Wednesday 31 August
08:45

Wednesday 31 August

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PL4
08:45 - 09:45

Plenary Lecture 4

08:45 - 09:45 Plenary Lecture 4. Hirofumi YAMADA (Kyoto, Japan)
Amphithéâtre
10:15

Wednesday 31 August

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MS6-II
10:15 - 12:30

MS6 : Oxide-based, Magnetic and other Functional materials
SLOT II

Chairmen: Etienne SNOECK (Toulouse, France), Maria VARELA (Madrid, Spain)
10:15 - 10:45 #6341 - MS06-S81 Atomic structures and dynamic behaviors of domain walls in ferroelectric thin films.
Atomic structures and dynamic behaviors of domain walls in ferroelectric thin films.

Ferroelectric materials are characterized by a spontaneous electric polarization that can be reoriented between different orientations by an applied electric field.  The ability to form and manipulate domains with different polarization orientations at the nanometer scale is key to the use of ferroelectric materials for devices such as nonvolatile memories.  The ferroelectric switching occurs through the nucleation and growth of favorably oriented domains and is strongly mediated by defects and interfaces. Thus, it is critical to understand how the ferroelectric domain forms, grows, and interacts with defects.  Here, we demonstrate the ability of studying the atomic structure and dynamic behaviors of ferroelectric domain walls using the state-of-the-art atomic-resolution scanning transmission electron microscopy (STEM) and in situ transmission electron microscopy (TEM) techniques.

Figure 1a shows a high-angle annular dark-field (HAADF) image of a BiFeO3 thin film in [100] pseudocubic orientation. With a quantitative analysis of this image, the displacement (DFB) vector was determined, which is measured by the displacement of the Fe position (which is located in the center of an oxygen octahedron) from the center of its four Bi neighbors. The vector DFB as the manifestation of the ferroelectric polarization in BiFeO3, points toward the center of the negative oxygen ions (Fig. 1b), and thus can be used to determine the polarization vector in the image plane. Using the polarization mapping technique based on HAADF imaging, we found that a localized vortex domain structure can be formed at the termination of 109° domain walls at the BiFeO3/TbScO3 interface. While in thicker (20 nm) BiFeO3 films (Fig. 1c), the vortex is accompanied by triangular domains consist of a mirrored pair of inclined 180° domain walls in conjunction with the previously existing 109° domain wall; no obvious triangular nano-domain patterns are observed in thinner (5 nm) films (Fig. 1d), and instead, the polarization vectors rotate smoothly to form a vortex core. These observations indicate that the configuration of the spontaneous flux-closure domains strongly depends on the size of the system, suggesting a potential method to tune the polarization vortex structures for practical devices.

Figure 2 shows in situ switching of domains in a 20 nm thick BiFeO3 films grown on TbScO3 substrate by applying a bias between a W probe and an epitaxial La0.7Sr0.3MnO3 bottom electrode. A charged domain wall (CDW) can be created by applying a bias (Fig. 2a). The initial stable structure (Fig. 2b) contained 109° and 180° domain walls separated by 10 nm at the substrate interface. The onset of domain wall motion occurred at a critical bias of 1.7 V (Fig. 2c). As the bias increases, the two 109° and 180° domain walls moved toward each other until they intersected (Fig. 2d). Then further shrinkage of the triangular domain led to upward motion of the triangular domain tip and resulted in the formation and elongation of a CDW (Fig. 2e,f). After the bias was removed, a stable state of the CDW remained (Fig. 2g). Such switching processes involving the evolution of CDWs result in significant changes in the local resistance of the film, suggesting the CDW may play an important role in future memory devices. 

   

References:

[1] C. T. Nelson, etc., Nano Letters 11 (2011), 828-834.

[2] This work was supported by the US Department of Energy (DOE) through the grant DE-SC0014430.

Xiaoqing PAN (, USA), Linze LI
Invited
10:45 - 11:00 #5897 - MS06-OP282 Polar-graded multiferroic SrMnO3 thin films.
Polar-graded multiferroic SrMnO3 thin films.

Engineering defects and strains in oxides provides a promising route for the quest of thin film
materials with coexisting ferroic orders, multiferroics, with efficient magnetoelectric coupling at
room temperature. Indeed, precise control of strain gradients would enable custom tailoring of
multiferroic properties, but presently remains challenging.

Here we use aberration-corrected scanning transmission electron microscopy (STEM) to explore

the existence of a polar-graded state in epitaxially-strained antiferromagnetic SrMnO3 thin films,

whose polar nature was predicted theoretically [1] and recently demonstrated experimentally [2].
Combining annular bright field (ABF) with high angle annular dark field (HAADF) imaging we
map at the atomic scale level the cation-oxygen dipole distortion far from (Figure 1) and near to
the domain walls (Figure 2). Local deformation analyses reveal a particular strain state where
flexoelectricity –the coupling between strain gradients and polarization– induce a rotation of the
in-plane ⟨110⟩ electric polarization due to ⟨001⟩ strain gradients. The analysis of the local Mn
oxidation state by electron energy loss spectroscopy (EELS) reveals that the strain gradients are
related to a gradual distribution of oxygen vacancies across the film thickness. Here we present a
chemistry-mediated route to induce flexoelectric polar rotations in oxygen-deficient multiferroic
films with potentially enhanced piezoelectricity.


References
[1] Lee, J. H.; Rabe, K. M. Phys. Rev. Lett. 2010, 104 (20), 207204.
[2] Becher, C.; Maurel, L.; Aschauer, U.; Lilienblum, M.; Magén, C.; Meier, D.; Langenberg,
E.; Trassin, M.; Blasco, J.; Krug, I. P.; Algarabel, P. A.; Spaldin, N. A.; Pardo, J. A.; Fiebig, M.
Nat. Nanotechnol. 2015, 10, 661–665.

Roger GUZMAN, Roger GUZMAN (Bellaterra, Spain), Laura MAUREL, Eric LANGENBERG, Andrew R. LUPINI, Pedro A. ALGARABEL, José A. PARDO, César MAGÉN
11:00 - 11:15 #5928 - MS06-OP283 Control of octahedral rotations via octahedral connectivity in an epitaxially strained [1 u.c.// 4 u.c.] LaNiO3/LaGaO3 superlattice.
MS06-OP283 Control of octahedral rotations via octahedral connectivity in an epitaxially strained [1 u.c.// 4 u.c.] LaNiO3/LaGaO3 superlattice.

ABO3 provskites represent a board spectrum of intriguing functionalities such as metal-insulator transitions, multiferroicity, colossal magnetoresistance and superconductivity owing to the strong correlation between charge, spin and orbital degrees of freedom [1]. In recent years, heterostructures and superlattices formed by two or more different ABO3 perovskites have  received intense research interest. Due to the electronic and structural reconstructions at the heterointerfaces, novel phenomena may emerge opening the way to novel functionalities that are not accessible in their bulk counterparts [2].  In ABO3 perovskites, the magnetic and electronic states are strongly coupled to the B-O-B bond angles and B-O bond lengths. Therefore, precise control of the BO6 octahedral rotations and distortions via epitaxial strain and interfacial octahedral connectivity offers a promising route to tailoring the material properties in a controllable way [3].

In our previous study of a [(4 u.c.// 4 u.c.) ×8] LaNiO3/LaGaO3 superlattice grown on (001) SrTiO3 by using aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM), we have shown that the response of NiO6 rotations to epitaxial strain in the LaNiO3/LaGaO3 superlattice is significantly different from that in LaNiO3 thin films [4]. In LaNiO3 thin films, epitaxial strain effectively modifies the NiO6 rotational magnitudes throughout the entire film therefore stabilizing new electronic and magnetic states [3]. However, in the [(4 u.c.// 4 u.c.) ×8] LaNiO3/LaGaO3 superlattice, the [100] and [010] rotational magnitudes of NiO6 and GaO6 relax to the bulk values of LaNiO3 and LaGaO3 even though the superlattice is still coherently strained.

Here, we investigated the local octahedral rotations in a [(1 u.c.// 4 u.c.) ×13] LaNiO3/LaGaO3 superlattice grown on a (001) SrTiO3 substrate. Figure 1 shows the atomic models of bulk LaNiO3 and LaGaO3 and the simulated [110] AC-HRTEM images. Figure 2 represents an experimental AC-HRTEM image near the surface of the superlattice. We found that, due to the octahedral connectivity at LaNiO3-LaGaO3 interfaces (see Fig. 1e), the octahedral rotations of NiO6 adopted the same [100] and [010] rotational magnitudes as the neighbouring GaO6 till the surface of the superlattice. Our results indicate that in LaNiO3 based superlattices, the octahedral rotations of NiO6 can be precisely controlled via interfacial octahedral connectivity when the thickness of the LaNiO3 layer is reduced to 1 unit cell.

[1] Imada M, Fujimori A, Tokura Y (1998) Metal-insulator transitions, Rev. Mod. Phys. 70:1309-1263

[2] Hwang HY, Iwasa Y, Kawasaki M, Keimer B, Nagaosa N, Tokura Y (2012) Emergent phenomena at oxide interfaces, Nat. Mater. 11:103-113

[3] Rondinelli JM, May SJ, Freeland JW (2012) Control of octahedral connectivity in perovskite oxide heterostructures: an emerging route to multifunctional materials discovery, MRS Bull. 37:261-270

[4] Qi HY, Kinyanjui MK, Biskupek J, Geiger D, Benckiser E, Habermeier HU, Keimer B, Kaiser U (2015) Local octahedral rotations and octahedral connectivity in epitaxially strained LaNiO3/LaGaO3 superlattices, J. Mater. Sci. 50:5300-5306

[5] We are grateful to Sabine Grözinger for cross-section TEM sample preparation. We gratefully acknowledge the financial support by the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the state Baden-Württemberg within the DFG: KA 1295/17-1 project.

Haoyuan QI (Ulm, Germany), Michael KINYANJUI, Xiaodan CHEN, Johannes BISKUPEK, Dorin GEIGER, Eva BENCKISER, Hanns-Ulrich HABERMEIER, Bernhard KEIMER, Ute KAISER
11:15 - 11:30 #6219 - MS06-OP287 Direct observation of interfacial coupling of oxygen octahedra and its impact on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures.
MS06-OP287 Direct observation of interfacial coupling of oxygen octahedra and its impact on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures.

La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) heterostructures have been intensively studied because of large values of tunnel magnetoresistance (TMR), which make it a promising material for spintronic devices based on magnetic tunneling junctions. However, the “dead-layer” behavior of the manganite layers, a quick decay of the magnetoresistance and metallicity with the decreasing thickness and eventually insulating behavior below certain critical thickness (4-7 u.c.), hinder the applications. The origins of this behavior were reported to be strongly related to the interfacial reconstruction, such as magnetic and orbital coupling, epitaxial strain, and carrier doping charge-effect.[1-2] Recently it has been also recognized that the metal-oxygen bonding modification due to oxygen octahedral tilt can also play an important role in the magnetism double-exchange interaction.[3-4] However, an atomistic understanding of how oxygen octahedral distortions are introduced to accommodate octahedral mismatch at a heterointerface is still a challenge because it requires a precise simultaneous determination of the positions of metal cations and oxygen.

We used the annular bright-field (ABF) imaging in a Cs-corrected scanning transmission electron microscope (STEM, JEOL ARM200F) to image directly both heavy cations and oxygen atoms in the real space. Two samples grown on cubic STO(100) substrate were studied: (a) a sandwich-like sample consisting of two LSMO layers of 10 and 20 u.c thick, separated by a 3 u.c thick STO middle layer; (b) a multilayer sample with LSMO-STO  consisting of 15 bilayers of 3-4 u.c LSMO and 6 u.c. STO. Magnetometry measurements showed that the LSMO layers of the trilayer sample had large Curie temperature for the onset of ferromagnetic order (larger than 250 K) while the LSMO-STO multilayer had a very low Curie temperature of less than 50 K and a very low magnetization, indicating very poor magnetic order. In Fig.1(a), a high magnification STEM-ABF image of LSMO at the 2nd layer (20 u.c.) is shown which is far from the strained interface and well present a bulk-like LSMO structure. The corresponding atomic model of LSMO with MnO6 tilt a-a-c- is also shown under the ABF image, where we can see clearly the oxygen octahedra with the anti-phase tilting of ~10o along [110] axis. The quantitative analysis of oxygen octahedral tilt angle is done for both samples, as shown in Fig.1 (b) and (c). At the interface between STO substrate and LSMO in both samples, a clear suppression of oxygen octahedral tilt of LSMO in the first 2-3 u.c. is found, and then the tilt gradually recovers when it is away from the interface, until reaching the bulk value at around 9th u.c. (as seen in the Fig.1 (b) 2nd LSMO layer). Moreover, the 3 u.c. thick LSMO layers in the multilayer are highly distorted and have a suppressed octahedral tilt of lower than 5o, which can well explain the degradation of their ferromagnetic order and transport properties. Interestingly, we also found that the top 2-3 u.c. of the STO have a non-zero apparent oxygen octahedra tilt, especially in the middle layer, of up to ~3o, and this finding correlates with the previous reported magnetic moment found at the interfacial Ti cations. [1]

Here, we directly evidenced the strong relation between the oxygen octahedral tilt and the magnetism. We found a greatly suppressed tilting in LSMO due to the interfacial coupling of adjacent oxygen octahedral of cubic STO, which gives rise to dramatic deterioration of the magnetic and transport properties when the LSMO layer is thinner than 3 u.c.. Therefore, for good TMR properties of LSMO-STO heterostructures, a critical thickness of LSMO of at least 7 u.c. (3 u.c. away from both interfaces) is required for preserving ferromagnetism, and a critical thickness of STO of at least 5 u.c. (2 u.c. away from both interfaces) for a good insulating barrier.

[1] F. Y. Bruno, et.al. Physical Review Letters 106 (14), 147205 (2011).

[2] R. Peng, et.al. Applied Physics Letters 104 (8), 081606 (2014).

[3] X. Zhai, et.al.  Nature Communications 5, 4283 (2014).

[4] Q. He, et.al. ACS Nano 9, 8412 (2015)

Xiaoyan LI (Orsay), Ionela VREJOIU, Michael ZIESE, Peter VAN AKEN
11:30 - 11:45 #6798 - MS06-OP294 A study of polymorph dynamics in mixed-phase BiFeO3 thin films via AFM and in-situ TEM applications of external stimuli.
MS06-OP294 A study of polymorph dynamics in mixed-phase BiFeO3 thin films via AFM and in-situ TEM applications of external stimuli.

Epitaxially strained BiFeO3 (BFO) thin films have seen an upsurge of interest over the past decade with the revelation of fascinating characteristics such as giant ferroelectric polarisation, exceptional piezoelectric and magnetoelectric responses to name a few.[1] The growth of BFO on substrates providing a large in-plane compressive strain which facilitates structural alterations of the material to that of a tetragonal-like phase (T-phase) and distorted rhombohedral-like phase (R-phase) has opened the door to a plethora of opportunities in terms of controlling nanoscale interfaces and expanding our understanding of solid state physics. In a recent review by Nagarajan et al. (2016),[2] it was advised that there are still a number of exciting possibilities as well as pending questions which need to be addressed before the functionality of mixed-phase BFO thin films can be driven to the next level.

Using both aberration-corrected TEM and STEM we have performed a high resolution study of the lattice parameters which characterise the ‘R’ and ‘T’ phase (Figure 1) and mapped the strain evolution (via geometrical phase analysis (GPA)) between polymorphs using the LaAlO3 substrate as reference. Having identified the metastable ‘T’ polymorph and the extent to which the thin film (and FIB-prepared lamella) thickness can influence the ratio of ‘R’ to ‘T’ polymorphs, we demonstrate the thermal energy required to expand the dimensions of the highly strained ‘T’ polymorph through in-situ heat cycling experiments (Figure 2), and explain the complexity involved in such a transition.

The relative ease through which transitions from one phase to another can be achieved varies depending on the application of stimuli selected. In this study we use a variation of techniques to explore the physical phenomena behind the functionality of mixed-phase BFO. We highlight the benefits of reversibly reorganising phases via the application of external stimuli with an AFM probe tip. Through precise SEM and FIB techniques we have identified the pre-written AFM areas and milled cross-sectional lamellae across these areas for post-AFM analysis using STEM. With this unique study we detect and compare structural differences between the as-grown polymorphs in a native lamella and polymorphs written through external stimuli via an AFM tip. We show differences in lattice parameters and strain evolution in the newly written polymorphs via nano-beam electron diffraction using a 2nm sized electron probe; this precision has allowed us to explicitly map the strain across a large area of these polymorphs. With the addition of EELS focusing on the Fe-L2,3 and O-K edges we show changes in ELNES features which suggest octahedral distortion variations between polymorphs.

By combining AFM and state-of-the-art STEM techniques in this distinctive way we provide an insight into the link between exciting interfacial behaviour seen via AFM at the polymorph boundaries on the microscopic scale, and structural polymorph changes seen via STEM on the atomic scale.

 

References

[1] R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21-29 (2007)

[2] D. Sando, B. Xu, L. Bellaiche and V. Nagarajan, Appl. Phys. Rev. 3, 011106 (2016)

Kristina HOLSGROVE (Belfast, United Kingdom), Martial DUCHAMP, Niall BROWNE, David EDWARDS, Nicolas BERNIER, Dipanjan MAZUMDAR, Marty GREGG, Amit KUMAR, Miryam ARREDONDO
11:45 - 12:00 #6826 - MS06-OP295 High resolution STEM analysis of temperature stable relaxors.
MS06-OP295 High resolution STEM analysis of temperature stable relaxors.

There is a need for dielectric materials with high relative permittivity (εr) values that can operate at high temperatures.  Such materials have uses in new automotive, aerospace and energy technologies where there are demands for capacitors to operate in increasingly demanding high temperature environments. One particular ceramic system that shows promise is (1-x)Ba0.8Ca0.2TiO3-(x)BiMg0.5Ti0.5O3 (BCT-BMT) [1].

Typical relaxor materials display a broad εr peak when plotted against temperature [2]. Their behaviour may be explained by the presence of nano-domains [3, 4], or polar nano-regions (PNRs), which act to broaden the dielectric peak through a distribution of relaxation times.  However, a special class of relaxors display εr plots with flat temperature responses up to 500 ºC.  These are termed temperature-stable relaxors and BCT-BMT is such a material.  This behaviour cannot be explained by current understanding of relaxor PNRs, and so further investigation of structural and chemical differences in the nanostructure is required.  One technique ideally suited to probing such features is atomic resolution scanning transmission electron microscopy (STEM).

Initial studies of the material with composition (Ba0.4 Ca0.1 Bi0.5) (Mg0.25 Ti0.75)O3, which displays typical temperature stable relaxor behaviour, have been carried out using aberration corrected STEM using a Nion UltraSTEM 100.  STEM high angle annular dark-field (HAADF) imaging at the atomic level suggests uniform crystallinity; however measurements of displacements of B-site atomic columns within the projected lattice on the nanoscale are detectable.  These localised, coherent B-site displacements (in the form of aligned nanodomains) may respond to changes in temperature in a different way to normal relaxor materials and their detection could go some way to explain the temperature stable behaviour.  Atomically resolved electron energy loss spectroscopy (EELS) spectrum imaging (SI) was used to identify potential chemical inhomogeneity which might also contribute to a distinctive polar nanostructure (Figure 1) and help explain the dielectric behaviour. However there are challenges associated with this type of EELS analysis: the Ca L2,3-edge was obscured by the tail of the C K-edge, the Mg K-edge is weak and the Bi M4,5-edge sits at a high energy loss.  Nevertheless, preliminary studies suggest significant column-to-column variations in HAADF intensity for both A and B sites in BCT-BMT for the very thinnest regions of the STEM specimen, suggesting significant short-range variations in composition. 

Present studies are concentrating on detecting any chemical inhomogeneities in the sample using STEM energy dispersive X-ray (EDX) mapping, which will complement EELS by being able to map the Ca, Mg and Bi elemental signals without overlap.  The new 300 kV FEI Titan3 Themis G2 STEM installed at Leeds University with its FEI SuperX EDX system is capable of producing elemental maps with sufficient resolution and sensitivity, as demonstrated in EDX maps shown in Figure 2.  Here, EDX mapping has been carried out on a different sample, in an area containing an interface between SrTiO3 and (0.7)BiFeO3 – (0.3)PbTiO3.  The positions of the columns of Bi are clearly visible, and it is anticipated that such capability will mean that more information can be gained about any possible chemical segregation in BCT-BMT.  

(1)  A Zeb and S J Milne. J. Am. Ceram. Soc., 96 [9] 2887-2892. (2013)

(2)  M Groting, S Hayn, K Albe J Solid State Chem 184, 2041-46. (2011)

(3)  C. A. Randall, D J Barber, R W Whatmore and P Groves.  J. Mater. Sci. 21,  4456. (1986)

(4)  A Feteira, D C Sinclair and J Kreisel. J. Am. Ceram. Soc. 93 (12) 4174-4181. (2010)

Michael WARD (Leeds, United Kingdom), Steven MILNE, Aurang ZEB, Faye ESAT, Nicole HONDOW, Andy BROWN, Rik BRYDSON, Sophia ANDERSSON, Ian MACLAREN, David HERNANDEZ MALDONADO
12:00 - 12:30 #7856 - MS06-S82 Strain-driven oxygen deficiency in multiferroic SrMnO3 thin films.
Strain-driven oxygen deficiency in multiferroic SrMnO3 thin films.

In the last decade, multiferroic materials exhibiting simultaneous magnetic and ferroelectric order with strong magnetoelectric coupling above room temperature have attracted great interest. This is motivated by the expectation of integrating them into low-power spintronic nanodevices, where the magnetic order is controlled by low energy consuming electric fields instead of magnetic fields. Manganese-based perovskite oxides are particularly promising for this purpose. Recently, it was suggested that strain-driven multiferroicity associated with spin-phonon coupling should arise in strained SrMnO3 (SMO): under epitaxial strain a polar instability in the ferromagnetic phase leads to a substantial energy lowering, which stabilizes the ferromagnetic-ferroelectric multiferroic phase over the bulk antiferromagnetic-paraelectric phase [1]. Thus, the spontaneous polarization is driven by the off-centering of the magnetic Mn4+ cations from the MnO6 octahedra and increases by expansion of the lattice.

However, the SMO structure tends to incorporate oxygen vacancies, which makes it very difficult to synthesize fully stoichiometric SMO samples. Moreover, for SMO films, a small biaxial tensile strain of 2% is predicted to increase the oxygen vacancy concentration by one order of magnitude at room temperature [2]. Understanding the role of the oxygen non-stoichiometry in this material is of fundamental importance for the development of multiferroic SMO thin films as perfectly stoichiometric highly resistive samples are required.

Here, we focus on the structural and electronic properties of several SMO thin films grown on different substrates, namely LSAO, LAO, LSAT, STO and DSO, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS). The structural parameters of the SMO films are obtained by geometrical phase analysis of HAADF-STEM images and are plotted as a function of nominal strain in Fig. 1. EELS data were acquired for all the samples: the obtained oxygen K-edge spectra show pronounced peak-height changes and energy shifts as a function strain (Fig. 2). The experimental results are interpreted with the aid of the all-electron density functional theory (DFT) code WIEN2k (see Fig. 3). Thus, the effect of oxygen vacancies on the O K and Mn L2,3 electron energy-loss near-edge structures (ELNES) is theoretically investigated [3].

[1] J. H. Lee and K. M. Rabe, Phys. Rev. Lett. 104, 207204 (2010).

[2] C. Becher et al. Nature Nanotech. 10, 661 (2015).

[3] This research was supported by the Swiss National Science Foundation under the project number 200021_147105. We thank J. Guo and P. Yu for providing the samples.

Marta D. ROSSELL (Dübendorf, Switzerland), Piyush AGRAWAL, Cécile HÉBERT, Daniele PASSERONE, Rolf ERNI
Invited
Amphithéâtre

Wednesday 31 August

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MS5-I
10:15 - 12:30

MS5: Energy-related materials
SLOT I - Different aspects

Chairmen: Wolfgang JÄGER (Kiel, Germany), Joachim MAYER (Aachen, Germany), Philippe MOREAU (IMN, Nantes, France)
10:15 - 10:45 #4442 - MS05-S76 Correlative atom probe tomography and electron microscopy on energy materials.
Correlative atom probe tomography and electron microscopy on energy materials.

We present recent progress in correlative methods for the joint analysis of samples by Atom Probe Tomography (LEAP 3000, LEAP 5000) and Electron Microscopy (Cs corrected Titan Themis). Measurements are conducted on the same Atom Probe sample tips and in some cases atomic resolution is reached.

Examples from functional and structural energy-related materials are presented including segregation effects in multicrystalline silicon solar cells and their relation to cell efficiency (Fig. 1), superalloys for advanced turbines and high strength steels (Fig. 2).

Andreas STOFFERS, Christian LIEBSCHER, Pyuck-Pa CHOI, Michael HERBIG, Christina SCHEU, Gerhard DEHM, Ivan POVSTUGAR, Dierk RAABE (Duesseldorf, Germany)
Invited
10:45 - 11:15 #8697 - MS05-S77 Combining advanced TEM techniques for full characterization of extended defects in creep-deformed single crystal superalloys.
Combining advanced TEM techniques for full characterization of extended defects in creep-deformed single crystal superalloys.

Single crystal superalloys are used for turbine blades in the hottest parts of gas engines, where they have to withstand high stresses at temperatures exceeding 1000°C. This is only possible because of the well-known γ/γ‘-microstructure of these alloys, forming the basis for excellent creep properties [1]. One of the research objectives is to gain a microscopic understanding of the elementary deformation mechanisms and defect processes that govern high-temperature creep and to use this knowledge for further improvement of single crystal superalloys. Beside Ni-base superalloys which are well established in applications new Co-base superalloys have attracted a lot of research interest since the discovery of stable γ/γ‘-microstructure in these alloys in 2006 [2]. Research on Co-base superalloys is still in its infancy, meaning that there is strong need for fundamental studies on elementary deformation mechanisms and defect processes.

In this contribution we give an overview of our recent research on extended defects that form upon high-temperature creep in Ni- and Co-base superalloys. We demonstrate that the combination of advanced transmission electron microscopy (TEM) techniques, employing dedicated diffraction, high-resolution and analytical methods, is generally required for full characterization of such extended defects and for obtaining a complete picture of their role in elementary creep processes.   

Figure 1 illustrates an example of full characterization of superdislocations in a Ni-base superalloy. First, large-angle convergent beam electron diffraction (LACBED) is employed for reliable determination of the Burgers vector [3], overcoming the well-known problems of conventional Burgers vector analysis of such dislocations. Secondly, using a new FIB cross-sectioning technique [4], the core structure of the same dislocation is studied by high-resolution scanning TEM (HRSTEM). This approach enables scale-bridging characterization of complex superdislocations on the micrometer and atomic scale.

Figures 2 and 3 summarize advanced TEM studies on extended faults that form upon creep of Ni-containing Co-base (CoNi) superalloys [5]. Inside of γ‘ precipitates a characteristic defect configuration is observed in which  superintrinsic stacking fault (SISF) loops are fully embedded in antiphase boundaries (APB). LACBED is used to determine the full Burgers vector (including sign and magnitude!) of the partial dislocation loop [5]. Pronounced segregation of alloying elements at SISF and APB is revealed by HRSTEM and energy-dispersive X-ray spectroscopy (EDXS). Based on these and other results, an elementary creep mechanism is proposed which is characterized by ½<112> slip involving a SISF→APB transformation [5]. Atomic diffusion at creep temperature appears to be a rate limiting step by gradually changing the fault energies which control the kinetics of the transformation.

[1] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, Cambridge, 2008.

[2] J. Sato, T. Omori, K. Oikawa, I. Ohnuma, R. Kainuma and K. Ishida, Cobalt-base high-temperature alloys, Science (2006), 312, pp. 90-91.

[3] J. Müller, G. Eggeler, and E. Spiecker, On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys – An application of the LACBED method to complex microstructures,  Acta Materialia (2016), 87, pp. 34-44.

[4] J. Müller and E. Spiecker, Correlative microscale and atomic scale characterization of individual defects and interfaces in Ni-base superalloys, Proceedings of the Microscopy Conference MC 2015, 6-11 September 2015, Göttingen, Germany, MS4.076, pp. 146-147.

[5] Y.M. Eggeler, J. Müller, M.S. Titus, A. Suzuki, T.M. Pollock, and E. Spiecker, Planar defect formation in the γ′ phase during high temperature creep in single crystal CoNi-base superalloys, Acta Materialia (2016), 113, pp. 335-349.

Financial support by the German Research Foundation (DFG) through the SFB-TR 103 “Single Crystal Superalloys” and the Cluster of Excellence EXC 315 “Engineering of Advanced Materials” is gratefully acknowledged.

Erdmann SPIECKER (Erlangen, Germany), Julian MÜLLER, Yolita M. EGGELER, Malte LENZ
Invited
11:15 - 11:30 #7067 - MS05-OP277 TEM and STEM investigations of Sr(Ti,Nb)O3-δ thermoelectric with the addition of CaO and SrO.
TEM and STEM investigations of Sr(Ti,Nb)O3-δ thermoelectric with the addition of CaO and SrO.

It is known that thermoelectric properties, i.e. figure of merit ZT of oxide-based polycrystalline thermoelectric materials can be improved by introducing planar faults into the microstructure of these materials. It is assumed that in-grown planar faults will reduce thermal conductivity without reducing electrical properties which would consequently increase the ZT value. In order to successfully tailor thermoelectric properties of chosen thermoelectric materials, it is prerequisite to know the structure and chemical composition of introduced planar faults. This is why we used HR TEM and HAADF STEM imaging with EDXS in order to study structure and chemical composition of the Ruddlesden-Popper-type (RP) planar faults1,2 in Sr(Ti,Nb)O3-d (STNO) thermoelectric material with the addition of SrO and/or CaO. All results were obtained in a Jeol ARM-200F with a CFEG and Cs probe corrector. HAADF imaging was performed at angles from 70 to 175 mrad (ADF from 42 to 168 mrad). EDX spectra were acquired using JEOL Centurio Dry SD100GV SDD Detector. TEM bright‑field images of pure STNO showed that the STNO solid solution grains contained no planar faults of any kind. Furthermore, the interfaces between the grains were clean with no observable interface phase. However, when SrO and/or CaO were added to the STNO, various nanostructured features were observed. In SrO‑doped STNO, one can observe three distinctly different regions, i.e. the STNO solid solution, the regions with ordered SrO faults and the region with a network of random SrO planar faults (Figure 1). In the ordered regions one SrO layer is always followed by two perovskite STNO blocks, which corresponds to the Sr3(Ti1‑xNbx)2O7 RP‑type phase in which Nb and Ti occupy the same crystallographic site. While the measured HAADF intensities across Sr atomic columns at the RP fault do not scatter significantly, the mixed (Ti1-xNbx)O6 atom columns on the other hand exhibit significant differences in measured intensities thus indicating variation in Nb and Ti content within a single mixed atom column (Figure 2). Semi-quantitative HAADF STEM of the perovskite matrix, i.e. the comparison of measured integrated intensities of the atom columns with the calculated intensities showed that that the Nb content on the Ti sites within the perovskite structure varied from app. X=0.05 to X=0.35 (from Sr(Ti0.95Nb0.05)O3-d  to Sr(Ti0.65Nb0.35)O3-d). When RP‑type planar faults are isolated they run parallel to the {001} low-index zone axes of the perovskite structure. A similar structural phenomenon was observed in STNO with excess of CaO. Again, ordered and/or random 3D networks of RP‑type planar faults were observed in the STNO grains (Figure 3). In very thin regions of CaO-doped STNO specimen many orthogonal loops of RP faults were observed that were not detected in SrO doped STNO (Figure 4). The EDX analysis from a single fault and from the matrix showed higher concentration of Ca at the fault. This is in agreement with previously reported investigations3 since smaller Ca ions are easier incorporated at the RP fault than in the perovskite matrix. The TEM and STEM investigations thus confirmed that the addition of SrO and/or CaO to the STNO perovskite solid solution is structurally compensated via the formation of RP‑type planar faults within the STNO grains. Finally, thermoelectric measurements confirmed that the existence of RP-type faults in the perovskite STNO matrix reduced the thermal conductivity of this oxide thermoelectric material.

REFERENCES:

[1]   S.N. Ruddlesden, P. Popper, Acta Cryst., 11, 54-55, 1958

[2]   S. Sturm, M. Shiojiri, M. Ceh, J. Mater. Res., 24(8), 2596-2604, 2009

[3]   S. Sturm, A. Recnik, M. Kawasaki, T. Yamazaki, K. Watanabe, M. Shiojiri, M. Ceh, JEOLNews, 37E 22, 2002

acknowledgements:

The authors acknowledge financial support from the Scientific and Technological Research Council of Turkey (TÜBITAK) under Fellows Program and from EU under Seventh Framework Programme under grant agreement n°312483 (ESTEEM2).

Miran ČEH (Ljubjana, Slovenia), Marja JERIČ, Sašo ŠTURM, Cleva OW-YANG, Mehmet ALI GÜLGÜN
11:30 - 11:45 #6640 - MS05-OP273 Atomic structure and electronic configurations of 2D thermoelectric cobaltates.
Atomic structure and electronic configurations of 2D thermoelectric cobaltates.

Among the numerous different oxide systems, the cobaltites present fascinating and complex physical properties directly correlated with their electronic structure. A strong interplay exists in these Co-based systems between (i) valence and spin states and (ii) low dimensionality and structural anisotropy, all of these yielding the emergence of unexpected magnetic and thermoelectric (TE) functionalities as illustrated in NaxCoO2 [1]. The misfit-layered cobalt oxides built from similar cobalt layers have triggered intensive works regarding their remarkable TE properties at high temperatures. A real driving force of this misfit family is the CoO2 2D hexagonal layers turning out to be at the origin of its metallicity combined with a large thermopower, and more largely of interesting magnetic and electronic phenomena [2].

Another cobalt oxide, Bi4Sr12Co8O28-δ, derived from the 2201-type cuprates reveal an original tubular structure. These cobaltates present a peculiar 2D structure composed of two defined sub-lattices interconnected through oxygen-deficient CoOxpillars. This complex network has the ability to accommodate a certain structural and chemical flexibility by tuning locally its oxygen stoichiometry and therefore modulates its concentration of charge carriers. Large oxygen stoichiometry variations are achieved in these Co-based systems after post-annealed synthesis inducing dramatic changes in their physical properties, e.g. the emergence of two magnetic transitions at ≈ 50 K and 450 K. As in misfit cobaltites, the Co-tubular family reveals peculiar transport properties and magnetic transitions while their local electronic configurations remain unresolved to date. In these bidimensional cobaltites, one of the pivotal questions is the key role played by each type of Co sites.

The recent advances in electron spectromicroscopy have considerably improved both spatial and spectral resolutions toward acquiring 2D elemental maps at sub-Angström resolution over large unit cells and more recently mapping mixed-valence state modulations down to the level of the atomic columns. Here we shed light on the [Bi2Sr2CoO6]n[Sr8Co6O16-δ] n=2 member by resolving its atomic structure using a Cs-corrected scanning transmission electron microscope (STEM), the NION UltraSTEM200, coupled with electron energy-loss spectroscopy (EELS). Relying on high-energy resolution, we map locally the Co mixed-valence state modulations and further probe the effect of electronic charge transfer on individual Co atomic columns by tracking the subtle evolution of the Co-L2,3 and O-K near-edge fine-structures which are highly sensitive to the mixed valence-states and the hybridization geometries (see Fig.).

[1] I. Terasaki, Y. and K. Uchinokura, Phys. Rev. B 56, R12685 (1997)

[2] S. H.bert, W. Kobayashi, H. Muguerra, Y. Br.ard, N. Raghavendra, F. Gascoin, E. Guilmeau, A. Maignan. Phys. Stat. Sol. A 210, 69-81 (2013)

[3] D. Pelloquin, A. C. Masset, A. Maignan, M. Hervieu, C. Michel, B. Raveau, J. Solid State Chem.148, 108-118 (1999)

Laura BOCHER (Orsay), Alexandre GLOTER, Odile STÉPHAN, Sylvie HÉBERT, Denis PELLOQUIN
11:45 - 12:00 #5768 - MS05-OP265 In-situ reduction of platinum alloys using environmental (S)TEM.
In-situ reduction of platinum alloys using environmental (S)TEM.

Platinum-cobalt amongst other platinum alloys has desirable properties for use in fuel cells, having higher activities and stability than current commercial catalysts which only use platinum. Research into structures of platinum-cobalt alloy nanoparticles has greatly benefited from electron microscopy, particularly Z contrast HAADF-STEM imaging which is able to resolve the nanoparticle and surface structure with atomic resolution [1].

Most of the electron microscopy has focused on the characterisation of as prepared particles which have underwent further treatment outside the microscope [1, 2]. These annealing experiments produce ordered structures on the surface and fully ordered cubic (Pt3Co) or tetragonal phase (PtCo) alloys depending on the composition and annealing temperatures and times. The studies have been aided by in-situ (S)TEM studies which focus on the segregation of platinum and cobalt is observed inside the microscope at atomic resolution [3, 4]. Although there are many studies, often accompanied by EELS and EDX there appear to be few studies on the initial interaction between platinum and other metals which may be more representative of the activation of a commercial catalyst which is produced in bulk.

Inspired by annealing experiments performed on thin films for magnetic applications [5], we took a different approach by using aberration corrected environmental STEM and a MEMS heating holder to investigate how platinum, cobalt and cobalt oxides interact upon initial annealing.  We sputtered thin films of platinum and cobalt to make model systems similar to those used in magnetic thin films. Figure 1 shows a low and high magnification image of the as deposited film. The high magnification image shows mostly platinum crystallography but several amorphous regions and CoO crystals in the background. By annealing in-situ, in hydrogen gas (to remove cobalt oxide, typically used as a cobalt precursor) we were able to form new structures. Figure 2 shows the film shortly after being at 450 °C and 700 °C. In both cases alloyed regions were generally found but only completely ordered particles were observed at 700 °C. The 700 °C annealing temperature produces a good distribution of nanoparticles which can be further explored. Similar experiments are to be carried out on other alloys including platinum-copper and platinum-nickel. Results from these experiments have importance to the development of fuel cells and other industrial processes. The behaviour of platinum-cobalt thin films under annealing may also find use in magnetic film research.

