas it is effected barely by the pulse frequency. The shear strength of SiC/Ti joint increases firstly and then decreases with increasing the pulse amplitude, duty ratio, pulse frequency, that is, there is an optimum value of shear strength. At low pulse frequency, a small amount of TiC phase formed can reduce the residual stress at the interface between SiC and Ti. When the pulse frequency or amplitude is too high, some large TiC particles were produced near the interface, and they induced the micro-cracks at the interface, so the joint performance was deteriorative. The TEM analysis of SiC/Ti diffusion bonding interface reveals that from SiC to Ti side, the interface structure is SiC/fine grain region(phase structure unknown)/ (TiC + Ti5Si3) / TiC / Ti.


Dongli SUN (Harbin, China)
08:00 - 18:15 #4600 - MS01-538 Ferromagnetic Nanocrystalline Coatings over Steel trough Laser Cladding of Fe and Ni-based Glass Former Alloys.
MS01-538 Ferromagnetic Nanocrystalline Coatings over Steel trough Laser Cladding of Fe and Ni-based Glass Former Alloys.

Fe-based bulk glassy alloy properties indicates that coatings can represent good applications opportunities for metallic glasses. The Fe72Nb4Si10B14 (at%) bulk metallic glassy (BMG) alloy in order to produce coatings over AISI 1020 mild steel substrate using spray forming and laser cladding processing routes. For the spray forming process an additional Ni brazing alloy was applied on the substrate before the deposition to improve adhesion. For the laser cladding process, different laser parameters, power (W) and scanning speed (mm/s), were tested, using a Yb fiber laser (up to 2kW), to verified the best condition for to obtained the coatings [1]. The coatings was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DCS). The Fe72Nb4Si10B14 and Ni59Nb35Sn6 (at%) coatings obtained by spray forming presented high fraction of amorphous phase when produced with thickness up to 1 mm), some porosity and low oxidation level. XRD analysis showed Fe-α, Fe23B6 and halo of amorphous phase, depending on thickness. In addition, these coatings presenting partial or complete glassy structure with high hardness around 1150 HV. The amorphous overspray powder of the BMG alloy obtained by spray forming, grain size < 45 µm, was used in order to produce coatings on AISI 1020 mild steel substrate by laser cladding of the pre-placed powder. SEM micrographs of the clad tracks showed that at 200 W and 100 mm/s, no crystalline phases were observed, indicating maintenance of glassy phase due to high glass forming ability (GFA) of this alloy. XRD analysis showed only halo of amorphous phases in this condition. Moreover, for higher powers and same scanning speed, the diffractograms showed halo of amorphous phases and the Fe-α and FeNbB crystalline phases. The coatings showed hardness ranging from 340-1180HV, depending on laser parameters. These results suggesting that processing routes are promising to fabricate coatings for industrial applications.


Conrado Ramos Moreira AFONSO (São Carlos - SP, Brazil), Marcos Fernandes DE CARVALHO, Fausto Lopes CATTO, Walter José BOTTA FILHO, Claudio Shyinti KIMINAMI
08:00 - 18:15 #4639 - MS01-540 Microstructural evolution and thermal stability of nitride-based metal/semiconductor superlattices for thermoelectric and hard-coating applications.
MS01-540 Microstructural evolution and thermal stability of nitride-based metal/semiconductor superlattices for thermoelectric and hard-coating applications.

A detailed analysis on the quality and microstructure of various metal/semiconductor superlattices employing HR(S)/TEM (high-resolution (scanning)/transmission electron microscopy) imaging and energy dispersive x-ray spectroscopy (EDX) mapping on as-deposited and annealed samples is presented.

Epitaxial metal/semiconductor superlattices are known to be promising candidates for compounds in electronic, photonic, and plasmonic devices, but are also of interest for applications as hard coatings, and in thermoelectric materials [1]. The crystalline quality of the superlattices, in terms of their defect density, phase purity, interface roughness, and stoichiometry of the individual layers, plays a crucial role with respect to the physical properties and thus the applicability of such superlattice stacks. It was recently shown that metal/semiconductor superlattices based on (Al,Sc)N as the semiconductor component can be grown epitaxially with low-defect densities by magnetron sputtering on [001]MgO substrates [2].

Phase formation and thermal stability studies of as-deposited and long-time annealed cubic TiN/(Al,Sc)N superlattices employing a combination of HR(S)/TEM and EDX mapping revealed intermixing of the TiN and (Al,Sc)N layers by interdiffusion of the metal atoms with increased annealing time [3].

Improved (Ti,W)N/(Al,Sc)N [4] and (Hf,Zr)N/ScN [5] superlattices were grown by magnetron sputtering and analyzed with various TEM methods, and their microstructural evolution as well as thermal stability becomes presented here. An example is show in Figure 1, which shows an overview of an improved cubic (Ti,W)N/(Al,Sc)N superlattice stack in cross-section STEM (a), and a typical HRTEM micrograph of the metal/semiconductor interface region, demonstrating the high epitaxial quality of the growth [4].

Figure 2 demonstrates the superior thermal stability of the (Zr,Hf)N- based systems as compared to previous TiN- based superlattices. EDX mapping at high-resolution before and after annealing at 950 °C for 120 hours reveals diffusion of the metal atoms in the TiN/AlScN system (b), while the Hf0.5Zr0.5N/ScN superlattice stays intact (d).

All experiments were conducted at Linköping’s image- and probe-corrected and monochromated FEI Titan3 60-300 microscope equipped with a Gatan Quantum ERS GIF, high-brightness XFEG source, and Super-X EDX detector, operated at 300 kV [6].

 

References:

 

1. T. D. Sands, C.J. Palmstrøm, J.P. Harbison, V.G. Keramidas, N. Tabatabaie, T.L. Cheeks, Y. Silberberg, Stable and epitaxial Metal/III-V semiconductor heterostructures, Mater. Sci. Rep. 5: 98–170, 1990.

2. B. Saha, S. Saber, G.V. Naik, A. Boltasseva, E.A. Stach, E.P. Kvam, T.D. Sands, Development of epitaxial AlxSc1-xN for artificially structured metal/semiconductor superlattice metamaterials, Phys. Status Solidi B, 252, 2, 251-259, 2015.

3. J. L. Schroeder, B. Saha, M. Garbrecht, N. Schell, T. D. Sands, and J. Birch, Thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/semiconductor superlattices, J. of Mater. Sci., 50: 3200-3206, (2015).

4. B. Saha, Y. R. Koh, J. Comparan,  S. Sadasivam, J. L. Schroeder, M.  Garbrecht, J. Birch, D. Cahill, T. Fisher, A. Shakouri, and T. D. Sands, Cross-plane thermal conductivity of (Ti,W)N/(Al,Sc)N metal/semiconductor superlattices, Phys. Rev. B, 93, 045311, 2016.

5. M. Garbrecht, J. L. Schroeder,L. Hultman, J. Birch, T. D. Sands, and B. Saha, Microstructural evolution and thermal stability of ZrxHf1-xN/ScN (x = 0, 0.5, 1) metal/semiconductor superlattices. submitted, 2016.

6. We acknowledge the Knut and Alice Wallenberg (KAW) Foundation for the Electron Microscope Laboratory in Linköping.


Magnus GARBRECHT (Linköping, Sweden), Jeremy L. SCHROEDER, Lars HULTMAN, Jens BIRCH, Timothy D. SANDS, Bivas SAHA
08:00 - 18:15 #4866 - MS01-542 Diamond-hexagonal silicon ribbons in silicon fins.
MS01-542 Diamond-hexagonal silicon ribbons in silicon fins.

The formation of diamond-hexagonal silicon [1-3] or Ge [4] is recently reported under different experimental conditions.  The dh-phase has potential for application in optoelectronic devices.  A transformation of diamond-cubic (dc) to diamond-hexagonal (dh) Si can occur during the wet oxidation treatment applied to densify the oxide fillings between silicon fins in nano-electronic devices [3]. The phase change is induced by the compressive stress due to the expanding oxide. A similar transformation is reported in Ge nanowires [4] under non-oxidizing conditions and related to tensile stress of the shrinking oxide filling. 

In the finfet structures, the transformation occurs at the base of the outer fins where the lateral stress is the largest and unbalanced between the wide and narrow oxide spacings (Fig. 1).  The outer fin moves outward and typically a step and bulge are formed at inner and outer side respectively.  In about half of the fins a thin dh-Si ribbon is formed across the full or partial width of the fin.  In the other cases only steps/bulges are present with defect-free dc-silicon in between.

To increase the volume of the dh-Si, in this work the oxidation time is doubled compared to the conditions in [3].  The phase transformation is investigated by high resolution HAADF-STEM at 120 kV in order to minimize the beam damage during the observations.  The longer treatment results in larger steps/bulges (Fig. 1b vs 1a). They are also formed for fins with larger spacings which are not modified in case of the standard oxidation time.  The average thickness of the dh-Si in the outer fins increases by a factor 2-3 while the transformed material in the outer fins generally becomes a mixture of several Si-polytypes (Fig. 2b).  Additionally the transformation also occurs in the second outer fins with thinner transformed slabs that are pure dh-Si phase (Fig. 2a).  The dh-Si is epitaxial to the silicon substrate with its c-axis horizontally across the fins i.e. (110)dc//(0001)dh and [-110]dc//[2-1-10]dhBoth (001)dc-Si and (115)dc-Si interfaces are present which are characterized by stepped (Fig. 3a) and flat (Fig. 3b) interfaces.  In the latter case the dh-lattice is ~4º rotated. The doubling of the oxidation time does not result in further consumption of the silicon on the fin sidewalls (Fig. 1), i.e. the oxidation rate is reduced by the stress which is therefore also not further increasing. The continued transformation of dc to dh-phase during the prolonged oxidation is therefore a time related phenomenon and indicates a relatively slow process.  Stress-retarded oxidation of Si sidewalls is previously reported in [5].  Although dh-Si is a metastable phase, once present, it remains stable during subsequent high temperature treatments even up to 1050ºC.  As the dh-Si is situated at the base of the fins it does not affect the transistor structures.

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Yang QIU, Hugo BENDER (Leuven, Belgium), Els VAN BESIEN, Min-Soo KIM, Olivier RICHARD, Wilfried VANDERVORST
08:00 - 18:15 #5004 - MS01-544 Transmission electron microscopy investigation of Mg-Zn alloy processed by severe plastic deformation.
MS01-544 Transmission electron microscopy investigation of Mg-Zn alloy processed by severe plastic deformation.

Magnesium alloys possess great potential owing to advantageous properties in various fields such as structural applications, electronic devices or hydrogen and thermal energy storage. Use of Mg alloys for biomedical implants also attracted enormous attention recently. Properties of Mg and its alloys can be significantly improved via design of alloys with precipitation hardening effect and grain refinement using severe plastic deformation (SPD) techniques. Mg-Zn based alloys are among the most important Mg alloys and have been investigated for more than a hundred years. With a most recent development of various SPD techniques, there is a number of processing routes which may significantly improve the alloys properties. However, fundamental knowledge about an impact of SPD on microstructure of these alloys at nanoscale is insufficient or missing in many cases.
In this work, deformation behaviour of an α-Mg matrix and Mg-Zn intermetallic particles in the Mg-12wt.% Zn alloy subjected to equal-channel angular pressing with applied back pressure (ECAP-BP) is characterized using transmission electron microscopy (TEM) techniques at nanoscale. Magnesium with 99.9% purity and an appropriate amount of high-grade Zinc were melted in a graphite crucible under an Ar atmosphere. The subsequent thermal treatment consisted of annealing at 320°C for 20 hours and quenching into warm water. The material was then processed by ECAP (4 passes via Bc route) with applied BP of ~400 MPa to prevent cracking during processing. The processing temperature was gradually decreasing from 200 °C to 185 °C, 170 °C and 155 °C for 1st, 2nd, 3rd and 4th ECAP-BP pass, respectively, to obtain ultra-fine grained structure.
Following TEM techniques are employed for micro- and nano- structural analysis: Bright-field (BF) imaging; selected area electron diffraction (SAED); high-resolution transmission electron microscopy (HRTEM); high-angle annular dark field - scanning transmission electron microscopy (HAADF-STEM) imaging; and electron energy-loss spectroscopy (EELS). All techniques were performed using an FEI Tecnai TF20 X-twin, which operated at 200 kV.

Acknowledgements

Financial support offered by CVUT SGS16/151/OHK3/2T/13 and GACR GBP108/12/G043 is greatly appreciated.


Martin NĚMEC (Prague 6, Czech Republic), Viera GÄRTNEROVÁ
08:00 - 18:15 #5020 - MS01-546 HRTEM investigation of dislocation/hydrogen interaction mechanisms in hydrided nanocrystalline palladium films.
MS01-546 HRTEM investigation of dislocation/hydrogen interaction mechanisms in hydrided nanocrystalline palladium films.

Thin Pd membranes constitute an enabling material in hydrogen permeation and sensing applications. During hydriding of Pd, as long as the H/Pd (atomic ratio) stays below αSSmax≈0.02, the α-Pd with face centered cubic (fcc) lattice will expand from 3.889 Å to 3.895 Å. When the ratio reaches 0.02 a β-phase, again fcc based, having a lattice constant near 4.025 Å appears. The initial volume of the Pd structure thus expands by about 10% due to α→β phase transformation which induces a large plastic deformation within the material. In the present study, we have performed detailed HRTEM characterizations of defect/hydrogen interactions on nc Pd thin films hydrided at low and high pressures for α-phase and β-phase transformations, respectively. The in-situ measurement of the evolution of the internal stress during hydriding of the nc Pd films shows that this internal stress increases rapidly in the compressive direction, and gradually reaches a constant value of 120 MPa tensile stress for α-phase transformation and 920 MPa compressive stress for β-phase transformation which affect the microstructure of the Pd film.

Figures 1a and 1b show HRTEM images of ∑3 {111} coherent twin boundaries (TBs) in Pd films before and after hydriding to α-phase, respectively. In contrast with Pd films hydrated to β-phase (see below), intrinsic or extrinsic stacking faults (SFs), dissociation of incoherent TB to form 9R and distortion of CTBs have not been observed in Pd films after hydriding to α- phase (Figure 1c). Surprisingly, an fcc→9R phase transformation at Σ3 {112} incoherent TBs as well as a high density of SFs (Figure 2a) have been observed after hydriding to β-phase indicating a clear effect of hydrogen on the stacking fault energy (SFE) of Pd. Ab-initio calculations of the effect of hydrogen on the intrinsic stable and unstable SFEs of Pd confirm the experimental observations. The experimental results confirm that hydrogen induced plasticity is mainly controlled by dislocation activity at higher hydrogen pressures. Shear type faulted loops rarely reported in nc materials were also observed within the Pd grains after hydriding to β-phase (Figure 2b). In order to investigate the stability of this shear type loops, different internal stress fields originating from the neighboring dislocation (dislocation "d3") and surface effects (image forces) have been computed using a Finite Element method (Figure 2c). Such calculations confirm that high attractive forces exist between the dislocation “d2” and “d3” forming the dipole. On the other hand, although the Peach Koehler force on the dislocation “d1” tends to extend the SF, the force magnitude is much smaller than the force induced by the fault on the partial segments. Therefore, an extra shear stress of +385MPa (τdis.) acting on the glide plane of the dislocation “d1” is required in order to counter balance the attractive force of the SF which thus explains the stability of this dislocation in the TEM thin foil after dehydriding. This shear stress can not be compensated by the negligible image force in such a thin foil. Moreover, no residual hydrides were detected using high resolution EELS. Therefore, the stability of glissile intrinsic SF loops in nc Pd films after dehydriding can thus be attributed to the presence of large internal stress heterogeneities typical of nc materials. Since the 9R phase and SFs are unexpected at high SFE Pd and considered as unstable phases, the stability of these defects was also investigated using in-situ HRTEM heating experiments at different temperatures and the critical temperature for removing these unstable SFs in the hydrided Pd film was determined.


Behnam AMINAHMADI, Gunnar LUMBEECK, Hosni IDRISSI, Renaud DELMELLE, Marc FIVEL, Thomas PARDOEN, Joris PROOST, Dominique SCHRYVERS (Antwerpen, Belgium)
08:00 - 18:15 #5166 - MS01-548 SEM-TEM study of low carbon steel subjected to conventional and severe plastic deformation.
MS01-548 SEM-TEM study of low carbon steel subjected to conventional and severe plastic deformation.

It is well known that plastic deformation induced by different conventional metal forming methods can significantly increase the strength of a metal. This increase of strength, at the same time, is always accompanied by a loss of ductility. However, a better combination of high strength and ductility can be obtained by severe plastic deformation (SPD) [1].

In the present work, both methods of plastic deformation are concerned. One is the Rastegaev upsetting method (conventional method) where cylindrical samples of Ø20x20 mm were compressed between parallel plates with imposed plastic strain  φR=0.4; 0.8; 1.2; 1.6; 1.91 (~2.0). The other is the discontinuous SPD method, which our lab developed, for upsetting square shaped billet by V-shape dies. In fact, the V-shape dies compression is a multistage process in which the sample is removed from the die after the first compression turn, rotated for 90° in an anti-clockwise direction, and then is returned into the dies [2]. The samples of 14x14x70 mm were compressed in eighteen turns with imposed strain from φSPD1=0.39 up to φSPD18=3.38. Maximum plastic strain of the conventional upsetting method (φR=1.91) is achieved after 4 turns in case of a discontinuous SPD method (φSPD4=2). The experimental compression tests for both of the methods were done on a hydraulic press using a normalized low carbon rod steel C15E with 0.14%C. The influence of the processing parameters on the microstructure was evaluated by SEM (JEOL JSM-6460LV) and by TEM (FEI Tecnai F20). The TEM sample (cross-sections) were prepared by FIB (Quanta 3D FEG) and in situ lift-out technique.

Fig. 1 shows the starting SEM microstructure of the undeformed low carbon steel C15E in a longitudinal and transversal direction. The microstructure consists of 85% ferrite and 15% pearlite. The starting average grain size of the ferrite was 19.1 µm. After upsetting by the Rastegaev method the microstructure is highly deformed in the direction normal to the applied compression, Fig. 2a. The deformation is a homogeneous trough the sample. On the other hand, during SPD the microstructure is deformed only in the central part of the sample (Fig. 2b), while in the region further from the center (about 6 mm) the deformation appearance of the microstructure is hardly visible. Furthermore, while the effective plastic strain is similar for both upsetting methods (Rastegaev φR=1.91 and SPD φSPD4=2) the microstructure of the SPD samples appears less deformed when compared to the Rastegaev sample. However, the TEM observation of the SPD samples reveals that the microstructure consists of elongated ferrite sub-grains having a width 0.2-0.3 µm, Fig. 3. The band boundaries are mostly in the low-angle misorientation and in the bands an interior dislocation cell boundaries are present. Additionally, beside a low-angle boundaries, the formation of grains with the high-angle boundaries could be noticed (indicated with arrows in Fig. 3). The presence of the newly formed equiaxed grains is confirmed by the diffraction ring pattern.

References:

[1] R. Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mat. Sci., 45, (2000), 103-189

[2] M. Vilotic, Doctoral thesis, University of Novi Sad, (2015)


Marko VILOTIC, Leposava SIDJANIN, Dragan RAJNOVIC (Novi Sad, Serbia), Milorad NOVOVIC
08:00 - 18:15 #5169 - MS01-550 Microstructure and fracture mode of ballistic perforated plates made of unalloyed ADI material.
MS01-550 Microstructure and fracture mode of ballistic perforated plates made of unalloyed ADI material.

Austempered Ductile Iron (ADI) is an advance engineering material produced from ductile cast iron by austempering heat treatment, through which a unique microstructure - ausferrite is obtained. Consequently, an excellent combination of mechanical properties could be achieved [1]. In this paper, ADI material has been evaluated as an alternative to steel for perforated plates applied in the ballistic protection of military vehicles [2].

The ADI materials were produced from an unalloyed ductile iron by austenitization at 900°C/2h, followed by 1 hour austempering at 275°C (ADI-275) or 400°C (ADI 400). The microstructure was observed on Leitz Orthoplan light microscope while fracture mode was studied by SEM JEOL JSM-6460LV, 20 kV equipped with EDS Oxford Instruments INCA system. To evaluate ballistic properties, perforated plates of the ADI materials, having thicknesses of 7 and 9 mm, were mounted in front of basic armour and 12.7x99 mm M8 API ammunition was fired from 100 m.

The microstructure of the ADI-275 and ADI-400 was fully ausferritic consisting of a mixture of ausferritic ferrite and retained austenite (9.8 and 26%, respectively). The ADI‑275 has an acicular morphology of ausferrite, while the ADI-400 had a more plate like morphology. Due to difference in ausferrite morphology and retained austenite amount the ADI-275 posses higher strength while ADI-400 is more ductile. After impact, intensive cracking occurs near the impact point, Fig. 1a and 2a. The size of the fractured fragments is considerably larger in the less ductile ADI-275, while ADI-400 is more plastically deformed. The microstructure after impact is shown in Fig. 1b and 2b. In ADI-400 martensite occurs in the area of the intensive plastic deformation, Fig. 2b. The martensite is formed through SITRAM effect (Strain Induced Transformation) [3]. In contrast, the SITRAM does not occur in ADI-275, where austenite carbon enrichment is higher, Fig 1b. The ADI-275 have a typical ductile fracture, with the dimpled surface covered with a layer in the form of drops, Fig. 1c. However, in ADI-400 a mixed fracture mode can be observed, a ductile fracture near the nodules and in other areas brittle, quasi – cleavage fracture, Fig. 2c. This brittle behaviour of ADI-400 is the result of the presence of martensite. The results of debris EDX analysis reveal a mixture of Cu, Ba, Mg and Al, i.e. material of the projectile jacket and products of the IM-11 incendiary mixture, Fig. 3.

Perforated plate made of the ADI-275, with a higher hardness and a lower ductility, were proved to be superior to the softer and more ductile ADI-400. The ballistic testing causes a SITRAM effect to occur in the ADI-400, causing a partial brittle fracture and thus lowering ballistic performance [4]. The perforated plates made of the ADI material have a larger damaged area, a lower cost of fabrication and a similar mass effectiveness than steel perforated plates [2, 4].

Acknowledgments:

The authors gratefully acknowledge research funding from the Ministry of Education, Science and Technological Development of the Republic of Serbia under grant number TR34015.

References:

[1] L. Sidjanin, D. Rajnovic, O. Eric, R.E. Smallman, Mater Sci Tech-Lond, 26/5, (2010), 567-571

[2] S. Balos, V. Grabulov, L. Sidjanin, M. Pantic, I. Radisavljevic, Mater Design, 31, (2010), 2916-2924

[3] J.L. Garin, R.L. Mannheim, J. Mater. Process. Tech, 143-144, (2003), 347–351

[4] S. Balos, I. Radisavljevic, D. Rajnovic, M. Dramicanin, S. Tabakovic, O. Eric-Cekic, L. Sidjanin, Mater Design, 83, (2015), 66-74


Dragan RAJNOVIC (Novi Sad, Serbia), Sebastian BALOS, Petar JANJATOVIC, Miroslav DRAMICANIN, Danka LABUS ZLATANOVIC, Olivera ERIC CEKIC, Leposava SIDJANIN
08:00 - 18:15 #5180 - MS01-552 The microstructure influence on abrasive wear behaviour of ductile irons.
MS01-552 The microstructure influence on abrasive wear behaviour of ductile irons.

In this paper, the wear rate of ferritic and pearlitic ductile iron, as well as three unalloyed ADI (Austempered Ductile Iron) materials with different ausferritic microstructure morphologies, was studied. Due to excellent combination of properties, the ductile irons and the ADIs are used for a number of applications, some of them relating to equipment exposed to abrasive wear in mining and agricultural industries.

The metal matrix microstructure of un-alloyed ductile iron (DI-F) was fully ferritic (Fig. 1a), while alloyed ductile iron (DI-P), as result of alloying with of Cu and Ni, had fully pearlitic microstructure (Fig. 1b). The ADI materials were obtained by austempering of unalloyed ductile iron at 300, 350 or 400°C for 1 hour (ADI-300, ADI-350, ADI-400, respectively). The microstructure of all ADI materials was fully ausferritic with 16, 24.9 and 31.4% of retained austenite. The ausferrite morphology changes from needle-like (acicular) at lower austempering temperatures to a more plate-like (feathery) at higher temperatures. Furthermore, due to microstructure the hardness of ADI-300 was highest, while ADI-400 was lowest. In order to determine an abrasive wear behaviour, the pin on disc wear tests were performed by SiC grinding paper with grit size P240, P500 and P800, and under 0.5, 1.3 and 2 kg loads. For microstructural characterization a “Leitz-Orthoplan” metallographic microscope was used.

After the wear test, in all cases, larger or smaller degree of plastic flow and tongue formation i.e. metal overlapping nodules is observed, Fig. 1c. As graphite nodules are very soft, they are easily covered by plastically deform material. The microstructure of ADI-300 and ADI-350 has not change after wear test, Fig 2a,b. In the case of the ADI-400 after wear testing at 2 kg loading and P240 grit paper a martensite is present in the microstructure, Fig 2c. The martensite was formed as a result of local pressure induced by the coarsest grit abrasive particles and maximal loading through stress assisted phase transformation (SATRAM) [2]. The results of wear rate represented by average weight loss of different material tested, as a function of grinding paper grit and loading are shown in Fig. 3a-c. The highest wear rate was obtained with the ferrite ductile iron (DI-F), while the lowest was for hardest ADI-300. However, ADI-400, in case when martensite form, exhibits better wear resistance.

It was found that the wear resistance primarily depends on materials’ microstructure, corresponding hardness and transformation during wear. In case of ADI materials, the SATRAM phenomenon play a major role in wear behaviour. However, the SATRAM phenomenon occurs only if appropriate conditions are fulfilled, namely: presence of metastable, low carbon-enriched, retained austenite; and local pressure on the metal matrix is sufficient, i.e. the SATRAM was detected only for ADI-400 at loads of 1.3 and 2 kg and at the coarsest abrasive grain size (P240). As a consequence, the wear rate of ADI austmpered at 400°C (the softest ADI tested) is equivalent to ADI austempered at 300°C (the hardest) [3].

Acknowledgment:

The authors gratefully acknowledge research funding from The Ministry of Education, Science and Technological Development of The Republic of Serbia under grant number TR34015.    

References:

[1] L. Sidjanin, D. Rajnovic, O. Eric and R. E. Smallman, Mater. Sci. Tech.-Lond., 26/5, (2010), 567-571

[2] C.Z. Wu, Y.J. Chen and T.S. Shih, Mater. Charact. 48, (2002) 43–54

[3] S. Balos, D. Rajnovic, M. Dramicanin, D. Labus, O. Eric-Cekic, J. Grbovic-Novakovic, L. Sidjanin, International Journal of Cast Metals Research, published online, doi: 10.1080/13640461.2015.1125982


Miroslav DRAMICANIN, Sebastian BALOS, Dragan RAJNOVIC (Novi Sad, Serbia), Danka LABUS ZLATANOVIC, Petar JANJATOVIC, Olivera ERIC CEKIC, Leposava SIDJANIN
08:00 - 18:15 #5182 - MS01-554 Superstructures In The Scheelite-Type Rare Earth Doped Tungstate Phases.
MS01-554 Superstructures In The Scheelite-Type Rare Earth Doped Tungstate Phases.

Introduction

The aim of the present work deals with the TEM study of scheelite related AWO4 compounds [1] showing modulated microstructures not revealed with synchrotron XRD investigations. These materials have potential applications in many fields such as photoluminescence, microwave, scintillator materials, humidity sensors and catalysis. Rare earth (RE) tungstate RE2(WO4)3 crystal phases are based on a cation-deficient superstructure of CaWO4. Cation substitution of RE are investigated in order to correlate the microstructure to physical properties especially for new potential applications in white-LEDs and lasers. In our study, a two-phase powder containing the monoclinic Ce2(WO4)3 and orthorhombic Ce10W22O81 structures [2], was obtained by complexing method using EDTA and citrate ions. In the latter phase, PED and HRTEM investigations showed a C2/c superspace group (SSG) doubling the smallest a cell parameter [3]. Additionally, another cerium tungstate structure substituted with strontium cations was also investigated revealing a (3+2)D incommensurately modulated structure.

 

Results

Ce10W22O81: polymorphism

ED patterns of highest symmetry (Fig. 1) showed reflection positions paired by a two-fold axis and indexed in the Pbnm SG (a = 3.6, b = 36 and c = 22.2 Å). A superposition of rows of weak reflections along [010] and present in the Fourier transforms obtained from HRTEM images, led to a doubling of the cell parameter a. With PED acquisitions, the superstructure reflections appeared linked together by an inversion center and were related to a C2/c SSG (Fig. 2). The structure was also identified from FFT analyzes of experimental HRTEM projections. The amplitude and phase error values were calculated using the symmetry rules of each plane group. The reconstructed density maps, after imposing the appropriate symmetry projections were used to extract reduced atomic coordinates of Ce atoms, WO5, WO6 and WO7 groups (Fig. 3). For similar structures, weak satellite reflections, were attributed to partial oxidation of RE elements, short-range order among the cations and slight changes in the oxygen positions [4-5]. Synthesized in air, the oxidization of Ce3+ precursor into Ce4+ limited to the grain surface, explains the contribution of both structures to the ED patterns.

Sr2+ substitution in Ce2(WO4)3: (3+2)D incommensurately modulated structure

ED patterns of Ce0.25Sr0.25WO4 along main zone axes were indexed in the scheelite tetragonal I41/a unit cell (as = bs = 5.44 Å and cs = 11.88 Å ≈ 2as). However, lower intensity reflections along [001] (Fig. 4), indicating a partially ordered distribution of vacancies and cations, needed the use of a supercell with 5 digit indices hk0mn corresponding to the diffraction wave vector H = ha* + kb* + lc* + mq1 + nq2, q1 ≈ 0.57a* + 0.80 b* and q2 ≈ -0.80a* + 0.57b* [6].

 

References

[1] Taoufyq, A.; Patout, L.; Guinneton, F.; Benlhachemi, A.; Bakiz, B.; Villain, S.; Lyoussi, A.; Nolibe, G. &

Gavarri J.-R. (2015). Journ. Of Mic. 00, 1-13

[2] Barker, R.S. & Radosavljevic Evans, I. (2008). Acta Cryst. B64, 708-712

[3] Patout, L.; Jacob, D. ; Arab, M. ; Pereira de Souza, C., & Leroux C. (2014). Acta Cryst., B70, 268–274

[4] Thompson, J. G.; Rae, A. D.; Bliznyuk, N. & Withers, R. L. (1999). J. Solid State Chem. 144,

240–246

[5] Marinder, B.-O. & Sundberg, M. (1984). Acta Cryst. C40, 1303–1306

[6] Arakcheeva, A. & Chapuis, G. (2008). Acta Cryst. B64, 12-25.


Loïc PATOUT (Marseille Cedex 20), Abdelali HALLAOUI, Aziz TAOUFYQ, Christian DOMINICI, Andrea PORTO CARREIRO CAMPOS, Claude ALFONSO, Ahmed CHARAI
08:00 - 18:15 #5216 - MS01-556 Study of strain fields induced by welding in nickel alloy 600 using in situ mechanical tensile test approach and standard EBSD.
MS01-556 Study of strain fields induced by welding in nickel alloy 600 using in situ mechanical tensile test approach and standard EBSD.

To evaluate the susceptibility to stress corrosion cracking (SCC) of stainless steel and nickel based alloy components, it is important to know the degree of plastic strain. Indeed, SCC is enhanced by the strain hardening induced by manufacturing and welding. We evaluate the residual plastic strain in a thick tube made of Ni-based alloy 600, generated by the welding operation with alloy 182 deposited metal. By using these data we can predict the crack growth rate and improve structural integrity assessments of components.

 

Hardness measurements or numerical stress analysis are usually applied to approximate plastic strain. Here, we use an innovative method by correlating deformation with mechanical characteristics from Electron Backscattered Diffraction (EBSD) [1] acquired during in-situ tensile test. We use a reference sample to calibrate and quantify the equivalent plastic deformation as a function of the average macroscopic deformation. Then, measurements are done on a sample after a welding operation and compared to the calibration data. The reference sample and the mock-up were manufactured with the same alloy 600. Tensile test specimen was prepared using Electrical Discharge Machining (EDM) and mechanical polishing to obtain an EBSD-quality finish. Micro-lithography was done to deposit markers on the surface.

In-situ experiment was done using a Tescan Mira 3 Scanning Electron Microscope (SEM) and a Micromecha tensile test machine allowing uniaxial loading at EBSD position.

Regarding [2], a preliminary work was done to optimize acquisition parameters and to check the statistical representativeness of the data (500 x 500 µm areas, step size = 250 nm). Data treatment consists in calculating for each point of the EBSD map the Kernel Average Misorientation (KAM). KAM gives local information about the plastic deformation [3]. In order to make the data quantitative, the KAM distribution is plotted. The shape of the distribution is very sensitive to the microstructure.

Results of the in-situ mechanical test are presented in figure 1. We found a logical and monotonic evolution of the misorientation distribution: the average, the position of the maximum and the width of the distribution are going up with the increase of the deformation (measured using markers), in accordance with [1]. We use those data to calibrate equivalent plastic deformation for the mock-up.

 

Fifteen measurements were performed on a weld mock-up with exactly the same SEM and EBSD acquisition parameters as used for the calibration experiment [2]. Location of the measurements were chosen in order to draw vertical and horizontal profiles regarding the welding interface.

We found least square fit method with a degree of freedom on the abscissa (up to 0,04°) as the best way to compare data from the mock-up and from the calibrated distributions. An example of a fit is given in figure 2. A map of the sample with the deduced values of equivalent plastic deformation is presented in figure 3.

 

From a material point of view, results show low levels of equivalent deformation. The maximum is 6,8%. Logical and monotonic decrease is observed going away from the front of the heat affected zone (HAZ, defined by the limit between large and small grains), the equivalent strain is 0 at 3 mm from the HAZ. Areas 4 mm away from the HAZ show an equivalent strain lower than the tensile test specimen in its initial state. This may be due to stress relief during heating by the welding operation, a problem of representativeness of the uniaxial tensile test compared to the welding operation or unexpected deformation of the reference test piece.

 

 

[1] Kamaya M. et al. (2015) Nucl. Eng. Des. 235, 173.

[2] Wright S. et al. (2011) Microsc. Microanal. 17, 316.

[3] Britton T. B. et al. (2010) Scripta Mater. 62, 639.


Julien STODOLNA (Moret sur Loing), Nicolas BRYNAERT, Thierry COUVANT
08:00 - 18:15 #5248 - MS01-558 Phase mapping of 2xxx-series aluminium alloys by scanning precession electron diffraction.
MS01-558 Phase mapping of 2xxx-series aluminium alloys by scanning precession electron diffraction.

2xxx-series Al alloys are Cu containing age-hardenable alloys, which are strengthened by numerous metastable precipitates formed during heat treatment. Many different precipitates exist, some of which are not well defined crystallographically. However, phases known to contribute particularly to strengthening and that exist in the over-aged condition are: θ’ and T1 in Al-Cu-Li alloys [1], and Ω and S in Al-Mg-Cu-Ag alloys [1]. These precipitates have various morphologies, ranging from long needles to thin plates, and coexist with inclusion particles as well as with dispersoids. The resulting microstructure is complex both in terms of coexistence and by precipitates deviating from simply defined phases. This makes complete characterization a demanding task for which techniques are required to enable statistical treatment of precipitate distributions in terms of their atomic structure. Here, we apply scanning precession electron diffraction (SPED) to heat-treated Al-Cu-Li and Al-Mg-Cu-Ag alloys, shedding light on the distribution of phases present and the complex interplay between these microstructural features.
    SPED involves scanning the electron beam across the specimen and recording a PED pattern at each point by rocking a focused probe in a hollow cone above the specimen, and de-rocking it below. In this way, integrated diffraction intensities are recorded in the geometry of a conventional electron diffraction pattern [2]. A 4D dataset is obtained comprising a 2D PED pattern at each position in the 2D scan region. Combined with subsequent data processing, this constitutes a powerful method for extracting valuable crystallographic information and orientation relationships in complex multiphase materials [3]. In this work, SPED was performed using a NanoMEGAS DigiSTAR scan generator fitted to a JEOL 2100F FEG-(S)TEM operated at 200 kV, with a precession angle of 1º and a step size of 4.5 nm. Typical datasets comprised 90 000 diffraction patterns (DPs), which were analysed using the open source platform HyperSpy [4] as described below.
    Obtained results from an Al-Cu-Li alloy are shown here as an example. All DPs in the SPED dataset were first summed (Fig. 1) and compared to a simulated DP, including the Al-matrix in the [001] orientation and the aforementioned θ’- and T1 precipitates (Fig. 2). This allowed identification of reflections associated with these particular phases. These phases are then visualised in ‘virtual dark-field’ (VDF) images, formed by plotting the intensity in pixels around selected reflections as a function of probe position (Fig. 3 and 4). For example, the thin T1-precipitate plates are seen on {111}Al planes inclined relative to [001]Al, and it is noted that even overlapping plates can still be discerned and visualized. The obtained VDF images exhibit a sharper, more consistent contrast between precipitate phases and the Al matrix as compared conventional imaging techniques, such as dark-field TEM. More sophisticated analysis applies machine learning in order to identify the main component patterns in the data, as well as their spatial localisation referred to as ‘loading maps’. These ‘loading maps’ look similar to VDF images but are obtained by an automated and objective approach, requiring little or no prior knowledge. This opens the possibility of identifying features with unexpected crystallographic structures. The analysis approaches demonstrated in this work offer important insight into the complex microstructures of these Al alloys.

[1] S.C. Wang and M. J. Starink, Int. Mater. Rev., 50:193-215, 2005. doi: 10.1179/174328005X14357
[2] R. Vincent and P.A. Midgley, Ultramicroscopy, 53:271-282, 1994. doi: 10.1016/0304-3991(94)90039-6
[3] P. Moeck et al, Cryst. Res. Technol., 46:589–606, 2011. doi: 10.1002/crat.201000676
[4] F. de la Peña et al, HyperSpy - 0.8.4, 2016. doi: 105231/zenodo.46897

RH and SW acknowledge funding from the Research Council of Norway 221714-FRINATEK. PAM and DNJ acknowledge the ERC 291522-3DIMAGE and an associateship from the Cambridge NanoDTC. The (S)TEM work was carried out on the NORTEM infrastructure at the TEM Gemini Centre, NTNU, Norway.


Jonas Kristoffer SUNDE (Trondheim, Norway), Sigurd WENNER, Antonius T.j. VAN HELVOORT, Duncan N. JOHNSTONE, Paul A. MIDGLEY, Randi HOLMESTAD
08:00 - 18:15 #5310 - MS01-562 Kinetics of precipitation in new generation of cobalt-based superalloys.
MS01-562 Kinetics of precipitation in new generation of cobalt-based superalloys.

Kinetics of precipitation in new generation of

cobalt-based superalloys

 

A. Azzam, T. Philippe, F. Danoix, A. Hauet, D. Blavette

Normandie Université, Groupe de Physique des Matériaux, UMR CNRS 6634,

Avenue de l’université BP 12, 76801 Saint Etienne du Rouvray, France

 

Keywords: Co-Al-W, Atom Probe Tomography, Transmission Electron Microscopy

Superalloys are key materials in aircraft engines and power generation systems. They are subjected to very high stress under temperatures in the range of 700-1000ᵒC. Currently Ni-based superalloys are the most widely used materials for high temperature applications. These alloys owe their excellent mechanical proprieties to their microstructure characterized by the presence of a high volume fraction (up to 70%) of thermodynamically stable,  coherent L12 ordered γ’ precipitates (Ni3Al or Ni3Nb type) embedded in a disordered fcc γ matrix. Compared to Ni based superalloys, conventional Co based alloys exhibit hot corrosion, oxidation and wear resistance but their applications are restricted to temperatures below 750ᵒC due to their instability at high temperatures.  In 2006 J. Sato et al. [1] discovered a new stable L12 ordered, Co3(Al,W) phase embedded in the disordered γ-Co solid-solution matrix. Mechanical properties (creep and plastic deformation, elastic property, structural stability) of Co-based alloy have been widely investigated but the precipitation process have been the subject of very few studies [2,3].

The aim of this work is to study precipitation kinetics in model CoAlW superalloys at the atomic scale. This work is focused on the kinetics transformation paths during precipitation. The temporal evolution of average size, volume fraction and number density of γ’ precipitates as well as that of phase composition has been studied as a function of aging time at 900°C employing three dimensional Atom Probe Tomography (APT). In addition Transmission and Scanning Electron Microscopy (TEM, SEM) have been used to complement APT studies. Different alloy compositions will be studied in this work, figures 1 and 2 show respectively bright field transmission electron micrographs of Co-9.7Al-10.8W (atomic %) alloy and dark field image of Co-9.1Al-7W alloy after annealing at 900°C for 10h. The microstructures reveal precipitates of cuboidal shape, respectively 100 nm (Fig 1) and 50 nm (Fig 2) in size, homogeneously distributed in the parent γ phase. The γ and γ'  phase compositions are major parameters controlling the properties of superalloys, that have been measured by APT. Figure 2 shows 3D reconstruction in the Co-7Al-9W aged at 900°C for 10h, showing γ' precipitates delineated by a 12 at.% W isoconcentration surface. Composition profile across γ/γ' interface reveals that W shows a very strong tendency to partition to the γ' phase unlike Al that exhibit a very weak tendency to partition between γ and γ' phases. The summation of Al content and W content in the γ’ phase is close to 25 atomic % that is to the expected stoichiometry of the γ' Co3(Al,W) ordered phase. It can this suggests that Al and W occupy preferentially the corner sites of the same sub-lattice in the ordered L12 structure. This presentation will come in more details on the temporal evolution of both the microstructure and phase composition.

References

[1] J. sato, T. Ohnuma, R. Kainuma, and K. Ishida, Science 2006, 312, p.90-91

[2] Peter J. Bocchini, Eric A. Lass, Kil-Won Moon, Maureen E. Williams, Carelyn E. Campbell, Ursula R. Kattner, David C. Dunand and David N. Seidman ., Scripta Materialia 2013, 68, p.563-566

[3] S. Meher, S. Nag, J. Tiley, A. Goel, Ultramicroscopy 2015, 148, p. 67-74


Ahmad AZZAM (Rouen), Didier BLAVETTE, Annie HAUET, Thomas PHILIPPE, Frederic DANOIX
08:00 - 18:15 #5330 - MS01-566 Interface formation between steels and alumina coatings in corrosive atmospheres.
MS01-566 Interface formation between steels and alumina coatings in corrosive atmospheres.

A way to increase application range and lifetime of metallic substrates is their protection by coatings. Here the effect of alumina coatings on common steel substrates having different Chromium content is examined. The background of the investigations is to expand the application possibilities of mechanically good steels to higher temperatures and corrosive atmospheres in the framework of the European project POEMA (“Production of Coatings for New Efficient and Clean Coal Power Plant Materials”) which was introduced to identify materials that can withstand the aggressive conditions arising during the oxyfuel process in modern coal power plants. This process is one possibility to reduce the CO2 emission by firing the coal in oxygen and recirculated flue gas.

Substrate materials were two steels of the POEMA project: P92 (9% Cr), and HR3C (25% Cr), and the steel K44X (19% Cr), not used in POEMA. It is interesting to examine the protection effect of the coating in this case, because this steel is developed to be used in air up to 1000° C. The alumina coatings were deposited by a sol-gel-process using boehmite, this is relatively simple and offers application possibilities for a wide technical range without special surface preparations. All samples were heat treated for 30 min at 650° C to remove the organic components and to start the crystallization process of the alumina. For P92 and HR3C, the following conditions simulated the oxyfuel process: 650° C in wet flue gas for 300 hrs for the steel P92, the same conditions for 2000 hrs for the steel HR3C. K44X was tested at 900° C in laboratory air for 500 hrs. After the first heat treatment of 30 min as well as after long-term testing, TEM characterizations were performed. The microstructure of the interfaces between steel and coating is of special interest to detect failure mechanisms, and identify diffusion and crystallization processes with the aim to understand the principle of action of the alumina coating.

The TEM samples were prepared by the lift-out-technique using a FIB Quanta 3D and were investigated in a STEM JEM2200FS at 200kV.

After the first heat treating process all steels show chromium oxide layers directly at the steel surface, they vary in thickness and are island-shaped in the case of the steel K44X. Chromium depletion arose more or less in the superficial zones of all samples. After the long-term tests, only P92 presents an intact interface, while K44X and HR3C showed breakaway oxidation. One can conclude: The positive impact of the coating and a high chromium content of a substrate are essential for the formation of protective chromium oxide layers. For long service times a renewal effect of the Cr-oxide layers is necessary, originated by the possibility of continuous Cr diffusion to the interface without forming Cr depletion. In this context the interaction with the other alloying elements of the particular steel is of importance.

 

This work was initiated and financially supported by the European project „Production of Coatings for New Efficient and Clean Coal Power Plant Materials“ (POEMA/FP7-NMP, 310436).


Ilona DÖRFEL (Berlin, Germany), Marianne NOFZ, Regine SOJREF, Maria MOSQUERA FEIJOO, Wencke SCHULZ, Nicole WOLLSCHLÄGER, Rene HESSE, Axel KRANZMANN
08:00 - 18:15 #5372 - MS01-570 Transmission electron microscopy of deformed Laves phase NbFe2.
MS01-570 Transmission electron microscopy of deformed Laves phase NbFe2.

Laves phases are the largest class of intermetallic phases, showing very high strength up to temperatures above 1000 °C, but being very brittle at room temperature. The mechanical behavior and the deformation mechanism of these phases is very much different from that of pure metals and is still not well understood [1, 2].

The unit layer of a Laves phase AB2 does not consist of only one plane but a slab of four planes, each of which is composed of either only A- or B-atoms. This package of four atomic planes can be sub-divided in one single B-atom layer and a triple layer of successive A-, B- and A-atom planes [3].

The present work summarizes results obtained from transmission electron microscopy on single-phase NbFe2 before and after compression tests at high temperatures. The material was produced by levitation melting and has hexagonal C14-type structure (hP12, P63mmc). The compressive stress-strain curves are characterized by a pronounced stress peak in the stress-strain curves at lower temperatures (up to 1200 °C) and by steady state flow at higher temperatures (above 1200 °C) [4].

Undeformed NbFe2 is almost free of dislocations. It is assumed that the lack of dislocations in the as-cast condition leads to the pronounced yield stress maximum observed during compression testing at lower deformation temperatures.

After deformation at 1200 °C new dislocations are introduced into the material. Widely-extended stacking faults on the basal plane dominate the microstructure. They are bounded either by partial dislocations or terminate at low angle grain boundaries (Fig. 1).

Material deformed at 1300 °C shows a high density of dislocations, which are split into pairs of partial dislocations that bound stacking faults on the basal plane (Fig. 2). The observation that most dislocations are split up into Shockley partial dislocations could indicate deformation by the synchroshear process [5], as the occurrence of such partial dislocations is essential for this mechanism. Synchroshear is based on the idea of a synchronous shear motion of two adjacent planes within the triple layer.

Dislocation networks (Fig. 3 and 4) indicate the activation of dislocation climb. The improvement of dislocation mobility at higher deformation temperatures leads to the absence of a pronounced stress maximum in the stress-strain curves.

The observed perfect dislocations are of (0001) 1/3<11-20> type, the dissociated Shockley partials are (0001) 1/3<10-10> type. The main deformation mechanism in the NbFe2 C14 Laves phase is basal slip.

 

[1] N. Takata et al., Intemetallics 70, 2016, 7-16

[2] W. Zhang et al., Physical Review Letters 106, 2011, 165505

[3] P. Hazzledine et al., Scripta Metallurgica et Materialia 28, 1993, 1277-1282

[4] S. Voss et al., Mater. Res. Soc. Symp. Proc.  1295, 2011, 311-316

[5] M. Chisholm et al., Science 307, 2005, 701-703


Michaela ŠLAPÁKOVÁ (Düsseldorf, Germany), Simon VOSS, Sharvan KUMAR, Christian LIEBSCHER, Frank STEIN
08:00 - 18:15 #5396 - MS01-572 Phase contrast transmission electron microscopy with hole-free phase plate for material analysis.
MS01-572 Phase contrast transmission electron microscopy with hole-free phase plate for material analysis.

In an analysis of material structures, transmission electron microscopy (TEM) stands out as an essential tool. A wide variety of information can be obtained from a sample by appropriately using TEM. Recently, phase contrast transmission electron microscopy (PC-TEM) with a hole-free phase plate (HFPP) has been established [1-4]. The HFPP is placed at the back focal plane of the objective lens. The incident beam passing through the thin film of HFPP generates secondary electrons, and they in turn lead to an electrostatic charging (potential). As the intensity distribution of electron beam on the HFPP has sharp peak at the center of optical axis, the charging occurs only at region of the beam crossover.  This causes an additional phase shift for the direct beam, resulting in high contrast images of samples. The difference of phase shift between scattered electrons and direct electrons, made by the charging, enhance the image contrast as the common phase microscopy. Thus, the contrast of materials made of light elements, e.g., polymer samples and biological samples, can be significantly enhanced by HFPP. Furthermore, we expect that it may be possible to derive phase, i.e., the mean inner potential, from the images obtainable from the PC-TEM with HFPP.  In this study, we demonstrate a couple of experimental results of PC-TEM with HFPP.

First example is an observation of nano-scale periodic structures of block copolymer (BCP). BCPs consist of light elements and thus often require “staining” of one of the phases for contrast enhancement under TEM observations. Because the staining might change nano-structures, it would be better not to use this method for increasing contrast. We used a field emission TEM (JEM-2200FS) operated at 200 kV with the PC-TEM with HFPP in this experiments. Figure.1 shows TEM images of a unstained block copolymer (poly(styrene-b-isoprene)) obtained with and without HFPP. The lamellar structure was hardly seen in the TEM images without HFPP at in-focus, i.e., Figures 1a. Under-focusing slightly enhanced the contrast between the polystyrene (PS) and polyisoprene (PI) phases in Figure 1b. In contrast, it was obvious that the use of HFPP significantly increased contrast of the two phases in lamellar structure.

Next, we explore the possibility of HFPP imaging for phase retrieval. As a first trial we selected the simple case of nanoparticles, for which we can assume a weak phase object approximation. As far as the phase values is small (typically <0.5 rad), we propose to retrieve the phase from two images with and without HFPP at the same defocus condition. The knowledge of the two images allows then for a simple inversion of the contrast transfer function that directly leads to a phase image of the area. Figure 2a and 2b show images of platinum nanoparticles with and without HFPP. Figure 2c is the phase image retrieved from the two images using the simple inversion. Phase images have been also retrieved for sets of images taken at different defocus (0, -40nm, -80 nm): the same phase images were obtained (though with a better signal/noise ratio at zero defocus). For comparison, an in line holography processing earlier validated on gold nanoparticles [5] has been applied: a good agreement on the phase value has been observed when comparing to the phase image of the same area obtained with and without HFPP. The measured phase values are close to 0.3 rd for 2-nm particles as expected from simple calculation based on the mean inner potential (VPt = 25 V). The advantage of the PC-TEM with HFPP is easier usability than conventional phase retrieving methods, such as off axis electron holography.

From these applications, we emphasize that PC-TEM with HFPP would be one of prospective analysis tools for materials characterization.

References

[1] M. Malac et al, Ultramicroscopy 118 (2012), p.77.

[2] Y. Konyuba et al, Microscopy and Microanalysis, 21.s3 (2015), p.1573.

[3] N. Hosogi et al, Microscopy 64.s1 (2015), p.126.

[4] M. Malac et al, U.S. Patent US 8,785,850.

[5] P. Donnadieu et al, Applied Physics Letters 94.26 (2009), p.3116.


Yuji KONYUBA (Tokyo, Japan), Hirofumi IIJIMA, Patricia DONNADIEU, Takeshi HIGUCHI, Hiroshi JINNAI, Naoki HOSOGI
08:00 - 18:15 #5593 - MS01-574 Delayering of 14 nm Node Technology IC with Xe Plasma FIB.
MS01-574 Delayering of 14 nm Node Technology IC with Xe Plasma FIB.

The burgeoning generation of electronic data and the growing need for fast processing is driving the development of unique architectures in microelectronic devices. High device performance, along with low energy consumption, decreasing device area and optimal production costs are the four basic tenets of operation in the microelectronics industry [1]. These rules have led to increasing area density of the elements in electronic devices and consequently to shrinkage of the elements to the nanometer scale.

 

State-of-the-art commercial electronic devices are based on 22 nm and 14 nm node technology and the next generation of 10 nm and 7 nm nodes are under development [1]. Mechanical polishing as a standard tool in the failure analysis of the devices does not meet the very strict requirements of these nodes. The thickness of metal interconnects and dielectric separating layers just above the transistors has shrunk to less than 100 nm. Therefore it is a challenge to stop the polishing process in a particular layer with the expected quality, accuracy and repeatability. Moreover, mechanical polishing is usually accompanied by surface artefacts like material chipping-off and site-specific fault excavation is usually impossible. Focused Ion Beam (FIB) technology has proven to overcome these restrictions. It offers the possibility to target a failure with nanometer accuracy in depth and the lateral direction. Therefore cross-sectioning, site-specific layer-by-layer excavation (delayering) and direct extraction of a Transmission Electron Microscopy (TEM) lamella containing the particular fault of interest have become standard methods in failure analysis.

 

We present delayering of an Intel Skylake processor (G4400) based on 14 nm node technology [2]. The delayering is performed with a Xe plasma FIB. Beam currents of Xe FIB up to 2 µA has extended the dimensions of the analyzed volume of interest to several hundred micrometers in general [3] while simultaneously enabling homogeneous delayering with nanometer accuracy. Xe FIB is advantageous also because interaction of inert Xe atoms with the material surface does not significantly alter its properties and surface contamination is negligible. Moreover Xe ions considerably reduce surface amorphisation when compared to Ga ions [4].  

 

Processor architecture is based on alternating metal and dielectric layers (Fig. 1). These layers have different sputtering rates when FIB delayering is applied. Uneven sputtering can be substantially suppressed by chemical means. In our experiments, water vapor was delivered to the point of patterning via a Gas Injection System (GIS) in order to equalize sputtering of metal interconnects and insulators. The result of processor delayering down to the first metal layer, just above the transistors is shown in Fig. 2. The damage-free surface of the transistor contact layer is ready for electrical probing.

 

Nanometer-sized elements in state-of-the-art electronics have posed a challenge also for imaging technologies. Clear observation of the very thin individual layers means suppressing the acceleration voltage of primary electrons to the sub-1 kV range, ideally to 500 V. At higher energies, the electron signals from different layers would intermix as they are generated in a volume comparable to the thickness of those layers. However, sub-nm resolution at low electron energies is a necessary condition due to the size of the observed features. High resolution pictures captured at 500 V are shown in Fig. 2. Low-kV imaging has verified highly homogeneous delayering of the processor as can be seen by the absence of large contrast changes in the delayered region.

 

[1] http://www.itrs2.net/

[2] http://www.intel.com/content/www/us/en/silicon-innovations/intel-14nm-technology.html

[3] T Hrnčíř et al, 38th ISTFA Conf. Proc. (2012), p. 26.

[4] T Hrnčíř et al., 41th ISTFA Conf. Proc. (2015), p. 60.


Jozef VINCENC OBOŇA, Tomáš HRNČÍŘ, Sharang SHARANG, Marek ŠIKULA, Andrey DENISYUK, Jiří DLUHOŠ (Brno, Czech Republic)
08:00 - 18:15 #5705 - MS01-576 Influence of metal additive Nb on the morphology of Vanadium Phosphorus Oxide Catalysts.
MS01-576 Influence of metal additive Nb on the morphology of Vanadium Phosphorus Oxide Catalysts.

Vanadium phosphorus oxide (VPO) catalysts have been considered as the most effective industrial catalysts for the oxidation of n-butane to maleic anhydride. The preparation methods of the catalyst, and reaction conditions and other factors will have different effects on the catalyst properties. In order to improve the performance of VPO catalyst, we often add co-catalyst: metal additives can not only improve the activity and selectivity of the catalysts to some extent, but also play important roles in reducing reaction conditions and prolonging the reaction time. The VPO catalyst precursors have been prepared through reducing pentavalent vanadium in organic phase, and then introducing the Nb element whose radius is similar to V4 + ionic by  the co-precipitation method. The effect of Nb additive on the morphology of VPO catalyst was studied by SEM and TEM, and the morphology of the catalyst was compared with that of the VPO catalyst without additive in the figure below.

SEM and TEM images of VPO catalyst precursor showed sheet structure with irregular size, and the transmission diffraction pattern suggested amorphous structure in Fig.1a-a’. Although there was a small amount of hexahedral structure for the VPO activated catalyst, the main morphology was granular structure different in size. After the addition of Nb additive, configuration of VPO precursor was regular diamond-shaped as shown in Fig.1c-c’. This kind of crystal takes the thickness of about 200nm, but the crystal size is reduced to 40nm for the Nb-VPO activated catalyst. It suggests that: the broken of tabular crystal is helpful to improve the specific surface area, thus to provide more active sites for the catalytic reaction and increase the catalytic activity. In addition, the catalyst will expose more crystal defects after fragmentation, and provide necessary conditions for the separation of the catalyst active site. Therefore, crushing effect by adding Nb into the organic phase is beneficial to improve the selectivity of maleic anhydride. On the basis of the previous work, a comparative analysis on the morphology VPO catalyst before and after the addition of Nb, we can draw a conclusion: the crystal phase of VPO precursor containing the Nb additive is VOHPO4∙0.5H2O, which is preferred to grow along the (001) crystal face. So the rhombic structure is more regular. For the Nb-VPO activated catalyst, phase composition is still (VO) 2P2O7, but there has been little VOPO4 phase. The synergy of V+4 and V+5 species in two crystal phases is the key to improve catalytic activity of VPO catalyst. The catalytic selectivity has been greatly increased after the addition of Nb.


Xiaopei MIAO (Beijing, China), Wenqing HUANG
08:00 - 18:15 #5730 - MS01-578 Effect of trace elements on the material properties of an aluminium casting alloy.
MS01-578 Effect of trace elements on the material properties of an aluminium casting alloy.

The aim of the project is to determine the fundamentally effects of impurities and individual micro-alloying elements (eg. as vanadium V, titanium Ti, calcium Ca, zirconium Zr, potassium K, phosphorus P and others) or combinations of these trace elements in aluminum alloys and their impact on the quality of aluminum castings.

As a result, limit values and tolerances for individual impurities (trace elements) should be defined and determined. Further on practical study methods should be developed that support on the one hand a reliable series production of high-quality alloys and castings and on the other hand the procurement of aluminum alloys by the foundries.

 

The casting trials in industry-related standards were performed on the base of a high purity alloy AlSi7Mg0,3 with systematic addition (30ppm, 300ppm, 3000ppm) of micro-alloying elements, controlled by chemical analysis with an emission spectrometer.

Technological properties as flowability, hot crack susceptibility and shrinkage cavity formation were evaluated. Static and dynamic material testing, like tensile tests, hardness measurements and Woehler curves was implemented. The thermo-physical properties as specific heat capacity, thermal expansion, temperature conductivity, density and heat conductivity were measured.

Phase calculations by ThermoCalc-Software, to simulate the formation of intermetallic phases and their impact on the microstructure are compared to classical metallography and additional SEM/TEM-investigations.

SEM/EDXS/EBSD-measurements for the micro-characterization of the structure and the composition of the intermetallic phases are made on cross sections (figures 1-4).

Out from regions/phases of interest TEM-lamellas are prepared with the FIB-technique and transferred to TEM for using EDXS/EELS/EFTEM and diffraction methods for the nano-characterization of these phases.

 

The authors want to thank the Austrian Research Promotion Agency (FFG) for financial support (PN845411).


Hartmuth SCHROETTNER (Graz, Austria), Thomas PABEL, Tose PETKOV, Sabrina MERTSCHNIGG, Anita ROSSMANN-PERNER
08:00 - 18:15 #5747 - MS01-582 Electron diffuse scattering in BNT-BKT-BT ternary compound.
MS01-582 Electron diffuse scattering in BNT-BKT-BT ternary compound.

Local structure of crystalline materials has been increasingly recognized to play a crucial role in understanding their functional properties. One important example is perovskite ferroelectrics where the structure often involves the presence of chemical short-range order, correlated atomic displacements and/or oxygen octahedral tilting disorder. In this case electron diffraction can prove to be a suitable technique for probing the local structure. The strong electron-matter interaction makes it possible to easily record weak superlattice reflections and diffuse scattering with good signal-to-noise ratio.

The ternary system xBi0.5Na0.5TiO3yBi0.5K0.5TiO3zBaTiO3 (BNT-BKT-BT) is a potential candidate for replacing Pb-based materials in piezoelectric applications1. Even though it is one of the most extensively studied lead-free piezoelectric systems, structural information about local distortions in this material is scarce. In this study Transmission Electron Microscopy (TEM) was employed in order to investigate the long- and short-range structure of this system in both real and reciprocal space. Two and three dimensional electron diffraction data was recorded. The true features of diffuse electron scattering in 3D as well as Bragg reflections were obtained by Rotation Electron Diffraction (RED) method2.

A projection of the three-dimensional reconstructed reciprocal space volume along <001>pc, is shown in Fig. 1(a). Superlattice reflections of both ½ooo and ½ooe type can be observed indicating the coexistence of two different octahedral tilting systems, anti-phase a-a-a- and in-phase a0a0c+ respectively (in Glazer notation3). The anti-phase a-a-a- tilting system is consistent with the long-range order space group, namely R3c. But, the in-phase a0a0c+ tilting system is consistent with P4bm space group indicating local deviations from the average space group. Furthermore two types of diffuse scattering features could be observed: (i) “asymmetric L-shaped” features around Bragg peaks extending along <100>pc directions clearly visible for Bragg peaks close to the direct beam, Fig. 1(b) and (ii) linear {h00}*, {0k0}* and {00l}* diffuse scattering ’rods’ that pass through the superlattice reflections as highlighted in Fig. 1(c). A possible explanation for the observed diffuse features is correlated displacements of the A- and B-cations along <111>pc and <100>pc chains coupled with disordered (in-phase/anti-phase) rotations of the oxygen octahedral4.

References

1 J. Shieh, K.C. Wu, and C.S. Chen, Acta Materialia 55, 3081–3087 (2007)

2 W. Wan, J. Sun, J. Su, S. Hovmöller and X. Zou, J. Appl. Crystallogr. 46, 1863-1873 (2013).

3 A. M. Glazer, Acta Cryst. B28, 3384 (1972).

4 J. Kreisel, P. Bouvier, B. Dkhil, P. A. Thomas, A. M. Glazer, T. R. Welberry, B. Chaabane and M. Mezouar, Physical Review B 68, 014113 (2003)

Acknowledgments

The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for providing the electron microscopy facilities and financial support under the project 3DEM-NATUR


Alexandra NEAGU (Stockholm, Sweden), Cheuk-Wai TAI
08:00 - 18:15 #5786 - MS01-584 Structural characterization of the high thermoelectric performance PbTe - PbSnS2 system and implications of its structural complexity in low lattice thermal conductivity.
MS01-584 Structural characterization of the high thermoelectric performance PbTe - PbSnS2 system and implications of its structural complexity in low lattice thermal conductivity.

Thermoelectric materials are promising alternative energy sources, suitable for applications in thermoelectric generators and refrigerators, due to their low cost and environmentally friendly heat-to-power generation. Their wide-scale utilization is limited because of their low efficiency; however, nanoscale inclusions can improve it by suppressing the lattice thermal conductivity.

The high performance PbTe-SnTe-PbS thermoelectric system forms a new PbTe - PbSnS2 composite with high n-type figure of merit. Electron Diffraction (ED) through tilting and Precession Electron Diffraction (PED) experiments as well as High Resolution Transmission Electron Microscopy (HRTEM) characterization of the thermoelectric composite PbTe + 25% PbSnS2 reveal that the system is nanostructured in a unique way, with PbSnS2 nanocrystals in the range of 80 to 500 nm in size. In most of the cases, they are endotaxially grown within the PbTe matrix (Figure 1).

Three independent crystal superstructures were observed for the PbSnS2 inclusions, originating from the same parent SnS-type structure. Specifically, the PbSnS2 inclusions appear with all three known structural modifications within the same matrix and often within the same nanocrystal. HRTEM images were qualitatively improved by Fast Fourier Transform (FFT) and inverse FFT (IFFT) processing, as well as by averaging of selected parts of the images. We then compared the processed images with computer image simulations and a very good resemblance between them was found. Figure 3 depicts a HRTEM image taken with the electron beam parallel to [110]PbSnS2 direction and is a part of a nanocrystal of about 300 nm in size. Two types of FFT patterns were observed and are shown as insets in Figure 3. Modified structural models for two of the superstructures observed in the PbSnS2 precipitates are proposed (one of them is presented in Figure 2).

Finally, evidence was also found for the growth process of the nanocrystals. The presence of some PbS nanocrystals implies that the growth process of the minor PbSnS2 phase starts from PbS crystals where SnS is gradually dissolved to eventually form PbSnS2 crystals.

Our findings suggest that this nanostructured thermoelectric composite exhibits unique structural complexity, which contributes to the low lattice thermal conductivity reported previously for these nanocomposite materials, by introducing extra scattering mechanisms. Phonon scattering can occur not only at the interfaces between the nanocrystals and the matrix, but also within the nanocrystals, due to their structural heterogeneity.

Acknowledgements

This work was supported by “IKY Fellowships of Excellence for Postgraduate Studies in Greece – SIEMENS Program”. Work at Northwestern is supported by the Department of Energy, Office of Science Basic Energy Sciences under grant DE-SC0014520.


Chrysoula IOANNIDOU, Christos LIOUTAS, Nikolaos FRANGIS (Thessaloniki, Greece), Steven GIRARD, Mercouri KANATZIDIS
08:00 - 18:15 #5819 - MS01-586 Severely plastically deformed Ti-Ni-Pd high-temperature shape memory alloys studied by TEM.
MS01-586 Severely plastically deformed Ti-Ni-Pd high-temperature shape memory alloys studied by TEM.

High-temperature shape memory alloys (HTSMA) have a broad range of prospective applications [1]. Grain refinement can substantially improve the material strenght and cyclic transformation stability [2]. It is the aim of the present work to study the structural evolution and phase transformations of a Ti50Ni25Pd25 HTSMA severely plastically deformed by high pressure torsion (HPT) using transmission electron microscopy (TEM) methods.

A Ti50Ni25Pd25 alloy prepared by arc melting was homogenized, solution treated, and water quenched. Disc shaped samples taken from the alloy were subjected to HPT (using a pressure of 6 GPa and 40 turns) carried out at room temperature (RT). HPT deformed samples were also subjected to isochronal heating (IH) to a temperature of 460°C followed by cooling to RT. TEM specimens were prepared by ion polishing (Gatan PIPS II) and analyzed in a Philips CM 200 transmission electron microscope.

 

The initial coarse grained TiNiPd HTSMA is fully martensitic since the thermally induced transformation from the cubic B2 austenite to orthorhombic B19 martensite occurs well above RT (the martensite finish temperature is ~ 130°C). Therefore, the HPT is applied to the martensitic state of the initial sample that is showing a self-accommodated morphology of twinned martensite (cf. Fig. 1).

 

After the HPT, a complex mixture of elongated crystals and amorphous bands has formed that have an average width of 40 and 15 nm, respectively. The thinnest amorphous bands have a width of about 5 nm only. While TEM bright field images hardly facilitate analysis of the deformation microstructure (cf. Fig. 2a), strong differences in scattering of the crystalline and amorphous phase (cf. the selected area diffraction pattern (SADP) of Fig. 2b) can be used to separately image these phases in dark field images: Fig. 2c was taken with strong diffraction spots (cf. C in Fig. 2b) of crystallites closely orientated to a Bragg-condition. In this case, the dark field intensity strongly depends on the specimen tilt and the position of the objective aperture. Since the lattice reflections form arc shaped segments in the SADP, it is concluded that the most of the crystals have rather similar orientation with respect to each other (the average rotational misorientation is about 20°). Most of the reflections correspond to B19 martensite. Weak reflections of the B2 austenite also arise. Fig. 2d was taken by allowing diffusely diffracted intensity of the amorphous phase to pass the objective aperture that is not superimposed by strong crystalline diffraction spots (cf. A in Fig. 2b; the maximum of the diffusely diffracted intensity was used occurring on a diffraction ring with a radius of about 4.6 nm-1). As expected, the dark field contrast of the amorphous bands is rather uniform and hardly depends on both the tilt of the specimen and the position of the objective aperture along the amorphous diffraction ring (as long as it is not superimposed by crystalline reflections). The present observations of a lamellar structure of elongated crystallites separated by thin amorphous bands indicates that the amorphization might preferentially occur at martensitic twin boundaries (cf. Fig. 1a) [3,4].

 

In samples subjected to IH after HPT, an ultrafine grained structure is formed by crystallization of the amorphous phase, as well as by recovery and grain growth that occurs in the austenitic state. During cooling to RT, the forward B2 to B19 transformation occurs in the ultrafine grains that frequently contain a twinned morphology of the martensite (cf. Fig. 3a and b). However, as compared to the case of coarse grains, the forward transformation is hindered (i.e. shifted to lower temperatures and incomplete) yielding retained austenite in some of the grains (cf. Fig. 3c).

 

[1] J.V. Humbeeck, Mater. Res. Bulletin 47 (2012) 2966-2988.

[2] K.C. Atli, I. Karaman, R.D. Noebe, A. Garg, Y.I. Chumlyakov, I.V. Kireeva, Acta Mater. 59 (2011) 4747-4760.

