Advanced quantitative transmission electron microscopy: materials research in several dimensions

Resume : In this contribution we will discuss various ways to visualize and quantitatively measure fields with the aid of a (S)TEM. While the techniques presented have been primarily developed for the imaging of magnetic structures, they have been shown to be useful for the imaging of electric fields, too. Both types of fields impose a phase modification on the primary electron wave, used in an electron microscope. The main job is then to register the phase modification by various techniques and to interpret the result. Rather than to simply observe an existing field distribution, one may impose external stimuli to the system using dedicated specimen holders. These allow the use of temperatures ranging from 10 K to room temperature and above and the application of DC and AC external magnetic and electric fields. Using a fast camera enables the experimentator to measure the temporal evolution of specific features after an external stimulus and to retrieve information on the dynamic behaviour. We will discuss different modes of operation of the (S)TEM for field imaging and illustrate them with examples.

Resume : Magnetic skyrmions are foreseen as building blocks for novel technologies that combine logic operations and non-volatile memories [1]. Skyrmions were first observed at low temperature in B20 compounds with broken inversion symmetry [2]. Néel-type skyrmions that are stable at room temperature were then obtained in multilayers of heavy metals / ferromagnets [3]. The size of the skyrmions in these layers can be tuned (from >100 nm down to a few tens of nm) by controlling their composition [4]. Consequently, the study of these small spin arrangements requires magnetic imaging techniques that provide both high spatial resolution and quantitative information. Here, we have investigated Ir/Fe/Co/Pt multilayers [4] that have been sputtered onto electron transparent membranes using off-axis electron holography [5]. We will discuss the influence of the tilt angle on the reconstructed phase images and the evolution of the domain structure in the presence of an external magnetic field. The results will be compared with phase images calculated from atomistic simulations. [1] A. Fert et al., Nat. Rev. Mater., 2, 17031 (2017) [2] S. Mühlbauer et al., Science, 323, 5916 (2009) [3] C. Moreau-Luchaire et al., Nat. Nanotechnol., 11, 444-448 (2016) [4] A. Soumyanarayanan et al., Nat. Mater., 16, 898-904 (2017) [5] A. Kovacs, R.E. Dunin-Borkowski, Handbook of Magnetic Materials (Elsevier), 27, 59 (2018) Acknowledgments: DARPA TEE program grant MIPR# HR0011831554 ; NRF - Investigatorship Ref. No.: NRF-NRFI2015-04

Resume : Recent developments in magnetic nano-technologies call for measurement methods that allow magnetic characterization with sufficient spatial resolution. Established methods such as x-ray magnetic ciruclar dichroism, spin-polarized tunneling microscopy, electron holography or magnetic exchange force microscopy offer invaluable insights into the magnetism of materials, yet each of them has certain limitations in quantitativeness, depth sensitivity or spatial resolution. An alternative measurement technique called electron magnetic circular (or chiral) dichroism (EMCD) offers a potential to reach atomic spatial resolution while transmitting electrons through the sample and, at the same time, it offers quantitative element specific magnetic information via sum rules. EMCD went through an intense development and as of today, quantitative magnetic information has been extracted from individual atomic planes or areas smaller than one square nanometer. We will review these latest experimental achievements and discuss the challenges for wider deployment of EMCD, such as ways to circumvent the problems with low magnetic signal strength and consequently low magnetic signal to noise ratio, suitable paths for data processing and the role of simulations in designing optimized EMCD experiments.

Resume : Differential phase contrast (DPC) is one of the most promising techniques for future research with scanning transmission electron microscopy (STEM). By determination of transferred momentum on a specimen site by detecting the electron beam deflection with a segmented detector, electric and magnetic field components can be measured and quantified. State-of-the-art aberration correction with hexapole Cs-correctors enables to obtain DPC-STEM images with a resolution smaller than typical atomic distances. While high-order aberrations are quite stable, low-order aberrations like defocus and two-fold astigmatism have to be corrected frequently. To analyze whether features in DPC images are due to present aberrations or specimen structure, thickness or tilt, a comparison of obtained DPC signals and simulations is necessary. We show on the example of SrTiO3 that multi-slice simulations reveal the influence of residual lens aberrations, specimen tilt and thickness on DPC images. For increasing lens aberrations and specimen thickness the DPC signal value is reduced, leading to a wrong quantification of electric fields. Specific lens aberrations and specimen tilts show characteristic changes of features from a DPC image at perfect conditions. In combination with the dark-field image, low-order lens aberrations and specimen tilt can be corrected and thus a perfect defocus can be found.