Acknowledgements

We acknowledge the EPSRC for funding of this project (EP/J018058/1 critical mass grant) and thank Ian Wright for assistance.


References

1. Chen, S., et al., Origin of Oxygen Reduction Reaction Activity on "Pt3Co" Nanoparticles: Atomically Resolved Chemical Compositions and Structures. Journal of Physical Chemistry C, 2009. 113(3): p. 1109-1125.

2. Wang, D.L., et al., Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nature Materials, 2013. 12(1): p. 81-87.

3. Xin, H.L.L., et al., Revealing the Atomic Restructuring of Pt-Co Nanoparticles. Nano Letters, 2014. 14(6): p. 3203-3207.

4. Chi, M., et al., Surface faceting and elemental diffusion behaviour at atomic scale for alloy nanoparticles during in situ annealing. Nat Commun, 2015. 6.

5. Sato, K., K. Yanajima, and T.J. Konno, Structure and compositional evolution in epitaxial Co/Pt core-shell nanoparticles on annealing. Thin Solid Films, 2012. 520(9): p. 3544-3552.

Michael WARD (York, United Kingdom), Ed BOYES, Pratibha GAI
12:00 - 12:15 #5266 - MS05-OP262 Investigation of the deformation mechanisms and defects in high-Mn austenitic steel.
Investigation of the deformation mechanisms and defects in high-Mn austenitic steel.

A new class of austenitic steels stabilized with high Mn contents (instead of Ni) exhibits exceptional mechanical properties, such as large energy absorption and high work-hardening rate, owing to secondary deformation mechanisms such as mechanical twinning-induced plasticity (TWIP) and martensitic transformation-induced plasticity (TRIP) favored for low stacking-fault energy (SFE) [1]. The interaction of dislocations with twin boundaries and martensite interfaces during mechanical deformation enhances the work hardening, i.e., a dynamic Hall-Petch effect, with total elongations exceeding 70% and ultimate tensile strengths in the GPa regime.

The influence of the strain rate, temperature, and changes in SFE on the deformation mechanisms in high-Mn austenitic steels has been investigated using electron backscattered diffraction (EBSD), electron-channeling contrast imaging (ECCI), conventional bright-field/dark-field imaging (BF/DF), and aberration-corrected high-resolution scanning/transmission electron microscopy (HRTEM/HRSTEM).  The TWIP/TRIP secondary deformation mechanisms are related to the low SFE exhibited in these materials.  Experimentally measured SFE from weak-beam-dark-field (WBDF) imaging provides the basis to understand how changes in SFE influence mechanical twinning versus transformation induced martensite [2-3].  However, adiabatic heating during deformation at high strain rates (20 -10,000 s-1) increases the SFE.  Quantifying the twin or martensite density by EBSD/ECCI and BF-DF images allows for comparison of the secondary deformation at different SFE, strain rates, and total elongation, but to study the details of the deformation mechanisms requires imaging at atomic resolution using aberration-corrected electron microscopy. 

Figure 1(a) shows the EBSD/ECCI experimental procedure.  EBSD identifies grains with a [110] orientation that are subsequently imaged using backscattered electrons where the incident beam strongly channels except for areas with twinning and martensite plates.  Although the EBSD/ECCI method provides for a statistical number of measurements, the DF imaging method shown in figure 1 (b) has the advantages of improved resolution and the ability to differentiate between the hexagonal ε-martensite and twins. The histograms in figures 1 (c-d) summarize the spacing between the planar defects and their thickness from a Fe-25Mn3Al3Si alloy deformed at a strain rate of 20 s-1 to a total strain of 20% using these two experimental methods.  Figure 3 is a HRTEM image produced with an image corrected FEI-Titan from a Fe-25Mn3Al3Si alloy deformed at a strain rate of 2x103 s-1 to a total strain of 18%. The image shows an example of an individual dislocation trapped at the planar defect interface as well as a local region of martenite (bottom left) together with the mechanical twins.  The high-angle dark field (HAADF) HRSTEM image in figure 3 from a Fe-16Mn14Cr0.3N0.3C alloy deformed to a total strain of 21% at a strain rate of 10-4 s-1 was acquired using the ER-C PICO operating at 300 kV.  Similar to the high strain rate material, the quasi-static deformed microstructure exhibits multiple mechanical twins with evidence of local hexagonal stacking (upper right). Advantages of the HRSTEM method compared to HRTEM images are more straight forward image interpretation from the HAADF amplitude contrast, the ability to image thicker samples, and the reduced sensitivity to local variation in crystallographic orientation.

 

[1] O Grassel, L Kruger, G Frommeyer, and LW Meyer,  Int. J. Plasticity,16(2000) p.1391

[2] D T Pierce, JA Jiménez, J Bentley, D Raabe, C Oskay and JE Wittig, Acta Mater 68(2014)238-53

[3] D T Pierce, J A Jiménez, J Bentley, D Raabe and J E Wittig, Acta Mater 100(2015)178-90

Acknowledgement - Financial support from the NSF DMR 0805295 and the DFG SFB 761 “Steel –ab initio” and research at the Ernst Ruska-Centre are gratefully acknowledged.

James WITTIG (Nashville, USA), Jake BENZING, Maryam BEIGMOHAMADI, Marta LIPINSKA, Joachim MAYER
12:15 - 12:30 #6390 - MS05-OP271 High resolution STEM and EELS investigation of N-doped carbon allotropes decorated with noble metal atom catalysts.
High resolution STEM and EELS investigation of N-doped carbon allotropes decorated with noble metal atom catalysts.

Graphene’s unique properties make this material an ideal catalyst support for use in the proton exchange membrane fuel cell (PEMFC). Graphene offers increased electrical conductivity and a larger surface area for catalyst deposition compared to other catalyst support material, such as carbon black.[1,2]. Utilizing graphene as the electrode support also results in increased chemical stability due to the spbonding; however, this precludes the availability of dangling bonds for chemisorption, thus leading to a poor Pt distribution and the formation of large nanoparticles (NPs). Functionalization can be used to introduce nucleation sites into the graphene lattice, where N-dopants have been shown to increase the Pt-C binding energy.[3] The atomic layer deposition (ALD) technique creates ultra-small NPs (<1 nm) which, when combined with the enhanced catalyst binding energy from the N-doped graphene support can produce stable Pt clusters/atoms, resulting in increased Pt utilization while subsequently reducing the cost.[4] 

To fully understand and design a more efficient PEMFC the material must be characterized at the atomic level. This can be accomplished by the use of aberration-corrected transmission electron microscopy (TEM). High resolution TEM (HRTEM) can be utilized to observe the structure of the graphene lattice, while high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) can be used to examine the Pt clusters’ size and distribution. Furthermore, electron energy loss spectroscopy (EELS) can be utilized to examine local chemical composition and bonding of the probed atoms from nanometer scaled areas in order to reveal the coordination of the N-dopant species in the graphene lattice.[4] Here we used a FEI Titan 80-300 Cubed TEM equipped with aberration correctors of the probe and imaging forming lens, and a monochromator for optimal imaging at a low accelerating voltage.

Low-energy condition HRTEM (figure 1) and STEM imaging were used to investigate thermally-exfoliated graphene (figure 2). From these observations, we deduced that the graphene lattice maintained its short range order; however, the long range order was lost due to the presence of steps, folds, defects, and incomplete exfoliation.[4] The mass-thickness dependence in HAADF imaging confirmed the presence of numerous steps and ledges in the N-doped graphene nanosheets. More importantly the Z-contrast in the HAADF images revealed that Pt was present as stable single atoms and clusters on the N-doped graphene and that Pt NPs were not detected.[4] Lastly, using EELS and the N-K edge fine structures, we probed the distribution of N-dopants within the nanosheets (figure 3). Significant variations in the local concentration of N-dopants were observed among the graphene sheets, in which an inhomogeneous distribution was discovered. Such effects have not been observed in previous broad beam techniques.[4] To futher investigate the effect of N-doping, mutliwalled N-doped CNTs (N-CNTs) were produced and ALD was utilized to deposit Pd. Initial investigations showed the stabilization of single Pd atoms with N-doping; however, small NPs were also formed (figure 4). It was demonstrated with EELS that many of the N-CNTs were filled with N2 gas. Using controlled evacuation of the multiwalled CNTs induced by the electron beam, we were able to reveal the intrinsic N-type doping within the CNT walls.

References

[1]Novoselov,K.S, Geim,A.K.  Science, 306,(2004),666.

[2]Lee,C. et.al. Science, 321,(2008),385.

[3]Holme,T. et.al. Phys.Chem.Chem.Phys, 12,(2010),9461.

[4]Stambula,S et.al. J.Phys.Chem.C, 118,(2014),3890.

Acknowledgements

This work is supported by NSERC via CaRPE-FC Network grant and a strategic grant to Western University and McMaster University. S. Stambula is grateful to NSERC for a Postgraduate Scholarship. The electron microscopy presented here was carried out at the Canadian Centre for Electron Microscopy, a National Facility supported by the Canada Foundation for Innovation under the MSI program, NSERC, and McMaster University.

Samantha STAMBULA (Hamilton, Canada), Matthieu BUGNET, Niancai CHENG, Andrew LUSHINGTON, Xueliang SUN, Gianluigi BOTTON
Salle Bellecour 1,2,3

Wednesday 31 August

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MS2-III
10:15 - 12:30

MS2: 1D and 2D materials
SLOT III

Chairmen: Raul ARENAL (Zaragoza, Spain), Ursel BANGERT (Limerick, Ireland)
10:15 - 10:45 #8632 - MS02-S71 Extreme Nanowires: The Smallest Crystals in the Smallest Nanotubes.
Extreme Nanowires: The Smallest Crystals in the Smallest Nanotubes.

A logical extension to fabrication of monolayer 2D materials such as graphene is creation of

'Extreme Nanowires' (i.e. Fig. 1), down to a single atom column in width.[1,2] In this limit, crystals

have fundamentally different physical characteristics and properties.[3-5] We and others have created

atomically regulated nanowires by confining them within the smallest diameter carbon nanotubes

(i.e. either single walled carbon nanotubes (SWNTs) or double walled carbon nanotubes (DWNTs)),

and are investigating their structural and electronic properties. These materials also provide an

ultimate benchmark for testing the most sensitive characterisation methodologies which, when

corroborated with suitable theory, will provide new data on physics at the most fundamental length

scale accessible to nanomaterials fabrication. The most powerful investigative tools for structural

investigation are aberraton-corrected Transmission Electron Microscopy (ac-TEM) and Scanning

Transmission Electron Microscopy (ac-STEM) and, here, we describe the application of these

methods to a variety of Extreme Nanowire systems.

One of the most crucial aspects of the role of electron microscopy in our investigation is the 2D

and 3D elucidation of the structure of quasi- or true 1D nanowires formed in SWNTs as these form

the primary source of information for density functional theory (DFT) and other ab initio theoretical

approaches to both structure and properties elucidation. When combined with real physical

measurements, this combined approach becomes even more powerful as we can start to piece

together how the fundamental physics of a crystalline nanowire changes once we constrain its width

down to one or two atoms in cross section. For example we recently record Raman Spectra from

2x2 atom thick HgTe nanowires embedded within 1.2-1.4 nm SWNTs[4] and found that we are able to

model the Raman-measured lattice phonons of this system based on a simple structural model

previously determined from two pairs of Exit Wave Reconstruction images which we also used to

make DFT predictions about the altered electronic structure of this system which is predicted to

change from a -0.3 eV semi-metal to a ~1.2 eV band gap semiconductor.[3] Following on from the

exciting recent work of Senga et al.[2] who imaged the first true 1D crystals of CsI in DWNTs we are

now modeling single atomic chain coils of tellurium formed within narrow SWNTs (Fig. 2).[6]

References

[1] J. Sloan et al Chem Commun (2002) 1319

[2] R. Senga et al Nature Mater 13 (2014) 1050

[3] R. Carter et al Phys Rev Lett 96 (2006) 21550

[4] J. H. Spencer et al ACS Nano 8 (2014) 9044

[5] C. Giusca et al Nano Lett 13 (2013) 2040

[6] To be published

Jeremy SLOAN (Coventry, United Kingdom), Reza KASHTIBAN, Sam MARKS, Richard BEANLAND, Ana SANCHEZ, Sam BROWN, Andrij VASYLENKO, Peter BROMMER, Krzysztof MORAWIEC, Slawomir KRET, Paulo MEDEIROS, James WYNN, Joe SPENCER, David SMITH, Quentin RAMASSE, Zheng LIU, Kazu SUENAGA, Andrew MORRIS, David QUIGLEY, Eric FAULQUES
Invited
10:45 - 11:00 #5976 - MS02-OP222 Defects in as-grown vs. annealed rutile titania nanowires and their effect on properties.
Defects in as-grown vs. annealed rutile titania nanowires and their effect on properties.

Semiconducting metal oxides play a key role in electrochemical and photo physical applications like photo catalysis and as electrode material in solar cells and Li-ion batteries. Among these metal oxides, hydrothermally grown rutile TiO2 nanowire arrays are promising as the 1 D structure possesses a large surface area and a directed electron path towards the substrate. However, the efficiency of these devices is also influenced by the defects inside the nanowires (dislocations, stacking faults, titanium interstitials and oxygen vacancies). A previous study showed that as-grown nanowires have a high density of lattice defects.[1] However, these defects can be removed by an additional annealing step. Latest findings demonstrate that hybrid solar cells where those annealed nanowires are incorporated have significantly higher power-conversion efficiency.[2] However, the removal of the defects is not fully understood so far.

In our present work, transmission electron microscopy (TEM) was used to study the changes within the nanowire during annealing. TEM investigations were performed at 200 kV using a JEOL JEM-2200FS and at 300 kV using a FEI Titan Themis 60-300. First results were obtained by ex-situ TEM analysis of as-grown TiO2 nanowires and nanowires, which were annealed at 500 °C for 4 h. These ex-situ analysis confirm that both, as-grown and annealed nanowires, have the rutile crystal structure. Defects, present in the as-grown state, can be removed by the thermal treatment. Concurrently, a structural transformation inside the nanowires occurs resulting in faceted voids of several nanometer in diameter. Using tilt series from -70° to +70° in high-angle annular dark-field (HAADF) scanning (S)TEM mode and the discrete iterative reconstruction technique (DIRT)[3], a TEM tomograph was obtained, which proved that these voids are solely formed inside the nanowire and not at the nanowire surface (Fig. 1). Further analysis including electron energy loss spectroscopy revealed changes of the oxidation state at the surface region of the voids during annealing, whereas the rutile TiO2 crystal structure was maintained.

HAADF-STEM in-situ heating experiments, performed in a JEOL JEM-2200FS with a DENSsolutions heating holder, enabled the direct observation of the structural changes inside the rutile TiO2 nanowires (Fig. 2). Using a slow heating ramp of 3.3°C/min, a sudden formation of these voids at around 500 °C could be observed. Heating at lower temperatures did not affect the structure of the nanowire and also an additional heating after the transformation (600°C, 1h) did not change the size and shape of the voids.  These ex-situ and in-situ observations are a decisive step to explain the mechanisms involved in this process in more detail.

The results of our TEM investigation were correlated to the properties of as-grown and annealed TiO2 nanowires. Here, we could show that the healing of the lattice defects upon annealing not only increased the performance of hybrid solar cells but also affects other properties of the nanowires e.g. chemical stability.

 

 

[1] A. Wisnet et al., Crystal Growth & Design 2014, 14, 4658-4663.

[2] A. Wisnet et al., Advanced Functional Materials 2015, 25, 2601–2608.

[3] A. Zurner et al., Ultramicrocopy 2012, 115, 41-49.

Alena FOLGER (Düsseldorf, Germany), Andreas WISNET, Christina SCHEU
11:00 - 11:15 #6494 - MS02-OP226 Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe2.
Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe2.

Off-axis electron holography is a powerful tool to measure electrostatic and magnetic fields at the nanoscale inside a transmission electron microscope. The electron wave that has passed through a thin specimen can be recovered from an electron hologram and the phase can be related to the specimen thickness or the electrostatic potential in and around the specimen. However, dynamical diffraction may cause a deviation from the expected linear relationship between phase and specimen thickness, which emphasizes the need for comparisons with corresponding computer simulations.

Here, we study few-layer-thick two-dimensional WSe2 flakes by off-axis electron holography. Voronoi tessellation is used to spatially average the phase and amplitude of the electron wavefunction within regions of unit-cell size (see Fig. 1). A determination of specimen thickness of the WSe2 is not possible from either the phase or the amplitude alone. Instead, we show that the combined analysis of phase and amplitude from experimental measurements and simulations allows an accurate determination of the local specimen thickness. Extremely thin specimens that are tilted slightly away from the [001] zone axis show an approximately linear relationship between phase and projected potential. If the specimen thickness is known, the electrostatic potential can be determined from the measured phase.

We used this combined approach to determine a value for the mean inner potential of 18.9 ± 0.8 V for WSe2, which is approximately 10% lower than the value calculated from neutral atom scattering factors. In this way, a comparison of high-resolution electron holography data with simulations has been achieved on a quantitative level, enabling an assessment of the experimental conditions under which electrostatic potentials can be extracted directly from the phases of measured wavefunctions.

The authors are grateful to L. Houben, M. Lentzen, A. Thust, J. Caron and C. B. Boothroyd for discussions, as well as A. Chaturvedi and C. Kloc from the School of Materials Science and Engineering, Nanyang Technological University, Singapore for providing the WSe2 crystals. We are also grateful to the European Research Council for an Advanced Grant and for funding by the German Science Foundation (DFG) grant MA 1280/40-1.

Florian WINKLER (Jülich, Germany), Amir H. TAVABI, Juri BARTHEL, Martial DUCHAMP, Emrah YUCELEN, Sven BORGHARDT, Beata E. KARDYNAL, Rafal E. DUNIN-BORKOWSKI
11:15 - 11:30 #6708 - MS02-OP229 Atomic relaxation in ultrathin fcc metal nanowires.
Atomic relaxation in ultrathin fcc metal nanowires.

Capillary forces affect the atomic structure, and can lead to radically different atomic configuration in nanoscale compared to bulk counterpart. In this work, we demonstrate such a phenomenon in ultrathin Au nanowires. These Au nanowires can be fabricated by a simple wet-chemical approach, using oleylamine as a capping agent facilitating the anisotropic crystal growth. Detailed investigation of the atomic structure shows that the close-packed plane normal to the [111]-wire axis undergoes wrinkling leading to the formation of a saddle surface1. This phenomenon was captured in ab initio simulations (Fig. 1) and was corroborated with aberration-corrected transmission electron microscopic studies (Fig. 2). The reason of such relaxation can be attributed to the anisotropic surface stress of the bounding facets leading to out-of-plane displacement of the atoms. In terms of electronic structure, such atomic scale structural relaxation forces the d-band of the Au nanowire towards higher energy, and opens up a tantalizing possibility of using them as nanoscale sensors2, as electronic states near Fermi energy become available for hybridization.

Furthermore, we have generalized this phenomenon for other FCC metal nanowires such as Cu, Ag and Pt. Our recent simulations show that the strain in the FCC nanowire exhibits a systematic variation with the resultant stress along the nanowire orientation (Fig. 3). These results shed light on the atomic structure of FCC nanowires and open up a possibility of experimental investigation of the atomic structure for other FCC nanowires in detail.

References:

1. A. Roy et al., Nano Lett., 14, 4859-4866 (2014)

2. A. Roy et al., J. Phys. Chem. C, 118, 676-682 (2014)

Ahin ROY (Fukuoka, Japan), Knut MÜLLER, Kenji KANEKO, Andreas ROSENAUER, Jörg WEISMÜLLER, Abhishek Kumar SINGH, N RAVISHANKAR
11:30 - 11:45 #6764 - MS02-OP231 Examination of inas/insb heterointerfaces in nanowires.
Examination of inas/insb heterointerfaces in nanowires.

Examination of InAs/InSb heterointerfaces in Nanowires

Antimony based semiconductor nanowires (NWs) have been widely studied in recent years due to their large spin-orbit coupling, high Landé g-factor, and single subband conductance. They may become a key material in the field of quantum information processing and are currently used in the search for Majorana fermions by electrical transport [1]. Theory predicts that Majorana bound states can be realized in a 1D semiconductor coupled to a superconductor when exposed to an external magnetic field and manipulated by electrostatic gates [2]. Using NWs as a mean to achieve this goal has become even more promising after a recent report on epitaxial semiconductor/superconductor NW interfaces [3,4]. InSb NWs are expected to be the optimal material in this endeavor as the large spin-orbit interaction, and the possibility to induce superconductivity, in InSb makes it feasible to drive the wires into the topologically protected regime [2,5]. Realization of Majorana bound states in semiconductor/superconductor NWs would open the route to perform quantum information processing in networks by exploiting the unique non-abelian braiding statistics and non-trivial fusion rules which are inherent in Majorana Fermions [6].

In this work we present a detailed investigation of the growth of Au-seeded InSb NWs grown by molecular beam epitaxy (MBE) on (111)B InAs substrate. As InSb has proven difficult to grow directly on InAs substrates, an InAs stem mediates the growth as seen in Fig. 1. In Fig. 2 the growth deformation in this region is analyzed using Geometric Phase Analysis (GPA) and the abrupt interface indicates that the compositional change occur rapidly. To explore the growth dynamics of InSb, the transition region between InAs/InSb is investigated in order to study how incorporation of Sb affects the system. This analysis is performed using HR-TEM, Dark-Field Electron Holography (DFEH) [7], and Energy Dispersive x-ray Spectroscopy (EDS). In Fig. 3 a DFEH image is obtained by interfering the diffracted beam from the InAs part of the NW with that from the InSb part. Using DFEH the in-plane deformation in the growth direction is shown in Fig. 4.  Comparing the results from the aforementioned techniques with parameters used in the MBE during growth gives insight into the incorporation dynamics of Sb in NWs. Understanding the dynamics of NW growth is crucial in order to optimize and utilize these in novel devices. InSb NWs with two-facet Al shell are also investigated to explore the epitaxy of the interface, since the interface plays the major role in utilizing InSb/Al heterostructures for topological superconductivity and devices.

References:

[1] Kitaev, A.Y., 2001. Unpaired Majorana fermions in quantum wires. Physics-Uspekhi44(10S), p.131.

[2] Leijnse, M. and Flensberg, K., 2012. Introduction to topological superconductivity and Majorana fermions. Semiconductor Science and Technology27(12), p.124003.

[3] Chang, W., Albrecht, S.M., Jespersen, T.S., Kuemmeth, F., Krogstrup, P., Nygård, J. and Marcus, C.M., 2015. Hard gap in epitaxial semiconductor–superconductor nanowires. Nature Nanotechnology10(3), pp.232-236.

[4] Krogstrup, P., Ziino, N.L.B., Chang, W., Albrecht, S.M., Madsen, M.H., Johnson, E., Nygård, J., Marcus, C.M. and Jespersen, T.S., 2015. Epitaxy of semiconductor–superconductor nanowires. Nature Materials14(4), pp.400-406.

[5] Mourik, V., Zuo, K., Frolov, S. M., Plissard, S. R., Bakkers, E. P. A. M., & Kouwenhoven, L. P. (2012). Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices. Science336(6084), 1003-1007.

[6] Alicea, J., Oreg, Y., Refael, G., von Oppen, F. and Fisher, M.P., 2011. Non-Abelian statistics and topological quantum information processing in 1D wire networks. Nature Physics7(5), pp.412-417.

[7] Hÿtch, M., Houdellier, F., Hüe, F. and Snoeck, E., 2008. Nanoscale holographic interferometry for strain measurements in electronic devices. Nature453(7198), pp.1086-1089.

Aske GEJL (Copenhagen Ø, Denmark), Thomas KANNE, Erik JOHNSON, Thibaud DENNEULIN, Wolfgang JÄGER, Jesper NYGÅRD, Peter KROGSTRUP
11:45 - 12:00 #6918 - MS02-OP235 Focused ion beam based highly resistive and reproducible sub-5nm nanogaps in thin gold wire.
Focused ion beam based highly resistive and reproducible sub-5nm nanogaps in thin gold wire.

The measurements of physical properties such as electrical characterization of single or few nano-objects like nanoparticles, nanoclusters and single molecules require the fabrication of nanoscale gaps between electrodes [1]. It is difficult to fabricate electrical contacts at this scale with standard lithography techniques but focused ion beam (FIB) can fabricate nanogaps by milling thin metallic wires on the insulation substrate. It is a not only a mask less technique but also enables to fabricate large numbers of nanogaps with reproducible properties at the room temperature [2]. The FIB milled nanogaps are employed to determine single or few nano-entities such as nanoparticles and molecules [3]. Due to limitation of ion beam diameter and profile, the size of FIB milled nanogap is more than 10 nm thus not applicable to measure single less than 10nm object. We have developed two strategies to create sub-5nm gaps 1) by a control and precise dosage of ions during milling 2) by electrodeposition of gold in 30nm FIB cut gaps

1) A series of nanogaps are milled with size range from sub-5 to 30nm in thin gold wires, through precise control of the applied ion dosages with a range between 4.6 and 9.2 x 1010 ions/cm  using focused gallium ion beam at 30KV. As shown in figure 1a, the nanogap is not fully milled with the ion dosage of 4.6×1010 ions/cm, whereas the large nanogaps are milled with high ion dosage of 9.2 x 1010 ions/cm having resistances in 100-1000TΩ range (figure 1b). With ion dosage of 5×1010 ions/cm the nanogap begin to show up in gold nanowire (figure 2), which is verified by the current-voltage (I-V) characteristic demonstrating the resistance up to 12 TΩ. The Fowler-Nordheim tunneling effect is also observed in the sub-5 nm. The application of Simmon’s model to the milled nanogaps and the electrical analysis indicates that the minimum effective nanogap size approaches to 2.3 nm

2) The 30nm FIB cut nanogaps (figure 1b) are connected to a potentiostat controlled by GPES software. A 10 µL drop of diluted gold chloride is added on top of a FIB cut gap. In one of the configuration when working electrode (WE) is connected to to one side of nanoelectrode setup, other side of setup is connected to reference electrode (RE) and counter electrode(CE) is placed inside the drop on silsicon dioxide as shown in figure 3, well defined and well-shaped reduction of gap size has occurred with the growth on both electrodes from deposition of gold atoms (figure 4). The calculated growth rate in the nanogap is in the order of 1 nm/s and growth rates depend directly on the deposition time. The current-voltage characteristics have demonstrated the open gap resistance of 5nm nanogap in the order of 300TΩ. The FIB templating also improves the shape of electrochemically grown nanogaps.

These two strategies allow us to create large number of nanogaps thus enabling to measure physical properties of a sub 5nm single nano-object.

References:

1.         Nicholas, P. and S. Kyung-Ah, Alligator clips to molecular dimensions. Journal of Physics: Condensed Matter, 2008. 20(37): p. 374116.

2.         Blom, T., et al., Fabrication and characterization of highly reproducible, high resistance nanogaps made by focused ion beam milling. Nanotechnology, 2007. 18(28): p. 285301.

3.         Jafri, S.H.M., et al., Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds. Scientific Reports, 2015. 5: p. 14431.

Syed Hassan Mujtaba JAFRI (Uppsala, Sweden), Hu LI, Ishtiaq Hassan WANI, Klaus LEIFER
12:00 - 12:15 #6942 - MS02-OP236 Exploring and Tailoring Structural Properties of the Two-Dimensional Family ofMXenes.
Exploring and Tailoring Structural Properties of the Two-Dimensional Family ofMXenes.

MXenes constitute a rather recent addition to two-dimensional materials, which exhibit a large range of tunable properties [1].  These materials can be synthesized in both bulk form as well as thin film structures. Although many of their properties remain unexplored they exhibit outstanding properties for applications in electrochemical energy storage devices, EESDs [2].

 

MXenes originate from a large family of naturally nanolaminated materials known as MAX phases, where M is a transition metal, A a group A element and X is either C or N, following the general formula Mn+1AXn., see e.g. the schematic in fig. 1a. Upon chemical etching, the A element leaves the MAX phase and results in single sheets, with surfaces functionalized, Tx, by O, OH and F to give the MXene formula Mn+1XnTx [3]. Bulk made MXene hence exhibit an extremely large surface area per volume, and are consequently attractive for energy storage. The material is easily intercalated and delaminated using standard methods.

 

To date, shy of 20 different 2D structures in this family has been synthesized and the wide range of choice of e.g. M elements enable a formidable playground for materials engineering. Among the more interesting structures synthesized to date, are those which are achieved while alloying different M elements to achieve (M1,M2)n+1Xn, see e.g. [4]. Through theoretical predictions and subsequent materials synthesis, we have been able to tune the material to exhibit out of-plane ordering as well as in-plane ordering, by means of repetitively alternating M elements. Additionally, by choosing the M elements carefully, we can incorporate those which are loosely bound and prone to chemical etching. This further enable a reduced MXene structure exhibiting in-plane vacancy ordering. The process is shown in fig. 1, where a MAX phase of 2M elements is schematically etched to produce the reduced, in-plane vacancy ordered structure as seen in cross-section (fig 1b) and plan-view (fig 1c). The corresponding synthesized structure is shown by HRSTEM methods in fig 2. These new MXenes additionally enable further tuning by vacancy ripening. Upon irradiating and heating the sample, the vacancies ripen to produce 2D sheets with a mesoporous structure, which exhibits a significant application in filtering and for enhancing ion mobilities in EESD applications.

 

This presentation will highlight the research front in this new family of materials, and present the opportunities given through its vast tailoring ability. Electron microscopy, and in particular aberration corrected STEM in combination with EELS and EDS has proven critical in the exploration of these new materials.

 

[1] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Adv. Mater., 23 4248 (2011).

[2] M. Ghidiu, M. Lukatskaya, M. Zhao, Y. Gogotsi, M.W. Barsoum, Nature 516, 78 (2014).

[3] L.H. Karlsson, J. Birch, J. Halim, M.W. Barsoum, and P.O.Å. Persson Nano Lett., 15 4955 (2015).

[4] B. Anasori, Y. Xie, M. Beidaghi, J. Lu, B.C. Hosler, L. Hultman, P.R.C. Kent, Y. Gogotsi, and M.W. Barsoum ACS Nano, 9 9507 (2015).

Per PERSSON (Linköping, Sweden)
12:15 - 12:30 #6944 - MS02-OP237 Probing structural and electronic properties of h-BN by HRTEM and STM.
Probing structural and electronic properties of h-BN by HRTEM and STM.

After the discovery of graphene and its consequences in the field of nanoscience and nanomaterials, there has been a growing interest in 2D materials and also their vertical stacking due to unique properties and potential applications.[1] For instance, it was shown the transport properties of exfoliated graphene supported by hexagonal boron nitride (h-BN) could approach the intrinsic graphene limits.[2] Nevertheless, studying the structural properties of 2D materials and 2D heterostructures is crucial to understand their physical and chemical properties. Our motivations have been to exploit state of the art aberration-corrected high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM) to study the structure and electronic properties of graphene (G), h-BN and G/h-BN heterostructures.

 

HRTEM analyses were conducted with a JEOL ARM microscope equipped together with a cold FEG, an aberration corrector for the objective lens and a One view camera (Gatan). Notably, we used high-speed atomic-scale imaging to study with unprecedented dynamics (up to 25 fps) the nucleation and growth mechanisms of triangular holes in h-BN under beam irradiation (Figure 1). The direct observation of B and N atom sputtering and surface reconstruction processes allow understanding how the triangular shape and orientation of holes are maintained during the growth. Interestingly, by studying the effects of the electron dose and the number of BN layers, we demonstrate that these atomic-scale processes are simultaneously driven by kinetic and thermodynamic effects. Further works are in progress to study the stability of h-BN/G stacking under electron-beam irradiation. 

 

STM analyses were carried out with low temperature STM at 4 K, on 2D heterostructures that consist in a few layers of graphene doped with nitrogen on thick exfoliated flakes of BN deposited on SiO2.  Remarkably, we show that STM allows identifying and characterizing ionization defects within the BN flakes below the graphene layers (Figure 2). This study opens new avenues to probe the electronic interactions between this two stacked materials.

[1] Geim & al., Nature, 499, 419 (2013)

[2] J. M. Xue & al., Nature Mater, 10, 282 (2011)

Ouafi MOUHOUB (Chatillon), Christian RICOLLEAU, Guillaume WANG, Hakim AMARA, Amandine ANDRIEUX, Nelly DORVAL, Frédéric FOSSARD, Pierre LAVENUS, Jerome LAGOUTE, Van Dong PHAM, Pai WOEI WU, Annick LOISEAU, Damien ALLOYEAU
Salle Prestige Gratte Ciel

Wednesday 31 August

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IM1-III
10:15 - 12:30

IM1: Tomography and Multidimensional microscopy
SLOT III

Chairmen: Sara BALS (Antwerpen, Belgium), Wolfgang LUDWIG (Lyon, France), Sergio MARCO (Paris, France)
10:15 - 10:45 #5714 - IM01-S33 3D Mapping of electric potentials and magnetic fields at the nanoscale using Electron Holographic Tomography.
3D Mapping of electric potentials and magnetic fields at the nanoscale using Electron Holographic Tomography.

Off-axis electron holography (EH) is a TEM technique that records the phase information of an electron wave transmitted through a thin specimen in an electron hologram. By reconstructing this phase information, it enables electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography to electron holographic tomography (EHT), in three dimensions (3D) [1,2]. Tomograms obtained by EHT provide the 3D mean inner potential (MIP) distribution of nanoscale materials from which the 3D morphology and the chemical composition can be inferred [3]. Moreover, functional potentials, e.g., introduced by doping of impurities in semiconductors, have been successfully revealed in 3D [4]. Recently, we succeeded in the 3D reconstruction of the axial component of the B-field prevailing in magnetic nanowires [5,6].

EHT as applied on magnetic samples proceeds as follows (see Fig. 1): (1) an electron hologram tilt series (ideally covering a range of 360°) is acquired, (2) the phase image tilt series is reconstructed from the holograms, (3) electric and magnetic phase shifts are separated by computing half of the sum/difference between opposite (180° tilted) projections, and (4) both the electric potential and the B-field component parallel to the tilt axis are reconstructed with tomographic techniques. Here, we report EHT studies achieved by means of tomography-dedicated TEM sample holders, in combination with advanced in-house developed software packages for acquisition, alignment and tomographic reconstruction.

Fig. 2 shows the 3D electric potential reconstruction of a GaAs/AlGaAs core-multishell nanowire (NW) grown by metalorganic vapour phase epitaxy (MOVPE) using an Au nanoparticle (NP) as catalyst. Such NWs may serve as novel coherent nanoscale light sources (lasers), because they provide an effective gain medium, low-loss optical waveguiding, and strong optical confinement for axially guided optical modes. The difference in the MIP allows discriminating between GaAs and AlGaAs within the NW. Longitudinal (Fig. 2b) and cross-sectional (Fig. 2e) 2D slices averaged over a well-defined thickness reveal not only the GaAs core and the AlGaAs shell, but also a 5nm thin GaAs shell within the AlGaAs, which acts as a quantum well.

Fig. 3 comprises two recent EHT studies revealing the B-field within a Co nanowire (NW) [5] and a Co2FeGa Heusler alloy NW [6] both with spatial resolution higher than 10 nm. The reconstructions of the dominant axial component of the magnetic induction exhibit a small inversion domain at the apex of the Co NW, whereas at the Co2FeGa NW, a magnetic dead layer of 10 nm width could be revealed.

The powerful approach presented here is widely applicable to a broad range of 3D electric and magnetic nanostructures and may trigger the progress of novel nanodevices.



[1] P A Midgley and R E Dunin-Borkowski, Nat. Mater. 8 (2009) p. 271.

[2] D Wolf, A Lubk, F Röder and H Lichte, Curr. Opin. Solid State and Mater. Sci. 17 (2013) p. 126.

[3] A Lubk, D Wolf, P Prete, N Lovergine, T Niermann, S Sturm and H Lichte, Phys. Rev. B 90 (2014) p. 125404.

[4] D Wolf, A Lubk, A Lenk, S Sturm and H Lichte, Appl. Phys. Lett. 103 (2013) p. 264104.

[5] D Wolf et al., Chem. Mater. 27 (2015) p. 6771.

[6] P Simon, D Wolf, C Wang, A A Levin, A Lubk, S Sturm, H Lichte, G H Fecher and C Felser, Nano letters 16 (2016) p. 114.

[7] We thank N Lovergine of University of Salento, Lecce for provision of the GaAs/AlGaAs core-multishell nanowire samples.

[8] This work was supported by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.

Daniel WOLF (dresden, Germany), Axel LUBK, Hannes LICHTE
Invited
10:45 - 11:00 #6431 - IM01-OP046 Quantitative elemental and bonding EELS tomography of a complex nanoparticle.
Quantitative elemental and bonding EELS tomography of a complex nanoparticle.

Comprehending the properties of complex nanoscale materials requires not just study of their morphology, but also determining the distribution and quantity of specific elements or phases, and the nature of bonding within these. Here we present quantitative 3D elemental and bonding mapping of a complex boron nitride based nanoparticle. This is achieved through a combination of electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM), novel EELS analysis methods and compressed sensing tomographic reconstruction [1].

For this study, a low-loss and core-loss STEM-EELS tilt series of spectrum images was recorded over the angular range -70° to 70° with a 17.5° tilt increment. The experiment was performed at 80 kV using a Tecnai Osiris with Gatan Enfinium spectrometer equipped with DualEELS. Figure 1a shows the high angle annular-dark field (HAADF) tilt-series images of the nanoparticle studied. The nanoparticle clearly possesses intricate structure, but the HAADF images are not fully revealing, obviating need for tomographic and analytical investigation.

EELS can achieve accurate absolute quantification of elemental composition without the need for standards, opening the door to quantitative analytical tomography. Moreover, the fine structure exhibited within the first tens of eV above an EELS ionisation edges is related to the local density of states, and hence, carries a wealth of information about the electronic environment of the ionised atom. However, direct measurement of the fine structure of pure compounds is only possible in homogeneous materials and in atomically resolved EELS of two-dimensional mono-layered materials. More commonly, EELS measurements comprise a linear combination of the fine structure corresponding to different atomic environments. Here we have devised a novel method to extract the fine structure of individual compounds from a multi-dimensional EELS dataset, based on a combination of curve fitting [2] and blind source separation [3]. Major practical complications with curve fitting for EELS quantification, especially in multi-dimensional datasets, are ill-conditioning and divergence of non-linear optimisation. To address this we have developed a new parallel Smart Adaptive Multidimensional Fitting (SAMFire) algorithm that learns the starting parameters from the dataset as the fitting progresses [4]. The analysis reveals that the particle is composed of boron (in different compounds), nitrogen, oxygen, carbon, silicon and calcium (Figure 1b, c). The EELS data analysis was performed using HyperSpy [5].