[3] M. Peterlechner, T. Waitz, H.P. Karnthaler, Scripta Mater. 60 (2009) 1137–1140.

[4] J.Y. Huang, Y.T. Zhuz, X.Z. Liao, R.Z. Valiev, Phil. Mag. Lett. 84 (2004) 183–190.

Financial support by the Austrian Federal Government within the framework of the COMET Funding Programme is gratefully acknowledged.

 


Semir TULIĆ, Michael KERBER, Mitsuhiro MATSUDA, Thomas WAITZ (Vienna, Austria)
08:00 - 18:15 #5820 - MS01-588 On the influence of the elemental addition Au on the semi-coherent interfaces in an Al-Cu alloy.
MS01-588 On the influence of the elemental addition Au on the semi-coherent interfaces in an Al-Cu alloy.

Aluminium light alloys are employed in commercial automotive and aerospace applications due to their high specific strength and corrosion resistance [1]. Precipitation hardening is one of the most important ways to improve the alloy performance. By tailoring precipitate size, aspect ratio, and distribution, the precipitation hardening can be significantly enhanced. The interfacial structure between precipitates and matrix is the critical factor being thought to manipulate the precipitate growth, but the fundamental understanding of these interfaces remains poor due to both limitations in atomic-resolution compositional characterisation techniques and computational capacity of first principle calculations. 

Al-Cu is a textbook binary alloy having precipitate strengthener θ′ (Al2Cu) phase, but recent work showed its semi-coherent interfaces is not as simple as previously thought [2]. In fact, a complex metastable θ′t phase is sandwiched in-between θ′′ and θ′ precipitates, indicating a non-intuitive energetically favourable phenomenon. Gold (Au) has strong negative solute formation enthalpy with aluminium and thereby its precipitation is directly linked to η′ and η phases without precursor Guinier–Preston (GP) zone [3]. How such element affects the interfacial structure is still unclear. In this work, we have used atomic-resolution high angle annular dark-field (HAADF) via aberration-corrected scanning transmission electron microscopy (AC-STEM) for detailed investigations of the influence of Au on the heterophase interfacial structure in an Al-Cu alloy. 

We have experimentally determined the effect of Au and ageing temperature on the complex interfacial structure between solid solution (α) and θ′ (Al2Cu). We have observed the sandwiched interfacial structure in Al-Cu-Au alloys aged at 200°C (See Fig. 1a) as discovered in Ref. [2], while in comparison some partially direct θ′-α interface was found (see Fig. 1b) in rare probability. However, Au addition was observed to clearly destabilise the complex interfaces (see fig. 1c) at higher temperature ageing (350°C), whereas the corresponding binary Al-Cu alloy still somehow displays complex interfacial structures (Fig. 1d). The complex interface was also proved to provide the first solution to the four-decades-old mystery where experimental precipitate coarsening rate was found to be hundreds of times that of theoretical predictions based on the direct θ′-α interface in the Ref. [4].

References:

[1] Williams JC et al. Acta Materialia 51(2003):5775-5799.

[2] Bourgeois L, et al. Physical Review Letters 111 (2013): 046102.

[3] Bourgeois, L et al. Acta Materialia 105 (2016): 284-293.

[4] Boyd JD et al. Acta Metallurgica 19.12 (1971): 1379-1391.


Yiqiang CHEN (Melbourne, Australia), Zezhong ZHANG, Chen ZHEN, Amalia TSALANIDIS, Matthew WEYLAND, Findlay SCOTT, Allen LES, Jiehua LI, Laure BOURGEOIS
08:00 - 18:15 #5821 - MS01-590 On the diffusion-mediated cyclic coarsening and reversal coarsening in an advanced Ni-based superalloy.
MS01-590 On the diffusion-mediated cyclic coarsening and reversal coarsening in an advanced Ni-based superalloy.

Polycrystalline nickel-based superalloys for turbine disc applications typically employ complex alloy chemistry in order to produce a high volume fraction of gamma-prime (γ′) precipitates for the optimisation of mechanical properties [1]. The precipitate coarsening causes a gradual loss of coherency between γ′ precipitates and γ matrix when materials serving elevated temperatures, therefore resulting in the degradation of its mechanical performance [2]. In this work, we report new experimental observations for diffusion-mediated secondary γʹ precipitate coarsening (See Fig. 1) within a near-zero misfit alloy RR1000 in a cyclic manner that these precipitates coarsen and split periodically [3].

Using absorption-corrected energy-dispersive X-ray (EDX) spectroscopy within the scanning transmission electron microscope (STEM) [4], compositional variations for secondary γ′ precipitates as a function of coarsening behaviour under have been investigated. We have observed clear cyclic variations in the elemental concentrations of Co, Ti and Al within the secondary γ′ as a function of ageing time. STEM/EDX spectrum imaging and electron tomography on individual secondary γ′ have revealed local enrichment of Co within the core of secondary γ′ (See Fig. 2). STEM-EDX analysis of the γ-γ′ interface revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix region near the γ-γ′ interface (See Fig. 3). Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, we show that elemental inhomogeneities, produced within both matrix and precipitates, are induced by complex interactions between thermodynamics and diffusion kinetics. These elemental inhomogeneities will likely affect the kinetics of coarsening and therefore must be taken into account when predicting the microstructure likely to be produced when the material is exposed to different heat treatment regimes. More generally, our findings suggest the importance of considering diffusion kinetics when attempting to understand the microstructural evolution of advanced superalloys. Our discovery renders the potential to retain the overall γ-γ′ coherence in nickel-based superalloys when exposed to elevated temperatures, and therefore to improve its creep properties.

 References:

[1] Reed, RC. Cambridge University Press, 2008.

[2] Acharya, MV et al. Materials Science and Engineering: A 381 (2004): 143-153.

[3] Chen YQ et al. Acta Materialia, (accepted) 2016.

[4] Chen, YQ, et al. Ultramicroscopy 144 (2014): 1-8.


Yiqiang CHEN (Melbourne, Australia), Rajan Prasath BABU, Thomas SLATER, Robert MITCHELL, Octav CIUCA, Michael PREUSS
08:00 - 18:15 #5835 - MS01-592 Study of disorders in zeolite ITQ-39 using structure projection reconstruction from through-focus series of HRTEM images.
MS01-592 Study of disorders in zeolite ITQ-39 using structure projection reconstruction from through-focus series of HRTEM images.

The structure of an aluminosilicate ITQ-39 has been determined by electron crystallography, it is one of the most complex zeolite ever solved [1]. ITQ-39 has a 3-dimensional channel system with intersecting 10- and pairwise 12-ring channels. It has a highly faulted structure and contains stacking disorder along [100], twinning along [010] and point defects. High-resolution transmission electron microscope (HRTEM) images were chosen to study these disorders. In order to improve HRTEM data quality, a through-focus series of 20 HRTEM images were collected and a structure projection image was reconstructed (see Fig. 1) by a contrast transfer function (CTF) correction algorithm using the software QFocus [2]. The through-focus series was acquired with a constant focus step (-53.3 Å) and the two-fold astigmatism was the same through the series. Defocus values and the two-fold astigmatism were then determined for all the images. CTF correction was performed on each image and the final reconstructed image was obtained by averaging all 20 CTF-corrected images (see Fig. 1b). Defocus determination revealed that the image series passed through the Scherzer focus condition and contained at least one image close to the Scherzer condition (Fig. 1a). By using structure projection reconstruction the noise in the images was significantly reduced and the structure and pore system were more obvious after the reconstruction. One big advantage is that there is no need to spend time to adjust to a certain focus (e.g. close to Scherzer focus), therefore the acquisition of HRTEM images can be done much faster which is very useful for beam sensitive samples.

References

[1] Willhammar, T. et al. Structure and catalytic properties of the most complex intergrown zeolite ITQ-39 determined by electron crystallography. 2012, Nature Chemistry, 4 (188-194).

[2] Wan W. et al. Structure projection reconstruction from through-focus series of high-resolution transmission electron microscopy images. 2012, Ultramicroscopy, 115 (50-60).

Acknowledgements

The project is supported by the Swedish Research Council (VR), the Swedish Governmental Agency for Innovation Systems (VINNOVA) and the Knut & Alice Wallenberg Foundation through the project grant 3DEM-NATUR and a grant for purchasing the TEM.  


Elina KAPACA (Stockholm, Sweden), Tom WILLHAMMAR, Wei WAN, Xiaodong ZOU, Manuel MOLINER, Cristina MARTINEZ, Fernando REY, Avelino CORMA
08:00 - 18:15 #5840 - MS01-594 UHVEM observation for the dual structure in spheroidal graphite of cast irons.
MS01-594 UHVEM observation for the dual structure in spheroidal graphite of cast irons.

The phenomenon that spheroidal graphite cast iron was formed by adding magnesium (Mg) was discovered in 1949 [1][2]. Since then, a number of researches have been made on this subject. Due partly to increase in research on the graphite-spheroidizing, there are various theories such as interfacial energy theory, nuclear theory, bubble theory,,, etc. Among them interfacial energy theory is becoming dominant as of now [3]. However the role of Mg as the graphite-spheroidizing agent has not been yet identified. And Mg particle is often observed inside of spheroidal graphite experimentally. In order to get the information on the internal structure of spheroidal graphite in cast iron, Ultra-High Voltage Electron Microscopy (UHVEM) observation was carried out.

The composition of the cast iron in this experiment was FCD450 (Fe:Bal., C:3.645, Si:2.533, Mg:0.0464, Mn:0.368, P:0.018, S:0.0114, Cu:0.176 (wt%), Yodoshi Co.). The position of Mg in spheroidal graphite was predicted from a surface observation result by EPMA. Focused ion beam (FIB) cut out into approximately rectangular parallelepiped shape carbon sample from a spheroidal graphite of FCD450. This sample was observed by UHVEM in Osaka Univ. at 2.0MeV.

Figure 1 shows a bright field image (BFI) of the internal structure of spheroidal graphite. The electron beam in acceleration voltage of 2MeV was transmitted approximately 8μm thickness of graphite (inclination of the incident beam: 30°). Figure 1 indicated that whole area was graphite including the internal structure of spheroidal graphite roughly composed of a core region surrounded by annual rings of a layered intermediate region (central domain) and a layered outer region with some radial factors (covering domain).

Only the covering domain has the radial and superimposed contrast. This radial factor originates from the metal inclusion that grew radially from the central domain. The shape of the external region of the central domain (without superimposed contrast) could be regarded as the almost truth sphere, which might become core for growing graphite crystal spherically. Primary crystal of graphite might be spheroidized by the solidification in liquid state, and crystal growth of graphite reaches the eutectic point at the external region of the central domain. After that, the covering domain formed by the cooperation growth between graphite and austenite.

The spheroidal graphite in cast iron has the dual structure consisting of a spherical central domain and a covering domain. This obtained data using by UHVEM has provided new knowledge of the internal structure of spheroidal graphite in cast iron. And this study might provide important visual data to identify a role of the Mg in the graphite-spheroidizing of cast iron.

 

 This work was supported by “Advanced Characterization Nanotechnology Platform, Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan” at the Research Center for Ultra-High Voltage Electron Microscopy (Nanotechnology Open Facilities) in Osaka University.

 

[1] A. P. Gagnebin, K. D. Millis and N. B. Pilling: The Iron Age, Feb. (1949) 77

[2] A. P. Gagnebin, K. D. Millis and N. B. Pilling: The Iron Age, Feb. (1949) 97

[3] S. Jung and H. Nakae: J.JFS, Vol.79-10 (2007) pp.605-615


Hidefumi MAEDA (Shiga, Japan), Kanako INOUE, Akira SUGIYAMA, Hidehiro YASUDA
08:00 - 18:15 #5887 - MS01-596 Quantitative analysis of AlN/SiC interfaces in AlGaN/GaN heterostructures grown on SiC.
MS01-596 Quantitative analysis of AlN/SiC interfaces in AlGaN/GaN heterostructures grown on SiC.

Wide band gap semiconductors, such as SiC and GaN, exhibit many attractive properties: a unique combination of the wide band gap, high breakdown field, high saturation velocity and the ability to form high quality AlGaN/GaN heterostructures with good transport properties make them ideal candidates for high power and high frequency applications. Hexagonal silicon carbide materials (SiC) are considered to be promising candidates for electronic devices as the third generation key materials for transistors. A typical example of a high performance device is the AlxGa1-xN/GaN heterostructure used as a high electron mobility transistor (HEMT) [1-2].

As 6-inch SiC wafers are being introduced into the market, a decrease of the substrate off-cut for SiC heteroepitaxy is desirable to reduce the manufacturing costs [3-4]. Therefore, multilayer (5 layers) and multicomponent structures (based on GaN and related materials) were grown on 6H-SiC (with a misorientation of 1 deg. off from the (0001) plane) substrates using the MOVPE method, for high power applications. The layers were grown epitaxially, as it was confirmed from the corresponding electron diffraction patterns. Several types of interfaces were observed between the layers that either ran parallel to the interface or formed V-shaped defects (e.g. the SiC/AlN, GaN/AlN, GaN/AlGaN interfaces etc.). Moreover, High Resolution TEM (HRTEM) images showed the existence of steps in the 6H-SiC/AlN interface. A typical HRTEM image where an atomic scale step is observed is shown in Fig. 1.

In this study, quantitative analysis of the 6H-SiC/AlN interface is presented based on experimental HRTEM micrographs, showing and proving the steps sites, the layers’ sequence and any strain relaxation situation existing. A structural model based on this analysis is proposed and simulated HRTEM images are also obtained.  The corresponding atomic models proposed are found to describe well the 6H-SiC/AlN interface, with the corresponding computer simulation images coinciding with the experimental HRTEM images. An example is shown in Fig. 2, illustrating the monolayer step observed in the 6H-SiC/AlN interface. In Fig. 3, a characteristic plot shows the phase shift of the fitting of the intensity’s distribution along a line that corresponds to the projection of a close-packing layer revealing the stacking sequence and therefore the starting position of the AlN epilayer. Moreover, the comparison of the sequence clearly shows the height of the step and any alteration on the stacking sequence between the two parts of the image. Finally, the computer simulated image, as shown in Fig. 4, coincides well with the HRTEM image shown in Fig. 1.

Acknowledgements

Supported by IKY Fellowships of Excellence for Postgraduate Studies in Greece-Siemens Program, the JU ENIAC Project Last Power Grant agreement no. 120218 and the Greek G.S.R.Τ., contract SAE 013/8 - 2009SE 01380012.

 

References

 [1] F. A. Ponce, C. G. Van de Walle, and J. E. Northrup, Physical Review B 53 (1996) 7473.

 [2] J.A. del Alamo, J. Joh , Microelectronics Reliability 49 (2009) 1200.

 [3] Leszczynski Mike, Prystawko Pawel, Kruszewski Piotr, Sarzynski Marcin, Plesiewicz Jerzy, Domagala Jarek, Gkanatsiou Alexandra, Lioutas Christos, Frangis Nicolaos, Polychroniadis Efstathios, Materials Science Forum 806 (2015) 73.


Alexandra GKANATSIOU (Thessaloniki, Greece), Christos B. LIOUTAS, Nikolaos FRANGIS
08:00 - 18:15 #5894 - MS01-598 Comparison of TEM and positron annihilation lifetime spectroscopy on tungsten exposed to mixed D plasmas.
MS01-598 Comparison of TEM and positron annihilation lifetime spectroscopy on tungsten exposed to mixed D plasmas.

Tungsten is currently a main candidate as a divertor plasma facing material in fusion devices. Even tungsten, however, will not be able to withstand excessive heat fluxes during off normal events such as ELM’s without significant erosion or other damages (e.g. melting or cracking) in the future fusion devices. One of the options to mitigate excessive power loads is by divertor impurity plasma seeding to dissipate the energy. Gases like Ar, Ne or N2 can be used for that purpose. Moreover, the presence of He as a reaction product from DT fusion reaction is also unavoidable. All these impurities will have an effect on tungsten behavior under plasma exposure. The expected impact of impurities on the surface morphology will change the tungsten erosion behavior. The presence of different subsurface defects (dislocation loops, voids) will influence the retention of hydrogen isotopes, which is of prime importance for the operation of a fusion device. The main focus of this work is to compare TEM analysis with positron annihilation lifetime spectroscopy to investigate the near-surface defects created by the impact of different plasmas.

Tungsten samples were mechanically polished to obtain mirror like surface and recrystallized at 1800°C for 1 h. Such prepared specimens were exposed in the linear plasma generator PSI-2 with an incident ion flux of about 1022 m-2s-1 and at an incident ion fluence of 5*1025 m-2, at a sample temperature of 500 K. Samples were biased to a potential of - 100 V resulting in incident ion energy of 70 eV. Pure D plasma (reference sample) and D plasma with additional impurities of He (3%), Ar (7%), Ne (10%) or N (~5%) were applied. The impurity concentration was controlled by spectroscopy, except for N for which it was estimated from the puffing rates.

The analysis covers the detection of subsurface defects and their density. TEM observation combined with positron annihilation techniques are employed to determine the thickness of the damaged zone and the presence and density of defects such as voids, dislocation loops and vacancies clusters.

After the plasma exposure, the surface morphology was investigated using scanning electron microscope (SEM) combined with a focused ion beam (FIB) utilized for cross-sectioning and thin lamella preparation for the transmission electron microscope (TEM) analysis. The reference sample exposed to a pure D plasma reveals at the surface the presence of two groups of blisters with a size of few mm and a few 100 nm. The presence of blisters is strongly correlated with the tungsten grain orientation. The addition of Ar and Ne results in surface erosion with different yields depending on grain orientation, confirmed also by electron backscattered diffraction (EBSD). Large blisters are present but show signatures of erosion. Less pronounced erosion is visible when adding N2. The presence of N in the plasma causes also blisters with cone-like shapes. The addition of He leads to the formation of flatter blisters and very fine nano-porosity on the surface.


Marcin RASIŃSKI (Jülich, Germany), Arkadi KRETER, Yuji TORIKAI, Grzegorz KARWASZ, Christian LINSMEIER
08:00 - 18:15 #5908 - MS01-602 Probing localized strain in solution-derived YBCO nanocomposite films.
MS01-602 Probing localized strain in solution-derived YBCO nanocomposite films.

The investigation of the atomic structure of individual defects is critical to the understanding and
precise controlling of the physical properties of materials. And although defects are sometimes
detrimental to functionality, in high temperature superconductors (HTS) are necessary for
providing pinning of magnetic flux and allowing high currents to be carried. Moreover, a strong
enhancement on the vortex pinning in HTS YBa2Cu3O7 (YBCO) films is also found to be
controlled by nanostrain [1], which is attributed to elastic distortions of the crystal lattice at the
nanoscale level. Using aberration-corrected Scanning Transmission Electron Microscopy
(STEM) we explore the complex defect landscape of YBCO nanocomposite thin films.
Combining High Angle Annular Dark Field (HADDF) with Low Angle Annular Dark Field
(LAADF) and local strain analyses we are able to map and quantify the lattice deformations
associated to the defects, which will ultimately determine their self-assembling behavior as well
as their mutual interaction. Our atomic scale investigation shows that the presence of mainly
randomly oriented nanoparticles generates incoherent interfaces within the epitaxial YBCO
matrix, which drastically increases the density of defects, yielding a ramified network of
inhomogeneously distributed nanostrained regions where the crystalline perfection of the
superconductor is perturbed.
Finally, we will compare the microstructure of conventional high-quality solution-derived
trifluoroacetate-YBCO nanocomposites with new fluorine-free films based on a novel transientliquid
assisted growth method (TLAG), which provides ultra-high growth rates with a consequent
influence on the defects landscape. Accordingly, TLAG envisages an enormous potential for lowcost
and high-performance coated conductors.


We acknowledge funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES, ERC-AdG-
2014-669504 ULTRASUPERTAPE and MINECO MAT2014-51778-C2-1-R.


References
[1] Llordés at al., Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in
high-temperature superconductors. Nature Materials, 11, 329 (2012).


Roger GUZMAN (Bellaterra, Spain), Jaume GAZQUEZ, Bernat MUNDET, Pablo CALLADO, Laia SOLER, Julia JAREÑO, Mariona COLL, Xavier OBRADORS, Teresa PUIG
08:00 - 18:15 #5946 - MS01-604 The comparison of grain boundaries in a nanostructured austenitic stainless steel annealed conventionally and under high hydrostatic pressure.
MS01-604 The comparison of grain boundaries in a nanostructured austenitic stainless steel annealed conventionally and under high hydrostatic pressure.

Abstract

Nanostructured metals and alloys are known to behave differently during annealing comparing to their micrograined counterparts. They exhibit lower thermal stability and often abnormal grain growth occurs. In the present work, the combination of severe plastic deformation processing  followed by subsequent conventional annealing and annealing under high hydrostatic pressure was applied. Samples were subjected to the heat treatment at 700°C for 10 minutes either under atmospheric or hydrostatic pressures of 2 or 6 GPa. The recrystallization process is investigated in TEM observations using HD2700 Hitachi. In samples after deformation one can notice a high density of nanotwins inside nanograins (Fig 1 and 2). In samples after annealing nanograins appear, the smallest of about 50 nm in the diameter (Fig 1 and 3). The main question refers to the differences in the microstructure after conventional annealing and annealing under high pressure. At this stage, one can predict that diffusion processes are suppressed by high pressure. The question arises as to what information about the grain boundary migration mechanisms can be drawn from high pressure studies. There are only a few papers where influence of high pressure on the grain growth [1] and migration of individual grain boundaries [2] was studied. It was found, that the activation energy and activation volume for grain boundary migration in aluminium bicrystals are larger than that for grain growth in aluminium polycrystals and depend on the grain boundary crystallography. Moreover, the high pressure had greater impact on the movement of high-angle grain boundaries than low-angle grain boundaries [3]. These theories refer to microstructural materials and they will be verified in the case of nanostructured materials.

 

References

[1] Hahn, H. and Gleiter, H. (1979). Scripta Metall., 14, 3-6.

[2] Molodov, D.A., Swiderski, J., Gottstein, G., Lojkowski, W. and Shvindlerman, L.S. (1994). Acta Metall. Mater., 42, 3397.

[3] Sursaeva V., Protasova S., Lojkowski W., Jun J. (1999). Textures an Microstruct., 32, 175-185

 

Acknowledgements

This work was supported by Polish NSC project No.UMO-2014/15/D/ST8/00532


Agnieszka KRAWCZYNSKA (Warsaw, Poland), Stanisław GIERLOTKA, Bogdan PALOSZ, Malgorzta LEWANDOWSKA
08:00 - 18:15 #5961 - MS01-606 Misfit of coherent precipitate phases in Al alloys measured by scanning transmission electron microscopy.
MS01-606 Misfit of coherent precipitate phases in Al alloys measured by scanning transmission electron microscopy.

The age-hardenable Al–Mg–Si alloy system is strengthened by needle-shaped β” phase precipitates that develop during heat treatment. The phase is fully coherent with the Al matrix, and induces a considerable strain field in the Al matrix, which is key to its hardening property. The lattice parameters of β” have been measured as an average over many precipitates using electron diffraction [1], but never systematically for specific precipitate shapes. Transmission electron microscopy is ideal for this task, although one must take care to avoid image distortions when accurate distances are to be measured. Multi-exposure scanning transmission electron microscopy (STEM) image series were processed to correct drift and scan distortions [2]. With such datasets, we can investigate the detailed atomic structure and ensure the precipitates to be defect-free, and at the same time measure relative atomic distances accurately. This enables the measurement of the misfit between the β” phase and the Al matrix, for which we employ the geometric phase analysis (GPA) technique [3]. As the misfit in the long direction of the particles is negligible, we concentrate on the a ([100]β’’) and b ([001]β’’) directions, which lie in the cross-sectional plane.

            24 precipitates with 15 distinct geometries were investigated [4]. Fig. 1 shows images of representative particles together with the average measured misfits for each geometry. The misfit is found to vary significantly between particles, in the range 1–7%. The misfit in either direction is inversely proportional to the particle width in that direction, as seen in Fig. 2(a). This works as an energy minimization mechanism, as the strain in the Al matrix is distributed evenly in its cross-sectional plane. The elasticity of the β” phase is similar to that of the Al matrix, which is in accordance with related literature [5]. The relative β”–Al misfit area is independent on particle shape, as shown in Fig. 2(b), and has a value of about 7%. A particle will grow during heat treatment until the strain from these extra 7% becomes too much to handle, and the particles loses coherency.

            The measured misfits give a good match to reported density functional theory (DFT) simulations for a β” phase with composition Mg5Al2Si4 within a range of precipitate geometries [6,7]. Two other likely compositions were tested, but were found not to fit with the shape-misfit relationship of the experimental data. The conclusions in this study will contribute to improving models for precipitation hardening from atomic to macroscopic scales.

 

[1] H.W. Zandbergen, S.J. Andersen, J. Jansen, Science 277 (1997) 1221–1225.

[2] L. Jones, H. Yang, T.J. Pennycook, M.S.J. Marshall, S. van Aert, N.D. Browning, M.R. Castell, P.D. Nellist, Adv. Struct. Chem. Imaging 1 (2015) 8.

[3] M.J. Hÿtch, E. Snoeck, R. Kilaas, Ultramicroscopy 74 (1998) 131–146.

[4] S. Wenner, R. Holmestad, Scripta Mater. 118 (2016) 5–8.

[5] A.G. Frøseth, R. Høier, P.M. Derlet, S.J. Andersen, C.D. Marioara, Phys. Rev. B, 67 (2003) 224106.

[6] F.J.H. Ehlers, S. Dumoulin, K. Marthinsen, R. Holmestad, Mater. Sci. Forum 794–796 (2014) 640–645.

[7] P.H. Ninive, O.M. Løvvik, A. Strandlie, Metall. Mater. Trans. A 45 (2014) 2916.

 

The authors would like to thank the Research Council of Norway (RCN) for funding of the FRINATEK project “Fundamental investigations of precipitation in the solid state with focus on Al-based alloys”. The TEM work was carried out on the NORTEM instrument JEOL ARM-200F, TEM Gemini Centre, Norwegian University of Science and Technology (NTNU), Norway. Acknowledgements are due to Dr. Lewys Jones for assistance with image processing.


Sigurd WENNER, Randi HOLMESTAD (Trondheim, Norway)
08:00 - 18:15 #6008 - MS01-608 Mechanical twinning and microstructure refinement in metastable titanium alloys.
MS01-608 Mechanical twinning and microstructure refinement in metastable titanium alloys.

Metastable beta titanium alloys gain increasing interest due to their unique properties such as high strength, good hardenability, high fracture toughness and excellent corrosion resistance. In this study two different metastable beta titanium alloys, Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL LCB) were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) technique. The main purpose of SPD methods is the insertion of large plastic deformation to the material without a change of the original shape of the material. The decreasing grain size and increasing the number of high-angle grain boundaries results in changes of mechanical and other physical properties. The main aim of this work is to analyse the effect of deformation on the microstructure and lattice defects. The initial stages of microstructure refinement were examined by electron backscatter diffraction (EBSD). The inverse pole map figure of Ti-15Mo alloy after N = ¼ HPT turns is depicted in the Figure 1.  It was shown that initial stages of deformation include misorientation within grains (see changing ‘colour’ along black line) and formation of low-angle sub-grain boundaries (black arrows). Moreover, it was proven that multiple twinning occurs in the material (Figure 2) which notably contributes notably to the microstructure refinement. Further microstructure refinement by HPT observed by transmission electron microscopy (TEM) and automated crystal orientation mapping (ACOM-TEM) will be also presented. 


Kristína VÁCLAVOVÁ (Prague 2, Czech Republic), Josef STRÁSKÝ, Petr HARCUBA, Jitka STRÁSKÁ, Josef VESELÝ, Veronika POLYAKOVA, Irina SEMENOVA
08:00 - 18:15 #6032 - MS01-610 EBSD Analysis of the Microstructural Evolution of Ni-Superalloys.
MS01-610 EBSD Analysis of the Microstructural Evolution of Ni-Superalloys.

Ni-superalloys are well known for their high strength, outstanding corrosion resistance and their resistance to both fatigue and creep, As a result, they are frequently used in gas-turbine and aero-engine applications. Inconel 625 also has a high resistance to chloride-ion stress-corrosion and is used in very aggressive environments. The strength of Inconel 625 is derived from the stiffening effect of molybdenum and niobium on its nickel-chromium matrix; thus precipitation-hardening treatments are not required. Control of the microstructure is very important with respect to the mechanical properties of the material. The microstructure changes during the hot rolling of the Inconel nickel-chromium superalloy 625 were investigated using FEG SEM. A detailed insight into the recrystallization behaviour during hot rolling was provided by an EBSD analysis (Figure 1). During the hot rolling the recrystallization starts on the grain boundaries, followed by the twin grain boundaries and, at even higher stresses, the recrystallization occurs on the (TiNb)CN phases inside the crystal grains. The EDS and EBSD results were able to explain the complex nature of the (TiNb)CN phases.


Matjaž GODEC (Ljublajna, Slovenia), Jaka BURJA, Bojan PODGORNIK, Franc TEHOVNIK
08:00 - 18:15 #6062 - MS01-612 Hexagonal patterning of 1-nm Gd nano-fibers based on dislocation templates in Mg-Gd alloys.
MS01-612 Hexagonal patterning of 1-nm Gd nano-fibers based on dislocation templates in Mg-Gd alloys.

Mg alloys, the lightest structural metal, generally suffer from low strength and poor deformability, and therefore severely restrict their widespread applications in automotive, aircraft, and aerospace industries. [1] The underlying reason for such mechanical behaviors is the anisotropic response inherent in the hexagonal close-packed (hcp) lattice of Mg. [2-3] Therefore, the key of advancing their applications is reducing the anisotropic behavior of different deformation modes, through regulating the relative activities of (easy) basal slips, (hard) non-basal slips, and twinning.

 

Herein, we demonstrate the new microstructure, the self-assembled hexagonal 1-nm Gd nano-fibers pattern within binary Mg-Gd alloys. [4] As shown in Fig.1, such hexagonal patterns are typically a few hundred nanometers in width and a few microns in length. These patterns are associated with dislocation templates. Such patterns include Gd-segregated dislocations with approximately 1 nm in diameter, which have a c-rod shape, and these in turn become effective inhibitors for basal slips since they have to cut these nano-fibers. On the other hand, non-basal slips suffer much less effect because the glide of non-basal dislocations has much less chances of cutting these Gd-segregations. Thus, these patterns can strengthen Mg alloys mainly through pinning basal dislocations; more importantly, tune the relative activities of basal and non-basal slips, and thus improve the deformability of Mg alloys. It is also worth mentioning that such patterning structure can be synthesized through a generally economical hot extrusion approach.

 

In summary, 1-nm Gd nano-fibers, with c-rod shape, are self-assembled into hexagonal patterns in Mg matrix, which can tune the relative activities of deformation modes and thus improve mechanical properties of Mg alloys. Our results open up a new path of engineering advanced Mg alloys though manipulating their microstructure.

 

 

References:

 

[1] I. J. Polmear, Light Alloys, 4th ed., Elsevier/Butterworth-Heinemann, Oxford, U.K., 2006.

[2] J. Koike, Acta. Mater. 51(2003), p. 2055-2065.

[3] J.-F. Nie, Metall. Mater. Trans. A 43(2012), p. 3891-3939.

[4] Y.X. Li, et al, (submitted).

 

Acknowledgments

 

We acknowledge the financial support from 1000Plan Professorship for Young Talents Program and the National Science Foundation of China (No. 51401124). We are grateful for the TEM specimens prepared by M. Shao and L. Jin. 


Guo-Zhen ZHU (Shanghai, China), Yangxin LI
08:00 - 18:15 #6073 - MS01-614 Graphite-to-diamond (13C) direct transition in a diamond anvil high-pressure cell.
MS01-614 Graphite-to-diamond (13C) direct transition in a diamond anvil high-pressure cell.

                 As the hardest material in nature, diamond is of great importance and interest for scientific studies. However, formation of a diamond is complicated process and requires extreme conditions. Bundy and Kasper (1967) for the first time synthesized a new form of carbon—hexagonal diamond – under conditions of static pressure exceeding about 13 GPa and temperature greater than about 1000°C [1]. At room temperature the crystal structure of graphite is stable up to pressure 15 GPa and loses some of the graphite features at higher pressure, forming metastable graphitic or amorphous phases [2]. Transition of polycrystalline graphite to diamond occurs after hydrostatic pressure treatment near 70 GPa [3]. The development of solid-state phase transitions, including those at the stage of nucleation and development of a new phase practically always is connected with the relaxation of elastic stress [4], and in case of graphite-diamond transformation the latter can play main role.

                The goal of the present work is the formation of diamond from graphite in direct phase transition in a diamond anvil high-pressure cell, where the relaxation of elastic stress can be realized by means of plastic deformation of the sample. The experiment was performed at room temperature without a catalyst.13С was subjected to the shear deformation under pressure of 25 GPa. The structure studies of the obtained material were made by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). In order to prevent the confusion of the diamond obtained in the experiment with one of the diamond anvils we used graphite composed of 13C carbon isotope atoms as a precursor. The diamond anvils consisted of conventional 12C diamond. Before TEM examination of each sample a Raman spectroscopy was used to verify that it contains only 13C (diamond) and no 12C. TEM and EELS were carried out using JEOL JEM-2010 high-resolution transmission electron microscope.

TEM analysis has shown that the samples obtained in the series of our experiments contain several phases of carbon simultaneously. After the high pressure treatment in shear diamond anvil cell (SDAC) there were observed some fragments of the sample, which contained both hexagonal and rhombohedral graphite (significant amounts of the last one), and also diamond and lonsdaleite. Fig. 1 shows the fragment, where the rhombohedral graphite presents.

             Fig. 2a shows the diamond structure fragment with {111}-planes composing 70º. Interplanar distances are 0.206 nm. Fig. 2b shows the EELS-spectrum which can be unambiguously attributed to a diamond.

           Thus, it was shown that 13С-graphite directly transforms into 13С-diamond (at least particularly) without a catalyst at room temperature after treatment in SDAC under pressure of 25 GPa.

1 - Bundy, F.P., Kasper, J.S. (1967) ‘Hexagonal diamond – A New Form of Carbon’, The Journal of Chemical Physics, Vol. 46 No.9, pp.3437-3446.

2- Bundy, F.P., Bassett, W.A., Weathers, M.S., Hemley, R.J., Mao, H.K. and Goncharov, A.F. (1996) ‘The pressure-temperature phase and transformation diagram for carbon’, Carbon, Vol. 34 No.2, pp. 141-153.

3- Schindler, T.L., Vohra, Y.K. (1996) ‘A micro-Raman investigation of high-pressure quenched graphite’, Journal of Physics: Condensed Matter, Vol. 8 No.21, pp. 3963-3963.

4 - Christian, J.W., (2002) The Theory of Transformations in Metals and Alloys, 2nd ed., Pergamon Press, Oxford.


Elizaveta TYUKALOVA (Moscow,Troitsk, Russia), Boris KULNITSKIY, Igor PEREZHOGIN, Alexey KIRICHENKO, Vladimir BLANK
08:00 - 18:15 #5051 - MS02-616 Measuring the number of layers in 2D materials with SEM and AFM.
MS02-616 Measuring the number of layers in 2D materials with SEM and AFM.

2D transition metal chalcogenides enable exciting new applications in electronic devices and show great promise to replace traditional silicon technology as functional building blocks [1]. However, in order to realize this potential there is a range of fabrication and integration challenges that have to be overcome and suitable, non-destructive characterization techniques are needed. Due to their high resolution, electron optical characterization in scanning electron microscopes (SEMs) and atomic force microscope is ideally suited. We show how a full structural and compositional characterization can be obtained by combining EDS, EBSD and AFM analysis.

 

The number of layers present in a 2D material is critical to its performance. As figures 1 and 2 show, we can obtain data of sufficiently high quality to non-destructively measure the number of layers in 2D MoS2 and WSe2 as well as from heterostructures containing both materials by processing EDS data obtained in the SEM. Figure 1 shows an SEM image of a flake of MoS2 with two regions, one with two layers of MoS2 and one with one layer as verified by step height measurements in the AFM and by Raman spectroscopy. EDS spectra acquired from the two different regions show a clear difference in the peak height of the overlapping Mo L-lines and S-K lines. The difference can be quantified by processing the data in a special software designed to calculate the thickness of thin films on substrates (AZtec LayerProbe) [2]. For the calculation, a density of 5.06 g/cm3 was assumes for MoS2. The resulting values shown in figure 1 correspond well to a theoretical interlayer distance of 0.65nm.

 

In order to test whether this method is also suitable for heterostructures of 2D materials, we obtained measurements from a sample where a flake of MoS2 had been transferred onto a flake of WSe2. In the region of where the two flakes overlap, Raman spectroscopy showed that while there is only a single layer of WSe2 present, MoS2 occurs in one layer and two layers. Figure 2 indicates the different regions of interest on the sample. As the W-M line overlaps closely with the Si-K line, the Se-K line was used for the layer thickness measurement. The results in figure 2b show that both the WSe2 layer and the MoS2 layer thickness can be accurately determined.

 

We also show that Kelvin Probe force measurements (KPFM) can be used to image the contrast between different layer thicknesses in both single layers and heterostructures (figure 2c). Further work is necessary to determine whether the work function measured by KPFM can be quantified.

 

In order to add crystallographic data revealing misalignment between flakes, we can use EBSD. IPF maps of an area that contains several flakes of exfoliated MoS2 clearly indicate significant misalignment between some of the flakes (figure 3). This may aid the understanding of the exfoliation process which is still widely used to produce 2D materials for research purposes.

 

Our results indicate the great potential of SEM and AFM for the characterization of devices based on 2D materials and indicate avenues of further work to establish them as means for failure analysis and production quality control.

 

References:

 

[1] S.Z. Butler et al., ACS Nano 7 (2013), p. 2898.

[2] C. Lang et al., Microscopy and Microanalysis 19 (2013), p. 1872.


Christian LANG (High Wycombe, United Kingdom), Matthew HISCOCK, Ravi SUNDARAM, Jonathan MOFFAT, Kim LARSEN
08:00 - 18:15 #5069 - MS02-618 Observation of thermal behavior of black phosphorus as a 2D material.
MS02-618 Observation of thermal behavior of black phosphorus as a 2D material.

 Recent works have focused on a black phosphorus (black P) joined as a family of 2D materials, because of its tunable band gap depended on a thickness from 0.3eV in a bulk to more than 1.0eV in a few layers black P. Very recently, the successful fabrication of a single layer black P from the bulk has been reported. Additionally, unique properties of atomic layers of black P have been demonstrated such as high mobility, large on-off rations, and anisotropic properties and so on. With these results, the black P as a 2D material is extremely attractive in electronic and optoelectronic applications. However, there is a lack of characterization of the few layers black P and the reported properties are still in theory. Especially, the basic parameters concerned with the thermal stability are not determined, though the most thermodynamically stable phosphorus allotrope is the black P. Therefore, it is important to investigate the thermal phenomena of the black P in a various temperature condition within an atomic scale. In this study, we studied the reaction behavior of a single crystal black P under in situ heating conditions. In addition, the microstructure of black P at different temperatures was analyzed using high-resolution TEM (HRTEM).

ACKNOWLEDGMENTS

This work was supported by KBSI (Korea Basic Science Institute) grants to J.-G. Kim (T36210)


Seung Jo YOO, Ji-Hyun LEE, Sang-Gil LEE, Jin-Gyu KIM (Daejeon, Korea)
08:00 - 18:15 #5107 - MS02-620 Characterizing periodic lattice distortions accompanying commensurate charge density waves in single-layer and few-layer 1T-TaSe2.
MS02-620 Characterizing periodic lattice distortions accompanying commensurate charge density waves in single-layer and few-layer 1T-TaSe2.

Due to the discovery of the exceptional electronic and physical properties of graphene, and thanks to hard-ware aberration correction in TEM, a new research area on the atomic structure of other two-dimensional (2D) layered materials has emerged such as transition metal dichalcogenides (TMDs) whose properties differ strongly from those of the semimetallic character of graphene. The diverse properties of TMDs depend on their composition. These materials can be semiconductors, semimetals or true metals, and superconductors. Metallic TMDs like 1T/2H-TaSe2, 1T/2H-TaS2 or 2H-NbSe2 can produce ―depending on temperature, doping and pressure ― so-called charge density waves (CDWs) [1]. These waves are periodic modulations of the charge density in a material accompanied by periodic lattice distortions (PLDs), forming a superstructure in the material. They can occur during a metal-insulator transition due to electron-phonon coupling. CDWs/PLDs in TMDs are of great interest, because they are a model system to understand phenomena like superconductivity, spin density waves, and metal-insulator transitions. Already since the 1970s it is known that bulk 1T-TaSe2 shows a commensurate CDW (CCDW)/PLD at temperatures below 473 K [1] and the CCDW/PLD is characterized by a  superstructure with a lattice parameter of a0 = 3.48 Å. Owing to the layered structure of the TMD materials, nowadays single layers of these materials can be mechanical exfoliated. However, so far neither much is known about the stability and the characteristics of CDWs/PLDs in single- layer TMDs, nor whether they do exist at all in single-layers and/or few-layers due to confinement effects.

Here we present the experimental characterization of PLDs in single-layer and few-layer 1T-TaSe2 as well as in 1T-TaSe2-graphene heterostructures using aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) and selected area electron diffraction (SAED). Experimental observations of PLDs in single-layer 1T-TaSe2 are a challenge due to difficulties in sample preparation and due to electron beam damage occurring during the TEM experiment. In 1T-TaSe2 / graphene heterostructures, graphene is used as a support material for imaging a few micrometer large exfoliated 1T-TaSe2 monolayer. Moreover, graphene sandwiching is used to reduce radiation damage effects [2].  We perform our TEM experiments at a low accelerating voltage of 80 kV, however, the material is not stable under the electron beam and low-dose operation is required. Figure 1 -shows that PLDs can be found in few-layer-thick 1T-TaSe2 graphene heterostructures, Figure 1(a) shows an AC-HRTEM image of this heterostructure and Figure 1(b) the corresponding fast Fourier transformation (FFT). We will also present a method to analyze PLDs in single-layer 1T-TaSe2 using atomic scale mapping. In addition we shall show our first experiments obtained with the new spherical and chromatic aberration-corrected SALVE instrument operating in the range between 20kV and 80kV at exceptionally high-resolution. Results proving better understanding of electron-sample interaction will be also discussed. [3]

References

[1] J. A. Wilson, F. J. Di Salvo, and S. Mahajan, Advances in Physics 24.2 (1975): 117-201.

[2] G. Algara-Siller, S. Kurasch, M. Sedighi, O. Lehtinen, & U. Kaiser, Applied Physics Letters 103.20 (2013): 203107.

[3] The authors acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany in the frame of the SALVE (Sub-Angstroem Low-Voltage) project.


Pia BÖRNER (Ulm, Germany), Michael KINYANJUI, Tibor LEHNERT, Janis KÖSTER, Ute KAISER
08:00 - 18:15 #5129 - MS02-622 Time-resolved imaging and analysis of single atom diffusion on graphene oxide.
MS02-622 Time-resolved imaging and analysis of single atom diffusion on graphene oxide.

Single atoms and small atomic clusters offer a range of novel, tunable properties for a number of applications such as selective catalysis [1]. Achieving precise control of the desired properties of these systems first requires an understanding of the interaction between the cluster and its support. Advances in aberration-corrected scanning transmission electron microscopy (STEM) mean that atomic resolution imaging and characterisation is now achievable for many materials. Observing individual atoms and small clusters remains difficult, however, due to low signal-to-noise ratio and beam-induced motions causing blurring during image acquisition. One route around these problems is to acquire rapid image sequences in an effort to reduce the electron dose and also to capture any dynamic behaviour of the atoms. Making use of the spatial and temporal correlations between frames, and using a novel processing method based on singular value thresholding [2], we have developed robust approaches to recover individual atomic positions, including STEM acquisition rates of 10 frames per second or better [3].

We have applied the approach to the study of catalytically-important copper atoms on few-layer graphene oxide (GO), where the presence of functional groups on GO may aid the control of deposited clusters by acting as preferential pinning sites. Processing and analysis of an annular dark-field STEM image sequence reveals a range of behaviours, with some strongly-pinned atoms and other more mobile atoms undertaking random walks on the surface (Figure 1b). Further investigation of the jump distances (Figure 1c) and mean-squared displacements (Figure 1d) reveals that the diffusion of adatoms on GO is anomalous. We combine this information with ab-initio DFT calculations to provide new insight into the formation and behaviour of small atom clusters under an electron beam, and the interactions between few-atom catalysts and high surface area supports.

References

[1] Tyo, EC, Vajda, S. (2015). Nat. Nanotechnol. 10, 577-588.

[2] Candes EJ, Sing-Long CA, Trzasko JD. (2013). IEEE Trans. Signal Process. 61, 4643-4657.

[3] Furnival T, Leary R, Midgley PA. (2016). Manuscript submitted.

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 291522-3DIMAGE.


Tom FURNIVAL (Cambridge, United Kingdom), Rowan LEARY, Eric C TYO, Stefan VAJDA, John Meurig THOMAS, Paul D BRISTOWE, Paul A MIDGLEY
08:00 - 18:15 #5456 - MS02-624 3D Investigation of InGaN Nanodisks in GaN Nanowires.
MS02-624 3D Investigation of InGaN Nanodisks in GaN Nanowires.

Nanoscaled structures like nanowires (NWs) can influence device characteristics (e.g. higher internal efficiency [1]), making them very suitable for the application in optoelectronic devices. Complex structures like InGaN nanodisks (NDs) embedded in GaN NWs can for example be used as active regions of tunable color and white light LEDs [2,3].

The GaN NWs investigated in this work have been grown via plasma-assisted molecular-beam epitaxy on n-type Si (111) substrates and contain 10x InGaN NDs. The growth direction was [000-1] and the NWs exhibit a hexagonal base with (1-100) planes (m-plane) forming the side facets. The TEM analysis of InGaN/GaN NWs grown under comparable growth parameters is reported in [4]. There it was shown that the ~4nm thick InGaN NDs exhibit a truncated pyramidal shape consisting of a (0001) central facet that is delimited by declining sixfold {10-1l} facets, where l can be -1, -2 or -3. In addition to this declined facets a continuation of the (0001) central facet towards the m-plane sidewalls of the NWs could be observed but is not described in the published STEM images [4]. Since these images contain projected information they do not deliver knowledge on the real shape of the NDs.

To obtain a deeper insight into the three dimensional geometry of the embedded InGaN NDs electron tomography is the method of choice.

Conventional sample preparation (spread NWs over a carbon film lying on a TEM grid) just allows tilting in a range of approximately +/-70° leading to a strong missing wedge effect [5]. The concomitant reduction of the resolution makes a meaningful reconstruction of the InGaN NDs impossible.  

To overcome this problem the sample needs to be prepared in such a way that a sample tilt of +/-90° is permitted. For this procedure we used a SEM (JIB 4601F, JEOL) with an integrated manipulator needle (Kleindiek). First of all a conventional FIB-lift-out-grid (Pelco, Ted Pella) with four narrow posts was trimmed with a scalpel in such a way, that the grid width was reduced from 3mm to less than 1.5mm and that just one post was left. On top of this post an electron beam induced, turret shaped tungsten structure was deposited in the SEM to create an exposed position on which the NW could be attached without any risk of shadowing effects during the tilt series in TEM. Using the manipulator needle a few NWs have been detached from the Si substrate and transferred to the FIB-lift-out-grid (cf Figure 1a). The NW that is most suitable oriented was brought closer to the top of the tungsten deposition. Since the attractive force between needle-NW and tungsten deposition-NW, respectively, are strong a deposition for connection is not necessarily required (cf Figure 1b).

Electron tomography measurements were performed in STEM mode (JEM 2200FS, JEOL using a model 2030, Fischione tomography holder) with a tilt range of -90° until +82° (the tilt angle limitation is due to a restriction of the TEM stage and not related to the sample geometry) and a tilt step of 2°. For reconstruction of the data the software IMOD [6] was used.

Figure 2 shows a section through the middle of a NW running parallel to the (11-20) plane (a-plane). Within this image two features can be observed. First the faceting of the InGaN NDs with an increased steepness of the inclination angle of the side facets for higher lying NDs can be seen. The inclination angle fits well to the above mentioned {10-1l} planes. Second the yellow circle marks a region where the afore-noted split of NDs appears. This is particularly interesting since this structure could be easily attributed to projection artifacts in conventional STEM images.

This example shows, that selecting a sample geometry which allows a tilt angle range as high as possible, is essential for obtaining the required resolution.

[1] D.J. Sirbuly et al., J. Phys. Chem. B 109 (2005) 15190-15213

[2] R. Armitage and K. Tsubaki, Nanotechnology 21 (2010) 195202-1-195202-7

[3] W. Guo et al., Appl. Phys. Lett. 98 (2011) 193102-1-193102-3

[4] Th. Kehagias et al., Nanotechnology 24 (2013) 435702-1-435702-14

[5] I. Arslan et al., Ultramicroscopy 106(11-12) (2006) 994-1000

[6] J.R. Kremer et al., J. Struct. Biol. 116 (1996) 71-76

[7] We acknowledge support of the German Science Foundation (DFG) in the framework of the Collaborative Research Centre “Structure and Dynamics of Internal Interfaces” (SFB 1083)


Katharina I. GRIES (Marburg, Germany), Julian SCHLECHTWEG, Andreas BEYER, Pascal HILLE, Jörg SCHÖRMANN, Martin EICKHOFF, Kerstin VOLZ
08:00 - 18:15 #5670 - MS02-626 Ultra-thin epitaxial selenide films: Structure and two-dimensional properties.
MS02-626 Ultra-thin epitaxial selenide films: Structure and two-dimensional properties.

Ultra-thin films of selenide compound epilayers were deposited epitaxially by molecular beam epitaxy (MBE) on AlN(0001) / Si(111) templates and were studied structurally using high resolution transmission electron microscopy (HRTEM), image simulations, and geometrical phase analysis (GPA). The films comprised Bi2Se3, MoSe2, and HfSe2 epilayers, as well as composite heterostructures of these materials.  Wurtzite AlN is a wide band gap semiconductor that strongly favors an excellent interfacial quality with the selenides under the employed growth conditions contrary to direct deposition on silicon that leads to interfacial amorphization and extended defects in the film. The structural observations were combined with angle-resolved photoelectron spectroscopy measurements.

Bi2Se3 is a topological insulator (TI), and the deposited films exhibited a surface Dirac cone making them promising for novel spintronics and quantum computing applications. HRTEM, combined with GPA, showed an epitaxial well-ordered interface with (0001) AlN. The interfacial periodicity was manifested by a 3:4 plane matching. No interdiffusion or chemical reaction was observed at the interface. High quality and large scale 2D films with thicknesses of 3 and 5 quintuple layers (QLs) were deposited [1]. The films contained only vertical and in-plane 180o rotational domain boundaries, as shown in Fig. 1.

In addition, high quality films of a few monolayers (MLs) of MoSe2 and HfSe2 compound semiconductors were deposited in extended scale by MBE directly on AlN(0001), showing promise for nanoelectronic device applications mediated by the van der Waals bonding [2]. In an alternative approach, Bi2Se3 was employed as buffer layer in order to maintain low growth temperatures that favor large scale manufacture. Furthermore, various combinations of alternating selenide layers were achieved, signifying a versatility towards advanced nanodevice possibilities and prospects for combined 2D semiconductor/TI applications. Cross sectional HRTEM, in conjunction with image simulations elucidated the interfaces between dissimilar materials. Variations in lattice spacings were obtained by GPA. Such a heterostructure is illustrated in Fig. 2.

 

[1] P. Tsipas, E. Xenogiannopoulou, S. Kassavetis, D. Tsoutsou, E. Golias, C. Bazioti, G. P. Dimitrakopulos, Ph. Komninou, H. Liang, M. Caymax, A. Dimoulas, ACS Nano, 8, 6614 (2014).

[2] E. Xenogiannopoulou, P. Tsipas, K. E. Aretouli, D. Tsoutsou, S. A. Giamini, C. Bazioti, G.P. Dimitrakopulos, Ph. Komninou, S. Brems, C. Hughebaert, I. P. Radu, A. Dimoulas, Nanoscale, 7, 7896 (2015).

 

Acknowledgement: Work partially supported by the ERC Advanced Grant SMARTGATE-291260- and the National program of excellence (ARISTEIA-745) through the project TOP-ELECTRONICS.


Calliope BAZIOTI, George DIMITRAKOPULOS, Polychronis TSIPAS, Evangelia XENOGIANNOPOULOU, Athanasios DIMOULAS, Philomela KOMNINOU (Thessaloniki, Greece)
08:00 - 18:15 #5738 - MS02-628 Quantitative low-voltage spherical and chromatic aberration-corrected high-resolution TEM analysis of beam-specimen interactions in single-layer MoS2 and MoS2/graphene heterostructures.
MS02-628 Quantitative low-voltage spherical and chromatic aberration-corrected high-resolution TEM analysis of beam-specimen interactions in single-layer MoS2 and MoS2/graphene heterostructures.

Sub-Angstrom resolution at medium accelerating voltages of 200-300 kV is routinely achieved in standard transmission electron microscopes by hardware correction of the spherical aberration of the imaging lenses [1, 2]. At such voltages, defective, very thin and/or light-element materials very often suffer from knock-on damage, demanding experiments at the lower voltage ends of the microscopes, which are by default 80kV resp. 60kV with a resolution limited to approximately 2Å. Often, however, the achievable resolution at these voltages is not sufficient to understand the electron beam-matter interactions. Consequently, there is a demand for instrumentation that allows atomic-resolution at much lower voltages [3]. Using a standard field-emission electron source, the resolution is strongly limited by the chromatic aberration of the imaging lens requesting hardware aberration corrector that corrects for both, spherical and chromatic aberrations of the objective lens [4, 5].

Here, we report results towards understanding voltage-dependent electron beam-sample interaction for single-layer MoS2 and MoS2-graphene heterostructures using our newly developed Cc/Cs – corrected Sub-Angstrom Low-Voltage Electron Microscope (SALVE) operating at voltages between 20-80kV [6]. Damage cross-sections are determined by directly counting the vacancies produced during the high-resolution TEM experiments obtained at defined voltages and with defined electron doses. As the vacancies are created by different damage mechanisms as knock-on damage, radiolysis, ionization, and chemical etching, we discuss our results in the light of these mechanisms, with the aim to separate their contributions. Similar 80kV Cs-corrected HRTEM studies have been performed earlier albeit with much lower resolution [7]: in this case the resolution is about half the resolution obtained at 30kV (Cc/Cs) (see for comparison Figure 1).  As the knock-on threshold energy for sulfur atoms in MoS2 has been calculated to be about 90 keV [8], the measured increase in the damage rates from 80 to 30 kV is attributed to radiolysis and ionization effects because of increased scattering cross-sections [9].

References

[1] M. Haider, S. Uhlemann, E. Schwan, H. Rose, B. Kabius & K. Urban, Nature 392, 768-769 (1998)

[2] O.L. Krivanek, N. Dellby, M.F. Murfitt, M.F. Chisholm, T.J. Pennycook, K. Suenaga, V. Nicolosi; Ultramicroscopy 110(8), 935 – 945 (2010)

[3] P. Hartel, M. Linck, H. Müller, S. Uhlemann, J. Biskupek, U. Kaiser, M. Niestadt and M. Haider, Microscopy and Microanalysis July 2016

[4] M. Haider, P. Hartel, H. Müller, S. Uhlemann, & J. Zach, (2010) Microscopy and Microanalysis, 16(04), 393-408.

[5] U. Kaiser, J. Biskupek, J. Mayer, J. Leschner, L. Lechner, H. Rose, M. Stöger-Pollach, A. Khlobystov, P. Hartel, H. Müller, M. Haider, S. Eyhusen, G. Benner; Ultramicroscopy 111 (8), 1239 – 1246 (2011)

[6] www.salve-projcet.de

[7] G. Algara-Siller, S. Kurasch, M. Sedighi, O. Lehtinen, U. Kaiser, Appl. Phys. Lett. 103(20) , (2013)

[8] H.P. Komsa, J. Kotakoski, S. Kurasch, O. Lehtinen, U. Kaiser, A.V. Krasheninnikov, Phys. Rev. Lett. 109, 035503 (2012)

[9] The authors acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany in the frame of the SALVE (Sub-Angstrom Low-Voltage) project


Tibor LEHNERT (Ulm, Germany), Johannes BISKUPEK, Janis KÖSTER, Martin LINCK, Ute KAISER
08:00 - 18:15 #5756 - MS02-630 In situ High Temperature ESEM study of carbon nanotubes reactivity under oxidative conditions.
MS02-630 In situ High Temperature ESEM study of carbon nanotubes reactivity under oxidative conditions.

Due to their extraordinary mechanical, electrical, optical, thermal … properties, single wall carbon nanotubes (SWCNTs) have attracted tremendous studies in the last 20 years. However, most of these properties are closely related with the defects present in the carbon layer and with the chirality of the SWNCTs. Two specific studies have been performed in order to better understand the high temperature behavior of SWCNTs, using the high temperature device attached with the environmental scanning electron microscope (HT-ESEM mode).

 

SWCNTs imaging (Fig. 1): Isolated SWCNTs cannot be observed directly in the HT-ESEM due to the relatively poor resolution of this technique. The SWCNTs are deposited on an insulating substrate and low voltage (1-3kV) charge contrast imaging is used to directly observe the CNTs. While the ESEM is not supposed to operate in such high voltage conditions at high temperature, aligned CNTs images have been recorded under various gas compositions and pressures using the gaseous secondary electron detector. These images have demonstrated the possibility to observe the SWCNTs at high temperature in the ESEM and have opened the path for further in situ experiments in the microscope chamber.

 

We studied the chemical reactivity of individual single-walled carbon nanotubes on oxide substrates toward oxygen etching using HT-ESEM (Fig. 2) coupled with AFM observations. Our in situ observations show that the reactivity of carbon nanotubes on substrates is different from that of free-standing ones. In particular, semiconducting nanotubes appear as or slightly more reactive than metallic ones, showing that the nanotube type has a secondary influence compared with that of the substrate. In addition, carbon nanotubes are not progressively etched from their ends as frequently assumed but disappear segment by segment. Atomic force microscopy before and after oxidation reveals that nanotube oxidation proceeds first by a local cutting leading to two separate nanotube segments, which is then followed by a rapid etching of the segment that has been electrically disconnected from others. In addition, our study shows that exposure to electron and laser beams can strongly increase the chemical reactivity of single-walled carbon nanotubes on substrates. These results are rationalized by considering i) the effect of substrate-trapped charges on the nanotube density of states close to the Fermi level, and ii) the effect of electron and laser beams on the density of these surface charges.

 

[1] H. A. Mehedi, J. Ravaux, Y. Khadija, T. Michel, S. Taïr, M. Odorico, R. Podor, V. Jourdain Oxidation Mechanism of Individual Single-Walled Carbon Nanotubes on Substrate Monitored by in situ Scanning Electron Microscopy. Nano Research (2016) 9 519-527


Hassan-Al MEHEDI, Renaud PODOR (ICSM, Marcoule), Johann RAVAUX, Thierry MICHEL, Said TAHIR, Khadija YAZDA, Michael ODORICO, Vincent JOURDAIN
08:00 - 18:15 #5780 - MS02-632 Precise STEM measurement of defocus and aberration in monolayer graphene.
MS02-632 Precise STEM measurement of defocus and aberration in monolayer graphene.

 Improvements to the Z height resolution in a scanning transmission / transmission electron microscope (S/TEM) are important for 3D reconstruction of local specimen topography and for quantitative image analysis using comparisons of experimental images to simulations.  The lateral position perpendicular to the direction of the electron trajectory in aberration corrected S/TEM instruments has a sensitivity comparable to most atomic spacing which can be directly determined in experimental images to an accuracy of a few pm.  However, the resolution in the Z direction parallel to the electron trajectory is of the order of only a few nm as determined by direct experimental measurement using confocal electron microscopy of extended objects [1] or in HAADF STEM images of point like objects [2].  This resolution is an order of magnitude larger most atomic spacing and therefore to accurately determine Z height information at atomic resolution, a method for precise measurement of the specimen height is required.

Toward obtain better accuracy for Z direction measurements, we have used the lateral magnification change of an ideal specimen of monolayer graphene, where in reciprocal space with a probe forming mode, the magnification of the lattice depends on the value of the defocus.  Ronchigrams have been recorded with a focused STEM probe of atomic dimensions at a large camera length of 80 cm at 80 kV [Fig. 1(a)].  The Ronchigram was divided into local angular areas to analyze the local magnification dependent on the probe-forming aberrations.  Importantly the lattice parameter of graphene is accurately known and changes in the lattice pattern with beam tilt or specimen tilt are smaller than for other, thicker crystalline specimens.  The local magnification in each local area was analyzed using an auto-correlation function [Fig.1(b)] [3].  Six peaks surrounding the center peak of the auto-correlation function from a graphene image were identified and were fitted with an oval [Fig.1(c)].  Using the fitted parameters describing the oval shapes at local angular areas, the distance between the probe and the graphene monolayer (defocus) was measured precisely to be - 58.52 nm.  Since the lattice parameter of a specimen is known, only one Ronchigram pattern is required to derive the value of defocus. Other aberration coefficients for the probe forming lens also can be calculated using this method [Table. 1].

[1] P. Wang, et. al. Physical Review Letters, 104, 200801 (2010).

[2] K .Benthema, et. al. Applied Physics Letters 87, 034104 (2005).

[3] H. Sawada, et al, Ultramicroscopy 108, 1467 (2008).


Hidetaka SAWADA (Oxford, United Kingdom), Angus KIRKLAND
08:00 - 18:15 #5800 - MS02-634 Revealing Formation Mechanism of Self-Induced InAlN Core-Shell Nanorods by Aberration-Corrected TEM.
MS02-634 Revealing Formation Mechanism of Self-Induced InAlN Core-Shell Nanorods by Aberration-Corrected TEM.

The interest in nanoscale structures has gained immense momentum due to their scientific and technological potentials. Among accessible nanostructures, semiconductor materials fabricated as one-dimensional nanorods (NRs) offer fascinating physical properties and engineering capabilities for active components in present and future nanoscale functional devices [1]. In particular, group-III nitride semiconductor NRs based on AlN, GaN, InN and their ternary alloys are attractive due to the widely tunable direct bandgap (0.64-6.2 eV), high crystal quality and improved light extraction efficiency. Recently, self-induced core-shell ternary nitride NRs have been demonstrated [2-5]. However, the understanding of the formation mechanism and self-induced separation remains debated. In this work we investigated the formation mechanism of self-induced core-shell InAlN NRs growth on an amorphous C substrate.

A number of InAlN samples were grown by magnetron sputtering epitaxy directly onto TEM grids which supports amorphous C films (substrates) with varying growth time. Apart from the otherwise identical growth conditions, the growth time (t) was varied in the steps of t=1, 2, 3, 5, and 20 min for the different samples. The obtained samples were subsequently investigated in plan-view projection using the doubly-corrected Linköping FEI Titan3 60-300. The microscope is equipped with a monochromated X-FEG high-brightness gun, efficient high solid angle Super-X EDX detector and ultrafast Gatan GIF Quantum ERS post-column imaging filter.

A time series of InAlN NR formation, at different nucleation and growth stage, imaged by plan-view STEM-HAADF together with corresponding SAED patterns, is shown in Fig. 1. The imaging conditions strongly promote image contrast dependence on the mass (Z number) and sample thickness with reduced diffraction contrast contributions. The bright features exhibit high mass and indicate that they are In-enriched. Al and In elemental distribution in the grown samples were examined using EDX elemental mapping as shown in Fig. 2. The elemental maps corroborate that the bright features (in Fig. 1) are In-enriched (InAlN) islands, while less bright areas represent Al-rich (InAlN) islands in the 1 and 2 min samples. The situation is somewhat more complex for the samples with extended growth time (3, 5, and 20 min), where the thickness contribution to the STEM-HAADF and EDX intensities must also be considered. Additionally, we performed EDX quantification, SAED pattern analysis, high-resolution TEM and STEM-HAADF imaging as well as statistical STEM-HAADF image contrast analysis.

By investigating different InAlN sample at different growth time we were able to follow NRs evolution process: from initial In-rich InAlN seed nucleation to final NR core-shell formation. This enabled us to derive NR formation scenario as a function of the growth time, see Fig. 3. The NR formation process can be divided into two distinct regimes. I) the nucleation and coalescence phase where In-enriched islands are surrounded by Al-rich, and II) the growth phase during which the In-enriched islands develop into cores and the surrounding Al-rich environment develop into shells around the cores, and the core-shell structure eventually reach a steady state NR growth. To account for the present observations we consider a number of factors affecting the NRs formation, including: adatom (In, Al, and N) surface kinetics (adsorption, desorption and surface diffusion), chemical potential, surface energy, thermal stability, and incoming flux (shadowing effect) during dual magnetron sputter epitaxy.

 

References:

[1] C. R. Eddy Jr., et al., J. Vac. Sci. Technol. A 31, 058501 (2013).

[2] C.-L. Hsiao, et al., Appl. Phys. Exp. 4, 115002 (2011).

[3] M. Gómez-Gómez, et al., Nanotechnology. 25, 075705 (2014).

[4] R. F. Webster, et al., Phys. Status Solidi C 11, 417–420 (2014).

[5] C.-L. Hsiao, et al., Nano Letters 15(1), 294-300 (2015).


Justinas PALISAITIS (Linköping, Sweden), Ching-Lien HSIAO, Lars HULTMAN, Jens BIRCH, Per O. Å. PERSSON
08:00 - 18:15 #5837 - MS02-636 EELS Investigation of the Work Function Reduction in Au decorated ZnO Nanotapers.
MS02-636 EELS Investigation of the Work Function Reduction in Au decorated ZnO Nanotapers.

The study of field emission (FE) from one dimensional (1D) nanostructures is emerging as a promising technology that can make a considerable contribution in the development of next generation devices such as electron microscopes, x-ray sources, flat panel displays and microwave devices. Generally, FE from carbon nanotubes has been extensively studied, although these have high work function (~5.2 eV) and low electron emission. The electron emission is also influenced by the density of state (DOS) of the emission site. The DOS can be increased by either some doping or by coating the surface with a much higher DOS material. Owing to the low work function, enhanced currents have been demonstrated in several ZnO systems, for example nanowires, nanopins, nanorods, tetrapod like, nitrogen implanted nanowires, Au coated ZnO nanowires etc. The Au-coated ZnO nanowires show low turn-on potential and excellent stability. The work function of ZnO can be altered either by hybridization with a donor organometallic, or inorganic molecule / polymer by the attachment of dipolar self-assembled monolayers perpendicular to the surface of ZnO, which induces charge transfer between the adsorbate molecule and substrate surface. The incorporation of Au ions on the surface modifies the surface morphology and that influences the enhanced field emission by lowering the Φ. The possibility of tuning the band gap by coating the ZnO surface with Au nanoparticles is very important because of its potential application in light emission devices in the ultraviolet (UV) region.

 

For this work, ZnO nanotapers samples have been coated using different Au particles size and analyzed using means of local conductive atomic force microscopy (CAFM) to measure the local conductance and also electron energy loss spectroscopy (EELS) in a STEM microscope to investigate the chemistry across the Au-ZnO interface as function of the Au particles size. The surface modification of the ZnO due to the Au decoration can alter the bandgap which might lead to potential applications for light emitting devices.  The bandgap was measured using high-energy resolution EELS analysis and appears to be dependent upon the Au particles size on the ZnO surface.

 

Figure 1 shows the ADF STEM image of the ZnO rod with some Au particles decorating the surface. EELS STEM was carried out across the Au-ZnO interface along the line shown in Figure 1. Figure 2 shows the O K-edge EELS spectra extracted from across the Au-ZnO interface and away in the ZnO region. The two spectra are clearly different. In particular, the peaks labeled as 2 and 3 in Figure 2 are much more defined than those in the spectrum extracted across the Au-ZnO interface. This is the clear sign that the incorporation of Au particles onto the surface modify the surface morphology and as result the Zn-O bonding. The same EELS STEM analysis was repeated across the Au-ZnO interfaces from samples decorated with different Au particles size and we have observed different features in the O K-edge in the EELS spectrum. This is evidence that the interaction with the Au particles modifies the ZnO surface and the effect is Au particle size dependent.


Avanendra SINGH, Paolo LONGO (Pleasanton, USA), Kartik SENAPATI, Ray TWESTEN, Pratap SAHOO
08:00 - 18:15 #5866 - MS02-638 Spontaneous formation of core–shell GaAsP nanowires with enhanced electrical conductivity.
MS02-638 Spontaneous formation of core–shell GaAsP nanowires with enhanced electrical conductivity.

The growth of GaAsP nanowires on GaAs (111)B substrates exhibit a core-shell heterosturture with P-enriched cores, which is attributed to Au catalysts enhancement of the local decomposition of PH3. These core–shell GaAsP nanowires exhibit enhanced electrical conductivity when compared with uniform GaAsP nanowires. This study provides an approach to enhance the electrical conductivity of III–V semiconductor nanowires.

Introduction: Ternary III-V epitaxial nanowires allow a continuous tuning of the bandgap, and they are also are required for fabricating complex radial/axial heterostructures devices.1 During the Au-catalyzed growth of III–V nanowires, group III and group V elements take different pathways to incorporate into the nanowires: group III elements through alloying with the Au catalyst, while group V elements through the triple phase line.2 In this study, the composition distribution between two group V elements was studied, and the electrical properties of these core–shell ternary GaAsP nanowires were investigated.

