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

Chairmen: Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX, FRANCE), Niels DE JONGE (Saarbrücken, GERMANY), Gerhard DEHM (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) (Paris, 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, UK), 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

Chairmen: Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX, FRANCE), Niels DE JONGE (Saarbrücken, GERMANY), Gerhard DEHM (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 (VILLEURBANNE CEDEX), 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 (Grenoble cedex 9), 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

Chairmen: Sara BALS (Antwerpen, BELGIUM), Wolfgang LUDWIG (Lyon, FRANCE), Sergio MARCO (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 (VILLEURBANNE CEDEX), 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, UK), 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 (ORSAY), 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

Chairmen: Catherine BOUGEROL (Grenoble, FRANCE), Vincenzo GRILLO (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 (Amsterdam, THE NETHERLANDS), 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, UK), 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

Chairmen: Rick BRYDSON (Leeds, UK), Marc SCHMUTZ (Strasbourg, FRANCE)
10:30 - 11:00 Cryo MEB. Roger A. WEPF (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, UK), 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, UK), 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, UK), 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, UK), 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

Chairmen: Joanne ETHERIDGE (Director) (Melbourne, AUSTRALIA), Jean-Christophe OLIVO-MARIN (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

Chairmen: David MCCOMB (Columbus, USA), Jérôme PLAIN (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 CEDEX)
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, UK), 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

Chairmen: David MCCOMB (Columbus, USA), Jérôme PLAIN (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

Chairmen: Joanne ETHERIDGE (Director) (Melbourne, AUSTRALIA), Jean-Christophe OLIVO-MARIN (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, UK), 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, UK), 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, UK), 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

Chairmen: Bettina BOETTCHER (Edinburgh, UK), Karen DAVIES (Staff Scientist) (Berkeley, USA), Guy SCHOEHN (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

Chairmen: Ilaria FERLENGHI (Structural Based Antigen Design) (Siena, ITALY), Kay GRUNEWALD (Oxford, UK), Olivier LAMBERT (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 (GRENOBLE CEDEX 9)
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

Chairmen: Grégoire MICHAUX (Rennes, FRANCE), Cristina PUJADES (co-chair) (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

Chairmen: Hans HEBERT (Huddinge, SWEDEN), Aurélien ROUX (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, UK)
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 (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, UK), 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, UK), 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, UK), 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, UK), 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 (Bagnols sur Cèze Cedex), 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 (Bagnols sur Cèze Cedex)
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 (VILLEURBANNE CEDEX), 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, UK), 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, UK), 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, UK), 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.