Tomographic reconstruction of the obtained tilt series of EELS elemental and bonding maps was performed using the FISTA algorithm with 3D total variation regularization [6]. Figure 2 displays a 3D visualization of the three boron compounds found in the sample, namely boron oxide, pure boron and boron nitride. Despite the small number of tilt series maps, the tomographic reconstruction reveals comprehensively the details of this complex 3D structure and provides new insight on the growth mechanism of the particle.

   

   

[1] R. Leary et al. Ultramicroscopy 131 (2013): 70–91

[2] T. Manoubi et al. Microscopy Microanalysis Microstructures 1.1 (1990): 23-39.

[3] F. de la Peña et al. Ultramicroscopy 111.2 (2011): 169-176.

[4] T. Ostasevicius et al. (in preparation).

[5] F. de la Peña et al. (2016). HyperSpy 0.8.4. Zenodo. 10.5281/zenodo.46897. http://hyperspy.org

[*] All authors acknowledge the support received from the European Union Seventh Framework Program under Grant Agreement 312483 – ESTEEM2 (Integrated Infrastructure Initiative – I3) . FDLP, TO, RKL and PM acknowledge support from the ERC under grant number 291522-3DIMAGE; and FDLP and CD under grant number 259619 PHOTO EM. RKL acknowledges a Junior Research Fellowship at Clare College. RA acknowledges funding from the Spanish MINECO (FIS2013-46159-C3-3-P), and from the EU under Grant Agreement 604391 Graphene Flagship.

Francisco DE LA PEÑA (Cambridge, United Kingdom), Tomas OSTAŠEVIČIUS, Rowan K. LEARY, Caterina DUCATI, Paul A. MIDGLEY, Raúl ARENAL
11:00 - 11:15 #6722 - IM01-OP052 STEM optical sectioning for imaging screw dislocation core structures.
STEM optical sectioning for imaging screw dislocation core structures.

The introduction of spherical-aberration correctors in STEM has allowed an improvement in spatial resolution up to the sub-angstrom scale also accompanied by a reduction of the depth of focus (due to the increase in probe convergence angles), which in a modern instrument is just a few nanometers, thus often less than the sample thickness. This can be exploited to extract information along the beam direction by focusing the electron probe at specific depths within the sample. This technique has already been used to observe the depth-dependence of the strain field due to the Eshelby twist associated with dislocations containing a screw component in thin STEM samples. The measurement of the magnitude of the displacement confirmed the screw Burgers vector for dislocations in GaN [1] and allowed the identification of a new dissociation reaction associated with mixed [c+a] dislocations [2]. The optical sectioning approach has also been applied to the direct observation of the c-component of the dissociation reaction of a mixed [c+a] dislocation in GaN by imaging a dislocation lying transverse to the electron beam [3].

Here we show how optical sectioning in high-angle annular dark-field (HAADF) STEM imaging conditions can be used to image the core structure of screw dislocations at atomic resolution. In particular, we evaluate using simulations whether the edge and screw displacements associated with the delocalization of ½[111] screw dislocations in body-centered cubic (BCC) metals [4] can be detected. In Figure 1 we show that the helicoidal displacements around a screw dislocation can be imaged with the dislocation lying transverse to the electron beam by optically sectioning the plane containing the dislocation.

In order to reveal how the edge components of the dislocation contribute to the observed contrast we have created two atomistic models, one using the anisotropic linear-elastic displacements around the dislocation, which is not capable of modelling the core delocalisation, and the other using the core structure relaxed using the Bond Order Potential for W which does predict a delocalised core. Figures 2 a) and b) show the respective HAADF simulated images. Figure 2 c) is the RGB image made from the (101) component of the Fourier Transform (FT) (shown in Figure 2 d)) of both images. It is possible to observe that the shifts in this Fourier component occur along two distinct lines lying parallel to [111]. The superposition of both filtered images shows that there is a discrepancy on both sides of the core between both models. It is therefore apparent that the delocalisation of the core can in principle be detected using electron-optical sectioning [5,6].

References

[1] J. G. Lozano, et al. Phys. Rev. Lett. 113 (2014) 135503.

[2] P.B. Hirsch, et al. Philosophical Magazine, 93 (2013) 3925.

[3] H. Yang, et al. Nature Communications, 6 (2015) 7266.

[4] P.B. Hirsch, Fifth Internat. Congs. Crystallography, Cambridge, 139 (1960)

[5] D. Hernandez-Maldonado, et al. manuscript in preparation

[6] The SuperSTEM Laboratory is the U.K National Facility for Aberration-Corrected STEM, supported by the Engineering and Physical Science Research Council (EPSRC). 

David HERNANDEZ-MALDONADO (Warrington, United Kingdom), Hao YANG, Lewys JONES, Roman GRÖGER, Peter B HIRSCH, Quentin M RAMASSE, Peter D NELLIST
11:15 - 11:30 #6413 - IM01-OP044 Vectorial field tomography from single projection for cylindrically symmetric nanostructures.
Vectorial field tomography from single projection for cylindrically symmetric nanostructures.

One-dimensional (1D) nanostructures have been regarded as the most promising building blocks for nanoelectronics and nanocomposite material systems as well as for alternative energy applications [1]. Magnetic nanowires with circular cross-section, are of utmost importance from theoretical and technological aspects [2]. 1D carbon-based nanostructures such as carbon nanotubes are amongst the best candidates for field emission displays and new high-brightness electron sources [3]. The confinement effects in 1D nanostructures can alter their properties and subsequently their behavior significantly. Hence it is necessary to understand the strong effect of their size on their three-dimensional (3D) properties such as the magnetic and electric fields associated with nanowires and nanotubes completely before they can be used in applications. There are currently very few methods, which have the capability to visualize the complete 3D fields associated with nanowires. In this work, we show that using a combination of symmetry arguments and electron-optical phase shift data obtained using TEM, it is possible to recover the entire 3D magnetic or electric field in and around nanowires and nanotubes from a single image.

The phase change φe+φm of an electron wave traveling through a thin foil can be expressed in terms of tomographic quantities [4]. The phase shift can be recovered experimentally using various techniques such as transport-of-intensity based methods or off-axis electron holography. Nominally, determining the 3D magnetic field or 3D elestrostatic potential requires recording a series of phase shift images as the sample is tilted about its axis. However, in a 1D nanostructure such as a nanocylinder that is uniformly magnetized along its long axis, the magnetic field possesses cylindrical symmetry with respect to the long axis. Similarly, carbon nanotubes under applied bias exhibit a cylindrically symmetric potential and electric field. Exploiting these conditions of cylindrical symmetry, we can reconstruct the full 3D magnetic or electric field associated with a nanowire from a single phase image using the inverse Abel transform. Simulations were performed for a uniformly magnetized sphere, which also possesses cylindrical symmetry to analyze the fidelty of the reconstruction algorithm. Fig. 1(a) shows the magnetic phase shift of such a uniformly magnetized sphere, (b) shows the derivative with respective to the horizontal axis (∂φm/∂x) showing the cylindrical symmetry, and (c) the 3D reconstructed magnetic induction using the single image method developed in this work. Experiments were performed on nickel nanowires that were uniformly magnetized to reconstruct the 3D magnetic induction. Similarly the 3D electric field was reconstructed from in-situ biased carbon cone nanotips under varying applied bias.

References

[1] C. M. Lieber, Solid State Communications, 107, 607 (1998).

[2] C. Chappert, A. Fert, F. N. Van Dau, Nature Materials, 6, 813 (2007).

[3] N. de Jonge, Y. Lamy, K. Schoots, T. H. Oosterkamp, Nature, 420, 393 (2002).

[4] C. Phatak, M. Beleggia, and M. De Graef, Ultramicroscopy, 108, 503 (2008)

Charudatta PHATAK, Ludvig DE KNOOP, Florent HOUDELLIER, Martin HYTCH, Aurélien MASSEBOEUF (Toulouse)
11:30 - 11:45 #6588 - IM01-OP051 Laboratory diffraction contrast tomography – applications and future directions.
Laboratory diffraction contrast tomography – applications and future directions.

Determining crystallographic microstructure of a given material in 2D can be challenging.  Further extending such an investigation to 3D on meaningful volumes (and without sample sectioning) can be even more so. Yet reaching this insight holds tremendous value for 3D materials science since the properties and performance of materials are intricately linked to microstructural morphology including crystal orientation. Achieving direct visualization of 3D crystallographic structure is possible by diffraction contrast tomography (DCT), albeit only available at a limited number of synchrotron X-ray facilities around the world. Recent developments, however, have made DCT possible on an X-ray microscope with a laboratory source. 

 

The introduction of diffraction contrast tomography as an additional imaging modality on the ZEISS Xradia 520 Versa laboratory X-ray microscope has opened up a whole new range of possibilities for studies of the effect of 3D crystallography on materials performance. The capability to link directly the crystallographic and grain microstructure information with that obtained via conventional absorption or phase contrast imaging, non-destructively in three-dimensions and all in the laboratory, creates a powerful and easy to access tool.   [1]  Using a polychromatic X-ray source, laboratory diffraction contrast tomography technique (LabDCT) takes advantage of the Laue focusing effect, improving diffraction signal detection and allows handling of many and closely spaced reflections. Additionally, LabDCT opens the way for routine, non-destructive and time-evolution studies of grain structure to complement destructive electron backscatter diffraction (EBSD) end-point characterization. Combination of grain information with microstructural features such as cracks, porosity, and inclusions all derived non-destructively in 3D presents new insights for materials characterization of damage, deformation and growth mechanisms. Furthermore, 3D grain orientation data is a valuable input into multi-scale, multi-layered modeling platforms that can virtually evaluate mechanical properties to produce high fidelity simulation results. Here, we introduce the LabDCT technique and demonstrate its unique capability through a selection of application examples for materials science as well as discuss innovative methods to extend the current capabilities of the technology for a better understanding of materials structure evolution in 3D.

 

[1] McDonald, S.A. et al. Non-destructive mapping of grain orientations in 3D by laboratory X-ray microscopy. Scientific Reports 5, 14665 (2015). doi: 10.1038/srep14665

Leah LAVERY (Pleasanton, USA), Christian HOLZNER, Hrishikesh BALE, Arno MERKLE, Samuel MCDONALD, Philip WITHERS, Yubin ZHANG, Dorte JUUL JENSEN, Peter REISCHIG, Erik LAURIDSEN
11:45 - 12:00 #5911 - IM01-OP040 Electron tomography based on highly limited data using a neural network reconstruction technique.
Electron tomography based on highly limited data using a neural network reconstruction technique.

Gold nanoparticles are studied extensively due to their unique optical and catalytical properties. Their exact shape determines the properties and thereby the possible applications. Electron tomography is therefore often used to examine the three-dimensional shape of nanoparticles. However, since the acquisition of the experimental tilt series and the 3D reconstructions are very time consuming, it is difficult to obtain statistical results concerning the 3D shape of nanoparticles. We propose a new approach for electron tomography that is based on artificial neural networks, which enables us to reduce the number of projection images with a factor of 5 or more1.

The application of neural network filtered backprojection method (NN-FBP)2 to electron tomography consists of two phases: (i) a learning phase, in which full tilt series and their corresponding reconstructions are used to calibrate the reconstruction algorithm and (ii) a reconstruction phase, in which large batches of limited tilt series (i.e. using fewer projection images) are rapidly reconstructed. The parameters of the NN-FBP are trained by high quality 3D reconstructions, based on 151 projection images, in the learning phase. In the reconstruction phase, a tilt series of only 10 projection images of a gold nanoparticle is used as input. As opposed to previous advanced reconstruction methods, specific prior knowledge is not explicitly used in the NN-FBP method. Also, since NN-FBP is based on the efficient Weighted Backprojection algorithm, it is computationally efficient as well, enabling high throughput of 3D reconstructions. We show that the NN-FBP reconstruction algorithm is able to yield electron tomography reconstructions based on highly limited data with a comparable quality to a reconstruction based on a full data series with a tilt increment of 1° (Figure 1).

The decrease in acquisition time and the use of an efficient reconstruction method enables us to examine a broad range of nanostructures in a statistical manner. Using the NN-FBP approach, the average radius of a batch of 70 nanospheres was obtained. These results confirm the reliability of the NN-FBP algorithm and demonstrate the possibility of combining electron tomography and statistical measurements (Figure 2).

 

[1]      Bladt, E.; Pelt, D. M.; Bals, S.; Batenburg, K. J. Ultramicroscopy 2015, 158, 81–88.

[2]      Pelt, D. M.; Batenburg, K. J. IEEE Trans. Image Process. 2013, 22, 5238–5251.

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

Eva BLADT (Antwerpen, Belgium), Daniel PELT, Joost BATENBURG, Sara BALS
12:00 - 12:15 #6191 - IM01-OP043 Electron tomography with sub-5-second temporal resolution for dynamic in situ transmission electron microscopy.
Electron tomography with sub-5-second temporal resolution for dynamic in situ transmission electron microscopy.

           Electron tomography (ET) is an important technique for the study of the three dimensional morphology, structure and chemical composition of nanoscale materials in the transmission electron microscope (TEM). Although ET is potentially a powerful tool for in situ three-dimensional TEM studies of catalytic reactions, growth processes, phase transformations, switching mechanisms and defect nucleation, motion and interactions, its application to dynamic experiments has been limited, since typical acquisition times for tilt series are typically tens of minutes or longer [1]. 

            We have recently demonstrated the rapid acquisition of a tomographic tilt series comprising approximately 3500 images between +30° and -70° in only 3.5 s, as shown in Fig. 1) [2]. Here, we apply the technique to the three-dimensional imaging of the thermal segregation of an InAs V-shaped nanomembrane [3]. The membrane was heated in situ in an FEI Titan TEM up to a temperature of 420 °C. Changes in membrane morphology were tracked by acquiring images on a Gatan K2-IS camera at 400 frames per second while continuously tilting the specimen between +60° and -65°. Each tilt series (i.e., one rotation in one direction) was acquired in approximately 4.3 s and comprised ~1700 frames. Selected frames are shown in Figs 2 and 3.

            The reconstruction of time-dependent (four-dimensional) tilt series can be performed using two approaches: i) Reconstruction of each individual tilt series and interpolation between each reconstruction in the time domain, resulting in a time resolution that is limited by the acquisition time for each tilt series (i.e., 4.3 s); ii) Model-based reconstruction, involving tracking of changes between individual frames within each tilt series, resulting in sub-second temporal resolution. We are presently pursuing both approaches. 

           We are grateful to Cory Czarnik and Ana Pakzad from Gatan Inc. for making the K2 IS camera available for the experiments and to the European Research Council for an Advanced Grant.

           References:

[1] S. Malladi et al. Nano Lett., 14: 384, 2014. DOI: 10.1021/nl404565j

[2] V. Migunov et al.  Sci. Rep., 5: 14516, 2015. DOI:10.1038/srep14516

[3] S. Conesa-Boj, et al. ACS Nano, 6: 10982, 2012. DOI: 10.1021/nn304526k

Vadim MIGUNOV (Jülich, Germany), Rowan K. LEARY, María DE LA MATA, Eleonora RUSSO-AVERCHI, Gözde TÜTÜNCÜOGLU, Anna FONTCUBERTA I MORRAL, Jordi ARBIOL, Rafal E. DUNIN-BORKOWSKI
12:15 - 12:30 #6521 - IM01-OP050 New approach for low dose electron diffraction tomography.
New approach for low dose electron diffraction tomography.

Due to strong interaction of electrons with matter, 3D electron diffraction tomography has been proven to be a reliable method to solve structures of very small crystals, compared to X-ray diffraction; even for small protein crystals [1]. 

Several procedures for automated acquisition of tomographic diffraction data sets have been described [2, 3], using precession technique or discrete beam tilt perpendicular to the goniometer tilt axis. However, those mentioned methods are mainly used for less beam sensitive materials and are not suitable for low dose applications.

Shi et al. [4-7] refined the collection of 3D data sets under low dose condition (less than 10 e-2). Here, the camera system is continuously acquiring diffraction data during a continuous tilt of the goniometer. This allows to completely scan the Fourier space. It turned out that the stability of the goniometer tilt speed and the limited flexibility of the tilt acquisition parameters are problematic.


Here, we present a newly developed automatic data acquisition system, combining real-time direct control of the TEM-deflection systems, the goniometer tilt and the acquisition of high-resolution diffraction patterns with a synchronized CMOS camera. This can be realized by the TVIPS Universal Scan Generator (USG), controlling eight TEM deflection coils, i. e. four beam deflection coils and four image deflection coils.

For a static goniometer alpha, the total beam tilt range (±3° to ±5°, depending on the TEM) can be fragmented by a defined beam tilt range, e.g. 0.5° (see Fig. 1). During the camera exposure time the beam is continuously tilted for this defined range (e.g. 0.5°). The continuous beam sweep during exposure time ensures the complete sampling of the Fourier space. 

Several crystal structures (Carbamazepin: C15H12N2O, bismuth oxychloride: BiOCl, Mayenite: Ca12Al14O33) have been successfully solved by this method. The total acquisition time for 190 high resolution (1Å resolution) diffraction patterns is about 15 minutes, with a total electron dose of about 10 e-2. The collected 3D electron diffraction data sets were processed using the EDT-PROCESS software [8].


 

References

1.       J. A. Rodriguez, M. I. Ivanova, M. R. Sawaya, D. Cascio, F. E. Reyes, D. Shi,S. Sangwan, E. L. Guenther, L. M. Johnson, M. Zhang, L. Jiang, M. A. Arbing, B. Nannenga, J. Hattne, J. Whitelegge, A. S.     Brewster, M. Messerschmidt, S. Boutet, N. K. Sauter, T.Gonen and D. Eisenberg: Structure of the toxic core of α-synuclein from invisible crystals, Nature (2015);  525 (7570).

2.       U. Kolb, T. Gorelik, C. Kübel, M.T. Otten and D. Hubert: Towards automated diffraction Tomography: part I- data acquisition, Ultramicroscopy (2007) ; 107(6-7):507-13.

3.       D. Zhang, P. Oleynikov, S. Hovmoller and X. Zou:Collecting 3D electron diffraction data by the rotation method, Z. Kristallogr (2010); 225 94–102.

4.       D. Shi, B.L. Nannenga, M.G. Iadanza and T. Gonen: Three-dimensional electron crystallography of protein microcrystals, eLife (2013); 2:e01345.

5.       B.L. Nannenga, D. Shi, j. Hattne, F.E. Reyes and T. Gonen: Structure of catalase determined by MicroED, eLife (2014); 3:e03600.

6.       B.L. Nannenga and T. Gonen:Protein structure determination by MicroED, Current Opinion in Structural Biology (2014); Volume 27:24–31.

7.       B.L. Nannenga, D. Shi, A.G.W. Leslie and T. Gonen: High-resolution structure determination by continuous-rotation data collection in MicroED, Nature Methods 11(2014); 927–930.

8.     M. Gemmi, P. Oleynikov: Scanning reciprocal space for solving unknown structures: energy filtered diffraction tomography and rotation diffraction tomography methods. Z. für Krist. 228 (2013) 51-58.

 

Reza GHADIMI, Hans TIETZ (Gauting, Germany), Peter OLYENIKOV
Salle Gratte Ciel 1&2

Wednesday 31 August

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IM7-I
10:15 - 12:30

IM7: Phase Microscopies
SLOT I

Chairmen: David COOPER (Engineer) (Grenobles, France), Christoph KOCH (Professor) (Berlin, Germany)
10:15 - 10:45 #8663 - IM07-S50 Limitations and challenges in off-axis electron holography of electromagnetic fields in nanoscale materials.
Limitations and challenges in off-axis electron holography of electromagnetic fields in nanoscale materials.

In contrast to most conventional transmission electron microscopy (TEM) techniques, which only allow the spatial distribution of image intensity to be recorded, off-axis electron holography allows the phase shift of the electron wave that has passed through an electron-transparent specimen to be measured. The phase shift can, in turn, be used to provide information about local variations in magnetic induction and electrostatic potential within and around the specimen. Recent developments in the technique include the reconstruction of electrostatic potentials and magnetic fields in three dimensions, the use of advanced specimen holders with multiple electrical contacts to study nanoscale working devices, improvements in the stability of transmission electron microscopes to optimise phase sensitivity and the development of new approaches for improving temporal resolution using both direct electron detectors and double exposure electron holography. We are currently using the technique to characterize electrostatic potentials and magnetic fields in a wide variety of nanoparticles, nanostructures and thin films that are subjected to electrical biases and externally applied magnetic fields, as well as to elevated and reduced temperatures. Figure 1 shows representative results obtained from a study of the thermomagnetic behaviour of nanoscale grains of magnetite during heating in situ in the TEM. The magnetic induction maps show first a horseshoe-like magnetic state and then magnetic phase contours that flow from the bottom to top of the grain at higher temperature.

 

An important limitation of backprojection-based algorithms for reconstructing magnetic fields in three dimensions is the presence of artefacts resulting from incomplete tilt series of phase images and the inability to include additional constraints and known physical laws. Accordingly, one of our aims is the development of a robust model-based approach that can be used to reconstruct the three-dimensional magnetization distribution in a specimen from phase images recorded as a function of specimen tilt angle using off-axis electron holography. In order to perform each reconstruction, we generate simulated magnetic induction maps by projecting best guesses for the three-dimensional magnetization distribution in the specimen onto two-dimensional Cartesian grids. Our simulations make use of known analytical solutions for the phase shifts of simple geometrical objects, with numerical discretization performed in real space to avoid artefacts generated by discretization in Fourier space, without a significant increase in computation time (Figs 2 and 3). Our forward simulation approach is used within an iterative model-based algorithm to solve the inverse problem of reconstructing the three-dimensional magnetization distribution in the specimen from tilt series of two-dimensional phase images recorded about two independent tilt axes. Results will be presented from studies of magnetite nanocrystals, lithographically patterned magnetic elements and magnetic skyrmions examined as a function of temperature and applied magnetic field. At the same time, we are developing a similar algorithm for the reconstruction of three-dimensional charge density distributions in materials. Preliminary results will be presented from studies of charge distributions in electrically biased needle-shaped specimens, which require the analysis of differences between phase images recorded using two applied voltages, in order to subtract the mean inner potential to the phase shift.

 

The above studies are part of a wider program of research aimed at recording off-axis electron holograms of nanoscale materials and devices in the presence of multiple external stimuli. Further examples will be presented from studies of electrically biased resistive switching devices and two-dimensional flakes of transition metal dichalcogenides, whose electrical properties can be influenced strongly by the presence of contamination and defects, as well as by their interfaces to metal contacts. We are grateful to J. Ungermann, M. Riese, G. Pozzi, W. Williams, A. R. Muxworthy, M. Farle, M. Beleggia, T. F. Kelly and N. Kiselev for valuable contributions to this work and to the European Research Council for an Advanced Grant.

Rafal E. DUNIN-BORKOWSKI (Jülich, Germany), Trevor P. ALMEIDA, Jan CARON, András KOVÁCS, Patrick DIEHLE, Florian WINKLER, Fengshan ZHENG, Amir H. TAVABI, Vadim MIGUNOV, Zi-An LI
Invited
10:45 - 11:00 #5876 - IM07-OP129 Direct Visualization of Magnetic Skyrmion by Aberration-Corrected Differential Phase Contrast Scanning Transmission Electron Microscopy.
Direct Visualization of Magnetic Skyrmion by Aberration-Corrected Differential Phase Contrast Scanning Transmission Electron Microscopy.

    Magnetic skyrmion is a topologically protected quantum spin texture. It is so stable and expected to be utilized for future memory devices featuring ultralow energy consumption. However, influences of structural defects in real materials remain to be elucidated in such practical applications. Since magnetic skyrmion is a nano-scale magnetic structure, visualization techniques with very high spatial resolution are essential to investigate such influences of nano-scale structural defects, such as edges, dislocations and grain boundaries on magnetic skyrmion. Here, we present the direct visualization of magnetic skyrmion lattice in a thin film specimen of FeGe1-xSix by aberration-corrected differential phase-contrast scanning transmission electron microscopy (DPC-STEM) taking advantages of a segmented annular all-field (SAAF) detector [1] connected via photomultiplier tubes with a high-speed numerical processer.

    Polycrystalline FeGe1-xSix was grown from FeGe0.8Si0.2 ingot by conventional solid-state reaction annealed at 900 °C for 100 hours.  A thin film specimen was fabricated from a bulk crystal by using an Ion Slicer (EM-9100IS, JEOL, Ltd.). For DPC STEM observations, we used a STEM (JEM-2100F, JEOL, Ltd.) equipped with a probe-forming aberration corrector (CEOS, GmbH) and a Schottky field emission gun operated at 200 kV.  This microscope was equipped with a SAAF detector.  We used a double-tilt liquid-nitrogen cooling specimen holder (Model 636, Gatan, Inc.).  Analysis of DPC STEM images was done either online by using a direct reconstruction system [2] implemented as an application of LabView software (National Instruments, Inc.) running on Windows or offline by using a program written in Digital Micrograph Scripting language (Gatan, Inc.).

    Figure 1a shows a schematic diagram of electron-optical system of DPC STEM used in the present study.  In-plane magnetization can be mapped by analyzing the Lorentz deflection of electron beam using segmented annular detector as schematically shown in Fig. 1b.  As shown in Fig. 2a, a set of four images is selected from sixteen images obtained by the segmented detector.  Such images are first converted into two images corresponding to a horizontal and a vertical component image of Lorentz deflection (Fig. 2b).  Finally, the two Lorentz deflection images are analyzed to show a in-plane magnetization vector, intensity, and magnetic helicity images as shown in Fig. 2c.  Figure 3 demonstrates the live reconstruction of in-plane magnetic field and intensity of a magnetic skyrmion lattice.  A unique structural relaxation mechanism in a magnetic Skyrmion domain boundary core, revealed by the technique for the first time [3], will be presented in detail.

 

References

 

[1] N. Shibata et al., J. Electron Microsc. 59 (2010) 473.

[2] N. Shibata et al., Sci. Rep. 5 (2015) 10040.

[3] T. Matsumoto et al., Sci. Adv. 2 (2016) e1501280.

[4] This work was supported by the Japan Science and Technology Agency SENTAN and Precursory Research for Embryonic Science and Technology. A part of this work was conducted at the Research Hub for Advanced Nano Characterization, The University of Tokyo, supported under “Nanotechnology Platform” (project No. 12024046) sponsored by MEXT, Japan. T.M. acknowledges support from GRENE from MEXT and N.S. acknowledges supports from the JSPS KAKENHI Grant number 26289234 and the Grant-in-Aid for Scientific Research on Innovative Areas "Nano Informatics" (grant number 25106003) from JSPS.

Takao MATSUMOTO (Tokyo, Japan), Yeong-Gi SO, Yuji KOHNO, Hidetaka SAWADA, Yuichi IKUHARA, Naoya SHIBATA
11:00 - 11:15 #6259 - IM07-OP134 Fabrication and characterization of a fine electron biprism on a Si-on-insulator MEMS chip.
Fabrication and characterization of a fine electron biprism on a Si-on-insulator MEMS chip.

For off-axis electron holography, an electrostatic biprism is usually located close to the selected area (SA) aperture plane of the transmission electron microscope (TEM). The application of a voltage to the biprism results in overlap of two parts of an incident electron beam and allows both the amplitude and phase of the electron wavefunction that has passed through a specimen to be recovered. The quality of the reconstructed electron wave depends directly on the information contained in the hologram. An off-axis electron hologram is characterized by its interference fringe spacing, contrast and overlap width. The interference fringe spacing and overlap width are determined by the electron optics of the TEM and by the deflection angle at the biprism. The interference fringe spacing is inversely proportional to the deflection angle, while the overlap width is influenced by the width of the biprism. In order to achieve as narrow a fringe spacing as possible with high fringe contrast, the biprism should be as narrow and stable as possible. Previous attempts to make ultra-narrow biprisms have included glass fibres coated with metal or patterned SiNx with focused ion beam. None of these attempts have provided a reproducible method of making ultra-narrow biprisms with perfect control over their dimensions.

.

Here, we illustrate an approach that can be used to fabricate a biprism that has a rectangular cross-section and is located between two counter electrodes that are at the same height. We pattern the biprism in the top Si layer of a Si-on-insulator (SOI) wafer. The wafer consists of a micron-thick single-crystalline Si layer that is isolated electrically from its substrate and can be left free-standing using an etching process. When combined with microelectromechanical systems (MEMS) processes, structures can be patterned down to nm scale in three dimensions. In this way, the width of the biprism and the distance to the counter-electrodes can be chosen to have dimensions down to ~100 nm. A further advantage of using an SOI wafer to fabricate a biprism is the large Young’s modulus of the single-crystalline Si biprism (170 GPa), when compared with that of a conventional biprism made from glass (~70 GPa). In addition, the two counter-electrodes can be biased independently. A schematic diagram and scanning electron microscopy (SEM) images of a biprism on an electrically-contacted MEMS chip are shown in Fig. 1, alongside a three-dimensional design drawing of a custom-made aperture rod.

.

In order to test its performance, the biprism was mounted close to the SA plane in a Philips CM20 TEM. The electron deflection was measured by recording the shift of a diffraction spot as function of applied voltage. The measured deflections are compared with predicted deflections and with similar measurements made using a conventional biprism on an FEI Titan TEM in Fig. 2. The deflection is a factor two greater for the new rectangular biprism for the same applied voltage. The measured interference fringe spacing, contrast and overlap width achieved using the new biprism are also shown in Fig. 2. Here, the maximum voltage that can be applied is limited by the distance between the biprism and the counter-electrodes, which can be increased in future designs.

.

In order to demonstrate the imaging capabilities of the new biprism, an off-axis electron hologram of a MoS2 flake was recorded in a Philips CM20 TEM. The hologram and the resulting reconstructed amplitude and phase are shown in Fig. 3. In the future, the biprism will be mounted in an image-aberration-corrected FEI Titan TEM, in which the electron optics offers greater flexibility in both normal and Lorentz imaging modes.

.

The authors acknowledge financial support from the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative (Reference 312483 ESTEEM2), the European Research Council for an Advanced Grant (Reference 320832 IMAGINE) and for Starting Grant (Reference 306535 HOLOVIEW) for financial support. We thank David Cooper and Helmut Soltner for valuable discussions and support.

Martial DUCHAMP (Jülich, Germany), Olivier GIRARD, Florian WINKLER, Rolf SPEEN, Rafal E. DUNIN-BORKOWSKI
11:15 - 11:30 #5495 - IM07-OP127 The measurement of off-plane magnetic field through electron vortex beams.
The measurement of off-plane magnetic field through electron vortex beams.

It is well known that a magnetic field produces a phase change proportional to the in plane magnetic field B [1]. Alas the out of plane magnetic field is more elusive. The only measurement that so far seems to produce  any out of plane information is the magnetic dichroism [2] as it actually measures the out of plane magnetization.
Unfortunately magnetic dichroism experiment is still a complicated experiment that requires a considerable control of the condition and of the material; moreover the measures of the magnetization , often with the objective on, is not directly a measure of the magnetic field itself.
However while a plane wave along the optical axis has no significant interaction with a magnetic field along z , vortex beams  with winding number l do feel such field through a Larmor phase effect .
We will demonstrate here that through the use of large vortex beams [3][4][5] we are able, for the first time, to measure the out of plane magnetic field generated by a magnetic pillar of Co [6].
We placed the vortex around the pillar ensuring as much as possible an axial symmetry and observed the phase effect.  The result is in good agreement with the typical magnetization expected for the pillar with an average field at surface of about 2T.
Fig 1 illustrates the A and B field produced by a magnetic pillar. The actual magnetic pillar fabricated by EBID deposition is illustrated in fig 1b. Fig 2 depicts the vortex beam density with L=200 as imaged in its focal plane. Thanks to an interferometric approach we were able to measure the magnetic phase contribution and bind it to the magnetic field in proximity of the sample. The magnetic phase is visible in the inset of fig 2 showing approximate axial symmetry.
The technique has been carried on in Low Mag mode with the main objective  lens off but there are no principle limitation to the application in conventional (S) TEM mode.
Moreover this promises  to be one of the best recognition of the importance of vortex beams in microscopy for material science.
[1] R E. Dunin-Borkowski, M. R. McCartney et al Science 282 (1998) 1868
[2] P. Schattschneider, S. Rubino Nature 441 (2006) 486
[3] J. Verbeeck, H. Tian  P. Schattschneider Nature 467 (2010) 301
[4] B. J. McMorran,  A. Agrawal  et al. Science 331 (2011) 192
[5] V. Grillo et al .  Phys Rev Lett 114, 034801 (2015)
[6] G. Pozzi, C. B. Boothroyd Appl. Phys. Lett. 108, 083108 (2016)

Vincenzo GRILLO (Modena, Italy), Tyler HARVEY, Jordan PIERCE, Federico VENTURI, Roberto BALBONI, Gian Carlo GAZZADI, Stefano FRABBONI, Benjamin MCMORRAN, Robert BOYD, Ebrahim KARIMI
11:30 - 11:45 #6834 - IM07-OP136 Direct comparison of differential phase contrast and off-axis electron holography for the measurement of electric potentials by the examination of reverse biased Si p-n junctions and III-V samples.
Direct comparison of differential phase contrast and off-axis electron holography for the measurement of electric potentials by the examination of reverse biased Si p-n junctions and III-V samples.

In this presentation we will compare differential phase contrast (DPC) [1] and off-axis electron holography [2] for the measurement of electrostatic potentials in semiconductor devices. DPC uses the lateral shifts of a convergent electron beam to determine the field in the sample whereas for electron holography the changes in potential is encoded in interference fringes that are formed using a biprism [1]. To fairly assess the relative sensitivity of the different techniques on the same specimen, a symmetrically doped p-n junction with a dopant concentration of 1 x 1019 cm-3 has been measured as a function of reverse bias applied in situ in the TEM.  Figure 1(a) and (b) shows maps acquired by DPC of the p-n junction with 0V and 4V reverse bias applied. At 0V, it is difficult to see the presence of the junction whereas for a reverse bias of 4 V the space charge region is now visible. Profiles acquired from across the junction for various reverse bias voltages can be seen in Figure 1(c).  Electron holograms were acquired and Figure 1(d) and (e) show reconstructed phase images of the junction at zero bias and 4 V reverse bias. Even though a low magnification has been used to obtain a large field of view at the expense of sensitivity, the junction is clearly visible in both of the phase images. Corresponding electric field profiles that have been calculated from the potential maps are shown in Figure 1(f).  These results show that off-axis electron holography has a significantly better sensitivity than DPC. However  the advantages of using DPC is that a large field of view has been obtained and it is not necessary to examine a region close to vacuum.

These techniques have also been applied to an InGaN/GaN system. Figure 2(a) shows a HAADF STEM image of the specimen. Figure 2(b) shows a potential map and (c) profile acquired by off-axis electron holography with a spatial resolution of 5 nm.  When using DPC, sub-nanometer spatial resolution is expected and maps of the electric field in the InGaN layers can be observed in Figures 2(d) and (e). Here the specimen has been tilted onto and away from a zone axis and a large variation in the measured signal is observed which is visible in Figure 2(f). In this presentation will discuss the effects of diffracted beams on measured DPC signal. We will also discuss the advantages and disadvantages of using DPC and electron holography for the measurement of electrostatic fields in a range of different doped and III/V semiconductor specimens and show improvements that have been applied.

 

Acknowledgements : This work has been funded by the ERC Starting Grant 306365 « Holoview ». The experiments have been performed on the platform nanocharacterisation at Minatec.

 

References

 

[1] N.H. Dekkers, Optik, 30 (1974) 452

[2] A. Tonomura, Reviews of Modern Physics, 59 (1987) 1

Benedikt HAAS (GRENOBLE CEDEX 9), David COOPER, Jean-Luc ROUVIÈRE
11:45 - 12:15 #8311 - IM07-S51 Electron interferometry techniques for strain analysis using a multi-biprism microscope.
Electron interferometry techniques for strain analysis using a multi-biprism microscope.

Electron interferometric techniques have progressed in the last years thanks to the development of multiple biprisms microscopes. Here, we will discuss some recent developments in the field of strain measurement carried out with the I2TEM microscope (In-situ Interferometry Transmission Electron Microscope) installed in Toulouse in 2012. The I2TEM is a Hitachi HF-3300 equipped with one pre-specimen electrostatic biprism, three post-specimen biprisms, an image corrector (CEOS B-COR for correcting off-axial aberrations) and two stages (objective stage and Lorentz stage above the objective lens).

In the dark-field off-axis scheme [1], an electron beam diffracted by an epitaxially grown region is interfered with a beam diffracted by the substrate thanks to a post-specimen biprism (Fig. 1(a)).  Fig. 1(b-d) is an example obtained on a p-MOSFET like transistor with SiGe source/drain. The deformation is recorded as a frequency modulation (FM) in the hologram (Fig. 1(c)) and it can be calculated from the gradient of the reconstructed phase image (Fig 1(d)).

In a recently proposed variant called differential phase contrast dark-field holography (DPC-DFEH) [2], a pre-specimen biprism is used to create two overlapping waves on the sample (Fig. 1(e)). The interference of beams diffracted by slightly distant regions is acquired in a defocused plane. The deformation is recorded as a phase modulation (PM) in the hologram (Fig. 1(f)) and the DPC phase is directly proportional to the deformation (Fig. 1(g)).

Another option is the 4-wave dark-field setup where two biprisms oriented perpendicularly are used to interfere three reference waves and one object wave (Fig. 1(h-j)). The holographic fringes are modulated in amplitude (AM) and each amplitude contour corresponds to a given displacement of the lattice planes [3].  It can provide live information if a sufficient fringe contrast is achieved.

In all cases, strain measurement requires a reference wave diffracted by a region of known lattice parameter (usually the substrate). One solution is the “tilted reference wave” (TRW) where a pre-specimen biprism and the condenser system are used to create an object-independent reference wave with an adjustable tilt angle [4]. Fig. 2(a,b) is an example acquired in the vacuum and Fig. 2(c,d) shows the dark-field configuration for strain measurement.