Experiment: GaAsP nanowires were epitaxially grown on the GaAs (111)B substrates using a horizontal flow MOCVD reactor at the pressure of 100 mbar with ultrahigh purity H2 as the carrier gas. Trimethylgallium (TMG) and was used as the group-III source, while PH3 and AsH3 were used as the group-V sources. Nanowires were grown at 500 °C and 420 °C for 30 min with a V/III ratio of 39.3 and a PH3/( PH3 + AsH3) ratio of 0.98. Electron microscopies were used to investigate the characteristics and transport measurements of grown nanowires.3

Results and discussion: SEM study showed that most nanowires synthesized at 500 °C grew vertically on the GaAs {111}B substrate with a tapered morphology. The nanowire quantitative analyses of EDS spectra taken from different sections indicate that the compositional distribution along the nanowire is uneven. The As/(P + As) ratio increases from 19 at% at the top to 22 at% at the middle and to 25 at% at the bottom of the nanowire. The tapered nanowire morphology suggests that lateral growth took place during the nanowire growth at 500 °C,3 which would lead to the formation of a shell. To clarify whether our tapered nanowires have a core–shell structure, TEM investigations were carried out on cross-sections of individual nanowires sliced from the tip, the middle and the bottom regions. Fig. 1(a) is a typical example of cross-section obtained from the bottom region of a nanowire, and shows a truncated-triangular shaped cross-section. Fig. 1(b) is a corresponding SAED. It should be noted that the As concentration in the centre (Fig. 1c) is similar to that of tip region of nanowire, indicating that the As concentration in the nanowire core is uniform along the nanowires. The EDS maps in Fig. 1(f, g) clearly identify the enriched P core and the enriched As shell. The formation of the core-shell GaAsP nanowires was attributed to two facts: (1) Since Au catalysts can enhance the decomposition of PH3.4 the P concentration around the nanowire catalysts should be higher than the general environment, leading to a relatively higher P concentration in the nanowire core; (2) the sticking coefficient of As adatoms is higher than that of P adatoms,5 and therefore, it is possible that As is preferentially incorporated on the sidewall of the formally formed core, leading to the higher concentration of As in the nanowire shells.

Vertically grown GaAsP nanowires at a temperature of 420 °C are uniform in their lateral dimension, as shown in Fig. 2(a). The compositional distribution along the nanowire is uniform with an As/(P + As) ratio of approximately 48 at% at the top (Fig. 2(c)) and approximately 47 at% at the bottom (Fig. 2(d)) of the nanowire. EDS maps in Fig. 2(f) and (g) shows the evenly distributed As and P across the nanowire cross-section, indicating that the GaAsP nanowires grown at 420 °C are homogeneous nanowires. Fig. 3 shows the I–V characteristics of the GaAsP nanowires synthesis at different temperatures. The homogeneous GaAsP nanowire grown at 420 °C show no electrical conductivity, while the core–shell GaAsP nanowires are capable of enhanced conductivity. It is likely that the band offset6 in core–shell nanowires could lead to the accumulation of carrier gas at core–shell interfaces, and in turn, the enhanced conductivity in undoped nanowires.

References:

1Y. Kim, H. J. Joyce, and et al, Nano Letters 6, 599 (2006).

2K. A. Dick, K. Deppert, and et al, Adv. Funct. Mater., 15, 1603 (2005).

3J. Zou, M. Paladugu, and et al, Small, 3, 389 (2007).

4M. A. Verheijen, G. Immink, and et al, J. Am. Chem. Soc., 128, 1353 (2006).

5Y. Y. Zhang, M. Aagesen, and et al, Nano Lett., 13, 3897 (2013).

6E. Dimakis, U. Jahn, and et al, Nano Lett., 14, 2604 (2014).


Wen SUN (Beijing, China), Yang HUANG, Yanan GUO, Zhiming LIAO, Qiang GAO, Hark Hoe TAN, Chennupati JAGADISH, Xiaozhou LIAO, Jin ZOU
08:00 - 18:15 #5902 - MS02-640 Nanoscale Chemical Mapping of GdX3@WS2 Nanotubes by EDS-STEM Tomography.
MS02-640 Nanoscale Chemical Mapping of GdX3@WS2 Nanotubes by EDS-STEM Tomography.

The hollow interiors of nanotubes could host the growth or filling of foreign elements/compounds to obtain hetero-structures. The growth of these materials in the confined one dimensional space lead to novel properties. Capillary filling serves as a method to enable filling of carbon nanotubes and inorganic nanotubes including those of BN and WS2.1, 2 In this work, considering the biocompatibility of WS2 and paramagnetic property of gadolinium (III) compounds, capillary filling is employed to obtain GdX3@WS2 nanotubes (X=Cl, Br, I).  The precise determination of the structure and composition is detrimental in its further application. Thus in the present study the morphology, structure and chemical composition of the synthesized GdI3 filled WS2 is investigated using aberration corrected scanning/transmission electron microscopy and associated spectroscopic techniques (EELS and EDS). The three-dimensional morphology is investigated using HAADF-STEM tomography but obtaining three dimensional composition information is non-trivial due to the presence of multiple high atomic number elements. Therefore, EDS-STEM tomography is employed in the present study to map the chemical composition in three dimensions.3 In order to reduce the beam induced effects on the specimen, tomography experiments were carried out at 80 kV in the present case. In view of the long duration of electron beam exposure necessary to perform EDS-STEM tomography, additional electron irradiation studies were carried out to optimize the EDS-STEM tomography conditions.

References

 [1]. Ronen Kreizman, Andrey N. Enyashin, Francis Leonard Deepak, Ana Albu-Yaron, Ronit Popovitz-Biro, Gotthard Seifert, and Reshef Tenne,  Adv. Funct. Mater., 20 (2010) 2459–2468 

[2].  Elok Fidiani, Pedro M. F. J. Costa, Anja U. B. Wolter, Diana Maier, Bernd Buechner, and Silke Hampel, J. Phys. Chem. C, 117 (2013) 16725−16733

[3]. Georg Haberfehlner, Angelina Orthacker, Mihaela Albu, Jiehua Li and Gerald Kothleitner, Nanoscale, 6 (2014) 14563–14569


Anumol E. A. (Braga, Portugal), Francis LEONARD DEEPAK
08:00 - 18:15 #5960 - MS02-642 Characterization of Branched Carbon Nanostructures.
MS02-642 Characterization of Branched Carbon Nanostructures.

Branched carbon nanostructures such as branched-Multi-Walled Carbon Nanotubes (b-MWCNTs, Fig. 1) are exotic types of carbon nanostructures whose technological potential have not yet been fully explored. Although MWCNTs have been used to improve the properties of composite materials, there are currently still two main problems remaining to be solved before MWCNT/composite materials can realize their full potential:-

(1) adequate dispersion of the nanotube-reinforcement material, and

(2) strong enough interfacial bonding between the nanotube-reinforcement elements and the composite matrix.

These problems can be addressed by utilizing branched-carbon nanostructures as it is known (from theory and simulation experiments) that branched fibres greatly enhance interfacial bonding e.g. the ancient process of adding straw to mud to make stronger bricks. It is well known that, in the case of carbon nanotube networks, junction resistance is the dominant limiting factor and so, a network of branched-carbon nanostructures would significantly reduce this network resistance. . Therefore, in addition to potential improvements in composite applications, the electrical properties of networks made of branched-carbon nanostructures could have major benefits to the existing commercial application of CNT/CNF reinforced composites in Conductive Static Dissipation (ESD) as well as potential use in Supercapacitors, Solar Cells and Li-Ion batteries.

Acknowledgements: S. M. acknowledges the continuing support of Prof Dr M. M. Kappes. This work was partly supported by World Premier International Research Center Initiative (WPI Initiative) from MEXT, Japan and we thank Dr Daisuke Fuijita and Dr Kiyotaka Iiayma for their support. We acknowledge Dr. Tony D. Keene, Southampton University for his support. We also thank Bayer Material Science A.G. for supply of MWCNT. The authors would like to acknowledge the contribution of COST Action CA15107.


Sharali MALIK (Karlsruhe, Germany), Yoshihiro NEMOTO, Guo HONGXUAN, Ariga KATSUHIKO, Jonathan HILL
08:00 - 18:15 #5997 - MS02-644 Covalent functionalization of Single-Wall Carbon Nanotube by ferrocene derivatives : localisation and identification of a single molecule.
MS02-644 Covalent functionalization of Single-Wall Carbon Nanotube by ferrocene derivatives : localisation and identification of a single molecule.

Depending on the rolling vector, single-wall carbon nanotubes (SWCNTs) can present metallic or semi-conducting properties, which is of strong interest for potential applications in electronic devices and sensors. Consequently, SWCNTs have been recognized as interesting candidates for developing electrochemical biosensors for several years. They are described as promoters of electron transfer between electrode and target molecules, increasing reaction rate and decreasing electrode response time [1,2]. To realise efficient bio-electrochemical sensor of D-glucose, ferrocene derivatives were covalently grafted on purified SWCNTs. Ferrocene active electrochemical center was separated from carbon nanotube by polyethylene glycol linkers of different chain length (3 to 5 etoxy links).

The functionalized SWCNTs were studied using an aberration-corrected STEM working at accelerating voltage of 80 kV (Jeol ARM200) to assess the efficiency of the functionalization and explore the selectivity of the grafting.

High-resolution HAADF and BF images were simultaneously recorded to determine the structure of the SWCNTs. By analysing the Fourier-transform of the images the chiral indices of a SWCNT can be determined using the same method than for ED pattern [3]. The comparison of information obtained from HAADF and BF images at different focus and simulations using QSTEM [4] shows that the chiral indices of a SWCNT can be assigned by measuring the tube diameter on HAADF and the chiral angle from BF images, for a focus range of a few nm. The simultaneous use of both detectors may allow for a lowering the dose of electrons for the recording of relevant data and so limit the irradiation damages, especially for small-diameter SWCNTs.

On HAADF, the strong signature of atoms heavier than carbon, at the end of a tiny over-contrast of about 1.4 nm long can be seen on some SWCNTs, as it is shown in the figure, in the red square. The structure of this SWCNT of 1.1 nm in diameter was determined as a chiral tube with (11,6) indices. QSTEM simulation of a single ferrocene-polyethylene-glycol molecule grafted on (11,6) CNT shows contrasts that match well with that of the experimental image. EELS spectrum images were realised to track oxygen K and iron L2-3 signals. As it can be seen in the figure, two iron atoms were detected in the area of analysis. Their localisation corresponds to that of the two strong contrasts in the HAADF image. In the EELS spectra the energy of the Fe L2-3 edges (close to 708 eV and 721 eV) is consistent to that usually recorded with XPS for Fe (II) in ferrocene groups [5]. Oxygen signal was also detected along the chain that links the iron atom to the SWCNT. The analysis of about thirty SWCNTs shows that the functionalization method affects both semiconducting and metallic tubes.

 

References :

[1] S. K. Vashist, D. Zhen, K. Al-Rubeaan, J. H. T. Luong, F. S. Sheu, Biotechnology Adv.

29 (2011) 169-88.

[2] F. Tasca, W. Harreither, R. Ludwig, J. J. Gooding, L. Gorton, Anal. Chem. 83 (2011)

3042-9

[3] H. Jiang, A. G. Nasibulin, D. P. Brown, E. I. Kauppinen, Carbon 45 (2007) 662–7

[4] C. Koch, PhD thesis, Arizona State University, May 2002

[5] T. Kitagawa, H. Matsubara, K.Komatsu, K. Hirai, T. Okazaki, T. Hase, Langmuir 29 (2013) 4275-81


Xavier DEVAUX (NANCY), Naoual ALLALI, Victor MAMANE, Manuel DOSSOT
08:00 - 18:15 #6040 - MS02-646 Details on the TiO2 nanotubes wall structure revealed by HRTEM.
MS02-646 Details on the TiO2 nanotubes wall structure revealed by HRTEM.

Details on the TiO2 nanotubes wall structure revealed by HRTEM

Valentin S. Teodorescu1, Leona C. Nistor1, Silviu Preda2, Maria Zaharescu2,

Marie-Genevieve Blanchin3

1.National Institute of Materials Physics, 077125 Magurele, Ilfov, Romania

2.Institute of Physical Chemistry“Ilie Murgulescu”, Romanian Academy

060021 Bucharest, Romania

3. ILM- Universite Claude-Bernard Lyon 1,  69622 Villeurbanne, France

 

Titanium oxide nanotubes were previously obtained by hydrothermal treatment using a crystalline precursor [1].  We have prepared titania nanotubes by the hydrothermal method starting with amorphous and crystalline sol-gel TiO2 precursors [2]. The hydrothermal treatment was realized in the presence of a 10 M NaOH solution at 140°C for various periods of time, from 24 to72 hours. The reaction yield was separated by centrifugation and washed alternately with distilled water and 0.1N HCl solution, down to pH ~6. The sample was dried at 110°C, for 12 h, in air. The HRTEM study was performed on specimens prepared on holey carbon grids.

 

Figure 1 show a low magnification image of an aggregate of TiO2 nanotubes obtained from an amorphous sol-gel precursor. The Na content, determined by the EDX, is about 3% for large aggregates, but is less than 1% in the case of small, transparent, nanotubes aggregates.

 

The high resolution study of the resulted TiO2 nanotubes, evidenced some interesting details about the nanotube formation. One is the presence of unrolled TiO2 foils. Figure 2 shows the presence of a TiO2 unrolled foil in the nanotubes aggregate. It is remarkable that the foil structure is much resistant to the electron beam irradiation compared to nanotubes . The most interesting is the spacing variation between the layers forming the nanotube wall, depending of the number of the layers in the wall.

The HRTEM study reveals the presence of TiO2 nanotubes with different number of layers in the wall, from 2 to 5, the majority having 3 layers in the wall. A comparative analyze of the wall details in the HRTEM images, evidenced clearly this variation of the spacing between the layers in the wall, depending on the number of layers: the higher the number of layers in the wall the smaller the spacing as revealed in figures 2 and 3a., On the other hand, the inner diameter of the nanotube becomes smaller as the number of layers grows. These measurements are shown in figure 3b.

We can explain these details supposing that the TiO2 foils formed from the amorphous precursor, are quite defected hindering the foil rolling. This effect explains the massive presence of unrolled TiO2 foils. On the other hand, the spacings in the rolling foils are also controlled by the foil defects, i.e. they become larger as the foil has more defects. Moreover, the rolling force between the TiO2 layers is more effective in the case of successive rolling of the foils, reducing the spacing between layers and leading to a smaller inner diameter of the nanotube.

References:

1. G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.-M. Peng: Preparation and Structure Analysis of

Titanium Oxide Nanotubes. Appl. Phys. Lett. 79, 3702 (2001).

 

2. S.Preda, V.S.Teodorescu, A.M.Musuc, C.Andronescu, M.Zaharescu,  Influence of the TiO2 precursors on the thermal and structural stability of titanate-based nanotubes, , J.Mater. Res. Vol 28(3),294-303,2013


Valentin Serban TEODORESCU, Leona Cristina NISTOR, Silviu PREDA, Maria ZAHARESCU, Marie-Genevieve BLANCHIN, Valentin Serban TEODORESCU (Bucharest-Magurele, Romania)
08:00 - 18:15 #6052 - MS02-650 Structure and topology of chemical vapour deposited graphene by scanning electron diffraction.
MS02-650 Structure and topology of chemical vapour deposited graphene by scanning electron diffraction.

Structural and topological features of graphene have been investigated widely in the (scanning) transmission electron microscope and include: grain structure [1], layer number and mis-stacking [2], dislocations [3] and out of plane buckling [4, 5]. Here we explore new insights offered by scanning electron diffraction (SED), including quantitative analysis of crystal orientation and local strain. SED involves scanning the electron beam across a specimen and recording a diffraction pattern at each point. This provides a four-dimensional (4d) dataset combining real and reciprocal space information with nanoscale spatial resolution [6]. SED can be performed over areas of a few square micrometres, and the rich 4d data can be analysed using a number of versatile schemes, as described below. This automated analysis enables numerous regions to be considered, an example of which is shown (Fig.1) from a graphene sample grown by chemical vapour deposition on copper [7].

  

‘Diffraction images’ can be formed by plotting the intensity of a particular sub-set of pixels in each diffraction pattern as a function of probe position to elucidate any variations in the diffraction condition. In Fig.1a, integration windows are selected around a particular set of first-order ((1,0)-type) and second-order ((1,1)-type) reflections to yield the ‘virtual’ dark field images in Fig.1b. These images reveal the local grain structure, in this case a grain in the lower right area of the map. They also show light/dark fringes associated with a small island (arrowed) as well as a fold (starred). This contrast can be attributed to deviation from perfect stacking between the island and the underlying graphene grain, or between layers in the fold. The contrast is understood in terms of variations in the interference condition for electrons scattered from atoms in each of the layers as their relative position varies spatially [5]. The most notable benefits of SED lie in further computational analysis. Orientation images can be produced by matching each diffraction pattern to a library of simulated patterns to automatically map the grain structure and determine the local orientation. All grains are then revealed, and the disorientation across grain boundaries can be determined (Fig. 1c). Strain and small orientation variations are also of considerable importance, and can be mapped with SED by comparing each pattern to an unstrained reference. Fig.1d shows up to 3% strain around the fold, as well as the rotation associated with the ~2º small angle grain boundary. Our approach can thus provide a comprehensive 'crystal cartography' of layered materials, paving the way for thorough understanding and exploitation of their unique structure and topology.

  

[1] Huang, P. et al., Nature, 2011, 496, 389-392

[2] Brown, L. et al., Nano Lett., 2012, 12, 1609-161

[3] Butz, B. et al., Nature 2014, 505, 533-537

[4] Yazyev, O. V., et al, Nature Nanotechnology, 2014, 9, 755-767

[5] Ovid’ko, I.A., Rev. Adv. Mater. Sci., 2012, 30, 201-224

[6] Moeck, P. et al., Cryst. Res. Technol., 2011, 46, 589-606

[7] Bae, S. et al., Nature Nanotechnology, 2010, 5, 574-578

  

We acknowledge funding from the EU Graphene flagship, the ERC (291522-3DIMAGE), ERC Hetero2D, the EC (312483-ESTEEM2), a Vice Chancellor’s award from the University of Cambridge, a Junior Research Fellowship at Clare College, a Royal Society University Research Fellowship, the Cambridge NanoDTC and GrapheneCDT.


Duncan N. JOHNSTONE (Cambridge, United Kingdom), Rowan K. LEARY, Alexander S. EGGEMAN, Stephen HODGE, Ugo SASSI, Domenico DE FAZIO, Andrea C. FERARRI, Paul A. MIDGLEY
08:00 - 18:15 #6090 - MS02-652 Optical and structural properties of facetted boron nitrides nanotubes.
MS02-652 Optical and structural properties of facetted boron nitrides nanotubes.

Luminescent devices operating at sub-250nm wavelength present a strong commercial interest. Applications such as antibacterial properties, high density optical storage or nanofabrication possibilies require reliable, portable and efficient devices. Actual Deep UltraViolet (DUV) sources are based on gaz active regions which are difficult to integrate in mobile devices, exhibit poor efficiencies and are harmful for environment. Thus, the development of solid state based DUV emitters is getting more and more relevant.

 

Hexagonal boron nitride is a wide band gap semiconductor (~ 6.5 eV), which meets a growing interest for DUV applications. In contrast to carbon nanotubes, Boron Nitride Nanotubes (BNNTs) are all semiconductors whatever their diameter and chirality and their luminescence emission occurs between 200 nm and 250 nm and is governed by strong excitonic effects. Until recently, the optical properties were poorly known due to both the scarcity of samples and suitable investigation tools. This situation has changed thanks to the development of dedicated photoluminescence (PL) and cathodoluminescence (CL) experiments running at 4K and adapted to the detection in the far UV range [1, 2, 3].

These previous studies on boron nitride nanotubes have mainly dealt with multi-wall BNNTs with a large number of walls (20-120 walls). These tubes luminesce between 226 and 234nm and this spectral range has been assigned, in hBN, to transitions involving defects. A critical point to further study the confinement effect on the excitonic transitions is therefore to elucidate the luminescence origin of these multiwalls. Furthermore it is important to investigate the luminescence of small diameter BNNTs (with a reduced number of walls), which actually appears to be very challenging.

 

Cathodoluminescence from a single BNNT with a large number of walls have been measured with a spatial resolution of about ten nanometers, thanks to an UV dedicated SEM system. Different areas along the tube were investigated, from which luminescence is detected at few wavelengths. From 224 to 228 nm, monochromatic cathodoluminescence images exhibit features, which can be linked to defects in the crystallographic structure, separately observed by Transmission Electron Microscopy (TEM) on the same tube.HRTEM observations and tomography experiments revealed that the BNNTs exhibit a peculiar shape. The section of the tube is polygonal with a number of facets between 6 and 9 (Fig 1). These facets forms an helix along the axis of the nanotube. An important consequence of this facetting is the formation of a large number of dislocations along the tube.

 

We will discuss the relations between these structural properties and the luminescence as shown on fig 2.

 

References

[1] P. Jaffrennou el al., J. Appl. Phys. 102  (2007) 116102

[2] P. Jaffrennou and al., Phys. Rev. B, 77 (2008), 235422.

[3] K. Watanabe and al., Phys. Rev. B, 79 (2009), 193104.


Aurélie PIERRET, Léonard SCHUÉ, Frédéric FOSSARD (CNRS-ONERA), Julien BARJON, Ovidiu ERSEN, Simona MOLDOVAN, François DUCASTELLE, Annick LOISEAU
08:00 - 18:15 #6100 - MS02-654 Spectroscopy on Black Phosphorus exfoliated down to the monolayer.
MS02-654 Spectroscopy on Black Phosphorus exfoliated down to the monolayer.

Black Phosphorus (P(black)) is a 2D semiconductor characterized by a direct band gap associated to high carriers mobility. The crystal is composed by tetravalent P atoms stacked by weak van der Waals interactions that can be exfoliated down to the monolayer using similar procedures than for graphene.  Studying pristine thin layers of P(black) is however challenging due to its strong degradation upon exposure to visible light in air.

 

In this study, we have investigated the chemistry of degradation using in-situ Raman spectroscopy, Transmission Electron Microscopy imaging and Electron Energy core-Loss spectroscopy (EELS) of mechanically exfoliated layers prepared in their pristine state in a glove box. The results highlight a thickness dependent photo-assisted oxidation reaction by adsorbed oxygen in water [1]. Using EELS, we have inspected the O K-edge and P L2,3 edge which gets shifted from 130.2 eV in pristine phosphorus to 136 eV in oxidized phosphorus. As shown in Fig.1, the thickness dependence to oxidation has been clearly revealed by comparing layers of different thicknesses before and after a 30s exposure to ambient air and light. On the basis of such experiments, we have proposed an oxidation mechanism involving electron transfer processes based on quantum confinement and found appropriate manipulation procedures opening a route to first Raman measurements on 1 to 5 pristine layers of P(black)[1].

 

We use also low-loss-EELS spectroscopy to investigate the angular dependence in the Brillouin zone of the dielectric response of exfoliated P(black) in the range [2-40 eV], taking advantages of the TEM-STEM Libra 200 machine at LEM. This machine is equipped with an electrostatic monochromator operating at 80 kV and makes possible the investigation of the angular dependence of the dielectric function at a nm scale and with an energy resolution below 100 meV. To this aim we applied the technique developed in [2] and fully adapted to the machine. Using this technique we have studied the onset of electronic excitations and the dispersion of the plasmons as a function of the q momentum for different crystallographic in plane orientations in mechanically exfoliated P(black) down to 2-3 layers. An example of w - q mapping recorded along the [002] q-direction is displayed in Fig.2 and reveals a large dispersion of the plasmon peak occurring at 19 eV at q= 0. The quantification of this dispersion is obtained by extracting the q dependence of this plasmon peak from the map as shown in Fig.3.  Q dependences along two different in plane directions of the layers, namely [200] and [002], are compared in Fig.4 and clearly reveal high anisotropy effects, which will be discussed with the help of suitable ab initio calaculations.

 

 

 

[1] A. Favron, E. Gaufrès et al, Nature Materials, 14,  (2015)

[2] P. Wachsmuth et.al., Phys.Rev.B (88), 075433 (2013)


Etienne GAUFRÈS (Châtillon), Alexandre FAVRON, Frédéric FOSSARD, Pierre LÉVESQUE, Anne-Laurence PHANEUF-L'HEUREUX, Sébastien FRANCOEUR, Richard MARTEL, Annick LOISEAU
08:00 - 18:15 #6105 - MS02-656 Structure and Energetics of Double-Wall Carbon Nanotubes.
MS02-656 Structure and Energetics of Double-Wall Carbon Nanotubes.

Single-walled carbon nanotubes (SWNTs) have shown oustanding capabilities in the realization of new functional devices but are extremely sensitive to any slight changes in their environment, altering their physical properties. A strategy to overcome this difficulty is to use double-walled carbon nanotubes (DWNTs), consisting of two concentric tubes. In order to better know the basic properties of this kind of tubes in linkage with their structure, we have developed a systematic and robust procedure using acHR-TEM (aberration corrected Transmitting High Resolution Electron Microscopy) to determine the atomic structure over one hundred DWNTs [1]. This procedure includes several steps which are summarized in Fig.1. Statistical analyses of their diameters and twist angle between inner and outer tubes shown in Fig.2 and Fig.3, reveal that some configurations are strongly favored whereas some others are never observed. These results reveal the existence of strong coupling between the two concentric tubes in a DWNT for the smaller diameters lower than 2 nm. To complete this analysis, we performed Monte Carlo calculations with an empirical energetic model in order to understand the nature of the coupling and explain the selectivity of observed patterns [2].

 

[1] M. Kociak, K. Hirahara, K. Suenaga, and S. Iijima, How accurate can the determination of chiral indices of carbon nanotubes be ? , Eur. Phys. J. B. 32 , 457, 2003.

[2] A. Ghedjatti, F. Fossard, E. Flahaut, J.-S. Lauret, A. Loiseau, Structural Configurations of Double-Walled Carbon Nanotubes Investigated by Transmitting Electron Microscopy  submitted


Ahmed GHEDJATTI, Frédéric FOSSARD, Guillaume WANG, Emmanuel FLAHAUT, Hakim AMARA, Jean-Sébastien LAURET, Annick LOISEAU (Chatillon)
08:00 - 18:15 #6159 - MS02-658 Revisiting Graphene Oxide Structure via Spatially-Resolved Electron Energy Loss Spectroscopy.
MS02-658 Revisiting Graphene Oxide Structure via Spatially-Resolved Electron Energy Loss Spectroscopy.

Graphene oxide (GO) is obtained by chemical oxidation and exfoliation of natural graphite. In the last decade, it has attracted a widespread interest for its mechanical strength, tunable optoelectrical properties, simple processability and its potential as precursor for a low-cost and large-scale production of graphene. Indeed, chemical and thermal treatments allow to almost completely remove the oxygen, yielding reduced graphene oxide (RGO). Nevertheless, after about 150 years, the atomic structure of GO and RGO is still greatly debated. At present, the most acknowledged model for GO considers a random functionalization of the carbon basal plane with epoxide and hydroxyl groups, forming graphitic and partially oxidized domains. However, no definitive evidence of this model has been reported due to the lack of chemical analysis at the proper scale. For these reasons, nanometrically spatially-resolved spectroscopy of GO and RGO is highly suitable.
In this work we provide the first chemical characterization of GO and RGO thin flakes at the scale of few nanometres, thanks to core Electron Energy Loss Spectroscopy (EELS) in a STEM microscope. A major issue is represented by the extreme sensitivity of these materials to illumination and the use of this technique on GO and RGO has been so far very restricted. A new experimental set up combining a liquid nitrogen cooling system at the sample stage, a low accelerated electron beam (60 keV) and a liquid nitrogen cooled CCD camera with a low read-out noise of three counts r.m.s. and a negligible dark count noise has allowed us to overcome this limitation. Optimal illumination conditions have been defined by monitoring the evolution of the sample under continuous illumination, defining a maximal electron dose before substantial chemical modification of the order of 103 e-Å-2 and hence a 3 nm lower limit on the hyperspectral spatial resolution. Chemical maps of the atomic oxygen content of few layers GO and RGO show well separated domains on the scale of tens of nanometres. Overall, the oxygen amount has been observed to vary within 10-50 at.% in GO and 5-20 at.% in RGO. Energy-Loss Near-Edge Structures (ELNES) at the carbon K-edge exhibit well-defined features related to C-O bonding, previously not reported. Moreover different oxidation levels in GO and RGO are characterized by specific ELNES profiles. The highly oxidized regions in GO (~50 oxygen at.%, i.e. 1:1 C/O ratio) correspond to a full functionalization of the carbon network. With the support of complementary DFT numerical calculations, we suggest a model for the highly oxidized regions consisting in a full functionalization with hydroxyls, forming a 2D-sp3 system.


Anna TARARAN (Orsay), Alberto ZOBELLI, Ana M BENITO, Wolfgang K MASER, Odile STÉPHAN
08:00 - 18:15 #6160 - MS02-660 EELS Observation on Spontaneously Grown Ag@Titanium Oxide Core Shell Nanowires.
MS02-660 EELS Observation on Spontaneously Grown Ag@Titanium Oxide Core Shell Nanowires.

To enhance functionality and usability of 1-D nanomaterials, the synthesis of composite nanowires such as core-shell nanowires is a good solution.  Ag-TiO2 core-shell nanowires have attracted much attraction, because Ag nanowires have unique properties of high surface plasmon resonance as well as good electrical conductivity, while TiO2 is an ideal material as photocatalysts.  In the absence of oxide precursors, templates, inoculants and surfactants, this study successfully prepared vertical Ag nanowires with a spontaneous ultra-thin TiO2 shell (~0.5 nm) on TiO2 substrate using a one step process.  STEM/EELS results demonstrate that this oxygen-deficient TiO2 layer is formed through the oxidation of Ti which is released from the substrate and segregated to the nanowire surface simultaneously with crystal growth of the nanowires (Fig. 1).  The EELS spectra suggest that the valence of Ti ions in this spontaneous oxide layer is between Ti3+ and Ti4+.


Jenn-Ming SONG (Taichung, Taiwan), Chi-Hang TSAI, Shih-Yun CHEN, Alexandre GLOTER
08:00 - 18:15 #6200 - MS02-662 3D structure and growth of inversion domain boundaries in nanowires of InRO3(ZnO)m (R = Al, Fe, Ga, In) - an electron microscopy study.
MS02-662 3D structure and growth of inversion domain boundaries in nanowires of InRO3(ZnO)m (R = Al, Fe, Ga, In) - an electron microscopy study.

Materials based on zinc oxide with additions of other transition and main group metal oxides offer a broad range of applications such as varistors, transparent conducting oxides (TCOs), gas sensors and dye-sensitized solar cells. They have good semiconducting and optical properties at low costs and easy availability.[1] The prime example is IGZO (indium gallium zinc oxide, InGaZnO4), which has garnered widespread attention for its use in flat-panel TFT displays. While the structure of these materials has been studied extensively for close to 50 years,[2] a model comprehensively describing the formation and growth of its unique structural features has not been proposed thus far. It is crucial to gain an understanding of the atomic arrangement and the growth mechanisms of basal and pyramidal inversion domain boundaries (IDBs).

ZnO nanowires (NWs) were grown on fused silica substrates via a thermal evaporation method and a metal-seeded growth mechanism.[3] Conversion of said NWs to IAZO, IGZO or IFZO NWs was performed by spin-coating with solutions of indium nitrate and aluminium nitrate, gallium nitrate or iron(III) acetylacetonate, respectively, in 2-methoxyethanol. For thermal decomposition of the solution droplets and subsequent reaction of the various oxide particles with ZnO at the NW surface, specimens were then annealed in a furnace in ambient air at 1000 °C.

High-angle annular dark field (HAADF) and BF/ABF STEM imaging at high resolution as well as spectroscopic analyses were performed using an advanced analytical TEM/STEM system (JEOL JEM-ARM 200CF equipped with a cold FEG, probe Cs corrector, X-ray (JEOL Centurio) and electron spectrometer (GATAN GIF Quantum ERS) attachments).[4]

The aforementioned synthesis method yields faceted ZnO NWs of various growth directions including [10-1 0] and [10-1 1]. Due to their distinct morphology, they offer two preferential sites for the reaction with R2O3 particles: large, planar {2-1-1 0} surfaces and kinks between {0001} and {10-1 1} facets. This allows to image the initial formation of basal and pyramidal IDBs and their growth into the bulk of the NW with the viewing direction either perpendicular or parallel to the direction of growth (see figure 1). In perpendicular direction, the location where the basal IDB and two ZnO {0002} lattice planes meet (dotted line, fig. 1a) appears sharp. Adjacent ZnO layers are displaced in direction of the polar c axis by up to 0.9 Å at the boundary. In parallel direction, the IDB appears to be sandwiched tightly between two ZnO planes, its image contrast gradually fading towards the outer edges. The surrounding atomic columns of ZnO appear slightly distorted. In a three-dimensional representation, it is visualized that both phenomena are the result of projections from different depth regions in the NW (see figure 2).

Elemental distributions of cations were mapped by X-ray spectroscopic imaging. From spatially resolved EDS analyses, we conclude that both trivalent cations (In and R) occupy exclusively those sites most energetically suitable for them. R-decorated pyramidal IDBs originate as a flat defect parallel to and adjoining the In-decorated basal IDB, while the surrounding wurtzite-structured ZnO domains are unaffected. From there, the flat defect grows into a dome-shaped and, finally, a pyramid-shaped defect. This process requires a displacement of R cations via cation vacancies and entails a considerable distortion of all surrounding tetrahedral cation sites (see figure 3).

References:

[1] D. P. Norton et al., Mater. Today 6 (2004), 34-40.

[2] I. Keller, W. Mader, Z. Anorg. Allg. Chem. 636 (2010), 1045-1049.

[3] H. Simon, T. Krekeler, G. Schaan, W. Mader, Cryst. Growth Des. 13 (2013), 572-580.

[4] H. Schmid, E. Okunishi, W. Mader, Ultramicrosc. 127 (2013), 76-84.


Gunnar SCHAAN (Bonn, Germany), Herbert SCHMID, Werner MADER
08:00 - 18:15 #6203 - MS02-664 TEM characterisation of diamond-hexagonal silicon nanowires.
MS02-664 TEM characterisation of diamond-hexagonal silicon nanowires.

The diamond-hexagonal phase (lonsdaleite) does not appear in the equilibrium pressure-temperature phase diagram of silicon. However, it has been observed in several cases (see review in1), especially  in samples that had undergone a stress treatment.2, 3 Its presence in semiconductor nanowires does not make consensus. In such objects, the occurrence of  the parent phase of wurtzite has been observed and explained in the case of (normally cubic) III-V semiconductors,4 but lonsdaleite has been ascertained in only very few cases: in Ge nanowires prepared under stress5 or after epitaxy of Si on GaP nanowires.6 In most cases of silicon nanowires grown by the vapour-liquid-solid (VLS) method, the proof of existence of that phase, given under the form of a transmission electron microscopy (TEM) high-resolution image or diffraction pattern, was disputable.1 This is due to the fact that, in most zone axes, hexagonal close packing (hcp) is very difficult to differentiate from face centred cubic (fcc) multiple twinning.1, 7 Even in the [110] cubic axis where hexagonal stacking can easily be recognised, the superposition of twinned orientations in the beam direction can lead to erroneous conclusion.1

 

Here, we present TEM recordings in the [110]C/[11-20]H zone axis of VLS silicon nanowires that cannot be interpreted in terms of cubic stacking. Those are the first unambiguous TEM observations of the lonsdaleite phase in VLS SiNWs to our knowledge. These nanowires differ from those usually studied by (i) their small size, as we find the phase in nanowires with diameters around 5 nm, (ii) the plasma-enhanced chemical vapour deposition technique (PECVD) we use for bringing gas radicals to the catalyst particles and (iii) the liquid tin we use as catalyst. Their small diameters, in particular, make it very unlikely that several different twin orientations be superimposed in the beam direction.

 

The figures present silicon nanowires (SiNWs) obtained after 2 min of growth. The SiNW in Fig. 1 has several polytypes, including 2H (lonsdaleite) and 6H silicon. The simulated images8 superimposed to the two regions of hexagonal symmetry confirm the structure. The amorphous contrast at the NW surface indicates the presence of oxide. Figure 2 shows the Fourier transform of the experimental image in Fig. 1(top-right) and diffraction patterns: (i) with a 10-nm beam centred on 6H (middle-right) and (ii) with a 20-nm beam encompassing both polytypes (bottom-right). These experimental diffractograms exhibit reflections characteristic of the 6H and 2H phases, as shown in the simulated diffractograms of the 6H (top-left) and 2H (bottom-left) geometries. Figure 3 exhibits the structure of a pure 2H silicon nanowire. The 90° bend is performed with no defect: the growth axis just changes from [0001] to [10-10]. The image at the bottom-left is in a slightly different orientation, giving a Fourier transform (bottom-right) that demonstrates that the structure is pure 2H. The nanowire in Fig. 4 presents the interesting feature of having started with a (twinned) cubic phase and having then switched to hexagonal phase, as if the latter were energetically favoured in the dynamic conditions of the growth.

 

1.         C. Cayron, et al., J. Appl. Crystallogr. 42, 242-252 (2009).

2.         R. H. Wentorf and J. S. Kasper, Science 139, 338-339 (1963).

3.         P. Pirouz, et al., Acta Metall. Mater. 38, 313-322 (1990).

4.         F. Glas, et al., Phys. Rev. Lett. 99, 146101 (2007).

5.         L. Vincent, et al., Nano Lett. 14, 4828-4836 (2014).

6.         H. I. T. Hauge, et al., Nano Lett. 15, 5855-5860 (2015).

7.         D. W. Pashley, et al., Phys. Stat. Sol. b 10, 153-163 (1965).

8.         P. Stadelmann, JEMS-Saas, http://www.jems-saas.ch/

 

Acknowledgements: jlm acknowledges fruitful discussions with Gilles Patriarche and Frank Glas, CNRS Marcoussis, France, and Olivier Hardouin Duparc, CNRS Ecole polytechnique, Palaiseau France.


Jean-Luc MAURICE (Palaiseau), Jian TANG, Ileana FLOREA, Frédéric FOSSARD, Pere ROCA I CABARROCAS, Erik V. JOHNSON, Martin FOLDYNA
08:00 - 18:15 #6218 - MS02-666 HAADF HRSTEM of one dimensional CuI crystals inside SWCNTs.
MS02-666 HAADF HRSTEM of one dimensional CuI crystals inside SWCNTs.

Reduced coordination in 1D crystals is obviously a consequence of the sterically confined space in which the encapsulated crystals have to grow. It was interesting to investigate the differences of the one dimensional crystals in comparison with  their three-dimensional unit cell.

In our previous investigations of 1DCuI@SWCNT HRTEM revealed hexagonal, monoclinic [1] and cubic [2] modifications of copper iodide. In cases of HRTEM investigations in order to obtain the 1D crystal structure it was necessary to get sets of micrographs (about hundreds) while HAADF HRSTEM method allows to determine 1D crystal structure with a help of a few micrographs, owing to an ability to designate various atoms according to their atomic numbers. Within a framework of this study the perspectives of HRSTEM method combined with local EELS for determination of atomic and electron structure of individual nanocrystals are shown. These analytical electron microscopy methods applied together have allowed to reveal a new type of 1DCuI@SWCNT nanocomposite structure and to identify a space group of 1D CuI unit cell in the nanotube channel which appeared to be Pmmm. The results of our investigation bear evidence of the necessity of using HRSTEM in both bright field and dark field modes simultaneously (Fig. 1). A formation of chemical bond between Cu3d and C2dz orbitals of the intercalated nanocrystal and the nanotube wall is accompanied by electron density transfer of ~ 0.09 e/atom towards C. Electron microscopy is performed in JEOL ARM 200F at 80 kV in HAADF HRSTEM mode. 

Acknowledgements

The work was supported by the Ministry of Education and Science of the Russian Federation within a framework of the program “Investigation and development on priory direction of scientific-technical complex of Russia for 2014-2020” (agreement № 14.585.21.0004, unique project identification number RFMEFI58514X0004).

1. Kiselev N.A., Kumskov A.S., Zakalyukin R.M., Vasiliev A.L., Chernisheva M.V., Eliseev A.A., Krestinin A.V.,  Freitag B., Hutchison J.L. // Journal of Microscopy. 2012. V. 246. P. 3. P. 309.

2. Kumskov A.S., Zhigalina V.G., Chuvilin A.L., Verbitskiy N.I., Ryabenko A.G., Zaytsev D.D., Eliseev A.A., Kiselev N.A. // Carbon. 2012. V. 50. № 12. P. 4696.


Andrey KUMSKOV (Moscow, Russia), Victoria ZHIGALINA, Nikolay KISELEV, Andrey ELISEEV, Nikolay VERBITSKIY, Alexander VASILIEV
08:00 - 18:15 #5172 - MS03-668 Sample preparation for in-situ biasing TEM experiments.
MS03-668 Sample preparation for in-situ biasing TEM experiments.

Knowledge about the potential distribution of semiconductor devices at the nanometer scale is important for the development of new device structures. Electron holography is a technique sensitive to the electrostatic potential with a high spatial resolution up to the sub nanometer scale [1, 2]. The potential especially in semiconductors is highly sensitive to  the generation of electron-hole pairs, damage caused by the electron and ion beam during investigation and preparation [3], and the biasing of the sample. In-situ Biasing of specimen within the transmission electron microscope (TEM) provides an external stimuli allowing to differentiate between various effects influencing the hologram. A possible investigation is the influence of reverse and forward bias on the width of the depletion region and on the potential difference between p- and n-sides of the diode [4, 5]. For the comparison between the the bulk device and the TEM- specimen we monitored the influence of the sample preparation on the electrical behavior.

 

For these investigations a silicon solar cell was chosen [6]. The used sample geometry is inspired by the experiments of A.C. Twitchett [7]. The first steps are cleaving and grinding of a wedge out of the wafer. Afterwards two copper clamps are bonded with conductive silver on the front and back contacts of the specimen. This is followed by the preparation of a TEM lamella with a focused ion beam (FIB). A sketch of the FIB prepared lamella is shown in Figure 1. The preparation of the biasing sample is finished by bonding of the clamped specimen on a custom build carrier chip with conductive silver. Figure 2 shows the finished sample.

 

After each preparation step, an IV measurement was performed and the corresponding curves are shown in Figure 3. As result, the influence of the FIB preparation on the whole sample is negligible (solid red and dashed blue). The measurements are not significantly changed before and after inserting the sample holder into the vacuum of the TEM (solid green and dashed orange). In reverse bias direction however a change of the slope after each bonding step is visible. Before bonding the parallel resistance is 70.1 kΩ. After bonding the slope of the current increases. This corresponds to  the presence of a parallel resistance. Conductive silver or residues from its solvent might shortcut the device and build a parallel resistance. This parallel resistance decreases from 10.3 kΩ to 1.95 kΩ after the bonding steps.

 

During investigations within the TEM an influence of the electron beam on the measured shortcut current was recognizable. Illuminating the FIB-cut lamella portion of the specimen (and partially the thicker parts below it (left of the lamella in Fig. 1) produced a current of -2.1 μA, while illuminating the hole near the specimen only produced an current of -8.8 nA.

 

1.    M. McCartney et al., Applied Physics Letters 65 (1994) 2603.

2.    K. Harada et al., Journal of Applied Physics 96 (2004) 6097.

3.    J.B. Park er al., Applied Physics Letters 105 (2014) 094102.

4.    A.C. Twitchett et al., Journal of Microscopy 214 (2004) 287.

5.    L.Z.-Y. Liu et al., Physica Status Solidi C 9 (2012) 704-707

6.    M. Mews et al., Energy Procedia 38 (2013) 855

7.    A.C. Twitchett et al., Physical Review Letters 88 (2002) 238302.

8.    We thank the Institute Silicon Photovoltaics of the HZB for providing us with a sample 

9.    We kindly acknowledge the support from the DFG within the Collaborative Research Center 787, project A4


Udo HÖMPLER (Berlin, Germany), Tolga WAGNER, Tore NIERMANN, Michael LEHMANN
08:00 - 18:15 #5274 - MS03-670 Combining Current Imaging and Electrical Probing for fast and reliable in situ Electrical Fault Isolation.
MS03-670 Combining Current Imaging and Electrical Probing for fast and reliable in situ Electrical Fault Isolation.

Within the last decades, the feature widths of semiconductor devices have become too small to be resolved by state-of the art optical microscopes, and other imaging techniques like Scanning Electron Microscopes (SEM) or Scanning Probe Microscopes (SPM) have become indispensable. For some analyses of device failures a combination of imaging and electrical measurements is required. Nowadays, one can find nanoprobers that are either based on SPM or SEM imaging techniques. For the SPM based nanoprobers, each probe consists of an Atomic Force Microscope (AFM) equipped with a conductive tip. For the SEM based systems, nanopositioners without force feedback are placed within a SEM.

Both approaches described above have advantages and disadvantages. Landing probes on a desired device is much easier for the SEM prober, as the SEM image gives immediate visual feedback. In contrast, the AFM prober requires many subsequent AFM images by each probe in order to align the probes with respect to each other and with respect to the device, which can be very time consuming. One advantage of the conductive AFM probing is the combination of topography images with electrical transport properties, i.e. images of local resistance (analogue to Scanning Tunneling Microscope (STM)). This allows easy identification of leakages or shorts on the chip.

In this work we present a very compact nanoprober that combines the advantages of SEM and SPM nanoprobers in one system.

The setup consists of a nanoprober with eight probes and an xyz substage mounted on the stage of an SEM (Zeiss Supra 40). Positioning the probes and the sample with nanometer precision as well as the controls for all electrical measurements are provided by a unified software interface. These can be used to quickly and easily probe contacts on technologies with line widths down to 10 nm and less. The probing experiments can include transistor characterization, electron beam induced current (EBIC) imaging, among others.

In order to locate leakages or shorts as described above, a voltage biased probe is scanned at constant height over the sample surface. The resulting current flow to a second probe or to the bulk contact can be simultaneously monitored with sub-pA resolution. A typical current image can be acquired in a few seconds in a scan range of up to 1.5 µm x 1.5 um.

Each probe (or the substage) can be selected as the “scanning” voltage source or the “stationary” current sink, which allows for a very flexible definition of the expected current path enabling a powerful method for the visualization of faults in the integrated circuits.

The combination of the SEM image (the probes can be observed while the scan takes place) and the resulting current images enables correlative microscopy. Figure 2 shows an overlay image of the SEM image with current images on a 22 nm chip. Depending on the polarity (+/- 1V) of the voltage biased tip, n-MOS or p-MOS contacts become visible in the current image.


Stephan KLEINDIEK (Reutlingen, Germany), Andreas RUMMEL, Klaus SCHOCK, Matthias KEMMLER
08:00 - 18:15 #5322 - MS03-672 Using a simplified in line holography method as a qualitative tool to detect local heterogeneities in HfO2 layers.
MS03-672 Using a simplified in line holography method as a qualitative tool to detect local heterogeneities in HfO2 layers.

Oxygen vacancies in high-k oxides are foreseen to have detrimental effects in devices like high-k metal gate MOS transistors [1] and beneficial ones in RRAM [2]. In this context techniques capable to characterize defects in ultra-thin (2 thin films and revealed the impact of the grain nanostructure on the electronic structures [4]. When traversing the sample electron waves experience a phase shift related to the local inner potential created by atoms. The reference technique to retrieve this phase shift is off axis electron holography. However as it is a delicate technique, the alternative in line holography based on the acquisition of a focal series can be of interest. The advantage of in line holography is to be easy to carry out in the course of a classical TEM study and to provide phase image on a large scale (up to several microns). The present work aims at examining the capabilities of a simplified approach of in line electron holography to evidence heterogeneities in a polycrystalline HfO2 layer.

The in line holography method consists first in acquiring images at different defocus (-D, 0, D) and then deriving the phase image using a post treatment that realizes the inversion of the phase transfer function. Various numerical treatments are possible, however a simple inversion process based on a small angle approximation can be achieved [5]. As a consequence of the approximation, the phase map has a limited spatial resolution, typically 1 nm for a 100 nm defocus. Also the inversion leads to a singularity at q = 0 that can be dealt with using a Gaussian edge filtering or more efficiently a Tikhonov regularisation (i.e. a q2/(q2+a2)2 filter) [6]. Depending on the parameter a, long range fluctuations can be amplified or cancelled out which have a significant impact on the value of the phase output. However if only a qualitative information on the phase map is desired, this simplified in line holography can be used as a tool to point out chemical or charge fluctuations within the sample.

Figure 1 shows a HRTEM image of a FIB lamella from an HfO2 layer grown by atomic layer deposition (ALD). A detailed analysis and FFT treatments have allowed to identify nanograins within the HfO2 layer as illustrated by Figure 1b: the grain size is in the 5-20 nm range. Focal series corresponding to the image in Figure 1 have been recorded. Using images at defocus of ±50 nm, the simplified in line holography treatment gives the phase images shown in Figure 2 (spatial resolution ~0.7 nm). Figure 2a and Figure 2b correspond to a parameter a equal to 0.07 nm-1 and 0.21 nm-1 respectively. As illustrated out by the marked areas in these figures, the phase images exhibit features independent from the numerical parameter a: here grain and grain boundary regions correspond to positive (blue and violet) or negative phase shift (green and red).

If diffraction effects are not involved, phase shifts of opposite sign should indicate effect of different nature on the local potential as can be expected from vacancies and chemical segregation. Of course, these defects will need further investigation by advanced TEM techniques to be identified. However the phase maps derived from the simplified in line holography approach can constitute a simple tool to evidence local heterogeneities in a complex microstructure.

  1. K. Shiraishi, et al., in: "Proc. of SISPAD", 306–313 (2006).
  2. P. Calka, et al., Nanotechnology 24, 085706 (2013).
  3. R. Krause-Rehberg. and H.S. Leipner, Positron Annihilation in Semiconductors: Defect Studies, Springer-Verlag. 1999.
  4. T. Mizoguchi et al. Journal of Physics Condensed Matter 21, 104212 (2009).
  5. P. Donnadieu et al, Applied Physics Letters 94, 263116, 2009
  6. P. Donnadieu et al, Journal of Nanoscience and Nanotechnology  11-10 (2011) p.9208.

Mathias ALEMANY, Patricia DONNADIEU (ST MARTIN D'HERES CEDEX), Amal CHABLI, Edgar RAUCH, Ben MAYALL, Nicolas BERNIER, Mickael GROS-JEAN, Marie-France BARTHE
08:00 - 18:15 #5371 - MS03-674 Quantitative observation of low energy electron channeling contrast from sub-nanometer thick surface layers using hexagonal Silicon Carbide single crystal.
MS03-674 Quantitative observation of low energy electron channeling contrast from sub-nanometer thick surface layers using hexagonal Silicon Carbide single crystal.

              A potential of low energy (< 1keV) electron channeling contrast imaging (LE-ECCI) by scanning electron microscopy (SEM) is demonstrated to characterize the crystallographic stacking sequence of hexagonal 4H-SiC single crystal within the first unit cell thickness from the topmost surface (< 1nm). It is also revealed that the LE-SEM signal intensity associated with the crystallographic orientation has enough sensitivity to even the change in Si-atom and C-atom (i.e. the change in surface polarity) at a primary electron energy of 0.4keV. The obtained data can be explained by an electron multiple scattering cluster (EMSC) theory where the interference of both incident and diffracted electron waves is considered within a finite size (a few nanometers) of an atomic cluster.

              So far, for characterizing local crystallographic orientation of bulk crystals using SEM, ECCI with a higher energy (> 10keV) of primary electron beam has been utilized in order to obtain larger interaction volume due to its deeper penetration depth [1]. The electron channeling contrast (ECC) from only a shallower surface region with lower energy has not been applied yet because of the consideration attributed to the lack of enough interaction volume in the lower energy regime. In fact, to our knowledge, the critical smallest volume (specimen minimum thickness) has not been investigated yet associated with ECC. In our previous study [2], a crystallographic orientation contrast from two bilayers thick surface layers of 4H-SiC single crystal was observed by changing an incident LE (< 1keV) EB direction with respect to the orientation of an atomically flattered 4H-SiC surface (Figs. 1). This contrast is considered to be the ECC in an extremely low electron energy regime. This result brings about the assumption that even the two bilayers thick interaction volume can contribute to the formation of ECC. In order to verify this assumption, in this study, crystallographic orientation dependence of SEM signal intensity was quantitatively measured on an atomically flat 4H-SiC surface. Furthermore, in order to exploit the Z-information, different polar faces, Si-terminated (0001) Si-face, and C-terminated (000-1) C-face were compared, where atomic sites of Si- and C-atoms are switched in the crystal. The obtained results were compared with the EMSC theory in which the concept of short-range periodicity is included [3]. This theoretical model is more applicable to reproduce the effect of shallower information depth of the LE-EB, compared with a Bloch-wave model utilized in the conventional ECCI in the higher electron energy regime.

              Fig.2 shows the representative SEM signal intensity from 4H-SiC Si-face and C-face as a function of the sample tilting angle obtained at a primary electron energy of 0.4 keV. Although the profile shapes seem to be different at a glance, peak positions are almost the same. Considering the fact that the atomic arrangement below the surface is identical both in Si-face and C-face SiC, except Si- /C-atomic sites in the crystal, the difference in the profile shape is ascribed to the contrast of backscattered electron yield [4]. The similar trend was also reproduced by the EMSC calculation.

 

Acknowledgement

Some of this work was financially supported by the New Energy and industrial Technology Development Organization (NEDO). The authors would like to thank Toyo Corp. for their experimental supports. The authors are grateful to Dr. Tomohiro Matsushita (Japan Synchrotron Radiation Research Institute) for his theoretical support.

 

References

[1] D. C. Joy, D. E. Newbury, and D. L. Davidson, J. Appl. Phys. 53, R81 (1982).

[2] K. Ashida, T. Kajino, Y. Kutsuma, N. Ohtani, and T. Kaneko, J. Vac. Sci. Technol. B, 33 (4) 04E104-1 (2015).

[3] T. Matsushita, F. Matsui, H. Daimon, and K. Hayashi, J. Elec. Spectrosc. Phenom., 178-179, 195-220 (2010).

[4] J. Cazaux, J. Appl. Phys. 112, 084905 (2012).


Koji ASHIDA (Sanda, Japan), Yasunori KUTSUMA, Tadaaki KANEKO
08:00 - 18:15 #5389 - MS03-676 Nanoscale quantitative characterization of 22nm CMOS transistor using Scanning Transmission Electron Microscopy (STEM).
MS03-676 Nanoscale quantitative characterization of 22nm CMOS transistor using Scanning Transmission Electron Microscopy (STEM).

    22nm Silicon-On-Insulator (SOI) complementary metal-oxide semiconductor (CMOS) technology has a number of performance boosters, such as third generation embedded DRAM, embedded stressor technology and 15 levels of copper interconnect.[1] In addition to geometric scaling, strain engineering in CMOS transistors has provided another enabler for device performance improvement. A compressive strain in PMOS channel can increase the hole mobility, and a tensile strain in NMOS channel can increase the electron mobility. In PMOS of 22nm SOI technology, epitaxial SiGe source/drain (S/D) is used to introduce compressive strain in the channel, and SiGe layer in the channel is used to control threshold voltage of device. Hence obtaining the nanoscale chemical composition and strain information of them is vital during the semiconductor development.

    In this work, energy-dispersive X-ray spectroscopy (EDX) in STEM is used to determine the Ge atomic concentration (%Ge) based on the Cliff-Lorimer ratio method.[2] The Cliff-Lorimer factor is calibrated by measuring a standard Si0.664Ge0.336 blanket sample with a relative error smaller than 1%. An improved high angle annular dark field (HAADF) STEM image is obtained by STEM with Drift Corrected Frame Integration (DCFI). DCFI technique integrates successive STEM images via calculating and correcting the drift from cross correlation. The produced STEM image has minimal drift and a high signal-to-noise ratio. It is analyzed by Geometrical Phase Analysis (GPA) to extract strain information.[3]      

    Figure 1(a) shows a typical PMOS transistor with <110> in-plane direction and <001> out-of-plane direction. SiGe channel and dual-layer embedded SiGe source/drain (S/D) can be clearly observed in HAADF-STEM mode. Since HAADF image intensity is proportional to atomic number Z1.7,[4] SiGe layers with different %Ge are clearly seen, and can be further related to %Ge measured from EDX. In Figure 1(b), EDX map shows that channel SiGe has around 22% Ge. In S/D the buffer layer has 20-21% Ge, and the main layer has 25 - 30% Ge. HAADF micrograph and the corresponding deformation maps are shown in Figure 2. A compressive strain of 0.3% in the <110> direction is observed in the channel Si. There is no deformation between channel SiGe and channel Si in the <110> direction, suggesting that channel SiGe is compressed in this direction to match underneath Si lattice. This agrees with the finding that a deformation as high as 1.9% in channel SiGe is observed in the <001> direction. In addition the channel Si shows a tensile strain of 0.3% in the <001> direction. Results show that a combination of STEM-based techniques, including HAADF-STEM, STEM-GPA and STEM-EDX, can reveal the nanoscale chemical composition and strain distribution of a transistor. These information are used to monitor and control the process.

 

References

[1] S. Narasimha et al., IEDM (2012) p. 331.

[2] W. Weng et al., Microscopy and Microanalysis 21(S3) (2015) p. 1087.

[3] J.-L. Rouvière et al., Ultramicroscopy 106 (2006) p. 1.

[4] O.L. Krivanek et al., Nature 464 (2010) p. 571.

[5] Acknowledgements: The authors thank Joshua Bell (GlobalFoundries) for providing samples, and John Miller (GlobalFoundries) for preparing TEM lamellae. The authors also thank John Bruley (IBM), Yun-Yu Wang (GlobalFoundries), Frieder Baumann (GlobalFoundries) and Michael Gribelyuk (GlobalFoundries) for valuable discussions.


Weihao WENG (Hopewell Junction, USA), Claude ORTOLLAND
08:00 - 18:15 #5454 - MS03-678 Evidence of metastable zinc blende phase and its influence in nanocrystalline ZnO film growth.
MS03-678 Evidence of metastable zinc blende phase and its influence in nanocrystalline ZnO film growth.

ZnO is a wide band gap semiconductor used in a broad range of technological applications such as gas sensing, piezoelectronics or transparent conductive oxides. Many II-VI compounds typically crystallize in a rock salt (RS), a wurtzite (WZ) or a zinc blende (ZB) phase. For ZnO it is well known that the most stable form under ambient conditions is the WZ crystal structure. The WZ structure can be described as an AaBbAaBb stacking sequence of close-packed Zn and O planes along the [0 0 0 1] axis, where the upper-case letters stand for Zn planes and the lower-case letters for O planes. The metastable RS phase forms under large hydrostatic pressure, while so far ZB has only been found during hetero-epitaxial growth on cubic substrates or in some free-standing nanostructures [1,2]. Here, using a combination of transmission electron microscopy (TEM) based automated crystal orientation mapping (ACOM) and high-resolution TEM (HRTEM), we present evidence of a metastable zinc blende phase in chemical vapor deposited (CVD) nanocrystalline ZnO films. We further show how this ZB significantly affects the growth in these films.

 

The vapor deposition growth of polycrystalline films usually begins with the formation of densely spaced nuclei on the substrate. During further film growth neighboring nuclei impinge on each other, leading to a growth competition between grains. Grains with their fastest growth direction normal to the substrate (i.e. [0 0 0 1]) overgrow otherwise oriented grains, resulting in a classical columnar growth morphology, as shown in Figure 1 a. Unexpected for such columnar film growth, however, is the observation that the growth of many columnar grains appears to stop, followed by a renucleation to form smaller grains as marked by circles in Figure 1 a. We investigated this behavior by ACOM (Figure 1 b), revealing a special epitaxial relationship of the new renucleating grains with the underlying columnar grains. The orientation data shows that the renucleating grains share a common [2 -1 -1 0] axis with their neighboring grains (Figure 1 c & d) and that the misorientation between the grains is close to 70°. This epitaxial relation has been further investigated by HRTEM, which reveals that the origin of this epitaxial orientation relationship can be attributed to a few nm sized core of ZB phase (Figure 2 a-d). This ZB region can form on top of the columnar grains by a simple change in stacking sequence to AaBbCcAaBbCc (Figure 2 e). We believe that the new WZ grains then nucleate on the {1 1 1} facets of this ZB core, producing the orientation relationship observed by ACOM. Furthermore, our analysis of the fast growth direction of the renucleating WZ grains using ACOM, proves that, unlike the initial columnar grains, the fast growth direction is no longer parallel to the [0 0 0 1] axis. We argue that this is due to a change from having a Zn-terminated (0 0 0 1) polar facet (as for WZ1 in Figure 2 e) to having an O-terminated (0 0 0 -1) facet (as for WZ2 in Figure 2 e) exposed to new adatoms. Polarity determination of the columnar and renucleating grains by convergent beam electron diffraction (CBED) confirms this hypothesis.

 

This nucleation of WZ on top of {1 1 1} ZB facets bears a strong resemblance with the formation free-standing ZnO tetrapod nanostructures [1]; indeed it is remarkable how a similar mechanism appears to play an important role in these compact thin films, even if the driving force for the formation of ZB on top of WZ ZnO remains still unclear [2]. We have further investigated [2 -1 -1 0] fiber textured CVD ZnO films, for which the same growth mechanism appears to be active and could explain the abundance of (1 0 -1 3) twin boundaries previously found in these films [3].

 

References and Acknowledgements

 

[1] Y. Ding et al., Applied Physics Letters 90 (2007), 153510

[2] L. Lazzarini et al, ACS Nano, 3, (2009), 3158-3164

[3] A. Aebersld et al., Ultramicroscopy 159 (2015), 112-123

The authors acknowledge funding from the SNSF, Grant Number 137833 (ZONEMproject). L Fanni, Dr A Hessler-Wyser and Dr Christophe Ballif of the IMT PV-lab, EPFL and Dr S Nicolay from CSEM, Neuchâtel are thanked for the samples and discussions. 


Arthur Brian AEBERSOLD (Lausanne, Switzerland), Duncan ALEXANDER, Cécile HÉBERT
08:00 - 18:15 #5706 - MS03-680 Quantitative evaluation of the (211)B GaAs/InAs quantum dot heterostructure.
MS03-680 Quantitative evaluation of the (211)B GaAs/InAs quantum dot heterostructure.

InAs QDs grown on high-index GaAs(h11) surfaces seem to exhibit superior optical properties compared to the usual QD growth on GaAs(001), due to their prominent piezoelectric field, which can be functional in nano-photonics and quantum computing. However, the morphology of the QDs, as well as their strain state and chemical composition, influence significantly the light emission and absorption, the lasing efficiency, and other optoelectronic properties of QD-based devices. In order to shed light to these effects, we have explored the nanostructure, the strain properties, and the related chemical composition of buried InAs QDs grown on (211)B GaAs surface employing quantitative HRTEM techniques.

The InAs QD layer was grown by MBE under the Stranski-Krastanow (S-K) regime at 480oC with a growth rate of 0.9 ML/s for 2s, over a 20 nm GaAs layer grown at 620oC. The corresponding BEP was 8.5x10-7 mbar. The QD layer was then overgrown by a 30 nm-thick GaAs cap layer, without growth interruption, by ramping up the temperature back to 620oC.

The QDs adopted an anisotropic pyramidal shape, elongated along the <111> direction and were delimited by the {110}, {100}, and {213} or {214} crystal facets [Fig. 1(a)], as clearly evidenced by the equivalent larger uncapped InAs QDs grown on (211)B GaAs surface under similar growth conditions [1]. Local strain measurements by the geometric phase analysis (GPA) method [2] showed that buried dots were pseudomorphically grown on GaAs without the presence of any interfacial or extended defects [Figs. 1(a) and 1(b)]. Assuming a plane stress state of the QDs, following the transformation of the elastic stiffness tensor in order to comply with growth along the [211] direction [3], we found a systematic increase of the local strain from the base area to their apex region [Figs. 1(c) and 1(d)]. Then, applying Vegard’s law, we calculated the chemical composition of the QDs that was found to exhibit an indium composition gradient along the growth direction, obviously suggesting gallium segregation inside the dots (Fig. 2). Even though the gradual increase of indium concentration is a common trend for all QDs, various In-content maxima (0.50 to 0.92) were measured at the apex area of different QDs. This variation can be attributed to the corrugated form of the (211) surface [4], resulting in local compositional fluctuations of the wetting layer at the nucleation sites of the QDs. Therefore, gallium segregation is already involved at the onset of the S-K growth. Furthermore, photoluminescence (PL) and μ-PL experiments, as well as simulations of the QDs’ transition energies, showed variations in the emission energy of the QDs, which is in line with a graded In-content along the growth direction instead of pure InAs, thus verifying the chemical composition profile of the QDs revealed by quantitative strain measurements.

 

[1] N. Florini, G. P. Dimitrakopulos, J. Kioseoglou, S. Germanis, C. Katsidis, Z. Hatzopoulos, N. T. Pelekanos, and Th. Kehagias, J. Appl. Phys. 119, 034304 (2016).

[2] M. J. Hÿtch, E. Snoeck, R. Kilaas, Ultramicroscopy 74, 131 (1998).

[3] T. Hammerschmidt, P. Kratzer, and M. Scheffler, Phys. Rev. B 75, 235328 (2007).

[4] R. Nötzel, L. Däweritz, and K. Ploog, Phys. Rev. B 46, 4736 (1992).

 

Acknowledgements

Work supported by the European Union (ESF) and Greek national funds - Research Funding Program: THALES, project "NANOPHOS".


Thomas KEHAGIAS (Thessaloniki, Greece), Nikoletta FLORINI, Joseph KIOSEOGLOU, George DIMITRAKOPULOS, Savvas GERMANIS, Charalambos KATSIDIS, Zacharias HATZOPOULOS, Nikolaos PELEKANOS
08:00 - 18:15 #5765 - MS03-682 TEM study of defect reduction in the growth of semipolar GaN grown on patterned substrates.
MS03-682 TEM study of defect reduction in the growth of semipolar GaN grown on patterned substrates.

III-nitride heteroepitaxial thin films and heterostructures suffer from the presence of large densities of structural defects, which are detrimental for the development of efficient devices. These defects result from differences between III-nitride films and foreign substrates (structure, chemistry, lattice parameters). Transmission electron microscopy (TEM) is the technique of choice for studying such crystalline defects. The understanding of the origin and behavior of structural defects may allow their tailoring and the development of low defect density materials.

III-nitrides are classically grown along the polar c-direction. In this case, internal electric fields play a major role in the properties of heterostructures. In order to eliminate or at least to reduce the influence of internal electric fields, growth along alternative directions with the c-direction in the growth plane (nonpolar) or inclined to it (semipolar) have been developed. Heteroepitaxial nonpolar and semipolar films contain large densities of basal plane stacking faults (BSF) and related partial dislocations together with prismatic stacking faults.

In this presentation, results of TEM studies of semipolar GaN films deposited on patterned substrates will be presented. Different TEM techniques from diffraction contrast classical imaging, high resolution TEM and scanning TEM to analytical techniques as energy dispersive X-ray spectroscopy (EDS) have been employed to obtain a complete view of semipolar GaN microstructures. The understanding of the nucleation and propagation of defects allowed us to develop several growth processes resulting in a drastic reduction of the defect density:

  • A 3-step growth process for (11-22) GaN deposited on patterned r-sapphire leads to high quality GaN films with dislocation densities as low as 7x107 cm-2 and BSF densities below 102 cm-1 (figure 1)1,2.
  • A method based on the introduction of Si at an intermediate stage of the growth (before coalescence of nucleation islands) allows, in the case of (10-11) GaN on patterned (001) 7° off-axis Si, blocking the propagation of dislocations (figure 2). A 4nm thick Si-rich (5% Si from EDS analysis) layer is revealed by HRSTEM (figure 3). This layer has the wurtzite structure of the surrounding GaN and does not introduce significant strain (as revealed by GPA analysis).
  • Selective growth on deeply grooved sapphire substrate results in GaN (11-22) bands with a dislocation density in the mid 106 cm-2 on 100µm-wide regions compatible with the fabrication of laser diodes.

Besides presenting results on the improvement of material quality through innovative growth processes, this presentation emphasizes the importance of TEM studies for the developments of heteroepitaxial semiconductors structures.

The authors acknowledge the support from GANEX (ANR-11- LABX-0014).GANEX belongs to the public funded “Investissements d'Avenir” program managed by the French ANR agency.

1 F. Tendille, P.De Mierry, P. Vennéguès, S. Chenot, M. Teisseire, J. Cryst. Growth 404 (2014) 177

2 P. Vennéguès, F. Tendille and P. De Mierry, J. Phys. D: Appl. Phys. 48 (2015) 325103


Philippe VENNÉGUÈS (CRHEA, Valbonne), Florian TENDILLE, Michel KHOURY, Philippe DE MIERRY, Nicolas MANTE, Jesus ZUNIGA PEREZ, Guy FEUILLET, Vincent DELAYE, Denis MARTIN, Nicolas GRANDJEAN
08:00 - 18:15 #5782 - MS03-684 Structure of short period In(Ga)N/GaN superlattices comprising ultra-thin quantum wells.
MS03-684 Structure of short period In(Ga)N/GaN superlattices comprising ultra-thin quantum wells.

InGaN quantum wells (QWs) grown along the polar c-axis are currently the principal structural elements of III-nitride optoelectronic device active regions. Further advancement of their operational wavelength range with high internal quantum efficiency in the green part of the spectrum may be facilitated through the use of short period In(Ga)N/GaN superlattices comprising QWs with a minimal number of monolayers (MLs). At the same time, ultra-thin InN/GaN QWs have recently generated considerable interest, due to the theoretical prediction that they may exhibit topological insulator properties [1], thus opening up new prospects for applications in quantum computing and spintronics. In recent work, two-dimensional electron gas properties were demonstrated in ML-thick, nominally InN QWs, as well as a temperature-independent behavior in the diagonal resistance, indicating the topological nature of the 2DES [2].

We have studied a series of In(Ga)N/GaN short period superlattice (SPS) heterostructures deposited at low growth temperatures. The examined samples comprised SPS with nominally 1, 2, and 4 ML QW thicknesses grown by molecular beam epitaxy (MBE) on (0001) GaN/sapphire MOVPE templates. High resolution transmission electron microscopy (HRTEM), and high resolution high-angle annular dark field imaging in scanning TEM mode (HRSTEM) have been employed in order to ascertain the compositional homogeneity due to issues such as the desorption, clustering, and diffusion of indium into the GaN barriers. Such issues may be aggravated by the large elastic strain associated with the pseudomorphic growth of the ultrathin QWs. On the other hand, it is also possible that the stress due to the misfit may in fact stabilize the growth of InN at unusually high temperatures. Fig. 1 illustrates HRTEM and HRSTEM images of a 1 ML InN / 10 ML GaN SPS and of a 4 ML-thick QW. Strain and compositional mappings were implemented by using peak finding with the peak-pairs method, and geometrical phase analysis. Z-contrast image calibration was performed using reference samples and image simulations.

The results were compared with energetic calculations of pertinent supercells using both molecular dynamics with a modified Tersoff interatomic potential, and ab initio density functional theory. In both cases, deviation from the biaxial stress state of InN was identified for these QWs in agreement with the experimental observations. 

 

[1] M. S. Miao Q. Yan, C. G. Van de Walle, W. K. Lou, L. L. Li, and K. Chang, Phys. Rev. Lett. 109, 186803 (2012).

[2] W. Pan, E. Dimakis, G. T. Wang, T. D. Moustakas, and D. C. Tsui, Appl. Phys. Lett. 105, 213503 (2014)

Acknowledgement: Work partially supported by the Sonata 8 (2014/15/D/ST3/03808) project  of the Polish National Science Centre.


George DIMITRAKOPULOS, Calliope BAZIOTI, Theodoros KARAKOSTAS, Joseph KIOSEOGLOU, Theodoros PAVLOUDIS, Slawomir KRET, Julita KOZIOROWSKA, Tadek SUSKI, Emmanouil DIMAKIS, Theodore MOUSTAKAS, Philomela KOMNINOU (Thessaloniki, Greece)
08:00 - 18:15 #5858 - MS03-686 Artefact-free top-down TEM lamella preparation from a 14 nm technology IC.
MS03-686 Artefact-free top-down TEM lamella preparation from a 14 nm technology IC.

Semiconductor industry continues to shrink sizes of the electronic devices. Currently commercial state-of-the-art technology node for integrated circuits is 14 nm, while 10 and 7 nm technology nodes are in the development stage [1]. Those integrated circuits are based on multigate transistors, where source-drain channel (“fin”) is surrounded by a 3D gate. Failure analysis process of such integrated circuits typically involves inspection of TEM lamellae prepared by FIB-SEM machines from a single transistor layer. One difficulty in preparation of such lamellae is different ion milling rates of materials of the integrated circuits. In case of conventional top-down FIB polishing this causes unwanted curtaining artefacts resulted mainly from the metal contacts above the transistor layer. One way to eliminate curtaining is so called backside (or inverted) polishing technique which involves lamella extraction, flipping and polishing by FIB through the silicon layer [2]. However, this technique is too much time consuming for a daily semiconductor industry process.

In this talk we present a new technique of curtaining-free lamella preparation. This technique allows for normal top-down FIB polishing through upper metal contacts while curtaining artefacts are eliminated due changing the incident angle of ion milling by rocking of the sample on a special stage.

In order to demonstrate this technique, we took the latest commercially available processor based on 14 nm technology node [3] (Intel Pentium G4400). The processor wafer was decapsulated and mechanically polished to remove the top metal contacts. Further delayering was performed by means of a specially developed technique of water-assisted Xe Plasma FIB etching. The later technique allows uniform damage-free delayering down to the first metal layer which is just above the transistor layer (Fig. 1a). The delayered sample was transferred to a FIB-SEM machine which was equipped with a Ga FIB column, an SEM column with immersion optics and a Rocking stage [4].

Initial steps of a lamella preparation involved a standard routine of Pt protection layer deposition, FIB trench milling, undercut and lamella transfer by a nanomanipulator to a TEM half-grid for the final thinning. Lamella thinning was performed by Ga FIB at 30 kV till reaching the lamella thickness of 150 nm. The last step of thinning down to the thickness of less than 20 nm was performed by 5 and 2 kV FIB polishing (see perpendicular “fin-cut” for illustration on Fig. 1b). The final lamella was prepared just in the middle of a single fin (so called “gate-cut”). Importantly that during the last steps of FIB thinning the lamella was milled from two directions on the Rocking stage (Fig. 2a). So due to continuous change of Ga beam incident angle (±15º) curtaining artefacts were eliminated.

Finally, the prepared lamella was transferred to a TEM microscope for observation. The observation demonstrated that even though the lamella was polished by Ga FIB by top-down technique through upper metal contacts it does not show evidence of any significant curtaining artefacts (Fig. 2b). The demonstrated result proved the potential of the technique of top-down lamella thinning on a Rocking stage.

References:

[1] http://www.itrs2.net/

[2] O. Ugurlu, M. Strauss, G. Dutrow, J. Blackwood, B. Routh, C. Senowitz, P. Plachinda, and R. Alvis, Proc. of SPIE 8681 (2013), p. 868107.

[3] http://www.intel.com/content/www/us/en/silicon-innovations/intel-14nm-technology.html

[4] T. Hrnčíř, J. Dluhoš, L. Hladík, E. Moyal, J. Teshima, and J. Kopeček, 40th ISTFA Conf. Proc. (2014), p. 136.


Andrey DENISYUK (Brno, Czech Republic), Tomáš HRNČÍŘ, Jozef Vincenc OBOŇA, Martin PETRENEC, Jan MICHALIČKA
08:00 - 18:15 #5915 - MS03-688 InAs/InSb: From Nanowires to Nanomembranes.
MS03-688 InAs/InSb: From Nanowires to Nanomembranes.

The control over crystalline defects is of extreme importance when growing functional materials, since their presence may alter the final behavior (opto-electronic properties) and evolution of the growing system (orientation and shape). In the present work we address both phenomena in the particular case of InSb related nanostructures.

On one hand, heterostructured architectures may show interfacial dislocations if reaching the coherency limit, which is dependent on geometrical constrictions of the systems in addition to the mismatch between phases. In this context, it is usually assumed that the mismatch strain in axial heterostructured nanowires is mainly relaxed by elastic distortion of the lattice, although theoretical calculations predict the formation of misfit dislocations[1]. It is remarkably important unveiling whether the partial/total lattice relaxation takes place through elastic and/or plastic mechanisms since both will affect the material performance. Interestingly, the location and shape of the heterointerfaces, as well as possible diffusion phenomena involved, may hinder the understanding of the actual relaxation mechanism, as we show in the case of axial InAs/InSb nanowires[2]. Contrary to most reported works, we find out the presence of misfit dislocations at the core of the system while there is a huge plane bending through the edges of the nanowires. 

On the other hand, we correlate the systematic observation of a lateral twin boundary with the morphological transition from nanowires to membrane-like systems, called nanosails. Based on the experimental data gathered, consisting on SEM and aberration-corrected STEM measurements, including polarity determination[3], we are able to establish the underlying defect-driving growth mechanism leading to the formation of membrane-like structures growing aside InAs/InSb nanowires[4], showing excellent transport properties. Possible instabilities during the growth may promote the sinking of the catalytic droplet to wet one sidewall, leading to the nucleation of the lateral twin that opens the way for the broadening of the system.

Acknowledgements: We acknowledge funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO MAT2014-51480-ERC (e-ATOM) and Spanish MINECO Severo Ochoa Excellence Program.

[1] F Glass, Physical Review B 74 (2006), p. 121302(R).

[2] M de la Mata et al., Nano Letters 14 (2014), p. 6614.

[3] M de la Mata et al., Nano Letters 12 (2012), p. 2579.

[4] M de la Mata et al., Nano Letters 16 (2016), p. 825

[5] S Bernal et al., Ultramicroscopy 72 (1998), p.135


María DE LA MATA (Bellaterra, Spain), Renaud LETURCQ, Sébastien R. PLISSARD, Chloé ROLLAND, César MAGÉN, Philippe CAROFF, Jordi ARBIOL
08:00 - 18:15 #5934 - MS03-690 Nano-characterization of switching mechanism in HfO2-based oxide resistive memories by TEM-EELS-EDS.
MS03-690 Nano-characterization of switching mechanism in HfO2-based oxide resistive memories by TEM-EELS-EDS.

Introduction: To answer the increasing need for data storage, several forms of memory called random access memories (RAM) have been developed. Oxide resistive RAM (OxRRAM), based on switching between a low and high conductive state, are considered as one of the most promising candidates for replacing FLASH technology in the next memory generation. Forming and breaking a nanometer-sized conductive area is commonly accepted as the physical phenomenon involved in the switching mechanism of OxRRAM [1]. Nevertheless, the nature of this filament is still highly debated because oxygen vacancies [2] on one side and and metallic migrations from the electrode on the other side [3] have been evidenced.By combining high spatial resolution and local chemical analyses coupling TEM with EELS and EDS, we propose to investigate and compare two approaches for confining and analyzing the filament of a state of the art OxRRAM device: (i) ex-situ polarization using a conductive atomic force microscopy (C-AFM) followed by FIB preparation and (ii) FIB preparation followed by in-situ polarization within the TEM. We will show the advantages and disadvantages of each approach, the principal challenge remaining the preparation of a TEM lamella which contain the nanometer-sized filament.

Ex-situ forming approach: This first approach consists to block the memory resistance in an operation state, then prepare a thin lamella by FIB before placing it in a TEM for analysis. Experiments are performed on small structures (< 100nm) with patterned top electrode to confine the filament in a sample whose size is suitable for TEM analysis. The resistance switching is realized with a C-AFM which allows both to localize the structure and to locally inject current in the memory (Fig. 1. a)). The I(V) curve of the forming operation (transition of a high resistance virgin state to a low resistance state) is shown in figure 1. a). A STEM image of the structure and the corresponding STEM-EELS map of the extracted Titanium signal are shown in figures 1. b) and 1. c), respectively. A Ti-rich region with a conical shape is clearly observed in the HfO2 layer (see the blue area inside the white dotted rectangle). This conical filament seems to connect the top electrode to the bottom electrode like previously reported [3]. We will discuss the limits of this method. How can we rigorously compare two states of the device in many lamellae with different thicknesses? Is the memory state stable over time or during the FIB preparation?

In-situ forming approach: In the second approach, the thin lamella is prepared from the memory. It is then loaded on a Nanofactory in-situ holder where a tip is used for contacting the device (Fig. 2 a)), applying a voltage and simultaneously monitoring the current. A protocol was developed to optimize sample preparation and electrical contacts with the probe and avoid mechanical stress and heating phenomena which can generate measurements artifacts or simply destroy the sample.  Figure 2. b) presents the I(V) curve of the forming operation obtained within the TEM. Clearly, the forming step of the memory has been completed with 100µA compliance. No Titanium migration in the HfO2 layer has been observed by EDS (Fig. 3 a) and b)). EELS measurements before and after in-situ forming are on-going to investigate the fine structure of the Oxygen K edge in HfO2 and explain the change of resistance.

Conclusion: Two complementary polarization protocols have been developed in this work. Using C-AFM to electrically test memories is relatively easy to setup but does raise questions about external contamination and analysis of commercially available devices. With the in-situ approach, the associated experimental work is much heavier but allows complex electrical testing without any external contamination. Ti migration in the HfO2 layer was observed with the first approach, which suggests that the memory operates as a CBRAM (conductive bridge random access memory). A different behavior seems to occur with the in-situ approach. On-going studies of the Oxygen K edge fine structures will probably help to explain the observed behavior. The reproducibility of these first results will also be checked.

References

1. Waser, R., et al., Advanced Materials, 2009. 21(25-26): p. 2632-2663.

2. Calka, P., et al., Nanotechnology, 2013. 24(8).

3. Privitera, S., et al., Microelectronic Engineering, 2013. 109(0): p. 75-78.


Tristan DEWOLF (GRENOBLE), Vincent DELAYE, Nicolas BERNIER, David COOPER, Nicolas CHEVALIER, Helen GRAMPEIX, Christelle CHARPIN, Eric JALAGUIER, Martin KOGELSCHATZ, Sylvie SCHAMM-CHARDON, Guillaume AUDOIT
08:00 - 18:15 #6011 - MS03-692 Mechanisms of polarity inversion during the MOVPE growth of III-nitrides on sapphire investigated by high resolution transmission electron microscopy.
MS03-692 Mechanisms of polarity inversion during the MOVPE growth of III-nitrides on sapphire investigated by high resolution transmission electron microscopy.

The most important compound semiconductors for applications in optoelectronics crystallize in the sphalerite or wurtzite structure, which contain polar axes. Though the polarity is of crucial importance, existing concepts of polarity control in wurtzite III-N films, grown on nonpolar substrates, are based on empiricism and a basic understanding of the elementary mechanisms behind is still missing.

A common concept is deposition of a buffer layer between the polar layer and the nonpolar substrate. In metalorganic vapor phase epitaxy (MOVPE) growth of III-nitrides such buffer layers are formed in three steps: first the surface of the sapphire is exposed to ammonia, commonly known as nitridation; then a thin layer of AlN or GaN is deposited onto the nitridated surface at relatively low growth temperatures, which is subsequently annealed at high temperatures. This classical process will result in high quality metal-polar films.

N-polar growth is achieved by nitridation of sapphire surface followed by layer deposition at high temperature. As opposed to metal-polar films, the resulting N-polar films are characterized by their rough surface morphologies, which is attributed to the presence of metal-polar inversion domains.

The importance of the nitridation step in improving structural and optical properties has been pointed out in numerous reports. While the chemical processes of the nitridation step were studied in detail, very little and contradictory work was presented on structural aspects that mediate polarity. This concerns the crystalline structure of the nitridation layer and the interface between the nitridation layer and the sapphire substrate. The event of aberration corrected transmission electron microscopes (TEM) now open new possibilities to resolve even single oxygen atomic column with high spatial resolution and thus to study this .

We performed a detailed study on the structure of nitridation layer and different buffer layers with respect to the polarity control by aberration corrected high resolution TEM as well as scanning TEM. We showed that sapphire nitridation results in a rhombohedral AlON-layer that converts the initially N-polar nucleated AlN to Al polarity. We performed contrast simulations for phase contrast imaging as well as for Z-contrast STEM imaging. The result of these simulations and the corresponding experimental images are shown in Fig.2.

The AlON layer, however, dissolves under high temperature growth conditions typical for III-nitrides, the initially N-polar AlN is reestablished and acts as a N-polar template. Therefore, we suggest that the role of the low temperature buffer is to protect the unstable AlON layer upon further growth.

The deeper understanding of the processes, governing the polarity inversion in III-N films, will allow optimizing growth conditions and improving the quality of N-polar thin films, therefore opening an access to the novel device concepts based on polarity engineering.

 Acknowledgments: N.Stolyarchuk acknowledge support from GANEX (ANR-11-LABX-0014). GANEX belongs to the public funded ‘Investissements d’Avenir’ program managed by the French ANR agency


Natalia STOLYARCHUK, Stefan MOHN, Toni MARKURT, Aimeric COURVILLE, Rosa DI FELICE, Philippe VENNÉGUÈS (CRHEA, Valbonne), Martin ALBRECHT
08:00 - 18:15 #6015 - MS03-694 Quantifying Mg doping in AlGaN layers.
MS03-694 Quantifying Mg doping in AlGaN layers.

     Gallium nitride and its alloys, AlGaN and InGaN, are essential but also challenging materials for the development of optoelectronic devices, such as visible or UV LEDs. The optical and electric properties of nano-objects such as 2D layers, quantum dots or nanowires are indeed directly affected by the concentration and the distribution of dopants. Mg is the most used p-type dopant for nitrides, however its impact on structural and optical properties is still not fully understood. In particular it is of the upmost importance to determine if it is homogeneously distributed or not. In metal organic chemical vapor deposition or hydride vapour phase epitaxy grown layers, for concentrations higher than 1018 cm-3, Mg has been observed to segregate into a variety of defects, inversion polarity, and pyramidal inversion domains [1][2][3]. In molecular beam epitaxy (MBE) grown layers, the question is not solved for such low concentrations. However, higher concentration of Mg causes clustering or incorporation in interstitial sites [4]. For this purpose, we have used atom probe tomography (APT), EDX both in SEM and TEM, and EELS with the aim to compare and determine which one is the most appropriate technique for providing qualitative and quantitative information on Mg low doping. The obtained results will be presented.

 

     Mg-doped Al0.2Ga0.8N 2D layers grown by MBE have been studied with a concentration around 3x1019 at.cm-3 according to SIMS measurements. The sample has been prepared by FIB either as a thin layer for EELS and EDX investigations, or as a sharp needle for APT experiments.

    

     For EDX as well as for EELS experiments, the main difficulty arises from the position of the emission line (Kα) or respectively absorption line of Mg (Mg K-edge), very close to that of Ga which is the main element of the sample. Adding to this, it has not been possible to observe Mg on TEM-EDX elemental maps.

 

     Thanks to the use of a Bruker FlatQUAD detector in SEM-EDX mode with a very large solid angle data collection which ensures a very high counting rate[5], it has been possible to detect Mg and its concentration has been estimated around 3.8 ±1.4 x1019 at.cm-3 (Figure [1]). This is in very good agreement with SIMS experiments.  

    

     Finally, we have performed APT analyses on the same samples. The mass spectrum clearly shows the presence of Mg, its isotopes and also its alloys (Figure [2]). The concentration has been estimated around 3.2 ±2.5 x1019 at.cm-3 after data treatment. The 3D reconstruction of Mg (Figure [3]) shows a homogenous distribution of Mg. Statistical analyses are currently performed using first and k-nearest neighbor method in order to determine if the distribution is actually homogenous or not at small scale, and to try to have a more accurate measurement of the Mg content according to the method developed by Thomas Philippe et al[6].

 

 

[1] S.E.Bennet et al. Study of Mg-doped AlGaN/GaN superlattices using TEM coupled to APT - Ultramicroscopy 111 (2011) 207-211)

[2] P.Vennéguès et al. Atomic structure of pyramidal defects in Mg-doped GaN – Phy.Rev.B, 68, 235214 (2003)

[3] N.Grandjean et al. Control of the polarity of GaN films using an Mg adsorption layer – J.Cryst.Growth 251 (2003) 460

[4] S.Pezzagna et al. Polarity inversion of GaN (0001) – J.Cryst.Growth 269,249 (2004)

[5] E. Robin et al., this conference proceedings

[6] T.Philippe et al. Clustering and nearest neighbour distances in atom probe tomography – Ultramicroscopy 109 (2009) 1304-1309


Lynda AMICHI (GRENOBLE), Isabelle MOUTON, Eric ROBIN, Vincent DELAYE, Nicolas MOLLARD, Philippe VENNÉGUÈS, Samuel MATTA, Julien BRAULT, Adeline GRENIER, Pierre-Henri JOUNEAU, Catherine BOUGEROL
08:00 - 18:15 #6019 - MS03-696 Structural quality of CH3NH3PbI3 perovskites for photovoltaic applications analyzed by electron microscopy techniques.
MS03-696 Structural quality of CH3NH3PbI3 perovskites for photovoltaic applications analyzed by electron microscopy techniques.

 

Hybrid Halide Perovskites (HPVK) are novel materials that have attracted attention in the last few years as promising materials in the photovoltaic field,[1, 2] as efficiencies higher than 20% have been reported[3].The HPVK can have different compositions, which allows the control of lattice parameters[4] and bandgap values in the visible to near infrared region[5]. In particular, CH3NH3PbI3 presents optimal electronic and optical properties such as direct optical band gap of 1.55 eV or long electron/hole diffusion length (100 nm)[6].

In this communication, we have analysed the structural quality of two CH3NH3PbI3samples by electron microscopy techniques. The samples have been grown using spin coating solution processing method. The details of the process can be found in[7]. The structure of the samples consists of a substrate of SiO2 (2µm), a TiO2 layer (40 nm), an active layer of HPVK and a poly-methyl methacrylate (PMMA) capping layer (1µm) in order to protect the sample from oxidation. The two samples studied (S500 and S100) have different HPVK thicknesses, 500 and 100 nm respectively, obtained by changing the speed in the spin-coating process [7]. The first analyses have been carried out by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which is sensitive to the atomic number of the atoms in the material. Fig. 1 a shows a HAADF-STEM image of the first sample, where the different layers of the structure can be observed. These layers have been studied by energy dispersive X-ray analysis (EDX) as exhibited in Fig. 1 b, and the layers sequence (SiO2, TiO2 and HPVK) has been confirmed. Thus, spectrum 3 in Fig. 1 b shows the presence of Pb and I, indicating that this layer corresponds to the HPVK, despite it shows a reduced size due to the ion bombardment during the sample preparation process for TEM. Electron diffraction patterns have been taken from the HPVK layer and, surprisingly, some areas have shown to be polycrystalline but others are amorphous. Previous analysis of the functional properties of these samples suggests that they are crystalline. Sample preparation to obtain the electron transparent specimen may cause a detrimental effect in the HPVK layer, producing the amorphization of a material. Because of this, the procedure of sample preparation is being optimized in order to analyse in detail the structural properties of this material.

References:

[1] H.J. Snaith, The Journal of Physical Chemistry Letters, 4 (2013) 3623-3630.

[2] H.-S. Kim, J.-W. Lee, N. Yantara, P.P. Boix, S.A. Kulkarni, S. Mhaisalkar, M. Grätzel, N.-G. Park, Nano Letters, 13 (2013) 2412-2417.

[3] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Progress in Photovoltaics: Research and Applications, 23 (2015) 1-9.

[4] A. Sadhanala, S. Ahmad, B. Zhao, N. Giesbrecht, P.M. Pearce, F. Deschler, R.L.Z. Hoye, K.C. Gödel, T. Bein, P. Docampo, S.E. Dutton, M.F.L. De Volder, R.H. Friend, Nano Letters, 15 (2015) 6095-6101.

[5] Y.H. Kim, H. Cho, J.H. Heo, T.S. Kim, N. Myoung, C.L. Lee, S.H. Im, T.W. Lee, Advanced Materials, 27 (2015) 1248-1254.

[6] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Grätzel, S. Mhaisalkar, T.C. Sum, Science, 342 (2013) 344-347.

[7] I. Suárez, E.J. Juárez-Pérez, J. Bisquert, I. Mora-Serõ, J.P. Martínez-Pastor, Advanced Materials, 27 (2015) 6157-6162.

Acknowledgements: This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-1-R, -2-R and CONSOLIDER INGENIO 2010 CSD2009-00013) and Junta de Andalucía (PAI research group TEP-946). The research leading to these results has received co-funding from the European Union.

 


Natalia FERNÁNDEZ-DELGADO (Cádiz, Spain), Miriam HERRERA-COLLADO, Francisco DELGADO-GONZÁLEZ, Emilio JUÁREZ-PÉREZ, Iván MORA-SERO, Isaac SUÁREZ, Juan MARTÍNEZ-PASTOR, Sergio MOLINA
08:00 - 18:15 #6033 - MS03-698 Structural analysis of GaSb/GaAs quantum rings for Solar Cells.
MS03-698 Structural analysis of GaSb/GaAs quantum rings for Solar Cells.

A type-II GaSb/GaAs quantum dot (QD)/quantum ring (QR) solar cell (SC) achieves higher photo-response than its type-I counterpart [1], as it supports an enhanced carrier recombination rate due to a larger separation between the electron and hole confinements [2]. This behavior leads to greater valence band offset [2] and, eventually, the solar cell is also able to function well into the near infrared (NIR) regions [3]. The stacking of several GaSb/GaAs QDs layers within the SC is essential to increase the photon absorption, however these heterostructures face a large lattice mismatch (7.8%) that causes a high local strain [4]. Because of this, sometimes QDs tend to relax by diffusing Sb from its center, followed by As/Sb exchange, giving place to nanostructures in the form of quantum rings (QR). [5].

In this communication, we analyze a GaSb/GaAs structure grown at 480ºC by molecular beam epitaxy (MBE) on a GaAs substrate. The GaSb layers (2.1 ML) are capped by two consecutive GaAs layers (10nm at 480ºC and 30nm at 580ºC), and this whole segment is repeated 10 times. The structural properties of this sample have been analyzed by diffraction contrast Transmission Electron Microscopy (TEM) in a JEOL 2100 LaB6 microscope, working at 200 kV. 220 bright field (BF) images of the GaSb layers have shown that no dislocations or other type of defects appear in the structure, and only some strain contrasts due to the lattice mismatch are observed. Fig. 1 shows a 002 dark field (DF) image of the sample that clearly shows two-lobe shaped nanostructures corresponding to the presence of QR. We have found that the average diameter of the QR is 14 ± 5 nm, with average diameter of the individual lobes of 4±2 nm and an average height of 3±2 nm. Also, it is worth highlighting that a vertical stacking of the QR is not observed which is a consequence of a reduced propagation of the strain to the subsequent QR layers because of the large thickness of the GaAs barrier layers. High angle annular dark field (HAADF) analyses using an aberration corrected electron microscope are in progress in order to obtain more detailed information about the composition and the strain in these heterostructures.    

References:

[1] Wei-Hsun Lin, Kai-Wei Wang, Yu-An Liao, Chun-Wei Pao and Shih-Yen Lin, Journal of Applied Physics, vol. 114, pp. 053509, 2013.

[2] A. Marent, M. Geller, A. Schliwa, D. Feise, K. Pötschke, D. Bimberg, N. Akçay and N. Öncan, Applied Physics Letters, vol. 91, pp. 242109, 2007.

[3] R. B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L. F. Lester and D. L. Huffaker, Applied Physics Letters, vol. 90, pp. 173125, 2007.

[4] P. J. Carrington, R. J. Young, P. D. Hodgson, A. M. Sanchez, M. Hayne and A. Krier, Crystal Growth & Design, vol. 13, pp. 1226-1230, 2013.

[5] E. P. Smakman, J. K. Garleff, R. J. Young, M. Hayne, P. Rambabu and P. M. Koenraad, Applied Physics Letters, vol. 100, pp. 142116, 2012.

 

This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-2-R and CONSOLIDER INGENIO 2010 CSD2009-00013) and Junta de Andalucía (PAI research group TEP-946). The research leading to these results has received funding from the European Union (PROMIS Marie Curie ITN network).


Atif Alam KHAN (Cádiz, Spain), Miriam HERRERA, Natalia FERNÁNDEZ-DELGADO, Denise MONTESDEOCA, Peter CARRINGTON, Hiromi FUJITA, Juanita Saroj JAMES ASIRVATHAN, Magnus WAGENER, Reinhardt BOTHA, Andrew MARSHALL, Anthony KRIER, Sergio MOLINA
08:00 - 18:15 #6037 - MS03-700 Analysis of semipolar InxGa1-xN/GaN heterostructures by WBDF and HRTEM imaging.
MS03-700 Analysis of semipolar InxGa1-xN/GaN heterostructures by WBDF and HRTEM imaging.

GaN-based optoelectronic structures have been successfully applied in the fabrication of blue light emitting diodes (LEDs) and laser diodes [1]. The fabrication of high-efficient green LEDs remains challenging which is referred to as the “green gap”. In GaN-based green LEDs, the active region consists of InxGa1-xN quantum wells (QWs) and GaN barrier layers (BLs) where the emitting wavelength is controlled by the In content (x).  However, In incorporation often results in inferior crystal quality due to the miscibility gap and lattice mismatch when the In content is increased [2]. Although conventional c-plane InxGa1-xN /GaN QWs have demonstrated the highest crystal quality,  the quantum efficiency remains low as a result of the polarization field along the c-axis which tilts the conduction and valence bands thus reducing the recombination probability (i.e., quantum confined Stark effect, QCSE). Semipolar InxGa1-xN/GaN structures, on the other hand, offer a good compromise between reduced QCSE and acceptable crystal quality [3].

In this study, we investigated the crystal quality of two sets of InxGa1-xN/GaN structures grown on (11-22) and (10-11)-semipolar planes. The samples were grown on sapphire by using metalorganic vapor phase epitaxy (MOVPE). The weak-beam dark-field technique (WBDF) was applied to investigate the distribution of the threading dislocations (TDs). Figure 1 shows the WBDF images of a (11-22)-semipolar sample. The GaN growth starts on the c-plane-like facets by MOVPE, forming triangular shaped stripes. The stripes subsequently coalesce forming a closed {11-22}-semipolar surface. TDs originate from the inclined c-plane-like GaN/sapphire interface and propagate in c-direction and then bend 90° to the a-directions. Most of the dislocations accumulate around the coalescence area and penetrate the QWs. The bending phenomenon is related to the triangular shape of the original GaN stripes and the SiNx interlayers deposited during the growth of GaN.

By using high-resolution TEM, we characterized the local structures of the QWs. Figure 2 shows the experimental HRTEM images of a (10-11)-semipolar sample acquired from the active region. Structural factors, such as the thickness of the QWs, the interfacial sharpness between the QWs and the BLs and the stacking faults formed with in the active region, can be analyzed with atomic resolution. Stacking faults are observed and confirmed to be the I2 intrinsic type. [4]

References

[1] Shuji Nakamura and Gerhard Fasol. The Blue Laser Diode-GaN Based Light Emitter and Lasers. Springer Berlin Heidelberg, 2000.

[2] F Scholz. Semipolar GaN grown on foreign substrates: a review. Semiconductor Science and Technology, 27(2):024002, 2012.

[3] J. S. Speck and S. F. Chichibu. Nonpolar and semipolar group III nitride-based materials. MRS Bulletin, 34:304312, 5 2009.

[4] We gratefully acknowledge financial support by the DFG (KA1295/22-1) and technical support by Sabine Grözinger in cross-section TEM sample preparation. 


Xiaodan CHEN (Ulm, Germany), Haoyuan QI, Yueliang LI, Tobias MEISCH, Ferdinand SCHOLZ, Ute KAISER
08:00 - 18:15 #6070 - MS03-702 Electron beam drilling rates of silicon crystal measured on various accelerating voltages and probe currents.
MS03-702 Electron beam drilling rates of silicon crystal measured on various accelerating voltages and probe currents.

 The design rule of the semiconductor device has been miniaturized to be 22/20 nm or 16/14 nm recently. When we analyze a structure of these miniaturized devices by scanning transmission electron microscopy (STEM), it is necessary to prepare a lamella with a thickness less than 20 nm. And the thin lamella is analyzed by energy dispersive x-ray spectroscopy (EDS) and/or electron energy-loss spectroscopy (EELS), which require longer dwell time than that of imaging due to small signal intensity. This lamella may receive the electron beam damage if a STEM probe is not optimized. The damage could be categorized to be two types, one is structure deformation of sample and the other is etching and/or migration of sample atoms, resulting in beam drilling of sample [1]. The latter is crucial for elemental analysis, since it significantly affect to results of quantitative analysis. Such phenomena were proposed, since the field emission STEM was commercialized [2,3]. In this paper, we report beam drilling of Si crystal depending on electron probe currents and accelerating voltages.

 We used a field emission electron microscope (JEOL, JEM-2800) equipped with a X-ray detection system, which include two large-sized silicon drift detectors (dual SDD, 100 mm2). A lamella of a silicon device was prepared with Ar ion milling (JEOL, Ion Slicer). The thickness of the lamella was measured to be approximately 15 nm by the EELS ratio method. To measure electron beam etching rate, we measured a decay of Si X-ray (Kα) count rates using a point analysis mode.

 Figure 1 shows the decay profiles on Si count rates for various probe currents at 200 kV. The decay rate (R) is express as R = R0*exp(-at), where R0 is initial decay rate, exp(-a) is decay coefficient at a certain probe current and an accelerating voltage, and t is elapsed time. Thus, we estimate the decay coefficient by fitting the decay profiles. Si X-ray count rates are decreased with the decay coefficients of 22.74 %/sec, 16.72 %/sec, 8.41 %/sec and 3.03 %/sec for probe currents of 1.50 nA, 0.96 nA, 0.50 nA and 0.22 nA. The measured decay coefficients are approximately proportional to probe current, through the electron densities under these probes are approximately constant to be 2.0 nA/nm2. We need to care to use a large probe current for analysis or imaging with long dwell time, since this result implies that the drilling rate is promoted when we uses larger current. Therefore, it is preferable to analyze with small current as possible for less damage of sample at 200 kV.

 Figure 2 plots decay of Si count rates for various accelerating voltages with probe current of 3.72 nA. The decay rates for 200 kV, 100 kV and 60 kV are measured to be 32.3 %/sec, 0.26 %/sec and 0.0 %/sec. The rates are not proportional to accelerating voltage. The drilling rate on accelerating voltage is related to threshold energy of sputtering of Si atoms. In addition to the low sputtering probability, the ionization cross section increases at lower energy, resulting in higher count rate at lower voltage. The initial count rates at 3.72 nA for 200, 100 and 60 kV were measured to be 12.7 kcounts/sec, 18.3 kcounts /sec, 29.0 kcounts /sec.

 In conclusion, we found out that elemental analysis in the low accelerating voltage is very effective for reduction of the electron beam damage as well as higher sensitivity due to larger ionization cross section of an element.

 

References

[1] RF Egerton, P Li and M Malac, Micron 35 (2004) p.399.

[2] LE Thomas, Ultramicroscopy 18 (1985) p.173.

[3] PA Crozier, MR McCartney and DJ Smith, Sur. Sci. 237 (1990) p.232.


Noriaki ENDO (Tokyo, Japan), Yukihito KONDO
08:00 - 18:15 #6077 - MS03-704 Holographic Measurement of Strain and Macroscopic Potentials in GaN Heterostrucutres.
MS03-704 Holographic Measurement of Strain and Macroscopic Potentials in GaN Heterostrucutres.

The phase of an electron wave transmitted through a sample can be measured with the transmission electron microscope (TEM) using electron holography. This phase information is used to determine electrostatic potential distributions within the specimen, e.g. a semiconductor device [1]. Increasing the resolution towards anatomic scale makes it further possible to measure the lattice constants and the strain state of the crystalline specimen.

In this contribution we present a simulation study on the obtainable phase information using the crystal structure, the macroscopic electrostatic potential, and the strain-state as input parameters. This input is used to compute the potential distribution within the specimen. Then, we use the multislice-algorithm to propagate the electron wave through the sample. The output of the simulation, i.e. the actual amplitude and phase of the exit-wave, is analyzed by Fourier filtering of selected reflections. Figure 1 illustrates this approach.

We apply this approach to simulate the measurement of the polarization induced electrostatic fields in a strained Al0.2Ga0.8N thin-layer, embedded in a GaN matrix. Figure 2a shows the scheme of the specific simulation setup described in this contribution. This material system is of particular interest, because of its applications to optoelectronics. The actual field-strengths in this material system are still a matter of scientific discussion.

For a holographic measurement in a TEM, a sample thickness of 200-300 nm is normally used. This has the advantage, that a large phase signal can be expected. But the thick samples lead to strong dynamic diffraction conditions and thus a complicated material contrast. Hence, sample-tilt and local thickness variations have a significant impact on the measurement result.

Figure 2b shows a typical simulation result in the case of a 10 nm Al0.2Ga0.8N thin-layer, grown on a c-plane of the Wurzite GaN matrix. The strain of the layer is set to 1% and the polarization induced field to 1.5 MV/cm, according to the order of magnitude expected from literature [2]. The polarization induced field corresponds to an interface charge density of ±7.7 *1012 cm-2.

The simulation results were filtered around the (0000)-beam and the ±(0002)-reflections (figure 3a). Typical results are shown in figure 3b. The phase profiles of the (0000)-beam and diffracted ±(0002)-beams show a slope within the Al0.2Ga0.8N - layer. This slopes contain information about the actual polarization field and strain of the layer. We show that, despite the dynamic diffraction conditions, the slope can be evaluated with the standard approach from kinematic diffraction theory yielding precisely the strain and field strength, as given as input to the model.

Nevertheless, one has to consider the dynamical diffraction conditions because they define the material contrast at the hetero interface and the actual signal amplitude within the Al0.2Ga0.8N - layer. By adjusting the sample thickness and sample tilt the conditions can be optimized with respect to the signal amplitude. We suggest to use wedge shaped samples in order to obtain more experimental flexibility.



[1] H. Lichte, M. Lehmann, Annu. Rev. Modern Physics 71 (2008) 016102

[2] O. Ambacher, J Majewski, J. Phys: Condens. Matter 14 (2002) 3399-3434

[3] We kindly acknowledge support from the DFG within the Collaborative Research Center 787, project A4.


Michael NARODOVITCH (Berlin, Germany), Tore NIERMANN, Michael LEHMANN
08:00 - 18:15 #6140 - MS03-706 In-situ observation of structural transition, ferromagnetic order breaking and magnetocrystalline anisotropy by EMCD in MnAs/GaAs(001).
MS03-706 In-situ observation of structural transition, ferromagnetic order breaking and magnetocrystalline anisotropy by EMCD in MnAs/GaAs(001).

        

        EMCD (Energy-Loss Magnetic Chiral Dichoism), an emerging technique based on energy-loss spectroscopy (EELS) in a transmission electron microscopy (TEM) [1-2], aims at measuring the element-specific local magnetic moment of solids at a nanometer scale. The signal of interest that brings the magnetic information comes from two chiral positions in the electron diffraction pattern, as sketched in figure 1(a).  

        In this work, EMCD is carried out to epitaxial MnAs thin films grown on a GaAs(001) substrate. As illustrated in figure 1(b-c), a breaking of the ferromagnetic order in MnAs thin film  is locally studied in-situ, together with the associated crystallographic transition from hexagonal α-MnAs to quasi-hexagonal β-MnAs, by modifying the temperature of the crystal inside the electron microscope. To achieve quantitative information, applying the sum rules [3] to the dichroic signal of magnetic anisotropic materials is accurately discussed [4].

       In addition, the orbital-to-spin moment ratio of -MnAs along the easy, hard, and intermediate magnetic axes is estimated by EMCD and compared to implemented density functional theory (DFT) calculations, as illustrated in table 1. The influence of the magnetocrystalline anisotropy is locally demonstrated [5]. This work in particular illustrates the feasibility of the EMCD technique for in-situ experiments, and proves its potential to explore the anisotropy of magnetic materials.

[1]. Schattschneider, P. et al. Nature 441, 486–488 (2006).

[2]. Warot-Fonrose, B. et al. Ultramicroscopy 108, 393–398 (2008).

[3] Calmels, L. et al. Phys. Rev. B 76, 060409 (2007).

[4] Fu, X. et al. Applied Physics Letters 107, 062402 (2015).

[5] Fu. X, et al. Phys. Rev. B 00, 004400 (2016).

Acknowledgement: This work is supported by the French national project EMMA (ANR12 BS10 013 01) and by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.


Xiaoxiao FU (Toulouse), Bénédicte WAROT-FONROSE, Rémi ARRAS, Dominique DEMAILLE, Mahmoud EDDRIEF, Virginie SERIN
08:00 - 18:15 #6154 - MS03-708 EDX-STEM phase mapping of semiconductor devices using multivariate statistical analysis tools.
MS03-708 EDX-STEM phase mapping of semiconductor devices using multivariate statistical analysis tools.

With the increasing miniaturization of electronic devices, high resolution structural and analytical characterization tools are necessary for the optimization of fabrication processes. Scanning transmission electron microscopy energy dispersive X-ray (EDX-STEM) spectroscopy is a well-established technique that has recently gained momentum thanks to the introduction of high-brightness electron sources and the Super-X EDX system (4 SDD detectors), allowing fast EDX mapping with high collection efficiency. While the traditional EDX data analysis consists in extracting the elemental map of each element present in the sample [1], it is often the case that the aim of the analysis is to investigate the spatial distribution, shape and thickness of the different chemical phases present in the sample.


Multivariate statistical analysis tools, such as non-negative matrix factorization (NMF) and independent component analysis (ICA), were shown to yield simplified interpretation of spectral datasets by rapid identification of phases (e.g. [2,3]). In this work, we applied NMF to the EDX analysis of Si/SiGe multilayers to validate the Sidewall Image Transfer (SIT) process developed for their patterning [4]. A FIB-prepared lamella was characterized in an FEI Titan Themis operating at 200kV and equipped with a probe corrector and 4 SDD EDX detectors. An EDX-STEM map was acquired with TIA, using a pixel size of 1nm and a dwell time of 20ms/pixel, and exported to hyperspy, a python-based software for hyperspectral data processing [5].


Spectral unmixing using NMF led to the identification of five chemical phases in the sample: Si, SiGe, SiO2, TiN and C (see the component spectra in Figure 1 and the corresponding loadings in Figure 2(a-e)). More specifically, NMF succeeded in: (1) separating the Si signal emanating from pure Si, SiGe and SiO2 layers; and (2) deconvoluting the C, N and O peaks. This greatly simplified the compositional analysis (Figure 2(f)), and allowed a more straightforward estimation of the thickness of the different layers, as shown in Figure 2(g).


NMF combined to EDX-STEM tomography was recently applied to superalloy systems for aerospace applications [6,7]. We will show that this approach has also the potential to address materials characterization challenges currently facing the semiconductor industry, such as the chemical analysis of dopants and impurities [8].

References

1. K. Lepinay et al. Micron 47 (2013), p. 43. 2. F. de la Pena et al. Ultramicroscopy 111 (2011), p. 169.
3. G. Lucas et al. Micron 52-53 (2013), p. 49.
4. S.Barnola et al. Proc. of SPIE 9054 (2014), 90540E-1.
5. www.hyperspy.org

6. D. Rossouw et al. Acta Materialia 107 (2016), p. 229.

7. S. Ikeshita et al. Micron 82 (2016), p.1

8. We thank Pierre Burdet and Francisco de la Pena for their help with hyperspy. The experiments were performed on the Nanocharacterisation platform at MINATEC.


Zineb SAGHI (Grenoble), Patricia PIMENTA-BARROS, Gael GORET, Tony PRINTEMPS, Nicolas BERNIER, Sylvain BARRAUD, Vincent DELAYE
08:00 - 18:15 #6177 - MS03-710 Radial and Axial Interfaces in III-V Heterostructured Nanowires.
MS03-710 Radial and Axial Interfaces in III-V Heterostructured Nanowires.

Nanoscale heterostructures are promising candidates with potential in several applications such as optoelectronics and electronics. III-V nanowires, in this context, are excellent examples as they offer an ample range of possibilities for modulating the properties.1 In addition, their morphology allows accommodation of different compounds with larger lattice mismatch compared to bulk.2 In this work, we study a heterostructured GaSb-InAs system in which InAs grows both radially and axially on the GaSb.

Nanowires are grown by metal organic vapor phase epitaxy (MOVPE) using size-selected Au aerosol particles,3 as catalysts, which were deposited onto B-oriented GaAs wafers. The nanowire growth was carried out in an AIXTRON 3x2” close coupled showerhead MOVPE growth reactor at set temperatures of 510°C for GaAs and GaSb and 460°C for InAs, respectively. Firstly GaAs stems were synthesized. Then GaSb axial segments were grown on GaAs stems followed by an InAs growth step which led to formation of a radial shell on the GaSb segments as well as an axial segment on top of GaSb.

The axial and radial heterointerfaces are thoroughly studied by means of aberration-corrected scanning electron microscopy (STEM) and atomic-resolution electron energy loss spectroscopy (EELS). We demonstrate that the sharpness of the interfaces is different and hence they exhibit different charge transport properties in axial and radial directions. In Figure 1, the atomic-resolution EELS maps at the radial interface are shown.4

Moreover, the polarity of the nanowires is determined by means of medium-/high-angle annular dark-field (MAADF/HAADF-STEM) imaging. It is shown that each accommodating material inherits the polarity from its base. An As-polar GaAs segment grows on the GaAs (111)B substrate. Then GaSb accommodates with Sb polarity, the heavier element is on top in the dumbbell units; and the InAs shell grows with As polarity (lighter element on top). Therefore, as shown in Figure 2, a switch in the intensity along the dumbbell units is observed along the diameter of the nanowires caused by the change in the chemical composition (from GaSb core to InAs shell). The EELS compositional maps also confirm this change.4

 

References

1 M.T. Bjork et al., Appl. Phys. Lett. 80, 1058 (2002).

2 F. Glas et al., Phys. Rev. B 74, 121302 (2006).

3 M.H. Magnusson et al, Nanostructured Mater. 12, 45 (1999).

4 R.R. Zamani et al, submitted (2016)

 

Acknowledgements

The authors acknowledge the financial support from the U.K. Engineering and Physical Sciences Research Council (EPSRC) for supporting at the SuperSTEM Laboratory, the National Facility for Aberration-Corrected STEM. Additionally, RRZ and KAD acknowledge the European Research Council (ERC) for funding the “NEWIRES” project under grant agreement number 336126.


Reza R. ZAMANI (Lund, Sweden), Sebastian LEHMANN, Fredrik S. HAGE, Quentin M. RAMASSE, Kimberly A. DICK
08:00 - 18:15 #6187 - MS03-712 Protecting copper TEM specimens against corrosion via e-beam induced carbon deposition.
MS03-712 Protecting copper TEM specimens against corrosion via e-beam induced carbon deposition.

Copper containing Transmission Electron Microscopy (TEM) specimens are vulnerable to corrosion during transfer from Focused Ion Beam (FIB) to TEM vacuum. The corrosion is an attack of the copper surface by sulfur from ambient laboratory air (present at ppb level). The sulfur attack leads to a CuS tarnish layer covering the specimen sidewalls, and holes are formed in the copper layers [1]. Hence the specimen has to be discarded for TEM analysis. Naturally, the native oxide of copper is protecting against sulfur attacks, but this oxide is removed during the specimen preparation by FIB. In this work, the protection efficiency of a carbon layer deposited in the FIB subsequently to the TEM specimen preparation, is studied.

The carbon layers are e-beam deposited in a FEI Helios460 dual beam FIB on a thinned copper specimen (Fig. 1). Sufficiently thin layers are achieved with a 15 pA, 5kV e-beam scanning for 60 s/120 s/180 s in an area of 2x5 µm2 with 300ns dwell time. For a full protection it is necessary to deposit carbon on both sidewalls of the specimen, which is done by rotating the FIB sample stage by 180 degree. The electron beam is incident to the TEM lamellae at an angle of 38º. The carbon layer quality and thickness are studied in a FEI Titan3 60-300 instrument on the sidewalls of a HfO2 layer in a cone shaped specimen prepared through a Si/100 nm HfO2 stack (Fig. 2). The HfO2 cones give suitable contrast to the carbon layer and exhibit little ion beam sidewall damage from ion beam thinning. A small layer of redeposited material is covering the cone sidewall with a maximum thickness of 1nm. As the corrosion from the ambient laboratory air is not very reproducible from day to day and varies in strength from no corrosion to severe damage to the specimen, we designed a better controlled experiment by simulating the corrosion via a forced sulfur attack. This is done by storing the copper specimens after the ion beam milling and carbon deposition with a sulfur flake for 10 min in a gelatin capsule, which creates a sulfur enriched ambient.

The effectiveness of the protection layer is shown on Fig. 1. The copper specimen has two areas where carbon was deposited on both sidewalls for 120 s and 60 s respectively. After the preparation, the specimen was stored for 10 min with sulfur and then immediately transferred to the TEM. This treatment resulted in complete corrosion of the uncapped areas, whereas the carbon protected regions remained in perfect condition for the 120 s deposition time. The 60 s deposition time shows little corrosion at the edge and thus does not offer a complete protection. The same carbon deposition conditions were used on the HfO2 cones to measure the carbon layer thickness  at the sidewall of the cones and to see the influence of an enduring beam interaction. The above mentioned deposition conditions lead to a thickness of 3/4/4.5 nm on the sidewall of the cones for a 60s/120s/180s deposition time, respectively (Fig. 3). A series of TEM images over 5 minutes did not show any increase of carbon layer thickness at the sample sidewall (Fig. 4). Henceforth the sample is suitable for enduring beam interactions in TEM mode. A 60s 25 % O2/75 % Ar plasma cleaning in a Fischione Instruments “Nanoclean 1070” system can remove the deposited carbon layer on the copper.

It can be concluded that 4 nm of carbon can protect the surface sufficiently against sulfur attack from ambient laboratory air. The e-beam deposition is done with low beam currents as used for SEM imaging during the specimen preparation and is suitable for e-beam sensitive materials .

Felix Seidel acknowledges the Institute for Promotion of Innovation by Science and Technology in Flanders (IWT) for his Ph.D. fellowship.

[1] Felix Seidel, Olivier Richard, Hugo Bender, Wilfried Vandervorst, “Post-ion beam induced degradation of copper layers in transmission electron microscopy specimens”, Semiconductor Science and Technology, Volume 30, Issue 11, article id. 114016 (2015)


Felix SEIDEL, Olivier RICHARD, Hugo BENDER (Leuven, Belgium), Wilfried VANDERVORST
08:00 - 18:15 #6193 - MS03-714 A mechanism for the introduction of threading dislocations in III-nitride epitaxial layers from closed basal stacking fault domains.
MS03-714 A mechanism for the introduction of threading dislocations in III-nitride epitaxial layers from closed basal stacking fault domains.

The crucial issue of extensive threading dislocation (TD) introduction in III-nitride compound semiconductor epitaxial layers remains unresolved so far since their densities cannot be justified on the basis of operation of Matthews-Blakeslee dislocation glide on pyramidal planes. This concerns primarily the TDs with a-type 1/3<11-20> Burgers vectors which constitute the principal TD population. Mosaicity is another reason for TD introduction due to the need for accommodation of small misorientations between nucleated islands. However, high TD densities have persisted despite the advancements in mosaicity suppression through growth involving two-dimensional step flow.

Based on transmission electron microscopy (TEM) observations, we propose a mechanism for the nucleation of a-type TD half loops from closed domains bounded by two superimposed intrinsic basal (0002) stacking faults (BSFs) of I1 type [1]. We have employed both diffraction contrast TEM and high resolution TEM (HRTEM) in cross sectional and plan-view geometries in order to verify the generality of the proposed mechanism by experimental observations on InGaN, AlGaN, and InAlGaN epilayers, as well as on multi-quantum wells (MQWs) grown by the MBE and MOVPE methods. Defect characterization was performed by the circuit mapping [2] and geometrical phase analysis (GPA) methods.

We have found that depending on the relative orientation of the sphalerite structural units that comprise the I1 BSFs, the BSF closed domain may be dislocation-free (Fig. 1) or may be surrounded by a 1/3<10-10> partial dislocation loop (Fig. 2). In the former case the structural units are mirror related, whereas in the latter they are oriented in parallel as shown in Figs. 1(a) and 2(a) respectively. Such closed domains are hexagonally-shaped with sides along the m-type <10-10> directions. When defected, they become unstable in response to the strain environment caused by the interfacial misfit or by the compositional grading. As a result they re-arrange in order to contribute to the strain relief through a Burgers vector reaction, so that the total in-plane Burgers vector of the loop is not zero, but corresponds instead to a misfit dislocation segment.  Such a reaction requires a concurrent emanation of an inverse TD half loop from nodes localized at two of the hexagon’s vertices, in agreement with the experimental observations. The I1 BSF superposition eliminates any Burgers vector component along the [0001] growth direction thus resulting in pure a-type TDs. The emanation of such TD half loops has been thoroughly verified by extensive observations.

 

[1] J. Smalc-Koziorowska, C. Bazioti, M. Albrecht, and G. P. Dimitrakopulos, Appl. Phys. Lett. 108, 051901 (2016).

[2] G. P. Dimitrakopulos, Ph. Komninou, and R. C. Pond, Phys. Status Solidi B 227, 45 (2001).

 

Acknowledgements

Work performed under the framework of the project BRIDGE: “Elimination of structural defects in nitride semiconductor layers (InGaN and InAlGaN) used as active layers in semiconductor lasers” funded by the Foundation for Polish Science in the frame of the EU operating program Innovative Economy, and in the framework of the Research Funding Program: THALES, project NitPhoto, performed under the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) supported by the EU (ESF) and Greek national funds.


Julita SMALC-KOZIOROWSKA, Calliope BAZIOTI, Martin ALBRECHT, Georgios DIMITRAKOPULOS (Thessaloniki, Greece)
08:00 - 18:15 #6208 - MS03-716 Carrier Localization at Atomic-Scale Compositional Fluctuations in Single AlGaN Nanowires with Nano-Cathodoluminescence.
MS03-716 Carrier Localization at Atomic-Scale Compositional Fluctuations in Single AlGaN Nanowires with Nano-Cathodoluminescence.

Considerable interest has been generated to develop highly efficient deep ultraviolet (DUV) emitters using AlGaN-based alloys with direct bandgaps between 3.4 – 6.1 eV for a broad range of applications. Conventional planar AlGaN DUV devices, such as electrically injected solid-state lasers and light-emitting diodes, experience limited efficiencies due to the high dislocation density and inefficient p-doping with increasing Al-content using Mg. Nanowire (NW) structures can be a promising alternative to enhance strain relaxation and p-doping, both mediated by the additional free surfaces. Exceptionally high internal quantum efficiencies have been reported in high Al-content AlGaN NWs [1, 2], suggesting strong charge carrier confinement at nanoscale kinetically-driven alloy inhomogeneities [3] that behave optically as quantum dots [4]. Recently, atomic-scale compositional modulations have been reported in high Al-content AlGaN NWs and suggested to act as localization centers to enhance radiative recombination [1, 2]. Comprehensive understanding of emission characteristics in such spontaneously-formed compositional fluctuations in AlGaN NWs, from directly correlating the localized optical response to structural/chemical properties at relevant lengthscales is still lacking.

 

High and low Al-content AlxGa1-xN p-i-n homojunctions were grown on Si-doped GaN NW templates on Si(111) substrates by plasma-assisted molecular beam epitaxy. Characterization on single NWs using nanometer-scale cathodoluminescence (CL) spectral imaging at 150 K was performed [5], subsequently correlated to structural information obtained with aberration-corrected scanning transmission electron microscopy (STEM). The low-Al AlGaN NWs (Sample A, nominally x = 0.11) exhibit an Al-rich shell that can passivate the surface. A high degree of homogeneity within the AlGaN core region is observed from high-angle annular dark-field (HAADF) Z-contrast imaging, and is further confirmed with electron energy-loss spectroscopy (EELS) [2]. Individual NWs in Sample A show the presence of up to three emission peaks highly delocalized along the NW length (Fig. 1f–h), two peaks originate from the AlGaN region (sharp 336 nm band-edge peak, broad and asymmetric peak with 360 nm maximum), and one from the n-GaN base (~355 nm).

 

With increased Al concentration (Sample B, nominally x = 0.88), extensive atomic-scale HAADF intensity fluctuations are present in the AlGaN regions, and these are indicative of strong Ga-rich/Al-rich compositional modulations, as validated using EELS elemental mapping at atomic-resolution [2]. The nature of the laterally discontinuous compositional fluctuations varies between p-, i-, n-doped AlGaN regions (Fig. 2a–e): single atomic layers occurring on c-planes along the growth direction in p-AlGaN and on semi-polar {10-13} planes in i- and n-AlGaN. Other nm-sized modulations and segregation are also observed. NWs in Sample B display a spectrally dense series of narrow lines blue-shifted (230 – 300 nm), and are drastically more spatially localized within the AlGaN regions in the wavelength-filtered CL images, relative to Sample A. Subsequent high-resolution (HR)STEM on these NWs shows that the different spectral behaviors (emission energy) can be correlated to positions along the NW with different types of compositional fluctuations. Specifically, the shortest wavelength peaks (230 – 240 nm) originate from volumes with atomic-scale Ga-rich c-planes in p-AlGaN (Fig. 2c, ROI 1 circled in 2g), while high intensity sharp peaks (250 – 290 nm) can be assigned to regions with atomic-scale modulations on inclined {10-13} planes or other nm-sized segregation (Fig. 2b,d). The presence/absence of extended defects and its role as localization centers will also be discussed [6].

 

[1] S. Zhao, X. Liu, S.Y. Woo et al., Appl. Phys. Lett., 107(4), 043101 (2015)

[2] S. Zhao, S.Y. Woo et al., Nano Lett., 15(12), 7801–7807 (2015)

[3] A. Pierret et al., Phys. Status Solidi - Rapid Res. Lett., 7(10), 868–873 (2013)

[4] M. Belloeil et al., Nano Lett., 16(2), 960–966 (2016)

[5] L. Zagonel et al., Nano Lett., 11(2), 568–573 (2011); L. Tizei et al., Appl. Phys. Lett., 105(14), 143106 (2014)

[6] This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), with additional funding from the Michael Smith Foreign Study Supplement (MSFSS).


Steffi Y WOO (Hamilton, Canada), Luiz TIZEI, Matthieu BUGNET, Songrui ZHAO, Zetian MI, Mathieu KOCIAK, Gianluigi A BOTTON
08:00 - 18:15 #6215 - MS03-718 Microscopy of thin AlN layers grown by MBE on (111) silicon.
MS03-718 Microscopy of thin AlN layers grown by MBE on (111) silicon.

After the discovery of graphene many other materials were subject of research in a 2D form among them some of them showed bandgap at room temperature. There were a lot of theoretical papers which predicted the properties of the appropriate candidates.  

AlN layer is widely used as a buffer layer for the epitaxial growth of III-Nitrides on SiC, Si and Al2O3 substrates. In this work, the formation of a few monolayers thick graphene-like AlN (g-AlN) layer on a (111)-oriented silicon substrate in ammonia molecular beam epitaxy (MBE) has been studied using various transmission electron microscopy (TEM) techniques.

Flat, ultrathin layer of AlN was deposited on an ammonia treated (111) Si substrate (1). Two samples with thick (15-25 monolayer) and thin (3-4 monolayer) AlN were prepared. Cross sectional TEM specimens were prepared using the conventional mechanical polishing and low-energy (<1 keV) low-angle Ar ion milling. TEM images (not shown) used to measure the AlN layer thicknesses as X and Y nm in the two samples. The chemical composition of the interface structure between the AlN and Si substrate was studied by spectrum imaging; combining energy dispersive X-ray spectroscopy (EDXS) and scanning TEM (STEM) in an aberration-corrected FEI Titan 80-200 ChemiSTEM microscope. Figure 1 shows the Si and N map of the sample containing 15-25 AlN monolayers. The elements distribution across the interface extracted as linescans and shown in Fig. 1b suggests nitrogen enrichment between the Si and AlN. The interface structure was further studied by aberration-corrected TEM using an FEI Titan 80-300 kV TEM operated at 300 kV. Figure 2 shows the boundary structure very clearly, where - based on the measured distances (3.3 Å and 1.9 Å) - we suppose that two layers of Si3N4 phase were grown onto the silicon probably due to the ammonia treatment prior the AlN deposition. The first three sheets of AlN was measured to be as 2.86 Å and 2.60 Å that are higher than the wurtzite-type AlN (2.5 Å) suggesting that the AlN does not immediately follow the wurtzite-type stacking.

The results promise not only the formation of few layer AlN with different properties from the thick layers, but also their possible integration into the silicon device technology.

 

Acknowledgements

Authors thank the support of the Hungarian National Scientific Foundation (OTKA) through Grant No. K 108869 and NN118914. I. C. and B. P thanks to the European Commission for providing support to access the ER-C facility through the ESTEEM2 project.

 

(1) RV.Mansurov, T.Malin, Galitsyn, K.Zhuravlev Journal of Crystal Growth 428 (2015) 93–97.


Ildikó CORA (Budapest, Hungary), András KOVÁCS, V.g. MANSUROV, T.v. MALIN, Yu.g. GALITSYN, Kostya ZHURAVLEV, Béla PÉCZ
08:00 - 18:15 #6236 - MS03-720 Anomalous contrast behavior for STEM HAADF imaging of ordered In<sub>0.33</sub>Ga<sub>0.67</sub>N monolayers.
MS03-720 Anomalous contrast behavior for STEM HAADF imaging of ordered In<sub>0.33</sub>Ga<sub>0.67</sub>N monolayers.

2-dimensional nanostructures consisting only of a single layer of material have attracted great research interest in the past years. Among them are e.g. transition metal dichalcogenides like MoSe2, WS2 and their ternary alloys. But also InxGa1-xN/GaN short period superlattices (SPSL) built up of InxGa1-xN monolayer quantum wells belong to this material class [1].

For such kind of nanostructures relevant structural parameters, which determine material properties and thus device performance, are interface quality, alloy composition and possible ordering phenomena. Characterization of these quantities, at atomic scale, is commonly performed by high angle annular dark field imaging using a scanning transmission electron microscope (STEM HAADF). In the past years even quantitative composition analysis at atomic scale by STEM HAADF imaging has been demonstrated for various material systems, including InxGa1-xN quantum wells [2]. What makes STEM HAADF imaging so attractive for that purpose is the generally valid monotonic relationship between image intensity and the mean atomic number Z of the probed material (commonly expressed by the Z1.7 rule of thumb for the image intensity).

However, in our combined experimental and theoretical STEM HAADF analysis of In0.33Ga0.67N/GaN SPSL consisting of ordered In0.33Ga0.67N monolayers we have observed an anomalous contrast behavior. Within the ordered In0.33Ga0.67N monolayers In atoms are arranged in a periodic √3x√3R30° structure, resulting in pure In atomic columns in a GaN matrix along the cross-sectional viewing directions of the wurtzite lattice (see Fig. 1). This has been experimentally confirmed ex-situ by high resolution (S)TEM and in-situ by reflection high-energy electron diffraction (RHEED). Expecting intuitively a high contrast in high-resolution STEM HAADF images of the ordered In0.33Ga0.67N monolayers, the experimental contrast between pure In and Ga atomic columns, however, was far below the Z1.7 rule of thumb. To verify this result, we have performed frozen phonon simulations of a relaxed structure model consisting of a √3x√3R30° ordered In0.33Ga0.67N monolayer coherently embedded in a GaN matrix. Although the simulations agree with our experimental finding, even on a quantitative level, the explanation for the low contrast is far from intuitive. Even more surprisingly, for specimen thicknesses above 45 nm a contrast inversion occurs, i.e. the peak intensity of pure In atomic columns becomes lower than that of adjacent Ga atomic columns (see Fig. 2). Our frozen phonon simulations reveal that the origin for this anomalous contrast behavior lies in a strongly enhanced de-channeling of the electron probe if it is positioned on the In atomic column of the ordered In0.33Ga0.67N monolayer. This in turn is caused by a complex interplay of increased disorder in the direct vicinity of the In atomic column in terms of chemistry (In atomic column is surrounded by material with a different atomic potential) and lattice periodicity (stronger local distortions around the In column because of differences in the In-N vs. Ga-N bond length).

 

[1] Suski et al., Appl. Phys. Lett. 104, 182103 (2014)

[2] Rosenauer et al., Ultramicroscopy 111, 1316 (2011)


Toni MARKURT (Berlin, Germany), Tobias SCHULZ, Xin Qiang WANG, Xian Tong ZHENG, Ding Yu MA, Martin ALBRECHT
08:00 - 18:15 #5179 - MS04-722 Investigation of the behaviour of codeposited pentacene:perfluoropentacene blends with different mixing ratios by TEM techniques.
MS04-722 Investigation of the behaviour of codeposited pentacene:perfluoropentacene blends with different mixing ratios by TEM techniques.

Organic semiconductors (OSCs) have gained a great interest in the last years since they are a promising alternative to conventional inorganic semiconductors. There have been successful demonstrations of their applications in OFETs [1,2] , OLEDs [3,4] and photovoltaic devices [5,6]. Among other OSCs, pentacene (PEN, C22H14) and perfluoropentancene (PFP, C22F14) blends attract a special attention since they can form donor/acceptor systems (p-n junctions) and are expected to be structurally compatible due to their similar molecular geometry. However, for good optical and structural properties of these junctions, a coupling between PEN and PFP at a molecular level is needed. For this reason, the understanding of the behaviour of this PEN:PFP intermixture is of prime interest.

Previous studies by global characterization methods  (XRD, GIXRD, RSM and PL) have demonstrated the intermolecular coupling of codeposited PEN:PFP [7-11]. Moreover, Hinderhofer et al. [9] reported that the evidence for coupling in all PEN:PFP blends (with independence of mixing ratio) is based on the formation of a new “mixed crystal” phase with exclusively equimolecular concentrations of these compounds. Non-equimolecular blends lead to a phase separation between the new “mixed crystal” and the respective pure phases. Until now PEN:PFP blends have only been studied by averaging characterization methods, whose spatial resolution is no high enough to get a microstructural information on the films. In consequence, local methods are also demanded to give information about the local crystallinity and crystallographic phases. The most commonly used technique to elucidate the structure at nanoscale dimension is TEM.

Here, we present a study of codeposited PEN:PFP grown on SiO2 using different mixing ratios: [1:1] equimolecular PEN:PFP (Fig. 1), [2:1] with excess of PEN (Fig. 2) and [1:2] with excess of PFP (Fig. 3). The SiO2 is an amorphous substrate which minimizes the molecule-substrate interactions. The characterization of the structure and morphology of these blends were performed by AFM and TEM techniques (using SAED patterns, conventional dark field (DF) and bright field (BF) pictures and STEM analyses). The SAED patterns taken from large length scales show polycrystalline character and diffraction rings of a “common phase” that systematically appears for all blend ratios used. This phase can be assigned to the new “mixed crystal” previously observed [9]. In addition, SAED measurements performed at small length scales (Fig. 4), display the possible monocrystalline diffraction pattern of this new “mixed phase” for the first time. This SAED pattern is rather similar to pure PEN in the [001] expected orientation normal to SiO2 substrates, suggesting that the crystalline structures of both (the “mixed phase” and pure PEN) should be very similar. Additional DF-TEM pictures reveal that no single domains at large scale (in the range of µm) can be found for the different phases (“mixed phase” and respective pure phase) in the case of non-equimolecular mixed blends. However, a grainy structure is visible (order of 100 -200 nm), suggesting that a large and homogeneous crystal for this “mixed phase” is not formed.

TEM characterization manifests as an useful tool to understand local and extended crystal orientation by a combination of imaging and diffraction techniques. In this study, three different blends of PEN and PFP are compared using TEM tools. Hence, diffraction techniques are used to obtain information on the arrangement of the not yet well-understood phase (new “mixed phase”) formed as consequence of the favorable coupling between the PEN and PFP.

[1] Jung, B. J. et al. Chem. Mater. 2011, 23, (3), 568-582.

[2] Sirringhaus, H. Adv. Mater. 2014, 26, (9), 1319-1335.

[3] Jou, J.-H. et al. J. Mater. Chem. 2015, 3, (13), 2974-3002.

[4] Kalyani, N. T. et al. Renew. Sustainable Energy Rev. 2012, 16, (5), 2696-2723.

[5] Anthony, J. E. et al. Adv. Mater.  2010, 22, (34), 3876-3892.

[6] Cao, W. et al. Energy Environ. Sci. 2014, 7, (7), 2123-2144.

[7] Broch, K. et al. Phys. Rev. B 2011, 83, (24), 245307.

[8] Salzmann, I. et al. Langmuir 2008, 24, (14), 7294-7298.

[9] Hinderhofer, A. et al. J. Chem. Phys. 2011, 134, (10), 104702.

[10] Breuer, T. et al. J. Chem. Phys. 2013, 138, (11), 114901.

[11] Anger, F. et al. J. Chem. Phys. 2012, 136, (5), 054701.

The authors gratefully acknowledge funding from the SFB 1083.


Rocio FELIX (Marburg, Germany), Katharina I. GRIES, Tobias BREUER, Gregor WITTE, Kerstin VOLZ
08:00 - 18:15 #5189 - MS04-724 Study of anelastic behavior of amorphous TiAl by atomic-level elastic strain measurement during in-situ TEM straining.
MS04-724 Study of anelastic behavior of amorphous TiAl by atomic-level elastic strain measurement during in-situ TEM straining.

Metallic glasses exhibit a number of superior mechanical properties such as high strength and high elastic limit that are a consequence of the amorphous nature of the structure [1]. Therefore, high interest exists in the characterisation of the structure of amorphous materials and the correlation to the mechanical properties. Due to the lack of structural order metallic glasses show also a time-dependent elastic behaviour. It is the aim of the present study to investigate this anelastic behaviour of an amorphous TiAl thin film by comparing macroscopic and microscopic strain measurements during tensile deformation in-situ in the transmission electron microscope (TEM).


Ti45Al55 films (150 nm thick) were synthesized by co-deposition of Ti and Al on a Silicon waver by DC Magnetron Sputtering. Photolithography and reactive Ion etching techniques were used to co-fabricate MEMS based tensile testing stages with freestanding thin films [2]. The special design of the samples allows macroscopic strain and stress measurements. Figure 1 shows the TiAl thin film next to the stress and strain gauges. Tensile tests were carried out in a Philips CM200 microscope at an accelerating voltage of 200kV. The samples were uniaxially strained in steps of 150 nm and bright-field images and selected area diffraction (SAD) patterns were recorded using Gatan Orius CCD camera. Microscopic strain tensor on atomic level was obtained from electron scattering images by tracing the shift of the maximum of the first broad diffraction halo during tensile loading.