Finally, a pre-specimen biprism can also be useful for electron diffraction techniques. For instance, one can create two parallel half cones on the specimen (splitting convergent beam electron diffraction, SCBED) with a controllable distance (Fig. 3(a)) [5]. Each spot in the diffraction pattern contains two lobes related to the regions crossed by the two probes. Fig. 3(b) shows an example of SCBED pattern series where the left and right lobes are related to unstrained and increasingly strained regions respectively.

[1]  M Hÿtch et al, Nature 453 (2008), 1086–1089.

[2]  T Denneulin et al, Ultramicroscopy  160 (2016), 98–109.

[3]  T Denneulin and M Hÿtch, J. Phys. D: Appl. Phys. 49 (2016), 244003.

[4]  F Röder et al, Ultramicroscopy 161 (2016), 23–40.

[5]  F Houdellier et al, Ultramicroscopy 159, Part 1 (2015), 59–66.

Acknowledgments

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. This work has been supported by the French National Research Agency under the "Investissement d'Avenir" program reference No. ANR-10-EQPX-38-01. The authors acknowledge the "Conseil Regional Midi-Pyrénées" and the European FEDER for financial support within the CPER program.

Thibaud DENNEULIN (TOULOUSE CEDEX), Falk RÖDER, Florent HOUDELLIER, Christophe GATEL, Etienne SNOECK, Martin HŸTCH
Invited
12:15 - 12:30 #5789 - IM07-OP128 Electron holography by means of tilted reference waves.
Electron holography by means of tilted reference waves.

Off-Axis Electron Holography permits the direct reconstruction of amplitude and phase of electron waves elastically scattered by an object (see, e.g., [1]). The technique employs the Möllenstedt biprism to mutually incline an object modulated wave and a plane reference wave to form an interference pattern at the detector plane. Limited coherence of the electron beam in presence of aberrations attenuates high spatial frequencies of the object exit wave spectrum, which is illustrated by the sideband envelope function for a non-corrected TEM in Fig. 1a.

In this work, we explore an extension of the conventional setup given by deliberately tilting the reference wave independent from the object wave. This allows the transfer of spatial frequencies beyond the conventional sideband information limit in Fig. 1a as predicted by a generalized transfer theory for Off-Axis Electron Holography [2]. This is because a reference wave tilted by q0 compensates the aberration impact on the spatial frequency q0 of the object wave spectrum. The resulting transfer envelope for a tilt of q0x = -10/nm perpendicular to the post-specimen biprism is shown in Fig. 1b, where the contrast maximum of the total envelope (TCC) is located at q0x. Thus, an off-axis hologram series with varying reference wave tilt allows in principle a linear synthesis of an effective coherent aperture with a radius reaching out beyond the conventional information limit. Furthermore, an object-independent measurement of aberrations as well as dark-field electron holography can be realized using this setup.

The experimental realization of an arbitrarily tilted reference wave is challenging and could be realized for the first time at the Hitachi HF3300C I2TEM at CEMES Toulouse for one direction [3]. We used an additional biprism placed in the illumination system. Three condenser lenses were adjusted to provide a demagnified image of the condenser biprism at the sample plane under parallel illumination (Fig. 2). The pre-specimen deflectors were adapted to maintain the incident wave vector of the object wave and to realize a tilt of the reference wave as a function of the condenser biprism voltage. Optimal condenser lens settings were found by means of paraxial ray tracing (Fig. 3) finally producing a mutual tilt of up to 20/nm at the object plane. We verified the kink-like phase modulation of the incident beam by means of holographic measurements. Contrast transfer theory including condenser aberrations and biprism instabilities was applied to explain detailed fringe contrast measurements. Finally, we have experimentally shown that dark-field holography [4] can be conducted with an object-independent reference.

 

[1]  H Lichte et al, Rep. Prog. Phys. 71 (2008) 016102

[2]  F Röder et al, Ultramic. 152 (2015) 63-74

[3]  F Röder et al, Ultramic. 161 (2016) 23–40

[4]  MJ Hÿtch et al, Nature 453 (2008), 1086–1089

 

Acknowledgments

We thank the Graduate Academy of the TU Dresden for the financial support. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure Initiative-I3).

Falk RÖDER (Dresden, Germany), Axel LUBK, Florent HOUDELLIER, Thibaud DENNEULIN, Etienne SNOECK, Martin HŸTCH
Salle Tête d'or 1&2

Wednesday 31 August

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IM10-I
10:15 - 12:30

IM10: Correlative microscopy
SLOT I

Chairmen: Yannick SCHWAB (Heidelberg, Germany), Paul VERKADE (Bristol, United Kingdom)
10:15 - 10:45 #8369 - IM10-S59 Correlative immuno Light Electron Microscopy (CLEM) of subcellular compartments.
Correlative immuno Light Electron Microscopy (CLEM) of subcellular compartments.

Correlative light and electron microscopy (CLEM) methods integrate light and electron microscopy on a single sample, literally bridging the gap between these two microscopy techniques. Most methods use fluorescent microscopy of thin or semi-thin sections to define a region-of-interest, which then is traced back in the EM to provide subcellular context information (e.g. membrane organization, non-labeled surroundings of the fluorescent structure). The most powerful application of CLEM is correlative live cell imaging-EM, by which dynamic information is inferred to structures seen in static EM pictures. By merging the strengths of the two techniques a novel and integrated type of image is created that combines parameters that cannot - or not easily - be obtained when using separate images of related events. However, because imaging requirements are intrinsically different between light and electron microscopy, creating conditions that are ideal for both modalities is a challenging process. Moreover, correlating fluorescent labeling to the cellular architecture seen in the EM is not always straightforward. The most pressing challenges in CLEM are currently therefore 1. development of sample preparation methods that are appropriate for both LM and EM imaging. 2. development of bi-modal probes that are visible in both LM and EM and 3. development of software that allows for rapid and accurate correlation of LM and EM images. 

The focus of our research is to develop new probes and CLEM pipelines that allow us to efficiently and with high accuracy define the three-dimensional (3D) ultrastructural context of fluorescently-tagged proteins previously localized in fixed or living cells. We apply our technologies to study the cellular pathways and mechanisms that control the cell’s digestive system – i.e. the endo-lysosomal system - in health and disease conditions. The main CLEM technology that we use in the lab is based on the use of immunogold labeling of ultrathin cryosections (the Tokuyasu technique) [1]. Most CLEM approaches, however, are restricted in their EM approach by the lack of 3D structural information. To overcome this limitation, we apply Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) as 3D-EM approach in a live cell-CLEM set up. To visualize endo-lysosomes in live cells we combine fluorescent tagged endo-lysosomal proteins (such as LAMP1-mGFP) with endocytic tracers (such as fluorescently labeled dextran). This approach enables live-cell tracking of specific endo-lysosomal compartments, after which the samples are fixed, stained and resin-embedded for FIB-SEM imaging. Figure 1 presents an example of Dextran-Alexa646 and/or LAMP1-mGFP labeled endo-lysosomal compartments in live cells (A) and in 3D-EM (D), providing the cellular context at ultrastructural resolution. In my presentation I will show various examples of both immunoEM and FIB.SEM-based CLEM approaches, which are designed to optimally image individual, membrane-bounded compartments. 

Judith KLUMPERMAN (Utrecht, The Netherlands)
Invited
10:45 - 11:00 #6922 - IM10-OP174 3D mapping of subcellular structures with super-resolution array tomography.
3D mapping of subcellular structures with super-resolution array tomography.

The combination of fluorescence light microscopy (FLM) and electron microscopy (EM) makes it possible to put molecular identity in its full ultrastructural context. With array tomography (AT) long ribbons of serial ultrathin sections are labelled via immunohistochemistry (IHC). After FLM imaging the same sections are processed for scanning electron microscopy (SEM) and re-analyzed. The resulting images are correlated and superimposed. Due to the serial nature of this approach it is possible to obtain large volumes of correlated multi-channel light and electron microscopic data. One drawback of this method is the large discrepancy of resolution between light and electron microscopy. To alleviate this we advanced AT for two super-resolution light microscopy techniques, Structured Illumination Microscopy (SIM) and direct Stochastic Optical Reconstruction Microscopy (dSTORM). We also devised a method for easy, precise, and unbiased correlation of EM images and super-resolution imaging data using endogenous cellular landmarks and freely available image processing software. Together, these advances make it possible to map even small subcellular structures with high precision and confidence. We applied our super-resolution AT approach in C. elegans to address an important problem in connectomics research: the mapping of gap junctions at connectomes.

Sebastian MARKERT (Würzburg, Germany), Sebastian BRITZ, Sven PROPPERT, Marietta LANG, Daniel WITVLIET, Ben MULCAHY, Markus SAUER, Mei ZHEN, Jean-Louis BESSEREAU, Christian STIGLOHER
11:00 - 11:15 #6596 - IM10-OP171 High-resolution identification of immuno-labelling nanoparticles on tissue using X-ray detection.
High-resolution identification of immuno-labelling nanoparticles on tissue using X-ray detection.

Cellular structure can be imaged at nanometer resolution using electron microscopy, but the biological molecules remain invisible in grayscale electron micrographs. With Correlative Light and Electron Microscopy (CLEM)[1], fluorescence recorded on a separate light microscope can be used to identify molecules in colour, but the resolution gap with electron microscope precludes accurate localization. The use of super-resolution fluorescence microscopy techniques combined with CLEM[2,3] holds great promise but is still one order of magnitude off in resolution compared to EM. In addition, accurate registration of the separate images may be challenging and correlation may require expert procedures to maintain, e.g., protein photo-switching under EM preparation conditions. Thus, while CLEM adds information represented in the colors of visible light, molecular identification and localization at the resolution of the electron microscope is still hampered. Therefore, we are exploring novel approaches to achieve color-EM using the electron beam to add information about the specimen to the electron micrographs.

Detection of electron-beam excited luminescence has been explored in the past. Cross-sections for visible luminescence (also called cathodoluminescence) are, however, small and most biological fluorescent labels are destroyed before sufficient signal has been collected to allow high-resolution localization [4]. Phosphorescent nanoparticles are considered as non-bleaching alternative, but particle sizes are not yet well-defined and bio-functionalization has in most cases still to be pursued[5]. 

Here, we explore detection of X-ray fluorescence to identify nanoparticle labels[6] at EM resolution in colour on tissue. We use Au and CdSe colloidal quantum dots, immuno-targeted to guanine quadruplexes, resp. insulin, as molecular labels in large-scale electron microscopy of pancreatic tissue. Energy dispersive detection of X-ray emission (EDX) during electron irradiation allows discrimination of both nanoparticles based on their elemental (Au vs Cd) composition. We will present electron microscopy images overlaid with false colour elemental maps displaying single nanoparticles. Separate resolution tests have shown that individual Au particles down to 3 nm in size can be detected using EDX, highlighting the potential for coloured identification of materials in biological electron microscopy, revealing both compositional and ultrastructural information at nanometer-scale resolution. 

[1] P. de Boer, J. P. Hoogenboom, B. N. G. Giepmans, Nature Methods 12, 503 (2015)

[2] Kopek, B.G., et al., Proc. Natl. Acad. Sci. U. S. A. 109, 6136 (2012)

[3] Johnson, E. et al., Scientific Reports 5 (2015)

[4] Niitsuma, J., Oikawa, H., Kimura, E., Ushiki, T. & Sekiguchi, T., J. Electron Microscopy 54, 325 (2005).

[5] Glenn, D.R. et al., Scientific Reports 2 (2012)

[6] A. Loukanov, N. Kamasawa, R. Danev, R. Shigemoto, K. Nagayama, Ultramicroscopy 110,366 (2010)

Marijke SCOTUZZI, Jeroen KUIPERS (Groningen, The Netherlands), Dasha I. WENSVEEN, Pascal DE BOER, Cornelis W. HAGEN, Ben N G GIEPMANS, Jacob P. HOOGENBOOM
11:15 - 11:30 #6676 - IM10-OP172 Role of Lamin B1 in structuring the cell nucleus in eukaryotic cells.
Role of Lamin B1 in structuring the cell nucleus in eukaryotic cells.

Lamin B1 (Lmnb1) is a major component of the nuclear lamina and together with other lamins (lamin A/C) plays a key role in the structure and function of the nucleus. Mutations in lamins and lamin-associated proteins have been indeed shown to cause various human diseases. There are indications that mammalian lamins A and B and drosophila Lamin Dm0 are involved in nuclear envelope formation and nuclear morphology (Prüfert et al. J Cell Sci 117, 6105). To explore the specific role of Lamin B1 in defining nuclear morphology we applied electron microscopy, electron tomography and correlative light electron microscopy (CLEM) on different cell types in which murine lamin B1 (Lmnb1) was silenced or human lamin B1 (LMNB1) was over-expressed.
To investigate how Lmnb1 deficiency affects nuclei of post-mitotic neurons, we analyzed the nuclear morphology of primary mouse cortical neurons derived from Lmnb1-null (Lmnb1Δ/Δ) embryos (Giacomini et al. Mol Biol Cell 27, 35). Nuclei of cultured Lmnb1Δ/Δ neurons were smaller and rounder than those of Lmnb1+/+ embryos (the area of Lmnb1Δ/Δ nuclei was 43% less than that of Lmnb1+/+). Moreover the Lmnb1Δ/Δ nuclei presented a rounder shape than those of Lmnb1+/+, as confirmed by increased circularity values (mean circularity was 0.62±0.16 n=28 and 0.9±0.87 n=39 for respectively Lmnb1+/+ and Lmnb1Δ/Δ nuclei). Taken together, these results indicate that optimal Lmnb1 levels are essential to maintain neuronal nuclear size and shape. Lmnb1 deficiency also affected the distribution and composition of nuclear pore complexes (NPCs) in mouse cortical neurons. Indeed in Lmnb1Δ/Δ neurons, the NPCs were distributed irregularly, with some regions of the nuclear envelope hosting groups of NPCs located close to each other and other areas devoid of NPCs (Fig.1A-C). Electron tomography performed on serial semi-thin sections showed that in Lmnb1Δ/Δ neurons the NPCs are distributed in parallel, closely packed rows (Fig.1D).
The effect of the LMNB1 over-expression on nuclear morphology was assessed in transfected HEK 293 and HeLa cells. HEK 293 cells transfected with a bicistronic plasmid containing LMNB1 and EGFP downstream of an IRES sequence were sorted by EGFP expression using fluorescence-activated cell sorting (FACS) and high pressure frozen and freeze substituted to maintain structural integrity as much as possible. To increase contrast at the nuclear membranes we set up a freeze substitution recipe with a pre-incubation step at -90°C in 2% hydrated acetone containing small percent of tannic acid and glutaraldehyde. The cell nuclei of HEK 293 over-expressing LMNB1 were lobulated, with highly folded nuclear membranes often associated with multi-membrane stacks and intra-nuclear membranes (Fig.2A). As in primary cortical neurons lacking Lmnb1, we observed clusters of NPCs at the nuclear membrane (Fig.2B). Immunolabeling performed on cryo-sections revealed the presence of LMNB1 on the membrane stacks associated with the nuclear envelope (Fig.2C). This result supports the idea that these membrane stacks are effectively multi-membrane-layered nuclear membranes. To better understand the fine localization of Lmnb1 we performed correlative light electron microscopy (CLEM) experiments on HEK 293 and HeLa cells overexpressing LMNB1 in fusion with EGFP and tdTomato downstream of an IRES sequence. The cells, grown on sapphire disks carbon coated with a finder grid pattern, were live imaged at the confocal laser scanning microscope and then high pressure frozen and freeze substituted (Fig.3). As for HEK 293, the cell nuclei of HeLa cells over-expressing LMNB1 were lobulated with highly folded nuclear membranes. The HeLa nuclei differed from HEK 293 nuclei for the absence of membrane stacks associated to the nuclear membranes and for the presence of peculiar morphologies (Fig.3A). Indeed our CLEM approach revealed the presence of highly folded intra-nuclear membranes delimiting large cytoplasmic regions (Fig.3B,D). This approach also revealed the presence of nuclei that branched apically in thin and long nuclear rods delimited by conventional nuclear membranes (Fig.3B,C). Altogether, our results indicate that proper levels of Lamin B1 are required to maintain structural integrity and morphology of the nuclear envelope.
Acknowledgement
We thank Roberta Ruffilli and all the EM Lab members for their support. Special thanks to Prof. Liberato Manna for supporting this research.

Roberto MAROTTA (Genoa, Italy), Tiziano CATELANI, Mattia PESCE, Caterina GIACOMINI, Sameehan MAHAJANI, Gasparini LAURA
11:30 - 12:00 #8741 - IM10-S61 An integrated structural cell biology to unravel the vesicle and membrane coat formation mechanism at the inner nuclear envelope.
An integrated structural cell biology to unravel the vesicle and membrane coat formation mechanism at the inner nuclear envelope.

Our group uses electron cryo microscopy (cryoEM) / tomography (cryoET) and combines these with complementary techniques to understand biological processes mechanistically. The power of such an integrated structural biology approach to cell biology at the macromolecular level will be exemplified along the group’s analyses of crucial steps in the molecular interactions between viruses and their host cells . The main emphasis will be on understanding mechanisms of membrane modulation in vesicle formation at the inner nuclear envelope in cargo egress from the nucleus [1,2].

Although nucleo-cytoplasmic transport is typically mediated through nuclear pore complexes, herpesvirus capsids exit the nucleus via a unique vesicular pathway. Vesicular nucleo-cytoplasmic transport might also be a general cellular mechanism for translocation of large cargoes across the nuclear envelope. Cargo is recruited, enveloped at the inner nuclear membrane (INM), and delivered by membrane fusion at the outer nuclear membrane. To understand the structural underpinning for this trafficking, we investigated nuclear egress of progeny herpesvirus capsids where capsid envelopment is mediated by two viral proteins (pUL31 and pUL34), forming the nuclear egress complex (NEC). Using a multi-modal imaging approach, we visualized the NEC in situ forming coated vesicles of defined size. Cellular electron cryo-tomography revealed a protein layer showing two distinct hexagonal lattices at its membrane-proximal and membrane- distant faces, respectively. NEC coat architecture was determined by combining this information with integrative modeling using small-angle X-ray scattering data. The molecular arrangement of the NEC establishes the basic mechanism for budding and scission of tailored vesicles at the INM. We further solved the crystal structure of the pseudorabies virus NEC. Fitting of the NEC crystal structure into the cryoEM-derived hexagonal lattice found in situ, provided details on the molecular basis of NEC coat formation and inner nuclear membrane remodeling. Altogether, this multimodal imaging study exemplified the power of an integrated structural cell biology approach to reveal novel mechanistic insights by combining in-situ data with information on the isolated macromolecules mediating the respective biological process of interest.

References:

[1] Hagen, C., Dent, K.C., Zeev-Ben-Mordehai, T., Grange, M., Bosse, J.B., Whittle, C., Klupp, B.G., Siebert, C.A., Vasishtan, D., Bäuerlein, F.J.B. Cheleski, J., Werner, S., Guttmann, P., Rehbein, S., Henzler, K., Demmerle, J., Adler, B., Koszinowski, U., Schermelleh, L., Schneider, G., Enquist, L.W., Plitzko, J.P., Mettenleiter, T.C., Grünewald, K. (2015) Structural Basis of Vesicle Formation at the Inner Nuclear Membrane. Cell 163: 1692–1701.

[2] Zeev-Ben-Mordehai, T., Weberruß, M., Lorenz, M., Cheleski, J., Hellberg, T., Whittle, C., El Omari, K., Vasishtan, D., Dent, K.C., Harlos, K., Franzke, K., Hagen, C., Klupp, B.G., Antonin, W., Mettenleiter, T.C., Grünewald, K. (2015) Crystal structure of the herpesvirus nuclear egress complex provides insights into inner nuclear membrane remodeling. Cell Reports 13: 2645–2652.

Christoph HAGEN, Tzviya ZEEV-BEN-MORDEHAI, Kyle DENT, Michael GRANGE, Jens B. BOSSE, Daven VASISHTAN, C. Alistair SIEBERT, Rainer KAUFMANN, Juliana CELESKI, Cathy WHITTLE, Lothar SCHERMELLEH, Barbara KLUPP, Gerd SCHNEIDER, Wolfram ANTONIN, Lynn ENQUIST, Thomas C METTENLEITER, Kay GRÜNEWALD (Oxford, United Kingdom)
Invited
12:00 - 12:15 #6031 - IM10-OP167 Mycobacterial protein weaponry studied by cryo-correlative microscopy.
Mycobacterial protein weaponry studied by cryo-correlative microscopy.

In correlative light and electron microscopy, the discerning power of fluorescence microscopy (FM) is combined with the ultra-high resolution of electron microscopy (EM). We fluorescently label protein complexes in cells, with the purpose of facilitating their identification in transmission electron tomograms. Our imaging strategy is to perform cryogenic selected volume tomography, continuously keeping the specimen in a vitreous state. To this end, we are developing a cryogenic workflow in collaboration with FEI, consisting of three microscopes: CorrSight light microscope (LM), Scios Dualbeam scanning electron microscope (SEM) with a focused ion beam (FIB) and Tecnai Arctica cryo–transmission electron microscope (TEM). The focus of my presentation is on the integration of the CorrSight light microscope into the workflow (1). This translates to performing cryogenic light microscopy and correlating the images with EM for guided ion beam milling and tomography (2).

The data we collected from cryogenic fluorescence microscopy of mammalian and bacterial cells on TEM grids shows great promise for complementing EM (Fig. 1). This is substantiated by alignment of LM and SEM images utilizing FEI MAPS software, allowing for fluorescence based navigation during SEM imaging (Fig. 2). My future efforts will focus on improving localization precision and correlation accuracy, to guide FIB-milling of sub-cellular structures in the SEM. FIB-milling is used to fabricate thin lamellae in cells, making them suitable for cryo-TEM. Our results indicate that tomograms can be recorded from lamellae using a 200kV cryo-TEM, but vitrification of the specimen is essential. The ultimate goal of our workflow is to obtain structural insight of the Type 7 Secretion System (T7SS), formed by the ESX-1 machinery. To this end, we will perform tomography of mycobacterial membranes inside professional antigen-presenting cells. The T7SS is an important part of the mycobacterial protein weaponry, as it mediates virulence and phagosomal escape (4, 5).

 

 

References

1. Arnold, J., J. Mahamid, A. de Marco, J.-J. Fernandez, T. Laugks, et al. 2016. Site-Specific Cryo-focused Ion Beam Sample Preparation Guided by 3D Correlative Microscopy. Biophysical Journal. 110: 860.

2. Mahamid, J., S. Pfeffer, M. Schaffer, E. Villa, R. Danev, et al. 2016. Visualizing the molecular sociology at the HeLa cell nuclear periphery. Science. 351: 969.

3. K.V. Korotkov, and W. Bitter. 2014. Take five — Type VII secretion systems of Mycobacteria. Biochimica et Biophysica Acta (BBA) - Molecular Cell Research. 1843: 1707.

4. Smith, S. 2004. Individual RD1-region genes are required for export of ESAT-6/CFP-10 and for virulence of Mycobacterium tuberculosis. Molecular Microbiology. 51: 359.

5. van der Wel, N., D. Hava, D. Houben, D. Fluitsma, M. van Zon, et al. 2007. M. tuberculosis and M. leprae Translocate from the Phagolysosome to the Cytosol in Myeloid Cells. Cell. 129: 1287.

Kristof NOTELAERS (Maastricht, The Netherlands), Casper BERGER, Nino IAKOBACHVILI, Delei CHEN, Axel SIROY, Hirotoshi FURUSHO, Raimond RAVELLI, Carmen LÓPEZ-IGLESIAS, Peter PETERS
12:15 - 12:30 #6998 - IM10-OP177 Imaging of Vitrified Biological Specimens by Confocal Cryo Fluorescence Microscopy and Cryo FIB/SEM Tomography.
Imaging of Vitrified Biological Specimens by Confocal Cryo Fluorescence Microscopy and Cryo FIB/SEM Tomography.

The investigation of vitrified biological specimens (i.e. samples that are plunge or high pressure frozen) enables the visualization of cellular ultrastructure in a near native fully hydrated state, unadulterated by harmful prepara­tion methods. Here, we focus on two recent cryo imaging modalities and discuss their impact on cryo correlative workflows. First, we present confocal cryo fluorescence microscopy, utilizing a novel confocal detector scheme with improved signal to noise ratio (SNR) and resolution. Second, we show volume imaging of multicellular specimens by cryo focused ion beam scanning electron microscopy (FIB/SEM).

Confocal laser scanning microscopes (LSM) are renowned for their optical sectioning capability, a feature enabled by utilizing a pinhole that rejects out of focus light. Closing the pinhole improves lateral resolution, but also causes less light to reach the detector leading to reduced signal to noise ratios. In cryo fluorescence microscopy, the situation is aggravated by the fact that current­ly no immersion optics are available and consequently only numerical apertures below NA 1 are possible. Here we combined Airyscan, a novel detector module (available for ZEISS LSM 780, 800 and 880) together with a cryo correlative stage (Linkam CMS 196) for cryo fluorescent imaging of vitrified cells prepared on electron microscopy grids (Figure 1a). The Airyscan detection module allows the spatially resolved detection of fluorescence light otherwise rejected by the pinhole in a standard confocal system. We show that under cryo conditions even without immersion optics a significant increase in resolution and SNR can be obtained with Airyscan compared to standard confocal images (Figure 1 b&c).

In FIB/SEM tomography three-dimensional volumetric data from biological specimens is obtained by sequentially removing material with the ion beam and imaging the milled block faces by scanning with the electron beam. Only recently, it has been shown that this imaging method can be applied also to frozen hydrated specimens [1]. Cryo FIB/SEM tomography allows the mapping of large multicellular specimens in the near native state and is particularly suited to analyze samples that require remaining hydrated. We demonstrate this approach by imaging the cryo immobilized specimens (e.g. mouse optic nerve) (Figure 2).

Both methods by themselves promise significant advantages for biomedical research by being able to investigate biological specimens in the near native fully hydrated state. Yet correlating both imaging modalities, LSM and FIB/SEM of vitrified samples, has the potential to provide even deeper insights into biological context. In contrast to correlative workflows using resin embedded samples, cryo imaging workflows do not have to find the optimal balance between preserving cellular ultrastructure and maintaining the functional integrity of fluorophores. Moreover, major mechanisms leading to irreversible bleaching of fluorescent molecules are suppressed at cryo temperatures [2]. The correlation between cryo light and electron microscopy data will greatly benefit from an increase in resolution in fluorescence imaging. Cryo Airyscan is a first step in that direction and can deliver three-dimensional optical sectioning data that can be used to reliably target cellular structures in a FIB/SEM microscope, before the structural context is explored by cryo FIB/SEM tomography.

References:

(1) Schertel A et al., “Cryo FIB SEM: volume imaging of cellular ultrastructure in native frozen specimens.” J Struct Biol. 2013 Nov; 184(2):355 60. doi: 10.1016/j.jsb.2013.09.024.
(2) Kaufmann R, Hagen C, Grunewald K. Fluorescence cryo microscopy: current challenges and prospects. Current opinion in chemical biology. 2014; 20:86 91. DOI: 10.1016/j.cbpa.2014.05.007

Andreas SCHERTEL (Oberkochen, Germany), Robert KIRMSE, Eric HUMMEL, Volker DÖRING, Michael SCHWERTNER, Ralf WOLLESCHENSKY, Wiebke MÖBIUS
Salon Tête d'Or

Wednesday 31 August

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LS9-I
10:15 - 12:30

LS9: Societal challenges and environment
SLOT I

Chairmen: Kesara ANAMTHAWAT-JONSSON (Reykjavik, Iceland), Béatrice SATIAT-JEUNEMAITRE (GIF SUR YVETTE CEDEX, France)
10:15 - 10:45 #8359 - LS09-S28 imaging via FIB-SEM tomography at nanoscale for tissue engineering applications.
imaging via FIB-SEM tomography at nanoscale for tissue engineering applications.

The use of electrospun nanofibers for guided bone regeneration or bone scaffolds are becoming increasingly important in tissue engineering for next-generation healthcare. However, understanding the interaction between the nanofiber scaffolds that organize into complex 3D organizations and cells for optimized growth is a recurrent problem yet to be overcome.

In this study we exploit high resolution 3D imaging using scanning electron microscopy (SEM) and focussed ion beam (FIB) microscopy to evaluate cryo-prepared electrospun nanofiber scaffolds. Specifically, a biodegradable electrospun nanofiber membrane fabricated from poly(D,L-lactide-co-glycolide) (PLGA) is used for guided bone regeneration for orthopaedic applications. The electrospun scaffolds are produced with randomly oriented and aligned nanofibers, creating a range of void sizes, which are crucial for controlling cells penetration into electrospun scaffolds. The secondary electron imaging in Figure 1 and 2 demonstrates osteoblasts spreading over the random and aligned nanofiber mat surface respectively and within the fibrous membrane after 4 days in culture. Osteoblasts tend to growth both along the principal fiber axis and migrate in all directions towards neighboring fibrous filopodia, localized at the edges of osteoblasts, help the sheets of cells to align in the nanofiber direction and participate in cell-cell adhesion. It is presumed that these filopodia–like–protrusions are responsible for osteoblast elongation and migration into the 3D network of the electrospun nanofiber mat.

The interaction between osteoblasts and osteoblast-derived mineralized nodule formation on the nanofiber membrane is visualized using 3D FIB-SEM imaging based on a ‘slice-and-view’ approach. The 3D imaging highlights a coherent interface at small length scales between osteoblasts and nanofiber surfaces through connections between osteoblast, their filopodia and the nanofibers that promote cell growth. These imaging results are supported by biochemical approaches that indicate the formation of proteins responsible for focal adhesion in osteoblasts.1 The presented 3D tomography therefore presents a new approach in high resolution visualization the cell growth on electrospun nanofibers, and potentially other biomaterials, that will develop and design new biomaterials for a range of clinically important applications including orthopaedics of this work.

Acknowledgments

The study was conducted within the funding from SONATA 8 project granted by National Science Centre in Poland, No 2014/15/D/ST6/02598 

References

1.   U. Stachewicz, T. Qiao, S.C.F. Rawlinson, F.V. Almeida, W.-Q Li, M. Cattell, A.H. Barber,  "3D Imaging of Cell Interactions with Electrospun PLGA Nanofiber Membranes for Bone Regeneration",  Acta Biomater, 27 (2015), 88-100.

Stachewicz URSZULA (Krakow, Poland), Piotr SZEWCZYK, Adam KRUK, Asa BARBER, Aleksandra CZYRSKA-FILEMONOWICZ
Invited
10:45 - 11:00 #6378 - LS09-OP035 Life cycle of carbon nanotubes in the body.
Life cycle of carbon nanotubes in the body.

The human body is increasingly exposed to nanoscale materials that are widely developed for industrial use or for biomedical applications. Moreover, these engineered nano-objects are not the only worrying source of exposure for humans, since anthropogenic nanomaterials released in the environments also constitute a growing threat to public health. Therefore, addressing the risks associated to nanomaterials is a complex and sensitive societal issue which requires identifying all the nano-objects to which humans are exposed and understanding their lifecycle in the organism.  Here, we reveal for the first time an entry pathway for multi-wall carbon nanotubes (MWCNTs) found in air pollution to the airways of Parisian children [1]. Given the key role of macrophages in the process of foreign substances, we have also exploited multidisciplinary know-how in materials and life sciences to unravel the way taken by these cells to degrade MWCNTs [2].

 

We used high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy to show the presence of MWCNTs in broncho-alveolar lavage-fluids (fig 1a) and inside lung cells (fig 1b) of asthmatic children. These CNTs are present in all examined samples (n = 64) and they are similar to those found in dusts and vehicle exhausts (fig 1c) collected in Paris and other parts of the world (USA, India…). These results strongly suggest that humans are routinely exposed to MWCNTs and demonstrate that advanced atomic-scale characterizations are essential to identify ultrafine particles in the environment or in the organism [1].

 

Like most nanomaterials, the “journey” of MWCNTs in the organism mainly ends in macrophages of lung after inhalation, or of liver and spleen after intravenous injection. In order to consider the reciprocity of the interactions between MWCNTs and macrophages, we have simultaneously examined the influences of intracellular environment on the atomic structure of nanomaterials and the biologic response of the cells to MWCNTs. Gene and protein expression profiles of macrophages exposed to MWCNTs allowed identifying the formation of reactive oxygen species (ROS) as a non-ambiguous causal process for triggering intracellular degradation of MWCNTs. Consistent with cellular approach, we could monitor with unprecedented nanoscale resolution the ROS-induced damages in MWCNTs using transmission electron microscopy in liquid (fig 2). Remarkably, we demonstrate that this in situ imaging of MWCNT degradation recapitulates the long term ROS-induced aging of MWCNT in macrophages and reveals the structural mechanisms of MWCNT transformation over time. More generally, such dynamical observations of nanomaterials under oxidative stress is a step forward for studying their behavior and reactivity in biological environment at the relevant scale. These mechanistic insights on the biological responses to nanomaterial exposure and the resulting nanomaterial transformations are of primary importance for both material scientists interested in optimizing the reactivity of nanostructures in biological environment and biologists anxious to evaluate the effects and potential risk of nanomaterials for the organism [2].

 

References:

[1] Jelena Kolosnjaj-Tabi et al, Anthropogenic Carbon Nanotubes Found in the Airways of Parisian Children, EBioMedicine 2 (2015), p. 3988.

[2] Dan Elgrabli et al. Carbon Nanotube Degradation in Macrophages: Live Nanoscale Monitoring and Understanding of Biological Pathway, ACS Nano 9 (2015), p. 10113.

Damien ALLOYEAU (LMPQ, Paris Diderot), Walid DARCHRAOUI, Dan ELGRABLI, Jelena KOLOSNJAJ-TABI2, Alberto BIANCO, Dominique BEGIN, Sylvie BEGIN, Christian RICOLLEAU, Fathi MOUSSA, Florence GAZEAU
11:00 - 11:15 #5826 - LS09-OP033 Application of FIB-SEM tomography for analyses of ceramic coatings deposited on titanium alloy and material-cell interface.
Application of FIB-SEM tomography for analyses of ceramic coatings deposited on titanium alloy and material-cell interface.

Biomedical engineering is a fast developing field of medicine, employing biomaterials to replace or augment damaged and diseased tissues. Growing population with osteoarthritis problems and extending life span contributes to increased demand for effective solutions in arthroplasty. Titanium alloys are the materials of the choice in case of elements of artificial joints. They are implanted directly into the bones. Long lasting performance of those implants depends on their integration with surrounding tissues. Surface modification of titanium alloys is performed to improve osseointegration. It can be achieved by deposition of porous, ceramic coatings. Such a structure facilitates bone cells ingrowth and stable anchorage of the implant in the bone. Understanding of the reactions at the interface between materials and cells will help to optimize the design of new implants with improved performance.

In current work, a titanium alloy (Ti6Al7Nb) was used as a substrate in micro-arc oxidation (MAO) process. As the effect of this electrochemical reaction the protective and bioactive coating composed of titanium dioxides, calcium titanate and calcium phosphates was formed on the metallic sample. The obtained complex ceramic, porous layer was investigated by SEM and SEM-EDS (Merlin Gemini II, Zeiss) and FIB-SEM tomography (NEON CrossBeam 40EsB, Zeiss) to analyze microstructure, chemical composition and internal structure, including pores network.

In vitro cell culture is a standard method to evaluate biocompatibility of new or modified materials. Thus, osteoblast-like cells line MG-63 were cultured for three days at the surface of coated titanium alloy samples. Subsequently cells attached to the ceramic coating were fixed, dehydrated and gold sputtered for electron microscopy investigations. Cells shape and distribution at rough and porous structure of the coating containing hydroxyapatite (HA) in the outer layer was investigated based on plain view SEM images (Fig. 1a). Area prepared for tomography by FIB milling of surrounding material is presented at Fig. 1b (marked by red square). FIB-SEM tomography enables to see both material surface and cells at nanometer scale (Fig. 1c). The 3D reconstruction allows for visualization and investigations of the interface between coated material and cell in the analyzed sample volume.

Acknowledgement

The study was conducted within the OPTYMED project granted by National Science Centre of Poland (no 2013/08/M/ST8/00332).

Joanna KARBOWNICZEK (Krakow, Poland), Adam GRUSZCZYŃSKI, Adam KRUK, Aleksandra CZYRSKA-FILEMONOWICZ
11:15 - 11:45 #8435 - LS09-S29 Metal Contaminated Environments And Interactions With Biota.
Metal Contaminated Environments And Interactions With Biota.

Contamination of the environment with metals has dramatically increased during the last century due to intense anthropogenic activities and increasing metal need. Living organisms present in soils and aquatic systems are affected by these metals, and some of them are able to develop some specific strategies to cope with metal toxicity. To decipher the impact of metals on these organisms, and particularly their trafficking and storage, the determination of their localization at the tissue and cell level as well as their chemical forms (speciation) are crucial.

In this context, we combine various imaging techniques including synchrotron micro X-ray Fluorescence (µXRF), Transmission Electron Microscopy combined with Energy Dispersive X-ray analysis (TEM-EDX), and Nano- Secondary Ion Mass Spectrometry (nano-SIMS) to locate metals, and X-ray Absorption Spectroscopy with focused and non focused beam (µXAS and XAS) to clarify the nature of metal species.

This approach will be illustrated by two environmental studies. The first one is focused on the green micro-algae Chlamydomonas reinhardtii, an eukaryotic photosynthetic model for metal stress. Chlamydomonas reinhardtii is tolerant to cadmium but the mechanisms involved in this tolerance are misknown. We will show that intracellular thiol ligands are involved in Cd binding at the intracellular level, and that these sulfur ligands are not the only tolerance mechanism implemented by the cell. The second environmental study is related to mining soils contaminated with Cd and Zn. Among the existing soil treatments, phytoremediation has emerged as an alternative sustainable technique for two decades. We will show here how pioneer plants used in a phytostabilization process interact with metals in the soil.

This presentation illustrates the interest of combining imaging and spectroscopy techniques to get a global view of metals in complex environmental systems.

Marie-Pierre ISAURE (PAU CEDEX 9)
Invited
11:45 - 12:00 #6142 - LS09-OP034 Revealing the internal structure of microbial mats by FIB/SEM tomography.
Revealing the internal structure of microbial mats by FIB/SEM tomography.