Figure 2 shows a characteristic diffraction pattern of amorphous TiAl from a selected area of 1.2 micrometer in diameter. The position of the first broad ring as a function of the angle χ is obtained by a Digital MicrographTM plug in. The evaluation procedure is described in detail in the contribution by C. Ebner et al [3]. The strain ε is calculated from the relative change of the maximum position q1(σ, χ) at a given stress with respect to the unloaded position q1(0,χ) by ε=(q1(0, χ)-q1(σ, χ))/q1(σ, χ). From a series of SAD patterns recorded from the same area at different stress levels during in-situ deformation the measured strain values and the corresponding fitted curve are plotted in Fig. 3. The maximum and minimum values of the curve increase with increasing stress; these values correspond to the principal strains e11 (parallel) and e22 (perpendicular to the loading direction), respectively. The macroscopic stresses parallel to the loading direction were calculated from the force gauges (cf. 2-3 in Fig. 1) of the MEMS device. Figure 4 shows the linear dependence of e11 and e22 on stress as expected from Hooke's law and reach 1% and -0.17% at the maximum stress, respectively. From the linear fit the Young's modulus E=185±2 GPa and the Poisson's ratio of ν=0.23±0.02 are obtained. The macroscopic strain values calculated from the gauges show the same trend but are systematically higher compared to the strain values obtained from reciprocal space measurements. Since the latter correlates with the modulus range of polycrystalline TiAl, the diffraction method traces the atomic-level strain and the difference to the macroscopic strain can be attributed to a non-affine and anelastic deformation resulting from topological rearrangements in metallic glasses. 


[1] A. L. Greer, Materials Today 12 (2009)14.
[2] W. Kang, J. Rajagopalan, M.T.A. Saif, Nanosc. and Nanotechn. Letters 2 (2010) 282.
[3] C. Ebner, R. Sarkar, J. Rajagopalan, C. Rentenberger, Proceedings of the EMC 2016, Lyon, France.

C. E. and C. R. acknowledge financial support by the Austrian Science Fund FWF: [I1309]. R. S. and J. R. acknowledge funding from the National Science Foundation (NSF) grants CMMI 1400505 and DMR 1454109.


Rohit SARKAR, Christian EBNER, Jagannathan RAJAGOPALAN, Christian RENTENBERGER (Vienna, Austria)
08:00 - 18:15 #5414 - MS04-726 Microscopic study of friction layer and the distribution of components in automotive brake pads.
MS04-726 Microscopic study of friction layer and the distribution of components in automotive brake pads.

Brake pad is a complex, multicomponent system consisting of reinforcing agents, abrasives, lubricants, binders, fillers and in some cases elemental metals. Numerous mechanical properties of this system are required and are related to the distribution of components in its volume. Friction layer is in contact with cast iron disc and distribution of single components there is crucial for the braking process. Due to high pressure and temperature achieved, various changes of chemistry and morphology during braking process occur. In this study commercially available automotive brake pads and newly developed brake pads modified with kaoline were investigated using light optical digital microscopy and scanning electron microscopy with EDS which enabled to create maps of elemental distribution. New unused brake pads and pads after AK master braking procedure were investigated. Combination of two microscopic techniques provided detailed information about the structure and its changes in such a complex system as brake pad is. This information will help to understand the process of friction layer formation and wear particles emissions.

Acknowledgement

The project has been financially supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 636592.  


Katerina DEDKOVA (Ostrava-Poruba, Czech Republic), Pavlina PEIKERTOVA, Tamara WILHELMI, Marcus MORBACH, Jana KUKUTSCHOVA
08:00 - 18:15 #5635 - MS04-728 Microstructure of glass ceramics synthesized from chromium waste.
MS04-728 Microstructure of glass ceramics synthesized from chromium waste.

The microstructure of different glass ceramic materials, obtained by thermal processing of vitrified products synthesized from tannery waste, was investigated using electron microscopy (TEM) techniques. Preliminary structural characterization was conducted by X-Ray Diffraction (XRD) while morphologies and compositions of the materials at the mesoscopic scale were attained using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS).  

A first series of materials were synthesized by sintering chromium rich ashes of tannery waste with low grade soda lime glass powder. Three different mixtures were prepared with proportions  30/70, 40/60 and 50/50 of Cr-ash over glass powder and sintered at 600°C, 800°C, 1000°C and 1200°C [1]. Depending on the temperature, crystalline phases with chromium and non-chromium content were solidified. At the low temperatures the resulting products were opaque ceramics with granular morphology and high porosity, as shown in Fig. 1. At the high temperatures they consisted of a vitreous matrix with dispersed crystalline phases having the morphology of typical glass-ceramics as those pointed by the white arrow in Fig. 2.

A second series of materials were synthesized using chromium ash (10wt%-20wt%) and SiO2, Na2O and CaO as vitrifying agents, in different relative proportions considering the low solubility of chromium inside the silicate melts [2]. Only the product with the lowest Cr-ash proportion was X-Ray amorphous with no indication of crystallites. HRTEM observations verified that the product retained the amorphous character even at the nanoscale since no nanostructured crystallites were detected. In all other vitrified products Eskolaite (Cr2O3) crystallites of hexagonal shape, shown in Figure 3, were grown in the melt. Devitrification of the as-casted products resulted to various crystalline phases dispersed within the vitreous matrix, depending on the initial batch composition. Eskolaite crystallites were not affected by the thermal processing and Devitrite (Na2Ca3Si6O16), Combeite (Na4Ca4Si6O18) and Wollastonite (CaSiO3) crystallites were grown. Figure 4 illustrates a HRTEM image showing a Wollastonite (CaSiO3) crystallite in the devitrified product with 13wt% Cr-ash, viewed along the [0 1 1] zone axis. This crystallite has the triclinic structure of 1A-CaSiO3.

[1] S. Varitis, P. Kavouras, G. Vourlias, E. Pavlidou,Th. Karakostas, Ph. Komninou, Production of Composite Materials by Mixing Chromium-Rich Ash and Soda-Lime Glass Powder: Mechanical properties and microstructure, Int Jour of Chem. Nucl. Mat. Metall. Eng. Vol:9, No:6, 2015

[2] S. Varitis, E. Pavlidou, P. Kavouras, G. Vourlias, K. Chrissafis, A. Xenidis,
Th. Karakostas, Devitrification routes of a vitrified chromium-loaded ash, Journal of Thermal Analysis and Calorimetry (2015) 121:203–208


Savvas VARITIS, Panagiotis KAVOURAS, George KAIMAKAMIS, Eleni PAVLIDOU, George VOURLIAS, Theodoros KARAKOSTAS (THESSALONIKI, Greece), Philomela KOMNINOU
08:00 - 18:15 #5729 - MS04-730 Effect of Ti addition in refinement of oxide dispersoids in Fe-Y2O3-Ti model ODS alloys during milling and subsequent annealing.
MS04-730 Effect of Ti addition in refinement of oxide dispersoids in Fe-Y2O3-Ti model ODS alloys during milling and subsequent annealing.

The oxide dispersion strengthened (ODS) steels have very high thermal stability and creep strength due to reinforcement of hard and stable nano-crystalline ceramic oxides in a metallic matrix which act as barriers to dislocation motion. These steels are candidate core structural materials for advanced fission and fusion reactors. The dispersoid size and their distribution in metal matrix play an important role in deciding physical and mechanical property of the steel. However understanding the formation mechanism and crystallographic structure of the nano-dispersoids is an ongoing activity of various research groups. The yttria based ODS steel, which contains 0.35wt% of yttria and 0.2 wt% of Ti amounting to ~ 0.56 vol % is very difficult to characterize by conventional XRD; TEM is possibly the only method to characterize the nano-sized (~2-5nm) dispersoids. But to understand the mechanism of formation of nano-dispersoid, crystallinity and structural changes one needs to employ both XRD and TEM. For this purpose, a Fe-15wt% Y2O3 –xwt%Ti (0, 5 and 15) type of concentrated alloy has been used. However, characterisation of samples prepared by conventional methods for TEM studies continues to be difficult owing to magnetic nature of ferritic steel. Hence FIB was employed to extract electron transparent samples which are of micrometer dimensions. The model ODS alloys were synthesized by ball milling for various time durations in inert atmosphere and with subsequent annealing at 1273K for 1h. The structural studies for the powders prior to milling as well as after milling and subsequent annealing were carried out using electron microscopy and XRD techniques. It was earlier reported by us [2] that yttria dispersoids gets refined and even amorphised during mechanical alloying (MA) and upon annealing yttria re-precipitates to micron size [2]. It is believed that Ti can inhibit the growth of nano-dispersoid during annealing by formation of Y-Ti-O complex oxides such as Y2Ti2O7 or Y2TiO5 or YTiO3 [3-5]. In this study XRD result revealed amorphisation of Y2O3/ Ti during milling for 60h in Ar atmosphere and evolution of Y2Ti2O7 andYTiO3 complex oxide upon annealing for 1h at 1273K, which is shown in figure-1. The scanning electron microscopy studies reveal the refinement of alloy powders from ~50 microns to sub micron level during milling, which is shown in figure 2. Figure 3(a) and (b) represent the Selected Area Diffraction pattern (SADP) of 60h milled powders of Fe-15wt% Y2O3 and Fe-15wt% Y2O3 -5wt%Ti respectively. Analysis of these patterns showed the 1st ring correspond to Y2O3 (411) was broader in case of the alloy without Ti as compared to alloy with Ti. Figure 3(c) and (d) represents the bright field (BF) TEM micrographs of Fe-15wt% Y2O3 -5wt%Ti model ODS after 60h of milling and subsequent annealing respectively, the corresponding SAD patterns are shown as inset. The analysis of BF-TEM micrograph showed the recrystallization of 100-200nm size crystallites upon annealing of milled powder, where as analysis of SADP reveal formation of Y-Ti-O complex oxides such as Y2Ti2O7, Y2TiO5 and YTiO3 in annealed alloy powder. EDS analysis from the 60h milled and annealed powder of Fe-15wt%Y2O3-5wt%Ti showed evolution of Y, Ti ,O rich complex oxide which is shown in figure 4. The results point towards a direct involvement of Ti in restricting amorphisation of yttria during milling, and formation of Y-Ti-O complex nano-dispersoid upon annealing. Details of these studies will be presented in the paper.

References

  1. L. Toualbi, M. Ratti, G. Andre, F. Onimus, Y. de Carlan, J. Nucl. Mater. 417 (2011) 225–228.
  2. Pradyumna Kumar Parida, Arup Dasgupta, K. Jayasankar, M. Kamruddin, S. Saroja, J. Nucl. Mater. 441 (2013) 331–336.
  3. A. Hirata, T. Fujita, Y.R. Wen, J.H. Schneibel, C.T. Liu, M.W. Chen, Nat. Mater. 10 (2011) 922.
  4. M.K. Miller, C.M. Parish, Q. Li, Mater. Sci. Technol. 29 (2013) 1174.
  5. H. Sakasegawa, L. Chaffron, F. Legendre, L. Boulanger, T. Cozzika, M. Brocq, Y. de Carlan, J. Nucl. Mater. 384 (2009) 115.

Pradyumna Kumar PARIDA (Kalpakkam, India), Arup DASGUPTA, K. G. RAGHAVENDRA, Sujay CHAKRAVARTY, K JAYASANKAR, Saroja SAIBABA
08:00 - 18:15 #5731 - MS04-732 Relationships between elaboration conditions, structural parameters and electrical properties in metal oxides nanometric periodic multilayers.
MS04-732 Relationships between elaboration conditions, structural parameters and electrical properties in metal oxides nanometric periodic multilayers.

Ti/TiOx and W/WOx multilayered thin films have been deposited by DC reactive sputtering using the reactive gas pulsing process (RGPP). It is implemented to produce regular alternations of metal-oxide compounds at the nanometric scale [1, 2]. Structure and growth have been investigated by High Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) Energy Dispersive X-rays Spectroscopy (EDX) and Energy Electron Loss Spectroscopy (EELS). Regularity of titanium and tungsten based alternations, quality of interfaces as well as oxygen diffusion through the multilayered structure have been examined taking into account the reactivity of oxygen towards the metals. Electrical measurements have been also carried out with the van der Pauw method to determine resistivity changes with temperature.

CTEM has been performed to determine the thickness of the periodic metallic and oxide layers from 3 to 40 nm. HRTEM experiments have been carried out to study the atomic structure of the periodic layers. The study of HRTEM images has allowed determining a growth model with the following series: (rutile-)TiO2, fcc-TiO and α-Ti for Titanium system. This result has been confirmed by SAED experiments. Chemical information was obtained from the core-loss EELS and EDX spectra. Core-loss study was particularly performed for TiOx samples to quantify the elemental composition from the Ti-L2,3 and O-K edges (Fig. 1).  The systematic presence of oxygen has been pointed out in the rich-metal sub-layer, corresponding to the TiO phase already pointed out by HRTEM. Concerning tungsten system, the layers appear to be mainly amorphous. Therefore, low-loss EELS study was performed and has allowed determining a growth model with the following series: W/W3O/WO3/W3O (Fig. 2).

The knowledge of the structural parameters has allowed determining a first relation between the elaboration conditions (control of the pressure value) and the structural parameters. Electrical and structural results have also been related to propose a law linking the resistivity values ρ to the structural parameters as total thickness etot, metal λmet and oxide λox layers thickness and metal elemental composition Cmet (Fig. 3).

[1] A. Cacucci et al, Thin Solid Films, 553 (2014) 93-97

[2] A. Cacucci et al, Acta Mater., 61 (2013) 4215-4225

 

ACKNOWLEDGMENTS

 This research is supported by LABEX project ACTION.


Valérie POTIN (DIJON CEDEX), Arnaud CACUCCI, Nicolas MARTIN
08:00 - 18:15 #5745 - MS04-734 Cryo-TEM observations and Xray mapping of polyoxometalate / polystyrene hybrid nanoparticles with surface reactivity.
MS04-734 Cryo-TEM observations and Xray mapping of polyoxometalate / polystyrene hybrid nanoparticles with surface reactivity.

            Polyoxometalates (POMs) are metal-oxygen clusters which can store electrons without altering their structure, so they are suitable for application in catalysis and materials science. In this project, we wished to synthesize surface catalytically active polystyrene-polyoxometalate (PS-POM) composite latexes through radical emulsion polymerization of styrene in water in the presence of amphiphilic POMs.

We used TEM analyses to demonstrate that polystyrene nanoparticles were stabilized by the POMs cluster at their surface. First, cryo TEM was used in order to determine the nanoparticles’ shape and size. Secondly the photo-catalytic activity of the POMs was evaluated. Upon UV irradiation, POMs can be photo-reduced, in presence of organic molecules (acting as sacrificial electrons donors). According to the literature, the POMs attached to the PS surface should be able to transfer their captured electrons to metal ions, such as silver ions, to generate metals nanoparticles. We tested this benchmark reaction in the presence of our hybrid latexes. By Cryo TEM, we observed that nanoparticles were formed as expected in short reaction times, and that they were only present at the surface of PS-POM composite latexes. In addition, Xray mapping at room temperature confirmed the presumed structure of the hybrid particles: POM clusters were only located at the particle surface by the detection of W atoms present in the used POM.

ref : CHEMISTRY-A EUROPEAN JOURNAL 21(2015)2948-2953


Jennifer LESAGE DE LA HAYE, Jean-Michel GUIGNER (Paris Cedex 05), Bernold HASENKNOPF, Emmanuel LACÔTE, Jutta RIEGER
08:00 - 18:15 #5750 - MS04-736 Micro- and nanostructural characterization of melamine-formaldehyde microcapsule shells using electron microscopy.
MS04-736 Micro- and nanostructural characterization of melamine-formaldehyde microcapsule shells using electron microscopy.

A systematic study has been carried out to compare the surface morphology, shell thickness, mechanical properties and binding behavior of melamine formaldehyde microcapsules of 5 to 30 μm diameter size with various amounts of core content by using scanning and transmission electron microscopy including electron tomography, in-situ nanomechanical tensile testing and electron energy-loss spectroscopy. It is found that porosities are present at the outside surface of the capsule shell but not at the inner surface of the shell. Nano-mechanical tensile tests on the capsule shells reveal that the Young’s modulus of the shell material is higher than that of bulk melamine formaldehyde and that the shells exhibit a larger fracture strain compared to the bulk. Core-loss elemental analysis of microcapsules embedded in epoxy indicates that during the curing process, the microcapsule-matrix interface remains uniform and the epoxy matrix penetrates into the surface microporosities of the capsule shells.


Hamed HEIDARI (Antwerpen, Belgium), Guadalupe RIVERO, Hosni IDRISSI, Dhanya RAMACHANDRAN, Seda CAKIR, Ricardo EGOAVIL, Mert KURTTEPELI, Amandine CRABBÉ, Tom HAUFFMAN, Herman TERRYN, Filip DU PREZ, Nick SCHRYVERS
08:00 - 18:15 #5759 - MS04-738 Transmission Electron Microscopy of a Poorly Soluble Drug, Felodipine.
MS04-738 Transmission Electron Microscopy of a Poorly Soluble Drug, Felodipine.

Many drugs that are currently in development exhibit poor dissolution properties due to low solubility. To increase the solubility of these drugs different technologies have been developed including solid dispersions, nano-crystals and lipid formulations. Use of solid dispersions can increase solubility by dispersing a drug within an amorphous polymer matrix, inhibiting re-crystallisation from the amorphous phase. This can be achieved by hot-melt extrusion (HME), spray drying and co-precipitation [1].

 

Transmission electron microscopy can be used to analyse different phases and morphologies within a solid dispersion which may not be found using conventional bulk techniques such as powder X-ray diffraction, differential scanning calorimetry and Fourier transform infra-red spectroscopy [2]. However, organic compounds can be easily damaged when exposed to the electron beam causing structural change, so low dose exposure is necessary to limit any damage that occurs [3].

 

The research here investigates a model system involving the drug felodipine and the copolymer copovidone. Solid dispersions of 15% and 50% felodipine were prepared by HME and then milled to form a powder. This powder was then dispersed in water, 3-4 droplets were placed onto separate copper continuous carbon TEM grids and left to dry. These were then examined by TEM, operated at 200kV, equipped with a CCD camera. The electron flux was set between 0.007 - 0.040 e− Ås−1 to limit any damage which may be caused by the high energy electrons [3].

 

The results show that within the 50% solid dispersions different morphologies and phases can be found, most areas show near amorphous diffraction patterns (Figure 1) but some show single crystal patterns (Figure 2). This suggests that the felodipine and copovidone are not always molecularly mixed and that small amounts of the felodipine had recrystallised after the initial HME process. Similarly, the 15% mixture had areas with varied morphologies and showed some signs of phase separation. The next step will be to quantify the identified phase fractions (amorphous, single crystal and nano-crystalline) within these solid dispersions.

 

References

[1] Huang Y and Dai WG., Fundamental aspects of solid dispersion technology for poorly soluble drugs. Acta. Pharma. Sinica. B 4(1) 18-25, (2014)

 

[2] Song Y et. al., Physiochemical Characterization of Felodipine-Kollidon VA64 Amorphous Solid Dispersions Prepared by Hot-Melt Extrusion. J. Pharma. 102(6) 1915-1923, (2013).

 

[3] S’ari M et. al., Analysis of Electron Beam Damage of Crystalline Pharmaceutical Materials by Transmission Electron Microscopy. J. Phys. Conf. Ser. 644, (2015).

 

[4] Lou et. al., Polymorph Control of Felodipine Form II in an Attempted Cocrystallization. Cryst. Growth Des. 9(3) 1254-1257, (2009).


Mark S'ARI (Leeds, United Kingdom), James CATTLE, Andy BROWN, Brydson RIK, Nicole HONDOW, Helen BLADE, Steve COSGROVE, Les HUGHES
08:00 - 18:15 #5796 - MS04-740 Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites.
MS04-740 Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites.

      Self-assembled nanostructures are promising for creating 2D and 3D superlattices with exceptional functionalities. Understanding the mechanisms driving the superlattice formation demands the underlying structural information. However, nanoscale structural modulations intrinsic to these superlattices are difficult to be characterized by conventional diffraction-based structure determination. A real-space, direct imaging method is necessary to probe the local structure characteristics, providing essential information for theoretical understanding and subsequent design of structure-property correlations.

     Using the aberration-corrected scanning transmission electron microscopy (STEM), we developed an optimized atomic-level bright-field (BF) condition to image the oxygen octahedra in perovskite oxides. We used multislice calculations to determine detector collection angles that allow oxygen octahedra to be imaged sensitively and robustly over large specimen thicknesses. These calculations also provided a calibration by which the octahedral-tilt angle can be measured quantitatively from the image of each octahedron.

     Applying this real-space octahedral-tilt mapping on Li0.5–3xNd0.5+xTiO3, a promising solid electrolyte in Li-ion batteries, we directly revealed an unconventional superlattices with 2D modulated octahedral tilting. A mathematical description of the octahedral-tilt modulation was derived based on the quantitative tilt maps, which explicitly identified the high-order harmonic character of the modulation. Using simultaneous annular-dark-field (ADF) imaging, we also mapped the lattice parameters unit-cell by unit-cell, uncovering highly-localized strain associated with the tilt modulation. Furthermore, we demonstrate the tunability of the tilt modulation by changing Li stoichiometry. Fascinatingly, we observe a reversible annihilation/reconstruction of the tilt modulation correlated with delithiation/lithiation process, suggesting the structural transformation that is associated with Li-ion conduction in this promising Li-ion conductor.1

     The above observations are largely inaccessible from conventional diffraction analysis,2 and lead to an unprecedented mechanically-coupled tilting competition model to explain the superlattice formation.1 Our real-space approach to quantify local octahedral structure and correlate it with strain can be applied to other advanced oxide systems.

 

Acknowledgements

This work was supported by the Australian Research Council (ARC) grants DP110104734 and DP150104483 and a Monash University IDR grant. The FEI Titan3 80-300 S/TEM at Monash Centre for Electron Microscopy was funded by the ARC Grant LE0454166. 

 

References:

[1] Y. Zhu, R. L. Withers, L. Bourgeois, C. Dwyer & J. Etheridge Nature Materials 14, 1142-1149 (2015). 

[2] A. M. Abakumov, R. Erni, A. A. Tsirlin, M. D. Rossell, D. Batuk, G. Nénert & G. V. Tendeloo Chem. Mater. 25, 2670–2683 (2013).


Ye ZHU, Ray WITHERS, Laure BOURGEOIS (Victoria, Australia), Christian DWYER, Joanne ETHERIDGE
08:00 - 18:15 #5801 - MS04-742 From zirconia to yttria: sampling the crystallographic and electronic phase diagram using sputter-deposited YSZ thin films.
MS04-742 From zirconia to yttria: sampling the crystallographic and electronic phase diagram using sputter-deposited YSZ thin films.

Yttria-stabilized zirconia (YSZ) is one of the most extensively investigated materials with a large field of applications due do its chemical inertness and mechanical as well as thermal stability. Moreover, it features a high ionic conductivity at elevated temperatures; it is consequently an ideal candidate as electrolyte in chemical sensors and solid oxide fuel cells. While there is a large number of works on this topic, most only deal with a single composition and thicker films. Here, we present a systematic study regarding the influence of the amount of yttria on the crystallographic and electronic properties of YSZ thin films to elucidate the thin film phase diagram. These epitaxially-grown thin films with a nominal thickness of 25 nm and compositions of 3 mol%, 8 mol%, 20 mol% and 40 mol% Y2O3 (from now on called 3YSZ, 8YSZ, 20YSZ and 40YSZ, respectively) were prepared on NaCl single crystals using our home-built direct-current ion beam sputter source.[1] The films, which could be floated off the sodium chloride to yield unsupported thin films for TEM studies, were shown to be homogeneous and impurity-free.

Although the distinction between the tetragonal and cubic phase of zirconium oxides is impossible with only selected area electron diffraction, due to both featuring planes with the same lattice spacings (see Figure 1a-d), it is possible to determine that 3YSZ and 8YSZ are tetragonal and 20YSZ as well as 40YSZ are cubic. For this, an analysis of the lattice parameter c, which is calculated from multiple diffraction rings for both, the tetragonal and cubic polymorphs, was performed, showing a stagnation in this parameter between 8 to 20 mol% yttria (see Figure 1e). Because a higher concentration of the larger atom (Y) is present, the cell volume still has to increase, which can only be explained if the total unit cell volume is still increasing, which means that the cell is expanding in the other two directions (a and b) instead, i.e. a phase transformation to the cubic cell is occurring.[1]

In order to obtain the direct band gaps, valence EELS experiments have been conducted (using only the direct beam) at an acceleration voltage of 60 kV to avoid Čerenkov losses.[2]. If the resulting band gaps (after Richardson-Lucy convolution of the spectra), which agree nicely with UV-photoelectron data, are plotted as a function of the amount of yttria present in the sample, a similar trend to that of the lattice parameter (see above) can be observed, as seen in Figure 2. Going from 3YSZ to 8YSZ, the band gap decreases from 5.6 eV to 5.3 eV, respectively, only to then increase again to almost 5.8 eV in the case of 20YSZ, coinciding with the tetragonal-cubic phase transition. Finally, 40YSZ features a much lower band gap of about 5.2 eV. In additional EELS experiments, the crystal field splitting between the eg and t2g bands (investigated using the oxygen K-ELNES) is also shown to feature a dependency of the dopant level.

Furthermore, a Gatan Vulcan Cathodoluminescence system is used to spectroscopically measure the Čerenkov radiation emitted from the sample when exposed to a 200 kV  beam. Determining the absorption edge seems to yield the distance from the valence band maximum to the gap states (present due to the oxygen vacancies), as determined by comparison with DFT band structures.

[1] Götsch, T.; Wallisch, W.; Stöger-Pollach, M.; Klötzer, B.; Penner, S. AIP Adv. 2016, 6, 025119

[2] Stöger-Pollach, M. Micron 2008, 39, 1092–1110


Thomas GÖTSCH (Innsbruck, Austria), Michael STÖGER-POLLACH, Simon PENNER
08:00 - 18:15 #5937 - MS04-744 Correlative SEM techniques for resolving complex microstructure of CoCrFeNiZrx High Entropy Alloys.
MS04-744 Correlative SEM techniques for resolving complex microstructure of CoCrFeNiZrx High Entropy Alloys.

High-entropy alloys (HEAs) introduce a new concept of developing advanced metallic materials with properties that conventional alloys, based on one principal element, cannot achieve [1]. Other multi-component metallic systems are quasicrystals (QCs), complex metallic alloys (CMAs) and bulk metallic glasses (BMGs).

HEAs are multicomponent mixtures of 4 to 9, and occasionally up to 20 chemical elements, in similar concentrations, ranging from 5 to 35 at.% each, where the high entropy of mixing can stabilize disordered solid-solution phases with simple crystal structures like a body-centered cubic (bcc), a face-centered cubic (fcc) and a hexagonal close-packed (hcp), in competition with ordered intermetallic phases and phase-segregated mixtures [2]. Though the average crystal structure of a HEA is generally simple, microstructure might be highly complex, as will be shown later in this presentation.

In our research, the HEA series CoCrFeNiZrx (x = 0.40, 0.45, and 0.50) has been investigated. Various SEM techniques were employed, including backscattered-electron (BSE) imaging, EDS point analysis, chemical mapping and EBSD.

The results reveal high complexity of the HEAs’ microstructure and helped us to determine the grains’ composition. Brief outline is presented in the four figures shown below. Figs. 1 and 2 show BSE SEM images. The coarsest interpretation is that the samples are composed of two different types of microstructures: (1) rounded dark phases and (2) fine interweaving of the light and dark phases. Higher magnification of the image in Fig. 2 shows the “invisible borders”, where the fine microstructure changes its character. EBSD investigations at lower magnification (2.000x) revealed that the “invisible borders” correspond to grain boundaries of larger grains (see Fig. 3 – showing different area than Figs. 1 and 2.), whereas more detailed EBSD results at higher magnification of 10.000x (Fig. 4) explained the fine grain structure inside the main grains. Therefore, grains’ sizes at two different scales are present in these samples. The effect of Zr content on the HEA microstructure was thus traceable in great detail.

 

[1] Y. Zhang, et al., Prog. Mater. Sci. 61, 1-93 (2014).

[2] P. Koželj, S. Vrtnik, A. Jelen, S. Jazbec, Z. Jagličić, S. Maiti, M. Feuerbacher, W. Steurer, and J. Dolinšek, Phys. Rev. Lett. 113, 107001 (2014).


Andreja JELEN (Ljubljana, Slovenia), Hwanuk GUIM, Janez DOLINŠEK
08:00 - 18:15 #5956 - MS04-746 Single crystals of V-amylose complexed with fatty acids and ibuprofen.
MS04-746 Single crystals of V-amylose complexed with fatty acids and ibuprofen.

Amylose, a mostly linear homopolymer of α(1,4)-D-glucose extracted from native starch, has the ability to form crystalline inclusion compounds with a large variety of small molecules. In these crystallosolvates, the so-called "V-amylose" occurs in the form of single helices and the ligands can be located inside the helices, in-between or both [Lourdin et al., in "Starch - Metabolism and Structure", Springer Japan, 2015, p. 61]. In this study, we have crystallized fractions of native and in vitro-biosynthesized amylose in the presence of various fatty acids and ibuprofen, a widely used anti-inflammatory drug [Yang et al., Starch-Stärke 65 (2013), 593]. The morphology, structure and stoichiometry of the resulting lamellar single crystals were characterized using TEM imaging, electron and X-ray diffraction, and solid-state NMR.

Dilute aqueous solutions of amylose (0.1 wt%) and fatty acids were briefly heated at 160°C, then kept at 90 or 100°C for 24 h. Crystallization occurred during the slow cooling to room temperature. Depending on the type of fatty acid and crystallization conditions, hexagonal or rectangular crystals, or a combination of both, were observed. Figure 1 shows examples of such lamellar crystals prepared from native amylose in the presence of stearic acid (C18). The hexagonal platelets crystallized by cooling the solution maintained at 90°C (Figure 1a) correspond to the so-called "VH" structure, i.e., the compact hexagonal packing of 6-fold amylose single helices (Figure 1b). The aliphatic part of the fatty acid would be included inside the helices while the polar head would remain outside [Godet et al., Carbohydr. Polym. 21 (1993), 91]. When the amylose/fatty acid mixture was kept at 100°C, the lamellar crystals formed upon cooling were rectangular and generally occurred as twinned assemblies (Figure 1c). The corresponding diffraction pattern suggested an orthorhombic unit cell (Figure 1d). After drying, the rectangular crystals yielded the diffraction pattern of the VH allomorph and cracks appeared parallel to the long axis, suggesting that a loss of guest and/or water molecules by evaporation promoted an anisotropic reorganization of the helices. This effect was well documented for crystals of V-amylose complexed with n-butanol [Helbert et al., Int. J. Biol. Macromol. 16 (1994), 207]. The formation of these two types of morphology and structure could be reproduced with fatty acids with different chain lengths (C10-C18).

A solution of amylose was mixed with preheated ibuprofen at 90°C and Vibuprofen crystals formed upon slow cooling. The complex grew as flower-like aggregates of long rectangular platelets (Figure 2a). The base-plane electron diffraction pattern (Figure 2b) is identical to that recorded from complexes of V-amylose with isopropanol [Nishiyama et al., Macromolecules 43 (2010), 8628]. The unit cell is thus orthorhombic and would contain 7-fold amylose single helices. Ibuprofen would be located inside and between the helices, along with a number of water molecules. This tentative model should provide a better understanding of the interaction of ibuprofen with a starch matrix as well as its controlled release.

Acknowledgement: We thank LabEx Arcane for financial support, the NanoBio-ICMG Platform (Grenoble) for granting access to the Electron Microscopy and NMR facilities, as well as W. Helbert and H. Chanzy for stimulating discussions.


Cong Anh Khanh LE (Grenoble), Jean-Luc PUTAUX, Yu OGAWA, Shivalika TANWAR, Florent GRIMAUD, Gabrielle VERONESE, Luc CHOISNARD
08:00 - 18:15 #5990 - MS04-748 Advanced TEM study of homogeneous flow and size dependent mechanical behaviour in highly ductile Zr65Ni35 metallic glass films.
MS04-748 Advanced TEM study of homogeneous flow and size dependent mechanical behaviour in highly ductile Zr65Ni35 metallic glass films.

Metallic glasses (MGs) exhibit outstanding mechanical and functional properties for numerous applications. However, although intensive research on the deformation and fracture mechanisms has been performed on metallic glasses for more than 20 years, the fundamental mechanisms governing the mechanical behaviour as well as the recently observed mechanical size effects in this class of materials are still not fully understood. Recently, amorphous Zr65Ni35 (%at) freestanding thin film MGs (TFMGs) deposited by magnetron sputtering have been deformed using a ‘’lab-on-chip’’ technique based on MEMS technology [1]. The results have shown that the ductility of the films is highly enhanced compared to bulk MGs and other TFMGs in the literature, and the plastic deformation occurs homogenously, i.e., without the observation of mature shear bands until fracture. In order to unravel the origin of these remarkable mechanical properties, the films have been investigated in-depth using advanced transmission electron microscopy (TEM).

Quantitative nanobeam electron diffraction (NBED) was used to investigate the relationship between the local atomic order and the activation of ‘’shear transformation zones’’ (STZs) [2]. The basic principle of NBED is shown in Figure 1a, consisting of a coherent electron beam with diameter of around 0.4 nm in order to produce two-dimensional diffraction patterns from atomic clusters with comparable size. Figures 1b, 1c and 1d exhibit NBED patterns with strong Bragg reflections which is the signature of a locally ordered region (i.e., atomic clusters) while in the NBED pattern of Figure 1e, only a diffuse background with speckles without Bragg reflections can be observed. High resolution HAADF-HSTEM and EELS revealed a heterogeneous microstructure with Ni-rich and Zr-rich regions exhibiting different atomic densities with characteristic length of 2-3 nm (Figure 2). Such behaviour can be attributed to the sputter deposition process involving very high cooling rates compared to bulk MGs. The results raise several fundamental questions that will be addressed: Does the nucleation of the STZs occur preferentially in regions with specific enriched chemical composition and atomic density? How will this affect the interaction between the STZs? How such features can be used to explain the exceptional high plastic deformation levels, the absence of shear bands and the delay of fracture in the Zr65Ni35 TFMGs used in the present work?     

References

[1] H. Idrissi, B. Wang, M.S. Colla, J.P. Raskin, D. Schryvers, T. Pardoen. Advanced Materials. 23 (2011) 2119

[2] A.S. Argon, Acta Metallurgica. 27 (1979) 47.


Hosni IDRISSI (Antwerpen, Belgium), Matteo GHIDELLI, Sébastien GRAVIER, Jean-Jacques BLANDIN, Jean-Pierre RASKIN, Thomas PARDOEN, Dominique SCHRYVERS
08:00 - 18:15 #6020 - MS04-750 Characterization of nanometric-sized participates formed during heat treatment of aluminium alloy with antimony.
MS04-750 Characterization of nanometric-sized participates formed during heat treatment of aluminium alloy with antimony.

New requirements to be met by modern alloys requires changing the currently used materials. This is accomplished by the use of newer and newer generation of  production technologies, or by modifying the chemical composition of the alloys presently used. During our research we examined the casting aluminum alloy with the addition of antimony. The chemical composition of the investigated alloys was Mg 5.3-5.8%, Si 1.7-2.8%, Mn 0.5-1.0%, Cr 0.2-0.6%, antimony content ranged from 0.1 to 0.3%, 0.3-0.5% 0.5-0.9% 0.9-1.2%. Aluminium and antimony form an intermetallic phase AlSb, which serve as the nuclei for crystallization growth of other precipitates, with complex structure and chemical composition. The size of participates that grown on the surface of AlSb is about 10-15 nm in diameter. Their large concentration and equal, homogenous distribution give the chance to obtain high mechanical properties of investigated material. The aim of the article is characterization of participates formed during age hardening of aluminium alloy with the addition of antimony, as the intermetallic phase component. Structural characterisation of precipitates are made by using simultaneous HR-TEM and HR-STEM imaging and EDS analysis. Obtained results were compared with computer simulations.


Krzysztof MATUS (Zbrosławice, Poland), Klaudiusz GOŁOMBEK, Mirosława PAWLYTA
08:00 - 18:15 #6109 - MS04-752 ACOM-TEM analysis of the effect of heating on the mineral nanocrystals in bone.
MS04-752 ACOM-TEM analysis of the effect of heating on the mineral nanocrystals in bone.

The evolution of the crystalline structure of bone mineral nanoparticles upon heating is a topic of interest in archeology, paleo-anthropology and forensic science. Archaeological bone remains contain a considerable amount of information which can be altered by heating (e.g. radiocarbon dating). Traces of heating are often observed on bone fragments, but it becomes quite difficult to distinguish its’ effect, particularly at temperatures below 600°C. Below this temperature, macroscopic structural parameters become inadequate to characterize the effect of heating and the nanoscale composite structure needs to be considered [1]. At the nanoscale bone consist of two principal components: collagen fibrils of ~ 100 nm in diameter and platelet-shaped calcium phosphate mineral crystals of about 5 x 50 x 100 nm3 dimensions.

The closest crystallographic structure that describes the bone mineral phase is hydroxyapatite, as determined by x-ray diffraction. However, significant differences in crystal structure with respect to the ideal hydroxyapatite are generally observed. Those have been shown to be related to modifications in crystalline chemistry by spectroscopy, as well as nanocrystal size and strain by X-ray scattering.

Therefore, the main difficulty stands in the necessity of analyzing both the nanocrystals morphology and organization as well as crystal structure which, generally, requires using different methods. Electron microscopy is the most widely used technique to visualize the nanocrystals in real space, while X-ray diffraction is generally used to analyze the crystal structure in reciprocal space, thus giving information on the crystal-chemistry disorder. However, the ideal method should provide both insights. Automated Crystal Orientation Mapping at TEM (ACOM-TEM, also known as ASTARTM tool from NanoMEGAS) [2] allows such complex study.

The ACOM-TEM method operates in scanning mode and relies on the comparison between the electron diffraction patterns collected at every scan position and the simulated patterns calculated for a given crystal structure in all possible orientations. Maps of the structural parameters can therefore be reconstructed which allow analyzing the crystal size and shape distribution in real space as well as the crystalline orientation. Different model crystal structures can also be tested to access the crystal chemistry fluctuations.

Bovine bone samples in control state as well as heated in vacuum to 9 temperatures from 100°C to 1000°C for 10 min were studied. An increase in nanoparticle size occurs upon heating (fig. 1) as also observed by X-ray scattering [3]. In order to test the sensitivity of the method, we found that hydroxyapatite describes the structure well in comparison with other apatite minerals with elemental composition compatible with bone biochemistry (brushite, monetite, tuite). The structure is better described by hexagonal space group P63/m in all the temperature range, contrary to results pointing to a monoclinic to hexagonal phase transition. Furthermore, the type of carbonate substitutions which are known to occur in bone (up to 7%w) was investigated and could be discriminated within the limits of the method sensitivity. Additionally the probability of various ionic substitutions (F-, Na+, Sr2+ and Cl-) in bone tissue is discussed.

Improvement of reciprocal space resolution and accessible q-range could allow studying even more subtle crystal-chemistry disorder in such complex materials like bone tissue. Current results are part of larger project aiming to understand the nanostructural characteristics of bone tissue and to identify key structural markers of pathological human bone [4], providing possible development of new diagnostic and pharmaceutical tools.

References:

1. Chadefaux C., Reiche I. Journ of nano research 8, 157-172 (2009).

2. Portillo J. et al. Mater Sci Forum 644, 1-7 (2010).

3. Gourrier A. et al. Archeo Sciences 35, 191-199 (2011).

4. Gourrier A. et al. J Appl Crystallogr 43, 1385-1392 (2010).


Mariana VEREZHAK (Grenoble), Edgar F. RAUCH, Muriel VERON, Pierre BORDET, Marie PLAZANET, Aurélien GOURRIER
08:00 - 18:15 #6119 - MS04-754 3-D areal functional parameters extracted from AFM data of polished dental tooth-restorative nanocomposites.
MS04-754 3-D areal functional parameters extracted from AFM data of polished dental tooth-restorative nanocomposites.

The analysis of contemporary areal functional surface parameters of functional parts in any system of elements, that are in contact with each other, is essential for the understanding of their frictional behaviour. Oral environment is a specific tribological system where a tooth or a tooth-restoration is exposed to the cyclic mechanical, thermal, and pH changes. 3-D areal texture parameters of tooth replacing materials are of a great importance for prediction and understanding of the materials’ tribological properties during their functional use.

 

The aim of this study was to conduct a detailed AFM morphological analysis of contemporary dental nanocomposites and to calculate specific 3-D surface texture parameters, after four dental polishing protocols.

 

Two representative materials were chosen for testing: nanohybrid (Filtek Z550, 3M ESPE - FZ550) and nanofilled (Filtek Ultimate Body, 3M ESPE - FUB). The polymerized composite samples were polished by four different dental polishing procedures: multi-step (MSP), single-step (SSP), multi-step followed by diamond paste (MSP+D), and single-step followed by diamond paste (SSP+D). For the texture analysis the atomic force microscopy was used (Veeco di CP-II). Using the software packages: Mountains Map® 7 (Digital Surf, Besançon, France,  available from: http://www.digitalsurf.fr) and Gwyddion 2.28 (available from: http://gwyddion.net/), the depth histograms, volume surface parameters, peak count histograms, watershed segmentation algorithms, average power spectral density were calculated to describe the 3-D surface properties of tested specimens.

 

These parameters offered the possibility of a more profound understanding of the structure-related functional properties of the surfaces of tested materials. The MSP created a more favourable surface texture on both tested materials. It had narrower height distribution and volume surface parameters which indicated the lower wear probability of materials polished by MSP than the SSP. The number of motifs after the watershed segmentation was the highest on the samples polished by MSP, due to the presence of fine surface irregularities. SSP created lower number of motifs, by cutting the surface details and creating deep and wide grooves, plane aspects of dales or hills. MSP or MSP+D mostly created more isotropic surface texture, which indicates that the MSP can be considered as a reliable method for dental composites polishing in comparison with the SSP or SSP+D.

 

Supported by Project TR 035020 and Project III-45016 of the Ministry of Education, Science and Technological Development of Republic of Serbia; and 3M (East) AG company and Mikodental – Shofu, Japan for Serbia.


Tijana LAINOVIĆ (Novi Sad, Serbia), Ştefan ŢĂLU, Sebastian STACH, Marko VILOTIĆ, Larisa BLAŽIĆ
08:00 - 18:15 #6130 - MS04-756 A new high pressure form of Ba3NiSb2O9.
MS04-756 A new high pressure form of Ba3NiSb2O9.

Quantum spin liquids (QSL) are an interesting state of matter and have inspired a great number of investigations into materials showing triangular nets of magnetic ions. In this work we present evidence that one of the targeted phases has indeed a triangular order of Ni ions, but only on a range of about 10 nm.

The QSL state was proposed as the ground state for a system of spins on a triangular lattice that interact with their nearest neighbors via an antiferromagnetic interaction. Recently, candidates from the family of 6H-perovskites Ba3MSb2O9 (M= divalent transition metal) have retained attention as QSL materials. The case of Ba3NiSb2O9 seems particularly interesting since it could represent an experimental proof of a QSL with spin S = 1. Three different structure types were reported for this compound. All of them are built up by the stacking along the c axis of layers made of either NiO6 or SbO6 octahedra and differ by the way layers are stacked via either corner or face sharing of the octahedra.

We focus here on the 6H-B form of Ba3NiSb2O9 which can be obtained by applying a High Pressure - High Temperature (HP-HT) treatment in the range 3 to 6 GPa at 600°C. This compound was reported to present no magnetic order down to 0.35 K, suggesting that it could be an experimental realization of a gapless QSL. In order to investigate this behavior by both nuclear magnetic resonance (NMR) and muon spin resonance (µSR) we synthesized this phase in HP-HT conditions.

X-ray powder diffraction on a laboratory source (Bruker D8) and neutron powder diffraction on the D1B beamline of the ILL (Grenoble, France) have been performed. The refinement of the structure using these data showed disorder of Ni and Sb atoms on the sites of the triangular lattice. In that case, the absence of magnetic ordering could be simply related to the structural disorder of the magnetic cation framework, in which the triangular planes are not preserved. For a correct interpretation of physical properties of the samples, it is thus mandatory to clarify the nature of the disorder and confirm (or not) the existence of Ni triangular planes in the structure. Therefore, we have undertaken a thorough investigation of the structural properties of 6H-B Ba3NiSb2O9 using precession electron diffraction and Z-contrast imaging.

STEM-HAADF Z-contrast imaging (figure 1a) shows rows of bright contrasts that can be linked to the structural model of Ba3NiSb2O9 (figure 1b). Figures 1c and 1d show the intensity profiles in the green and pink boxed areas, respectively, with theoretical intensities based on the atomic numbers superimposed. As expected, the contrast corresponding to the Ba atoms (green) shows the highest intensities. The intensity in the center of the profiles corresponds well to a Sb atom (yellow). However, on the two remaining positions, where the 6H-B structure has either a Sb or a Ni atom (blue) the expected intensities differ from the observed ones. The observed intensities on the Ni/Sb sites can be interpreted as Sb and Ni being both present in the atomic columns.

Precession electron diffraction on a thin particle showed that the symmetry of the crystal isn’t compatible with the P63mc space group proposed for 6H-B. The [010] zone axis (figure 2a) shows no mirror symmetry perpendicular to the c* direction and the existence of the 003 reflection. This is incompatible with a 6-fold axis or a c glide plane in the crystal. The correct space group must therefore be trigonal.

Figure 2b shows a structure compatible with all our observations. Here the Ni and Sb atoms are on distinct sites, different from those reported for the 6H-A and 6H-B forms, thus breaking the mirror symmetry perpendicular to c and the c glide plane.

In conclusion, Ni-triangle planes can effectively be present in this structure but only in domains of up to 10 nm in size. At the larger scale seen by X-rays and neutrons, typically several 10s nm we observe an average of several domains and the average symmetry becomes hexagonal with a statistical Ni/Sb disorder on the cation sites of the face sharing octahedra.


Holger KLEIN (Grenoble), Céline DARIE, Christophe LEPOITTEVIN, Stéphanie KODJIKIAN, Pierre BORDET, Claire COLIN, Oleg LEBEDEV
08:00 - 18:15 #5053 - MS05-758 Advanced TEM of BiCu1-xOS oxysulfide: copper deficiency and electronic properties.
MS05-758 Advanced TEM of BiCu1-xOS oxysulfide: copper deficiency and electronic properties.

The BiCuOX (X=S, Se) oxychalcogenides have attracted much attention because of their properties which are performing thermoelectric material ( copper deficient BiCu1-xOSe phase) and along the BiCuOSe-BiCuOS solid solution, one of candidate for possible transparent conductor (BiCuOS) and also superconductivity for BiCu1-xOS structure.

An oxysulfide series of nominal compositions BiCu1-xOS with x<0.20 has been prepared and its structural properties characterized by combining XPRD and TEM techniques. It is found from XPRD that this oxysulfide is crystallized in the P4/nmm SG with a3.87Å, c8.6Å unit cell parameters.

In order to investigate the crystal structure and chemical nature of defect structure, such as dislocations and possible intergrowth at atomic level, which mightbe responsible for particular properties,  advanced TEM was carried out. The high resolution [100] HAADF-STEM images of BiCuOS perfect crystal are given in Fig. 1a.The careful inspection of the high resolution HAADF-STEM image reveals the presence of local variation in the intensity of the atomic columns in some of the weak brightness rows located in between two high brightness zig-zag rows (Fig1b) It is clear from the nature of HAADF-STEM contrast that the bright zig-zag dots correspond to Bi atomic columns whereas the less bright dots correspond to the Cu ones. The corresponding intensity plot profiles made along the Cu atom rows shows clear drop of the peak intensity with respect to the adjacent peaks. This is observed every two atomic columns (black arrow heads). The peaks of lower intensity definitely correspond to lower amount of Cu atoms in the column. Accordingly, the presence of Cu vacancies should also create re-arrangements of the S atoms in the structure.  The larger sensitivity to the light elements of ABF-STEM images evidences it (Fig. 1c, insert in bottom panel). In the latter, some of the S atoms are slightly displaced from original position. They also exhibit less contrast with respect to the other S atomic columns suggesting S-vacancies. Therefore, the presence of Cu vacancies was confirmed by HAADF-STEM studies and shows that BiCu1-xOS tends to adopt a constant amount of copper vacancy corresponding to x=0.05 and is general to all the BiCu1-xOS (x≤0.15) samples.

 Moreover, for larger Cu deficiencies (x>0.05 in the nominal composition), other types of structural nanodefects are evidenced from HAADF-STEM imaging such as oxysulfides of the “BiOS” ternary system which might explain the report of superconductivity for the BiCu1-xOS oxysulfide (Fig.2) Intergrowths between different structures can be evidenced at the nanoscale level, which are related to deficient Cu regions. Interestingly, the layers stacking in the “I” region shows a continuous phase transformation from tetragonal BiCuOS structure to the orthorhombic copper free BiOS oxysulfide and, finally, to the cubic Bi2O3 oxide (see structural mode in the left bottom panel). The structures are epitaxial intergrowth, showing that, by adding sulfur to the Bi2O3 oxide, and then Cu, the material can evolve from the oxide to BiCuOS oxysulfide in only few unit cells. The region noted II shows a nanoparticle (~15nm) without copper which can be identified as the Bi2O2S structure surrounded on both its left and right sides by Bi2O3 layers.

This is in contrast with the TEM investigation of  “nominal” BiCuOSe oxyselenide. TEM results evidence that layer stacking in the structure is very regular. Neither intergrowth nor dislocation can be observed. More importantly, neither Cu nor other vacancies, including oxygen or sulfur  were found in the case of BiCuOSe .

The lack of copper explains why Bi2O3 can play the role of buffer layer to epitaxially adapt BiCuOS to Bi2O2S. The presence of defects with Bi2O2S composition, very close to that of the Bi3O2S3 superconductor with TC=4.5K, also explains why superconductivity could have been observed in Cu deficient BiCuOS oxysulfide reported previously.

Finally, our study shows that the BiCuOS structure tends to adapt a limited amount of Cu deficiency corresponding to “BiCu0.95” through the partial occupancy of the Cu sites in the (ab) plane. For nominal contents of copper smaller than 0.95, other kind of defects are created implying nano-epitaxy between the “BiCuOS” phase and both “Bi2O2S” and “Bi2O3” phases. The origin of the BiCuOS lack of structural flexibility for Cu amounts deviating from ~5% could be related to a very rigid skeleton imposed by the epitaxial relation between the two types of structural layers in contrast to the isostructural selenides. Consequently, this limitation of doping via Cu vacancy in BiCuOS calls for other types of chemical control to inject carriers in the conduction [Cu2S2]2- layers.


Oleg LEBEDEV (Caen), David BERTHEBAUD, Emmanuel GUILMEAU, Antoine MAIGNAN
08:00 - 18:15 #5347 - MS05-760 Nanoscale characterization of Co and Co-B catalytic coatings before and after catalytic tests for the sodium borohydride hydrolysis.
MS05-760 Nanoscale characterization of Co and Co-B catalytic coatings before and after catalytic tests for the sodium borohydride hydrolysis.

The use of hydrogen as a potential future energy carrier is limited due to the problems of its storage. Hydrolysis of hydrogen storage materials such as sodium borohydride (NaBH4, SB) has been one of the most investigated approaches for hydrogen generation. SB is stable in dry air and combines lightweight with high hydrogen content (10.8 wt%).

NaBH4 + 2H2O   →   4H2 + NaBO2  (1)

Although spontaneous, the SB hydrolysis (reaction 1) needs catalysts to occur at appreciable rates. Co has demonstrated to be a good choice because its compromise between activity and cost.  However, its major drawback is related to stability: these materials deactivate upon cycling. Despite the great number of works reporting Co and Co-B based catalysts, the nature of the active phase and deactivation mechanisms are still under intense discussion. 

We have recently reported the preparation of supported Co metallic catalysts as thin films by magnetron sputtering for sodium borohydride hydrolysis [1]. Magnetron sputtering is a very versatile technique used in this work to fabricate Co and CoB catalytic coatings under different deposition conditions and supported on different substrates (i.e. silicon and polymeric membranes). In this work we have been able to study by electron microscopy the catalytic coatings as grown on the wires of the polymeric membranes. The structural and compositional characterization by SEM and (S)TEM techniques has been performed before and after the catalytic tests (19 wt% SB in NaOH 4 wt%, 90 min reaction time).  Fig. 1 and 2 show the SEM morphology of a Co thin film on the polymeric membrane before and after the catalytic test, respectively, showing the growth of a new layer onto the catalysts upon operation. Further nano-analysis of the structure and compositional distribution have been performed by (S)TEM techniques coupled to EELS. They also reveal the formation of fiber/nanoflake-like nanostructures onto the catalytic coatings (Fig. 3 and Fig. 4). Compositional analysis have pointed out the formation of Co-borates and most likely cobalt oxide/hydroxide nanoflakes (i.e. CoO(OH)) which could be the origin of the leaching and deactivation mechanisms of the Co-based catalysts for the investigated reaction.

 

References

[1] Paladini, M. et al. Applied Catalysis B-Environmental 2014, 158, 400-409.

 

Acknowledgements

This work was supported by the Spanish MINECO (project CTQ2012-32519 and CTQ2015-65918), CONSOLIDER FUNCOAT+ (MAT2015-69035-REDC), Junta de Andalucía (PE2012-TEP862) and CSIC (PIE201460E018). The authors also acknowledge the Laboratory for Nanoscopies and Spectroscopies (LANE) at the ICMS for the TEM facilities and I. Rosa for the TEM samples preparation. AMB thanks to Talent-Hub Program funded by the Junta de Andalucía and the European Commission under the Co-funding of the 7thFramework Program in the People Program (Marie Curie Special Action).


Ana M BELTRÁN (Sevilla, Spain), M PALADINI, V GODINHO, G.m. ARZAC, M.c. JIMENEZ DE HARO, A FERNÁNDEZ
08:00 - 18:15 #5445 - MS05-762 Characterizing blends of Zn/Pc-C60 for Organic Photovoltaic Cells using energy-filtered Backscattered Electron (BSE) imaging in combination with Low Voltage Scanning Electron Microscopy (LVSEM).
MS05-762 Characterizing blends of Zn/Pc-C60 for Organic Photovoltaic Cells using energy-filtered Backscattered Electron (BSE) imaging in combination with Low Voltage Scanning Electron Microscopy (LVSEM).

Blends of conjugated polymer zinc-phtalocyanine (ZnPc) and electron acceptor fullerene (C60) are used as an active layer of organic photovoltaic (OPV) cells in bulk heterojunction architectures. In this study, the morphology of co-evaporated small molecule blends of ZnPc (ZnC32H18N8) and C60 is investigated with Low Voltage Scanning Electron Microscopy (LVSEM). Compared to SEM studies carried out at standard conditions, these LVSEM studies present the advantages of a lower modification of the polymer structures during observation and of a better material contrast between the components which consist mainly on carbon. Energy-filtered SEM imaging in combination with low primary beam energies (Ep) allows to detect the low loss backscattered electrons (LLBSE). These LLBSEs undergo a small number of inelastic scattering events, and therefore, they lose only a small amount of energy. The material contrast between the blend components is detectable and enhanced for low Ep. These fine contrast differences obtained from the LLBSE are originated in the very shallow regions of the blend surface, and not from the volume where multiple inelastic scattered electrons, like the secondary electrons (SEs), are produced [1]. The blend morphology is imaged using a novel energy selective backscattered (EsB) electron detector whose grid voltage is set to high values in order to cut off the lower energy SEs, which are responsible for the topography information in the SEM images. This energy-filtering technique allows to detect uniquely the LLBSEs, which mainly carry material contrast information. High atomic number (Z) elements have a higher backscatter electron coefficient for Ep ≥ 1 kV [2]. Therefore, higher Z elements appear brighter due to a higher BSE emission. The material contrast is optimized and quantified as a function of the Ep, the EsB grid voltage and the working distance (WD) at low landing energies. Figure 1 illustrates how topography information dominates the images for EsB grid voltages below 300 V, due to influence of SEs. The material contrast is obtained for EsB grid voltages ≥ 500 V. In Figure 2, the in-lens SE image a) also shows the blend topography: long rods interlaced with each other and in between, cube-shaped particles. For the identical sample region, the image detected with the EsB detector b) shows the material contrast between rods and particles. The bright rods are identified as the ZnPc phase, while the darker cube-shaped component is assumed to correspond to C60. Depending on the SEM working conditions, the systematic study of the material contrast reveals that small differences in composition can be detected using the LLBSEs. Summarizing, it is possible to image the material contrast difference even for challenging materials like the ZnPc-C60 active layer in OPV cells by optimizing Ep, EsB grid voltage and WD values.

 

Acknowledgements: The authors kindly thank Carl Zeiss Microscopy GmbH, Oberkochen, Germany, for their collaboration. This study was supported in part by the German Science Council Center of Advancing Electronics Dresden, EXC1056, CFAED.

References

[1] H. Jaksch, European Microbeam Analysis Society (EMAS), France, 2011, 255-269.

[2] A. M. D. Assa’d, M. M. El Gomati, Scan. Micro. Mat., 1998, 12, 185-192.

[3] D. C. Bell, N. Erdman, Low Voltage Electron Microscopy: Principles and Applications, John Wiley & Sons, United Kingdom 2013.

[4] A. Garitagoitia Cid, R. Rosenkranz, E. Zschech, Adv. Eng. Mat., 2015, 10.1002/adem.201500161.

 


Aránzazu GARITAGOITIA CID (Dresden, Germany), Mona SEDIGHI, Markus LOEFFLER, Willem F. VAN DORP, Ehrenfried ZSCHECH
08:00 - 18:15 #5465 - MS05-764 Growth of novel Pt 3D networks on tungsten suboxide electrodes and their effect on the performance of fuel cells.
MS05-764 Growth of novel Pt 3D networks on tungsten suboxide electrodes and their effect on the performance of fuel cells.

Green energy gains in importance in the spotlight of current research activities. Solar, wind and water are promising energy sources and they are already in use in different ways. Their practical disadvantage – their dependence on daytime or seasonal conditions – requires an intermediate storage of the produced energy. A convenient carrier could be hydrogen (H2). Fuel cells (FCs) can then be used to convert the chemically bound energy in H2 into electrical power as well as heat. One of the most simple redox reactions, the oxidation of H2 and reduction of oxygen (O2) followed by the formation of water (H2O), is the basis of this promising route of energy conversion and electricity production. In our research work we develop and investigate novel electrode materials for improved and long-term stable FC performance. Tungsten suboxide (WO3-x) is chosen as support material due to its ability to minimize poisoning of the platinum (Pt) catalyst. For most efficient hydrogen spill over and electron transport, the interface between the catalyst and the support material is of great interest. In addition, the distribution and shape of the catalyst on its support are important. To address these questions we used various electron microscopy techniques. Via scanning electron microscopy (SEM) we obtained overview images of the Pt catalyst particles and their distribution on the WO3-x support. Detailed analysis regarding growth and architecture of the Pt catalyst was performed with the help of a transmission electron microscope (TEM) equipped with a monochromator and Cs-corrector for the condensor system (Titan Themis 60 – 300 kV). The preparation of electron transparent samples was performed by site specific focused ion beam (FIB) sectioning.

Pt is deposited on the WO3-x support by a wet chemical approach and the different growth stages were investigated after varied deposition times. SEM images revealed octahedral and truncated octahedral shaped 3D Pt catalyst morphologies on the WO3-x support with lateral dimensions of 2 – 4 µm. Depending on the deposition time of the Pt precursor (4 min, 20 min, 60 min) their appearance differs (Fig. 1). TEM and STEM images revealed highly porous 3D networks formed by Pt nanorods with a width of a few nanometers (Fig. 2a). These networks develop as a result of the reduction of the catalyst precursor in H2 atmosphere. High resolution TEM (HRTEM), HRSTEM and electron diffraction experiments revealed that the Pt rods have different orientations. The nanorods are connected to each other as visualized by TEM tomography (Fig. 2b). The formation of the Pt catalyst 3D network takes place in a 2-step procedure: firstly octahedral or truncated octahedral shaped compact crystals of the precursor are formed on the WO3-x support. These crystals are then reduced to metallic Pt with side products evaporating in the second step in the presence of H2. The corresponding redox reaction starts at the outside of the bulk crystal and penetrates to its inside while reducing the precursor step by step. This is shown in Fig. 3a where the outer area of the displayed 3D catalyst is formed by a network of Pt nanorods while its interior is still compact. Energy dispersive X-Ray (EDX) analysis and STEM reveal that small Pt particles are embedded in a reduced form of the amorphous precursor matrix (Fig. 3b). The long-term stability and performance continuity of the Pt/WO3-xsystem as an anode of high-temperature polymer-electrolyte-membrane FCs was demonstrated in accelerated and continuous FC operation times of up to 2000 h. Via SEM and different TEM based techniques the improved degradation behavior compared to standard carbon support material was shown.1

   

  

 (1)          Heinzl, C.; Hengge, K. A.; Perchthaler, M.; Hacker, V.; Scheu, C. Journal of The Electrochemical Society 2015, 162, F280.


Katharina HENGGE (Duesseldorf, Germany), Christoph HEINZL, Markus PERCHTHALER, Marina WELSCH, Christina SCHEU
08:00 - 18:15 #5715 - MS05-766 HRSTEM and EELS study of NMC aging mechanisms for renewable energy application.
MS05-766 HRSTEM and EELS study of NMC aging mechanisms for renewable energy application.

With the constant development of renewable energy, the associated energy storage devices need to improve their lifetime (15-20 years), security and be of low cost. Among the possible materials for such applications, Li[Ni1-x-yMnxCoy]O2 (NMC) is a good candidate due to its lower cost and better thermal stability than NCA for example. Obtaining a better understanding of the aging mechanisms of NMC is then critical to create reliable prototypes. In this aim, NMC materials have been investigated after drastic cycling.

 

To investigate the microstructural and chemical evolutions of the NMC before and after cycling, we performed combined SAED, HRSTEM-HAADF, EDS and EELS study using a TECNAI F20-STWIN and a JEOL ARM 200 CF Cs-corrected both equipped with an EDX and EELS. By compiling the different information, we were able to highlight several modifications:

  • a preferential dissolution1,2 of first manganese then nickel with the cycling,
  • A preferential dissolution of the material along the lithium diffusion path resulting in the creation of cracks,
  • Microstructural evolution from a layered structure to a spinel3 and a rock salt one (see figure 1),
  • Changes in the oxidation state of the manganese and cobalt (see figure 2).

These evolutions show a progressive degradation of the NMC during cycling until the apparition of phase (rock salt type) with limited cycling performance. Knowing this extreme behavior will help to adapt new generation (electrode formulation, …) of prototype cells.

Acknowledgement:

This work was funded by ANR (Agence Nationale de la Recherche) and was part of the VISION project.

 

Références

1.        Vetter, J. et al., J. Power Sources 147, 269–281 (2005).

2.        Pieczonka, N. P. W. et al., J. Phys. Chem. C 117, 15947–15957 (2013).

3.        Boulineau, A. et al., Nano Lett. 13, 3857–3863 (2013).


Carine DAVOISNE (Amiens), Mohamed BEN HASSINE, Cécile TESSIER, Goran DRAZIC, Loic DUPONT
08:00 - 18:15 #5720 - MS05-768 On the morphology of Li2S deposits in Li-S batteries.
MS05-768 On the morphology of Li2S deposits in Li-S batteries.

Lithium-Sulfur batteries are thought to be the future of Li-ion batteries thanks to their high theoretical specific capacity (1675 mAh.g-1) and low-cost. However, the real-life application of Lithium-Sulfur batteries is still hindered by limitations such as the formation of highly insulating and insoluble species at the cathode (e.g. Li2S), mechanical instability of the electrodes and capacity fading due to polysulfides shuttle effect [1]. It is crucial for the development of Lithium-Sulfur batteries to gain a better understanding on the chemical and microstructural changes which take place at the electrodes upon electrochemical cycling.

We will present the results of our SEM and (S)TEM investigation on the microstructural, crystallographic and chemical properties of the species formed at the cathode during galvanostatic cycling at different discharge rates. We have cycled a Carbon/Sulfur composite material using a Swagelok type cell with Li metal anode with 1M LiTFSI-TEGDME/DIOX electrolyte. All materials were handled in O2-free conditions.

The studied cathode is formed by carbon nanoparticles having 30 to 50 nm diameter and two types of morphology; regular solid and irregular core-shell-like. HR TEM imaging associated with spectroscopy techniques show that sulfur forms a uniform, amorphous coating on the carbon particles.

After full discharge at slow rate (C/20), a porous desert-rose-like deposit was observed, whereas at faster rate (1C) a particulate conglomerate discharge product was locally concentrated on the C/S composite (Fig. 1); we infer these discharge products to be Li2S deposits.

At the nanometric scale, both slow and fast cycling result in the deposition of a 1 to 2 nm thick Solid Electrolyte Interphase (SEI) onto the carbon particles. Moreover, slow cycling promotes the formation of cubic (Fm3m) Li2S nanocrystals up to 30 nm in size (Fig.2).

These observations suggest that fast and slow discharge rates promote the formation of amorphous and crystalline Li2S, respectively.

 

Acknowledgments

ALISTORE-ERI, HELiS and RS2E consortiums are gratefully acknowledged for their financial support.

 

References

[1]       Bruce et al., Nat Mater 11 (2012) 19.


Mattia GIANNINI, Afef MASTOURI, Arnaud DEMORTIÈRE, Goran DRAŽIĆ, Carine DAVOISNE (Amiens)
08:00 - 18:15 #5741 - MS05-770 HRTEM and STEM-EELS study of thin films Pt-CeOx nanocatalysts for on-chip fuel cell technology.
MS05-770 HRTEM and STEM-EELS study of thin films Pt-CeOx nanocatalysts for on-chip fuel cell technology.

Platinum (Pt) is a versatile element in catalysis that efficiently mediates a multitude of chemical reactions. Unfortunately, Pt is a rare noble metal and its high price exceeding that of gold, limits large-scale applications. Therefore, not surprisingly, reducing amount of Pt is the major driving force in catalysis research. There are two strategies to tackle this challenge: to replace noble metal by others, less expensive materials; and to use platinum as efficiently as possible. In this study, we handle both of them by growing extremely porous Pt-CeO2 structures prepared It was shown that sputtered thin cerium oxide films containing Pt, which had been deposited on the anode side of a fuel cell, exhibited a higher specific power compared to a conventional Pt−Ru catalyst [1]. Besides the large scale fuel cells, there is also an increasing interest in miniature fuel cells fabricated on silicon, which could be used as an on-chip power supply for portable electronic devices.

In this study, nanometric Pt-ceria thin films were characterized by TEM after elaboration by physical vapor deposition on various substrates (silicon, carbon foils, intermediate carbon films). The deposited layers exhibited different morphologies linked to the different substrates. It is shown that the roughness of the layers is dependent on the deposition conditions, the amount of deposited material and the type of the carbon substrate. The change of these parameters results in growth of flat, mushroom-like or noodle-like structures (Fig. 1). By the optimization and the suitable combination of materials we can tune the morphology of the catalysts. In addition to the substrate type, many effects as the formation of carbides or silicates at the interface, an interaction of ceria with platinum and the presence of the porosity influenced also the structure and the chemistry of the deposited layers [2,3].

In all samples, crystallites corresponding mainly to CeO2 and to a less extent to CeC2 crystallographic structures were observed (Fig. 1). STEM-EELS measurements have been carried out on layers grown on silicon with and without intermediate carbon layer. Data analysis of the M4,5 white lines of cerium have pointed out a variation of cerium oxidation state from Ce4+ to a mixture of Ce3+ and Ce4+ depending of the localization of the measurement (Fig. 2).

[1] V. Matolin et. al, Langmuir 26, 12824-12831 (2010)
[2] J. Lavkova et. al, Nanoscale 7, 4038-4047 (2015)
[3] M. Dubau et. al, ACS Appl. Mater. Interfaces 6, 1213-1218 (2014)

 Acknowledgement: This research is supported by EU within FP-7-NMP-2012 project chipCAT under Contract No. 310191.The authors acknowledge the support by the Czech Science Foundation under grant No.13-10396S and J.L. is grateful to the Conseil Regional de Bourgogne (PARI ONOV 2012).


Valérie POTIN (DIJON CEDEX), Jaroslava LAVKOVA, Martin DUBAU, Iva MATOLINOVA, Vladimir MATOLIN
08:00 - 18:15 #5758 - MS05-772 High temperature and in situ study of SrO surface precipitation on perovskite ceramics.
MS05-772 High temperature and in situ study of SrO surface precipitation on perovskite ceramics.

Solid oxide fuel cells (SOFC) convert gaseous fuels, e.g. H2, into electricity through an electrochemical process. Their conversion efficiencies are not limited by the Carnot cycle and pollution levels in the exhaust gas are significantly lower than that of traditional technologies. SOFC cathode materials require a very precise balance of material properties in order to function at operating temperatures (~600 – 800°C). A number of systems fulfil the requirements, but there are numerous challenges these materials face during manufacture and operation. Of particularly concern is the negative impact secondary phase formation at the surface has on the reduction of oxygen. In a large number of perovskite systems used for SOFC cathodes the A-site is occupied by lanthanum (La3+) and it is often doped with strontium (Sr2+) to introduce oxygen vacancies, which generates ionic conductivity, and electronic species leading to mixed conductivity, essential for operation as aN SOFC cathode. The crystal chemistry of these perovskite structures can be described as an alternated stacking of SrO and LaO2 layers.  It is believed that the charge difference between the lanthanum and strontium changes the state of the B transition metals (i.e. B2+/3+) to preserve charge neutrality and in turn creates dipole moments SrO and LaO2 layers. This creates an alternating electric field throughout the material, resulting in a large surface charge, which the system attempts to reduce by depleting the surface of lanthanum and segregating strontium [1].

Continuous surface precipitation was observed on polished La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) ceramics using high temperature environmental scanning electron microscopy (HT-ESEM) up to 1000°C. Several experiments were performed under different atmospheres: vacuum, 300 Pa O2, H2O and air. A characteristic image series recorded under water vapor is reported on Figure 1. The surface precipitation phenomenon can be clearly observed. The composition of the precipitates determined from the X-ray maps recorded on samples cooled at room temperature is SrO (Fig. 2). The evolution of the surface occurs in three distinct stages: (1) rapid growth of precipitates on grain boundaries, at defect points as well as at the centre of the grains, (2) agglomeration of the precipitates due to surface transport phenomena and (3) Continuous coarsening during the heat treatment. It is clear that at the beginning the precipitation process, the SrO precipitates appear to grow with crystallographic direction as, within each grain, they tend to orientate themselves in a similar direction. It is also clear that the initiation of the SrO precipitation and the density of the precipitates depend on the LSCF grain orientations. When comparing the EBSD maps with the HT-ESEM images (Figure 3a), it is obvious that the precipitation process and grain growth are directly linked with the grain orientations and presence of twinning inside the grains (Fig. 3bcd). Twin planes exist in many grains. Average twin plane width measured across a particular grain (Fig. 3b) is very close to the average particle width (measured centre to centre), 0.6 and 0.5µm respectively. This result suggests that twin planes in this orientation are fast diffusion pathways. Other grains display homogenous precipitate growth across the grain surface and it is predicted that knowledge of the twin habit plane can be used to describe why these grain orientations display different behavior.

From these new data sets, a very precise and original description of the surface precipitation has been proposed.

[1] W. A. Harrison, Phys. Rev. B, 83, 155437 (2011)


Mathew NIANIA, Renaud PODOR (ICSM, Marcoule), Ben BRITTON, Stephen SKINNER, John KILNER
08:00 - 18:15 #5777 - MS05-774 Aluminum nanoflakes: relations between microstructure and reactivity.
MS05-774 Aluminum nanoflakes: relations between microstructure and reactivity.

Aluminum powders are used as a component in propellant formulations, explosives and pyrotechnics [1]. Their reactive and propulsive properties are linked to the exothermic reaction of aluminum with an oxidant. Powders with a high specific surface area (higher than 10 m2/g) i.e. aluminum nanopowders (Al-NPs) have a higher reactivity than micronic powders. Moreover, we have shown that shape and nanostructuration of the grains influence the reactivity [2]. The reactivity of the powders is characterized by thermogravimetric (TG) analyses. In this work, the powders which are obtained by high energy ball milling, show a flake like morphology (fig. 1). The influence of the milling parameters and additives on the morphology, structure and reactivity of the powder was  investigated. Among the milling parameters, we have changed the size of the balls, the atmosphere of the process and the post-treatment of the powder.

The nanoflakes are highly dispersed in lateral size and thickness;  the biggest ones have micronic lateral sizes (1-4 mm) and thickness less than 200 nm, and the smallest ones 100 nm in lateral size and 30 nm in thickness (fig. 2). The nanoflakes contain several crystallites with an orientation relationship; a [110] texture was observed (fig. 3). They are surrounded by an amorphous layer of aluminum oxide, whose thickness depends on the size of the crystallite. Small crystallites exhibit thicker amorphous layer at the edges than on basal planes (fig. 4), this could induce different oxidation rates of the powder. To remove the passivation agent used in the milling process, the powder is rinsed several times. This is a rather tedious process, so a simple annealing of the powder was tested as an alternative to remove paraffin. Heating the powder at 420°C led to an early crystallization of the Al2O3 amorphous layer, which is an unwanted effect. In-situ heating in a transmission electron microscope, under inert atmosphere, are in progress, in order to follow the crystallisation of the amorphous alumina layer and the different phase transitions, which are predicated to play an important role in the oxidation mechanism [3].

 

 

[1] E.L. Dreizin, Prog. Energy Combust.Sci. 35 141 (2009).

[2] B. André, M.V. Coulet, Ph. Esposito, B. Rufino, R. Denoyel , Mat. Lett110 108 (2013).

[3] M.A. Trunov et al. Combust. Flame 140 310 (2005).

 

Acknowledgments:

Authors acknowledge financial support from the Agence Nationale de la Recherche  (ANR) and the Direction Générale des Armées (DGA) (Grant No ANR-13-ASTR-0032).


Véronique MADIGOU (LA GARDE), Christine LEROUX, Pierre-Henri ESPOSITO, Renaud DENOYEL, Marie-Vanessa COULET
08:00 - 18:15 #5799 - MS05-776 Imaging of deformation of a polypropylene separator due to immersion in electrolyte or due to heating.
MS05-776 Imaging of deformation of a polypropylene separator due to immersion in electrolyte or due to heating.

Pore size of separators is one of the most important factors for Li-ion batteries. The separator must electrically isolate the anode and the cathode of the battery, but should allow Li ions to pass through easily. Large pores degrade the extent of isolation but small pores degrade the permeability; thus, a balance is needed for optimal performance. The pore size of the separator is determined well before its incorporation into the battery, but the pore size may not be maintained in the electrolyte. We, therefore, observed the structures of the separator before and after immersion in the electrolyte.

Figures 1(a) and 1(b) show atomic force microscopy (AFM) images of a 25-mm-thick single-layer polypropylene separator before and after immersion in the electrolyte, respectively. The figures clearly show the lamellar alignment and the the fibrillae between the lamellae. The spacing between the lamellar layers was around 0.8 mm before immersion, but expanded to 1.4 mm after immersion. The pores expanded like sponge when they immersed in the liquid electrolyte. Expansion of the separator occurs when it is bound tightly inside the battery. Thus, expansion of the pores of the separator must be considered while fabricating good batteries.

One of the most important abilities of the separator is a shutdown ability. When a large current flows between the electrodes, the resultant heating partially melts the separator, and fills the pores. As a result, Li ions cannot pass through the portion of the separator, and the large current is shut down. Thus, shutdown by melting of the separator is an important factor for safety. To investigate melting of the separator, we observed the separator while heating.

AFM images obtained at 398 K show the deformation of the separator. Expansion of the lamellar layers and the filled pores were imaged. The melting band normal to the lamellar layers was extended in the direction of the fibrillae. The melting bands should be formed by fusion of the fibrillae and lamellae, and by conjunction. Partial melting around 390 K is adequate for shutdown. These results prove the good performance of the separator.


Keiji TAKATA (Osaka, Japan)
08:00 - 18:15 #5882 - MS05-778 Quantitative compositional mapping on the nanoscale over large fields of view in thin film solar cells from earth abundant elements.
MS05-778 Quantitative compositional mapping on the nanoscale over large fields of view in thin film solar cells from earth abundant elements.

Solar power is projected to become the largest single global source of electricity generation by 2050, with photovoltaic devices constituting the majority of this market share [1].  Next generation materials based on material systems with direct band gaps fabricated in thin films are predicted to make considerable contributions to this market with the promise of significantly reducing materials costs and solar cell bulk.  At present, one of the most promising thin film absorber material is CuInxGa(1-x)Se2 (CIGS), which has been shown to achieve a solar conversion efficiency in the laboratory of 21.7% [2].  However, the high cost of the raw materials in this system has driven research to find more cost effective alternatives  [3].

One such alternative material system based on the crystallographic kesterite family Cu2ZnSn(S,Se)4 promises to be highly cost competitive due to the use of earth abundant elements.  However, this system is currently less mature than CIGS with a record solar conversion efficiencies of 12.6% [4] or 11.6% [5] if no sulphur is used.  The challenges impeding improvement are related to chemical and nanoscale structural inhomogenities that lead to a reduction in the open circuit voltage of the system [6].  A full understanding of these defects and, hence, progression in fabrication of these materials thus requires a characterization technique capable of spatially mapping out compositional, structural, and electronic trends with nanoscale spatial resolution.

In this contribution, we present our progress towards the development of a methodology that will allow researchers to directly address these challenges with nanometer spatial resolution over micron sized areas of sample.  We accomplish this through two stages.  First, a sample preparation methodology enabling a very large field of view while retaining a reasonable lamellae thickness utilizing the Focused Ion Beam is presented.  Preliminary results of this methodology are presented in figure 1, revealing a field of view of many microns over a sample that is under 80 nm thick in the absorber layer.  Second, we employ correlative EDX and EELS spectroscopic imaging techniques over large areas, focusing on features such as grain boundaries and pores. The simultaneous use of both spectroscopy techniques allows for two independent quantitative assessments of composition, reducing systematic errors. It also assists in the identification and elimination of artifacts that may arise through the data treatment.  The result is large area maps with optimized uncertainties from a materials system containing many difficult and overlapping spectroscopy edges. These techniques are then applied to state-of-the-art samples with very high efficiencies [7].

[1]          International Energy Agency: Technology Roadmap: Solar Photovoltaic Energy. 2014.

[2]          Jackson, P. et al.: Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%. Phys. Status Solidi RRL 9 (2015) 28.

[3]          European Comission: Report on critical raw materials for the EU. 2014.

[4]          Wang, W. et al.: Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 4 (2014) 1301465.

[5]          Lee, Y.S. et al.: Cu2ZnSnSe4 Thin-Film Solar Cells by Thermal Co-evaporation with 11.6% Efficiency and Improved Minority Carrier Diffusion Length. Adv. Energy Mater. 5 (2015) 1401372.

[6]          Mitzi, D.B. et al.: Prospects and performance limitations for Cu–Zn–Sn–S–Se photovoltaic technology. Phil. T. Roy. Soc. A 371 (2013) 20110432.

[7]          Giraldo, S. et al.: Large Efficiency Improvement in Cu2ZnSnSe4 Solar Cells by Introducing a Superficial Ge Nanolayer. Adv. Energy Mater. (2015) 1501070.

[8]          Hubert, M. et al.: ROBPCA: A New Approach to Robust Principal Component Analysis. Technometrics 47 (2005) 64


Thomas THERSLEFF (Uppsala, Sweden), Sergio GIRALDO, Haibing XIE, Paul PISTOR, Edgardo SAUCEDO, Klaus LEIFER
08:00 - 18:15 #5975 - MS05-780 Combined TEM/STEM and In-situ c-AFM Characterization of 2D Nanoflake-like Heterostructures for Energy Storage and Conversion Applications.
MS05-780 Combined TEM/STEM and In-situ c-AFM Characterization of 2D Nanoflake-like Heterostructures for Energy Storage and Conversion Applications.

Honeycomb-like hematite nanoflakes/branched polypyrrole nanoleaves heterostructures with a 3D complex structure have been synthesized and employed as high-performance negative electrodes for asymmetric supercapacitors application. The detailed TEM-STEM characterization and deep EELS chemical analysis at the nanoscale has been combined to elucidate the mechanisms underlying the formation and morphology evolution of core-branch Fe2O3@PPy heterostructures.[1] In addition, we have studied the mechanism of converting MoO2 nanoflakes into 2D free-standing MoS2 electrode by sulfurization process for water splitting. In this way, the atomic resolution aberration corrected HAADF STEM reveals the sulfurization mechanism in an unprecedented detail, together with EELS chemical maps. Furthermore, in-situ electrical measurements have been performed by means of c-AFM on the MoO2/MoS2 nanoflakes surfaces in order to detect their electronic active sites.

 

Acknowledgments

We acknowledge the funding from Generalitat de Catalunya 2014 SGR 1638, 2014 SGR 797 and MINECO coordinated projects between IREC and ICN2 TNT-FUELS and e-TNT (MAT2014-59961-C2-2-R). Xuan Zhang is grateful for financial support from China Scholarship Council. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program.

 

Reference:

 [1] Peng-Yi Tang, Li-Juan Han, Aziz Genç, Yong-Min He, Xuan Zhang, Lin Zhang, José Ramón Galán-Mascarós, Joan Ramon Morante, Jordi Arbiol, Nano Energy, 22, 189-201 (2016).


Peng-Yi TANG (Bellaterra, Spain), María DE LA MATA, Li-Juan HAN, Albert VERDAGUER, Aziz GENÇ, Yong-Min HE, Xuan ZHANG, Lin ZHANG, José Ramón GALÁN-MASCARÓS, Joan Ramon MORANTE, Jordi ARBIOL
08:00 - 18:15 #6023 - MS05-782 Analysis of the binder and carbon black distribution in graphite electrodes for lithium ion batteries using electron dispersive X-Ray spectroscopy and energy selective backscatter electrons.
MS05-782 Analysis of the binder and carbon black distribution in graphite electrodes for lithium ion batteries using electron dispersive X-Ray spectroscopy and energy selective backscatter electrons.

Lithium ion batteries (LIBs) as mobile storage systems become more important in the future. At the same time, the demands on LIBs, such as high capacity in combination with high dis-/charge rates, low weight, long life time and cycle stability, are rising. This presents great challenges to the internal structure of the LIB, especially to the electrodes. Here, graphite is the most common material, which is used within an anode. The anode itself consists also of carbon black for the electro conductivity and binder for cohesion (particle and carbon black) and adhesion (electrode on current collector). But both are electrochemical inactive. Hence the amount must be as small as possible. However, the distribution of the binder and the carbon black has not been discussed in detail in the literature, in particular for low amounts. Therefore the distribution of two different types of binder poly vinylidene fluoride (PVDF) and a mixture of styrene-butadiene rubber (SBR) and sodium salt of carboxymethylcellulose (Na-CMC), within graphite anodes is investigated by scanning electron microscopic (SEM) methods in this work. The investigated electrodes are about 350 µm in thickness. To facilitate analysis in the SEM smooth cross-sections were prepared by argon ion polishing.

In case of the PVDF binder the distribution can be visualized based on its fluorine content. Therefore electron dispersive X-Ray spectroscopy (EDXS) mapping was used and drying related gradients in the binder distribution could be identified (Figure 1b). High binder concentrations were usually found in areas of high carbon black concentration. However, the rather poor lateral resolution of EDXS (about 1 µm) impedes more detailed investigations. On the other hand the detection of energy selective backscattered electrons (ESB) can be used to obtain element specific information at lateral resolutions comparable to conventional secondary electron images. By optimizing the grid voltage of the ESB – detector and the electron energy, it was possible to obtain high resolution images, in which fluorine rich regions appears bright (Figure 1a). It can be shown that not only the binder but also the carbon black differs in contrast compared to the graphite particles (Figure 2b). The obtained ESB – images were evaluated with image manipulation software to mark the particles and the carbon black (Figure 2d). Due to an about 100 times faster acquisition compared to EDXS, images spanning a large part of the cross-sections could also be obtained in a rather short time, but the evaluation is not that simple and cannot be automated like EDXS - mappings.

Since the SBR – binder as such does not contain a suitable element for mapping, osmium tetroxide (OsO4) staining was used to selectively mark the double bonds of the SBR – binder. Thereafter, localization of the binder was possible by mapping the Os distribution with EDXS. Unfortunately the osmium concentration was too low to visualize the SBR – binder by ESB.


Lukas PFAFFMANN (Eggenstein-Leopoldshafen, Germany), Marcus MÜLLER, Werner BAUER, Frieder SCHEIBA, Stefan JAISER, Michael BAUNACH, Philip SCHARFER, Helmut EHRENBERG
08:00 - 18:15 #6029 - MS05-784 Atomic resolution HR(S)TEM and EDXS analyses of GaInAs/GaSb and GaInP/GaSb bond interfaces for high-efficiency solar cells.
MS05-784 Atomic resolution HR(S)TEM and EDXS analyses of GaInAs/GaSb and GaInP/GaSb bond interfaces for high-efficiency solar cells.

The use of direct wafer bonding to combine semiconductor materials that have a large lattice mismatch is especially beneficial for high efficiency multi-junction solar cells. Multi-junction solar cells that have been fabricated by wafer bonding are of particular interest since efficiencies of up to 46% have been obtained [1] and efficiencies of up to 50% are within reach for concentrator solar cells based on III-V compound semiconductors. Fast atom beam activation is used as a pre-treatment to remove oxides and contamination before bonding [2]. Aberration-corrected transmission electron microsocpy (TEM) analyses of GaAs/Si interfaces have previously been applied successfully to support the implementation of bonding concepts for the development of high-efficiency solar cells [3].

 

Here, we investigate cross-sectional specimens of GaInAs/GaSb and GaInP/GaSb bond interfaces in wafer-bonded multi-junction solar cells, in order to obtain an improved understanding of their interface structures and thermal stability, by combining aberration-corrected high-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF STEM), energy-dispersive X-ray spectroscopy (EDXS) in the STEM and in situ TEM heating experiments.

 

Figures 1a-e shows results obtained from the GaInP/GaSb bond interface. Fig.1a shows the interface at low magnification. Figure 1b shows an HRTEM image, which reveals an amorphous interface layer (~1 nm thick). Figure 1c shows an atomic resolution HAADF STEM image of the bond interface structure and a digital diffractogram (inset), revealing a nearly perfect structural orientation relationship between the two crystalline layers. When correctly positioned with respect to the HAADF image, elemental maps extracted from EDXS spectrum images (Figs 1d and 1e) reveal that a high level of Ga is present at the interface. The Ga can be attributed to the pre-treatment procedure and bonding conditions. I  situ thermal treatment of this interface results in pronounced interdiffusion for temperatures above 225°C (not shown here).

 

Figures 2a-c show the GaInAs/GaSb bond interface, which is decorated by pores and cavities that extend along the interface by more than 10 nm. As a result of the use of misoriented wafers for bonding, the crystal lattices are rotated with respect to each other by a few degrees (Figs 2b and 2c).

 

Our results confirm that the advanced imaging and spectroscopic methods of aberration-corrected (S)TEM are advantageous for characterizing the morphology, elemental distribution and structure of layers and bond interfaces for the monitoring, control and optimization of different concepts used for fabricating high-efficiency solar cells. Out results are also of interest for assessing electrical conductivity phenomena at these interfaces.

 

1. F. Dimroth et al., IEEE Journal of Photovoltaics, 6, 343 (2016).

2. E. Stephanie et al., J. Appl. Phys. 113, 203512 (2013).

3. D. Häussler et al., Ultramicroscopy, 134, 55 (2013).

 


András KOVÁCS (Juelich, Germany), Martial DUCHAMP, Felix PREDAN, Frank DIMROTH, Rafal DUNIN-BORKOWSKI, Wolfgang JÄGER
08:00 - 18:15 #6076 - MS05-788 Analysis of nanoscale band gap fluctuations in Cu(In,Ga)Se2 solar cells by VEELS.
MS05-788 Analysis of nanoscale band gap fluctuations in Cu(In,Ga)Se2 solar cells by VEELS.

Thin film solar cells based on CuInGaSe2 (CIGS) absorber layers have demonstrated conversion efficiencies of more than 22% and they manifest a promising potential for the development of highly efficient, flexible and low-cost solar cells [1]. However, even though such high efficiencies are reached, still many fundamental properties that govern the performance are not yet clearly understood. The interfacial regions of the thin film layer stack and also the bulk of the polycrystalline CIGS absorber layer contain a high variety of structural and chemical inhomogeneities which consequently affect the electronic properties of the photovoltaic device in various ways. In order to push the achievable efficiency in a targeted way further towards the theoretical limit, a better understanding of local electronic loss mechanisms related to compositional inhomogeneities is needed. Tools that allow for characterizing the electronic properties in CIGS in the nanometer range are therefore sought.

Valence electron energy loss spectroscopy (VEELS) provides access to electro-optical properties down to the nanometer scale which makes it a very promising technique for local electronic characterization of CIGS solar cells. Besides others, information about the band structure and in particular about the local band gap energy may be extracted from VEEL spectra. However, various artifacts may influence the measured VEEL spectra which complicates the data interpretation and requires careful consideration of the results. In case of CIGS, the average band gap energy is rather small, i.e. 1.1-1.4 eV for typically used CIGS compositions, which pushes the required energy resolution towards the achievable limits of state-of-the-art electron sources [2].

Within this work, the applicability of VEELS for probing local band gap fluctuations in CIGS is comprehensively assessed. Based on the results obtained on different microscopes we discuss the achievable precision and accuracy in correlation with the performance of the respective instrument as shown in Figure 1. This analysis suggests that a monochromator is required to reliably identify the small band gap variations in CIGS. Finally, we use a microscope equipped with a second generation monochromator to study the electronic properties in CIGS solar cells at specific sites such as grain boundaries and interfaces.

 

[1] K. Maraun, 2015. Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency: 22.3%, http://www.solar-frontier.com/eng/news/2015/C051171.html (accessed 12.15.15).

[2] D. Keller et al., Local Band Gap Measurements by VEELS of Thin Film Solar Cells. Microsc. Microanal. 20, 1246–1253 (2014). 


Debora KELLER (Duebendorf, Switzerland), Stephan BUECHELER, Patrick REINHARD, Fabian PIANEZZI, Marta D. ROSSELL, Darius POHL, Alexander SURREY, Bernd RELLINGHAUS, Fredrik HAGE, Quentin RAMASSE, Rolf ERNI, Ayodhya N. TIWARI
08:00 - 18:15 #6095 - MS05-790 Microstructural evolution of 9Cr-ODS steel during high temperature deformation.
MS05-790 Microstructural evolution of 9Cr-ODS steel during high temperature deformation.

Scanning and transmission electron microscopy (TEM) was applied to study the structure of oxide dispersion strengthened (ODS) martensitic 9Cr steel before and after mechanical deformation at elevated temperatures. The tensile tests were performed in the temperature range of 25°C to 800°C at a nominal strain rate of 10-3 s-1. High angle annular dark field (HAADF) scanning TEM (STEM) with energy-dispersive X-ray (EDX) spectroscopy for the determination of the composition and conventional TEM with selected area diffraction for the crystallographic investigations were applied.

Figure 1 shows an overview HAADF STEM image (a) with corresponded schematic (b) of the investigated steel in tempered conditions. Owning to the composition and heat treatments, the structure of the steel typically consists of tempered martensite with a high density of hierarchically organized internal interfaces. During quench, the prior austenite grains are sub-structured by martensitic laths and packets. This transformation causes a large local deformation of the matrix, resulting in a high dislocation density.

The following tempering process allows the recovery of the martensitic structure with transformation into a ferritic structure, precipitation of solute atoms and recovery of the dislocation cell structure into subgrain structure. Therefore, the final microstructure consists of some portions that are composed of typical laths (with several µm in length and 0.25 ± 0.06 µm in width) and others portions that are filled with subgrains (with size of 0.29 ± 0.13 µm). The non-regular shaped M23C6 carbides precipitate are arranged preferentially at prior austenite grain boundaries, packet and lath boundaries. HRTEM analysis of M23C6 (here not shown) reveals that many carbides have fcc structure. The relation between the carbide and ferrite is: M23C6 || Fe. The detail EDX analysis shows that carbide consist of Fe and Cr. The fine ODS particles are mostly revealed inside the boundaries of laths and subgrains, yet on them as well. Figure 2 shows HAADF STEM image of the ODS particle (a) with linescan EDX profiles across the particle (b). The detail EDX analysis of many ODS particles shows that they have composition of Y2Ti2-xO7-2x.

For investigation on the deformed state, TEM samples were prepared from the gauge section region near the fracture surface of the tempered martensitic state specimens. At room temperature, the deformed microstructure shows similar characteristics as those typically observed for an as-received state, but the dislocations are mostly pinned to the ODS particles. At elevated temperatures, structural evolution becomes prominent with the large decrease in dislocation density and the appearance of polygonal subgrains that replace the original lath structure. In addition, coarsening of M23C6 carbides was observed with increase of testing temperatures. Figure 3 presents TEM bright-field (a) and (110) dark-field (b) images from the same place of sample obtained after the tensile test at 800°C. The equiaxed α-Fe has high dislocation density at the grain boundaries (area 1) and dislocations that are pinned to the ODS particles at various locations (marked by arrows). The partially illuminated grain in Figure 3b confirms the presence of low-angle grain boundary LGB that appears to be undergoing recovery process.


Dimitri LITVINOV (Eggenstein-Leopoldshafen, Germany), Ankur CHAUCHAN, Jarir AKTAA
08:00 - 18:15 #6103 - MS05-792 EFTEM measurements of the sp2/sp3 ratio on SiC/SiC pyrolytic carbon interphases before and after irradiation.
MS05-792 EFTEM measurements of the sp2/sp3 ratio on SiC/SiC pyrolytic carbon interphases before and after irradiation.

We investigated the behaviour of a specific component of silicon carbide based ceramic matrix composites (SiC/SiC) under irradiation, namely the pyrolytic carbon (PyC) interphase linking the matrix and fibres together. SiC/SiC is studied for its potential as a new nuclear fuel cladding to replace the zirconium based alloys currently used. The hybridization of the carbon atoms found in this interface layer is studied using energy filtered TEM (EFTEM) to determine the ratio between sp2 and sp3 bonding states (R-ratio), before and after irradiation with ions and neutrons. Indeed, the initial state of the interphase is graphite-like PyC, in which the electrons occupy the π bonding states of the sp2 orbitals. The ratio between the π and σ occupied levels is different in each carbon allotropes, be it amorphous, graphite or diamond. Radiation damage is known to amorphise and open micro-cracks in nuclear graphite [1], [2]. In cases where the graphitic interphase would become completely amorphous, both the mechanical and thermal properties of the composite would be impacted.

In this work, TEM lamellae prepared from both pristine and neutron irradiated SiC/SiC have been examined. These samples are extracted from actual prototype cladding sections produced by General Atomics. In addition to this, the samples prepared from pristine material have been subsequently irradiated at the in-situ TEM JANNuS facility in Orsay, France [3]. Using the unique capabilities of this apparatus, self-ion irradiations with both Si+ and C+ were performed. Damage levels approaching one displacement per atom (dpa) were reached whilst observing the effects of said ions at the same time with a TECNAI G2 20 Twin TEM. The EFTEM measurements were carried out at EPFL, Lausanne, using a Schottky FEG JEOL 2200FS with an in-column omega filter. All of these measurements were carried out as close as technically possible to magic angle conditions [4]. The ratio between the peaks corresponding to the π* and σ* anti-bonding states can be quantified with different techniques, two of which are used in the present work: the two window (TWM) [5], whereby the absorption spectrum is numerically integrated over two energy ranges or windows are centered on the aforementioned peaks and spectrum fitting using Gaussian functions [6] is also used to ensure the robustness of the measurement.

The maps displayed in Figure 1 and 2 show an overall decrease of the R-ratio after irradiation of the samples, meaning that even sub-dpa damage levels are already inducing an amorphisation of the PyC interlayer. Additionally, the overall layer is homogeneously amorphised, as further shown with the histograms displayed in Figure 3.

The authors would like to thank Westinghouse and General Atomics for providing the prototype cladding tubes within the CARAT research program as well as JANNuS-Orsay part of the CSNSM, Orsay, France which is part of the EMIR French accelerators network, where the in-situ experiments were carried out.

References:

[1]     K. Wen, J. Marrow, et B. Marsden, « Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG) », J. Nucl. Mater., vol. 381, no 1‑2, p. 199‑203, oct. 2008.

[2]     W. Bollmann et G. R. Hennig, « Electron microscope observations of irradiated graphite single crystals », Carbon, vol. 1, no 4, p. 525‑533, juillet 1964.

[3]     M-O. Ruault, J-F. Dars, P. Stroppa/CEA, A.Gonin/CEA, « Plate-forme de multi-irradiation JANNuS Jumelage d’Accélérateurs pour les Nanosciences, le Nucléaire et la Simulation ». .

[4]     C. Hébert, P. Schattschneider, H. Franco, et B. Jouffrey, « ELNES at magic angle conditions », Ultramicroscopy, vol. 106, no 11‑12, p. 1139‑1143, oct. 2006.

[5]     J. Bruley, D. B. Williams, J. J. Cuomo, et D. P. Pappas, « Quantitative near-edge structure analysis of diamond-like carbon in the electron microscope using a two-window method », J. Microsc., vol. 180, no 1, p. 22‑32, oct. 1995.

[6]     Z. Zhang, R. Brydson, Z. Aslam, S. Reddy, A. Brown, A. Westwood, et B. Rand, « Investigating the structure of non-graphitising carbons using electron energy loss spectroscopy in the transmission electron microscope », Carbon, vol. 49, no 15, p. 5049‑5063, décembre 2011.


Loïc FAVE (Villigen PSI, Switzerland), Cécile HÉBERT, Manuel Alexandre POUCHON
08:00 - 18:15 #6124 - MS05-794 FIB-tomography of graphite anode particles for lithium ion batteries.
MS05-794 FIB-tomography of graphite anode particles for lithium ion batteries.

Graphite as anode material in lithium ion batteries (LIB) dominates with a share of 90%, whereby this share is divided in 55% for natural graphite and 45% for synthetic graphite. Before these material are used in a LIB, a spherodization process in a particle design mill is performed. This process is fully mechanical, using impact and shear forces. The raw material turned into the spherical graphite consists of comparably big flakes with diameters to about 300µm.

The motivation for the spherodization of the graphite flakes lies in the improvement of their performance for LIBs. Lithium ions intercalate into the anode material while charging the battery, but the ions penetrate the graphite through crystal defects. Is the anode made out of graphite flakes, intercalation only happens from the sides of the flakes where the crystal lattice is broken, ions cannot intercalate from the top. Spherodization breaks the crystal lattice and offers therefore many openings for intercalation. Furthermore, the spherodization helps to face the solid electrolyte interface (SEI) problem. The SEI forms in the first cyclization of a LIB and is a chemical reaction with the electrolyte and the anode material, its thickness is just a few nanometres. This process binds lithium ions which are therefore no longer available and the capacity of the LIB decreases. Spheroids have the best volume-to- surface ratio and therefore are helping to minimize the capacity decrease.

Due to this mechanical shaping process, about 50% of the initially introduced raw material will not be spherodized. The grade of the spherodization is controllable via the rotational speed of the mill, whereby a higher speed means higher mechanical forces as described above.

Fig. 1 shows the result of FIB investigations on natural graphite material regarding the porous nature of the particles. FIB cross sectioning revealed that the spherodization process is rather a folding of the raw graphite flakes (see Fig. 2). The arising pores are therefore of elongated shape. For the spherodized synthetic graphite almost no porosity was observed by cross sectioning in comparison to the sperodized natural graphite. To learn more about the porous properties of the spherical graphite, FIB-tomography was utilized. It was observed that the graphite spheres included two different types of pores. A “classical” pore type isolated to the outside named “closed pores”, and pores with a connection to the surface of the particle named “open pores”. With FIB-tomography the volume fractions of both porosities were determined following the relation Φ=VP/V, where Φ is the porosity, VP the volume of the pore (open or closed or the combined pore volume) and V the volume of the particle consisting of the graphite and the complete (open plus closed) pore volumes. The FIB-tomogram analyses were carried out both for commercial spherical graphite samples (Fig. 3) as well as for spherical graphite samples prepared by the authors (Fig. 4). The results for the various samples are compared and discussed.

 

Acknowledgement:

We thank the German Federal Ministry of Education and Research (BMBF) for financial support (Project: Li-EcoSafe, contract no. 03X4636).


Manuel MUNDSZINGER (Ulm, Germany), Manfred RAPP, Ute GOLLA-SCHINDLER, Mario WACHTLER, Ute KAISER
08:00 - 18:15 #6133 - MS05-796 Application of photoconductivity effect in BaTiO3/TiO2 hybrid heterostructure for solar cells.
MS05-796 Application of photoconductivity effect in BaTiO3/TiO2 hybrid heterostructure for solar cells.

TiO2 is known as material with excellent chemical and photochemical stability. It exists in three different crystalline phases: anatase, rutile  and brookite. It was found that anatase is photocatalytically more active than rutile due to its large surface area. This activity depends not only on the phase of TiO2 but also on the crystallite size and porosity. One of the most applied synthesis is electrochemical eteching of titanium foil to obtained thin TiO2 nanotubes array. The morphology parameters, e.g., nanotube length, diameter, smoothness, depend on the anodization conditions, such as voltage, electrolyte composition, temperature, and duration. After anodization, the amorphous nanotubes can be annealed to increase the electron mobility, sensitized with dyes or polymers to increase solar photon absorption, and doped or surface-functionalized to adjust the density of states. Because of these properties the titania nanostructures can be used for photo-catalysis, in solar cells (DSSC) and sensors. On the other hand BaTiO3 is a well known and widely investigated dielectric material. Barium titanate is also used for electronic devices in technological ceramic industry because of its ferroelectric, thermoelectric and piezoelectric properties when it assumes the tetragonal structure. As such it have application in production of capacitors, positive temperature coefficient resistors, dynamic random access memories, electro mechanics and nonlinear optics. In this research we will study effect on optical and electrical properties of hydrothermal growth of BaTiO3 nanoparticles at the surface of TiO2 nanotubes obtained by electrochemical etching of titanium foil and titanium film deposited on FTO glass. Due to ferroelectric effect of BaTiO3 we believed the charge separation in this type of heterostructure will be improved compared to pristine TiO2 nanotubes. The nanotubes will be synthesized by electrochemical oxidation of Ti-foil. The parameters of the TiO2 nanotubes will be finely tuned together with the size of BaTiO3 nanoparticle, with the aim to obtained highest photoconductivity effects and allowing the optimization of device fabrication for different types of solar cells hybrid solar cells. Preliminary results show that TiO2 nanotubes have diameter around 80 nm and grown BaTiO3 nanoparticles 30–50 nm (Fig. 1 and 2). AFM measurements confirmed the size of BaTiO3 nanoparticles and local measurements of piezoelectric effect shown that in ours system exists preferred polarization in the direction of applied external electrical field. This study will be performed with the aim to optimize the dimensions of TiO2 and BaTiO3 nanoparticles to increase the efficiency of solar cells. For the structural characterization of all the titanate nanostructures we will used conventional and analytical transmission electron microscopy (TEM) techniques, scanning electron microscopy (SEM), XRD and Raman spectroscopy, Impedance spectroscopy, SPM microscopy and UV-Vis spectroscopy.

Acknowledgments

Center of Excellence for Advanced Materials and Sensing Devices, Croatian Science Foundation (hrzz) and European Social Fond (ESF) for the financial support of this research.

 


Milivoj PLODINEC (Zagreb, Croatia), Iva ŠRUT-RAKIĆ, Andreja GAJOVIĆ, Ana ŠANTIĆ, Marc-Georg WILLINGER
08:00 - 18:15 #6134 - MS05-798 FIB-SEM tomography imaging and 3D structure quantification of supported metal catalysts with bi- and tri-modal meso-macroporosity.
MS05-798 FIB-SEM tomography imaging and 3D structure quantification of supported metal catalysts with bi- and tri-modal meso-macroporosity.

Materials with hierarchical porosities extending over several length scales, from the nanometer to the macrometer range, attract great attention as solid catalysis. They offer possibilities to enhance the rate of diffusional access of reactants to, and products from, the catalyst active sites. Even though several methodologies based on e.g. soft- and hard-templating, or spontaneous assembly, have been demonstrated to synthesize meso-macroporous catalysts [1], their structure is typically assessed on a qualitative basis and not much is known about how to deliberately control key structural features of their different porosity levels such as modality, pore size or spatial interconnectivity. Here we have applied FIB-SEM tomography, coupled to quantitative 3D image analysis, to visualize and quantify the internal architecture of high-surface area, multi-modal meso-macroporous RuCo/γ-Al2O3 catalysts, which are highly interesting in the production of synthetic liquid fuels from natural gas.[2] The spatial resolution offered by FIB-SEM tomography is particularly suitable to image macroporous networks, with pores ranging from 50 nm to several micrometer in size.

 Different series of gamma-Al2O3 support materials, displaying either highly interconnected, channel-like or mostly isolated, globule-like macroporous networks, and essentially identical specific surface areas and mesoporities, were synthesized via soft- and hard-templating from nanosized boehmite precursors. Next, highly dispersed RuCo nanoparticles (10-15 nm) were incorporated by incipient wetness impregnation with nitrate precursors in solution to obtain the supported catalysts. The overall texture (specific surface area, pore volume, mesopore size distribution) of the materials was assessed by means of N2 adsorption and Hg intrusion porosimetries. Volumes of ca. 400 µm2 of resin-embedded catalyst particles (400-600 µm) were imaged using a ETD(SE) detector (2kV, 0.17 nA) and Ar-FIB milling (30 kV, 21 nA) in a Helios Nanolab dual-beam microscope (FEI).

 3D reconstruction of the tomograms allowed a precise visualization of the internal macropore networks in the catalysts, overcoming limitations of alternative intrusion porosimetry methods which probe exclusively unconstrained macropores connected to the outer surface of the particles. Access to the internal structure of the materials made it possible to identify composition ranges for the synthesis gels suitable to prepare catalyst materials with either bi-modal meso-macroporous or tri-modal meso-macro-macroporous structures. In addition, several structural parameters of relevance for molecular diffusion could be quantified by image analysis after tomogram segmentation (Figure 1). The skeleton of the macropore system was computed through a morphology-preserving thinning algorithm. Macropore connectivities were quantified using an algorithm based on the Euler characteristic of the corresponding set of voxels, while a local trabecular thickness algorithm was applied on the set of voxels corresponding to the mesoporous Al2O3 phase to quantify the diffusion distances for primary reaction products through the mesoporous network towards the connected macropores.[3]

 Our imaging and structure quantification approach guided the synthesis of catalysts with similar macropore volumes and mesopore diffusion distances albeit notably different macropore connectivities (Figure 2), enabling a systematic investigation of the relative significance of these parameters for the catalytic performance under industrially relevant reaction conditions.

References:

[1] C.M.A. Parlett, et al., Chem. Soc. Rev. 42 (2013) 3876.

[2] A.Y. Khodakov, et al. Chem. Rev. 107 (2007) 1692.

[3] M. Doube, et al. Bone 47 (2010) 1076.


Nicolas DUYCKAERTS, Mathias BARTSCH, Axel LORKE, Ferdi SCHÜTH, Gonzalo PRIETO (Mülheim an der Ruhr, Germany)
08:00 - 18:15 #6147 - MS05-800 Calcination of pd nanocatalysts on alumina: ex-situ analysis versus in-situ environmental TEM.
MS05-800 Calcination of pd nanocatalysts on alumina: ex-situ analysis versus in-situ environmental TEM.

      

 

                                                                      

Sustainable chemistry, reduction of pollution, greenhouse gases control, oil refinery, liquid and solid waste management, are essential societal topics related to what is called Environmental catalysis [1], a sector which is booming rapidly for the past 10 years. So, developing innovative catalysts is a very important aspect ahead of us. To develop more advanced catalysts, we have to understand the catalysts genesis in all the stages of preparation (impregnation, drying, calcination and reduction of the active phase). Nowadays Environmental Transmission Electron Microscopy (ETEM) enables dynamic studies of the catalysts down to the nanometer and even atomic scale., at high temperatures and under gaseous environment.

               This contribution deals with the dynamic evolution of palladium nanoparticles (NP) supported on alumina during the catalyst preparation process. Here, the calcination step is studied in a FEI TITAN-ETEM microscope operated at 300 kV. A Pd/delta alumina catalyst was investigated at different temperatures and under different gas pressures in order to follow the particles size evolution. Owing to the small size of the Pd NPs in the range of 1-3 nm, we mainly used the STEM-ADF imaging mode. Preliminary observations during heating under high vacuum up to 600°C show that particles as well as the supporting media remain stable and apparently not damaged by the electron beam under nominal and atomic resolution imaging STEM conditions (see Fig. 1). Some EELS (Electron Energy-Loss Spectroscopy, Gatan Imaging Filter Tridiem Ers) analysis was performed in situ in order to ascertain the chemical nature of the observed particles. During this work it appeared essential to perform measurements systematically on the same areas at different temperatures. Fig. 2 is a typically illustration of the in situ evolution of palladium particles under oxygen partial pressure at different temperatures. From such micrographs, the NP size evolution was quantified and compared with post-mortem TEM observations after ex-situ experiments performed at same temperatures but atmospheric pressure in the course of the catalyst synthesis (Fig. 3).

This comparison shows that all measurements appear to be consistent except those performed in the bright field TEM imaging mode, where larger particles sizes are obtained, most probably due to irradiation effects which were further evidenced by a high-induced mobility of NPs on their supporting media. This ETEM study brings direct in situ information on the transient stages that cannot be followed by post mortem experiments after ex-situ treatments; in particular, we will also discuss about the crystallographic evolution of the Pd NPs during the calcination process [2,3].

 

 

[1] G. Centi, P. Ciambelli, S. Perathoner, and P. Russo, “Environmental catalysis: trends and outlook,” Catalysis Today, vol. 75, no. 1-4, pp. 3-15, July 2002.

[2] The CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) is acknowledged for its guidance in the ETEM project which was financially supported by the CNRS, the Région Rhône-Alpes, the ‘GrandLyon’ and the French Ministry of Research and Higher Education.

[3] thanks are due to DENS solutions for allowing experiments with the Wildfire MEMS-based heating holder.

 

 


Siddardha KONETI (VILLEURBANNE CEDEX), Lucian ROIBAN, Thierry EPICIER, Anne-Sophie GAY, Priscilla AVENIER, Amandine CABIAC, Florent DALMAS
08:00 - 18:15 #6173 - MS05-802 High resolution EDS study of Ba2Y1+xNb1-xO6-δ sintered compounds.
MS05-802 High resolution EDS study of Ba2Y1+xNb1-xO6-δ sintered compounds.

We present here the study of Ba2Y1+xNb1-xO6-δ (x = 0, 0.05, 0.1, 0.15, 0.2) compounds with potential application as ion-conducting materials in Solid Oxide Fuel Cells and/or Proton Conducting Fuel Cells. Yttrium is used here as an acceptor-doping element in substitution to niobium to create oxygen vacancies by charge compensation. Nanopowders have been prepared by a freeze-drying route and dense ceramics were obtained after shaping and treatment at high temperature. X-ray diffraction analysis shows that single phase compounds are obtained for x values equal or below 0.2. Transport properties indicate that, in spite of a significant concentration of oxygen vacancies, the ion conductivity remains low in oxidative atmosphere. A higher conductivity is observed in humidified hydrogen showing that the material gets partially reduced in such conditions.

High resolution STEM images and EDS chemical mapping show very well order of Y and Nb in Ba2YNbO6 compound (x = 0), however when x = 0.1 a pronounced cationic exchange is observed between Y and Nb (Figure 1). Based on these results and Molecular Dynamics calculations, we noticed that the Y on Nb site induce an oxygen vacancies created by Yttrium doping. When the doping level is low, a long range diffusion would suppose that oxygen vacancies also locate close to Niobium atoms which does not seem to be possible in this material. Thus, the only way for oxygen vacancies to participate to long range ion diffusion is to percolate from a site close to a Y dopant to another site close to a Y dopant.

These results were presented at an international conference EMRS Warsaw (2015) and gave rise to a publication that is being prepared.

 

Acknowledgments:

This work has benefited from the financial support of the LabeX LaSIPS (ANR-10-LABX-0040-LaSIPS) managed by the French National Research Agency under the "Investissements d'avenir" program (n°ANR-11-IDEX-0003-02). We acknowledge the French National Research Agency (MATMECA Equipex project) for microscope financial support.

 


Mohamed BEN HASSINE (Antony), Anastasia IAKOVLEVA, Paul HAGHI ASHTIANI, Guilhem DEZANNEAU
08:00 - 18:15 #6182 - MS05-804 Investigation of pristine Li1.2Ni0.13Mn0.56Co0.13O2 by advanced TEM.
MS05-804 Investigation of pristine Li1.2Ni0.13Mn0.56Co0.13O2 by advanced TEM.

Layered Li-transition metal (TM) oxides are very promising materials for new Li ion battery cathodes. Compounds with an increased content of Li and Mn are particularly interesting because they exhibit a high capacity of 200-300 mAhg‑1 even after an initial drop in capacity during the first charging cycle.[1] The structure of such compounds and their capacity degradation over multiple cycles is not fully understood due to the complexity of the crystallographic phases, the ambiguities in the diffraction data and the presence of additional phases.

The investigated compound Li1.2Ni0.13Mn0.56Co0.13O2 is derived from LiNi0.33Mn0.33Co0.33O2, (NMC). Due to the higher capacity of the Li-rich compound, it will be referred to as high energy NMC (HENMC).[2] The chemical formula of HENMC can be rewritten as Li2MnO3 · LiNi0.33Mn0.33Co0.33O2 which implies that this compound is a mixture of trigonal NMC and monoclinic Li2MnO3. In NMC, there are two different cation layers which are exclusively occupied by Li and TM respectively. In Li2MnO3 one third of the TM sites are occupied by Li. HENMC must be either a phase mixture with distinct domains of the two oxides or a solid solution of the two, with “Li rich” positions in the TM layers and an overall reduced, monoclinic symmetry (Figure 1).[3] The exact nature of HENMC and similar materials is crucial to understand the de‑lithiation processes during charging cycles.

The layered structure and its relation to other phases was investigated with a combination of electron diffraction in bright-field and scanning modes with high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) imaging. Due to the high structural similarity of NMC and Li2MnO3, all lattice distances which can be found for NMC can also be found for Li2MnO3; as seen in the respective electron diffraction patterns (Figure 2). HAADF-STEM imaging reveals the presence of Li-rich positions in the TM layers as indicated by a lower intensity of the related atom columns (Figure 3). Due to the stacking faults of the TM layers, the interpretation of contrast in any other orientation becomes by far less intuitive as we will demonstrate. A LiTM2O4 spinel phase is found on the surface quite frequently. Its relation to the bulk phase was derived from high resolution images. STEM-EELS revealed the presence of surface reduction due to oxygen depletion in HENMC.

This work thus provides structural information about the synthesized material in its pristine state to facilitate understanding the changes the material undergoes during the (de‑)lithiation segments  of electrochemical cycling. Further investigations on such effects are currently being carried out and will be discussed.

 

[1] B. L. Ellis, K. T. Lee, L. F. Nazar, Chem. Mater. 2010, 22, 691-714.

[2] C. S. Johnson, N. Li, C. Lefief, J. T. Vaughey, M. M. Thackeray, Chem. Mater. 2008, 20, 6095-6106.

[3] H. Yu, H. Zhou, J. Phys. Chem. Lett. 2013, 4, 1268−1280.


Christian WIKTOR (Hamilton, Canada), Hanshuo LIU, Meng JIANG, Yan WU, Xingyi Yang YANG, Gianluigi A. BOTTON
08:00 - 18:15 #6227 - MS05-806 The corrosion of Zr(Fe,Cr)2 Secondary Phase Particles within Zircaloy-4 under Simulated Nuclear Reactor Conditions.
MS05-806 The corrosion of Zr(Fe,Cr)2 Secondary Phase Particles within Zircaloy-4 under Simulated Nuclear Reactor Conditions.

Using Scanning Transmission Electron Microscopy (STEM) coupled with Dual Electron Energy Loss Spectroscopy (DualEELS), the corrosion and incorporation of Secondary Phase Particles (SPPs) into the oxide layer of Zircaloy-4 material has been investigated.

Primarily, this work has focussed on studying Zr(Fe,Cr)2 particles, and tracking their changes in morphology as they undergo oxidation.

It has been seen that these precipitates typically present themselves as very faceted structures within the α-Zr metal matrix, and exhibit the hexagonal Laves phase structure. At this point, the body of the SPP tends to contain more Cr than Fe, however, Fe is often seen to segregate to the outside rim of the particle, and occasionally also to sit preferentially in regions within the body of the particle, resulting in Fe enriched areas, as can be seen in Figure 1.

Upon oxidation of the SPP, an out-of-plane expansion is seen to occur due to the volume expansion on the formation of the oxide, often resulting in particles that are quite elliptical in cross section with the major axis in the out-of-plane direction, as depicted in Figure 2.  It can be seen that the elemental distribution does not remain homogeneous, as in the original SPP, and, at this point, three principal phases have been identified in the oxidised SPPs. The first thing to oxidise (i.e. the cap of the precipitate) is very Cr-rich and is probably mainly Cr2O3 with some Fe substitution – this must consume a reasonable proportion of the Cr in the SPP. Subsequently, the main body of the particle itself is found to oxidise to a Zr, Cr oxide with an approximate 3:2 ratio of Zr:Cr, and very little Fe content.  The result of the formation of this majority phase and the rejection of the Fe is the precipitation of veins of Fe-rich material.  Early in the process, this Fe remains at least partially metallic. Even when the SPP is well encapsulated in the oxide scale, Fe-rich veins have been observed to be very low in oxygen.

This nanoanalytical approach reveals the true complexity of the oxidation of these intermetallic compounds, and, understanding how these precipitates corrode in simulated nuclear reactor conditions prior to service is crucial in analysing how the addition of these alloying elements affects the macroscopic properties of the material to corrosion.

Acknowledgements

KJA is grateful to the EPSRC and AMEC Foster Wheeler for the provision of PhD studentship.


Ian MACLAREN (Glasgow, United Kingdom), Kirsty J. ANNAND, Mhairi GASS
08:00 - 18:15 #6234 - MS05-808 Interfacial properties of ceria films on yttria-stabilized zirconia.
MS05-808 Interfacial properties of ceria films on yttria-stabilized zirconia.

Ceria (CeO2) and yttria-stabilized zirconia (YSZ) are two promissing electrolyte materials used in solid oxide fuel cells due to their high ionic conductivities. Theory predicts that the formation energy of oxygen vacancies is decreased at free surfaces and internal interfaces. Oxygen vacancies are expected to segregate at interfaces and thus might provide an easy path for rapid ion conduction.1 Aims of the present work are: (i) to obtain insights into the structure and chemistry of interfaces between ceria films and YSZ substrates, and (ii) quantitative assessments of ceria/zirconia intermixing and oxidation state of Ce at interfaces. Analytical scanning transmission electron microscopy (STEM) is the method of choice for a comprehensive materials characterization in terms of structure and chemistry down to atomic levels.2 In the present study high-resolution electron spectroscopic imaging was performed in an advanced TEM/STEM system (JEOL JEM-ARM200CF) equipped with a cold field-emission gun, a probe Cs-corrector (DCOR, CEOS/Heidelberg), and X-ray and electron spectrometer attachments (JEOL Centurio SDD; GATAN GIF Quantum ERS). Electron energy-loss spectroscopy (EELS) in STEM enables to probe the local oxidation state of Ce ions in interface regions by utilizing valence sensitive features in the energy-loss near-edge fine structures of Ce-M4,5 edges.

Continuous epitaxial films of pure and 10 mol% Gd doped ceria were grown by pulsed laser deposition on (111) YSZ substrates (Fig. 1a). The film-substrate lattice mismatch is accommodated by misfit dislocations (extra atomic planes in the YSZ substrate).3 Atomic column-resolved EEL spectroscopic images (ESI) were acquired in selected regions of interest (ROI) across ceria-YSZ interfaces. ESI enables the visualization of Ce4+àCe3+ reduction in narrow interface regions using multiple linear least-square (MLLS) fitting methods (Fig. 1c). ESI setup parameters: ROI 134x39 pixels, pixel size 0.051nm, dwell time 0.02 s/pixel, probe current 140 pA, camera length 15 mm, collection angle 110 mrad, energy resolution 0.5 eV, total acquisition time 108 s. Chemical and valence changes across interfaces were quantitatively assessed by EELS line scans (Figs. 2,3) with acquisition parameters: probe size 0.1 nm, probe current 140 pA, convergence angle 28 mrad, collection angle 110 mrad, number of measured points 160, step width 45 pm, range 7 nm, dwell 1 s/pixel, acquisition time 163 s. Parallel recording of EELS and HAADF signals in STEM-EELS enables a precise correlation of EELS spectra and structural features at atomic levels regardless of eventual sample drift. Thus, atomically resolved EEL spectra were extracted column-by-column from line scans (integration windows 0.25 nm) and quantitatively evaluated for each individual (111) atom plane crossed by the line scan. The Ce-M5/M4 intensity ratios were measured by the second derivative method3 (Fig. 3a), from which the fraction of Ce3+ can be deduced (ratios of 0.95 and 1.26 correspond to Ce4+ and Ce3+ in CeO2 and CePO4 reference materials, respectively)3. Chemical profiles indicate a Ce/Zr intermixing zone extending over seven (111) lattice planes (Fig.3b).

No noticeable differences were observed between pure and Gd doped ceria films. In both doped and non-doped ceria films, Ce4+ is gradually reduced in a region of 7 to 9 (111) lattice planes wide with a maximum fraction of 0.9 to 1.0 for Ce3+ in a single atomic layer at the interface (Fig. 3a).  Assuming charge balance, the presence of Ce3+ ions is seen as evidence of oxygen vacancy formation in narrow interface regions.

In summary, it is concluded that advanced analytical TEM/STEM methods enable the elucidation of local non-stoichiometry, which is crucial not only for understanding charge transport mechanisms in these hetero-structured materials, but also for understanding the catalytic properties of ceria.3,4

References

  1. M. Fronzi et al.: Phys. Rev. B 86 (2012) 085407
  2. H. Schmid, E. Okunishi and W. Mader: Ultramicroscopy 127 (2013) 76
  3. K. Song et al.: APL Materials 2 (2014) 032104
  4. KS and PvA acknowledge funding from the PhD student exchange program between the Max Planck Society and the Chinese Academy of Sciences and the Natural Sciences Foundation of China (Grant No. 51221264). The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement no 312483 (ESTEEM2).

 


Herbert SCHMID, Kepeng SONG, Paolo LONGO, Elisa GILARDI, Giuliano GREGORI, Kui DU, Joachim MAIER, Peter VAN AKEN (Stuttgart, Germany)
08:00 - 18:15 #6253 - MS05-810 Effects and implications of photon reabsorption phenomena on confocal micro-photoluminescence measurements in crystalline Si.
MS05-810 Effects and implications of photon reabsorption phenomena on confocal micro-photoluminescence measurements in crystalline Si.

Confocal micro-photoluminescence (PL) spectroscopy has become during the last years one of the most important tools for carrying out novel studies of advanced solar cell concepts with micron resolution. This work presents a comprehensive study about the effect of photon reabsorption in confocal micro-PL spectroscopy measurements. First, supported by theoretical calculations, we study the dependence of reabsorption phenomena on the different setups and experimental parameters, i.e. excitation wavelength, pinhole aperture, numerical aperture (NA) of focusing lenses. Second, we analyze the effects of reabsorption on the emission line-shape of the resulting micro-PL spectra. Finally, in order to prove the importance and implications of this study, we present a current and relevant application, namely the estimation of doping densities in crystalline Si (c-Si) via micro-PL, where reabsorption processes must be taken into account to avoid the misinterpretation and misquantification of the obtained micro-PL data.

Figure 1(a) shows the normalized PL emission spectra calculated with our theoretical model (based on the generalized Plank's law) for photons spontaneously emitted at different X distances from the front surface (see inset diagram). The variation in the reabsorption level is evidenced by a decrease in the PL signal between 1000 and 1100 nm. This is correlated to the increase of Si absorption coefficient at lower wavelengths (see the black dashed curve). Figure 1(b) shows confocal micro-PL spectra recorded from a p-type c-Si wafer by changing the collection point distance from the front surface, i.e. 0 (on focus), 100 and 200 µm. The measured PL spectra of Figure 1(b) are qualitatively the same than the calculated spectra of Figure 1(a), proving the consistency of our theoretical model.

When the collection point is positioned at the surface (common conditions in micro-PL measurements), the reabsorption level is dependent on the selected experimental parameters, which define the extension of the collection volume, and hence, the available photon travelling distance. This is observed in Figure 2(a) for different NA values, and in Figure 2(b) for different pinhole apertures. The higher the confocality, the lower the reabsorption.

The effect of reabsorption on the PL spectra line-shape can have important implications in some type of applications. One of these examples is the estimation of doping densities in c-Si via micro-PL spectroscopy, where the band gap shift that takes place in heavily doped c-Si can be studied at room-temperature by monitoring the center of mass of the PL spectra. Figure 3 shows the study and quantification of doping densities in a laser-doped region (LDR) in c-Si. We present data for two lenses with different NA. A picture of the studied LDR is shown in Figure 3(a), and the change in the center of mass of the PL spectra recorded along the LDR is depicted in Figure 3(b). The large shift between the two curves is linked to a change in reabsorption level. As expected, the two curves are constant outside the LDR, but the center of mass increases remarkably inside the LDR, showing, apparently, an increase in doping density. By means of the calibration curves obtained from c-Si wafers with different doping densities (see Figure 3(c)), we can transform the center of mass profiles depicted in Figure 7(b) into the doping density profiles of the LDR under study (see Figure 7(d)). Now, the two curves show a very similar and reliable doping density profile across the LDR, reaching doping levels around 7x1019 cm-3 at the very center of the processed region. This example proves the importance of considering reabsorption effects in all those micro-PL studies that are based on the analysis of the PL spectra line-shape and position.

 

Acknowledgements

This work was supported by the project HERCULES that has received funding from the European Union's Seventh Programme for Research Technological Development and Demonstration under Grant agreement no. 608498.


Abel ROIGÉ (Gif-sur-Yvette), Alexandre JAFFRÉ, José ALVAREZ, Thibaut DESRUES, Delfina MUÑOZ, Isidro MARTÍN, Ramon ALCUBILLA, Jean-Paul KLEIDER
08:00 - 18:15 #6278 - MS05-812 Progress in analysing lithium ion battery materials in the SEM.
MS05-812 Progress in analysing lithium ion battery materials in the SEM.

New and existing materials for lithium ion batteries are being studied extensively with the aim of increasing their storage capacity and lifetime. While the SEM is an important tool in the study of these materials, progress is held back by lack of techniques for characterising the distribution of Li. Detection of Li-K X-rays from metallic Li with a special windowless EDS detector was first shown by Burgess et al. [1] and using a flat-field holographic grating WDS by Terauchi et al. [2] and of Li compounds by Hovington et al. [3]. Here we show the current progress in characterising Li-ion battery materials with EDS by exploring how lithiation can be studied on graphite anodes with a windowless detector and on lithium containing ceramics using a conventional detector.

 

Graphite anodes were measured using a 100 mm2 windowless EDS detector X-Max Extreme. After charging, the graphite particles contain intercalated lithium between graphene layers. Lithium compounds also form on the surface as solid-electrolyte interphase (SEI) and lithium dendrites after charge-discharge cycles. On some particles no Li signal was detected, whilst EDS spectra from others showed a significant Li peak. Fig. 1 shows the same lithiated graphite particle before and after prolonged exposure to the electron beam. The particulate matter formed during exposure to the electron beam is clearly visible on the surface of the particle in Fig. 1b. From the EDS spectrum we can conclude that significant amounts of Li are present close to the surface of the particle based on the information depth for graphite at 3 kV. The origin of this Li is being investigated, in particular whether it is formed from Li in the graphite samples, from the surface SEI layers or from Li dendrites deposited during cell cycling and whether Li diffusion depends on the grain orientation. Importantly, the results indicate that Li is highly mobile under the influence of the electron beam and therefore any quantitative measurements have to be interpreted with caution.

 

Without measuring Li X-rays directly, it is possible to estimate the thickness of a hypothetical surface layer of lithium. To show this, we studied a sintered pellet of Li1.4Al0.4Ge1.6(PO4)3 (LAGP), which is a solid electrolyte with an ionic conductivity of 0.3 mS/cm at room temperature. A part of the pellet was contacted with lithium foil for several days, after which the lithium foil was removed before the EDS measurement. We compared EDS spectra from the sample surfaces with and without exposure to lithium based on the assumption that Li has transferred from the Li foil to the sample, forming a Li rich surface layer. Comparing the EDS spectra from the regions with and without exposure to lithium shows a distinct attenuation of the O K, Ge L, Al K and P K lines in the region contacted with lithium (Fig. 2). The height of the carbon peak which can be attributed to surface contamination is of approximately equal height in both spectra, ruling out a geometric effect such as shadowing or surface tilt. AZtec LayerProbe calculates the thickness of a hypothetical layer of lithium on LAGP based on the attenuation of X-rays emitted from inside the sample. Assuming a layer of metallic lithium (ρ=0.53 g/cm3) covers the contacted part of the sample, LayerProbe calculates that a thickness of 100-150 nm of lithium would result in the observed attenuation of the X-ray signals from Li1.4Al0.4Ge1.6(PO4)3. In contrast, LayerProbe did not detect attenuation of the O K, Ge L, Al K and P K lines for the part of the sample which had not been contacted with lithium.

 

Our results indicate the great potential of SEM/EDS for the characterisation of lithium ion battery materials. They show that while it is possible to detect Li X-rays from those materials with a specially designed EDS detector, the results may be difficult to interpret due to the mobility of Li under the electron beam. However, it is also possible to study lithiation processes indirectly, by using the attenuation that a hypothetical surface layer of lithium exerts on the X-ray emissions of other elements in the sample.

 

References:

[1] Burgess S, Li X, Holland J. 2013 Micro Anal 27:S8–S13.

[2] Terauchi M, Takahashi H, Handa N et al. 2012. J Electron Microsc 61:1–8.

[3] Hovington P, Timoshevskii V, Burgess S. et al 2016, Scanning, in print. 


Christian LANG (High Wycombe, United Kingdom), Andy NAYLOR, Felix RICHTER, Christoph BIRKL, Stefanie ZEKOLL, Simon BURGESS, Gareth HUGHES, David HOWEY, Peter G. BRUCE
08:00 - 18:15 #6326 - MS05-814 Revealing phase separation and crystallinity in small molecule solar cells using 3D electron microscopy.
MS05-814 Revealing phase separation and crystallinity in small molecule solar cells using 3D electron microscopy.

Transmission Electron Microscopy (TEM) can be utilized to understand the morphology of organic bulk heterojunction (BHJ) solar cells and thus aid in improving device performance. We have previously shown that phase separation and formation of crystallinity is to be expected during co-evaporation of small molecule BHJ layers [1]. Using Electron Spectroscopic Imaging (ESI) [2] and electron diffraction, we found a significant influence of substrate and substrate temperature on the morphology of the photoactive layer during the fabrication of F4ZnPc:C60 BHJs. Whether or not the device is fabricated as inverted [3] or non-inverted cell influences crystal growth and, thus, phase separation during film formation. We have found that heating the substrate during BHJ film formation leads to an increase in efficiency for the inverted cell, whereas the non-inverted device shows no improvement. While the ESI measurements showed that substrate heating facilitates phase separation of the two materials, the difference in efficiency of the different device architectures could not be explained by this. Electron diffraction data indicated that crystallinity plays a role here.

Since conventional ESI and electron diffraction only provide information about material distribution and crystallinity in a two-dimensional projection of the BHJ layers, high-resolution electron tomography was performed to gain insight into the three-dimensional structure. F4ZnPc:C60 was co-evaporated onto layers of neat F4ZnPc and C60, respectively. The measurements were performed under low-dose and LN2-cryo conditions in an FEI Titan Krios. This was necessary to preserve the sample, and foremost its crystallinity, since carbon-based materials, like C60, are prone to severe damage by electron irradiation.

Figure 1 shows a bright-field TEM image of the BHJ on C60 (gold fiducials, seen in black, were used for tilt-series alignment). All images of the acquired tilt-series show crystalline areas such as the ones marked (A,B,C). The crystalline spacing seen here can be identified in the power spectra as characteristic for C60 (red: 0.85 nm, green: 0.5 nm and blue: 0.44 nm). As illustrated, such crystallinity can also be visualized in high-resolution electron tomograms, albeit only for smaller volumes at quite high magnification.

To obtain a statistically significant distribution of crystallinity for different cell architecture and cell fabrication, larger volumes need to be analysed. For a given detector size, one needs to apply lower magnifications which results in lower resolution. However, the signature of pure crystals at these imaging conditions are a low variance in 3D, i.e. crystal distributions can easily be obtained from segmented 3D variance maps. A slice through the tomographic reconstruction of such samples can be seen in figure 2. Here, a BHJ film on C60 substrate is compared with a similar section through a tomogram of the BHJ on F4ZnPc. The gold fiducial indicates the top of the BHJ film. The homogeneous, aka crystalline areas are highlighted (red overlay). From the distribution of crystallinity we deduce, that large C60 crystals are found in both device architectures causing a very rough film surface. In the inverted device, these crystals can extend throughout the whole film, using the polycrystalline C60 substrate as seed for crystal growth, whereas the non-inverted BHJ showed C60 crystals starting somewhere in the middle of the film. Correlating this data with device performance, we find that C60 crystals which have grown throughout the BHJ layer are crucial for efficient devices.

References:

[1] D. Nanova, Adv. Funct. Mater. 25, 6511–6518 (2015).
[2] M. Pfannmöller et al., Nano Letters 11, 3099–3107 (2011).
[3] J. Meiss et al., Adv. Funct. Mat. 22, 405-414 (2012).
[4] This work was supported by the German Ministry of Research and Education, collaborative project „LOTsE“, grants 03EK3505L (W.K.) and 03EK3505K (R.R.S).


Anne Katrin KAST (Heidelberg, Germany), Johan ZEELEN, Lars MÜLLER, Pirmin KÜKELHAN, Diana NANOVA, Robert LOVRINCIC, Wolfgang KOWALSKY, Rasmus R. SCHRÖDER
08:00 - 18:15 #6340 - MS05-816 GENESIS Rouen : An open platform for the study and nano analysis (Atom probe, SEM Cross beam station and TEM (in situ straining, temperature, tomography) of irradiation effects in radioactive materials for nuclear application.
MS05-816 GENESIS Rouen : An open platform for the study and nano analysis (Atom probe, SEM Cross beam station and TEM (in situ straining, temperature, tomography) of irradiation effects in radioactive materials for nuclear application.

Nuclear materials undergo degradations due to neutron/matter interaction. The degradation initially occurs at atomic scale. The French GENESIS platform, divided among three sites in France: GPM Rouen, CIMAP Caen and CEA Saclay, gathers experimental nanoanalysis tools devoted to irradiated and radioactive materials. Results allow understanding, modelling, and finally simulation of the long time behaviour of the nuclear materials.

In GPM Rouen, this platform will give access to tomographic atom probe, transmission electron microscopy with in-situ experiments (heating, in-situ straining tests, tomography) and a dual beam station for sample preparation in active materials irradiated in nuclear power plants. The samples may reach an activity of 200 MBq. These characterizations will allow the extraction of reliable data necessary to develop and validate the multi-scale numerical simulation of metal, oxide, ceramic or glass materials.

Facilities in GPM Rouen are open since March 2016. The study of irradiated and radioactive materials will begin in September 2016.

Characteristics of the techniques will be described with some results that may be achieved as well as the protocol access.

 

GENESIS is supported by the Région Haute-Normandie, the Métropole Rouen Normandie, the CNRS via LABEX EMC and the French National Research Agency as a part of the program “Investissements d’avenir” with the reference ANR-11-EQPX-0020.


Auriane ETIENNE (ST ETIENNE ROUVRAY CEDEX), Philippe PAREIGE, Bertrand RADIGUET, Cristelle PAREIGE, Fabien CUVILLY, Emmanuel CADEL
08:00 - 18:15 #6342 - MS05-818 Investigation of the cathode material Li9V3(P2O7)3(PO4)2 for Li-batteries using Cs-corrected HRTEM.
MS05-818 Investigation of the cathode material Li9V3(P2O7)3(PO4)2 for Li-batteries using Cs-corrected HRTEM.

New cathode materials for Li-ion batteries are investigated assiduously to achieve higher performance in terms of energy density life time and safety. In pursuit, a polyanion-based lithium vanadium monodiphosphate, Li9V3(P2O7)3(PO4)2 [LVPP] was identified. This layered material poses distinct advantages like higher energy density, thermal stability over the more commonly used metal oxides, owing to the strong binding of the phosphate anions. Theoretically the extraction of maximum 6 Li ions per unit cell is possible, thus giving out a theoretical capacity of 173mAhg-1 through complete oxidation of vanadium from its initial +3 to final +5 state. A facile synthesis method of LVPP has been designed. To comprehend the structure and its functionality, transmission electron microscopy (TEM) with image Cs-corrector was used (FEI Titan 80-300kV), allowing atomic resolution even at lower accelerating voltages [2] (in this case at 80kV), where lower knock-on damage is expected.

To preserve as good as possible the original quality of the synthesized powder material, the TEM specimen was prepared without additional grinding using three methods: simply spread of powder particles on the carbon foil, sonicate powder in alcohol and fishing particles on the carbon foil and spread of particles, which were crashed after cooling in liquid nitrogen, on the carbon foil.

The crystallographic structure of LVPP corresponds to the trigonal symmetry (space group P-3c1) with a = b = 90°, g = 120°, a = b = 9.724 Å, c = 13.596 Å [3]. VO6 octahedra and PO4 tetrahedra, connected to the Li atoms, are organized in two layers generating a 2D ionic conductivity. Fig.1d) shows the simulated structure in [0001]- and [-2021]-projections where V atoms are red, P atoms blue, O atoms light rosa and Li atoms yellow.

HRTEM images of thin particles edges at 80 kV and 300kV are shown in Fig. 1 a-c). To get interpretable TEM micrographs usually the crystalline specimen has to be aligned during imaging in a particular pre-selected crystal orientation. However LVPP is very sensitive to the electron beam with the result that practically the crystal could not be oriented. This behaviour was found independent of the accelerating voltage of 80kV or 300kV. Fortunately, some particles had by chance a favorable crystal orientations and qualitatively interpretable TEM images were obtained (see Fig. 1). We confirmed the expected crystallographic structure for the synthesized particles by corresponding image simulations using JEMS. Moreover we found no significant volume change of the structure during electron-beam driven delithiation.

 

[1] M. Haider et al., Ultramicroscopy75 (1998), p.53.

[2] U. Kaiser et al., Ultramicroscopy 111, (2011), p.1239.

[3] Xue Miao et al., RSC Adv., 2015, 5, p. 243.

 

Acknowledgement: This work was supported by the German Ministry for Education and Research (BMBF) in the frame of the joint research project "Li-EcoSafe - Development of economical and safe lithium ion batteries".