Microbial mats are macroscopically visible communities of microorganisms, which play an important role for the interaction of microorganisms with their environment [1]. Important examples are mats dominated by Fe(II)-oxidizing  bacteria, which are typically found in wet, Fe(II)-rich environments such as lake sediments and mine tunnels [2,3].

The microscopic constituents of microbial mats can be analyzed using 2D microscopy approaches such as SEM. Other properties such as the microbial community and elemental composition are accessible using bulk analytical methods [e.g. 3]. However, little is known about the 3-dimensional structure of these mats.

Samples of two microbial mats from an abandoned mine tunnel were analyzed by FIB/SEM tomography using a Zeiss Auriga CrossBeam instrument. The samples were embedded in resin. Metal ions and minerals within the sample were responsible for contrast generation. The voxel size was 30 x 30 x 30 nm³. The resulting 3D reconstructions show the original arrangement of the microscopic constituents in great detail (Fig. 1,2). The two mats were dominated by different types of Fe(II)-oxidizing bacteria, which formed characteristic structures that were visible in the two respective volumes. A large part of both volumes appeared void.

The results of this analysis provide explanations for a series of macroscopically observed effects, such as the physical stability of the mats, high water content, efficient interaction with the feeding water, and the creation of a habitat for a variety of different microbial species. In addition, the results represent a valuable reference for the detection of microfossils in sedimentary rocks.

 

References

[1] WE Krumbein et al. (2003), in Fossil and recent biofilms. Springer Netherlands, pp. 1-27

[2] EJ Fleming et al. (2014), ISME J 8, pp. 804-815

[3] C Heim et al. (2015), Front Earth Sci 3

Fabian ZEITVOGEL (Tuebingen, Germany), Martin OBST, Birgit SCHROEPPEL, Claus J. BURKHARDT
12:00 - 12:15 #8754 - LS09-OP035b Environmental issues addressed by analytical TEM and X-rays absorption spectroscopies.
LS09-OP035b Environmental issues addressed by analytical TEM and X-rays absorption spectroscopies.

Fe-bearing minerals are widespread in earth surface environments, including soils, sediments and aquifers. Owing to their large specific area, these minerals can efficiently scavenge inorganic pollutants such as arsenic, nickel, etc. Metabolic activities of native microorganisms are capable of weathering either directly by enzymatic reduction, or indirectly through the reaction of biogenic hydrogen sulfides with Fe(III)-minerals. Metals/metalloids associated with such ferric solids phases can be released into solutions or be sequestrated into biogenic Fe(II)-containing minerals that result from direct and indirect bacterial transformation of Fe(III)-minerals. Here we investigate, using X-ray absorption spectroscopy (XAS) and high resolution transmission electron microscopy (HRTEM), the role (i) of Fe(II)-bearing minerals such magnetite and mackinawite nanoparticles to scavenge inorganic pollutants (arsenic or  nickel), and (ii) of aluminum-rich ferrihydrites in arsenic scavenging in river-bed sediments from a circumneutral river (pH 6−7) impacted by an arsenic-rich acid mine drainage (AMD). We therefore present recent investigations of natural and model systems that illustrate the relation between the crystal chemistry of metal/metalloids and their speciation and distribution at the Earth’s surface. Our findings show the importance of nanocrystalline Fe-minerals in delaying the long-term impact of inorganic pollutants natural environments.

Yuheng WANG, Areej ADRA, Maya IKOGOU, Georges ONA-NGUEMA, Farid JUILLOT, Nicolas MENGUY (PARIS), Guillaume MORIN
Salle Gratte Ciel 3
12:30

Wednesday 31 August

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SSW5
12:30 - 14:15

European and National Microscopy Networks
Lunch Workshop

12:30 - 12:35 Introduction. Thierry EPICIER (LYON, France)
12:35 - 12:50 Electron microscopy in INSTRUCT: the key role for a center on image processing. José L. CARRASCOSA (Madrid, Spain)
12:50 - 13:05 ESTEEM2: An European Integrated Infrastructure of Electron Microscopy facilities. Etienne SNOECK (Toulouse, France)
13:05 - 13:15 Atomic Resolution Cluster - a national electron microscopy infrastructure for Sweden. Crispin HETHERINGTON (Research Engineer) (Lund, Sweden)
13:15 - 13:25 Presentation of the French Microscopy Infrastructure “METSA”. Mathieu KOCIAK (Orsay, France)
13:25 - 13:35 The Spanish TEM network. César MAGÉN (Zaragoza, Spain)
13:35 - 13:45 France Bio Imaging. Jean SALAMERO (PARIS, France)
13:45 - 13:55 The French Infrastructure for Integrated Structural Biology (FRISBI). Bruno KLAHOLZ (Illkirch, France)
13:55 - 14:05 The mission for inter-disciplinarity at CNRS: microscopy networks. Catherine CLERC (Paris, France)
14:05 - 14:15 Wrap-up. Etienne SNOECK (Toulouse, France), José L. CARRASCOSA (Madrid, Spain), José Maria CARAZO (Spain)
Amphithéâtre
14:15

Wednesday 31 August

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IM8-I
14:15 - 16:30

IM8: Spectromicroscopies and analytical microscopy
SLOT I

Chairmen: Gerald KOTHLEITNER (Graz, Austria), Anders MEIBOM (Lausanne, Switzerland), Bénédicte WAROT-FONROSE (Toulouse, France)
14:15 - 14:45 #8326 - IM08-S53 Dynamics of magnetic domain walls and skyrmions studied by high resolution XMCD-PEEM microscopy.
Dynamics of magnetic domain walls and skyrmions studied by high resolution XMCD-PEEM microscopy.

Photoemission electron microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM) is a powerful synchrotron radiation technique that can be used to observe magnetic configurations of nano-objects with high spatial resolution. Combined with time resolution, using stroboscopic detection, the magnetisation dynamics of such objects can be studied with sub-nanosecond resolution.

We will describe the study of domain wall (DW) dynamics and magnetic skyrmions in multilayers of composition Pt/Co/MOx (M=Al, Mg), characterised by a large perpendicular magnetic anisotropy (PMA) and anti-symmetric exchange interaction (Dzyaloshinskii-Moriya interaction, DMI). Such interaction favours non-collinear magnetic textures such as chiral Néel walls and magnetic skyrmions.

In systems with large DMI, such as Pt/Co/AlOx with ultrathin Co, domain walls acquire a Néel chiral structure and move at large speeds when driven by current pulses. Most microscopic studies of domain wall dynamics have been performed using quasi-static measurements, where the DW position and structure are imaged before and after the excitation pulse. In order to better understand the details of the interaction between current and DW magnetisation, we have observed DWs during the application of the current pulses, by synchronising the current pulses with the x-ray pulses, in stroboscopic mode. The measurements reveal that the DWs move without inertia, showing a practically vanishing mass. The negligible inertial effects can be explained taking into account  the narrow domain wall width induced by the large PMA, the large damping parameter and the stabilising effect of the longitudinal field associated to the Dzyaloshinskii-Moriya interaction.

Magnetic skyrmions are nanometer scale whirling spin configurations, predicted in the 80's but only recently observed with high resolution magnetic microscopies. Their small size, topological protection and the fact that they can be moved by very small current densities call for applications to novel memory and logic devices where skyrmions are the information carriers. Here we report on the experimental observation of magnetic skyrmions in Pt/Co/MgO sputtered ultrathin magnetic nanostructures, stable at room temperature without applied magnetic field. XMCD-PEEM measurements allowed us to observe such skyrmions and to demonstrate their chiral Néel structure. The skyrmion sizes are typically of the order of 120 nm. Our experimental observations are well reproduced by micromagnetic simulations and numerical modelling. This allows the identification of the physical mechanisms governing the size and stability of the skyrmions, which are keys for the design of devices based on their manipulation.

REFERENCES

J. Vogel at al. Phys. Rev. Lett. 108, 247202 (2012)

O.Boulle et al., Nature Nanotechn. 11, 449 (2016)

Stefania PIZZINI (GRENOBLE CEDEX 9), Jan VOGEL, Nicolas ROUGEMAILLE, Fausto SIROTTI, Julio CESAR, Dayane CHAVES, Onur MENTES, Michael FOESTER, Lucia ABALLE, Andrea LOCATELLI, Olivier BOULLE
Invited
14:45 - 15:00 #5590 - IM08-OP139 Ten years of EMCD: what has been achieved.
Ten years of EMCD: what has been achieved.

Energy loss magnetic chiral dichroism (EMCD), established in 2006 [1] celebrates its 10th anniversary. EMCD is the TEM equivalent of the X-ray magnetic circular dichroism (XMCD) technique routinely applied on synchrotron beam lines for the study of magnetic moments. The EMCD signal is detected as an asymmetry in the energy filtered diffraction pattern, or alternatively as a slight difference in the fine structure of energy loss spectra for particular momentum transfers in the inelastic interaction (Fig. 1). The extremely high spatial resolution of modern TEMs makes EMCD interesting for spintronic and micromagnetic applications. In  the last decade, the technique has evolved into a reliable tool demonstrating nm-resolution, site selectivity and separation of spin and orbital moments via sum rules.

One of the consequences of EMCD is that the outgoing inelastically scattered probe electrons have topological charge. Structurally, they are identical with vortex electrons that can be routinely created in the electron microscope [2]. Such vortices are characterized by a spiraling wavefront and a phase singularity at the center, similar to optical vortices that were first described by Nye & Berry [3]. Owing to their short wavelength, these matter waves can be focused to atomic size. Another novel aspect is their magnetic moment, quantized in multiples of the Bohr magneton, independent of the electron spin. These features make electron vortices extremely attractive as a nanoscale probe for magnetic materials.

The discovery of vortex electron beams has spurred efforts to use them for EMCD because of their intrinsic chirality. It became soon clear that atom-sized vortices are needed to achieve this goal [4], as shown in Fig. 2. At the time of writing, the closest successful approach to such beams is the shaping of the incident wave front with a Cs corrector such that it matches the point group symmetry of the selected atomic column, revealing  EMCD signals with atomic resolution [5]. A promising alternative is the use of holographic vortex filters in the outgoing beam to detect spin polarized transitions [6], as sketched in Fig. 3. Other than in the standard EMCD geometry, this approach does not require a precise alignment of the crystal, and would thus allow the study of nanocrystalline and amorphous materials.

 

Acknowledgements: The financial support of the Austrian Science Fund (I543-N20, J3732-N27) and of the European research council, project ERC-StG-306447 is gratefully acknowledged.

 

[1] P. Schattschneider et al., Nature 441 (2006)  486

[2] J. Verbeeck et al.,Nature 467 (2010) 301

[3] J. F. Nye and M. V. Berry, Proc.Roy. Soc. A 336 (1974) 165

[4] P. Schattschneider et al., Ultramicroscopy 136 (2014) 81

[5] J. Rusz et al., Microscopy and Microanalysis 21 (2015) 499

[6] T. Schachinger et al., submitted; also this conference

Peter SCHATTSCHNEIDER (Wien, Austria), Thomas SCHACHINGER, Stefan LÖFFLER
15:00 - 15:15 #6365 - IM08-OP154 Nanoscale maps of magnetic behavior using STEM-EMCD.
Nanoscale maps of magnetic behavior using STEM-EMCD.

Key to the advancement of magnetic materials is the exploitation of emergent magnetic behavior arising from nanoscale confinement.   A comprehensive understanding of this behavior requires a technique capable of resolving it and mapping its relationship to parallel nanoscale property domains such as local structural imperfections, chemical environment, and interfacial proximity.  The relatively young electron absorption spectroscopy technique known as Electron Magnetic Circular Dichroism (EMCD) [1] promises to meet these criteria, and it has already demonstrated qualitative [2] and quantitative [3, 4] results for parallel electron probe illumination on sample regions in the sub 100 nanometer range.  While this already represents a spatial resolution superior to what can be achieved with x-rays [5], the use of parallel electron illumination means that high spatial resolution can only be achieved through the use of energy filtered diffraction patterns [3, 6] or energy filtered TEM imaging techniques [7].

In this work, we describe our progress in the development of an experimental methodology utilizing convergent electron probes for the purposes of acquiring EMCD datasets.  We name this method STEM-EMCD (see figure 1) and argue that it has two significant advantages over parallel beam illumination schemes.  First, the use of a convergent probe allows for the acquisition of arbitrarily large datasets, extending the effective acquisition time on a region of interest well beyond what is feasible for a single-shot experiment using parallel illumination.  This substantially improves the signal to noise ratio of EMCD spectral pairs, as shown in figure 1, and can be employed in such a way to mitigate significant beam damage.  Second, the diameter of the electron probe can be reduced to one nanometer or less.  If appropriate multivariate statistical methods are used to exploit spectral redundancy in the datacubes, it becomes possible to accurately approximate the raw data using a significantly reduced parameter space, dramatically suppressing the influence of statistical noise.  We demonstrate that this allows for sum rules to be applied on individual spectral pairs, enabling real-space maps of magnetic transitions to be generated with a spatial resolution approaching that of the beam diameter, as shown in figure 2.  We conclude with a discussion of some of the challenges still faced by the STEM-EMCD method as well as its prospects for directly addressing some of the most elusive questions in contemporary research in nanomagnetism.

[1]          Schattschneider, P. et al.: Detection of magnetic circular dichroism using a transmission electron microscope. Nature 441 (2006) 486.

[2]          Schattschneider, P. et al.: Magnetic circular dichroism in EELS: Towards 10 nm resolution. Ultramicroscopy 108 (2008) 433.

[3]          Lidbaum, H. et al.: Quantitative Magnetic Information from Reciprocal Space Maps in Transmission Electron Microscopy. Phys. Rev. Lett. 102 (2009) 037201.

[4]          Wang, Z.Q. et al.: Quantitative experimental determination of site-specific magnetic structures by transmitted electrons. Nat. Commun. 4 (2013) 1395.

[5]          Fischer, P.: Frontiers in imaging magnetism with polarized x-rays. Condens. Matter Phys. 2 (2015) 82.

[6]          Loukya, B. et al.: Electron magnetic chiral dichroism in CrO2 thin films using monochromatic probe illumination in a transmission electron microscope. J. Magn. Magn. Mater. 324 (2012) 3754.

[7]          Lidbaum, H. et al.: Reciprocal and real space maps for EMCD experiments. Ultramicroscopy 110 (2010) 1380.

[8]          Thersleff, T. et al.: Quantitative analysis of magnetic spin and orbital moments from an oxidized iron (1 1 0) surface using electron magnetic circular dichroism. Sci. Rep. 5 (2015) 13012

Thomas THERSLEFF (Uppsala, Sweden), Shunsuke MUTO, Jakob SPIEGELBERG, Ján RUSZ, Klaus LEIFER
15:15 - 15:30 #6839 - IM08-OP161 Surface oxidation issues in CO oxidation bimetallic surfaces studied by NAP-XPS.
Surface oxidation issues in CO oxidation bimetallic surfaces studied by NAP-XPS.

Near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) [1] is a modification of XPS so that surfaces can be studied in presence of gases and not only in vacuum. This is necessary when you want to access chemical and electronic properties of catalytic surfaces in realistic reaction conditions (variable P, T). We have focused on the study of bimetallic catalytic surfaces and their chemical/electronic surface properties during the oxidation of carbon monoxide. Indeed, the low temperature catalytic oxidation of carbon monoxide to carbon dioxide is an important reaction used in several areas and in particular for removing the CO traces from the H2 combustible in fuel cells. Pt and Au are well known catalysts for the CO oxidation reaction, however on both metals the oxygen dissociation remains the rate determining step. Associating Pt or Au with a second metal, that facilitates the oxygen activation while keeping free sites for CO adsorption on the surface, appears as an effective way to improve the catalytic performances of these two metals. The surfaces Pd70Au30(110) and Pt3Sn(111) have been chosen on the basis of the performance of real catalysts (bimetallic nanoparticles supported on oxides).

The NAP-XPS experiments were performed at ALS beamlines 9.3.2 & 11.0.2 (Berkeley, USA) and, more recently, at SOLEIL beamline TEMPO (St Aubin, France). NAP-XPS allows a fine characterization of electronic properties of catalytic surfaces including reactants, products and inert spectator species. In addition, NAP-XPS can also detect the gas phase species near the surface and can therefore monitor the reaction rates, at least qualitatively. The inherent variable photon energy of synchrotron sources enables depth profiling measurements which can be used to explore the segregation of species into the subsurface region under gas environments.

In the case of Pd70Au30(110) we have a very rich Au-surface (~ 85% of Au as determined by LEISS) due to Au segregation to the surface at thermodynamic equilibrium. By environmental STM [2] we could notice that the surface reorganizes at atomic level and eventually roughens just after adsorption of the reactive gases both under oxygen and CO. This is followed by a reversal of segregation where Pd migrates to the surface as it is determined by varying the photon kinetic energy in NAP-XPS. For Pt3Sn(111) we studied the two terminations (2x2) and (√3x√3)R30° obtained at different annealing temperatures. For both surfaces we have identified the presence of bridge and top sites during adsorption of CO. Oxygen adsorption studied by PM-IRRAS shows a higher interaction of oxygen with Pt3Sn(111)-(√3×√3)R30° than with Pt3Sn(111)-(2x2). By NAP-XPS we observe, in both surfaces, the partial segregation of Sn to the surface with increasing temperature in the presence of oxygen and under reaction conditions [3]. Finally we studied the evolution of the surfaces by NAP-XPSunder reaction conditions. A general result was observed for the studied bimetallic surfaces: the activity of the surface is higher in presence of chemisorbed oxygen (that we can compare to a“loose”oxide) but it rapidly decreaseswhen stoichiometric oxides are formed on the surface at higher temperatures (Figures 1 & 2).

References
[1] D.F. Ogletree et al., Rev. Sci. Instr. 73 (2002) 3872; M. Salmeron, R. Schlögl, Surf. Sci. Rep. 63 (2008) 169

[2] F.J. Cadete Santos Aires, C. Deranlot, Proc. EUREM12, Vol.III, P. Ciampor, L. Frank, P. Tománek, R. Kolarik, Brno 2000, I263; Y. Jugnet et al., Surf. Sci. 521 (2002) L639; M.A. Languille et al., Catal. Today 260 (2016) 39.

[3] Y. Jugnet et al., J. Phys. Chem. Lett. 12 (2012) 3707.

Eric EHRET, Bongjin Simon MUN, Marie-Angélique LANGUILLE, Jean-Jacques GALLET, Fabrice BOURNEL, Céline DUPONT, David LOFFREDA, Francisco José CADETE SANTOS AIRES (VILLEURBANNE CEDEX)
15:30 - 15:45 #4647 - IM08-OP138 Multidimensional Analysis of Local Compositional and Valence Fluctuations in the Model Complex Oxide La2MnNiO6.
Multidimensional Analysis of Local Compositional and Valence Fluctuations in the Model Complex Oxide La2MnNiO6.

Propelled by rapid advances in synthesis, characterization, and computational modeling, materials science is fast progressing toward a “materials-by-design” paradigm. While we can envision a very large number of materials combinations, we are unable to synthesize them in practice because existing characterization and modeling approaches fail to capture the inherent complexities of such systems. This shortcoming is amplified by the fundamental disconnect between highly local and volume-averaged structure-property models resulting from electron microscopy and X-ray diffraction investigations, respectively. Here we show how complementary analysis techniques can reveal previously overlooked local compositional and valence fluctuations around secondary phases in an oxide thin film, yielding powerful new insight into low-pressure thin film synthesis. We explore the model complex oxide, La2MnNiO6 (LMNO), which possesses a hierarchy of structural, chemical, and magnetic ordering across multiple length scales. This material shows great promise for next-generation spintronics and thermoelectrics, but its implementation is hindered by a poor understanding of the underlying structure that governs its macroscale magnetic performance. Using aberration-corrected scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (STEM-EDS), we confirm the onset of cation ordering upon annealing; however, three-dimensional composition mapping using atom probe tomography (APT) reveals a fine distribution of NiO secondary phases, as shown in Figure 1. Electron energy loss spectroscopy (STEM-EELS) mapping of the chemical environment surrounding these particles shows significant fine structure changes, indicating a reduction in Mn valence, as shown in Figure 2. We consider these results in light of ab initio calculations, which show that the NiO phase and a transition region with reduced Mn valence is an inevitable result of low-oxygen pressure growth processes that are commonly used in fabrication of complex oxide heterojunctions. We argue that kinetic limitations on the reincorporation of NiO nuclei “locks” them into the film structure during synthesis; the resulting nanoscale network disrupts the long-range ferromagnetic ordering of the matrix, degrading macroscale magnetic properties. This array of experimental and theoretical techniques allows us to better understand the relationship between structure and magnetic properties, illustrating the need for a correlative, multidimensional approach to thin film characterization.

Steven SPURGEON, Yingge DU, Timothy DROUBAY, Arun DEVARAJ, Xiahan SANG, Paolo LONGO (Pleasanton, USA), Pengfei YAN, Paul KOTULA, Vaithiyalingam SHUTTHANANDAN, Mark BOWDEN, James LEBEAU, Chongmin WANG, Peter SUSHKO, Scott CHAMBERS
15:45 - 16:00 #5996 - IM08-OP148 Atomic-scale investigation of interface phenomena in two-dimensionally Sr-doped La2CuO4 and La2CuO4/ La2-xSrxNiO4 superlattices.
Atomic-scale investigation of interface phenomena in two-dimensionally Sr-doped La2CuO4 and La2CuO4/ La2-xSrxNiO4 superlattices.

Atomic-scale investigation of interface phenomena in two-dimensionally Sr-doped La2CuO4 and La2CuO4/ La2-xSrxNiO4 superlattices

 

Y. Wang, Y. E. Suyolcu, W. Sigle, U. Salzberger, F. Baiutti, G. Gregori, G. Cristiani, G. Logvenov, J. Maier, and P.A. van Aken

 

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

 

Physics phenomena at interfaces of complex oxide heterostructures have stimulated intense research activity due to the occurrence of a broad range of electric and magnetic functionalities that do not pertain to the constituting single phases alone. Interface effects have been proven to be a powerful tool for improving or even inducing novel functionalities [1, 2]. In the case of interface superconductivity, the interatomic structure relaxation and charge transfer play a key role [3].

 

In this work, we combine atomic-resolved quantitative STEM imaging with analytical STEM-EELS/EDX analysis to investigate the interface effects in La2CuO4 (LCO)-based hetero-structures, i.e. superlattices of Sr two-dimensionally (2D) doped LCO, and LCO/La2-xSrxNiO4 (LSNO) hetero-structures, both exhibiting Tc up to 40 K, despite its non-superconducting constituents. HAADF images of the structure are displayed in Fig.1 confirming high-quality epitaxy. A detailed study of the cation distribution and concentration at the doped interfaces was performed by EDXS and EELS analyses. In the case of Sr-2D doped LCO superlattices, the analysis shows that Sr cations undergo an asymmetric redistribution, as a result of the MBE growth process. Oxygen K-edge fine structure analysis reveals that the hole concentration profile on the downward side of the nominal SrO layer is clearly decoupled from the Sr-profile indicating an accumulation of positive charges compensating the negatively charged SrO planes (Fig.2).

 

These results were supplemented by quantitative analysis of atomic-resolved high-angle annular dark-field (HAADF) and annular bright-field (ABF) images, which were simultaneously acquired. From these images the local lattice and copper-apical-oxygen distortions at the interfaces were evaluated. The relation of these results with the observed Tc will be discussed.

 

References:

 

[1] A.Gozar et al., Nature 455 (2008), p.782.

[2] F.Baiutti et al., Nat. Commun. 6 (2015), p. 8586.

[3] Y.Wang et al., ACS Appl. Mater. Interfaces (2016) DOI :10.1021/acsami.5b12813

[4] 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, Y. Eren SUYOLCU, Wilfried SIGLE (Stuttgart, Germany), Ute SALZBERGER, Federico BAIUTTI, Giuliano GREGORI, Georg CRISTIANI, Gennady LOGVENOV, Joachim MAIER, Peter VAN AKEN
16:00 - 16:15 #6003 - IM08-OP149 A Correlative Methodology based on SIMS for Advanced Materials Characterization.
A Correlative Methodology based on SIMS for Advanced Materials Characterization.

Development and innovations of modern high-performance materials rely crucially on the repertoire of characterization methods available. While individual techniques provide information on a specific aspect of the investigated material, a comprehensive understanding requires a correlative approach in which complementary information are needed from exactly the same Region of Interest (RoI). Therefore, correlative methodologies combining several techniques are indispensable in a wide range of scientific disciplines including materials science [1].

In contrast to the traditional EDX and EELS spectroscopies, SIMS is well-known for high-sensitivity (ppm), high dynamic range and more interestingly, isotope analysis for all the elements of the periodic table. This presentation will highlight the benefits of correlating SIMS imaging with complementary analysis using Electron Back Scattered Diffraction (EBSD) and Atomic Force Microscopy (AFM). As the SIMS images do not reveal crystallographic orientations of the grains in the microstructure, studies that aim to correlate structural defects such as twinning and grain boundary orientation in a multiphase material require that the strengths of SIMS are coupled with that of EBSD to probe the local crystallographic characteristics. As a first step, we correlate the SIMS with EBSD imaging to complement specific chemical characteristics seen in SIMS with the corresponding crystallographic characteristic captured in EBSD.

To further characterize the material and for correct interpretation, the exact topography of the RoI is required. We do topographic analysis using an AFM on the exact location where SIMS and EBSD analysis are carried out. In this way, a three-way correlative methodology is developed. As each of these analyses is done on separate instruments, specific methodological strategies are employed to overcome the challenges.

To illustrate this methodology, we selected the topic of hydrogen embrittlement in a TWinning Induced Plasticity (TWIP) steel [2]. As conventional techniques such as EDX are not capable of mapping hydrogen distribution, the SIMS approach brings a particularly strong advantage to this study. Prior to the SIMS analysis, the steel samples were electrochemically charged with deuterium to distinguish it from hydrogen naturally present in the sample. The SIMS images of H- and D- distributions are obtained from a Cameca NanoSIMS50 with a Cs+ primary ion beam. Then, EBSD and AFM analyses are carried out on the exact location to correlate SIMS chemical characteristics with crystallographic defects such as twins, grain boundaries and sample topography.

In this presentation, we will introduce and demonstrate this correlative paradigm for materials characterization. We will also present the scientific and technical challenges and discuss the strategies to go beyond the current state-of-the-art.

References:

T. Wirtz et al, Nanotechnology, Vol. 26, 434001, 2015.

L. Mosecker et al, Mat. Sci. Engr. A, 642, p71-85, 2015.

Acknowledgments: This work was co-funded by the Luxembourg National Research Fund (FNR) by the grant C13/MS/5951975.

Santhana ESWARA (Esch-sur-Alzette, Luxembourg), Rong HU, Lluís YEDRA, Jean-Nicolas AUDINOT, Alexander SCHWEDT, Cem TASAN, Joachim MAYER, Dierk RAABE, Tom WIRTZ
16:15 - 16:30 #6458 - IM08-OP158 Absorption-induced enhancement of X-ray contrast by soft X-ray emissions.
Absorption-induced enhancement of X-ray contrast by soft X-ray emissions.

Contrast in atomically-resolved EDX elemental mapping in real space or in reciprocal space channelling patterns, derived from characteristic X-ray emissions excited by a given probe, is generated by the variation in emission rate as the incident wave function is scanned over the target atoms in a crystal. This is achieved either by raster scanning a focused coherent probe in real space at a fixed orientation (conventional EDX mapping), or by a systematic scan in angle of a collimated beam, the entire EDX spectrum being collected for each pixel (ALCHEMI). Whilst the signal is essentially governed by the probability density of the probe wave function on each atom site, delocalization of the primary ionization event may lead to a reduction in contrast since the electron wave function is effectively sampled over a finite region.  The total ionization potential may be described in real space as a Lorentzian function of half-width b, and the event becomes more localized with an increase in X-ray energy. Thus coherent contrast provides a map in real or reciprocal space of the electron beam interaction with the ionization potential of a specific atomic species.

However incoherent scattering by an absorptive potential leads to a progressive increase in a separate incoherent background component, this being associated with an EDX spectrum which is representative of the overall specimen composition. Generally speaking, coherent contrast is generated within the top 10 – 30 nm, and this initial contrast is progressively undermined by the incoherent background contribution in the form of an absorptive potential in atomic resolved maps or thermal diffuse scattering Kikuchi band contrast in channelling patterns [1].

The relative detected proportion of coherent compared with incoherent signal is altered by X-ray absorption within the specimen. It is envisaged that strong absorption of soft X-rays (described by a small mean free path λ) will enhance coherent contrast in both lattice image and channelling patterns compared to that derived from more energetic X-rays. High energy X-rays should have better contrast in the coherent signal due to increased localization, but this begins to be dominated by an incoherent signal that builds up with thickness. Softer X-rays may have somewhat diminished coherent contrast due to greater intrinsic delocalization, but this is offset by a more strongly attenuated incoherent signal with increasing thickness.

X-ray channelling patterns near the orientation were obtained from GaAs using 300 keV electrons together with a custom-built acquisition script run on an FEI Titan TEM. Fig. 1 shows PCA noise-reduced patterns compared with simulations in which both delocalization and absorption are taken into account. Close inspection reveals greater overall dynamic contrast occurs for softer X-rays. Experimental K/L ratio maps are shown in Fig. 2 (tilt 26o and take off angle 21o), being consistent with the calculation where the central As K/L contrast ratio starts to diminish with increasing thickness due to X-ray absorption (λK = 16 μm, λL = 0.15 μm, (bK= 0.024 Å, bL= 0.13 Å) .  A similar but smaller effect occurs for the mapped Ga K/L ratio (λK = 42 μm, λL = 1.2 μm,  bK= 0.027 Å, bL= 0.15 Å) since Ga L-shell X-rays are less strongly absorbed than the As L. Without X-ray absorption, a central enhancement remains for all thicknesses, and the harder X-rays retain more contrast.  Fig. 3 shows the separate coherent and incoherent components together with the total contribution. Note the incoherent contrast is similar for all excitations.

In conclusion, experimental observations are consistent with a model that predicts an increase in contrast from soft X-rays compared with more energetic X-rays, thus demonstrating that X-ray absorption may play a greater role than delocalization in determining overall contrast. It is anticipated that X-ray absorption, enhanced by grazing angle specimen-detector geometry, may be useful in enhancing the coherent/incoherent scattering for soft X-rays. Calculations have shown analogous effects occur in atomically-resolved X-ray STEM imaging [1].

[1] C.J. Rossouw, ‘Channelling and atomic resolution STEM using X-ray emissions with absorption’, Microsc. Microanal. 21 (2015), 1077 - 1078.

Chris ROSSOUW, David ROSSOUW (Dundas, Canada), Andreas KORINEK, Steffi WOO, Gianluigi BOTTON
Amphithéâtre

Wednesday 31 August

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IM3-I
14:15 - 16:30

IM3: New Instrumentation
SLOT I

Chairmen: Emmanuel BEAUREPAIRE (Polytechnique, Paris, France), Christian COLLIEX (Orsay, France), Jörg ENDERLEIN (Göttingen, Germany), Andreas ENGEL (Delft, The Netherlands), Ernst H.K. STELZER (Professor) (Frankfurt am Main, Germany)
14:15 - 14:45 #8633 - IM03-S38 New developments in direct electron detecting cameras and their importance for cryo-EM.
New developments in direct electron detecting cameras and their importance for cryo-EM.

Direct electron detectors have played a key role in the recent increase in the power of single particle electron cryomicroscopy (cryo-EM). In this talk, I will summarise the background to these recent developments, give a practical guide to their optimal use, and discuss future directions.

Richard HENDERSON (Cambridge, United Kingdom)
Invited
14:45 - 15:00 #6293 - IM03-OP095 Direct detection and electron counting - A beginning of a new era for electron microscopy.
Direct detection and electron counting - A beginning of a new era for electron microscopy.

Over the last decade, great progress has been made in the instrumentation of transmission electron microscopy (TEM) through the introduction of aberration correctors, electron energy monochromators, and a wide variety of TEM specimen holders designed for in-situ applications. With these new capabilities, a wealth of advanced experimental data can be easily generated by advanced TEM systems.  However, the development of a revolutionary image capture device has been lacking; this effectively makes imaging detectors the main bottleneck when striving to achieve the full potential performance of an advanced TEM.

Traditional image detectors use scintillator and optical transfer path (fiber-coupling or lens) to convert high energy electrons to photons that are subsequently transferred to the imaging sensor to form an image.  One of the disadvantages of this indirect detection approach is the loss of image resolution and sensitivity during the electron-photon conversion and the photon transfer as additional noise sources that significantly degrade the signal-to-noise ratio (SNR) of the image detector.

Very recently direct detection imaging devices have been successfully developed based on technology advancement made in CMOS (complementary metal oxide semiconductor) design and manufacturing, high speed data architectures, vastly increased memory densities and speed, etc. The elimination of scintillator and subsequent optical transfer path has significantly improved the detective quantum efficiency (DQE) – a critical measure of SNR for resolution and sensitivity of an imaging device. The state-of-the-art direct detection imaging device further boosts the DQE and image quality under extremely low beam conditions by electron counting at high speed (e.g. 400 fps @ 4k x 4k resolution) to eliminate the sensor readout noise and minimize the electron scattering noise. Figure 1 shows the comparison of DQE measurement at 300 kV for K2 direct detection cameras operated in electron counting (solid blue) and linear (dotted blue) mode, and scintillator/fiber optical coupled cameras (purple). It is clear that electron counting has significantly restored the DQE performance of direct detection cameras for all frequencies.

This new generation of direct detection imaging device has revolutionized the field of cryo-electron microscopy (cryoEM) in structural biology and is starting to impact many applications in electron microscopy of materials science, for example, in-situ microscopy, 4D STEM, imaging beam sensitive materials, quantitative measurement of radiation damage or quantitative electron microscopy, etc. Direct detection and electron counting are poised to advance electron microscopy into a new era.

Ming PAN (Pleasanton, USA)
15:00 - 15:15 #6818 - IM03-OP099 Characterisation of the Medipix3 detector for electron imaging.
Characterisation of the Medipix3 detector for electron imaging.

ABSTRACT

Hybrid pixel sensors, originally developed for particle physics, incorporate advanced analogue processing and digital conversion circuitry at the individual pixel level. Medipix3 [1] is an example of such a sensor and we have investigated its performance as an imaging detector for transmission electron microscopy (TEM). Measurements were performed with electron beam energies in the range, 60–200 keV on a JEOLARM200cF TEM/STEM [2] utilising a 256x256 pixel Medipix3 detector with 300 µm thick Si sensor layer.

In order to characterise the Modulation Transfer Function (MTF) and the Detective Quantum Efficiency (DQE) performance, 32 repeated datasets were acquired containing images of free space and a knife-edge for known beam current conditions at each energy across the full range of relevant Medipix3 energy threshold values. Data was acquired in Single Pixel Mode (SPM) and in Charge Summing Mode (CSM) [3], where, in the latter mode, effects from charge spreading in individual electron events are corrected for on the detector.  We have also measured DQE(0) using the methods described in [4].

At high lower threshold (THL) DAC values the MTF for this counting detector in single pixel mode is better than the theoretical maximum due to the reduction in the effective pixel size [4] as shown in Figure 1. However, the DQE at such high THL DAC values in single pixel mode is significantly reduced as seen in Figure 2, seeing as many real electron events are now not counted as the charge is deposited in more than one pixel and therefore falls below the threshold for detection.  Consequently, there is a balance to be made between optimizing DQE and MTF, depending on the exact requirements in the given application.

As is shown in Figure 3, the use of the CSM allows the achievement of a much higher MTF whilst retaining high DQE by using a lower threshold DAC value.  This therefore offers additional benefits over the more conventional SPM, thus allowing very high efficiency imaging whilst preserving maximal detail in the images, which is particularly beneficial for minimizing the required electron dose to the sample required to produce interpretable data, with obvious applications in beam sensitive materials.

 

ACKNOWLEDGEMENTS

This work was funded by the EPSRC (Fast Pixel Detectors: a paradigm shift in STEM imaging, EP/M009963/1).  Financial support from the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative (Ref 312483-ESTEEM2) is gratefully acknowledged.

 

 

REFERENCES

[1] R. Ballabriga, M. Campbell, E. Heijne, X. Llopart, L. Tlustos, W. Wong, Medipix3: A 64 k pixel detector readout chip working in single photon counting mode with improved spectrometric performance, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 633, Supplement 1, May 2011, Pages S15-S18, ISSN 0168-9002, http://dx.doi.org/10.1016/j.nima.2010.06.108.

(http://www.sciencedirect.com/science/article/pii/S0168900210012982).

[2] S. McVitie, D. McGrouther, S. McFadzean, D.A. MacLaren, K.J. O’Shea, M.J. Benitez, Ultramicroscopy 152, 57 (2015).

[3] Pennicard D., Ballabriga R., Llopart X., Campbell M. and Graafsma H., Simulations of charge summing and threshold dispersion effects in medipix3, Nucl. Instrum. & Meth. In Physics Research A 636 (2011) 74-81.

[4] McMullan G., Cattermole D.M., Chen S., Henderson R., Lloport X., Summerfield C., Tlustos L. and Faruqi A.R., Electron imaging with Medipix2 hybrid pixel detector, Ultramicroscopy 107 (2007), 401-413.

Jamil MIR, Jamil MIR (Oxford, United Kingdom), Robert CLOUGH, Ruaraidh MACINNES, Christopher GOUGH, Richard PLACKETT, Hide SAWADA, Ian MACLAREN, Dima MANEUSKI, Val O'SHEA, Damien MCGROUTHER, Angus KIRKLAND
15:15 - 15:30 #5291 - IM03-OP084 Ptychographic phase reconstruction and aberration correction of STEM image using 4D dataset recorded by pixelated detector.
Ptychographic phase reconstruction and aberration correction of STEM image using 4D dataset recorded by pixelated detector.

In scanning transmission electron microscopy (STEM), one can obtain a variety of STEM images such as bright-field (BF) and annular dark-field (ADF) STEM images by changing the shape of the scintillator. However, the intensity distribution of convergent beam electron diffraction (CBED) patterns at the detector plane is yet to be fully utilized. Meanwhile, direct electron detectors with fast frame rate have recently been commercialized and used in electron microscopy. Such detectors, when used for recording CBED pattern images for each STEM probe position, are called pixelated STEM detectors. With the obtained 4-dimensional (4D) dataset, any shape of STEM detector can be synthesized in a post processing by a free selection of the integration area. Therefore, we can synthesize variety of STEM images such as differential phase contrast (DPC) and annular bright field (ABF) images, if we once record the image signal with a pixelated detector.