 


Dorin GEIGER (Ulm, Germany), Prasanth BALASUBRAMANIAN, Marilena MANCINI, Peter AXMANN, Margret WOHLFAHRT-MEHRENS, Ute KAISER
08:00 - 18:15 #6366 - MS05-820 TEM characterization of irradiated beryllium.
MS05-820 TEM characterization of irradiated beryllium.

It is suggested that beryllium will be used as a neutron multiplier material in the Helium-cooled Pebble Bed (HCPB) European concept of a breeding blanket of demonstration power reactor DEMO. Long-term irradiation tests in high-neutron-flux nuclear research reactors yield information about the evolution of the microstructure of beryllium pebbles under close-to-fusion conditions (temperature, damage dose, helium and tritium production) excluding 14 MeV neutrons which are not present in the neutron spectra of fission reactors. The previous microstructural characterisation of irradiated Be performed in the course of high dose irradiation program (HIDOBE I) show the temperature depended formation of cavities inside the materials [1,2]. In the recent study was investigated beryllium pebble which was irradiated at 750°C. The target preparation of TEM specimens was performed using focused ion beam (FIB) which enables TEM investigation of defined areas such as secondary phases or grain boundaries.

Fig 1 shows a TEM image which demonstrates preferable formation of voids with sizes up to 1µm along the grain boundary. The edges of the voids correspond with crystallographic planes of the Be matrix, but because both halves of the bubble grow in two differently oriented grains, their shape is more irregular. The areas with high density of 30-70 nm large voids are located on the distance of 0.5 µm - 1.3 µm on both sides of the grain boundary. The areas of 0.5-1.5 µm thickness close to the grain boundary without any visible void can be named as void denuded zone. These zones formed because grain boundaries act as a sink for vacancies and interstitials that form in the nearest area. On the other hand, this effect promotes also the formation of large voids direct on the grain boundary. The similar effect was already observed in polycrystalline neutron irradiated tungsten.

The voids in beryllium usually show shapes of flat discs which are located in the basalt plane of hexagonal Be [1]. In the Fig. 2 (a) and (b) the same area is imaged which was tilted to the angle of ≈60° (-29° till +30° alpha tilt of goniometer). It is clearly seen that shape of the voids changes from narrow strip at -29° to the hexagonal faceted void at +32° alphq tilt. These results enable determination of the void size and thickness distribution histogram. Imaging of lamella in an oxygen map (Fig. 3) reflects the topography of beryllium surface which has been oxidized by contact with air. The formation of this thin oxide layer enables imaging of voids which got contact to the foil surface during FIB preparation. In Fig. 3a HAADF image of an area with numerous voids is presented. Be is oriented with the c axis in the image plane as it marked in the image. The open voids got a surface decoration and are visible as narrow strips. The voids which do not have contact with a foil surface remain invisible in the oxygen map without any decoration of the void’s surface with oxygen or other impurity elements. This imaging allows correct calculations of the size distribution of radiation induced voids in irradiated beryllium.

Conclusion: Observation of preferable formation of voids on the grain boundary and a void-free zone in the area next to the grain boundary. TEM imaging of the voids depend on the orientation of the image plane that varies from a regular hexagon to an elongated shape as a line segment with an intermediate state as a rectangle.

References:

[1] M. Klimenkov, et. al Journal of Nuclear Materials 455 (2014) 660–664

[2] M. Klimenkov, et. al Journal of Nuclear Materials 443, (2013) 409-416

[3] A. Hasegawa, et al. Fusion Engineering and Design 89 (2014) 1568-1572


Michael KLIMENKOV (Eggenstein-Leopoldshafen, Germany), Jan HOFFMANN, Peter KURINSKY, Viacheslav KUKSENKO, Pavel VLADIMIROV, Vladimir CHAKIN, Anton MÖSLANG
08:00 - 18:15 #4463 - MS06-822 Adiabatic shear loading in thermal spray coatings studied by EBSD.
MS06-822 Adiabatic shear loading in thermal spray coatings studied by EBSD.

A basic principle during the formation of thermal spray coatings is the phenomenon of “adiabatic shear instability” which influences splat formation, bonding or oxidation of intersplat regions, grain refinement and dynamic recrystallization as result of thermal softening gaining over work hardening upon the impact of accelerated particles of the spray feedstock. Deposits of uniform composition, low porosity and suitable bonding to the substrate are obtained by proper adjusting the window of deposition parameters with respect to the occurrence of adiabatic shear instabilities. However, a lack of knowledge on how low-ductile intermetallic compounds and hard oxide particles behave at high strain rates obtained during the thermal spray process has to be stated.

The present work shows the interfacial features of several coatings obtained at typical spray parameters. The examples include coatings formed from combinations of ductile or hard particles on hard or ductile substrates [1-4]. Electron backscatter diffraction (EBSD) was used to show the influence of adiabatic loading on the microstructural evolution. Kikuchi pattern quality maps (Fig. 1a-d) provide information about dynamic recrystallization and the occurrence of unchanged spherical particles, fragmented shells as well as particles agglomerated by sintering. EBSD kernel average misorientation (KAM) is effective in resolving grain regions subjected to deformation. KAM maps of the next and next-next neighbor pixels (Fig. 2a-d) show shear bands and dynamic recrystallization to a different degree in high-ductile aluminium (a) and low-ductile iron aluminide (b); hard layers of zirconia (c) and alumina (d) confirm expectedly marginal misorientation, but clearly indicate that the impinging particles induce different alterations of the steel substrates subjected to different pretreatments. Shear bands have been formed on the hills of the laser-cut structure; dynamic recrystallization occurred on top of the highly disturbed sandblasted surface.

 

  1. R. Drehmann et al. (2014) Investigation of bonding mechanisms of cold gas-sprayed Al coatings on Al2O3. Mater. Sci. Eng. Techn. 45 (6): 476–485.

  2. N. Cinca et al. (2015) Influence of spraying parameters on cold gas spraying of iron aluminide intermetallics. Surf. Coat. Techn. 268: 99–107.

  3. P. Sokołowski et al. (2015) Advanced microscopic study of suspension plasma sprayed zirconia coatings with different microstructures. J. Therm. Spray Techn. DOI: 10.1007/s11666-015-0310-7.

  4. T. Lampke et al. (2011) Alumina coatings obtained by thermal spraying and plasma anodizing — a comparison. Surf. Coat. Techn. 206: 2012–2016.


Dagmar DIETRICH (Chemnitz, Germany), Nuria CINCA, Pawel SOKOLOWSKI, Lech PAWLOWSKI, Josep GUILEMANY, Thomas LAMPKE
08:00 - 18:15 #4503 - MS06-824 Microstructure characterization of bioinspired multilayer coatings on advanced polymer PEEK type materials as well as on Carbon Fiber Composites (CFC).
MS06-824 Microstructure characterization of bioinspired multilayer coatings on advanced polymer PEEK type materials as well as on Carbon Fiber Composites (CFC).

The ultimate goal for a materials engineer is to learn from the lessons of nature and to apply this knowledge to new materials and design. In nature materials are often optimized to manage any damage that could occur. The occurrence of damage is expected and accepted as a fact of life. Natural materials can cope with damage due to in- built healing abilities. The ability to heal damage is one of the remarkable properties of biological systems and living organisms. One recurring goal of material development was to emulate the materials of nature. In the frame of the presented research, the coatings design was inspired by nature. The first part of the paper deals with the microstructure characterization of a natural shell. The coatings design was based on the nacre structure. The design of materials and structures inspired by nature involves special challenges not encountered before. Traditional design has followed disciplinary lines. Bioinspired design required mulitidisciplinary teams of engineers (design and materials), and scientists (biology, molecular biology, tribology, and micro-/nano- structure characterization). Given the widespread interest in the structure and mechanical properties of abalone shell, there have been some attempts to fabricate nacre like multilayered structure. Advanced hard coating concepts like multilayer coatings, multicomponent solution hardened layer materials, new metastable layer materials, nanocrystalline layer materials or superlattice films became increasingly important for wear protection under extreme and complex loads. Among these advanced coatings, the multilayer concepts seems to be the most versatile and promising with respect to properties and performance in almost all the fields of application. The main objective of the research was to improve the bio- activity and tribological properties of advanced polymer PEEK type materials (Polyetherethkethone) as well as Carbon fiber Composites (CFC). It was performed through the development of biomimetic, self- healing polymeric and ceramic multilayer coatings. Bio- engineering tests considered cell material interaction. In the initial analysis, associated with cytotoxic effect, fibroblasts were applied. The selected materials were undertaken the hemocompatibility. The analysis were executed based on the rules described in ISO 10993-4, dynamic blood-material interaction. The final analysis were focused on induced stem cells. Coatings were also subjected to complex mechanical tests based on indentation, ball- on disc wear and scratch adhesion tests. The wear test revealed the friction coefficient in the range of 0.3 up to 0.4 under 5N and 2500 cycles.Before and after bio- tribological tests the detailed microstructure/ nanostructure analysis of the coatings was performed in order to determine the tribological behavior of the coatings under the influence of a mechanical load and under biological conditions. Microstructural diagnosis was carried out using a high resolution transmission microscopy technique (HRTEM). It showed that cracking was initiated at the coating/substrate interface and the energy of brittle cracking was reduced due to the plastic deformation at each Cr interlayer interface (Fig. 1).

Keywords: multilayer coatings, wear, TEM microstructure characterization

Acknowledging to the research project which was financed by the National Science Centre (Polish- Narodowe Centrum Nauki, abbr. NCN) No:

v  2012/06/M/ST8/00408

v  2014/15/B/ST8/00103


Lukasz MAJOR (Cracow, Poland), Juergen M. LACKNER, Marta JANUSZ, Marcin KOT, Boguslaw MAJOR
08:00 - 18:15 #4550 - MS06-826 Scanning Transmission Electron Microscopy Investigation of LaAlO3/SrTiO3 Bi-Interfaces.
MS06-826 Scanning Transmission Electron Microscopy Investigation of LaAlO3/SrTiO3 Bi-Interfaces.

The interfaces between complex oxides can generate fascinating properties that are not observed in the single compounds. A significant example is the high-mobility 2-dimensional electron liquid (2DEL) detected at the interface between two good band-gap insulators, a LaAlO(LAO) thin film grown epitaxially on (001) TiO2-terminated SrTiO3 (STO) single crystal [1]. The 2DEL formation is understood in the framework of the polar catastrophe scenario for which electrons are transferred at the interface in order to minimize the built-in potential generated by the contact between the polar planes of LAO and the neutral ones of STO. According this model a fraction of Ti3+, with 3d1 configuration, should be stabilized in proximity of the interface.

LaAlO3/SrTiO3 bi-Interfaces, here discussed, are multilayer structures with a STO film and a second LAO thin film subsequently grown on the top of the first LAO thin film. Such system displays three inequivalent interfaces - two of which are conducting: LAO / STO substrate and LAO / STO film, for STO thickness ≥ 8 nm [2,3]. Our work is driven by the effort to understand the 2DEL formation at the LAO / STO film interface. For this purpose bi-interfaces with thick (12 nm ≈ 30 unit cells (uc)) and thin (6 nm) STO film were investigated and discussed in parallel. High-angle annular dark-field (HAADF) imaging as well as electron energy-loss spectroscopy (EELS) were performed in an aberration corrected Nion UltraSTEMTM Scanning Transmission Electron Microscope (STEM). The possibility to combine HAADF, an incoherent and Z-sensible technique ideal to investigate distortions and defects, and EELS, a spectroscopy capable to probe valence states with atomic spatial resolution, makes STEM a powerful tool to understand interfaces. Specifically in STO the hybridization between the 3d band of Ti and the 2p of O results in a features-rich spectroscopy.

According the HAADF images collected in STO thick bi-Interfaces, coherent growth, with no obvious defects or dislocations, was observed at the bottom and the middle interface whereas a periodic network of edge dislocations were identified at the top interface pointing out to a relaxed LAO / STO film and to a strained LAO / STO substrate interface. Ti fine structure corroborates the HAADF observations since evidences of orbital reconstruction i.e. a shift of ≈ 60 meV towards higher energy of the orbital-field edge L3-eg, are observed at the LAO / STO substrate and not at the LAO / STO film interface. Generally Ti-L2,3 fine structure is known to be a spectroscopic fingerprint of the strain state of the interfaces. Besides strain, roughness and polarity of the interfaces are key features. In order to determine the termination plane sequences, a large energy range (1.9 keV) for the EELS data was used collecting simultaneously all the meaningful edges from Ti-L2,3 (at ca. 450 eV) to Sr-L2,3 (at ca. 1950 eV). These atomically resolved elemental maps show that the insulating interface(s) is(are) the sharpest, indicating that the cation intermixing may play a role in the response of the system to the occurrence of the 2DEL.

  1. Ohtomo et al, Nature 427 (2004).

  2. Danfeng Li et al., APL Materials 2 (2014).

  3. “A/B” indicates that the material “A” is grown on the top of the material “B”.


Giulio TIERI (ANTONY), Alexandre GLOTER, Danfeng LI, Stefano GARIGLIO, Jean-Marc TRISCONE, Odile STÉPHAN
08:00 - 18:15 #5046 - MS06-828 The complex structure of catalyst particles used in fluid catalytic cracking.
MS06-828 The complex structure of catalyst particles used in fluid catalytic cracking.

The refining of crude oil by conversion of its heavy fraction into gasoline and light olefins represents an important industrial process providing a valuable source for fuel and basic chemicals. The catalyst for this reaction, the so-called fluid catalytic cracking (FCC), is a complex composite of multiple components, including zeolites, clay, and various minor additives with certain catalytic or structural functions. The many reaction and regeneration cycles during long-term operation change the structure of the FCC particles, leading to a decrease of their catalytic activity. To understand this deactivation, it is essential to derive a three-dimensional model of an individual catalyst particle, to quantify the porosity, and to identify metal deposition and other structural changes happening during the process. In addition to 3D imaging with high resolution X-ray tomography [1], we here present the results obtained with different electron microscopy methods that were applied to gain information about the local structure and the elemental distribution with high resolution.

The commercially-deactivated FCC catalyst studied here is a mixture of a La-exchanged zeolite type Y, metakaolin clay, an aluminum silicate, and comprises almost spherical particles with diameters between ca. 50 and 100 micrometer (Fig. 1a). For the investigation of the particles’ internal structure, cross-sections were prepared. The spheres were embedded in a resin and the obtained cylindrical blocks cut into slices. For TEM and STEM investigations, these slices were mechanically thinned and finally ion-milled till a hole in the center was generated [2].

SEM images of these cross-sections reveal a porous and partly fibrous interior structure that is surrounded by a dense wall (Fig. 1b) which is iron rich according to EDXS mapping, (Fig. 1c). The HAADF-STEM image (Fig. 2a) was taken at the rim of such a particle. The outermost layer at the left appears bright, dense and two to five micrometer thick. It mainly consists of silicon oxide enriched with iron (Fig. 2b-d). The loss of porosity in the outermost layer during the catalytic operation originates from the formation of a dense phase there. Furthermore, the EDXS mappings reveal areas in the inner part of the particles that contain mainly Ti, Si and Al. This finding agrees with the results of x-ray diffraction studies that identified zeolite and titanium oxide in addition to a small amount of crystalline aluminum silicate.

It can be concluded that an equilibrated FCC particle as investigated here is covered by a dense newly-formed iron-enriched silicon oxide layer while a freshly prepared, catalytically more active FCC particle has a highly porous outermost shell [3].

[1] J. C. da Silva et al. ChemCatChem 2015, 7, 413–416.

[2] E. Müller and F. Krumeich, Ultramicr. 2000, 84, 143-147.

[3] Electron microscopy was performed at the Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich.


Frank KRUMEICH (Zurich, Switzerland), Ana DIAZ, Andreas MENZEL, Mirko HOLLER, Manuel GUIZAR-SICAIROS, Dario FERREIRA SANCHEZ, Daniel GROLIMUND, Jeroen VAN BOKHOVEN, Yuying SHU, Wu-Cheng CHENG
08:00 - 18:15 #5049 - MS06-830 TEM investigations of Al/AlOx/Al Josephson junctions.
MS06-830 TEM investigations of Al/AlOx/Al Josephson junctions.

The development of fully-operational quantum computers is one of the major goals in information science. Quantum computers rely on the quantum-coherent evolution of their constituents, the quantum bits (qubits). For this purpose all sources of decoherence have to be identified, to eliminate them as far as possible, or to reduce their effect, e.g., by an optimized qubit design. Superconducting qubits based on Josephson junctions (JJs) provide the most advanced platform. Their coherence is limited by low-frequency charge fluctuations, flux noise, and critical current fluctuations (frequently with a 1/f spectrum) [1,2]. Much progress has been made in the last 15 years reducing their effect, leading to an increase of the coherence times by five orders of magnitude. Still a major source of decoherence appears to be environmental two-level systems (TLS) and other imperfections within the amorphous AlOx-layer [3,4] of Al/AlOx/Al-based JJs. Many properties of the TLS have been probed experimentally, however, identifying their true microscopic nature remains an open problem. Recent transmission electron microscopy (TEM) investigations showed the potential of these techniques for analyzing and improving the properties of JJs and qubits [5,6].

In this work Al/AlOx/Al-layer systems for JJs were analyzed in an FEI Titan³ 80-300. Different samples were fabricated by electron beam physical vapor deposition with varying oxidation parameters like, e.g., oxidation time to, oxygen pressure po, UV-enhanced oxidation and thermally enhanced oxidation. The morphology of the samples was analyzed by high-resolution TEM (HRTEM). Fig. 1 shows an AlOx-layer fabricated at room temperature with to = 12.5 min and po = 0.0145 mbar, resulting in an oxide layer with an average thickness of 1.9 nm. Fig. 1a shows a smooth oxide layer with homogenous thickness at an Al/AlOx interface of a single Al-grain. Fig. 1b shows the same layer at an Al-grain boundary in the lower electrode layer, where the oxide layer is bent illustrating the negative influence of grain boundaries. Thickness variations due to grain boundaries can also be found. Furthermore, the bonding characteristics between Al- and O-atoms were analyzed by electron energy loss spectroscopy (EELS). The electron loss near edge structure of the Al-L2,3 edge is shown in Fig. 2 for the AlOx-layer (green spectrum) and crystalline a- and g-Al2O3 reference samples (black and red spectra). The first maximum of the absorption edges is located at 77.3 eV and 80.2 eV which is characteristic for tetrahedral- and octahedral-coordinated Al-sites. By analysing the peak intensities [7] the fraction of tetrahedral-coordinated Al-sites can be determined to 40 at%. Although the AlOx-spectrum is similar to the expected spectra for amorphous Al2O3, there are some unexpected features which can be connected to medium range order (82 – 90 eV). By comparison of the data obtained from differently fabricated samples the influence of individual fabrication parameters can be analyzed to improve the qubit properties by optimization of the fabrication process.

 

References:

[1] J. Clarke, F. K. Wilhelm, Nature 453, 1031 (2008)

[2] J. Martinis et al., Phys. Rev. Lett. 95, 210503 (2005)

[3] C. Müller et al., Phys. Rev. B 80, 134517 (2009)

[4] M. Choi et al., J. Appl. Phys. 113, 044501 (2013)

[5] V. V. Roddatis et al., J. Appl. Phys. 110, 123903 (2011)

[6] L. J. Zeng et al., J. Phys. D: Appl. Phys. 48, 395308 (2015)

[7] J. Bruley et al., Microsc. Microanal. Micrustruct. 6, 1 (1995)


Stefan FRITZ (Karlsruhe, Germany), Reinhard SCHNEIDER, Lucas RADTKE, Martin WEIDES, Dagmar GERTHSEN
08:00 - 18:15 #5065 - MS06-832 Analysis of white-luminescent mesoporous carbonized silica under electron irradiation by TEM-CL system.
MS06-832 Analysis of white-luminescent mesoporous carbonized silica under electron irradiation by TEM-CL system.

     Mesoporous carbonized silica (MPCS) exhibits continuous white photoluminescence (PL) like the solar emission spectrum [Fig.1 (a)], which attracts attention as a new luminescent material, consisting of ubiquitous elements alone. The PL intensity of MPCS, however, is still insufficient for practical use and it is necessary to clarify the origin of the white luminescence to improve its emission intensity. Since MPCS has a nanoscale honeycomb structure and the emission intensity and its color rendering depend on the pore size, it was thought that the nanostructure is responsible for the light emission. In the present study we conducted TEM-cathodoluminescence (CL) experiments on MPCS in order to investigate relationship between luminescence properties and nanoscale honeycomb structure.

     An MPCS sample was ground and dispersed in ethanol, a drop of which was dripped on a microgrid. TEM-CL experiments were carried out by a Jeol JEM2100 STEM equipped with a Gatan Vulcan TEM-CL system, in which high efficiency light collecting mirrors are implemented, covering both upper and lower sides of the sample over the solid angle of as large as 7.3 steradian. Using this system, it was found that the CL spectral profile was different from that of PL, consisting of several primary peaks around 450 nm, 620 nm and 650 nm [Fig.1 (b)]. Several samples with different synthesis process and carbon content were examined at electron accelerating voltages of 200 kV and 100 kV to investigate the change in the CL spectrum by electron irradiation.

     It was found that the CL spectral profile changed with the structural change by electron irradiation at an accelerating voltage of 200 kV, as shown in Fig. 2. At the initial stage of irradiation the nano-structure of MPCS hardly changed, though the intensity of the 620-650 nm peak was quickly reduced. By prolonged irradiation, on the other hand, the spectral intensity of the intermediate wavelength region (500-600 nm) between the two primary peaks was increased, followed by the increased integrated intensity of overall spectrum as the honeycomb structure collapsed. It is thus concluded that the origin of the luminescence was attributable to generation and annihilation of point defects rather than nanoscale honeycomb structure itself.

     Electron irradiation is supposed to break chemical bonds by electronic excitations and/or displacement of host atoms through inelastic and elastic electron scattering, respectively. It is generally known that the inelastic scattering probability is increased and the knock-on damage probability is decreased with decreasing the accelerating voltage, where a threshold accelerating voltage exists for the displacement of an atom by elastic scattering (knock-on), depending on the atomic number of the displaced atom. At the accelerating voltages of 200kV and 100 kV, well above and below the threshold for Si and O (~150 kV) in SiO2 respectively, the intensity of 620-650 nm peak was decreased both in case of 100 kV 200 kV, though its rate was larger at 100 kV than that at 200 kV, as shown in Fig.3. This suggests that the defect specie giving rise to the 620-650 nm peak should collapse by an electronic excitation effect rather than knock-on damage. Besides, the increase in the intensity the intermediate wavelength region observed in Fig. 2 was not observed at 100 kV. It is hence presumed that the origin of the emission in the intermediate wavelength region should be the generation of a defect related to oxygen deficiency by knock-on damage.

     Moreover, in order to confirm how carbon incorporated affects the emission spectrum of MPCS, CL spectrum of a non-carbonized mesoporous silica (MPS), having the same honeycomb structure as MPCS was acquired, as shown in Fig.4. The CL spectral profile of MPS hardly differed from that of MPCS. It has not become clear how the carbon in MPCS affects the light emission.

     The generation and annihilation of point defects in MPCS by electron irradiation should be modeled by a set of reaction rate equations on the basis of the present observations, which is reported in further detail.

 

[1]K. Sato, Y. Ishikawa, Y. Ishii, S. Kawasaki, S. Muto, and Y. Yamamoto: Jpn. J. Appl. Phys. 51 (2012) 082402

[2]T. Yoshino, 1994. Irradiation Effect in Materials. (SHOKABO Tokyo, 1994), p. 25-26


Kazuki OGUNI (Nagoya, Japan), Shunsuke MUTO, Yukari ISHIKAWA, Koji SATO, Yosuke ISHII, Shinji KAWASAKI
08:00 - 18:15 #5090 - MS06-834 Convergent-Beam EMCD: Benefits, Pitfalls, and Applications.
MS06-834 Convergent-Beam EMCD: Benefits, Pitfalls, and Applications.

Energy-loss magnetic chiral dichroism (EMCD) [1] is a state-of-the-art technique to measure magnetic properties on the nanoscale using TEM and EELS. Since its first experimental realization a decade ago, it has seen tremendous progress and many applications, including the analysis of phase transitions [2] or of magnetic nanoparticles [3,4].

EMCD exploits the spin-orbit interaction in the sample that gives rise to different probabilities for the transfer of ±1 ħ of orbital angular momentum (OAM) to the probe beam. The net OAM of the probe beam is then measured interferometrically in the diffraction plane. The classical EMCD approach uses an incident plane wave and a “point-like” detector placed on the Thales circle through two diffraction spots. This, however, has two major shortcomings: on the one hand, it limits the best achievable spatial resolution to the size of the selected area aperture; on the other hand, it places the detector far off-axis where the intensity is very low. Thus, it is difficult or even impossible to use for many applications, including many application-relevant cases such as nanoparticles, interfaces, defects, or beam-sensitive materials where long exposure times are not possible.

Here, we investigate the benefits of using a convergent incident beam for EMCD [5,6] by simulating the EMCD effect using the multislice algorithm [7] together with the mixed dynamic form factor (MDFF) approach [8] for a 10 nm Fe sample oriented in a systematic row condition including the (2 0 0) diffraction spot and an incident beam energy of 300 kV. As shown in fig. 1 for an Fe model system, a visible EMCD signal occurs up to large convergence angles as used in contemporary, Cs-corrected STEMs. However, the region of large EMCD signal is “pushed out” away from the Thales circle towards the rim of the diffraction disks with increasing convergence angles. This is natural as the overall effect is expected to average out inside the diffraction disks. At the same time, it gives a good rule of thumb regarding the optimal position of the detector.

In addition, we investigate the signal-to-noise ratio (SNR) as a function of convergence and collection angles. For practical applications, the SNR actually plays a much more important role than the theoretical expected EMCD effect as even a high EMCD signal is useless if it is well below the noise level. As shown in fig. 2, the SNR is highest for medium convergence and collection angles of the order of the Bragg angle (~7 mrad for the present model system), while the EMCD effect under these conditions is only slightly smaller than for the “classical” case of small convergence and collection angles and a detector positioned on the Thales circle. Thus, it is not necessary (and, indeed, counter-productive) to use a “point-like” detector and parallel illumination which severely limits the recorded intensity as well as the spatial resolution.

Our analysis shows that convergent beam EMCD is not only possible, but actually is superior to classical EMCD in several aspects – most notably spatial resolution and SNR. This makes it the ideal tool for characterizing magnetic properties on the nanoscale, including the technologically relevant question of how the magnetic behavior changes at interfaces.

 

Acknowledgements: Financial support by the Austrian Science Fund (FWF) under grant nr. J3732‑N27 is gratefully acknowledged.

 

[1] Schattschneider et al., Nature 441 (2006) 486
[2] Rubino et al., J. Mater. Res. 23 (2008) 2582
[3] Schattschneider et al., J. Appl. Phys. 107 (2010) 09D311
[4] Stöger-Pollach et al., Micron 42 (2011) 456
[5] Schattschneider et al., Phys. Rev. B 78 (2008) 104413
[6] Löffler and Hetaba, submitted
[7] Kirkland, Plenum Press 1998
[8] Löffler et al., Ultramicroscopy 131 (2013) 39


Stefan LÖFFLER (Wien, Austria), Walid HETABA
08:00 - 18:15 #5226 - MS06-836 TEM/STEM investigation of the crystallization kinetics of GeTe and Ag4In3Sb67Te26.
MS06-836 TEM/STEM investigation of the crystallization kinetics of GeTe and Ag4In3Sb67Te26.

Phase-change materials are promising candidates for non-volatile electronic memory applications already successful established as rewritable optical data storage (Raoux, 2009; Meinders et al., 2006). The writing speed and the stability of the phase-change data storage is mainly defined by the crystallization kinetics. Therefore knowledge of the crystallization kinetics of phase-change materials is mandatory. Lee and coworkers have demonstrated by fluctuation electron microscopy (FEM) that the crystal nucleation of Ge2S2bTe5 and AgInSbTe is influenced by their medium range order (MRO) of the amorphous phase (Lee et al., 2014). In our work we measured the crystal growth velocities by brightfield transmission electron microscopy (TEM) and we measured the variance of the diffracted intensity as measure of the MRO by FEM to prove a relation between the MRO and the growth velocity of the investigated phase-change materials. The measurements of the growth velocities and the MRO were done for GeTe and Ag4In3Sb67Te26 (AIST) in the amorphous sputtered as‑deposited and in the amorphous melt‑quenched state (Figure 1 and Figure 2).

The 30 nm thick amorphous GeTe or AIST layer is embedded in a supporting layer stack on a 500 µm thick silicon substrate (Figure 2). The 100 nm thick ZnS‑SiO2 capping layer on top of the AIST layer prevents oxidation. The 10 nm thick ZnS‑SiO2 layer below the phase-change material layer decreases together with the capping layer the necessary power to melt the phase-change material layer by laser irradiation. The 50 nm thick Si3N4 layer is an etch stop needed to prepare the TEM specimens by etching. The silicon substrate delivers sufficient heat dissipation for melt quenching. The GeTe or AIST layer is either investigated in the amorphous as-deposited or in the amorphous melt‑quenched state. Melt-quenched amorphous marks are produced by cooling a laser molten area of the phase-change layer rapidly back to room temperature.

The growth velocities were measured by brightfield TEM in a FEI Tecnai F20 at 200 kV. Electron transparent specimens were prepared by mechanical grinding, dimple grinding and etching with KOH from the layer stack described above. In this process parts of the Si substrates were removed to leave few hundred µm in diameter wide homogenously thick electron transparent windows. The specimens were alternated between heating in a heating furnace of a differential scanning calorimeter or heating in an oil bath for higher temperatures and the TEM to measure the size of the imaged grains. The measured grains were fitted by a circle. From a linear fit of the increase in radius of the fitted circles the growth velocities were calculated.

FEM measurements were conducted in a FEI Titan dedicated to scanning transmission electron microscopy (STEM) at 300 kV (Heggen et al., 2016). A coherent almost parallel around 2 nm in diameter sized electron probe was used to generate the 500-1000 nano area electron diffraction patterns per specimen used for the FEM analysis. The measured TEM specimens of as‑deposited and melt‑quenched GeTe and AIST were prepared from the layer stack described above as cross section lamellas by focused ion beam (FIB). The lamellas were produced in a FEI Helios dual beam scanning electron microscope/FIB system.

 

References

Heggen, M., Luysberg, M. & Tillmann, K. 2016 Journal of large-scale research facilities JLSRF, 2.

Lee, B.-S., Shelby, R. M., Raoux, S., Retter, C. T., Burr, G. W., Bogle, S. N., Darmawikarta, K., Bishop, S. G. & Abelson, J. R. 2014 J Appl Phys, 115, 063506.

Meinders, E. R., Mijiritskii, A. V., Pieterson, L. & Wuttig, M. 2006 Optical data storage : phase-change media and recording, Dordrecht: Springer.

Raoux, S. 2009 Phase change materials : science and applications, New York, NY: Springer.

 

Acknowledgesments

We kindly acknowledge the funding from the DFG in the framework of the SFB 917 “Nanoswitches”.


Manuel BORNHÖFFT (Aachen, Germany), Julia BENKE, Julian PRIES, Paul M. VOYLES, Matthias WUTTIG, Joachim MAYER
08:00 - 18:15 #5363 - MS06-838 Hollow carbon spheres as ideal supports for hydrogen spillover studies using ruthenium and cobalt nanoparticles.
MS06-838 Hollow carbon spheres as ideal supports for hydrogen spillover studies using ruthenium and cobalt nanoparticles.

Introduction

Cobalt Fischer-Tropsch Synthesis catalysts are generally doped with small amounts of noble metals that serve as reduction promoters to enhance the catalytic activity of the cobalt active sites [1]. This is because the promoter metals are able to dissociate hydrogen gas at a low temperature which then also lowers the reduction temperature of the cobalt oxide to cobalt. This then prevents easy deactivation of the cobalt catalysts that can be induced by high temperature activation. Hydrogen spillover has been invoked to explain the observed effect of these metals as promoters on cobalt catalysts. Two types of hydrogen spillover processes can be envisaged; (1) primary hydrogen spillover, whereby the promoter [i.e., initiator] is in contact with the cobalt oxide [i.e, acceptor] and the dissociated hydrogen atoms can move from the initiator through the direct interface to interact with the acceptor and (2) secondary hydrogen spillover, in this process the initiator and the acceptor materials are separated by some distance and hydrogen spillover can only happen by the dissociation of the hydrogen molecule on the initiator followed by a migration of the atomic hydrogen on a carrier (or catalyst support) to the acceptor material [i.e., cobalt oxide] [2]. Few model catalysts exist that can provide direct evidence of the existence of a type of hydrogen spillover that is dominant on Fischer-Tropsch like catalysts. In this study mesoporous hollow carbon spheres (MHCS) were used as model supports to study whether both the primary and secondary hydrogen spillover were prominent during catalyst activation and Fischer-Tropsch synthesis.

Experimental

MHCS were prepared as shown in Fig 1 (a). Three Co catalysts (15% loading) were prepared (1) Ru@MHCS@Co, with Ru nanoparticles and Co nanoparticles separated by the carbon shell, (2) CoRu/MHCS, Ru and Co co-precipitated outside MHCS and (3) Co/MHCS, Co outside MHCS. Materials were thoroughly characterized using electron microscopy before being tested under Fischer-Tropsch conditions at 220 oC and 10 bar.

Results and Discussion

Scanning electron microscopy (SEM) analysis of the silica template and hollow carbon spheres gave respective average sizes of 340 nm and 290 nm, thus showing that the silica spheres shrunk as they were heated up to 900 oC before the carbonization process (Fig 1(b,c) and Fig 2 (a,b)). The resulting hollow carbon spheres retained their spherical nature hence showing no significant breakage of the MHCS. Transmission electron microscopy (TEM) analysis of the materials showed that indeed the spheres were hollow and they had Ru nanoparticles with an average size of 4.1 nm embedded on its walls (Fig 1(e) and Fig 2 (c)). The loaded Co nanoparticles had an average particles size of approximately 5.9 nm on all three catalysts (Fig 1(f) and Fig 2 (d)). MHCS show a distinct roughness under TEM imaging suggesting high porosity of the materials which is necessary to allow reactants to access the encapsulated Ru nanoparticles. TEM tilting over a single axis proved that all the Ru nanoparticles are encapsulated inside the MHCS. Loading of Co nanoparticles outside the MHCS allowed for decoupling of the spillover effects from those that require direct Ru and Co direct contact. Electron Probe Micro-Analysis (EPMA) large area mapping analysis proved that the metal nanoparticles are well dispersed on the MHCS and thus was ideal materials to study the spillover process (Fig 3). The Fischer-Tropsch catalytic reaction of the three catalysts was compared and gave a Co time yield in terms of carbon monoxide and hydrogen conversion to hydrocarbons as follows; CoRu/MHCS > Ru@MHCS@Co  Co/MHCS. Electron microscopy has therefore helped in following the preparation of a functional material where the promoter effects of Ru using MHCS could be evaluated. I was also observed that a close proximity of Ru and Co nanoparticles was vital for an improved catalytic performance when compared to the case where the Ru and Co nanoparticles were separated by a potential hydrogen transporting material.

References

[1] Beaumont, S.K.; Alayoglu, S; Specht, C; Michalak, W.D; Pushkarev, V.V; Guo, J; Kruse, N; and Somorjai, G.A. Journal of the American Chemical Society, 2014, 136:28, 9898-9901.

[2] Conner Jr, W.C; and John L. Falconer J.L. Chemical reviews, 1995, 95:3, 759-788.


Tumelo PHAAHLAMOHLAKA (Johannesburg, South Africa), Linda JEWELL, Neil COVILLE
08:00 - 18:15 #5394 - MS06-840 Visualization of 2-dimensional potential map in multilayer organic electroluminescence materials by phase-shifting electron holography.
MS06-840 Visualization of 2-dimensional potential map in multilayer organic electroluminescence materials by phase-shifting electron holography.

Electron holography (EH) is a TEM method which can quantitatively measure electromagnetic fields of various samples [1-3]. In this study, we tried observing a local two-dimensional electric distribution, formed in multilayer organic electroluminescence (OEL) quantitatively with phase-shifting EH [4-5] by HF-3300EH Cold-FE TEM operated at 300 kV equipped with multiple biprism system.
An OEL multilayer sample (CuPc/α-NPD/Rubrene/Alq3/LiF/Ag) was fabricated on a Si substrate using a vacuum evaporation method. Each layer’s thickness and the surface morphology of the OEL multilayer sample were evaluated by X-Ray Reflectometry (XRR) and Atomic Force Microscopy (AFM), respectively.
The sample for phase-shifting EH observation was fabricated by focused ion beam (FIB) technique. A part of the multilayer sample, which formed a multilayer structure on a Si substrate, was picked up by the microsampling technique and was fixed onto a W deposition on the mesh for TEM observation. Then, a thin film sample of thickness 450 nm was fabricated by FIB processing. Generally, the OEL sample is vulnerable to water, and the structure may change in quality in reaction to atmospheric water vapor, depending on the formed materials. Therefore, in this study, after thin film processing, contact with the atmosphere was prevented by using an air protection mesh holder.
Figure 1 shows a TEM image, and a hologram by double-biprism EH technique [6]. The model structure of an OEL multilayer is inserted in the TEM image. Sample thickness was about 450 nm. From the TEM image, some contrast is observed in the position of the CuPc layer, but we cannot confirm the image contrast corresponding to the other layers. 
Figure 2 shows the result of the visualization of the 2-dimensional potential map of an OEL multilayer. We can clearly observe the contrast of the OEL multilayer, as shown in the reconstruction image (Figure 2b). Because the inner potential of the materials in each layer is constant, we can consider phase shifts to be an electric potential change in the sample. In the future, we will investigate the accuracy of this experimental data by comparing our data with a simulation.

References

[1] Z. Wang et al., Appl. Phys. Lett., 80 (2002) 246.
[2] T. Hirayama et al., Appl. Phys. Lett., 63 (1993) 418.
[3] K. Yamamoto et al., Angewandte Chemie., 49 (2010) 4414.
[4] Q. Ru et al., Appl. Phys. Lett., 59 (1991) 2372.
[5] Q. Ru et al., Ultramicroscopy, 55 (1994) 209.
[6] K. Harada et al., Journal of Electron Microscopy, 54 (2005) 19.


Takeshi SATO (Hitachinaka-shi, Japan), Kazuo YAMAMOTO, Miki TSUCHIYA, Katsuji ITO, Noriyuki YOSHIMOTO, Yoshifumi TANIGUCHI
08:00 - 18:15 #5399 - MS06-842 Heterogeneous interfacial chemical nature and bonds in an Al matrix composite reinforced with W-coated diamond particles.
MS06-842 Heterogeneous interfacial chemical nature and bonds in an Al matrix composite reinforced with W-coated diamond particles.

Over the last decade, a diamond particle reinforced Al matrix, namely diamond/Al, composite has been developed for thermal management applications, mainly in the microelectronic industry. This composite has demonstrated an excellent combination of a Thermal Conductivity (TC) as high as 600 W/mK and a Coefficient of Thermal Expansion (CTE) lower than 10 ppm/K, being compatible with that of electronic components. However, considering the TC (~ 1800 W/mK) of the diamond particles incorporated, it is clear that the overall TC enhancement has not been completely exploited in the composite. One then points at the diamond/Al interface, which should provide good adhesion as well as maximal Interfacial Thermal Conductance (ITC) in order to facilitate thermal exchange across the diamond/Al interface.

Our analytical modelling has recently predicted that introduction of a W interface nanolayer is one of the most efficient ways to achieve high ITC, which provides a practical guide for interface engineering. Accordingly, a cost-effective sol-gel process has been tentatively used to deposit the W coating for diamond surface metallization. Compared with the diamond/Al counterpart, TC of the composite with such a W nanolayer improved more than 20 %. In this work, Scanning Transmission Electron Microscopy (STEM)/Energy-Dispersive X-ray spectroscopy (EDX) and Precession Electron Diffraction (PED) have been performed in order to investigate interface configurations of a W-coated diamond/Al composite. The aim is to study the effect of interface formation, reaction and diffusion on the ITC.

The results indicate that the deposited W coating is discontinuous, and consists of nanoparticles with a size in the range 30-400 nm and homogenously covering the surface of the diamond particle (Figs. 1a and 1b). The average coating thickness is estimated to be around 200 nm (Fig. 1c). A STEM/ADF image in Fig. 1d shows that the formed diamond/Al interface has a heterogeneous configuration at the nanoscale where Al grain contrasts and a particle with high Z contrast are revealed. STEM/EDX mapping in Fig. 2 displays a W and Al rich interfacial particle. Visible O traces can be related to fine microstructural features. Alternatively, a 'clean' diamond/Al interface is tightly-adhered and is not rich in O (Fig. 3). As shown in Fig. 4, PED zone-axis patterns recorded from the W and Al rich particles are indexed to be the Al12W phase (Cubic, I2/m-3, No. 204). Such different chemical nature of the bonds at the interface can have a pronounced effect on the local ITC [1,2].

References:

[1] Ji, G., Tan, Z.Q., Shabadi, R., Li, Z.Q., Grünewald, W., Addad, A., Schryvers, D., Zhang, D. Materials Characterization, 89, 132-137 (2014)

[2] Ji, G., Tan, Z.Q., Lu, Y.G., Schryvers, D., Li, Z.Q., Zhang, D. Materials Characterization, 112, 129-133 (2016)


Ji GANG (Villeneuve d'Ascq), Tan ZHANQIU, Lu YINGGANG, Schryvers DOMINIQUE, Li ZHIQIANG
08:00 - 18:15 #5503 - MS06-844 Novel MAX resembling Phase Mo2Ga2C.
MS06-844 Novel MAX resembling Phase Mo2Ga2C.

MAX phases are layered transition metal carbides/nitrides with specific properties e.g. electrical/heat conductivity and machinability as metals. The MAX phases have hexagonal structure with a general Mn+1AXn formula, where M is an early transition metal, A is an A-group element (mostly IIIA and IVA) and X is either C and/or N1,2. Their unit cells consist of M6X octahedral, interleaved by layers of A elements. The difference between them is in the number of M layers separated by the A element layers: 2 layers for 211 structure, 3 layers for 312 and 4 layers for 413. All MAX phases reported to date have only one A layer. So far more than 70 ternary Mn+1AXn phases were found2. The MAX family, however, is still being extended as new phases are discovered and/or by creating quaternary and higher, solid solutions. While challenges with their crystal structural determination are addressed,3,4 also exploration is made for phases that do not follow the traditional Mn+1AXn chemistry. In this work, we present a novel crystal structure: Mo2Ga2C.5

     EDX measurement revealed that the Mo/Ga ratio of the material is approximately 1, which implies a new phase. The detailed atomic structure of the new phase was characterized with analytical high resolution scanning electron microscopy. Both Z-contrast images and EDX mappings along [11-20] and [10-10] demonstrate that this phase is layered with two Ga layers separating each 2 Mo layers. Thus, a new structure is found, which has hexagonal structure with lattice parameters of a=3.034Å, c=18.081Å and space group of P63/mmc (194). The structure does not follow the MAX formula Mn+1AXn, but is related to Mo2GaC. It can be described as a Ga pair replacing Ga in Mo2GaC structure, shifted in opposite directions along the c axis, positioned exactly on top of each other and at 4f positions instead of 2c positions. The final detailed atomic coordinates are obtained by using XRD Rietveld refinement and are listed in Table 1. The highlight of the work is not only finding a new phase, but also giving directions for a potential new family of related structures, on par with the MAX phases.

 

Refs.

1. M. W. Barsoum and M. Radovic, Annu. Rev. Mater. Res. 41, 195 (2011).

2. P Eklund, M Beckers, U Jansson, H Högberg, and L Hultman, Thin Solid Films 518, 1851 (2010).

3. Z Liu, E Wu, J Wang, Y Qian, H Xiang, X Li, Q Jin, G Sun, X Chen, J Wang, and M Li, Acta Mater. 73, 186 (2014).

4. B Anasori, M Dahlqvist, J Halim, EJ Moon, J Lu, B. Hosler, EN Caspi, SJ May, L Hultman, P Eklund,

Johanna Rosen and MW Barsoum, J of Appl. Phys. 118 (2015) 094304.

5. C Hu, CC Lai, QZ Tao, J Lu, J Halim, L Sun, J Zhang, J Yang, B Anasori, J Wang, Y Sakka, L Hultman, P Eklund, J Rosen, and MW.Barsoum, Chem. Commun. 51, (2015) 6560.

 

We acknowledge support from the Swedish Research Council (project grants #621-2011-4420, 642-2013-8020, and 621-2014-4890), the Swedish Foundation for Strategic Research through the Synergy Grant FUNCASE Functional Carbides for Advanced Surface Engineering (CC L., J R., P E., MW B., J H.), the Future Research Leaders 5 Program (P E., J L.), and the ERC Grant agreement [no. 258509] (J R.). The Knut and Alice Wallenberg Foundation supported the Electron Microscopy Laboratory at Linköping University operated by the Thin Film Physics Division.


Jun LU (Linköping, Sweden), Chunfeng HU, Chung-Chuan LAI, Quanzheng TAO, Joseph HALIM, Lars HULTMAN, Per EKLUND, Johanna ROSEN, Michel BARSOUM
08:00 - 18:15 #5728 - MS06-846 Nanograins in polycrystalline ferroelectric Hf0.5Zr0.5O2 films on Si substrate.
MS06-846 Nanograins in polycrystalline ferroelectric Hf0.5Zr0.5O2 films on Si substrate.

Structure of ultrathin ferroelectric Hf0.5Zr0.5O2 films in composites TiN/Hf0.5Zr0.5O2/Si were investigated by TEM, HRTEM, electron diffraction and X-ray EDS in a FEI Tecnai Osiris microscope (200 kV X-FEG field emission gun, 1.2 mm spherical aberration, 1.2 mm chromatic aberration, 0.24 nm resolution at Scherzer defocus and 0.14 nm information limit) and a JEOL ARM 200 F Cold FEG (200 kV, CEOS hexapole type aberration corrector, 0.075 nm resolution at Scherzer defocus and 0.046 nm information limit, -2 mm spherical aberration). JEMS [1] simulation of HRTEM images, TED patterns and HRTEM diffractograms were performed for the phase analysis of the nanograins in the Hf0.5Zr0.5O2polycrystalline film.

Ultrathin Hf0.5Zr0.5O2 films (2.5-2.7 nm thick) were grown by the Atomic Layer Deposition (ALD) technique on the highly doped n-type Si (r=0.005 Ohm×cm) substrates and they possess ferroelectric properties [2]. SAED patterns showed that the polycrystalline film contains grains with monoclinic structure (sp.gr. P21/c) and orthorhombic structure (sp.gr. Pbc21). Ferroelectric behavior was assigned to the presence of the grains with the orthorhombic non- centrosymmetric space group Pbc21.

HRTEM and HRTEM image simulation was done in order to distinguish monoclinic grains from the orthorhombic non-centrosymmetric ones and to find possible mutual orientations for better understanding the growth mechanism. The lattice parameters of both phases are very close and the presence of the orthorhombic phase in the mixture can be revealed due to the 111 ring (d111o=0.296 nm) in the SAED pattern (Fig.1b) which locates between -111 and 111monoclinic reflections (d-111m= 0.316 nm, d111m=0.284 nm). High resolution JEOL ARM 200 F Cold FEG microscope with the Cs corrector allowed us to obtain good-quality HRTEM diffractograms and to find grains with different structures and orientations.

In ultrathin Hf0.5Zr0.5O2 films, adjacent grains of 15-25 nm in size (Fig.1c) with monoclinic and orthorhombic structures are overlapped and nevertheless reveal crystallographic relationships. The monoclinic grain has the zone axis close to [-11 52] which is parallel to the [112] zone axis of the orthorhombic grain (Fig.2) and the monoclinic (13-2) plane is parallel to the orthorhombic (-311) plane creating 6.5 % lattice misfit. The overlapped area of about 5 nm wide is between two grains where two crystals of approximately 1.0 – 1.5 nm thick are superimposed. The HRTEM simulation and structure modeling performed for the conditions used showed good agreement with the experimental HRTEM images (Fig.2).

[1] P.Stadelmann. Java Electron Microscopy Software (JEMS). http://www.jems-saas.ch/.

[2]A.Chernikova, et al.(2016) ACS Applied Materials & Interfaces, DOI: 10.1021/acsami.5b11653.


Elena SUVOROVA (Moscow, Russia), Anna CHERNIKOVA, Sergei ZARUBIN, Andrei ZENKEVICH, Christian RICOLLEAU
08:00 - 18:15 #5743 - MS06-848 Structural investigation, corrosion properties and adhesion behavior of magnetron-sputtered nanocomposite TiC/a:C thin film coatings.
MS06-848 Structural investigation, corrosion properties and adhesion behavior of magnetron-sputtered nanocomposite TiC/a:C thin film coatings.

Due to the continuous rise in living standards, the continued growth of the world population and the development of medical science an ever-increasing need for materials especially suited for bio-implant applications. In case of an implant material a wide variety of parameter (adhesion, corrosion, structural, biological and mechanical properties) to must be considered.  Thus, a TiC / amorphous C (TiC/a:C) nanocomposite thin film as bio-coating was developed by simultaneously depositing Ti and C on TiAl6V4 and Titanium wafers (blasted and unblasted) using DC magnetron sputtering system at room temperature to improve the aforementioned properties. Furthermore, in order to achieve the higher osseointegration, the TiC/a:C thin film was coated with ~500-600 nm thick biogenic HAp coating by electrospraying. The relationship between the structural, mechanical, adhesion and corrosion properties of TiC/a:C nanocomposite thin film was investigated. The film’s composition and morphology were studied by Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD). In all cases, combination of columnar TiC nanostructure and thin amorphous carbon was showed (Fig.1a). In the case of Ti content above ~ 18 at%, the presence of the fcc TiC nanocrystals were confirmed by electron diffraction (Fig. 1b.) and by XRD (Fig.2). The mechanical characteristics of the thin film were investigated by nanoindentation technique while the simulated body fluid (SBF) was developed initially to evaluate the surface structural changes of the film. The applied pH was 7.25. The potential changes in the surface structure of the thin film were investigated by Scanning Electron Microscope (SEM) which does not show special structure change in the film’s surface after the corrosion test and the calcination at 900 °C (Fig. 3). The highest hardness ~ 26 GPa and modulus of elasticity ~ 140 GPa was observed in case of the film prepared at ~ 40 at% Ti content which consisted of ~ 20 nm width TiC columns separated by 2-3 nm thin a:C layers. Overall, the results demonstrated that the best choice for a protective nanocomposite coating would be the TiC/a:C thin film with ~60 at% a:C and ~40 at% Ti contents which is sufficiently corrosion-resistant, hydrophobic, nontoxic, biocompatible and mechanically stable.

 

Acknowledgments

Nikolett Oláh thanks to Young Research Fellowship of Hungarian Academy of Sciences (FIKU) for the support. Authors thank to Levente Illés and Endre Zsolt Horváth from MTA EK for the SEM and XRD measurements.


Nikolett OLÁH (Budapest, Hungary), Zsolt FOGARASSY, Attila SULYOK, Eszter SÁFRÁN, Mónika FURKÓ, Tamás CSANÁDI, Katalin BALÁZSI
08:00 - 18:15 #5823 - MS06-850 Scanning electron microscopy study of platinum catalyst gauzes treated in air, ammonia and NH3 in air.
MS06-850 Scanning electron microscopy study of platinum catalyst gauzes treated in air, ammonia and NH3 in air.

     Ammonia oxidation with air on platinum catalyst gauzes is widely used in chemical industry for synthesis of nitric acid. It is well known that during this process the gauzes undergo deep structural rearrangement of surface layers (catalytic etching) leading to a loss of platinum and decrease of catalytic activity. To elucidate the contributions of individual reactions of О2 and NH3 with the platinum surface in the catalytic etching of platinum catalyst gauzes during the NH3 oxidation, we carried out detailed investigation of the surface microstructure of platinum catalyst gauzes treated in air, ammonia, and in the reaction medium (NH3+O2). The platinum catalyst gauzes used in the study were made from a polycrystalline wire with d ≈ 82 μm, with the chemical composition  (in wt.%) 81% Pt, 15% Pd, 3.5% Rh and 0.5% Ru. A laboratory flow reactor made of a quartz tube with the inner diameter of 11.2 mm was used at the feed (ca.10% NH3 in air) flow rate 880–890 l/h, the gauze temperature 860±5 °C and total pressure ca.3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM-6460 LV (Jeol) in the modes of secondary electrons (SE) and backscattered electrons (BSE) at beam energy 20 keV.

     Different microstructure of the polycrystalline catalyst wires was observed by SEM after the treatment of the platinum gauzes at T ≈ 860 °C for 50 h in air, ammonia and the reaction medium (ca.10% NH3 in air). A grid structure consisting of dark bands with the width 0.5–1.0 μm located along the grain boundaries separating the surface of bright grains was formed on the surface of the wires during the interaction of platinum gauzes with air (Fig. 1a,b). Increased concentrations of Rh, C and O (22.1, 31.6, 40.1 at.%, respectively) were observed in the dark areas. For comparison, the concentrations of these elements in the bright areas were 5.3, 0.3 and 13.7 at.%, respectively. The grid structure was registered on the wire surface by both the SE and BSE modes. This indicates that such the structure is spreading deep into the bulk of the wire because the depths of the analysis are significantly different in these modes, ≤10 and ~300–500 nm, correspondingly. The grid structure is formed during decomposition of the surface film including the graphite-like layer on the Rh2O3 oxide film covering the surface of the grains and grain boundaries in the bulk of the polycrystalline wire. The surface films are first removed from the surface of the grains and then from the grain boundaries as a result of carbon atom diffusion from these areas to the surface.

     A micrograin structure with 1–30 μm grains was formed on the wires surface during the reaction of NH3 with the platinum catalyst gauzes (Fig. 2a,b). No carbon was observed on the surface of this structure. Apparently, the reaction of ammonia with the carbon and oxide films is very fast leading to the destruction of the surface films on the grain surface and in the grain boundary areas. Fast destruction of the graphite-like and oxide films in the grain boundaries leads to growing the grains to ca.30 μm. A dramatic structural transformation of the surface layer of platinum catalyst gauzes (catalytic etching) with the formation of a rough layer containing microcrystals and porous aggregates with the size as large as 10–20 μm separated by extended voids with the width ca.1–10 μm occurs during the catalytic NH3 oxidation with air (Fig. 3a,b). At T ≈ 860 °C the reaction of gaseous NH3 molecules with Oads atoms at the grain boundaries and other surface defects with the formation of gaseous NO leads to a sufficient local overheating of the surface whith initiates the release of metal atoms on the surface. Pits, pores and facets grow on the surface due to these processes. An intense release of metal atoms from the grain boundaries results in the formation of extended voids between the grains, which are gradually reconstructed into faceted crystalline agglomerates with through pores formed during the growing and merging of pits. A prolonged occurrence of these processes leads to the formation a rough corrosion layer on the wire surface in the platinum catalyst gauzes (Fig. 3a,b).

Acknowledgement

This work was supported by Russian Academy of Sciences and Federal Agency of Scientific Organizations (project 44.1.17).


Aleksei SALANOV (Novosibirsk, Russia), Evgenii SUPRUN, Alexandra SERKOVA, Olga SIDELNIKOVA, Elena SUTORMINA, Lyubov ISUPOVA, Valentin PARMON
08:00 - 18:15 #5827 - MS06-852 Three-dimensional core-shell ferromagnetic nanowires fabricated by focused electron beam induced deposition.
MS06-852 Three-dimensional core-shell ferromagnetic nanowires fabricated by focused electron beam induced deposition.

Functional nanostructured materials often rely on the combination of more than one material to confer the desired functionality or an enhanced performance of the devices. One of the challenges for Focused Electron Beam Induced Deposition (FEBID) technology is the fabrication of three-dimensional (3D) heterogeneous nanostructures which can be applied in magnetic memories, logic and sensing [1]. A novel procedure to create nanoscale heterostructured materials in the form of 3D core-shell nanowires by FEBID technologies has been developed. This new approach has been applied to synthesize standing nanowires with cylindrical metallic ferromagnetic cores of Co- and Fe-FEBID (less than 100 nm thick) coated with a protective Pt-FEBID shell ranging 10-20 nm of uniform thickness, using Co2(CO)8, Fe2(CO)9 and CH3CpPt(CH3)3 precursor gases. This architecture aims at minimizing the degradation of magnetic properties caused by the natural surface oxidation of the core to a non-ferromagnetic material. This is a key issue in such thin ferromagnetic objects with a high surface-to-volume ratio [2].

The structure, chemistry and magnetism of FEBID nanowires with cores of Co and Fe have been characterized in Pt-coated and uncoated nanostructures. Scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) experiments have revealed that the surface oxidation is suppressed from the magnetic cores and confined to the Pt layer, while keeping the cylindrical shape of the nanowire (Figure 1). Local magnetic states of uncoated and coated Co- and Fe-FEBID nanowires in remanence have been obtained by off-axis Electron Holography (EH). After substraction of the phase contribution of the mean inner potential and thanks to the cylindrical shape of the cores, quantitative magnetic induction maps have been obtained which demonstrates that the average magnetization of the ultrathin coated cores is strengthened up to 30% in the thinnest nanowires (50-nm-thick cores) with respect to unprotected ones (Figure 2).

 

References:

[1] A. Fernández-Pacheco, et. al. Sci. Rep., 2013, 3, 1492.

[2] L. A. Rodríguez, et. al. Beilstein J. Nanotechnol., 2015, 6, 1319-1331.

 

Acknowledgements:

J. P.-N. grant is funded by the Ayuda para Contratos Predoctorales para la Formación de DoctoresConvocatoria Res. 05/06/15 (BOE 12/06/15) of the Secretaría de Estado de Investigación, Desarrollo e Innovación in the Subprograma Estatal de Formación of the Spanish Ministry of Economy and Competitiveness (MINECO) with the participation of the European Social Fund. Financial support from MINECO through the project MAT2014-51982-C2 and from regional Gobierno de Aragón through project E26 with European Social Fund funding is acknowledged.


Javier PABLO-NAVARRO (Zaragoza, Spain), César MAGÉN, José María DE TERESA
08:00 - 18:15 #5850 - MS06-854 Electron-beam-induced structural phase transition related to oxygen vacancy ordering in epitaxial La2/3Sr1/3MnO3 films.
MS06-854 Electron-beam-induced structural phase transition related to oxygen vacancy ordering in epitaxial La2/3Sr1/3MnO3 films.

Functional oxides with a perovskite crystal lattice of type ABO3 may possess corresponding oxygen-deficient modulation structures, which can be used to tailor material properties including magnetism, ferroelectricity, and superconductivity. One prototypical example is the brownmillerite crystal structure of type ABO2.5 [1-5], which due to its high ionic conductivity could find applications in solid oxide fuel cells, oxygen-separation membranes, gas sensors and other devices requiring anion diffusion. Brownmillerites have been derived from perovskite materials using topotactic reduction [1], optimized film growth [2,3], and oxygen getters [4]. Here, we demonstrate that the evolution of the perovskite-brownmillerite phase transition can be fully controlled and monitored in epitaxial La2/3Sr1/3MnO3 (LSMO) films using electron-beam irradiation in a transmission electron microscope (TEM) [5].

Atomic-scale real-time TEM imaging reveals that the structural transition is driven by an incessant ordering of electron-beam induced oxygen vacancies in every second MnOx plane. This local depletion of oxygen reduces the coordination of Mn cations, causing a vertical displacement of the La/Sr ions. A map of the out-of-plane lattice spacing corroborates this point (Figure 1(b)). Over-irradiation of the brownmillerite phase induces a second transition to a perovskite-like structure with disordered oxygen vacancies and a significantly enhanced out-of-plane lattice compared to the original LSMO film (Figure 1(c)). Additional information on the distribution of oxygen vacancies in the three structural phases of LSMO is obtained by HRTEM under negative Cs imaging (NCSI) conditions [6]. Compared to the original perovskite LSMO (see Figure 2(a) and inset), the NCSI contrast from brownmillerite LSMO (Figure 2(b) and inset) manifests a depletion of oxygen and predominant tetrahedral coordination of Mn in every other MnOx layer. The modulation structure disappears when the LSMO crystal transforms into the oxygen-deficient perovskite-like structure with enhanced out-of-plane lattice parameter (Figure 2(c) and inset). In this case, the oxygen is randomly distributed, which is facilitated by oxygen diffusion from MnO6 octrahedra to MnO4 tetrahedra during the second structural phase transition. Electron energy loss spectroscopy and energy-dispersive x-ray spectroscopy further confirm our findings [5]. 

This work was supported by the Academy of Finland (Grant Nos. 260361 and 252301) and by the European Research Council (ERC-2012-StG 307502). 

     

[1] T. G. Parsons, H. D’Hondt, J. Hadermann, M. A. Hayward, Chem. Mater. 2009, 21, 5527.

[2] Y. M. Kim, J. He, M. D. Biegalski, H. Ambaye, V. Lauter, H. M. Christen, S. T. Pantelides, S. J. Pennycook, S. V. Kalinin, A. Y. Borisevich, Nat. Mater. 2012, 11, 888.

[3] H. Jeen, W. S. Choi, J. W. Freeland, H. Ohta, C. U. Jung, H. N. Lee, Adv. Mater. 2013, 25, 3651.

[4] J. D. Ferguson, Y. Kim, L. Fitting Kourkoutis, A. Vodnick, A. R. Woll, D. A. Muller, J. D. Brock, Adv. Mater. 2011, 23, 1226.

[5] L. D. Yao, S. Majumdar, L. Äkäslompolo, S. Inkinen, Q. H. Qin, and S. van Dijken, Adv. Mater. 2014, 26, 2789.

[6] C. L. Jia, M. Lentzen, K. Urban, Science 2003, 299, 870.


Lide YAO (Espoo, Finland), Sayani MAJUMDAR, Laura ÄKÄSLOMPOLO, Sampo INKINEN, Qi Hang QIN, Sebastiaan VAN DIJKEN
08:00 - 18:15 #5886 - MS06-856 In vivo visualizing the generation of hypochlorous acid using a novel selective fluorescent probe.
MS06-856 In vivo visualizing the generation of hypochlorous acid using a novel selective fluorescent probe.

Introduction Hypochlorous acid (HOCl) is one of the most important reactive oxygen species, and plays a vital role in physiological events and diseases. Many fluorescent probes have been synthesized to detect HOCl. However, most of them were difficult to be distinguished from endogenous fluorophores in biological samples in vivo and can only be employed under one-photon microscopy, often with drawbacks during further application. To enhance the sensing sensitivity and reduce photodamage, we developed a novel iridium(III) complex-based fluorescent probe for in vivo visualizing the generation of HOCl at the cellular level.

Methods A novel iridium(III) complex-based Fc (ferrocene) dual-signaling chemosensor (Ir-Fc) was designed and synthesized as a highly sensitive and selective fluorescent probe for the in vivo recognition and detection of HOCl. The absorption and fluorescent properties were investigated under simulative physiological conditions. It is well documented that during hepatic ischemia-reperfusion (I/R) injury, HOCl is generated by neutrophils and diffuses into hepatocytes, causing oxidant stress-mediated injury. Therefore, we used a BALB/c mouse hepatic I/R injury model to test this probe. DermaInspect system (JenLab GmbH, Jena, Germany) was used for in vivo multiphoton microscopy (MPM) imaging of the liver after injection of 0.2 mL Ir-Fc (50μM) from the portal vein. Systemic toxic effects were evaluated by examining the pathological changes of the major organs of mice.

Results Figure 1A shows the reaction mechanism of the fluorescent response of Ir-Fc toward HOCl. Ir-Fc exhibited a strong positive fluorescent response only in the presence of HOCl, whereas negligible fluorescent signals were observed upon the additions of other reactive oxygen/nitrogen species and metal ions (Figure 1B). The dose-dependent fluorescent enhancement followed a good linear relationship with HOCl concentration (Figure 2). In mouse liver with I/R injury, reduced autofluorescence was detected by MPM, indicating the hepatocyte necrosis (Figure 3D). As shown in Figure 3F, remarkable enhancement of the red fluorescence was observed in hepatocytes with decreased autofluorescence, indicating the reaction of Ir-Fc with endogenous HOCl molecules. No obvious toxic effects were observed in histological examination of major organs after Ir-Fc injection.

Conclusion Ir-Fc has good biocompatibility, low cytotoxicity, high specificity to HOCl, and rapid “off-on” fluorescence. This complex is suitable as an in vivo HOCl detection probeat the cellular level.

Acknowledgements This work was supported by grants from National Health and Medical Research Council (APP1049979).


Haolu WANG (Brisbane, Australia), Germain GRAVOT, Xiaowen LIANG, Camilla THORLING, Run ZHANG, Xin LIU, Michael ROBERTS
08:00 - 18:15 #5904 - MS06-858 MLLS fitting on plasmon pic for mapping hydrides in a Zr alloy with a complex αZr+βZr acicular microstructure obtained by water quenching.
MS06-858 MLLS fitting on plasmon pic for mapping hydrides in a Zr alloy with a complex αZr+βZr acicular microstructure obtained by water quenching.

Zr alloys are used to manufacture fuels claddings in nuclear plants. These tubes have to insure safety and prevent any fuel leak even in severe conditions such as Loss Of Coolant Accident. In such conditions the fuel cladding tubes are submitted to high temperature steam oxidation before core reflooding. This results in an oxidation, α-β phase transformations and partitioning of oxygen/hydrogen in a very complex microstructure (Fig. 1). At 1200°C, the hydrogen has low affinity for α-O and a high solubility in the β phase and most of the hydrogen moves towards the β-phase and is expected to remain in the prior β-phase after quenching. Macroscopic analysis (Elastic Recoil Detection Analysis-ERDA) evidenced hydrogen enrichment of the β-phase and depletion of the α-O. Because of the lack of spatial resolution of this technic, studying sub-micrometric hydrides needles in such complex mixture of α-O, β-Zr phases, highly strained needs to perform TEM (Fig.2). Furthermore, very thin specimens are needed to limit features overlapping in the lamella thickness. The multiphase sample cannot be thinned using electropolishing and has to be prepared using ions. In another hand, the high reactivity of zirconium with hydrogen implies accurate preparation conditions. Any additional hydrides formation during thinning is achievable with a very high vacuum in the thinning devices and with cold traps. This is achieved by a two steps specimen preparation combining Focused Ion bean and low kV Ar+ ions cleaning. A 100-150 nm thick lamella is first extracted using FEI Helios Nanolab Dual Beam at 30kV. We performed post FIB cleaning and thinning in PIPS II from GATAN with Ar ions at 500V during 60 min to eliminate the 20-30 nm amorphous and Ga+ implanted layer on both sides of the samples and to reach a thickness of about 40 nm (Fig.2). Diffraction studies are very complicated and time consuming even on such thin specimens. Based on the difference of plasmon energy of α-O, β-Zr in comparison with hydrides (ζ : 17,3-17,5 eV, γ : 18,3 eV, δ : 19,2 eV and ε : 19,6 eV reported in [1, 2 & 3]), hydrides are identified as the δ-ZrH2 and mapped using on MLLS fitting on plasmon pics. Since α-O and β-Zr phase have the same plasmon energy, the ambiguity between these two allotropic Zr compounds is solved using Fe and Cr mapping by EDXS with Super X detectors in a XFEG TECNAI OSIRIS. The high brightness of the XFEG gun and the high angle collection of the four EDXS detectors allows us to distinguish α-Zr from β-Zr based on their Fe and Cr solubility difference (few percent in β-Zr, and few hundred ppm in α-Zr [4, 5]). Correlation between all these analytical technics and microdiffraction patterns give us the possibility to confirm ERDA analysis concluding that sub-micrometric needles of hydrides, identified as δ-ZrH2, where mainly located at the interface between residual β-Zr and α-Zr (Fig. 3).


Laurent LEGRAS (Moret sur loing), Elodie TORRES, Marie Christine BAIETTO, Jean DESQUINE, Andréa CABRERA-SALCÉDO, Martine BLAT YRIEX
08:00 - 18:15 #5924 - MS06-860 FIB and TEM study of nanometric tribofilm formed on stainless steel during fretting-impact tribologic tests in simulated Pressurized Water Reactor conditions.
MS06-860 FIB and TEM study of nanometric tribofilm formed on stainless steel during fretting-impact tribologic tests in simulated Pressurized Water Reactor conditions.