The 4D dataset can also be used for the advanced image processing techniques such as ptychography, which has been shown to provide high efficiency for reconstructing the phase image of an object [1,2]. Using ptychography, not only the phase contrast can be enhanced but also the effect of lens aberrations to the image such as defocus can be corrected by the post processing using the information collected by a pixelated detector.

Experiments were performed using an aberration corrected microscope (JEOL JEM-ARM200F) equipped with a pixelated detector (pnDetector pnCCD), the fast direct electron detector, which can record images at a speed of 1,000 fps in a full frame mode (264 x 264 pixels). Binning or windowing can increase the speed. The camera was placed below the ADF detector to enable simultaneous recording.

Figure 1 shows STEM images of a monolayer graphene obtained at 80 kV. Fig. 1a shows an ADF image obtained with the probe current of approximately 0.2 pA and the dwell time for a pixel of 0.5 ms. Because of a combination of low dose and residual uncorrected aberrations, the lattice contrast of graphene is almost buried in the noise originated from 50 Hz commercial frequency. Fig. 1b shows a reconstructed phase image using ptychography. The image contrast is significantly improved compared to the simultaneous ADF image, but the contrast transfer of the image (see the Fourier transform displayed in the inset) is anisotropic, resulting in uncertain positions of carbon atoms. This is because there remains large two-fold astigmatism and defocus in the image shown in Fig. 1(b), because we could not adjust those by observation of the ADF image due to the weak image signal. Fig. 1c is a phase image in which the aberrations are corrected through post processing using the same 4D dataset by applying correction functions in the spatial frequency domain. The image is no longer anisotropic and the carbon atomic positions can be unambiguously determined. Although the same amount of electron dose is used to form the ADF and the corrected phase images, the result clearly shows the benefit of ptychographic phase reconstruction in improving image signal to noise and being able to correct aberrations through post processing.

Figure 2 shows the ptychographic phase maps of the 4D dataset at a certain spatial frequency. With an aberration-free electron probe, they would be flat phase on the two sidebands, and the phase difference between the bands be π. In Fig. 2a, the map of the original dataset, there is a phase gradient inside each sideband because the aberrations were present in the electron probe. Fig. 2b is the correction function that compensates for the aberration seen in Fig. 2a. Fig. 2c is the corrected phase map and corresponding to the image in Fig. 1c. Here, only defocus and two-fold astigmatism are corrected but corrections of other higher order aberrations are in principle possible.

Figure 3 shows through focus images created in the same way as above. As we know the full information on the electron wave at the condenser aperture plane, we can induce any aberrations such as defocus.

References

[1] PD Nellist et al., Nature, 374 (1995) p. 630.

[2] TJ Pennycook et al., Ultramicroscopy, 151 (2015) p. 160.

Acknowledgement

PDN and HY acknowledge funding from the EPSRC through grant number EP/M010708/1.

Ryusuke SAGAWA (Tokyo, Japan), Hao YANG, Lewys JONES, Martin SIMSON, Martin HUTH, Heike SOLTAU, Peter NELLIST, Yukihito KONDO
15:30 - 15:45 #5383 - IM03-OP086 A Compressive Sensing based acquisition design for quantitative ultra-low dose high-resolution imaging and spectroscopy in the STEM.
A Compressive Sensing based acquisition design for quantitative ultra-low dose high-resolution imaging and spectroscopy in the STEM.

Typically, the Nyquist frequency determines the sampling rate required to resolve a specific feature in a dataset. In practice this requires oversampling by twice the maximum relevant frequency. As such this provides a lower limit for the electron dose necessary to acquire the data experimentally. However, by applying the compressive sensing (CS) principles of sparsity and incoherence, this limit can be overcome and therefore a substantial reduction of the total electron dose can be achieved. CS theory then allows a recovery of the essential image information from randomly undersampled images. The theory of CS has been successfully applied in a number of areas, e.g. astronomy, MRI scanning, electron tomography [1], and most recently first experimental designs for STEM have been reported [2,3]. Despite these developments, CS acquisition schemes able to recover quantifiable information from atomically resolved images of beam-sensitive materials, time-changing processes and spectroscopic datasets have so far not been established  for STEM.
Here we demonstrate such a CS-based STEM acquisition implementation and benchmark its performance at atomic resolution using a variety of materials, including complex oxide ceramics with an inhomogeneous distribution of point defects as illustrated in fig. 1, but also highly beam-sensitive catalysts with atomic resolution [4]. The implementation relies on custom-built hardware and software which controls an electrostatic beam shutter to blank the electron beam during all but a few randomly chosen pixels through a regular image (or indeed spectrum image) acquisition. The total electron dose used to form a whole dataset can thus be tailored according to the electron damage threshold of the sample under investigation whilst maintaining all other acquisition parameters identical to regular data acquisition, allowing a seamless transition from ‘fully sampled’ to ultra-low-dose conditions. It will be shown that datasets acquired in this fashion down to a few % of the total incoming dose (see fig. 2) can be reconstructed to yield a truthful atomic resolution representation of the sample, using a variety of reconstruction algorithms [5,6]. We show that this implementation of CS can also be extended to 2D spectroscopy mapping and simultaneous HAADF image acquisition.. These results open the door to practical ultra-low dose high-resolution imaging and spectroscopy in the STEM for very beam sensitive samples, where the level of sampling can simply be chosen depending on the damage threshold of the material being investigated [7].
References
[1] Z. Saghi et al., Nano Letters 11 (2011), 4666
[2] Q.M. Ramasse et al., 15th Frontiers of Electron Microscopy in Materials Sciences (FEMS); 2015, September 13-18; Tahoe City, California, p76;
[3] A. Béché et al. , Appl. Phys. Lett. 108 (2016)
[4] D. Mücke-Herzberg et al. submitted
[5] A. Stevens et al. , Microscopy 63 (2014), 41-5;
[6] J. Ma. et al. submitted
[7] SuperSTEM is the UK EPSRC National Facility for Aberration-Corrected STEM, supported by the Engineering and Physical Science Research Council. PNNL, a multiprogram national laboratory, is operated by Battelle for the U.S. Department of Energy under Contract No. DE-AC05-76RLO1830. PAM, ZS and RL acknowledge funding under ERC Advanced Grant 291522-3DIMAGE.


Dorothea MUECKE-HERZBERG (Daresbury, United Kingdom), Patricia ABELLAN, Michael SARAHAN, Iain GODFREY, Zineb SAGHI, Rowan LEARY, Andrew STEVENS, Jackie MA, Gitta KUTYNIOK, Feridoon AZOUGH, Robert FREER, Paul MIDGLEY, Nigel BROWNING, Quentin RAMASSE
15:45 - 16:00 #6174 - IM03-OP094 Compressed sensing for beam sensitive materials imaging in Scanning Transmission Electron Microscopy.
Compressed sensing for beam sensitive materials imaging in Scanning Transmission Electron Microscopy.

Transmission electron microscopy (TEM) is a very powerful technique to investigate materials down to their atomic components. Its versatility allows quantifying samples from their shape to the nature of their constituents and surroundings. However, the strong interaction of the electron beam with matter potentially induces damages in samples under investigation, especially for those composed of soft matter such as zeolites, metal organic frameworks (MOFs) and most life science samples. Current workarounds involve the reduction of the beam intensity, such as in the so-called low dose imaging, or increase of the detector performances as provided e.g. by direct electron detectors.

Recently, improvement in signal processing lead to the development of compressed sensing [1, 2], a numerical algorithm based on the assumption that real life images are sparse in some particular well-chosen basis. Images can then be expressed with much less components than what required by the Nyquist sampling theorem. By extension, not all pixels in a given image are necessary and only a random selection of them is sufficient to retrieve the original image with fidelity. By definition, compressed sensing is a very dose efficient technique as only parts of the sample need to be exposed to the electron beam to reconstruct a faithful image. So far, only theoretical implementations of compressed sensing were investigated in the TEM community, focused mostly on the case of tomographic reconstructions [3].

Very recently, we demonstrated the first physical implementation of compressed sensing in a Scanning TEM (STEM) based on the use of a solenoid as a fast beam blanker [4]. The solenoid is placed in the condenser plane of a STEM in a specially designed condenser aperture holder with feedthrough electrical contacts. By synchronizing the STEM signal with the current source driving the solenoid, we successfully acquired compressed images by shifting the beam away from the region of interest on blanked pixels. The case of both medium scale imaging (Fig.1) and high resolution imaging (Fig. 2) was investigated and reconstructed using the SPGL1 algorithm [5] with a signal compression of 80%. The present setup, although still quite far from low dose imaging conditions, has lead to successful imaging of a cross grating and a SrTiO3 sample. Latest results revealed that a checkerboard pattern, the most demanding pattern for the fast beam blanker, could be acquired with a dwell time of 40 µs and a beam deflection of 225 nm, as shown in Fig. 3.

By further improving the experimental setup, we expect reducing the acquisition dwell time to values within the µs range. Such improvements will offer possibilities for realizing improved images and tomography experiments of electron beam sensitive materials.

 

[1] Candes E. J. et al., Communications on Pure and Applied Mathematics, 59 (2006) 1207-1223.

[2] Donoho, D., IEEE Transactions on Information Theory 52 (2006) 1289-1306.

[3] Saghi Z. et al., Advanced Structural and Chemical Imaging 1 (2015) 1-10.

[4] Béché A., Goris B., Freitag B. and Verbeeck J., APL 108 (2016) 093103.

[5] Van den Berg E. and Friedlander M. P., SIAM Journal on Scientific Computing, 31 (2008) 890-912.

 

Aknowledgments: A.B and J.V. acknowledge funding from the ERC Grant No. 278510 VORTEX and under a contract for an Integrated Infrastructure Initiative No. 312483 ESTEEM2. B.G. acknowledges the FWO for a postdoctoral research grant.

Armand BÉCHÉ (Antwerp, Belgium), Bart GORIS, Bert FREITAG, Jo VERBEECK
16:00 - 16:15 #5719 - IM03-OP087 Implementing electron energy-gain spectroscopy in scanning transmission electron microscope.
Implementing electron energy-gain spectroscopy in scanning transmission electron microscope.

Spectro-microscopy techniques like electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) are routinely used nowadays for optical characterization of nanostructures with wonderful spatial resolution. With advanced monochromators, the energy resolution in EELS can be as good as 10 meV [1]. Nevertheless, with monochromators, there is a trade-off between the signal intensity and the energy resolution. Conversely, the spatial resolution in common optical microscopes is diffraction limited, though the spectral resolution is very often down to the sub meV regime. Hence, it is a high time to look for a technique, which can combine the spectacular spectral resolution of optical probes and spatial resolution of the electron probe. The possibility of integrating these two probes was first anticipated by Prof. Archie Howie in 1999 [2], and a few years later, a compact theoretical formalism of the technique named electron energy-gain spectroscopy (EEGS) was proposed [3].

In EEGS, electrons pick up energy from external illumination, which is very often the coherent output of a finely tuned laser. Although the electron and photon do not couple linearly in free space due to energy-momentum mismatch, the presence of a nanostructure can break the mismatch by providing the extra momentum through induced light fields, like the evanescent plasmonic field of the nanostructure. Employing synchronized femtosecond pulses of electrons and very intense pulse of photons, electron energy gain (EEG) has been demonstrated, a technique commonly called as photon-induced near-field electron microscopy (PINEM) [4]. It has shown that electrons can absorb or emit photons and the resulting spectra consist of peaks positioned at energies integral multiples of the laser photon energy on both sides of the zero loss peak. Multiphoton absorption/emission is a non-linear phenomenon, occurring at very high laser peak intensity (~ GWcm-2), but this can also be triggered at comparatively lower laser intensity, if the laser wavelength is tuned at particle plasmon wavelength [5]. The multiphoton emission and absorption probabilities depend only on the temporal ratio of the electron and photon pulse and the delay between them. The laser can be described as a coherent photon state, which excites a coherent plasmon state and the electron can absorb photon resulting in gain. And the reverse process is a stimulated photon emission by the electron (SEELS) in which the electron loses one photon energy. For large number of incident photons, these two probabilities are the same [5]. For a moderate laser intensity the EEGS or SEELS probability is few ten times larger than the EELS probability [3,5].

In our lab, we are developing an EEGS setup in one of the existing STEM (VG HB 501). The goal is to do spectroscopy by varying the laser energy and detecting the energy gain of electrons. We use a paraboloid mirror (used in CL) to focus the laser output on the sample surface, which can be used over a wide wavelength range without any change of optics. In a normal EEG set up, a pulsed laser is used as excitation source and a pulsed gun is used to detect only those electrons which goes through energy gain or loss in interaction with the laser pulse. In this work we are not using a pulsed gun. A pulsed gun is expensive to make, pulse duration is not easily tunable. A high brightness and thus high spatial resolution is not achieved easily with pulsed gun. We will use a Q-switched pulsed dye laser with tunable wavelength (570-900 nm) and typical pulse duration of approximately 30 ns.

The instrumental details and some preliminary results will be discussed. We expect that geometries like nanostars (Figure 1(b)) which can confine a huge amount of field might be a promising candidate that can yield much stronger EEGS signals at moderate laser intensity.

References:

[1] O. L. Krivanek et al. Nature. 2014, 514, 209.

[2] A. Howie, Inst. Phys. Conf. Ser. 1999, 161, 311.

[3] F. J.  García de Abajo, and M. Kociak, New J. Phys. 2008, 10, 073035.

[4] B. Barwick et al.  Nature 2009, 462, 902.

[5] A. A. Garcia, and F. J.  García de Abajo, New J. Phys. 2013, 15, 103021.

 

Pabitra DAS (ORSAY), Jean-Denis BLAZIT, Marcel TENCÉ, Luiz TIZEI, Hugo LOURENÇO MARTINS, Christian COLLIEX, Mathieu KOCIAK
16:15 - 16:30 #5951 - IM03-OP088 The concept of quantum electron microscopy.
The concept of quantum electron microscopy.

Following a recent suggestion [1] that interaction-free measurements are possible with electrons, we have analyzed the possibilities to use this concept for imaging of biological specimen with reduced damage. We have also made preliminary designs for an atomic resolution interaction-free electron microscope, or “quantum electron microscope” [2], for one example see figure 1.

The idea of interaction-free measurements follows Elitzur and Vaidman [3], who proposed that an opaque object may be observed by detecting a photon that did not interact with that object. This concept uses a Mach-Zehnder interferometer in which one branch of the particle wave passes through the specimen (“specimen beam”) while the other part follows another path (“reference beam”).  For a substantial reduction of the interaction, the amplitude of the wave passing through the specimen must be small and the interaction must be repeated many times. To implement this concept in an electron microscope would require a number of unique components not found in conventional transmission electron microscopes. These components include a coherent electron beam-splitter or two-state-coupler, and a resonator structure to allow each electron to interrogate the specimen multiple times. A two-state-coupler has the function of moving the electron wave slowly between the reference beam and the specimen beam, as in a Rabi-oscillation. We have come up with four different choices for this: a thin crystal, a grating mirror, a standing light wave and an electro-dynamical pseudopotential.

Figures 2 shows a typical example of the theoretical output of a measurement, where for each pixel there is signal in the reference beam, the specimen beam and the inelastic channel, possibly detected in an energy loss measurement. Figure 3 shows the advantage of interaction free measurement in the detection of the presence of a dark pixel as compared to dark field microscopy. In the analysis of the image contrast that interaction-free microscopy can create, our tentative conclusion is that transparent specimen can be detected, but different transparencies cannot be distinguished [4]. Other modes of operation of a microscope with multiple interactions with the specimen, however, may enable this.

 

This research is funded by the Gordon and Betty Moore Foundation.

 

1] Putnam, W.; Yanik, M. Phys. Rev. A 2009, 80, 040902.

2] Kruit, P.; R. G. Hobbs, C-S. Kim, Y. Yang, V. R. Manfrinato, J. Hammer, S. Thomas, P. Weber, B. Klopfer, C. Kohstall, T. Juffmann, M. A. Kasevich, P.Hommelhoff, K. K. Berggren. Ultramicroscopy (2016), doi:10.1016/j.ultramic.2016.03.004.

3] Elitzur, A. C.; Vaidman, L. Found. Phys. 1993, 23, 987–997.

4] Thomas, S.; Kohstall, C.; Kruit, P.; Hommelhoff, P. Phys. Rev. A 2014, 90, 053840.

Pieter KRUIT (Delft, The Netherlands), Karl BERGGREN, Peter HOMMELHOFF, Mark KASEVICH
Salle Bellecour 1,2,3

Wednesday 31 August

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MS1-I
14:15 - 16:30

MS1: Structural materials, defects and phase transformations
SLOT I

Chairmen: Patricia DONNADIEU (ST MARTIN D'HERES CEDEX, France), Randi HOLMESTAD (Trondheim, Norway), Simon RINGER (Sydney, Australia)
14:15 - 14:45 #5681 - MS01-S65 The Role of Interfacial Structure and Defects in Precipitation Pathways in Aluminium Alloys.
The Role of Interfacial Structure and Defects in Precipitation Pathways in Aluminium Alloys.

Precipitation hardening in aluminium alloys provides well-known examples of how identical materials can develop very different microstructures through different processing conditions or the addition of trace alloying elements. Desirable microstructures for useful materials properties such as high mechanical strength often involve the precipitation of metastable phases from a supersaturated solid solution. A textbook case is the precipitation of Guinier-Preston (GP) zones in Al-Cu alloys [1] and the subsequent formation of θ” and θ’ metastable phases [2]. More than 75 years after the original work by Guinier and Preston, the atomic-scale mechanisms behind the solid-solid phase transformations associated with the nucleation and growth of those phases remain unknown. Such an understanding is required if one is to move towards rational design of new high-performance alloys aided by modern computational techniques.

In the last eight years aberration-corrected scanning transmission electron microscopy (STEM) has enabled significant progress in the determination of bulk and interfacial structures of alloy precipitates. Here we present recent work combining STEM observations and computer simulations for the structural and energetics characterisation at the atomic scale of precipitate phases and their interfaces.  We will show that even classic binary alloy systems such Al-Cu, Al-Au and Al-Ag can reveal surprising characteristics. For example, the two isostructural phases θ’ (Al2Cu) and η (Al2Au) display vastly different interfacial structures (see Fig. 1) [3-5]. These differences can be explained by the calculated defect energies for Cu and Au solute in aluminium [5]. Another unexpected finding was that of a new intermediate phase, denoted Z, in the Al-Ag system. This phase is coherent with the Al matrix and consists of alternating bilayers of Ag and Al (see Fig. 2) [6]. The structure of the Z phase is analogous to that of Ag segregating on θ’  precipitates (see Fig. 3) [7]. Finally, we will present examples of precipitation pathways being altered through the introduction of certain alloying additions or lattice defects.

The authors acknowledge funding from the Australian Research Council (DP150100558 and LE0454166), computational support from the Monash Sun Grid cluster, the National Computing Infrastructure and Pawsey Supercomputing Centre funded by the Australian Government, and the use of facilities within the Monash Centre for Electron Microscopy.

[1]   A. Guinier, Nature 142 (1938) 569; G. D. Preston, Nature 142 (1938) 570.

[2]   J.M. Silcock, T.J. Heal, and H.K. Hardy, J. Inst. Met. 82 (1953) 239.

[3]   L. Bourgeois, C. Dwyer, M. Weyland, J.F. Nie and B.C. Muddle, Acta Mater. 59 (2011) 7043.

[4]   L. Bourgeois, N.V. Medhekar, A.E. Smith, M. Weyland, J.F. Nie and C. Dwyer, Phys. Rev. Lett. 111 (2013) 46102.

[5]   L. Bourgeois, Z. Zhang, J. Li and N.V. Medhekar, Acta Mater. 105 (2016) 284.

[6]   Z. Zhang, L. Bourgeois, J.M. Rosalie and N.V. Medhekar, to be submitted.

[7]   J.M. Rosalie and L. Bourgeois, Acta Mater. 60 (2012) 6033.

Laure BOURGEOIS (Victoria, Australia), Zezhong ZHANG, Yong ZHANG, Yiqiang CHEN, Julian ROSALIE, Jiehua LI, Christian DWYER, Nikhil MEDHEKAR
Invited
14:45 - 15:00 #6138 - MS01-OP208 Analytical electron tomographic investigations revealing self stabilization of core-shell precipitates through opposing diffusion processes.
Analytical electron tomographic investigations revealing self stabilization of core-shell precipitates through opposing diffusion processes.

For a thorough understanding of a material, investigations at the nanoscale are often essential. Analytical techniques like electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) in scanning transmission electron microscopy (STEM) can reveal important chemical information necessary for the development and improvement of high-tech materials. The integrative character of the signal acquired through transmission, however, might hide important morphological details of the material, relevant for its properties. Those details can be revealed through electron tomography, where the data is acquired at different tilt angles and, after alignment, reconstructed to form a full 3D model of the material under investigation.  The combination of both techniques, analytical STEM and tomography, gives full insight into structure and composition of a material [1].

 

In this study an industrially cast aluminum alloy, containing scandium (Sc) and zirconium (Zr) rich nano-precipitates, was investigated at different stages of ageing. High resolution STEM and analytical EELS and EDX tomographic investigations were carried out. The resulting 3D elemental reconstructions delivered otherwise inaccessible information on the samples’ chemistry and structure (figure 1). Additionally, EDX-spectra of a long aged and electron-beam re-solidified (EBRS) sample were reconstructed channel by channel, resulting in a data cube, in which each voxel (volume pixel) contained an entire spectrum. Via a high angle annular dark field (HAADF) reconstruction and 3D masking, the voxels of the core and the different shell regions of a nano-precipitate were extracted and summed to obtain pure spectra of those volumes, thus overcoming the intrinsic limitations arising from the integrative character of analytical STEM. Comparisons of these results with data from high resolution STEM imaging, HAADF tomography and data from literature have led to the conclusion -opposed to previous believes [2]- that a self-limiting diffusion process takes place within the precipitates stabilizing their core-shell structure (figure 2).

 

 

[1] G. Haberfehlner et al, Nanoscale 6 (2014), p. 14563.

[2] E. Clouet et al. Nature Materials 5 (2006), p. 482.

 

 The authors thank the Austrian Cooperative Research Facility, the European Union (7th Framework Programme: ESTEEM2) and the Austrian Research Promotion Agency FFG (TAKE OFF project 839002) for funding.

Angelina ORTHACKER (Graz, Austria), Georg HABERFEHLNER, Johannes TAENDL, Maria Cecilia POLETTI, Bernhard SONDEREGGER, Gerald KOTHLEITNER
15:00 - 15:15 #5970 - MS01-OP206 Characterization of multicomponent Al alloys by TEM, HAADF-STEM, EELS and DFT.
Characterization of multicomponent Al alloys by TEM, HAADF-STEM, EELS and DFT.

Light, environmentally friendly and recyclable materials are of increasing importance, such as in the automotive industry where the age-hardenable Al-Mg-Si alloys are of high interest. By understanding better how microstructure responds to changed processing or composition, costs can be cut and macroscopic properties tailored for particular applications. For example, by reducing the solute amount a softer material will allow higher extrusion speeds, however, on the expense of material strength. Recently, it was shown how adding back a lower at% of ‘exotic’ elements like Li, Cu, Ag and Ge compensates such strength loss [1]. Age hardening is ascribed to dislocation resisting strain-fields set up in the aluminium during nucleation and growth of the metastable, nanoscale, semi-coherent precipitate needles in the three <100> Al directions [2]. The replacement elements change precipitate structure and growth conditions.

Precipitate statistics based on transmission electron microscopy (TEM) can elucidate how material strength is related to precipitate number densities, sizes and volume fractions; see example a bright field TEM micrograph in Fig. 1 (a) used for this purpose. The elemental contrast in probe Cs corrected high angle annular dark field scanning TEM (HAADF-STEM) greatly assists structural characterization of individual precipitate cross sections [3], enabling identification of precipitate type and the composition of individual atomic columns. See example of a β’’ precipitate in Fig. 1 (b).

            Because elements with similar atomic number (Z) express very small intensity differences, electron energy loss spectroscopy (EELS) elemental mapping has been applied to aid resolution. It became clear that EELS in combination with HAADF-STEM can resolve the hexagonal network of Si columns [4] well, see Fig. 2. It was also found that a significant proportion of the other atomic columns were mixed.

A pressing matter in material design is enhanced control with precipitate characteristics which vary strongly with alloy composition and processing. Density functional theory (DFT) simulations have a great potential for broadening the understanding concerning precipitate nucleation, growth and final microstructure, and how they are connected. Here we have investigated binding between solute atoms and to vacancies, along with volume misfits (see Fig. 4) in the Al lattice, in a venture to relate these parameters to precipitation.

 

References

 

[1] E. A. Mørtsell, C. D. Marioara, S. J. Andersen, J. Røyset, O. Reiso and R. Holmestad, Metallurgical and Materials Transactions A, vol. 46, no. 9, pp. 4369 - 4379, 2015.

[2] C. D. Marioara, H. Nordmark, S. J. Andersen and R. Holmestad, J. Mater. Sci., vol. 41, pp. 471 - 478, 2006.

[3] P. D. Nellist and S. J. Pennycook, "The principles and interpretation of annular dark-field Z-contrast imaging," Advances in Imaging and Electron Physics, vol. 113, pp. 147 - 203, 2000.

[4] S. J. Andersen, C. D. Marioara, R. Vissers, A. L. Frøseth and P. Derlet, in Proc. 13th Eur. Mocr. Congress (EMC) 2, 2004.

Eva MØRTSELL (Trondheim, Norway), Sigmund ANDERSEN, Calin MARIOARA, Jostein RØYSET, Jesper FRIIS, Randi HOLMESTAD
15:15 - 15:30 #6363 - MS01-OP212 Probing the heterogeneous nucleation interface of TiB2 in Al alloys.
Probing the heterogeneous nucleation interface of TiB2 in Al alloys.

The grain refinement of Al alloys has been extensively investigated for several decades both in industry and academia, not only through attempts at developing efficient grain refiners, but also with a view to achieve a better understanding of the grain refinement mechanism [1-7]. The addition of Al-Ti-B grain refiners (e.g. Al-5Ti-1B, wt. %) during the grain refinement of Al alloys has been widely investigated due to these compounds’ higher nucleation potency and wide potential for industrial applications. Various theories regarding the grain refinement mechanisms of Al-Ti-B refiners have been proposed [1-4]. Despite differences between all these theories, it is generally accepted that Ti has multiple roles within the Al melt. One role is to create an enriched Ti region leading to the formation of an Al3Ti monolayer necessary for the heterogeneous nucleation of Al on the stable boride substrates (TiB2) [5, 6]. The other role is to act as an effective growth restrictor factor [7]. It is the combined effects of the enhancement in nucleation site numbers and of the growth restrictions that result in the formation of desirable, small uniform equiaxed Al grains. It is well accepted that Ti is more likely to segregate to the TiB2 / a-Al interface affecting its structure and thereby the constitutional undercooling at the solid-liquid interface [5, 6]. The presence of the Al3Ti monolayer is still fervently disputed [5-7]. To resolve this disagreement, an atomic scale experimental investigation on the heterogeneous nucleation interface between TiB2 and α-Al in conventional castings (e.g. in alloys containing < 0.15 wt. % Ti that have not been melt spun) would be required.

This paper employs atomic scale high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) to probe the heterogeneous nucleation interface of TiB2 in Al alloys, with a special focus on the partitioning of solute elements (Ti) to TiB2. A significant Ti partitioning to the whole surface of TiB2 was unambiguously observed in the commercial grain refiner (Al-5Ti-1B), as shown in Figure 1 and Figure 2. However, such prevalent Ti partitioning was not observed on the basal planes of TiB2 in Al-Si based alloys with high Si concentrations, which could be used to explain why Si poisoning occurs. In order to avoid / reduce the Si poisoning, CrB2 was added together with TiB2. In this case, a significant Cr partitioning to the whole surface of TiB2 was observed, which may minimize the lattice mismatch with Al and / or improve the stability of a Al3Ti monolayer on TiB2, as shown in Figure 3. This investigation provides a clearer picture about the heterogeneous nucleation interface between TiB2 and Al and further develops heterogeneous nucleation theory.

 

Jiehua Li gratefully acknowledges the financial support from the Major International (Regional) Joint Research Project (No. 51420105005) from China. The SuperSTEM Laboratory is the U.K. National Facility for Aberration-Corrected Scanning Transmission Electron Microscopy, supported by the Engineering and Physical Sciences Research Council (EPSRC).

[1]   M. Easton and D. StJohn, Metall. Mater. Trans. A., 30 (1999)1613.

[2]   T.E. Quested and A.L. Greer, Acta Mater., 53(2005)2683.

[3]   D. Shu, B.D. Sun, J. Mi and P.S. Grant, Acta Mater., 59(2011) 2135.

[4]   D.H. StJohn, M. Qian, M.A. Easton and P. Cao, Acta Mater., 59(2011)4907.

[5]   N. Iqbal, et al. Acta Mater., 53(2005)2875.

[6]   P. Schumacher and A.L. Greer, Mater. Sci. Eng. A, 178(1994)309.

[7]   M. Easton and D. StJohn, Metall. Mater. Trans. A., 30(1999)1625.

Jiehua LI (Leoben, Austria), Fredrik HAGE, Quentin RAMASSE, Peter SCHUMACHER
15:30 - 16:00 #8644 - MS01-S66 Atom probe tomography of early stage clustering in Al alloys.
Atom probe tomography of early stage clustering in Al alloys.

Nano-scaled early stage Si or Mg containing clusters in Al-Mg-Si alloys are experimentally extremely difficult to observe and atom probe tomography (APT) is the only technique to visualize and chemically measure these clusters today. Two examples for the technological importance of understanding clustering during natural aging in the Al-Mg-Si system are presented. First, a quenching procedure is shown to significantly alter the overall excess-vacancy concentration consequently altering solute clustering kinetics. Second, trace element additions are shown to trap excess-vacancies and hence to reduce the freely available vacancy concentration, which results in decreased solute clustering. The modification and understanding of early stage clustering is commercially important since it has strong implications on the strength evolution during industrial heat treatments. However, restrictions remain in achieving the ultimate quantification of such small solute aggregates by APT. Especially the surface migration of Si atoms is an open issue. However, an accurate adjustment of measurement and analysis parameters is known to minimize this problem. Nevertheless, clustering directly after quenching has not been possible to investigate because of the time required for sample preparation and sample transfer into the analysis chamber. Here we present a new strategy how the measurement of as-quenched conditions and very early stages of natural aging can be realized via APT. This can be accomplished via sample production at cryogenic temperatures to suppress diffusion. The shown procedure includes cryogenic focused ion beam (FIB) preparation and direct cryogenic transfer of fresh APT samples into the analysis chamber of an atom probe. Results on as-quenched and in-situ naturally aged Al-Mg-Si alloys are presented. Furthermore, the effect of FIB induced clustering at cryogenic temperatures is discussed.

Stefan POGATSCHER (Leoben, Austria), Phillip DUMITRASCHKEWITZ, Stephan S.a. GERSTL
Invited
16:00 - 16:15 #6041 - MS01-OP207 Crystallographic mapping in engineering alloys by scanning precession electron diffraction.
Crystallographic mapping in engineering alloys by scanning precession electron diffraction.

Crystallographic, compositional and morphological complexity in modern engineering alloys necessitates the use of sophisticated tools for multi-scale materials characterisation. Here, we develop scanning precession electron diffraction (SPED) for mapping crystalline phases in engineering alloys. SPED involves scanning the electron beam across the specimen and recording a PED pattern at each point by rocking a focused probe in a hollow cone above the specimen and de-rocking the beam back to the optic axis below. In this way, integrated diffraction intensities are recorded in the geometry of a conventional electron diffraction pattern [1]. A 4D dataset is obtained comprising a 2D PED pattern at each position in the 2D scan region, which can be analysed in a number of ways. Most simply, ‘virtual diffraction images’ can be formed by plotting the intensity of a sub-set of pixels in each PED pattern as a function of probe position to elucidate variations in the diffraction condition in a versatile post-acquisition scheme. Phase and orientation maps can also be formed by matching each PED pattern to a library of simulated patterns [2]. Here, we use this approach to determine the phases of precipitates in a nickel base superalloy and to identify orientation relationships existing between these phases. To do this we explore the orientation data in disorientation space where the rotation axis and angle between the two crystallographic bases is plotted (Figure 1). This automated analysis enabled treatment of multiple precipitates yielding a more representative view of the microstructure compared to conventional SAED methods.

 

New methods for strain mapping and phase characterisation based on machine learning were developed as part of this work to extract further insight into microstructural features. Strain maps were obtained by comparing each pattern to an unstrained reference and used to explore the strain distribution between precipitates in aluminium alloys (Figure 2). These SPED based strain maps offer a greater field of view as compared to methods based on atomic resolution imaging whilst retaining nm-scale spatial resolution. This yields unique insights such as the ability to map the interaction of strain fields associated with multiple precipitates, which can be seen in Figure 2. Phase characterisation, on the other hand, addresses the challenge of determining the chemistry and crystallography of phases in the microstructure that are often embedded and overlap in projection. We apply machine learning algorithms to SPED [3] and STEM-EDX [4] data acquired from the same region to achieve a correlated crystallographic and chemical characterisation of a Ti-Fe-Mo alloy with a nanometre scale lamellar microstructure (Figure 3). This approach learns component signals (spectra or patterns), which make up the particular dataset, together with their associated loading at each real space pixel. An efficient representation of the data is therefore found with minimal prior knowledge and signals from overlapping crystals are separated to achieve phase specific characterisation. Combined, the analysis approaches developed in this work provide comprehensive 'crystal cartography' of engineering alloys paving the way to better understanding of relationships between processing, structure and properties.

 

[1] R. Vincent, and P. A. Midgley, Ultramicroscopy, 1992, 53, 271-282

[2] E. Rauch et al, Zeitshrift fur Kristallographie, 2010, 225, 103-109

[3] A. S. Eggeman et al, Nature Communications, 2015, 6, 7267

[4] D. Rossouw et al, Nano Letters, 2015, 15, 2716-2720

 

The authors acknowledge: the ERC (291522-3DIMAGE), the European Commission (312483 - ESTEEM2), Rolls-Royce plc (EP/H022309/1), the EPSRC (EP/H500375/1),  BMWi (20T0813), and the Research Council of Norway (197405-NORTEM & 221714-FRINATEK).

Duncan N. JOHNSTONE (Cambridge, United Kingdom), Alexander J. KNOWLES, Robert KRAKOW, Sigurd WENNER, Antonius T. J. VAN HELVOORT, Randi HOLMESTAD, Howard STONE, Catherine RAE, Paul A. MIDGLEY
16:15 - 16:30 #5784 - MS01-OP201 Absolute quantification of nano-scale precipitates in steel using DualEELS.
Absolute quantification of nano-scale precipitates in steel using DualEELS.

Many steels rely on dispersion hardening with transition metal carbides or carbonitrides. TixV(1-x)CyNz precipitates are used in the high-Mn steels investigated in a recent project [1]. Consequently, the quantitative characterization of such precipitates is essential to understand the interdependency of alloy composition, thermomechanical treatment and final mechanical properties. 

In an earlier paper, the extraction of a precipitate spectrum image (SI) by the subtraction of matrix contribution was demonstrated using the capabilities of DualEELS [2]. The combination of closely spaced edges, weak edges and strong ELNES makes absolute quantification of such a precipitate SI challenging.

Thus absolute experimental cross-sections, derived from ceramic standards of TiC0.98. TiN0.88, VC0.83 and VN0.97, are used. A wedge shaped lamella of the standard is prepared by focused ion beam milling and an SI is recorded over a region where t/λ varies from 0.2 to 0.8. Using the procedures in [1], the low loss and high loss regions are spliced and the result Fourier logarithmically deconvolved to give a single scattering distribution.   At each energy loss, the resulting spectral intensities per unit energy, normalized by the zero loss intensity, are plotted against the product of the metal atoms per volume in the standard multiplied by the local thicknesses. The slope of this line is the absolute differential cross-section appropriate to the probe and collection angles used. An absolute value of λ is required to convert the t/λ map to a t map. This is measured using a needle shaped specimen.  Its t/λ is measured at one orientation and its t is measured from the image width after rotation through 90o, giving a direct measurement of λ.   The cross-sections for the individual edges can be separated out by fitting a suitable background before the non-metal edge and using its Hartree-Slater calculated cross-section to extrapolate it under the metal edge. The non-metal edge can then be corrected for the sub-stoichiometry.

The precipitate has the same rocksalt structure as the standards but is a quaternary. With the four standards, two cross-sections are obtained from each edge. In principle, a cross-section appropriate to the precipitate composition can be made for each edge using an appropriately weighted average. However, because the N and Ti edges are so close, the edges from TiN0.88 are difficult to separate and are not used here.  Thus the Ti cross-section from TiC0.98 and the N cross-section from VN0.97 are always used. Fortunately, the Ti metal fraction and the N non-metal fraction are only ~0.3 and so this causes no issues. To complete the standards for a multiple linear least squares (MLLS) fit to the precipitate SI, shapes for the background and the residual carbon and oxygen edges left after the matrix subtraction are required. The latter can be extracted from the average matrix spectrum.

Figure 1 shows the result of such an MLLS fit and the residuals are extremely low. The maps of the fitting coefficients for the four elements are shown in Figure 2. When normalized by the zero loss intensity, they give the number of each atom type per unit area. Using the volume of the unit cell appropriate to the composition, the “thickness” occupied by each atom type can be found. The sum of the Ti and V “thicknesses” is the precipitate thickness and a profile of this is shown in the lower part of Figure 3. Outside the precipitate, the values are essentially zero, as expected, and have low noise, indicating the sensitivity of the technique. The values of x, y and z can also be found and profiles of them are shown in the upper part of Figure 3.   The region outside the precipitate has been zeroed. The values are essentially constant across the precipitate and become noisy at the edge where the signals themselves are low and hence noisy.

 

References:

[1] G. Paul et al Erste Erkenntnisse zum Ausscheidungsverhalten von Mikrolegierungselementen in Hoch-Mangan Stählen; Ed. G. Petzow, (2012) Sonderbände der praktischen Metallographie, vol. 44

[2] J. Bobynko et al Ultramicroscopy 149 (2015) 9-20.

 

Acknowledgements:

EEC Funding from the RFCS Fund for PrecHiMn (RFSR-CT-2010-00018) and PreTiControl (RFSR-CT-2015-00013); Prof W. Lengauer (TU-Wien) for the provision of carbide and nitride standards; Dr G. Paul (thyssenkrupp Steel Europe AG) for the provision of the vanadium steel sample.

Ian MACLAREN (Glasgow, United Kingdom), Bianca SALA, Joanna BOBYNKO, Alan J. CRAVEN
Salle Prestige Gratte Ciel

Wednesday 31 August

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MS0-II
14:15 - 16:30

MS0: Nanoparticles: from synthesis to applications
SLOT II

Chairmen: José CALVINO (Cadiz, Spain), Goran DRAZIC (Head of microscopy group) (Ljubljana, Slovenia), Christian RICOLLEAU (Professor) (Paris, France)
14:15 - 14:45 #7887 - MS00-S64 Studying membrane proteins in intact cells using nanoparticle labels and liquid-phase electron microscopy.
Studying membrane proteins in intact cells using nanoparticle labels and liquid-phase electron microscopy.

Cells have receptor proteins in their plasma membranes ‘listening’ to chemical signals from the outside world. These signals consist of ligands, small molecules that bind specifically to a receptor. But how those signals are interpreted and lead to decisions is incompletely understood mainly on account of limitations of present analytical methods. It is typically extremely difficult to directly see how endogenously expressed individual proteins respond to ligand binding in the intact cell, which can lead, for example, to the formation of protein complexes triggering signaling processes. Much knowledge about cellular function has been obtained via biochemical methods but these analyze pooled material from many thousands of cells and the knowledge is thus based on population averages. But we need to look at the individual cell in order to understand the fundamentals of how a cell interprets a signal. Studying membrane proteins at the nanoscale in intact eukaryotic cells is now possible using liquid-phase scanning transmission electron microscopy (STEM) [1, 2]. The key step is to specifically label the proteins of interest in a one-to-one ratio with small probes combined with nanoparticles, for example, gold nanoparticles or quantum dots. Cells in liquid are then placed in a microfluidic chamber enclosing the sample in the vacuum of the electron microscope, and are imaged with STEM. It is not always necessary to enclose the cells in the microfluidic chamber. For some studies, it is sufficient to obtain information from the thin outer regions of the cells, and those can be imaged with high resolution using environmental scanning electron microscopy (ESEM) with STEM detector [3]. Liquid-phase STEM was used to explore the formation of the epidermal growth factor HER2 at the single-molecule level in intact SKBR3 breast cancer cells in liquid state [4]. HER2 is a membrane protein and plays an important role in breast cancer aggressiveness and progression. Data analysis based on calculating the pair correlation function from individual HER2 positions revealed remarkable differences in functionality between different cellular regions, and between cells with possible relevance for studying cancer metastasis and drug response.

 

Acknowledgements: We thank E. Arzt for his support through INM.  Research in part supported by the Leibniz Competition 2014.

 

References:

[1]     de Jonge, N., Peckys, D.B., Kremers, G.J., Piston, D.W., Proc. Natl. Acad. Sci. 106, 2159-2164, 2009.

[2]     de Jonge, N., Ross, F.M., Nat. Nanotechnol. 6, 695-704, 2011.

[3]     Peckys, D.B., et al., Sci. Rep. 3, 2626: 1-6, 2013.

[4]     Peckys, D.B., Korf, U., de Jonge, N., Sci. Adv. 1, e1500165, 2015.

Niels DE JONGE (Saarbrücken, Germany), Diana PECKYS
Invited
14:45 - 15:00 #5945 - MS00-OP183 Correlative In situ imaging and spectroscopy of a bimetallic PdCu catalyst using Synchrotron and Electron Optical Beam Lines.
Correlative In situ imaging and spectroscopy of a bimetallic PdCu catalyst using Synchrotron and Electron Optical Beam Lines.

Supported metal nanoparticle catalysts are key materials for diverse conversion processes and are widely used throughout the chemical industry. Bimetallic catalysts frequently offer improvements in activity, selectivity, and stability compared to their monometallic equivalents [1]. The nanoparticle (NPs) distributions are generally not homogeneous and not all NP morphologies show equal activity for a given reaction [2]. Characterisation of the elemental distribution and oxidation state is therefore of great importance for understanding the alloying and segregation phenomena in bimetallic catalysts and hence optimising the performance of such materials. However, such characterisation usually requires the sample to be imaged at high vacuum and room temperature and the chemical reactions of interest rarely occur at such benign conditions. Therefore, we have conducted in situ environmental TEM study [3-4] with complementary bulk analysis using temperature programmed reduction (TPR) and in situ X-ray Absorption Near Edge Spectroscopy (XANES) to gain in depth insights into the chemical and morphological changes experienced by TiO2 supported PdCu nanoparticle catalysts when exposed to relevant elevated temperature in 1 bar reducing atmosphere. Recent progress with environmental cell and microscope design has enabled us to follow the evolution of bimetallic NPs using in situ elemental imaging capabilities [5-6].
The PdCu catalyst was prepared by incipient wetness co-impregnation of copper and palladium nitrates on titania followed by calcination to yield the unreduced form of the catalyst. Conventional STEM-XEDS elemental mapping indicates that the catalyst comprises ~10 nm large Pd containing particles with smaller ~1 nm size Cu clusters dispersed across the titania support. The XANES spectra of the calcined material (Figure 1) indicate that both Pd and Cu are initially present as oxides. The Temperature Control Reduction (TPR)-XANES of the Pd (Figure 1a) shows complete reduction to Pd(0) after room temperature hydrogen treatment and that the metal remains Pd(0) throughout the remaining temperature points. By contrast, the Cu-K edge reduction profile (Figure 1b) shows a small degree of reduction at lower temperatures, likely due to Cu associated with the Pd clusters, and that the sample contains both Cu oxide and metallic Cu up to 250 °C. Figure 2 shows XEDS elemental mapping of TiO2 supported PdCu catalysts. After introducing a reducing H2 atmosphere and heating to 250 °C, NPs start to be resolvable in the Cu distribution visible as local regions of higher intensity (Figure 2e-h). At 550 °C, where TPR-XANES indicates that copper oxides are no longer present, Cu NPs are formed with Pd particles generally containing Cu but at varying concentrations. The heterogeneity of the localized composition is striking and in some cases Janus NPs are formed with various degrees of phase separation between the two metals (Figure 2j-l).
In summary, correlative studies on a bimetallic PdCu catalyst using in situ high temperature gaseous environmental conditions in both synchrotron and electron-optical instruments has allowed the identification of the processes by which the active phases of these catalysts evolve. This has provided invaluable insights to the complementary bulk XANES and TPR characterisation including the subtle changes in the Cu-K edge. This correlative approach has enabled an unambiguous identification of the states of the catalysts and helped to elucidate the segregation phenomenon which gives rise to their properties.

References

[1] J. H. Sinfelt, Acc. Chem. Res. 10, (1977) p. 15.
[2] P. J. Dietrich et al., ACS Catal. 4, (2014) p. 480
[3] S. Giorgio et al, Ultramicroscopy 106, (2006), p. 503
[3] J. F. Creemer et al, Ultramicroscopy 108, (2008), p. 993
[4] N. J. Zaluzec et al, Microsc. Microanal. 20, (2014), p. 323
[5] E. Prestat et al, Microsc. Microanal. 21 (2015), p. 247-248
[6] Research supported by EPSRC Grants #EP/G035954/1 and EP/J021172/1, the DTR Agency Grant HDTRA1-12-1-003, the BP Innovation Fund and ICAM project at Manchester and the U.S. DoE, Office of Science, DE-AC02-06CH11357 at Argonne National Laboratory

Eric PRESTAT (Manchester, United Kingdom), Matthew A. KULZICK, Paul J. DIETRICH, Eric DOSKOCIL, Matthew SMITH, M. Grace BURKE, Sarah J. HAIGH, Nestor J. ZALUZEC
15:00 - 15:15 #6307 - MS00-OP188 Local Information Powers Heterogeneous Catalysis Research: TEM Investigation of Industrial Relevant Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis.
Local Information Powers Heterogeneous Catalysis Research: TEM Investigation of Industrial Relevant Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis.

In a simplified picture a heterogeneous catalyst can be described as a high energy material, which is composed of two important parts: bulk and surface. The bulk is defined by a certain crystal and electronic structure as well as defect concentration, and reflects the real structure of the catalyst. It is responsible for the formation and stabilization of the active surface. Although physical and chemical information on the real and surface structure can be obtained by integral methods, important local fluctuations from the ideal structure, which are essential for the catalytic performance, in the catalyst composition may be overlooked.1 This highlights the significance of transmission electron microscopy (TEM) as the tool for the local description of the nano- and/or mesoscale of heterogeneous catalysts.

Here, we focus on a transmission electron microscopic (TEM) description of the local nano- and mesoscale of industrial relevant Cu/ZnO/Al2O3 catalyst for methanol synthesis. Methanol, one of the most important industrial chemicals, is produced from syngas (CO, CO2, H2) at high pressure (50-80 bar) and elevated temperature (240-280 °C) and is considered as a prospective key-compound for chemical energy and hydrogen storage. The working mechanisms of this industrial methanol catalyst have been investigated extensively.2 However, its actual active phase is still debated controversially. On the one hand, the catalytic odyssey may be caused by material, pressure and time gaps, which often do not mirror industrial relevant conditions. On the other hand, the diversity of results indicates the difficulty of investigating and understanding this complex catalyst system, in particular due to the lack of visual local information.

Our local observations tackle a detailed description of the mesostructure of the catalyst, the real structure of Cu nanoparticles and the formation of a special polymorph of layered ZnO on top of the Cu nanoparticles after reductive activation on industrial relevant ternary Cu/ZnO/Al2O3 catalyst.1b In addition, we will highlight the temperature, gas and time dependent evolution of this metastable layered ZnO polymorph from minutes to months (Figure 1).3

In conclusion, the results demonstrate the power of local TEM investigation in heterogeneous catalysis research. In combination with complementary integral data, they allow a detailed understanding of the system, which would be otherwise hard to obtain. The findings illustrate that methanol synthesis can be considered as an interface mediated process between Cu and ZnO.

References

(1)           (a) Lunkenbein, T.; Girgsdies, F.; Wernbacher, A.; Noack, J.; Auffermann, G.; Yasuhara, A.; Klein-Hoffmann, A.; Ueda, W.; Eichelbaum, M.; Trunschke, A.; Schlögl, R.; Willinger, M. G. Angewandte Chemie International Edition 2015, 54, 6828(b) Lunkenbein, T.; Schumann, J.; Behrens, M.; Schlögl, R.; Willinger, M. G. Angewandte Chemie International Edition 2015, 54, 4544(c) Holse, C.; Elkjær, C. F.; Nierhoff, A.; Sehested, J.; Chorkendorff, I.; Helveg, S.; Nielsen, J. H. The Journal of Physical Chemistry C 2015, 119, 2804.

(2)           Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B.-L.; Tovar, M.; Fischer, R. W.; Nørskov, J. K.; Schlögl, R. Science 2012, 336, 893.

(3)           Lunkenbein, T.; Kandemir, T.; Girgsdies, F.; Thomas, N.; Behrens, M.; Schlögl, R.; Frei, E. Angewandte Chemie International Edition 2016, to be submitted.

 

Thomas LUNKENBEIN (Berlin, Germany), Elias FREI, Julia SCHUMANN, Malte BEHRENS, Marc Georg WILLINGER, Robert SCHLÖGL
15:15 - 15:30 #6593 - MS00-OP192 Synthesis and Characterization of Ag2Se-Based Hybrid and Ternary Semiconductor Nanocrystals with potential termoelectric applications.
Synthesis and Characterization of Ag2Se-Based Hybrid and Ternary Semiconductor Nanocrystals with potential termoelectric applications.

Ag2Se is a heavy-metal free, low band gap semiconductor that is currently attracting much attention in several fields. Remarkably, its high Seebeck coefficient and its low thermal conductivity have made of Ag2Se a definitely promising thermoelectric material. Considering that material nanostructuration often entails a significant increase of the ZT figure of merit with respect to the bulk, the synthesis and study of Ag2Se nanocrystals and of its nanostructured derivatives seems exciting and definitely deserves an effort.

With regard to the synthesis of metal-Ag2Se hybrid nanocrystals, very few examples are currently available in the literature and none of them has addressed their potential applications.

The aim of this work is studying the ternary Au-Ag-Se system at the nanoscale in order to figure out appropriate synthetic protocols for the controlled growth of hybrid and compositionally-controlled Ag2Se-based nanocrystals and perform an in-depth structural characterization of the materials obtained while also studying the thermoelectric potential of this new ternary nanosystem.

Pau TORRUELLA BESA (Barcelona, Spain), Mariona DALMASES, Víctor FERNÀNDEZ-ALTABLE, Andreu CABOT, Maria IBÁÑEZ, Jordi LLORCA, Lluís LÓPEZ-CONESA, Gemma MARTÍN, Sónia ESTRADÉ, Francesca PEIRÓ
15:30 - 15:45 #5856 - MS00-OP182 Understanding the Self-Assembly of a Janus-type POM–POSS Co-Cluster from low-dose STEM.
Understanding the Self-Assembly of a Janus-type POM–POSS Co-Cluster from low-dose STEM.

Understanding the self-assembly process of novel cluster-assembled materials (CAMs) is a prerequisite to optimize their structure and function. Janus-type molecules are of particular interest as the provide the possibility to combine dissimilar properties. In this study, two kinds of unlike organo-functionalized inorganic clusters (polyoxometalate: POM, polyhedral oligomeric silsesquioxane: POSS) have been covalently linked by a short organic tether to form a dumbbell-shaped Janus co-cluster. Their supramolecular assembly in nanocrystals has been analyzed in 2D and 3D using low-dose HAADF-STEM techniques to understand the assembly process based on the structure and the structural variations. The results have been compared to coarse grained molecular simulations.

STEM tomography reveals a honeycomb superstructure constructed by hexagonal close-packed cylinders of the smaller POSS cluster and an orderly arranged framework of the larger POM cluster either in single or bicrystals (Fig. 1). A more detailed analysis (Fig. 2) reveals a bilayer arrangement of the POM-POSS clusters within the hexagonal framework as the favorable structural motive. However, locally significant structural variations are observed, both in terms of the POM neighbor coordination (Fig. 2E) as well as for the tessellation of the supercell arrangement (Fig. 2F). This self-organization demonstrates two synergistic effects controlling the structure formation: phase separation of the two immiscible blocks favoring a hexagonal framework, while crystallization or at least ordered packing of the POM clusters favors an extended bilayer leading to a geometrically frustrated structure. Close to the growth surface, this frustration leads to large structural variations with an increasing number of clusters in the basic motive, while it becomes more defined close to the core of the nanoparticles (Fig. 2D).

A series of coarse grained molecular simulations reveals that the size asymmetry of the two clusters has a significant effect on the self-assembled structure. Ratios between 1.5 and 1.8 lead to hexagonal structures in good agreement with the experimental results. This understanding opens up new opportunities for generating novel CAMs for advanced applications.

 

References

(1)        C. Ma et al, Angew. Chemie. Int. Ed. Engl, 2015, 54, 15699-15704.

 

Acknowledgement

We gratefully acknowledge the financial support by the National Natural Science Foundation of China for grants (21174080, 21274069, 21334003, and 21422403), the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry and the Karlsruhe Nano Micro Facility (KNMF) for access to the electron microscopic equipment.

Christian KUEBEL (Eggenstein-Leopoldshafen, Germany), L.t. YAN, Wei WANG
15:45 - 16:00 #6671 - MS00-OP193 Electron microscopy investigations of cation exchange in colloidal PbSe/CdSe nanocrystals.
Electron microscopy investigations of cation exchange in colloidal PbSe/CdSe nanocrystals.

Cation exchange is a very powerful method for creating heterogeneous nanocrystals (NCs), such as core-shell or rod-tip nanostructures. Here we present an overview of the experimental and simulation efforts made to elucidate the atomic mechanism underlying cation exchange in PbSe/CdSe nanocrystals.

When pure PbSe nanorods are in a colloidal solution, adding Cd precursors molecules to the solution will lead to the exchange of Pb ions by Cd ions, resulting in PbSe/CdSe core-shell nanorods. High-resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis of partly exchanged PbSe/CdSe core-shell nanorods showed clearly defined interfaces in the nanorods, dominated by PbSe-rocksalt/CdSe-zinc blende {111}/{111} crystal planes [1]. However, from an atomic-resolution 3D reconstruction of the PbSe core within such a core-shell nanorod [2], it became clear that the interfaces are not as straight and defect-free as they appear to be when considering only 2D projections.

The opposite process of Cd-for-Pb cation exchange was recorded in real time when PbSe-tipped CdSe nanorods were heated in-situ in the transmission electron microscope [3]. Stills of a movie recorded in HAADF-STEM mode at a temperature of 180 °C are shown in Figure 1. Here the PbSe domains at the tips grew at the expense of the CdSe domains, a process driven by the evaporation of Cd (or Cd molecular complexes), as confirmed by energy-dispersive X-ray spectroscopy (EDX) chemical mapping. It became clear that cation exchange is a very dynamic process whereby excess Pb cations were consumed from Pb-oleate complexes that were covering the surface of the NCs.

Finally, very recently a force-field molecular dynamics (FF-MD) simulation study was conducted which elucidates the role of both cation and anions in the underlying physical mechanism [3,4]. Figure 2 shows the result of a simulation whereby two types of interfaces are present in the nanorods, exhibiting very different defectiveness of the atomic planes at both sides of the interfaces.

Future opportunities for studying cation exchange, including Liquid Cell investigations and performing cation exchange of 2D of 3D assemblies of nanocrystals, are discussed as well.

References

[1] M. Casavola, M.A. van Huis, S. Bals, K. Lambert, Z. Hens, D. Vanmaekelbergh, Chemistry of Materials 2012, 24, 294–302.

[2] S. Bals, M. Casavola, M.A. van Huis, S. Van Aert, K.J. Batenburg, G. Van Tendeloo, D. Vanmaekelbergh, Nano Letters 2011, 11, 3420-3424.

[3] A.O. Yalcin, Z. Fan, B. Goris, W.-F. Li, R.S. Koster, C.-M. Fang, A. Van Blaaderen, M. Casavola, F.D. Tichelaar, S. Bals, G. Van Tendeloo, T.J.H. Vlugt, D. Vanmaekelbergh, H.W. Zandbergen, M.A. van Huis, Nano Letters 2014, 14, 3661-3667.

[4] Z. Fan, L.-C. Lin, W. Buijs, T.J.H. Vlugt, M.A. van Huis, accepted for publication.

Anil YALCIN, Bart GORIS, Zaochuan FAN, Thijs VLUGT, Alfons VAN BLAADEREN, Daniel VANMAEKELBERGH, Sara BALS, Marijn VAN HUIS (Utrecht, The Netherlands)
16:00 - 16:15 #6673 - MS00-OP194 Combined characterization of cobalt aggregates by HAADF-STEM electron tomography and Anomalous X-ray scattering.
Combined characterization of cobalt aggregates by HAADF-STEM electron tomography and Anomalous X-ray scattering.

Fischer-Tropsch synthesis (FTS) is a well-known process, which permits to produce clean fuels from a synthetic gas (CO+H2). The syngas can be obtained from natural gas or by gaseification of lignocellulosic biomass, providing an interesting environment-friendly alternative to oil. It is generally admitted that the catalytic properties (activity and selectivity) of the cobalt-based catalysts used for the FTS depend on the size distribution of the cobalt nanoparticles [1]. However, in high-loaded catalysts, nanoparticles tend to form large aggregates, modifying the quantity of accessible active sites. Their impact of the catalytic properties is still uncertain.

The aim of this study is thus to lead to a detailed description of the multi-scale structure of Cobalt-based catalysts. A multi-technique approach is proposed combining transmission electron microscopy in different modes (dark field, HAADF-STEM and electron tomography) and Anomalous Small-Angle X-Ray Scattering (ASAXS). The project focuses on two Co-based catalysts, supported on alumina and containing 15%wt Co. After impregnation, the catalyst was dried and calcined in air. The first sample is the oxide catalyst. The second one is obtained after reduction under pure H2 flow.

BF-TEM, DF-TEM and HAADF-STEM were used to characterize both the Co crystallites size and the aggregates size distribution. Electron tomography acquisitions were performed in HAADF-STEM mode [4] in order to obtain a 3D visualization of the Co aggregates. Quantitative analysis of the segmented aggregates was performed.

The ASAXS technique is yet little used for the characterization of the active phases of catalysts [2, 3]. Hence, ASAXS experiments have been performed on SWING Beamline of SOLEIL Synchrotron, slightly below the Co K-edge at 3 different energies. The exploitation of the ASAXS data was successfully performed by means of using X-Ray scattering theory, and allowed calculating the size distribution of the spherical nanoparticles and aggregates.

First, the complementarity between ASAXS and electron tomography techniques has been demonstrated: they bring new insights on the multi-scale structure of Cobalt-based catalysts.

The ASAXS technique has permitted to characterize simultaneously the size distribution of the nanoparticles and the aggregates, from a few angstrom scale to a scale of several hundreds of nanometers (Figure 1). Size distribution of the aggregates are in good agreement with HAADF-STEM results (figure 3).

Secondly, the advantage of the electron tomography has been highlighted. Electron tomography gives access to a unique 3D visualization of the aggregates (see figure 2). It has revealed that the morphology of the aggregate is changing during reduction: in the oxide state, they are compact, even if internal porosity is observed, whereas in the reduced state, they present a porous sponge-like structure, more or less compact. Calculation of porous volume, pore size and mean size of walls (nanoparticles forming the aggregates) can also be carried out, for a quantitative comparison.

Even if the sintering of the particles is often evoked during the reduction step, it appears that another phenomenon occurs at the aggregate scale : ASAXS and electron tomography show that, contrary to their size, the internal density of the aggregates does evolve. Indeed, the aggregates of the reduced sample are airier with a larger porosity. Is it due to the change of the crystallographic structure from Co3O4 to Co0 or to internal stresses that break and open the aggregate structure ?

Aknowledgments :

Electron tomography acquisitions were partially performed at « Centre Technologique des Microstructures de l’Université Lyon I». The authors thank X. Jaurand for his support.

References :

[1] G.L. Bezemer, et al., Journal of the American Chemical Society, 128 (2006).

[2] H. Brumberger, et al., Journal of Applied Crystallography, 38 (2005).

[3] P. Georgopoulos, J.B. Cohen, Journal of Catalysis, 92 (1985).

[4] I. Arslan, et al., Journal of the American Chemical Society, 130 (2008).

[5] P. Munnik, P.E. de Jongh, K.P. de Jong, Journal of the American Chemical Society, 136 (2014).

[6] P. Munnik, et al., ACS Catalysis, 4 (2014).

Anne-Sophie GAY (Solaize), Séverine HUMBERT, Anne-Lise TALEB, Véronique LEFEBVRE, Charline DALVERNY, Guillaume DESJOUIS, Thomas BIZIEN
16:15 - 16:30 #4672 - MS00-OP179 Formation of bimetallic clusters in superfluid helium nanodroplets analysed by atomic resolution electron tomography.
Formation of bimetallic clusters in superfluid helium nanodroplets analysed by atomic resolution electron tomography.

Metallic nanoparticles consisting of a few thousand atoms are of large interest for potential applications in different fields such as optics, catalysis or magnetism. Structure, shape and composition are the basic parameters responsible for their properties. To reveal these parameters in three dimensions at the nanoscale, electron tomography is a powerful tool. Advancing electron tomography to atomic resolution in an aberration-corrected transmission electron microscope is challenging though [1–3], and the ultimate goal of resolving position and type of each single atom inside a material remains elusive.

Here we demonstrate atomic resolution electron tomography on silver/gold core/shell nanoclusters grown in superfluid helium nanodroplets [4,5] (Figure 1a). Superfluid helium droplets represent a versatile, novel tool for designing such nanoparticles, allowing fine-tuned synthesis of pure or composite clusters for a wide range of materials. Using ultra-high vacuum conditions and getting on without solvents or additives compared with chemical synthesis, the method delivers high purity materials, which can be well controlled in terms of size and composition.

Analytical TEM investigations reveal smaller clusters mainly consisting of a single silver core, surrounded by a gold shell, whereas larger clusters contain two or more silver grains embedded in a gold matrix [6] (Figure 1b&c). The measured transition between single- and double-core growth appears at a cluster size of about 5000 atoms and similar values are found when cluster agglomeration inside the He-droplet is simulated for the used process parameters [7] (Figure 1d).

One cluster with two silver cores was analysed three-dimensionally for its atomic structure, shape and composition [6] (Figure 2). We identify gold- and silver-rich regions in three dimensions and we are able to estimate atomic positions inside the nanocluster. Two silver cores are visible as darker regions, separated by gold with a minimal thickness of 2–3 atomic layers.  The cluster appears in a highly symmetric multiply twinned structure, shaped roughly as an icosahedron, which is structurally modified due to binding of the cluster to the surface.

This work demonstrates estimation of atomic positions within nanoparticles in 3D without any prior information, while at the same time information about the local elemental composition is retrieved at near-atomic resolution. Our results give insight into the growth and deposition process of composite nanoclusters created in superfluid helium droplets. This understanding will allow fine-tuning of process parameters for optimizing nanoparticle properties.

 

[1]          M.C. Scott et al., Nature. 483: 444–447 (2012).

[2]          B. Goris et al., Nat. Mater. 11: 930–935 (2012).

[3]          C.-C. Chen et al., Nature. 496: 74–77 (2013).

[4]          A. Boatwright et al., Faraday Discuss. 162: 113–124 (2013).

[5]          P. Thaler et al., Phys. Rev. B. 90: 155442 (2014).

[6]          G. Haberfehlner  et al., Nat. Commun. 6: 8779 (2015).

[7]          P. Thaler et al., J. Chem. Phys. 143: 134201 (2015).

 

This research has received funding from the European Union within the 7th Framework Program (FP7/2007-2013) under Grant Agreement no. 312483 (ESTEEM2) and from the European Commission and the Styrian Government within the ERDF program (FP7/2007-2013). We gratefully acknowledge support from NAWI Graz.

Georg HABERFEHLNER (Graz, Austria), Philipp THALER, Daniel KNEZ, Alexander VOLK, Ferdinand HOFER, Wolfgang E. ERNST, Gerald KOTHLEITNER
Salle Gratte Ciel 1&2

Wednesday 31 August

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IM6-I
14:15 - 16:30

IM6: Quantitative Diffraction
SLOT I

Chairmen: Tatiana GORELIK (Mainz, Germany), Damien JACOB (UMET, Lille, France)
14:15 - 14:45 #8327 - IM06-S46 Nanoscale Crystal Cartography using Scanning Electron Diffraction.
Nanoscale Crystal Cartography using Scanning Electron Diffraction.

Electron diffraction, and its use to study the nanoscale crystallography of materials, has had something of a renaissance in recent years. The ease with which electron diffraction patterns can now be acquired with modern instrumentation, coupled with developments in electron optical techniques, faster detectors and sensitive cameras have all contributed to microscopists looking again at what novel information electron diffraction can provide. In this presentation, we will focus on scanning electron diffraction (SED), whereby electron diffraction patterns are acquired at each real space pixel, so that following a raster scan across a region of interest, a rich 4D data set is obtained that contains a wealth of information about the phases present, their orientation, defective regions and strain. Automation of pattern acquisition and sensitive recording devices also enable very fast data collection and open new avenues for recording unique crystallographic data from highly beam-sensitive materials [1]. SED can also be applied whilst precessing the beam [2] and this combination, known, in general, as scanning precession electron diffraction (SPED), which may lead to more accurate measurements of local crystallography through access to higher order reflections and greater sensitivity to in-plane rotations, strain, etc.

 

Figure 1 shows an example of SED analysis, with and without precession, considering the effects of strain brought about by the addition of Sb in a GaAs nanowire, whose wire axis is parallel to the axis in the zinc blende structure ([0001] in wurtzite). Fig 1 (a) shows a series of ‘virtual dark field’ (VDF) images created by plotting the intensities of particular reflections, from an unprecessed pattern series, as a function of real space position. The contrast seen in the VDF resembles the contrast seen in conventional DF images showing changes in intensity brought about through planar bending in the doped region. In Fig 1(b) the strain components are plotted, derived from measuring the distortions of precessed diffraction patterns relative to a reference (undistorted) region well away from the Sb region. The magnitude and direction of the strain is as expected from finite element modelling.

 

In reality, any 2D crystallographic map such as those shown in Figure 1, is really a projection of a 3D structure. As such, in order to be sensitive to changes in strain and orientation in 3 orthogonal directions, it becomes necessary to tilt the sample and ultimately to record a full tilt series of crystallographic information so that, using tomographic methods, a complete 3D reconstruction of the local crystallography is made possible [3]. Recently, we have combined SPED and tomography to reconstruct both real space morphology and orientation maps in three dimensions, to interrogate the local crystallography of sub-volumes of material and to determine, for example in the case of a Ni-base superalloy, a novel 3D orientation relationship between a matrix phase and an embedded carbide particle, see Figure 2. To extend this to map strain, as illustrated in Fig 1 (b), into 3D is considerably more challenging as such ‘tensor tomography’ demands a new approach to recover, in general, six strain components at every voxel. We will discuss progress in this regard and show how a model-fitting approach may be the best route forward [4].

 

[1] E.F. Rauch and L. Dupuy, Arch. Metall. Mater. 50, 87–99 (2005).

[2] P.A. Midgley and R. Vincent, Ultramicroscopy, 53, 271–282 (1994).

[3] A.S. Eggeman, R. Krakow and P.A. Midgley, Nature Commun. 6, 7267 (2015).

[4] The authors acknowledge the many people who have contributed to this work including R. Leary. R. Krakow, J. Barnard, J.M. Thomas, A. van Helvoort, Z. Saghi, M. Benning. C. Schoenlieb, J. Ma, O. Messe, C. Rae, A. Knowles, H. Stone, A. Ferrari, S. Hodge, U. Sassi, D. de Fazio. 

 

The authors acknowledge funding from the Royal Society and from the ERC grant 291522-3DIMAGE. DJ acknowledges an an Associateship from the Cambridge NanoDTC.

Paul MIDGLEY (Cambridge, United Kingdom), Duncan JOHNSTONE, Sung-Jin KANG, Alex EGGEMAN
Invited
14:45 - 15:15 Electron ptychography: what we can do with 'redundant' data. John RODENBURG (Sheffield, United Kingdom)
Invited
15:15 - 15:30 #6385 - IM06-OP120 Deformation mapping in a TEM: Dark Field Electron Holography, Nanobeam Electron Diffraction, Precession Electron Diffraction and GPA compared.
Deformation mapping in a TEM: Dark Field Electron Holography, Nanobeam Electron Diffraction, Precession Electron Diffraction and GPA compared.

The properties of nanoscaled materials can be changed by applying strain and as such there is an interest in the accurate measurement of deformation with nm-scale resolution. This was until recently considered as a difficult problem. However, the last ten years has seen a great deal of development in techniques that can be used to measure deformation with the required resolution [1,2]. Today there are many different approaches which can be used to recover valuable information about the deformation. Each of these techniques has strengths and weaknesses and requires different set ups in the electron microscope [2]. In this presentation we will present dark field electron holography, nanobeam electron diffraction (NBED), precession diffraction (NPED) and the geometrical phase analysis (GPA) of TEM and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images. We will then discuss which technique is most suitable for different types of materials problems and benchmark their performance with respect to the accuracy of the measurements, precision and spatial resolution.

Figure 1(a) shows a HAADF STEM image of a 100-nm-thick Si test specimen containing 10-nm-thick SiGe layers with different Ge concentrations. As the specimen has been grown epitaxially we expect no deformation to be measured in the ex direction. However, due to the expanded lattice parameter of the SiGe layers relative to the Si reference, tensile deformation is expected in the ez direction. Figure 1 shows deformation maps that have been acquired by (b) GPA of HAADF images (c) dark holography and (d) precession diffraction. These are compared to finite element simulations that are shown in Figure 1(e). These results reveal that all of the different techniques provide accurate measurements of the deformation. Figure 2(a) shows a HAADF STEM image of a SiGe test device structure with a gate length of 35 nm. Finite element simulations showing the expected deformation in the struture is shown in Figures 2(b) and (c). Deformation maps are shown in Figure 2(d) and (e) for precession diffraction, (f) and (g) for dark holography and (h) and (i) for GPA of HAADF STEM images [4]. Again, accurate measurements of the deformation are made, but the precision and spatial resolution depends on the experimental technique that has been used. As well as presenting state of the art results from a range of strained materials we will highlight improvements that are required for all of the different techniques in order to optimise their performance and provide the best possible measurements of deformation.

Acknowledgements : These experiments have been performed on the platform nanocharacterisation (PFNC) at Minatec. The work has been funded by the ERC Starting Grant 306365 « Holoview ».


References
[1] M. Hytch et al. Ultramicroscopy 74, 131–146 (1998)
[2] M. Hytch et al. Nature 453, 1086-1089 (2008)
[3] D. Cooper et al. Micron 80, 145-165 (2016)
[4] D. Cooper et al. Nano letters 15, 5289-5294 (2015)

David COOPER (Grenobles), Nicolas BERNIER, Jean-Luc ROUVIERE
15:30 - 15:45 #6564 - IM06-OP121 The use of Correlation Coefficient maps to enhance visibility of internal structure for nanocrystalline thin foils.
The use of Correlation Coefficient maps to enhance visibility of internal structure for nanocrystalline thin foils.

Orientation and phase are routinely determined with the automated ACOM tool developed for Transmission Electron Microscopes [1]. With this attachment, the beam is scanned over the area of interest and all the diffraction patterns are collected and kept in memory for further off-line analysis. The present work concerns a novel approach that makes use of the memorized data to compute a structural image of the sample through a straightforward post-processing algorithm that consists in weighting the similarities between the neighbor diffraction patterns [2].

 

The successive diffraction patterns acquired within a given crystal are anticipated to be nearly identical, while an abrupt change is expected when the beam is crossing a grain or a phase boundary. Plotting the value of a correlation coefficient that compares the intensities of every pixels of the neighbor diffraction patterns produces a contrasted picture in which all structural features that modify the local diffracting conditions are highlighted.

 

Fig. 1 gives a typical example where grain boundaries for a polycrystalline sample are retrieved. Of particular interest is the fact that the grain boundary contrasts are directly related to the boundary inclination. Indeed, the sharp changes in diffraction patterns expected for boundaries parallel to the electron beam are associated to a strong contrast. A weak and extended contrast indicates qualitatively a gradual modification of the diffracting signal as expected for inclined boundaries. Quantitative evaluation needs different processing [3].

 

The correlation coefficient is sensitive to any structural component that modifies the diffracting conditions. This is valid for dislocations, too. Two of them appear in the upper grain in figure 1.

Moreover, the correlation coefficient is less sensitive to non-visibility conditions. This is because it is constructed on the difference between the intensities of all reflections including the faint ones. In particular, if the sample is in a so-called two beam condition, the main reflection g remains unchanged when the beam is crossing a dislocation line whose Burgers vector is normal to g. This reflection will be dominant in the bright field image and the dislocation will not be visible. Being constant, g will not contribute to the correlation coefficient. By contrast faint reflections that always exist in the diffraction pattern - even in two beam conditions - will be affected by the distortion around the defect line. Figure 2 compares the virtual bright-field image and the correlation coefficient map for a deformed ferritic steel sample. The thin foil is slightly bended (less than 2°, mainly from left to right) so that the contrast conditions are not homogeneous in the micrograph and part of the structural information is missing. The correlation coefficient is less sensitive to the exact illumination conditions and consequently additional dislocations appear in the map.

 

 

[1] E.F. Rauch, M. Véron, Mater. Charact. 98 (2014) 1–9.

[2] Á.K. Kiss, E.F. Rauch, J.L. Lábár, Ultramicroscopy 163 (2016) 31–37.

[3] Á K. Kiss E F. Rauch B Pécz J Szívós and J L. Lábár, Microsc Microanal 21(2015) 422–435

Edgar RAUCH (ST MARTIN D'HERES CEDEX), Akos KISS, Janos LABAR
15:45 - 16:00 #6060 - IM06-OP118 Electron diffractive imaging using fork-shaped grating masks.
Electron diffractive imaging using fork-shaped grating masks.

An electron vortex beam is propagating electron which carries an orbital angular momentum (OAM) [1]. Because an electron has an electric charge and a mass, electron vortex beam is considered to have a magnetic moment and a moment of force, with which imaging of magnetic materials and manipulation of nano-sized objects are expected [2,3].

Electron vortex beams are generated using amplitude holograms such as binary masks of forked gratings [2] and spiral zone plates [4], and using phase holograms of forked gratings as their diffracted waves when the holograms are illuminated by a plane wave [2]. Recently, spiral phase plates with a smooth and continuous variation of the thickness of Si3N4 membranes were successfully prepared by nano-fabrication techniques, which were not realized in the first report by Uchida and Tonomura [1].

In the present study, the forked grating masks are used as a selected area aperture for electron diffractive imaging, which imposes a real space constraint. Diffractive imaging is lens less imaging for reconstructing the wave field from a diffraction pattern. Diffractive imaging requires a constraint in real space, or “support region”, which corresponds to a beam illuminated area of the specimen. The support region has so far been restricted by a narrow beam itself or a single circular hole inserted at the first image plane of microscopes. In the present study, the support region is restricted by a forked grating. Forked gratings provide not only a transmitted peak but also discrete Bragg peaks which contain rich information of the object located in the support region, and thus one can expect more reliable phase retrieval than the case using a hole mask.

The patterns of the forked gratings with a 5 μm diameter were designed by computer hologram whose Burgers vector of b = 1. The forked gratings were fabricated from 1μm thick PtPd films deposited on 50nm thick Si3N4 membranes by using a focused ion beam (FIB) instrument. The forked gratings were introduced into a position of selected-are aperture of a transmission electron microscope. Diffraction patterns were taken by a Gatan imaging fileter with a 16 bit 2k x 2k CCD camera.