Wear is one of the degradation mechanism observed on some component of PWR reactor. It was mainly studied based on wear tests in various conditions and SEM observations of wear surface. In this paper we present results obtained by TEM on wear scars observed on cross-sectioned samples. Tribologic tests were performed at two temperatures using a fretting impact tribometer on 304L stainless steel tubes in an aqueous solution similar to the primary circuit fluid presents in Pressurized Water Reactors (PWR). The evolution as a function of number of cycles of the surface of the wear scar at 25°C and 75°C was measured by SEM showing that wear kinetic saturated at 25°C and is higher at 75°C (Fig.1). By modifying acceleration voltage from 3kV to 15kV and by using different detectors in SEM, we showed that a scaling oxide with wear scratches is formed in both conditions covering a surface fraction higher at 25°C than at 75°C (darker areas on backscattered images in Fig.2). Cross sections were extracted from unworn surface and from the center of the wear scars obtained after 20000 cycles using a Helios Nanolab Dual Beam FIB .The tribofilm and underlying metal structure were studied by EELS in a XFEG TECNAI OSIRIS TEM equipped with a Gatan Quantum filtered imaging system. A cold work layer coming from the manufacturing process is observed at the surface of the unworn specimen. It consists in a 500 nm thick outer layer constituted of nanometric grains and inner highly strained grains. We observed that during tribologic tests this layer is evolving being recovered and recrystallized. Because of the initial heterogeneity of strain degree at the outer surface of the tube, it is difficult to conclude on a potential different evolution of the cold work layer as a function of temperature. On those cross sections, both scaling oxide layers, appearing amorphous, and stacks of multilayered compound forming bumps are observed. EDXS and EELS mapping were also performed to study the oxide observed by SEM on the wear scars (Fig. 3). At 25°C, the scaling oxide observed on the surface is found to be amorphous and Cr and Fe rich. Chemical shifts of Cr L2,3 edge and O K Near Edge Structures (NES) evolve from the surface and the interfaces to the centre of this oxide layer whereas Fe L2,3 edge remains unchanged and characteristic of oxidized Fe. This indicates that oxidation state of Cr is changing at interfaces where oxide is scaling. At 25°C, a continuous nanometric and polycrystalline almost composed of pure Ni metal film is also between this oxide areas and locally at Cr oxide/metal interface. Bumps at the wear scar surface observed by SEM are stacks of nanometric Ni metal and more or less oxidised 304L films. At 75°C, the growth of Cr/Fe amorphous oxide and Ni rich film is also observed, but the Cr/Fe rich oxide seems more porous and the Ni enrichment only reaches 40% and is found to be oxidised. Facing wear kinetics measured during tribologic tests, the modification of the tribofilms composition depending on temperature might be the reason of the difference of wear kinetics observed between 25°C and 75°C. This very new results should be implemented by a deeper analysis (via modelling and test samples) of fines structures of 0 and Cr edges to give a better understanding of the competition between oxidation and mechanical wear processes.


Laurent LEGRAS (Moret sur loing), Jean-Louis MANSOT, Guillaume PERILLAT, Andi MIKOSCH CUKA
08:00 - 18:15 #5925 - MS06-862 Deep investigation of antiphase-boundaries defects in rare-earth nickelates.
MS06-862 Deep investigation of antiphase-boundaries defects in rare-earth nickelates.

Transition metal oxides (TMO’s) are highly sensitive systems to external fields due to strong electron correlations and high polarizability of the metal-oxygen bond. This makes it possible to tune their macroscopic properties by inducing slight distortions at their unit cell structure. A paradigmatic example is the rare earth (RE) nickelates, which present a tunable metal-to-insulator transition (MIT) and resistive switching (RS) effect, placing them as a new plausible alternative for current non-volatile memories. Here, we use aberration-corrected Scanning Transmission Electron Microscopy (STEM) combined with Electron Energy Loss Spectroscopy (EELS) to correlate both structure and electrical properties as a function of the RE cation specie (La, Sm, or Nd), substrate mismatch and film thickness. Nanoscale investigations show that Nikelate thin films require chemical and structural reconstructions in the form of antiphase boundaries (APBs) to compensate the mismatch with the substrate. This 2D defect suppresses one Ni-O plane either in the in-plane or in the out-of-plane direction, changing locally the lattice spacing. The APBs landscape is evaluated on RE-nickelate epitaxial thin films grown by chemical solution deposition onto LaAl(LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT), SrTiO3 (STO) or LaAlO3 (LAO) substrates with thicknesses that range from 6nm to 50nm, as shown in Figure 1 and 2. This structural study is also complemented with electrical and advanced XRD measurements that reveal a sharp MIT transition which is shifted when substrate, thickness or RE ionic radius is modified.

We acknowledge the financial support from MICINN (COACHSUPENERGY, MAT 2014-51778-C2-1-R, project co-financed by Fondo Europeo de Desarrollo Regional (FEDER).


Bernat MUNDET (Barcelona, Spain), Júlia JAREÑO, Jaume GÁZQUEZ, Juan Carlos GONZÁLEZ, Xavier OBRADORS, Teresa PUIG
08:00 - 18:15 #5943 - MS06-864 Structural, chemical and strain features of misfit dislocation cores in ultrathin La0.7Sr0.3MnO3 epitaxial films deposited on LaAlO3.
MS06-864 Structural, chemical and strain features of misfit dislocation cores in ultrathin La0.7Sr0.3MnO3 epitaxial films deposited on LaAlO3.

Heteroepitaxial interfaces in thin film complex oxides have attracted considerable attention in recent years due to their influence on the physical properties of these materials, in particular due to the possibility of tuning bulk functional properties [1]. The most common strain relaxation mechanism between lattice mismatched heterostructures is the formation of misfit dislocation arrays at the interface. These misfit dislocations play an interesting role as they can form self-organized patterns on the nanometre scale which can behave differently from bulk material [2]. An understanding of the structural and chemical consequences associated with the strain fields of dislocations at oxide interfaces is important as they may determine the functionality of these oxides in ultrathin films and multilayers.

In this work, misfit dislocations in epitaxial films of La0.7Sr0.3MnO3 (LSMO), a half-metal ferromagnet, grown on [001] LaAlO3 (LAO) single crystal substrates were investigated. A detailed study of atomic-scale structural and chemical changes associated with the strain field of dislocations was performed using aberration-corrected scanning transmission electron microscopy (STEM) combined with atomic resolution spectroscopy techniques; electron energy-loss (EELS) spectrum imaging and energy dispersive x-ray (EDS) spectral mapping. Special attention was paid to ultrathin films of only a few nanometers thickness where the strain field of the dislocations affects surface topography and current [3].

A STEM high angle annular dark field (HAADF) of a cross-section of a 7nm film exhibiting dislocated interface is shown in Figure 1a, where spacing between dislocations around 20 nm can be observed. A STEM-HAADF image of the dislocation core structure with the Burgers circuit yielding a Burgers vector bx = aLAO[100], parallel to the interface, is shown in Figure 1b. The lattice strain maps (Figure 1c), taken from the experimental images using the Geometrical Phase Analysis (GPA) method [4], show core splitting in two partials, along with compressive and tensile regions extending into the substrate and the film, respectively. Spectroscopy analyses of cross-sections reveal a chemically rough interface between the substrate and film and show that the dislocations are not located at the interface but one or two unit cells above the interface with the substrate. Analysis of the EELS spectrum images suggest that the lanthanum composition at the vicinity of the dislocation core is enhanced (Figure 2).

These results provide an insight into how the intrinsic strain associated with a defect, in this case misfit dislocations, may produce chemical changes which may be useful in the development of ordered functional patterns in complex oxide thin films.

 

Acknowledgements:

We acknowledge financial support from the Spanish MINECO (MAT2011-29081-C02, MAT2012-33207 and MAT2013-47869-C4-1-P). N. B. thanks to the Spanish MINECO for financial support through the FPI program.

 

References:

[1] D. G. Schlom, et al., J. Amer. Ceram. Soc., 91, 2429-2454 (2008)

[2] M, Arredondo, et al., Adv. Mater., 22, 2430–2434 (2010)

[3] F. Sandiumenge, et al., Advanced Materials Interfaces, under review

[4] M. J. Hÿtch, et al., Ultramicroscopy 74, 131–146 (1998)


Núria BAGUÉS (Manresa, Spain), José SANTISO, Bryan D. ESSER, Robert E. A. WILLIAMS, David W. MCCOMB, Zorica KONSTANTINOVIC, Alberto POMAR, Lluís BALCELLS, Felip SANDIUMENGE
08:00 - 18:15 #6013 - MS06-866 Nanoscale texture analysis of d-HDDR processed Nd-Fe-B powder particles.
MS06-866 Nanoscale texture analysis of d-HDDR processed Nd-Fe-B powder particles.

A strong texture in polycrystalline Nd-Fe-B powder particles processed by dynamic hydrogenation disproportionation desorption and recombination (d-HDDR) method is of importance for enhanced macroscopic magnetic properties [1]. This heavily depends on the processing parameters such as the hydrogen partial pressure [2]. The initial and the final step of the HDDR process has already been extensively studied by electron backscatter diffraction (EBSD) technique in the scanning electron microscope (SEM) [3]. However, for SEM-EBSD analyses, e.g. after the early and full disproportionation step, the spatial resolution as well as the signal of the Kikuchi patterns was not sufficient for a thorough local texture analysis. Therefore, analyses on the nanometer scale have been carried out in a transmission electron microscope (TEM) using orientation and phase mapping via ASTAR/DigiSTAR (“EBSD inside the TEM”). In this case, a spatial resolution of up to one nanometer can be achieved. Data evaluation was carried out using the Matlab® MTEX toolbox yielding, for example, pole figures. Using this technique it is possible to map the phase distribution of α-Fe, NdH2, and Fe2B as well as their texture in order to reveal an inter-phase textural relation during the different HDDR process steps. The current understanding is that solely the Fe2B phase is responsible for the texture transfer. The obtained results for the different process steps and parameters will be interpreted with respect to the existing models and the texture analysis results on α-Fe and NdH2 from ASTAR/DigiSTAR and EBSD will be compared to evaluate the statistical reliability.

[1] H. Sepehri-Amin, T. Ohkubo, K. Hono, K. Güth, and O. Gutfleisch, „Mechanism of the texture development in hydrogen-disproportionation–desorption-recombination (HDDR) processed Nd–Fe–B powders“, Acta Mater., Bd. 85, S. 42–52, Feb. 2015.

[2] K. Güth, J. Lyubina, B. Gebel, L. Schultz, and O. Gutfleisch, „Ultra-fine grained Nd–Fe–B by high pressure reactive milling and desorption“, J. Magn. Magn. Mater., Bd. 324, Nr. 18, S. 2731–2735, Sep. 2012.

[3] K. Güth, T. G. Woodcock, L. Schultz, and O. Gutfleisch, „Comparison of local and global texture in HDDR processed Nd–Fe–B magnets“, Acta Mater., Bd. 59, Nr. 5, S. 2029–2034, März 2011.

 Acknowledgements

The authors acknowledge financial support from the LOEWE research cluster RESPONSE (Hessen, Germany) and MagHem (Japan).


Michael DUERRSCHNABEL, Enrico BRUDER, Konrad GÜTH, Roland GAUSS, Oliver GUTFLEISCH, Leopoldo MOLINA-LUNA (Darmstadt, Germany)
08:00 - 18:15 #6018 - MS06-868 Study of ferroelectric-antiferroelectric phase coexistence in La-doped PZT ceramics.
MS06-868 Study of ferroelectric-antiferroelectric phase coexistence in La-doped PZT ceramics.

Due to their wide applications in microelectromechanical systems and energy storage devices lead zirconate titanate–based antiferroelectrics have a significant technological and commercial importance1,2. It has been already established that in the PbZrO3–PbTiO3 (PZT) solid solution at the Zr-rich end of the phase diagram, an antiferroelectric/ferroelectric (AFE/FE) phase boundary exists3. At this phase boundary, switching from the AFE to the FE state using an applied field or stress induces large effective strains or charges. In order to broaden the phase transition and reduce the free energy difference between the AFE and FE phases these Zr-rich PZT solid solutions are often doped with La, Nb or Sn. Generally doping with La was found to increase the stability range of the antiferroelectric orthorhombic phase. Moreover previous studies have shown that there is a region of coexistence of the AFE/FE phases for La-doped PZT where their functional properties are improved4.

In this study Transmission Electron Microscopy (TEM) was employed in order to investigate in detail the AFE/FE phase coexistence region for a number of Pb1-xLax(Zr0.9Ti0.1)1-x/4O3 (PLZT x/90/10) compositions prepared with the mixed oxide solid state reaction method, with x=0.025, 0.030, 0.032, 0.033, 0.035, 0.040. Previous studies5,6 based on XRD and anelastic and dielectric spectroscopy measurements have suggested for 0 ≤ x < 0.020 a rhombohedral FE phase, for 0.025 ≤ x < 0.035 a phase coexistence region of orthorhombic and rhombohedral phases and for x ≥ 0.035 an orthorhombic AFE phase. The TEM study revealed that all investigated compositions contained grains that in the SAED patterns showed satellite spots along <110>pc directions with a periodicity of 8-9 (110) spacings. These satellite spots are associated with a long-period ordered incommensurate antiferroelectric structure. The incommensurate modulations spots along the <110>pc shown in the SADPs can be expressed as ha* + kb* + lc* ± 1/n(a*+ b*). For lower La-content (x=0.025, 0.030) also grains that do not contain the incommensurate antiferroelectric phase at all have been found suggesting chemical inhomogeneity. On the other hand, for higher La-content (x > 0.030) all investigated grains presented the incommensurate modulated spots. For these compositions most grains presented a multi-domain configuration with alternating AFE-FE domains as shown in Fig. 1(a). Domains denoted with odd numbers (1, 3, 5) are AFE while domains denoted with even numbers (2, 4) are FE. The presence of satellites spots in the DP pattern of AFE domains is always accompanied by the presence of stripes in the BF and DF images, perpendicular to the direction of the spots as seen in Fig. 1(b) and (c). Based on these results for the investigated samples, La-dopant  impurities may only induce a competition between the antiferroelectric and ferroelectric ordering due to the disruption of long-range dipolar interactions.

References

1 G. H. Haertling, Ferroelectrics, 75, 25-55 (1987).

2 X. Hao, J. Zhai, L. B. Kong and Z. Xu, Progress in Materials Science, 63, 1–57 (2014).

3 T. Asada and Y. Koyama, Physical Review B 70, 104105 (2004).

4 J. Knudsen, D.I. Woodward and I. M. Reaney, J. Mater. Res., Vol. 18, No. 2, (2003).

5 F. Craciun, F. Cordero, I. V. Ciuchi, L. Mitoseriu, and C. Galassi, Journal of applied physics 117, 184103 (2015).

6 I.V. Ciuchi, F. Craciun, L. Mitoseriu, C. Galassi, Journal of Alloys and Compounds 646, 16-22 (2015).

Acknowledgments

The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for providing the electron microscopy facilities and financial support under the project 3DEM-NATUR.


Alexandra NEAGU (Stockholm, Sweden), Ioana Veronica CIUCHI, Liliana MITOSERIU, Carmen GALASSI, Cheuk-Wai TAI
08:00 - 18:15 #6048 - MS06-870 Direct holographic depth- and lateral- imaging of nanoscale magnets generated by ion impact.
MS06-870 Direct holographic depth- and lateral- imaging of nanoscale magnets generated by ion impact.

The alloy Fe60Al40 is described by a paramagnetic B2 structure in its ordered phase, which transforms into a ferromagnetic A2 structure by chemical disordering that can be induced locally by ion irradiation [1,2]. This mechanism allows writing arbitrary magnetic nanostructures on paramagnetic thin films e.g. by means of a focused ion beam available in novel scanning ion microscopes. However, reproducible fabrication of nanoscale magnets requires knowledge about the depth and lateral distribution of the induced magnetization in dependence on irradiation parameters.

Off-Axis Electron Holography provides suitable insights by revealing the local distribution of the projected magnetic flux density with nanometer resolution [3]. By means of the coherent superposition of an electron wave passing through the object with one passing through vacuum, interference fringes can be formed at the detector plane encoding the amplitude and phase of the electron wave. The phase of an electron wave shifted by electric and magnetic fields of the object permits direct field mapping at the nanometer scale.

In cross-sectional samples of irradiated thin films, we studied the effect of the kinetic ion energy ranging from 5-30 keV on the depth distribution of the induced magnetization [4]. In agreement with irradiation damage simulations [2], we found a magnetized film adjacent to the ion entrance surface growing in depth with increasing kinetic ion energy. We conclude that a homogeneous magnetization depth distribution in a 40 nm thick film requires a kinetic Ne+ ion energy of at least 20 keV. The resolution of the ion beam nano-pattering is mainly limited by the effect of lateral ion scattering blurring the magnetization distribution at the pattern edges. To study this effect, we fabricated 500 nm wide magnetized stripes separated by non-ferromagnetic (i.e. non-irradiated) spacers (Fig. 1) using a focused Ne+ ion beam (2 nm probe size) at 25 keV in a helium ion microscope [5]. The flux distribution at the stripe facets is an indicator for the effect of lateral scattering but is difficult to directly interpret in terms of magnetization because of the superposition with stray fields. Therefore, we applied a magneto-static model for the field distribution around the nanoscale magnet as a function of the magnetization blurring, which returns a width of lateral scattering of about 20 nm fitting best to experimental results [4].

[1] J Fassbender et al, Phys. Rev. B 77 (2008) 174430.

[2] R Bali et al, Nanoletters 14 (2014) 435-441.

[3] H Lichte et al, Rep. Prog. Phys. 71 (2008) 016102.

[4] F Röder et al, Sci. Rep. 5 (2015) 16786.

[5] G Hlawacek et al,  J. Vac. Sci. Technol. B 32 (2014) 020801.

 

Acknowledgments

 

We thank the Ion Beam Center at Helmholtz-Zentrum Dresden-Rossendorf for providing the necessary facilities. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative- I3).


Falk RÖDER (Dresden, Germany), Gregor HLAWACEK, Sebastian WINTZ, René HÜBNER, Lothar BISCHOFF, Hannes LICHTE, Kay POTZGER, Jürgen LINDNER, Jürgen FASSBENDER, Rantej BALI
08:00 - 18:15 #6148 - MS06-872 Magnetic coupling of 4f and 3d moments by EMCD: application to DyFe2-based superlattices.
MS06-872 Magnetic coupling of 4f and 3d moments by EMCD: application to DyFe2-based superlattices.

            The emerging technique EMCD, Energy-Loss Magnetic Chiral Dichoism, aims to measure the element-specific magnetic moment of solids at a nanometer scale using electron energy-loss spectroscopy in a transmission electron microscope (TEM) [1-3]. The magnetic moment information is carried in the electron energy-loss spectra obtained from two specific positions of particular electron diffraction patterns. In the last few years, EMCD has been extensively applied to probe 3d moments in 3d transition metal. Our work explores its application to both 4f moment and 3d moment by investigating Dy-M4,5 edges and Fe-L2,3 edges respectively  in DyFe2/YFe2 superlattice.

            As illustrated in figure 1, DyFe2 and YFe2 layers appear alternatively to form a superlattice with a specific bilayer repeat distance. Both DyFe2 and YFe2 crystalize in the same structure, making the superlattice actually a single crystal. For EMCD experimental setup, a specific three-beam condition has been settled in the TEM to detect the signals of Fe-L2,3 edges and Dy-M4,5 edges in the same condition. Spectra and dichroic signals are shown in figure 2. The influence of the dynamic diffraction effect on the Fe-L3 and Dy-M5 EMCD signal amplitudes is precisely analyzed, and it will be shown in this presentation that the opposite sign of these two peaks unambiguously proves the antiparallel alignment of net Fe 3d and Dy 4f moments. In addition, EMCD sum rules specified for M4,5 edges have been derived and their application conditions to Dy-M4,5 edges will be discussed in detail.

            This work evidences that the EMCD technique is an effective tool to probe 4f moment and to study magnetic moment coupling in magnetic materials.

 

[1]. Schattschneider, P. et al. Nature 441, 486–488 (2006).

[2]. Warot-Fonrose, B. et al. Ultramicroscopy 108, 393–398 (2008).

[3]. Fu, X. et al. Phys. Rev. B, in Press, (2016)

 

Acknowledgement: This work is supported by the French national project EMMA (ANR12 BS10 013 01) and by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.


Xiaoxiao FU (Toulouse), Bénédicte WAROT-FONROSE, Karine DUMESNIL, Rémi ARRAS, Virginie SERIN
08:00 - 18:15 #6149 - MS06-874 Controlling the grain size of polycrystalline TiO2 films grown by atomic layer deposition.
MS06-874 Controlling the grain size of polycrystalline TiO2 films grown by atomic layer deposition.

Titanium dioxide (TiO2) is a widely used material for photocatalytic, optical, electrical and medical applications. Atomic layer deposition (ALD) represents an excellent technique for the synthesis of thin films due to its precise thickness control, simplicity, high conformity of obtained films and reproducible growth of defect-free films.

It was shown recently that photo-catalytic TiO2 films grown on cellulose-based and porous substrates can be used in water purification systems [1]. Its photo-catalytic activity strongly depends on the crystal structure and the grain size of the film, i.e. the TiO2 films must have a well-defined anatase phase with large polycrystalline grains [2]. It was recognized that plasma enhanced ALD (PEALD) growth of the TiO2 film can produce the anatase phase even at low deposition temperatures [3]. This result is important for the growth of thin polycrystalline films on temperature-sensitive materials, such as organic substrates. On the other hand, the grain size is shown to depend critically on the type and the morphology of substrates [4].

We have investigated the effects of thin intermediate layers, grown by ALD on silicon substrates, on the grain size of the TiO2 films, grown by thermal ALD and PEALD technique in a wide temperature range, from room temperature up to 300oC. Amorphous TiO2 films were obtained with thermal ALD for temperatures below 150oC, while the anatase phase with crystalline aggregates has been identified on films synthesised with PEALD at low temperatures. We show that the size of crystallites can be greatly enlarged if grown on an intermediate layer of amorphous Al2O3. The films were characterised by a range of analytical techniques, including scanning electron microscopy with energy dispersive x-ray spectroscopy, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and x-ray diffraction.

References

[1] M. Knez, K. Nielsch, L. Niinistö, Adv. Mater. 19, 3425 (2007).

[2] J. G. Chen, Surf. Sci. Rep. 30, 1 (1997).

[3] S. Ratzsch, E.-B. Kley, A. Tünnermann, A. Szeghalmi, Nanotechnology 26, 024003, (2015).

[4] R. L. Pruurunen, et al., J. Nanosci. Nanotechnol. 11, 8101 (2011).


Ivna KAVRE PILTAVER (Rijeka, Croatia), Ivana JELOVICA BADOVINAC, Iva SARIC, Robert PETER, Gabriela AMBROZIC, Ales OMERZU, Mladen PETRAVIC
08:00 - 18:15 #6169 - MS06-876 Strain-structure-property relation in Co-super tetragonal BiFeO3 heterojunctions.
MS06-876 Strain-structure-property relation in Co-super tetragonal BiFeO3 heterojunctions.

Bismuth ferrite (BiFeO3) has received a lot of attention as a rare multiferroic material showing simultaneous ferroelectric and antiferromagnetic orders at room temperature. Strain engineering offers a new avenue to tune the coupling between various orders, and provides access to some new crystalline phases, which are not stable in bulk BiFeO3 (BFO). The super tetragonal phase (T-phase) with a giant axial ratio (c/a~1.27) is one such strain engineered phases, obtained through a biaxial compressive strain on BFO (>-4%) [1]. This phase exhibits a large polarization (~150 µC/cm2) that was exploited for applications such as ferroelectric tunnel junctions with colossal electroresistance [2]. The interface between ultrathin films of T-phase BFO and the bottom electrode of (Ca,Ce)MnO3 has been thoroughly investigated recently [Marinova et al. Nano Lett. 15, 2533-2541 (2015)]. However, the interface between the ferroelectric and the top electrode of Co used in devices remains to be investigated. 

In this context, we synthesized Co-BFO-CCMO heterostructures using pulsed laser deposition on the (001) surface of three different substrates (NdGaO3, LaSrGaO4, YAlO3). Structural details were probed and compared through high resolution STEM imaging in both annular bright field (ABF) modes to obtain information about lighter atoms, and in high angular annular dark-field modes. Furthermore, by correlating local chemical information obtained from STEM EELS, with ABF STEM images, we detect features that are consistent with the existence of oxygen vacancies in ultrathin T-phase BiFeO3. We also investigated the interface quality between Co and T-BFO using EELS and EDX, and correlate these observations with our transport measurements.

 

1. B.Dupe, I.C.Infante, et al., Phys. Rev. B, 81, pg: 144128 (2010)

2. H.Yamada, V.Garcia et al., ACS Nano, 7, pg: 5385 (2013)


Pavan NUKALA (Massy-Palaiseau), Anke SANDERS, Cecile CARRETERO, Stephane FUSIL, Agnes BARTHELEMY, Brahim DKHIL, Vincent GARCIA, Manuel BIBES
08:00 - 18:15 #6172 - MS06-878 Impact of high energy electrons on nitrides for nanocathodoluminescence.
MS06-878 Impact of high energy electrons on nitrides for nanocathodoluminescence.

Nitride quantum well (QW) light emitting diodes (LEDs) exhibit high efficiencies relative to traditional lighting sources. The quantum wells serve to confine carriers and lead to higher device efficiencies. To achieve the highest device efficiencies requires optimisation of the active quantum wells and hence experimental observations of the structural and optical characteristics. Recently, by imaging the cathodoluminescence (CL) in a scanning transmission electron microscope (STEM), the structural and optical properties were correlated with a spatial resolution of a few nanometres [1], an approach referred to as nanoCL. NanoCL has also been used to reveal quantum confinement in high brightness LEDs [2], and optimise the device structure to achieve the highest device efficiency.

 

Whilst the improved spatial resolution associated with nanoCL can reveal optical characteristics on the nanoscale, the higher electron energies can lead to structural damage. Here we investigate the effect of high energy electrons on the luminescent intensity. An exponential decline in the luminescent intensity is observed over time with exposure to the electron probe. Spectral time profiles are performed over a range of accelerating voltages from 80 to 200 keV to study the variation in the luminescence with the electron energy. We observe that the luminescent intensity is substantially more stable at lower accelerating voltages and predict a threshold for damage below which there is no damage. NanoCL can thus potentially provide a reliable approach to the study of functional optoelectronic devices.

[1] L.F. Zagonel, S. Mazzucco, M. Tencé, K. March, R. Bernard, B. Laslier, G. Jacopin, M. Tchernycheva, L. Rigutti, F.H. Julien, R. Songmuang, and M. Kociak, Nano Lett. 11, 568 (2011).

 [2] J.T. Griffiths, S. Zhang, B. Rouet-Leduc, W.Y. Fu, A. Bao, D. Zhu, D.J. Wallis, A. Howkins, I. Boyd, D. Stowe, M.J. Kappers, C.J. Humphreys, and R.A. Oliver, Nano Lett. 15, 7639 (2015).


James GRIFFITHS (Cambridge, United Kingdom), Siyuan ZHANG, Jeremy LHUILLIER, Dandan ZHU, David WALLIS, Ashley HOWKINS, Ian BOYD, David STOWE, Colin HUMPHREYS, Rachel OLIVER
08:00 - 18:15 #6207 - MS06-880 The influence of Yb and Bi doping on the thermoelectric properties of Mg2Si0.4Sn0.6 studied using transmission electron microscopy.
MS06-880 The influence of Yb and Bi doping on the thermoelectric properties of Mg2Si0.4Sn0.6 studied using transmission electron microscopy.

Current research in thermoelectric materials is focused on increasing the figure of merit ZT=(S2σ/κ)T (where S is the Seebeck coefficient and σ is the electrical conductivity) by maximizing the power factor PF (S2σ) and/or minimizing the thermal conductivity (κ). Attempts to maximize the PF include the development of new materials and optimization of existing materials by doping and nano-structuring. A reduction in thermal conductivity can be achieved by alloying, by producing disordered or complex unit cells or by nanostructuring. Here, we investigate a Bi-doped and a Bi- and Yb- doped Mg2Si0.4Sn0.6 alloy. We discuss the influence of composition, crystal structure and microstructure on the thermoelectric performance of the materials, in order to assess new opportunities for enhancing the performance of bulk nano-structured composite materials.

 Samples were produced by powder metallurgical processes, starting from a stoichiometric mixture of a melt-spun Mg or Mg-Yb pre-alloy and Si, Sn and Bi powders. After performing high energy milling to mix the components homogeneously under a protective Ar atmosphere, the material was simultaneously compacted and synthesized during a FAST process.

 Pure Mg2Si0.4Sn0.6 is an n-type semiconductor with a low value of  σ. S is negative between room temperature and 600 °C. σ increases approximately linearly with Bi concentration. An optimized doping content leads to a value for σ of 140000‑180000 S/m and a value for S of ‑150 µV/K at room temperature. Strong doping results in degeneracy of the semiconductor. Therefore, σ decreases with temperature, while S increases. The temperature dependence of κ shows two “branches”. In samples that have an optimized Bi doping concentration, κ decreases from room temperature to approximately 400 °C due to a dominant phonon-phonon scattering mechanism, with a minimum of 2 W/mK. At higher temperatures, thermal excitation of charge carriers across the band gap increases κ. Bi-Yb-doped Mg2Si0.4Sn0.6 shows a larger ZT than the Yb-free sample.

We prepared specimens for high-resolution transmission electron microscopy (HRTEM) using an FEI Helios Nanolab 400s focused ion beam (FIB) dual-beam system. HRTEM images were acquired at 300 kV using an FEI Titan 80-300 TEM equipped with a spherical aberration (Cs) corrector on the objective lens. High-angle annular dark-field (HAADF) scanning TEM (STEM) images and elemental maps were acquired at 200 kV on an FEI Titan G2 80-200 TEM equipped with a Cs corrector on the condenser lens system.

 An inspection of the microstructures of the materials by TEM reveals a homogeneous Mg2Si0.4Sn0.6 matrix and a similar grain size distribution in both samples. The average grain sizes are in the range 1‑3 μm, which shows that an improvement in the thermoelectric properties of the Bi- and Yb- doped alloy cannot be attributed to grain size. High spatial resolution energy-dispersive X-ray spectroscopy (EDXS) shows that the elemental distribution inside the grains differs from that at the grain boundaries. Our results show that Yb does not form a solid solution with Mg2Si0.4Sn0.6, but instead forms distinct grains by reacting with Bi and Sn. The formation of Bi-rich precipitates in Bi- and Yb- doped Mg2Si0.4Sn0.6 reduces the Bi content in the otherwise homogeneously doped matrix. Some oxygen enrichment in the region of the grain boundaries, associated with the formation of MgO and SiOx, was observed in both samples. Sn and Si nanoscale precipitates were detected in the Bi-doped sample.

Acknowledgment: The authors are grateful to the German Science Foundation (DFG) for funding through the collaborative project SPP 1386.[r1] 

 [r1]shall not exceed 4800 characters


Maryam BEIG MOHAMADI (Aachen, Germany), Amir Hossein TAVABI, Rafal Edward DUNIN-BORKOWSKI, Georg PÖHLE, Vicente PACHECO
08:00 - 18:15 #6216 - MS06-882 Towards Conductivity Measurements in Battery Materials Using Scanning Electron Microscopy.
MS06-882 Towards Conductivity Measurements in Battery Materials Using Scanning Electron Microscopy.

There are several ways of determining the conductivity of bulk materials, especially known in the field of solid state physics [1]. The most commonly used procedure is a four-point-probe measurement. By applying voltage on two of the probes, a current can be measured with the remaining two probes.

We introduce a new method for determining the conductivity of battery electrode materials such as graphite using an SEM. A constant current of electrons is injected into a specimen by the electron beam. A  Pt pad which is sputtered on the specimen surface is contacted with a micromanipulator. (cf. fig. 1). The specimen itself serves as a current divider (cf. fig. 2). The beam current is diminished by the secondary and backscattered electron current and is thus

Iin = I0 - ISE - IBSE (1)

The absorbed current, measured with a picoammeter via the micromanipulator, and the transmitted current, measured with a second picoammeter, sum up to the total effective injected current

Iin = Iabs + Itrans (2)

To spatially resolve the electrical conductivity [2] of the battery electrodes, the electron beam is scanned parallel to the rectangular Pt pad in various distances. The specimen itself is of constant thickness which allows us to assume the resistance R2 along the height axis of the specimen to be constant (cf. fig. 3). The absorbed current Iabs can be expressed in dependence of the distance d between the electron probe and the Pt pad as

Itrans(d) = IinR2*((ρd/A) + R2)-1 (3)

By varying the distance d we extrapolate the resistivity ρ and therefore the conductivity of the material. A is the contact area between the Pt pad and the specimen surface. It can either be measured directly or also extrapolated from eq. 3.

In a first step, it is necessary to measure the effective electron beam current  remaining in the sample (cf. eq. 1). Therefore it is crucial to quantitatively determine the secondary electron and backscattered electron current. The secondary electron yield is highly dependent of the sample topography. Thus, one of the preconditions is to observe samples with identical topography and measure experimentally the absorbed current. To obtain identical sample topographies, a Focused Ion Beam (FIB) system is used. This makes it possible to polish the sample surface and get plane topographies. The experimental results of ISE and IBSE can be cross-checked using Monte-Carlo simulations.

The use of a Gas Injection System (GIS) allows for sputtering platinum contacts on top of the specimen. We use these platinum pads as contact areas [3] for micromanipulators to read out absorbed currents.

We introduce the basic principle of measuring currents in an SEM using this new technique and show first results for state-of-the-art Li-Ion battery electrodes.

 

[1] Sun, Ling; Wang, Jianjun; Bonaccurso, Elmar (2013): „Conductivity of individual particles measured by a microscopic four-point-probe method“. In: Sci. Rep. 3.

[2] Park, Myounggu; Zhang, Xiangchun; Chung, Myoungdo u. a. (2010): „A review of conduction phenomena in Li-ion batteries“. In: Journal of Power Sources. 195 (24), S. 7904-7929.

[3] Marlow, Gregory S.; Das, Mukunda B. (1982): „The effects of contact size and non-zero metal resistance on the determination of specific contact resistance“. In: Solid-State Electronics. 25 (2), S. 91-94.


Sebastian STURN (Ulm, Germany), Ute GOLLA-SCHINDLER, Jörg BERNHARD, Manfred RAPP, Mario WACHTLER, Ute KAISER
08:00 - 18:15 #6270 - MS06-884 Analytical STEM Study of Dy-doped Bi2Te3 Thin Films.
MS06-884 Analytical STEM Study of Dy-doped Bi2Te3 Thin Films.

Breaking the time-reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs) [1,2] is crucial for unlocking exotic physical states and exploring possible device application. Doping the prototypical 3D-TI Bi2Te3 with transition metal ions can lead to ferromagnetic ordering at low temperatures [3,4]. We present the study of incorporation of dysprosium (Dy) into Bi2Te3 with the intent to achieve higher ferromagnetic ordering temperatures and higher magnetic moments [5].

 

Dy-doped thin films were grown on c-plane sapphire substrates by molecular beam epitaxy (MBE) [6]. Samples with a Dy concentration of x ≤ 0.113 were selected for high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX-) STEM investigations [5]. A (Dy0.113Bi0.887)2Te3 film was carefully detached from the substrate using a droplet of glue, and thin TEM lamellae were cut in cross-sectional geometry using an ultramicrotome. Thin sections were obtained using an oscillating water-filled diamond knife to cut approximately 40 nm thick slices that were captured on Cu grids covered with a lacey carbon film.

 

A high crystallinity (Dy0.113Bi0.887)2Te3 thin film acquired at 60 kV is shown in an HAADF-STEM image in Figure 1. The characteristic crystal structure consisting of stacked quintuple layers separated by van der Waals gaps is clearly recognizable. A higher magnification HAADF-STEM image with the overlaid structural model is presented in Figure 2a. EDX line-scans were acquired traversing the van der Waals gap between the adjacent quintuple layers (see arrow in Fig. 2a). The corresponding intensity profiles of Bi-M, Te-L and Dy-L X-ray emission lines along the arrow (in Figure 2a) are presented in Figure 2b. The intensity of the Dy-L signal is precisely following the Bi-M signal, indicating the substitutional incorporation of Dy atoms on Bi sites and the absence of Dy in the van der Waals gaps. No evidence for cluster formation or local phase segregations could be detected [7].

 

References:      

[1] CL Kane and EJ Mele, Phys Rev Lett 95 (2005), 146802.

[2] BA Barnevig et al., Science 314 (2006), 1757.

[3] J Choi et al., Phys Stat Sol (b) 241 (2004), 1541.

[4] Y Chen et al., Science 329 (2010), 659.

[5] SE Harrison et al., Sci Rep 5 (2015), 15767.

[6] SE Harrison et al., J Phys: Condens Matter 27 (2015), 245602.

[7] The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement No. 312483 (ESTEEM2).


Vesna SROT (Stuttgart, Germany), Piet SCHÖNHERR, Birgit BUSSMANN, Sara E. HARRISON, Peter A. VAN AKEN, Thorsten HESJEDAL
08:00 - 18:15 #6286 - MS06-886 3D-TEM microstructure analyses of anisotropic and isotropic aerogels of TiO2 nanoparticles.
MS06-886 3D-TEM microstructure analyses of anisotropic and isotropic aerogels of TiO2 nanoparticles.

Aerogels are fascinating materials with low density and high surface area with great application potential in battery materials, fuel- and solar cells and many more.(1-3) These porous structures are conventionally prepared by sol-gel chemistry and subsequent supercritical drying.(4) In the case of TiO2 nanoparticles, the ability of the crystalline nanoparticles to undergo oriented attachment is fundamentally important for the self-assembly into three-dimensional interconnected networks. Such structures (Fig.1) are prepared by destabilizing highly concentrated dispersions of well-defined TiO2 nanocrystals. The gelation results in a percolating network of ultrafine structures which is preserved by supercritical drying. This processing technique allows for the preparation of crystalline, isotropic and translucent aerogels with broad pore size distributions and disordered pore arrangements (Fig.1).(5)

The aim of our study is to produce structures from destabilized dispersions and subsequent unidirectional freeze-drying. Compared to the aerogel route, this process results in a different macro- and microstructure consisting of about 3 µm large units of compacted aerogel structures. Here, the pores are of a more ordered nature due to the freeze drying procedure.

In this work, aerogels prepared by supercritical drying and structures made via freeze-drying were analyzed microstructurally. Using TEM tomography, we study the morphology of the 3D networks of the nanocrystals within both structures. Critical performance parameters like pore size, connectivity and tortuosity of those structures are analyzed.

 

  1. Lawrence W. Hrubesh, Journal of Non-Crystalline Solids, 1998, 225, 335.
  2. F. Rechberger, G. Ilari and M. Niederberger, Chem. Commun., 2014, 50, 13138.
  3. N. Gaponik, A.-K. Herrmann and A. Eychmüller, J. Phys. Chem. Lett., 2011, 3, 8.
  4. F. J. Heiligtag, M. D. Rossell, M.J. Süess and M. Niederberger, J. Mater. Chem, 2011, 21, 16893.
  5. F. J. Heiligtag, et al., Nanoscale, 2014, 6, 13213.

Fabian GRAMM (Zurich, Switzerland), Elena TERVOORT, Alla SOLOGUBENKO, Clara MINAS, Felix RECHBERGER, Florian BOUVILLE, Andre STUDART, Markus NIEDERBERGER
08:00 - 18:15 #5110 - MS07-888 Investigation of the near fields of sputtered Au thin films, used for surface enhanced Raman spectroscopy, using the AFM and DDA.
MS07-888 Investigation of the near fields of sputtered Au thin films, used for surface enhanced Raman spectroscopy, using the AFM and DDA.

Surface enhanced Raman spectroscopy (SERS) is a powerful technique that uses metal nanostructures (Au, Ag, Cu) to gain in the best case single molecule sensitivity [1], because these nanostructures give rise to a huge enhancement of the Raman signal of molecules in their immediate vicinity. Most of the enhancement is attributed to the enhancement of the electric field near the surface of the metal, because the Raman signal in this case scales approximately proportional to the fourth power of the electric filed strength [2]. Therefore a good knowledge of both the geometry of the metal nanostructure and the electric near field it causes is crucial for the understanding of a SERS-substrate.

 

A combination of AFM and the discrete dipole approximation (DDA) is a promising approach for a better understanding of solid SERS substrates. Using the AFM the geometry of a nanostructure on a flat substrate or a structured surface can be measured very accurately and the DDA is a well-established method for solving scattering problems for arbitrary shapes, which makes little assumptions about the sample other than the target geometry [3]. Furthermore, using the FFT-accelerated DDA a very large number of dipoles (>106) can be simulated on a desktop computer [4], allowing for a discretization of comparatively large scatters on a nm scale.

 

We have written a Matlab program that uses the DDA to calculate the near field from a given AFM image of a nanostructure composed of a single material. The incident field and dielectric constant of the material can be chosen arbitrarily. We have tested the program against Mie theory (fig. 1) and shown that the error of the electric field strength is reasonably small two discretization steps away from the surface (fig.2). The average error of the electric field strength two discretization steps away from the surface is smaller than 5 % (fig.2), with the maximum error being smaller than 10 % (fig.1; apart from the strip around x=0, where the relative error appears large due to |EMie|≈0, but the absolute error is actually very small).

 

This program is used to simulate SERS substrates, produced by sputtering thin Au-films on glass slides (fig. 3). These substrates give rise to small but regular (across the substrate) enhancement with enhancement factors of the order of 102 – 103. Our goal is to demonstrate the feasibility of using AFM based DDA simulations to study near field enhancement, by predicting the enhancement factors of several substrates.

 

Literature:

[1] Xu, H., Bjerneld, E.J., Käll, M. and Börjesson, L. (1999), Spectroscopy of Single Hemoglobin Molecules by Surface Enhanced Raman Scattering, Phys. Rev. Lett. 83, 4357

 

[2] Garcia-Vidal, F.J. and Pendry, J.B. (1996), Collective Theory for Surface Enhanced Raman Scattering, Phys. Rev. let. 77, 1163

 

[3] Yurking, M.A. and Hoekstra, A.G. (2007), The discrete dipole approximation: an overview and recent developments, Journal of Quantitative Spectroscopy and Radiative Transfer, 106(1), 558-589

 

[4] Goodman, J.J., Flatau, P.J. and Draine, B.T., Application of the fast-Fourier-transform techniques to the discrete-dipole approximation, Optics Letters 16(15), 1198-1200


Harald FITZEK (Graz, Austria), Jürgen SATTELKOW, Peter PÖLT
08:00 - 18:15 #5878 - MS07-890 Angle-resolved cathodoluminescence polarimetry on plasmonic nanostructures.
MS07-890 Angle-resolved cathodoluminescence polarimetry on plasmonic nanostructures.

Plasmonic metamaterials and metasurfaces have the ability to influence the propagation, confinement, and emission of light on a deep-subwavelength scale. Many of the optical properties of such materials are encoded in the spectrum, the angular intensity distribution, and the polarization of the far-field emission. Angle-resolved cathodoluminescence (CL) imaging spectroscopy (ARCIS) is a powerful platform for studying these properties, as it combines nanoscale excitation resolution, with the capability to measure both spectra and the angular emission intensity distribution. In particular, we use a 30 keV electron beam as a well-defined broad-band excitation source which is sensitive to the optical density of states. This method has been used to study the spectral and angular optical properties of a large variety of dielectric and plasmonic nanostructures. However, thus far we were only able to measure emission intensities and had to disregard the vectorial polarization nature of the light emission. The emission polarization contains valuable information, which can be used to identify multipoles, separate TM and TE modes in waveguides, characterize the coherence of an emission source etc.

Here, we demonstrate a novel CL polarimetry technique in which we retrieve the Stokes vector, i.e. the full polarization state of the far-field emission, as function of angle [1]. To that end, we extend our setup to include a quarter-wave plate (QWP) and a linear polarizer in the beam path (see Figure 1 for a schematic representation of the setup). By taking six measurements with the appropriate combinations of QWP and polarizer angle we retrieve the polarization distribution in the detection plane. By applying a correction for the aluminum parabolic mirror optic we then find the emission polarization distribution.

This approach is applied to gold plasmonic bull’s eye gratings  which were fabricated using focused-ion-beam milling in a single-crystal gold substrate (see Figure 2(a) for an SEM image). These bull’s eyes can coherently couple out the Surface Plasmon Polaritons (SPPs) that are excited by the electron beam. Because the electron beam can be positioned at will, we can study the effect of exciting the bull’s eye at different positions. For central excitation, the grating is driven in phase leading to an azimuthally symmetric pattern and a radial polarization distribution, as expected from symmetry (see Figure 2(b-c)). However, when we excite off-center the patterns become significantly more complex, showing multiple lobes and alternating regions in angular space in which the polarization goes from circular to linear. To demonstrate the applicability to chiral structures, we move to spiral bull’s eyes with different handedness, and show that their chirality is reflected in the field distributions. The validity of the polarimetry technique is verified by measuring transition radiation which has a characteristic radial polarization distribution, similar to a vertical dipole source. This work paves the way for polarimetry measurements on a myriad of metallic and semiconductor nanophotonic geometries for characterization and better understanding of their optical properties.

 [1] C. I. Osorio, T. Coenen, B. J. M. Brenny, A. Polman, and A. F. Koenderink, ACS Photonics 3, 147−154 (2016)

 


Toon COENEN (Delft, The Netherlands), Clara OSORIO, Benjamin BRENNY, Albert POLMAN, Femius KOENDERINK
08:00 - 18:15 #6088 - MS07-894 Plasmonic edge and breathing modes in aluminum nanotriangles.
MS07-894 Plasmonic edge and breathing modes in aluminum nanotriangles.

The optical properties of metallic particles at nanometric scale have raised a great interest in scientific community due to the high promising technological applications such as optical communication and storage or quantum optics1. It is well known that optical properties of metallic nanoparticles are dominated by surface plasmons that are collective electron oscillations at a metal-dielectric interface which can be exploited to manipulate light. In a metallic nanoparticle these oscillations are confined by the boundaries of the particles, resulting in discrete modes of oscillations (plasmon modes) which can be tuned by shaping the geometry of the nanoparticle. Recently, a detailed analysis of surface plasmons in flat structures1 has allowed classifying plasmon modes in two groups. The first group corresponds to the so-called edge modes which are localized at the periphery of the nanoparticle. These edge modes are well known in the literature and have been reported for several geometries, including nanorods and nanotriangles. On the other hand the second group of modes corresponds to the so-called breathing modes (or cavity modes) which are localized on the center of the nanoparticles. Nowadays breathing modes have just been reported and explained for simple structures as disks2 and squares3. More complex structures remain to be understood.

In this work we study aluminum nanotriangles (edge length size ranging from 125 to 622 nm) by electron energy loss spectroscopy (EELS) coupled with a transmission scanning electron microscope (STEM) in order to understand the complexity of plasmon modes in this kind of structures. Behind this geometry, a rich variety of edge and breathing modes are observed ranging from 1 to 5 eV (figure 1a). Thanks to the high spatial and energy resolution of STEM-EELS technique, we were able to generate with high level of precision, plasmon maps for all modes (figure 1b). In order to understand the breathing modes in triangular nanoparticles, we propose an analytical model considering the interference of reflected waves at the boundaries of triangular cavities which allowed us to explain the symmetry of lobes shown in maps of figure 1b. Furthermore, plasmon modes dependence with nanotriangles size will be shown and interpreted based on our analytical model and rigorous theoretical simulations.

 

References:

(1) Schmidt, F.-P.et al. Universal dispersion of surface plasmons in flat nanostructures. Nat. Commun. 5, 3604 (2014).

(2) Schmidt, F.-P.et al. Dark plasmonic breathing modes in silver nanodisks. Nano Lett. 12, 5780–5783 (2012)

(3) Edson P. Bellido. et al. Electron Energy-Loss Spectroscopy of Multipolar Edge and Cavity Modes in Silver Nanosquares. ACS photonics. DOI: 10.1021/acsphotonics.5b00594


Alfredo CAMPOS (Orsay), Davy GERARD, Jerome MARTIN, Jerome PLAIN, Julien PROUST, Arnaud ARBOUET, Mathieu KOCIAK
08:00 - 18:15 #6096 - MS07-896 Luminescence properties of hexagonal boron nitride layers.
MS07-896 Luminescence properties of hexagonal boron nitride layers.

Hexagonal boron nitride (hBN) is a wide band gap semiconductor (6.4 eV), which can be synthesized, as graphite, its carbon analog, as bulk crystallites, nanotubes and layers. These structures meet a growing interest for deep UV LED and graphene engineering [1]. Knowing better the intrinsic properties of this material is therefore highly desirable. We attempt to have a better comprehension of the optical and electronic properties of thin BN layers, in correlation with their structural properties and to better know, how these properties can be further exploited for the characterization of these nanostructures and how electronic properties of graphene can be impacted by underlying BN layers.

 

To this aim, we combined TEM structural analyses performed in a monochromated Libra 200 TEM and cathodoluminescence experiments at 4K using a dedicated set-up implemented in a JEOL FEG-SEM and adapted to the detection in the far UV range [2, 3]. Data recording is available in a spectroscopic mode or in a hyperspectral imaging mode thanks to the imaging capability of the microscope.

 

In this work, we have first investigated the luminescence properties of different hBN sources (HPHT [4], PDCs [5] and commercial samples) in the near band edge energy range (5 - 6 eV). In this energy range, luminescence properties are governed by strong excitonic effects and consist of D and S lines [2, 3]. Emission related to D lines (5.3 – 5.5 eV) has been proved to be due to structural defects, such as grain boundaries, as identified by TEM [3]. In defect free areas, D lines completely vanish and S lines (5.75 – 5.9 eV) only are observed. As shown in Fig.1, S lines display similar features whatever the sample source. S emission consists of four main lines S1 – S4. Although the exact nature of these lines is still a debated issue, their constant observation in various kinds of samples let to identify them as the intrinsic luminescence of the bulk material [6].

 

In a second step, we studied luminescence in thin layers, obtained by mechanically exfoliating small crystallites. Exfoliated flakes were reported on SiO2 substrates for AFM thickness measurements and luminescence experiments. We first have shown that the transfer procedure on the substrate can highly impact the luminescence. Indeed if reported flakes display folds or ripples, excitons get trapped on these defects. S line emission vanishes and emission is dominated by D lines and is highly localized at the defects [7]. We used this effect as a check of the structural quality of the flakes and in such a way we managed to prepare defect free flakes, that is with no D band in their emission spectrum, with various thicknesses from 100L to 6L from both an HPHT crystal and a commercial powder. As shown in Fig.2, their S-emission dramatically changes, when reducing the number of layers. The relative intensity of S3-4 lines progressively decreases whereas the one of S1-2 lines increases. This rise of the S1-2 line is accompanied by that of related phonon replica corresponding to the E2g mode [3] as outlined by the dashed lines in Fig.2. In the thinnest layers, emission is therefore restricted to the S1-2 line only, identified as a signature of the 2D confinement [6].

 

[1] C.R. Dean et al., Nature Nanotechnology, 5, 722-6 (2010)

[2] P. Jaffrennou el al.,  Phys. Rev. B 77  (2008)  235422

[3] A. Pierret et al, Phys. Rev. B, 89 (2014) 035414.

[4] Y. Kubota et al.. Science 317, (2007) 932

[5] S. Yuan et al, Scientific Reports 6 (2016) 203388

[6] L. Schué et al,  Nanoscale (2016)

[7] L. Schué et al in preparation (2016)


Léonard SCHUÉ, François DUCASTELLE, Frédéric FOSSARD (CNRS-ONERA), Julien BARJON, Annick LOISEAU
08:00 - 18:15 #4462 - MS08-898 EDS/EBSD studies and HR-EBSD pattern analysis on pre-Inca ceramic fragments recovered during San José de Moro Archaeology Program.
MS08-898 EDS/EBSD studies and HR-EBSD pattern analysis on pre-Inca ceramic fragments recovered during San José de Moro Archaeology Program.

Pre-Inca civilizations like the coastal cultures Moche and Nazca (Early Intermediate) and the inland culture Wari (Middle Horizon) were agrarian societies which supported indigenous elites of impressive wealth, power, and organization. With the expansion of the Wari Empire, the polychrome style and technique of Nazca propagated to the other cultures [1, 2]. High status burials, most of the Late Moche Fine Line ceramics and a large corpus of ceramics with Wari-derived decoration have been recovered in San José de Moro since 1991 [1]. The degree of transfer of procedures in this highly interactive scenario is of special interest: is there a limitation to decoration or is it adopted by the local potters also regarding the formulation of the ceramic bodies? In this context the relative amount, size and type of incorporated non-plastic inclusions as temper are important.

Two kinds of ceramic artefacts from Peru were compared: fragments of a baker in Wari Viñaque style (Wari, Ayacucho, Peru [3]) and a canteen in Mochica Polícromo style (San José de Moro, Jequetepeque valley, Peru [1]). Bulk composition and elemental distribution were analyzed by combined scanning electron microscopy and energy-dispersive X-ray spectroscopy on cross-sections (Fig. 1). Analyzing appropriate regions of interest in the element maps evidenced composition differences of the fired clay and the mineral inclusions (Fig. 1, 2). On principle, electron backscatter diffraction is appropriate for mineral phase analysis of the artifacts, but apart from quartz the crystallographic identification by automatic indexing commonly fails due to the micro granular aspect and the porosity of the body, weak diffraction patterns and varying composition of the feldspars. Image analysis of the Kikuchi pattern quality maps (Fig. 1) reveals the fractions of silty crystalline inclusions and of micro-pores in the clayey matrix. However, the clear crystallographic identification of non-plastic inclusions was possible by setting the electron beam at distinct crystals of interest, averaging accumulated patterns and comparing the resulting analysis with dynamical pattern simulation thereby identifying mostly quartz, ilmenite, magnetite, albite, epidote and kaersutite (Fig. 3), sanidine and apatite. Thus additional indications for the use as well as the provenience of the raw clays, the formulation of the ceramic material and the firing conditions can be provided.

We greatly acknowledge L.J. Castillo Butters, La Pontificia Universidad Católica del Perú and R. Chapoulie, Université Bordeaux Montaigne for giving the opportunity to contribute to the study and providing the artifacts.

  1. L. J. Castillo (2012) The multidimensional relations between the Wari and the Moche states of northern Peru. Boletin de Arqueologia PUCP 16: 53-77.

  2. D. Menzel (1964) Style and time in the Middle Horizon, Nawpa Pacha, 2: 1–105.

  3. D. Collier (1955) ARCHEOLOGY: Excavations at Wari, Ayacucho, Peru. Wendell C. Bennett. American Anthropologist, 57: 646–647.


Dagmar DIETRICH (Chemnitz, Germany), Gert NOLZE, Thomas MEHNER, Daniela NICKEL, Thomas LAMPKE
08:00 - 18:15 #4589 - MS08-900 Analysing Jan Steen's pigments using SEM-EDX quantitative X-ray mapping.
MS08-900 Analysing Jan Steen's pigments using SEM-EDX quantitative X-ray mapping.

The Seventeenth Century Dutch artist Jan Steen is famous for his humorous genre scenes in which he treats life as a vast comedy of manners. However, for such a renowned painter, there is surprisingly little technical information about his paintings. In an effort to correct this deficiency we have carried out a detailed technical examination of the paintings of Jan Steen held at the Mauritshuis as part of the Partnership in Science cooperation between Shell Nederlands B.V. and the Mauritshuis. An important part of this work was the characterisation of paint samples using SEM-EDX quantitative X-ray mapping. Compared to measuring individually selected pigment particles this approach has a number of advantages, namely: (i) all particles are measured; (ii) intra-particle details are discernible; and (iii) by using the spectral imaging method the complete data set is available for retrospective examination.

An example where this approach has proved particularly useful is in the study of Smalt. This is a cobalt containing potash glass that was used widely in Seventeenth Century European oil paintings as a blue pigment. The Smalt used in 12 Jan Steen paintings has been examined. Using quantitative X-ray mapping we are able to clearly identify the Smalt particles by using a Silicon/Potassium overlay, as shown in Figure 1. It is also possible to determine the composition of each Smalt particle by extracting the X-ray spectrum associated with each particle and quantifying using standardless methods. Using this approach made it possible to characterise the composition of over 350 Smalt particles. A summary of the results is shown in table I. These reveal that the composition of the Smalt used by  Jan Steen did not vary significantly over the course of his career. In addition, by using the mapping approach it is possible to discern differences in the Potassium distribution within individual particles.  For example, in Figure 1, a number of the larger Smalt  particles have a higher concentration of Potassium in their core which shows up as a region of yellow in the false colour map. To show this variation more clearly a linescan of the Potassium net counts has been extracted from the data which reveals that there is approximately twice as much Potassium in the centre of the particle. In addition, the concentration is constant across most of the centre region of the particle. This could prove helpful in obtaining a more reliable estimate of the original Potassium concentration in these particles so that a better comparison can be made between the degraded and non-degraded state of the Smalt. 

In conclusion, this work shows that quantitative X-ray mapping has a number of important advantages over measuring particles individually and, crucially,  it is now possible to use this approach routinely because with silicon drift detectors it is takes minutes to collect high quality data as compared to the many hours needed with the previous generation of detectors.

Acknowledgements

The analysis was possible due to the financial support of Shell Netherlands B.V.


Ralph HASWELL (Amsterdam, The Netherlands), Jesse WOUTERS, Sabrina MELONI
08:00 - 18:15 #5171 - MS08-902 Trapping of helium in nano-bubbles from 920 Ma euxenite crystals revealed by STEM-EELS analysis.
MS08-902 Trapping of helium in nano-bubbles from 920 Ma euxenite crystals revealed by STEM-EELS analysis.

The study of radiation damage resulting from alpha-decay from U and Th chains in radiocative U-Th minerals is of tremendous importance. Indeed, this damage critically affects the long term behavior of minerals. It is caused by both the ejection of recoil-nuclei and alpha-particles, leading to fundamental modifications in the physical and chemical properties of minerals. If the recoil-nuclei induced defects have been extensively studied, the consequences of the alpha-particles ejection have been less investigated. In particular, the (Y,REE,U,Th)-(Nb,Ta,Ti) oxides, like euxenite, fergusonite, pyrochlore, zirconolite, are known to contain nanometric spherical voids or bubbles, interpreted to contain radiogenic helium. However no direct evidence of the trapping of helium in these voids has been shown up to now. In this study, in-situ analyses by STEM EELS on individual nano-bubbles from an euxenite crystal, sampled in its host c. 920 Ma old pegmatite in Norway, deliver, for the first time, a positive identification of helium and an estimation of helium pressure in such bubbles. The chemically unaltered euxenite crystal proves amorphous and homogeneously speckled with bubbles ranging from 5 to 68 nm in diameter, around a log-normal distribution centred at 19 nm. The euxenite contains 9.87 wt% UO2 and 3.15 wt% ThO2. It accumulated a theoretical alpha-decay dose of 3.46 x 1020 α/g (i.e. 170 He/nm3), at a dose rate of 11926 α/g/s. This corresponds to production of 0.23 wt% He. The density of He inside the bubbles, estimated from EELS data, ranges from 2 to 45 He/nm3, leading to a pressure of 8 to 500 MPa. The proportion of produced He trapped in bubbles is about 10%. The bubbles, acting as traps, clearly influence He diffusion. They may contribute to the swelling of euxenite during amorphization and to the fracturing of the host rock. These results suggest that both dose and dose rate are key parameters for the nucleation, growth and coalescence of He bubbles. Simulation of the behaviour of high-level nuclear waste glasses by extrapolation from actinide doped glass or externally irradiated materials (very high dose rates) may not be predictive on the macroscopic effects (swelling, fracturing), the change of properties of material in which He accumulates, and the presence/size of He bubbles. Furthermore, because alteration is promoted by amorphization, fluid interaction with euxenite crystals saturated with He bubbles will mobilize and redistribute He through the entire rock. Helium may be infused in other minerals, e.g. zircon or apatite, the most utilized minerals for (U-Th)/He thermochronology, and may completely disturb (U-Th)/He dating.


Anne-Magali SEYDOUX-GUILLAUME (Saint Etienne Cedex 02), Marie-Laure DAVID, Kevin ALIX, Lucien DATAS, Bernard BINGEN
08:00 - 18:15 #5188 - MS08-904 Understanding Single-Crystal Paleomagnetism: A Multiscale Approach.
MS08-904 Understanding Single-Crystal Paleomagnetism: A Multiscale Approach.

The development of the single-crystal paleomagnetic approach has resulted in a new paradigm for paleomagnetism, enabling some of the most challenging paleomagnetic problems to be tackled for the first time: from measurements of the magnetic fields in the early solar system 1 to the search for evidence of Earth’s oldest magnetic field 2. Until now, characterization of the magnetic signal carriers in such unique samples has relied solely on two-dimensional cross sections of crystals measured using either TEM or SEM. This leaves many unanswered questions about the three-dimensional properties of the magnetic ensemble, without which single-crystal paleomagnetic measurements can be subject to uncertainties and errors caused by magnetic anisotropy or the presence of secondary magnetic minerals 3,4. We present a multi-scale paleomagnetic approach, which employs both nondestructive and destructive tomographic techniques combined with correlative magnetic measurements and micromagnetic modeling. This multi-scale approach addresses many of the shortcomings inherent in current single-crystal paleomagnetic studies by providing a sound physical framework for interpreting the magnetic behavior. We demonstrate this concept using results from focused ion beam nanotomography (FIB-nT) combined with finite element micromagnetic modeling to reconstruct the magnetic architecture of a single crystal from a chondritic meteorite 5 (Fig. 1A and B). Magnetic anisotropy in the grain arises from the sheet-like arrangement of Fe nanoparticles forming along sub grain boundaries (Fig. 1B).  Using individual particle geometries from the FIB nT volume micromagnetic modeling reveals new insights into the fundamental rock magnetic behaviour of particles with realistic shapes (Fig. 1C). We extend this methodology further by performing a correlative study of a zircon grain from the Bishop Tuff formation. The quantum diamond magnetometer (QDM) allows us to measure and map out localized magnetic sources in the zircon grain (Fig. 2A).  Correlative measurements of the sample using the SEM and EDS do not identify all the magnetic sources originating on the sample surface. Using high-resolution x-ray microscope (XRM) tomography we observe the presence of buried magnetic signal carriers in inclusions below the polished surface of the zircon grain (Fig 2B). Using our correlative workflow we can determine the relationship between magnetic signal carriers and the host zircon grain as well as other mineral inclusions. We demonstrate that understanding single-crystal paleomagnetism requires quantitative multiscale tomographic characterization in order to have confidence in the magnetic measurements reported.

 

References:

1 R.R. Fu, B.P. Weiss, E.A. Lima, R.J. Harrison, X.-N. Bai, S.J. Desch, D.S. Ebel, C. Suavet, H. Wang, D. Glenn, D. Le Sage, T. Kasama, R.L. Walsworth, and A.T. Kuan, Science. 346, 1089 (2014).

2 J.A. Tarduno, R.D. Cottrell, W.J. Davis, F. Nimmo, and R.K. Bono, Science. 349, 521 (2015).

3 T. Berndt, A.R. Muxworthy, and K. Fabian, J. Geophys. Res. Solid Earth 121, 15 (2015).

4 B.P. Weiss, A.C. Maloof, N. Tailby, J. Ramezani, R.R. Fu, V. Hanus, D. Trail, E.B. Watson, T.M. Harrison, S.A. Bowring, J.L. Kirschvink, N.L. Swanson-Hysell, and R.S. Coe, Earth Planet. Sci. Lett. 430, 115 (2015).

5 J.F. Einsle, R.J. Harrison, T. Kasama, P.Ó. Conbhuí, K. Fabian, W. Williams, L. Woodland, R.R. Fu, B.P. Weiss, and P.A. Midgley, Submitted (2016).

Acknowledgements:

J.F.E., P.A.M. and R.J.H. would like to acknowledge funding under ERC Advanced grant 320750- Nanopaleomagnetism. P.A.M. would also like to acknowledge funding under ERC Advanced grant 291522 - 3DIMAGE. This work was performed (in part) at the South Australian node of the Australian National Fabrication Facility under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers. 


Joshua Franz EINSLE (Cambridge, United Kingdom), Roger R. FU, Jeff GELB, Benjamin P. WEISS, Richard J. HARRISON, Paul A. MIDGLEY
08:00 - 18:15 #5816 - MS08-906 Soil traces in forensic practise.
MS08-906 Soil traces in forensic practise.

Introduction

Requirements for analysis of relicts of soils, abrasions, or contamination of clothing, footwear, human body, vehicle, tools, etc., are relatively frequently encountered in forensic practice. There are two basic types of tasks - We can compare these traces with reference samples of soils from places where the traces may have occurred, or a more complicated task -forecasting an unknown location of the origin of a pedological trace. In both cases, it is a relatively complex examination involving a wide range of individual investigations, whereas to a forensic lab can be submitted practically any material produced by activities of a human being and nature. Therefore, an organic part, pollen, relicts of soil microorganism shells, etc. are assessed separately within the analysis of pedological traces

Methodology

Basic techniques are optical light microscopy in transmitted and reflected light, polarization, fluorescence, next are utilised SEM/EDS (WDS), XRF, FTIR, Raman spektroscopy, XRD, micro XRD, etc.

Technique of cathode luminescence (CL) capable of distinguishing material based on its genesis, thus usually also according to a different place of occurrence, were introduced to differentiate mineralogical phases of practically identical chemical composition, optical characteristics, structure and similar inclusions.

Biological material frequently present in the traces is analysed separately, plant and animal relicts are analysed, including microscopic shells and relics.

Anthropogenic material (glass fragments, slag, construction materials, etc.) is analysed separately by other techniques and may increase the probability measure of a match (hit) between traces and reference samples.

For forecasting of unknown locations, where the contamination by soil may have occurred, systems of geographic information (GIS) are used, in which data are connected with detailed geological maps, both uncovered and covered, and with pedological maps.

The complex of methodologies is based on mineralogical and physical-chemical methods better embracing particularities of soil composition than some applied procedures arising from purely chemical base. Nevertheless, the introduced system at the same time is not omitting even biological and anthropogenic materials that usually represent a very important comparative material. Genetic methods that could compare both information from plant fragments and soil microorganisms are a big potential into the future.

Most of the analytical methods described yield quantitative data for data comparison. One of the possibilities bringing to soil analysis quantitative data that can be used e.g. for statistical comparison of traces and reference samples among each other are systems of automatic analysis of mineral grains (based on SEM/EDS). These systems have been available at the market for several years, nevertheless, to date samples of mineral grains recovered from soil traces have been prepared into casting compounds and then have been cut into polished section forms. Only then were samples analysed in systems of automatic mineralogical analysis. Our laboratory for the first time carried out in cooperation with the producer a number of detailed experiments with analysis of samples prepared directly from mineral grains of a soil trace, fixed at an adhesive stub with carbon target for SEM in diameter of 30mm. The experiments were at first conducted with model samples prepared from known mineral phases and then with samples from common soils. Several series of random samples were taken in a final stage from different places and the ability to determine differences among individual sampling points was tested. It was found that the differences are traceable also among sampling sites that were at a distance less than 20m. This technology, of course, is not a panacea, but appropriately compliments a complex analysis of soil phases.

Summary

The use of new methods considerably extends possibilities of typical microscopic procedures in a forensic field and allows to obtain the needed quantitative data in a forensic analysis of pedological phases, differentiation of analogous mineral phases, or the option of analysis of the organic phase directly in SEM chamber.

The technology described suitably supplements the complex analysis of soil phases.

 

Acknowledgements - microanalytical methods at ICP were supported by projects: VD20062008B10, VD20072010B15, VG20102015065, VF20112015016, VF20122015027, VI20152020035.


Marek KOTRLÝ (Praha, Czech Republic)
08:00 - 18:15 #6046 - MS08-908 Forensic microscopy - a basic method of microtraces examination.
MS08-908 Forensic microscopy - a basic method of microtraces examination.

Microscopy constitutes one of the pillars in forensic investigation of traces and samples from a crime scene. Currently, among classic methods of the examination of microtraces using particularly visible radiation is so-called optical microscopy, be it in the mode of incident light or transmitted light. Simple polarized microscopy can lead an experienced expert to identification of objects. In some cases, it is necessary for trace or sample identification to use also combination with fluorescent microscopy.

Electron microscopy and microanalysis belong to crucial applications in investigation traces from crime scene in forensic practise. SEM with EDS/WSD allow a rapid screening and gaining initial information for a wide range traces, including a view of their surface and evaluation of morphology.

Microscopic methods are widely employed to analyse of so-called “microtrases”, it means latent traces (latents), unperceivable by naked eye, but always related to a criminal case. They can involve gunshot residues (GSR) and post-blast residues (PBR), soils and mineral grains. Microscopy is further applied also in other fields - for example in examinations of handwriting strokes (intersecting lines), biology (morphological evaluation of bio objects), ballistics (composition of bullets), etc.

Investigation of paints and pigments is another important field. Each of these used microscopic methods have their advantages and disadvantages, a great asset of optical microscopy is a view and assessment of samples in their natural colours. By methods of optical microscopy we can not only observe morphology of the surface and possible visible marks, but we can also study stratigraphy in samples of paints and pigment in colours of individual layers, which are important features for comparison. Electron microscopy allows to image traces or samples in the shades of grey, however, its options to discriminate and at the same time possibilities of spectral microanalysis are essential for an analysis.

A further significant field of the use of microscopic and spectroscopic forensic methods is the detection of forgeries of works of art. Qualification of crimes includes a wide range of violations of the law, from an illegal export of works of art, determination of the ownership right, theft of an artwork, to a fraudulent production of forgeries of prominent authors and their introduction to sale. Thanks to a combined use of microscopic and spectral techniques, it is possible to collect key arguments for comparative analysis of the applied materials, assess morphological and structural marks of pigments, to identify bonding agents and judge painter´s technique of the work. In the Czech Republic, forgeries of outstanding Czech artists of the 20th century are encountered most frequently, such as those of Jan Zrzavý, Emil Filla, Josef Čapek and Pravoslav Kotík. In 2007, two paintings of Pravoslav Kotik with a similar theme appeared on the Czech market that sparked controversy over the authenticity of both artworks and subsequently the whole counterfeiting and with it associated trafficking  practices were revealed. The proposed contribution deals with comparative microscopic studies of pigments taken from of works of art.

From other microscopic applications, microscopic and spectroscopic forensic methods are important in assessing fragments of car paints found at a crime scene.

Acknowledgements: Microanalytical methods at Institute of Criminalistics Prague were supported by grant-aided projects of the Czech Republic Ministry of Interior RN 19961997008, RN 19982000005, RN 20012003007, RN 20052005001, VD20062008B10 a VD20072010B15, and the Project of Security Research – Development of selected methods for forensic identification of persons and objects” - VF2012201507.


Ivana TURKOVA (Prague, Czech Republic)