Figures 1(a) and 1(b) show a TEM image of the forked grating and its diffraction pattern, respectively. Figures 1(c), 1(d), 1(e) and 1(f) show retrieved amplitudes of image and diffraction pattern, and retrieved phases of image and diffraction patterns, respectively. The retrieved diffraction amplitude (Fig. 1(d)) shows a good agreement with the experimental amplitude (Fig.1(b)). A flat phase in the retrieved phase of the image (Fig.1(e)), which is consistent to the plane wave incidence, is well reproduced. A deformation of the wavefront of incident electron beam by a quadruple magnetic field produced by a stigmator of the intermediate lens of the microscope is also successfully visualized by the present method.

A wavefront deformed by a specimen is also reconstructed. We used Au nano-plates as test specimens. Figures 2(a) and 2(b) show an experimental TEM image and diffraction pattern of a Au nano-plate. Figures 2(c), 2(d), 2(e) and 2(f) show retrieved amplitudes of the image and diffraction pattern, and retrieved phase of the image and diffraction pattern, respectively. The retrieved diffraction amplitude (Fig. 2(d)) shows a good agreement with the experimental amplitude (Fig. 2(b)). The thickness of the Au nano-plate is obtained from the phase map (Fig. 2(e)), which is consistent to the thickness determined by electron holography. We discuss how the phase retrieval using a forked grating mask is effective by comparing with the cases using a regular grating and a single circular hole.

Saitoh KOH (Nagoya, Japan), Nambu HIROKI, Uchida MASAYA
16:00 - 16:15 #5912 - IM06-OP116 Inversion of dynamical scattering from large-angle rocking-beam electron diffraction patterns.
Inversion of dynamical scattering from large-angle rocking-beam electron diffraction patterns.

A method for ab-initio structure factor retrieval from large-angle rocking-beam electron diffraction (LARBED)[1] data of thin crystals is described and tested.
This method determines crystal structure factors and specimen thickness from the intensities of the diffraction spots alone, solving a nonlinear least-squares problem. No additional information, such as atomicity or information about chemical composition, have been made use of.

In addition to a demonstration of the method on 120 keV experimental data of SrTiO3, where 456 structure factors have been retrieved, we analyze the dependence of the success of the reconstruction on specimen thickness and tilt range by applying this algorithm to simulated data. Our numerical experiments show that the dynamical inversion by gradient optimization works best if the beam tilt range is large and the specimen not too thick. At specimen thicknesses which allow for moderate multiple scattering, the large tilt amplitude effectively removes local minima in this global optimization problem, making ab-initio structure factor retrieval possible.

In addition, a dynamic parallelism framework  based on the Compute Unified Device Architecture (CUDA) is introduced, reducing the runtime of the optimization routine by two orders of magnitude when compared to running on a CPU.

The authors acknowledge funding by the Carl Zeiss Foundation as well as the German Research Foundation (Grant no. KO 2911/7-1 and SFB 951).

[1] Koch, C.T., 2011. Aberration-compensated large-angle rocking-beam electron diffraction. Ultramicroscopy, 111, 828–840. doi:10.1016/j.ultramic.2010.12.014

Feng WANG (Berlin, Germany), Robert PENNINGTON, Christoph KOCH
16:15 - 16:30 #5947 - IM06-OP117 Optimization of NBED simulations to accurately predict disc-detection measurements.
Optimization of NBED simulations to accurately predict disc-detection measurements.

Scanning TEM (STEM) in combination with a small convergence angle (nano-beam electron diffraction, NBED) can be used, e.g., to determine the local strain state of a sample by measuring the distance between two non-overlapping Bragg discs in the diffraction pattern [1]. Automated disc-detection algorithms [2] reveal a high precision which allows for measuring small shifts of diffracted discs relative to the undiffracted disc to determine local strain with a spatial resolution below 1 nm. Simulation of NBED patterns is an important tool to optimize the precision of an experimental measurement as well as to predict the resolution of measured strain profiles at interfaces [3]. In this context, it is important to optimize such simulations to describe experimental measurements as accurately as possible. The present study shows by a comparison of experiment and simulation that for both types of simulations, the absorptive-potential and the frozen-lattice method, deviations to experiment occur if additional effects, such as electron noise or the influence of the modulation-transfer function (MTF) of the CCD camera, are not taken into account.

Fig 1(a) shows the undiffracted 000 disc of an experimental tilt series in pure silicon which was compared with simulations (Fig 1(b) shows the raw absorptive-potential simulations). The selected experimental disc in Fig 2(a) shows smooth edges in contrast to the raw simulation (b) with much sharper edges. Fig 2(c) shows the simulated disc after adding background and applying MTF and noise. It is shown that results for 000-disc detection of the modified simulation agree sufficiently well with experiment for all tilts giving a reliable tool to investigate the effect of sample conditions on NBED-disc detection.

With these optimized simulations, we separately investigated three effects on the 000-disc detection procedure: the effect of (1) scanning into a strained layer (without composition change), (2) scanning into a layer of different material (but without changes in the crystal structure), and (3) the effect of varying specimen thickness.

The latter effect occurs in experiment, e.g., in case of selective layer etching. A GaAs supercell was set up with [001] electron beam direction with two plateaus on the top and on the bottom surface as shown in the schematic illustration in Fig. 3(a). Different NBED disc-detection algorithms [2] were used to detect the deviation of the 000-disc position from the initial center for a scan along the [100] direction across the plateaus. Fig. 3(b) shows the deviations in μrad where each line stands for a certain detection algorithm (Selective Edge Detection [2] and Radial Gradient Maximization [2], both with and without radius fitting) - in all cases the slopes on the surface lead to a measured deviation of the disc position of up to 30-40 μrad. This reveals a consequence for experimental measurements: On the one hand, if non-homogeneous surface topology is an unwanted factor, it can lead to artifacts in disc-position determination. On the other hand, it also might offer the possibility for precise measurements of topology or sample orientation.

[1] F Uesugi et al., Ultramicroscopy 111(2011), p. 995-998

[2] K Müller et al. Microsc. Microanal. 18 (2012), p. 995-1009

[3] C Mahr et al., Ultramicroscopy 158 (2015), p. 38-48

[4] This work was supported by the German Research Foundation (DFG) under contract number RO 2057/11-1 and MU 3660/1-1.

Tim GRIEB (Bremen, Germany), Florian Fritz KRAUSE, Christoph MAHR, Knut MÜLLER-CASPARY, Dennis ZILLMANN, Marco SCHOWALTER, Andreas ROSENAUER
Salle Tête d'or 1&2

Wednesday 31 August

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LS2-II
14:15 - 16:30

LS2: Cell organisation and dynamics
SLOT II

Chairmen: Isabelle ARNAL (Grenoble, France), Chris HAWES (Oxford, United Kingdom), Eija JOKITALO (University of Helsinki, Finland)
14:15 - 14:45 #8262 - LS02-S08 Dissecting the neuronal microtubule cytoskeleton using optical nanoscopy.
Dissecting the neuronal microtubule cytoskeleton using optical nanoscopy.

Cellular organization depends on the cytoskeleton, a mechanical network of biopolymers that controls cell shape and strength, as well as on motor proteins that can move over these biopolymers to deliver cargo to specific subcellular compartments. Nevertheless, the precise mechanisms that control cytoskeletal organization, the function and dynamics of different motor proteins, and the precise functions of subcellular positioning are still poorly understood. In my lecture, I will highlight novel probes and methodology for the super-resolution imaging of the cytoskeleton. These approaches allow us to better resolve cytoskeletal organization in dense cellular compartments, such as the axons and dendrites of neurons. These technologies hold great promises for exploring cellular organization in health and disease.

Lukas KAPITEIN (Utrecht, The Netherlands)
Invited
14:45 - 15:15 #8650 - LS02-S09 Ultrastructural analysis of spindle architecture in C. elegans.
Ultrastructural analysis of spindle architecture in C. elegans.

Mitosis and meiosis are essential mechanisms for any eukaryotic cell in order to divide and propagate. During both processes, microtubules are reorganized to assemble into bipolar spindles and align the condensed DNA in preparation for chromosome segregation. While the dynamic aspects of spindle assembly have been intensively studied by light microscopy, detailed ultrastructural analyses of spindle organization are largely missing. For an in depth understanding of both mitotic and meiotic spindle architecture, a quantitative 3-D analysis of spindles is urgently needed. Using the early C. elegans embryo as a model system, the aim of our work is to better understand the functional organization of bipolar spindles. We apply dual-axis, serial-section electron tomography to quantitative analyze spindle parameters. This approach takes advantage of imaging embryos at about 5-8 nm resolution and a genetically amenable model system, in which background structure and a genome-wide screen for factors involved in cell division has been completed.

Thomas MÜLLER-REICHERT (dresden, Germany)
Invited
15:15 - 15:30 #6888 - LS02-OP016 Local order and conformation of nucleosomes in solution and in interphase nuclei analysed by cryo-electron microscopy of vitreous sections.
Local order and conformation of nucleosomes in solution and in interphase nuclei analysed by cryo-electron microscopy of vitreous sections.

Despite spectacular advances in our knowledge of chromosome dynamics and large scale organisation, little is still known about how the 10 nm nucleosome bead-on-string fibre is packed in the cell nucleus. While a folding into 30 nm fibres has long been assumed, there is now accumulating evidence that no such structure exists in situ (1), except in the cases of chicken erythrocytes and echinoderm sperm cells (2). Our knowledge of the nucleosome structure itself is still limited to crystallographic structures obtained with engineered particles (3). There is also increasing evidence that there would be no unique nucleosome structure but a whole dynamic family with variations of the composition and conformation of the particle (histone variants, post translational modifications, stoichiometry) (4).

Using cryo-electron microscopy of vitreous sections (CEMOVIS), we explore in parallel the conformation and local ordering of nucleosomes both in the interphase nuclei of human cell lines HT29 (Figure 1A) and KE37 and in concentrated solutions of isolated native particles in presence of low or high added salt concentrations (15 or 150 mM NaCl) (Figure 1B).

In both cases, individual nucleosomes can be identified. Striated patterns, indicative of the presence of piles of stacked particles are observed, but much more locally in cell nuclei (piles of 2 to 4-5 particles only). We analyse these patterns by comparison with the different projections of the DNA from the canonical crystallographic structure of the nucleosome core particle (2) to determine their orientations. On side view of the nucleosomes, we measured the distance between the DNA gyres around the particle (pitch P of the DNA superhelix). This reveals that both in vitro in solution and in situ in interphase nuclei, the nucleosome conformation is not unique, but fluctuates. In concentrated solution, both the supramolecular order (5) and the conformation depend on the ionic environment. In particular, the nucleosome is on average more “open” at low added salt concentration (with P = 30.5Å), than at high salt, where its conformation resembles that found in crystals formed by reconstituted recombinant particles (P = 28 Å).  Surprisingly, in situ, the conformation of the nucleosome is also more “open” (31Å), matching the results found in vitro at low salt concentration.

 

(1) McDowall et al (1986) EMBO J., 5, 1395-1402; Elstov et al (2008) PNAS 73, 1897-1901; Nishino et al (2012) EMBO J. 31, 1644-1653.

(2) Scheffer et al (2011), PNAS 108, 16992-1997; Woodcock (1994) J. Cell Biol. 125, 11-19.

(3) Lüger K. et al (1997) Nature, 389, 251-259 ; Davey CA. et al (2002) J. Mol. Biol. 319, 1097-1113.  

(4) Zlatanova J. et al (2009) Structure 17, 160-171; McGinty & Tan (2014) Chem Rev 115, 2255-2273 ; Ngo & Ha (2015) NAR, 43, 3964-3971.

(5) Mangenot et al (2004) J. Mol. Biol. 333, 907-913.

Amélie LEFORESTIER (Paris-Saclay), Nicolas LEMERCIER, Françoise LIVOLANT
15:30 - 15:45 #6897 - LS02-OP017 Lipidic structures in the cell nucleus – role of lamins?
Lipidic structures in the cell nucleus – role of lamins?

Spatial ordering of the cell nucleus is critical for controlling fundamental nuclear processes such as gene expression, DNA replication, and RNA processing. So far, mostly protein complexes have been found as important for this ordering. We describe novel structures containing phosphatidylinositol 4,5-bisphosphate (PIP2) which seem to contribute as well. Ultrastructural studies demonstrate the PIP2-positive structures propagating through the nucleolus and stretching into the nucleoplasm where PIP2 is enriched in interchromatin granules, and also form previously undescribed 50-100 nm roundish “lipid islets”. We show that PIP2 islets are evolutionary conserved structures enriched in carbon-rich compounds and surrounded by nucleic acids and protein-containing constituents. By means of electron microscopy and super-resolution light microscopy, we show that at the periphery of the islets, PIP2 co-localizes or is located in the immediate vicinity with nascent transcripts, lamin A, LAP2α, and markers of transcriptionally active and inactive chromatin. We demonstrate that PIP2 islets are sensitive to RNase treatment, while their disruption affects the level of transcription. Direct binding and mobility assays also revealed nucleoplasmic interactions between PIP2 and nuclear myosin 1 (NM1), which is a part of chromatin remodelling complex B-WICH and promotes Pol I and Pol II transcription. Furthermore, PIP2-dependent recruitment of lamin A into an NM1-bound lipo-protein complex was demonstrated. We analysed the lamin A/NMI/PIP2 complex in more detail and revealed some specific lamin isoforms as components of this complex. Based also on the results of microscopic localization and NMI PIP2-dependent mobility, we hypothesize that this complex participates in the formation of intranuclear domains involved in fine-tuning of gene expression patterns.

Acknowledgements: This work was supported by GACR (15-08835Y, 15-08738S, 16-03346S), TACR (TE01020118), the Human Frontier Science Program (RGP0017/2013), and by institutional grant (long-term conceptual development of the scientific organization; RVO: 68378050). We acknowledge the LM CF and EM CF of the Microscopy Centre, IMG AS CR supported by the MEYS CR (LM2015062 Czech-BioImaging)

Vlada PHILIMONENKO (Prague 4, Czech Republic), Margarita SOBOL, Lenka JAROLIMOVÁ, Zuzana LUBOVSKÁ, Jana SCHRENKOVÁ, Alžběta KALENDOVÁ, Anatoly PHILIMONENKO, Ondřej ŠMÍD, Pavel HOZÁK
15:45 - 16:00 #6635 - LS02-OP014 Visualizing translocation of pre-ribosomal particles through nuclear pore complexes by electron tomography.
Visualizing translocation of pre-ribosomal particles through nuclear pore complexes by electron tomography.

            Exchanges between the nucleus and the cytoplasm are ensured by the nuclear pore complexes (NPCs), massive multi-molecular structures that form channels through the nuclear envelope. The molecular determinants that underlie the translocation of macromolecules through NPCs have been thoroughly studied, but less is known about the topology and the dynamics of the translocation events. The precursors to the ribosomal subunits, which ensure protein synthesis in the cells, are one of the biggest and most abundant nuclear export substrates. Most steps of ribosome synthesis take place in the nucleolus, but pre-ribosomes (the precursors to the ribosomal subunits) leave the nucleolus and are exported into the cytoplasm at the end of the maturation process. In yeast Saccharomyces cerevisiae, the ribosome synthesis rate is estimated to reach 4000 subunits/minute in exponential growth phase. How cargoes of such large size and complex structure cross the barrier of the nuclear pore complex so efficiently remains poorly understood. In addition, the interactions of large cargoes with the NPC as well as their path in the central channel have been much debated and several models have been proposed, calling for experimental observations at high resolution.

            Using ultrafast high-pressure freezing, cryo-embedding and electron tomography, we could detect for the first time unlabelled large RNPs translocating through the NPC in yeast. Their size, abundance and morphological resemblance with both cytoplasmic ribosomes and nucleolar RNPs indicated that they were mainly pre-ribosomes. In support to this hypothesis, the presence of these particles in NPCs was strongly diminished upon inhibition of ribosome synthesis in an RNA polymerase I mutant strain. Over 700 NPCs were observed in the tomograms: the occupancy rate of NPCs with RNP particles was only ~5-6%, suggesting that pre-ribosome nuclear export was not limited by the number of NPCs. Although pre-ribosomes are synthesized in the nucleolus, they were detected with equal probability in NPCs located on the nucleolar and the nucleoplasmic sides of the nucleus, indicating no specialized function of either class of NPCs with respect to ribosomal export. To examine the position of the translocating particles relative to the NPC structure, we fitted a 3D reconstruction of the NPC from Dictyostellium discoideum into the tomograms (figure 1). On the nuclear side and within the core scaffold of the NPC, pre-ribosomes follow the NPC central axis, but they deviate from this position when they reach the NPC cytoplasmic ring, which suggest that they interact with the cytoplasmic nucleoporins. Finally, to get access to the dynamics of this process, we established a model of pre-ribosome nuclear export through NPCs according to a Jackson queueing network. Fitting of this model to the experimental data allowed us to approximate the translocation time of pre-ribosomes through the NPC to 70-150 ms. The rate limiting step appears to be the passage of the NPC core scaffold. These electron tomography data from ultrafast frozen samples not only deliver the first high-resolution view of the trajectory of pre-ribosomes through the NPC at nanometer scale, but also provide some dynamic parameters when combined with a probabilistic model of nuclear export.

Franck DELAVOIE, Vanessa SOLDAN, Jean-Yves DAUXOIS, Pierre-Emmanuel GLEIZES (Toulouse)
16:00 - 16:15 #5870 - LS02-OP011 Bright-field STEM tomography of thick biological sections.
Bright-field STEM tomography of thick biological sections.

By performing electron tomography in the scanning transmission electron microscope (STEM), it is possible to obtain 3D reconstructions at a resolution of around 10 nm from stained plastic-embedded sections of eukaryotic cells in 1–2 µm thick sections. This is achievable because there are no imaging lenses after the specimen when the electron microscope is operated in STEM mode, so that chromatic aberration of the objective lens does not compromise the spatial resolution when there is strong multiple inelastic scattering [1-4]. In STEM tomography it is necessary to minimize geometrical broadening of the probe by selecting a small probe convergence angle of ~1 mrad. Furthemore, by using an axial bright-field detector instead of a standard high-angle annular dark-field detector, it is possible to reduce resolution loss caused by multiple elastic scattering in thick specimens [2-4].

 

Recently, we have applied axial bright-field STEM tomography to various biological problems, including: mechanism of vesicle release in presynaptic rod bipolar cell ribbon synapses in retina [5]; ultrastructural changes in postsynaptic densities in hypocampal neuronal cultures when specific scaffolding proteins are knocked out [6]; and ultrastructure changes that occur on activation of human blood platelets, small anucleate blood cells that aggregate to seal leaks at sites of vascular injury [7].

 

Electron tomograms were acquired with an FEI Tecnai TF30 transmission electron microscope equipped with a field-emission gun and operating at an acceleration voltage of 300 kV. Specimens were prepared by conventional or freeze-substitution techniques with osmium tetroxide fixation, and sections were cut to a thickness of between 1 µm and 1.5 µm and stained with uranyl acetate and/or lead citrate, before being coated with carbon and gold nanoparticles, which served as fiducial markers. Dual-axis tilt series were acquired using a Gatan bright-field STEM detector over an angular tilt range of ±60° and for some specimens ±68° with a 2° tilt increment. Tomograms were reconstructed using the IMOD program [8] and surface rendered using FEI Amira 3D software.

 

The capabilities of bright-field STEM tomography are illustrated in 3D reconstructions of blood platelets (Figure 1), which shows specimen and detector geometry, as well as orthoslices through a 1.5 µm thick section of a plastic-embedded, frozen and freeze-substituted preparation of platelets in early stage of activation. Of particular interest are the morphological changes that occur in alpha-granules, which package important proteins that are released on platelet activation [9]. Structural changes that occur in the early stages of alpha-granule activation are not understood due to difficulties in controlling the physiological state of platelets and in visualizing membranes at the nanoscale throughout entire platelets. Visualization of the 3D structure from the STEM tomogram (Figure 2) reveals that, on early activation, tubules extend from decondensing alpha-granules and form pores with the plasma membrane, whereas other a-granules remain in their condensed unactivated state [7].  These results from blood platelets, and a range of other biological systems, demonstrate that STEM tomography can visualize large cellular structures in their entirety at a spatial resolution of around 10 nm. In the case of blood platelets, it is possible to reconstruct complex interconnected membrane systems in almost complete cells.

 

This research was supported by the intramural program of NIBIB, NIH.  The authors thank Drs. B. Storrie, I.D. Pokrovskaya, J.A. Kamyskowski, and A.A. Prince for the blood platelet specimens.

 

[1] A.E. Yakushevska et al, J. Struct. Biol. 159 (2007) p. 381.

[2] M.F. Hohmann-Marriott et al, Nature Methods 6 (2009) p. 729.

[3] A.A. Sousa et al, Ultramicroscopy 109 (2009) p. 213.

[4] A.A. Sousa et al, J. Struct. Biol. 174 (2011) p. 107.

[5] C.W. Graydon et al, J. Neurosci. 34 (2014) p. 8948.

[6] X. Chen et al, Proc. Natl. Acad. Sci. USA 112 (2015) p. E6983.

[7] I.D. Pokrovskaya et al, J. Thrombosis Haemostasis 14 (2015) p. 572.

[8] J.R. Kremer, D.N. Mastronarde, J. Struct. Biol. 116 (1996) p. 71.

[9] J. Kamykowski et al, Blood 118 (2011) p. 1370.

Richard LEAPMAN (Bethesda, USA), Qianping HE, Gina CALCO, Bryan KUO, Jake HOYNE, Guofeng ZHANG, Maria ARONOVA
16:15 - 16:30 #5574 - LS02-OP010 Advantages and pitfalls for transmission electron microscopic studies in the identification of extracellular vesicles.
Advantages and pitfalls for transmission electron microscopic studies in the identification of extracellular vesicles.

Although the story of extracellular vesicles (EVs), these cell-derived submicron structures started about 70 years ago, we had to wait for the first transmission electron microscopic (TEM) evidence of their existence till 1981.

By now it has been proved that the release of membrane vesicles is a process conserved in both prokaryotes and eukaryotes. Compelling piles of evidence support the significance of exosomes, microparticles/microvesicles or ectosomes and apoptotic bodies in a broad range of biological processes including e.g. intercellular communication, signaling processes, the transfer of genetic information, and also  in case of pathological events, from inflammation till tumor development. Not surprisingly, the possibility of EV-related biomarker and therapeutic applications made this research field attractive for clinical and pharmaceutical research, too.

However, in spite of growing interest, the classification of EVs is still under debate. The numerous protocols, applied by the different research teams, made the comparison of the results more complicated; moreover several frequently used and up-to-date particle enumeration techniques such as nanoparticle tracking analysis or tunable resistive pulse sensing are not suitable to distinguish vesicles and e.g. protein aggregates in similar size. Falsified experimental results may occur easily, especially in case of body fluid samples or culture media, in which protein aggregates, which share biophysical parameters with EVs, may contaminate vesicle preparations.

Purity and type of the fractions should be investigated with different techniques depending on the size range of the investigated vesicle population. For detection of exosomes, the frequently used flow cytometry is not the best choice, while by means of AFM, dynamic light scattering, MS techniques or microfluidic device, mentioned only few among the applied possibilities, can reveal several features of the investigated vesicle samples. However, even the most sophisticated combination of different techniques cannot be complete without transmission electron microscopy. It is the only technique which is able to visualize vesicular and non-vesicular particles in the whole size range of the vesicle population, and if we want to use EVs for research and therapy, we have to learn, how TEM can be applied for their accurate characterization.

Acknowledgements

This work was supported by grant NK 84043.

Agnes KITTEL (Budapest, Hungary), Xabier OSTEIKOETXEA, Barbara SÓDAR, Krisztina PÁLÓCZY, Tamás BARANYAI, Zoltán GIRICZ, Edit Irén BUZÁS
Salon Tête d'Or

Wednesday 31 August

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IM9-I
14:15 - 16:30

IM9: Super resolution in light microscopy
SLOT I

Chairmen: Cristina FLORS (Madrid, Spain), Suliana MANLEY (Lausanne, Switzerland)
14:15 - 14:45 #8374 - IM09-S57 Information in localisation microscopy.
Information in localisation microscopy.

Localisation microscopy is a powerful tool for imaging structures at a lengthscale of tens of nm, but its utility for live cell imaging is limited by the time it takes to acquire the data needed for a super-resolution image. The acquisition time can be cut by more than two orders of magnitude by using advanced algorithms which can analyse dense data, trading off acquisition and processing time.

Modelling the entire localisation microscopy dataset using a Hidden Markov Model allows localisation information to be extracted from extremely dense datasets. This Bayesian analysis of blinking and bleaching (3B) is able to image dynamic processes in live cells at a timescale of a few seconds, though it is very computationally intensive, requiring at least several hours of analysis. We demonstrate the performance of 3B on various live cell systems, including cardiac myocytes and podosomes, showing a resolution of tens of nm with acquisition times down to a second.

While analysing higher density images can improve the speed at which the data required for a super-resolution reconstruction is acquired, there are still limits to the speed which can be achieved. Unlike in conventional fluorescence microscopy, these limits are not just set by the properties of the microscope, but are determined by the structure of the sample itself. This is because the local sample structure affects how quickly the information necessary to create an image of a certain resolution can be transmitted through the optical system. The theoretical limits will be discussed, and the effect on live cell experiments demonstrated.

Susan COX (London, United Kingdom)
Invited
14:45 - 15:15 #8687 - IM09-S58 Robust sub-diffraction imaging using SOFI and engineered genetically-encoded labels.
Robust sub-diffraction imaging using SOFI and engineered genetically-encoded labels.

Time-lapse sub-diffraction imaging of cells labeled with genetically-encoded labels continues remains a challenging task. We have focused our efforts on developing techniques that work well under challenging conditions, combined with the semi-rational engineering of "smart" fluorescent proteins. In this presentation I will discuss our work in developing live cell sub-diffraction imaging by combining superresolution optical fluctuation imaging (SOFI) with new photochromic probes based on the enhanced green fluorescent protein, EGFP. In particular, I will discuss our recent progress in enhancing the temporal resolution of SOFI imaging while also providing reliability estimates for the obtained images [1], followed by our work on engineering and understanding improved fluorescent proteins with properties specifically tailored to sub-diffraction imaging. In particular I will discuss our recent rsGreens [2], which display robust optical performance while also expressing well in E. coli and HeLa cells. Since our rsGreens are based on EGFP, they should be widely useful in existing experiments. In the last part I will highlight ongoing work on how these developments can be applied to live cell interaction sensing.

[1] Vandenberg et al, Biomed Opt Expr 2016
[2] Duwé et al, ACS Nano 2015

Peter DEDECKER (Leuven, Belgium)
Invited
15:15 - 15:45 #8901 - IM09-S58b Super-resolution technologies to probe nuclear architecture.
Super-resolution technologies to probe nuclear architecture.

Chromosomes from bacteria to humans are organized at the sub-megabase scale into genomic domains. We will present examples of how super-resolution microscopies can be combined with new DNA labeling technologies and genome-wide chromosome capture methods to shed light into the mechanisms of formation and regulation of genome architecture. In particular, we will present our recent advances in multifocus plane microscopy for 3D single-molecule localization microcopy, two-color STORM to study genome structure, and high-thoughput 3D-SIM-based image analysis to investigate chromosome segregation.
Marcelo NOLLMANN (Montpellier)
Invited
15:45 - 16:00 #5778 - IM09-OP163 Nanoscale topography of cells and vesicles in adhesion revealed by quantitative Total Internal Reflection Fluorescence Microscopy.
Nanoscale topography of cells and vesicles in adhesion revealed by quantitative Total Internal Reflection Fluorescence Microscopy.

Total Internal Reflection Fluorescence (TIRF) microscopy is becoming a widespread technique to study cellular processes occurring near the contact region with the glass substrate [1]. The characteristics of TIRF microscopy are directly related to the singular properties of evanescent waves, such as the exponential decay of the electric field along the z direction. This providing a selective excitation of fluorescent molecules close to the interface. Determination of the accurate distance from the surface to the plasma membrane constitutes a crucial issue to investigate the physical basis of cellular adhesion process [2]. However, quantitative interpretation of TIRF pictures regarding the distance z between a labeled membrane and the substrate is not trivial. Indeed, the contrast of TIRF images depends on several parameters. The first one is obviously the distance z, which separates the dye molecules from the surface, as the emitted fluorescence signal is mainly governed by the exponential decay of the evanescent wave. But TIRF images contrast is also affected by unknown parameters such as the local concentration of dyes, their dipole moment orientation and consequences on their related angular emission pattern (which influences the detection efficiency ηd), and also their absorption cross section (σabs) and their fluorescence lifetime (τ). Moreover, these last three parameters (ηd, σabs, τ) can be strongly altered as a function of z near the surface [3].

To get around this problem, we propose two strategies allowing us to map the membrane-substrate separation distance with a nanometric resolution (typically 10 nm). The first one is dedicated to study the adhesion of Giant Unilamellar Vesicles (GUVs), which are often used as a biomimetic system to reproduce cells spreading. This approach, called normalized TIRF, is based on dual observation, which combine epi-fluorescence microscopy and TIRF microscopy: TIRF images are normalized by epi-fluorescence ones [4, 5]. Figure 1 shows an example of a negatively charged GUV completely spread on a thin layer of a positively charged polyelectrolyte (PDDA) recovering the coverslip. The second technique is devoted to explore the adhesion of living cells. This last is called variable-angle TIRF (vaTIRF) microscopy. vaTIRF is an old technique introduced in the middle of 80s and quickly forgot due to the high complexity of the first experimental setup used. We propose an improved straightforward version of vaTIRF microscopy adapted to modern TIRF setup. This technique involves the recording of a stack of several TIRF images, by gradually increasing the incident angle θ on the sample. We developed a comprehensive theory to extract the membrane/substrate separation distance from fluorescently labeled cell membranes [6], as illustrated in figure 2 for a MDA-MB-231 cell in adhesion on fibronectin. Finally, we demonstrate that these two techniques (nTIRF and vaTIRF) are useful to quantify the adhesion of vesicles and cells from weak to strong membrane-surface interactions, achieved on various functionalized substrates with polymers or proteins, such as collagen and fibronectin.

[1] D. Axelrod, Total Internal Reflection Fluorescence Microscopy, Meth. in Cell Biol. 89, 169-221 (2008).

[2] E. Sackmann and A-S. Smith, Physics of cell adhesion: some lessons from cell-mimetic systems, Soft Matter 10, 1644-1659 (2014).

[3] J. Mertz, Radiative absorption, fluorescence, and scattering of a classical dipole near a lossless interface: a unified description, J. Opt. Soc. Am. B 17, 1906-1913 (2000).

[4] M. Cardoso Dos Santos, R. Déturche, C. Vézy and R. Jaffiol, Axial nanoscale localization by normalized total internal reflection fluorescence microscopy, Opt. Lett. 39, 869-872 (2014).

[5] M. Cardoso Dos Santos, C. Vézy and R. Jaffiol, Nanoscale characterization of vesicle adhesion by normalized total internal reflection fluorescence microscopy, accepted in BBA Biomembranes (2016).

[6] M. Cardoso Dos Santos, R. Déturche, C. Vézy and R. Jaffiol, Nanoscale topography of cells in adhesion revealed by variable-angle total internal reflection fluorescence microscopy, submitted in Biophys. J. (2016).

Marcelina CARDOSO DOS SANTOS, Cyrille VÉZY, Rodolphe JAFFIOL (TROYES CEDEX)
16:00 - 16:15 #6038 - IM09-OP164 Turning Off Photoinduced Electron Transfer In Green Fluorescent Proteins For Super-Resolution Microscopy.
Turning Off Photoinduced Electron Transfer In Green Fluorescent Proteins For Super-Resolution Microscopy.

Under blue light irradiation green FPs can act as light-induced electron donors in photochemical reactions with various electron acceptors. These reactions are accompanied by green-to-red photoconversion (oxidative redding) which is among the processes significantly contributing to the photostability of many GFP family proteins. We suppose that inhibition of redding will have similar consequences in super-resolution microscopy (PALM) as it does in epifluorescent microscopy, namely, enhanced photostability and average fluorophore brightness. Understanding of the primary mechanisms of electron transfer (ET) occurring in excited state allows to control the photostability. In our recent work [1] we consider two mechanistic hypotheses:

1) ET proceeds via direct ET to a surface-docked oxidant (tunnel mechanism)

2) Photoinduced ET proceeds via a hopping mechanism (in which the first step is ET from the chromophore to some intermediate acceptor, and the second step is ET from this acceptor to an outside oxidant molecule)

Hopping mechanism appears to be the most likely process according to the quantum-mechanical calculations. One of the approaches to reduce ET via hopping mechanism is an elimination of the potential internal electron acceptors, located in the side chain of fluorescent protein. We determined such critical residues in the polypeptide chain of PAGFP using quantum-chemical simulations, and changed them to amino acid that is not able to be an efficient electron acceptor, namely, leucine. According to both epifluorescent and TIRF-experiments’ data, this substitution leads to the highly improved photostability in comparison with the original PAGFP. It has also been shown that in TIRF-experiments PAGFP Y145L had significantly lower blinking rate than PAGFP (see Fig.1). This property could be potentially used in SPT (single particle tracking) technique, where transitions to the dark states impose serious constraints and appear to be a drawback.

[1]. Bogdanov et al., JACS, 2016, in press

Anastasia MAMONTOVA (Moscow, Russia), Alexey BOGDANOV, Alexander MISHIN, Natalia KLEMENTIEVA, Konstantin LUKYANOV
16:15 - 16:30 #6636 - IM09-OP165 Fractals in the nucleus – understanding chromatin organisation with 3D SMLM.
Fractals in the nucleus – understanding chromatin organisation with 3D SMLM.

Abstract

Chromatin presents a unique challenge in structural biology.  Its fundamental units, DNA and the nucleosome, are well-characterised at the Ångstrom level.1  Yet, the higher-order organisation of the chromatin ensemble, particularly during interphase, remains largely to be determined.  Single molecule localisation microscopy (SMLM) appears uniquely positioned to answer many remaining questions about chromatin structure – it is possible to determine structures to accuracies of better than 20 nm,2 and analysis of spatial point data has been used by some groups to determine the scaling behaviour and fractal dimension of chromatin.3  This is of particular interest when considering nuclear molecular transport and gene activation: it has been suggested that cells use the very structure of chromatin to regulate transcription and expression.4

 

Whilst imaging has been able to shed light on the general organisation of chromatin and other fibrous structures in the cell, there is a lack of methods to fully characterise their folding and looping behaviour.  To date, most SMLM analysis has focussed on clustering-based algorithms such as Ripley’s K function or more recently Bayesian clustering.5  It is clear, however, that different approaches are needed to tackle the analysis of elongated or fibrous structures within cells, and in three dimensions.

 

Here, we demonstrate the power of 3D pair correlation analysis combined with careful labelling of regions of interest in chromatin, showing that the fractal dimension of chromatin can be determined across the entire nucleus for structures 30 nm in size upwards.  This provides a platform for analysis complementary to imaging, describing differential folding behaviour across the nucleus, yielding results comparable to Hi-C and other recent methods.  In addition to applying spatial statistics to probe chromatin structure on the nanoscale, we incorporate photon statistics into the analysis.  This allows us to draw conclusions about chromatin structure with greater confidence by adding a weighting to reduce the influence of data points localised with lower precision than others.

 

Understanding how chromatin fibres fold at all stages in the cell cycle is crucial to the future development of drugs and novel treatments.  More and more, drugs target specific loci yet in many cases lack the ability to be directed to the target of interest with great accuracy.6  We hope that by understanding the organisation of different regions of the genome, drugs can be designed to be delivered to their targets in ‘smarter’ ways.

 

References

1.  Flors, C. & Earnshaw, W. C. Super-resolution fluorescence microscopy as a tool to study the nanoscale organization of chromosomes. Curr. Opin. Chem. Biol. 15, 838–44 (2011).

2.  Deschout, H. et al. Precisely and accurately localizing single emitters in fluorescence microscopy. Nat. Methods 11, 253–66 (2014).

3.  Récamier, V. et al. Single cell correlation fractal dimension of chromatin. Nucleus 5, 75–84 (2014).

4.  Bancaud, A. et al. Molecular crowding affects diffusion and binding of nuclear proteins in heterochromatin and reveals the fractal organization of chromatin. EMBO J. 28, 3785–3798 (2009).

5.  Rubin-Delanchy, P. et al. Bayesian cluster identification in single-molecule localization microscopy data. Nat. Methods (2015). doi:10.1038/nmeth.3612

6.  Rajendran, L., Knölker, H.-J. & Simons, K. Subcellular targeting strategies for drug design and delivery. Nat. Rev. Drug Discov. 9, 29–42 (2010). 

Christophe LYNCH (Reading, United Kingdom), Stanley BOTCHWAY, Stephen WEBB, Ian ROBINSON
Salle Gratte Ciel 3
16:45

Wednesday 31 August

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SSW3
16:45 - 18:45

Specific scientific workshop MIRID
Microscopy of Ion Radiation Induced Defects and changes in structure and properties of materialstereology and 3D Image Analysis in Microscopy

Moderators: Isabelle MONNET (Chercheur/ Madir Group) (CAEN CEDEX, France), Elena SUVOROVA (Moscow, Russia)
Invited speakers: Pierre-Eugene COULON (Research Ingenior) (Palaiseau, France), Jacques HERMAN O'CONNELL (Port Elisabeth, South Africa), Ritesh SACHAN (Oak Ridge, USA), Christina TRAUTMANN (Darmstadt, Germany)
Salle Tête d'or 1&2

Wednesday 31 August

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SSW4
16:45 - 18:45

Specific scientific workshop Stereology
Stereology and 3D Image Analysis in Microscopy

Moderator: Lucie KUBÍNOVÁ (Pragues, Czech Republic)
Animators: Karl-Anton DORPH-PETERSEN (Associate Professor) (Aarhus, Denmark), Alain HAZOTTE (Lorraine, France), Maxime MOREAUD (Solaize, France), Eric PIRARD (Liege, Belgium)
Salon Tête d'Or

Wednesday 31 August

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SSW2
16:45 - 18:45

specific scientific workshop MLMF
Management of Large Microscopy Facilities

Moderators: Ferdinand HOFER (Graz, Austria), Joachim MAYER (Aachen, Germany)
Salle Gratte Ciel 3