Resume : The high spatial resolution analysis of the magnetic properties of epitaxial perovskite magnetic oxide films is important both for providing a fundamental understanding of magnetism and for the development of novel spintronic device. Epitaxial La1-xSrxMnO3 (x=0.3~0.4) films (LSMO) continue to be widely investigated due to its colossal magnetoresistance, high spin polarization and a relatively high Curie temperature of 370 K, which make LSMO a potential materials for applications on magnetic sensor and recording devices. Here, we use off-axis electron holography to measure the magnetic fields in a cross-sectional epitaxial LSMO film on a Nb-doped SrTiO3 substrate with high spatial resolution. The magnetic field measurements are performed both at room temperature and at cryogenic temperature in an aberration-corrected transmission electron microscope operated in Lorentz mode. Particular emphasis is devoted to the magnetic ordering at the interface between ferromagnetic phase and paramagnetic phase as a function of temperature and external magnetic field. Such high-resolution quantitatively mapping of magnetic fields in LSMO films promises to provide direct experimental information to better understanding not only device performance but also the correlations between local microstructure and magnetism.

Resume : In order to fully exploit structure-property relations of nanomaterials, a 3D characterisation at the atomic scale is often required. A successful approach to reach this goal is to count the number of atoms in each atomic column from 2D ADF STEM images. So-called scattering cross-sections have been introduced corresponding to the total fraction of scattered intensity attributed to each atomic column. Their high sensitivity in combination with a statistical analysis enables us to count atoms with single-atom sensitivity. These atom counts can be used to create an initial atomic model which serves as an input for an energy minimization to obtain a relaxed 3D reconstruction of a nanostructure. This method opens up new opportunities towards in-situ 3D characterisation. In order to track changes at the atomic scale, we propose the use of hidden Markov models. Scattering cross-sections resulting from a time series of images are then considered as a dynamic network enabling us to measure changes in the 3D atomic structure in terms of transition probabilities. To count atoms with single-atom sensitivity, a minimally required electron dose has been shown to be necessary, while on the other hand the risk of knock-on damage, induced by the high energy electrons, puts an upper limit on the tolerable dose. To optimise the electron dose, a statistical framework combined with physics-based modelling of the dose-dependent processes is proposed and experimentally verified. This model enables an investigator to theoretically predict, in advance of an experimental measurement, the electron dose resulting in an unambiguous quantification of nanostructures in their native state with the highest attainable precision. For beam-sensitive materials, where the optimal electron dose can be low, the images will exhibit a limited signal-to-noise ratio and additionally, they will show a very weak contrast in the presence of light elements. In order to reliably detect the presence or absence of atomic columns, a combination of model fitting and model-order selection is proposed to assign the most probable atomic structure.

Resume : High-energy electrons that are used as a probe of specimens in scanning transmission electron microscopy (STEM/TEM) exhibit a complex behavior due to multiple scattering. Among other things, understanding the dynamical effects is needed for a quantitative analysis of atomic-resolution images and spectroscopic data. Ensuing advancements in instrumentation, including pixelated electron detectors and high-energy electron optics, require that theory and computational technique developments also go in tandem. In this work, state-correlation functions are computed within the multislice approach that allow to elucidate behaviors of transversely bound states in crystals. These states play an important role as the present-day aberration-corrected STEM instruments can couple a large fraction of probe-current density into 1s-states bound to individual atomic-columns. Unlike the typical treatment of the bound states, we show that bound states are generically unstable and decay monoexponentially with specimen depth. Their attenuation is accompanied by a resonant intensity transfer to Bessel-like wavefunctions that appear as Laue rings in the far-field diffraction patterns. Behaviors of bound states are also quantified when thermal effects are included, as well as point defects in a form of substitutional atoms. Practical applications of the new framework of analysis presented in this study allows for a more accurate quantitative information retrieval about a specimen via feature detection in pixelated electron detectors, efficient utilization of electron vortex beams, and to gauge dynamical scattering effects in atomic-resolution electron energy-loss spectroscopy.

Resume : The electrostatic potential of a material determines many of its fundamental properties. Off-axis electron holography (OAEH) is an established technique that can be used to record an electron wavefunction in a transmission electron microscope (TEM) with high spatial resolution. Momentum-resolved scanning TEM (MR-STEM) is an emerging technique, which has recently been applied to the measurement of electric fields. Both techniques are sensitive to the electrostatic potential and offer prospects for the quantitative measurement of fundamental material properties with atomic spatial resolution. However, in each case the direct interpretation of the recorded signal in terms of the projected electrostatic potential is only applicable if the phase of the electron wavefunction is proportional to the electrostatic potential. For thicker specimens, dynamical diffraction and thermal diffuse scattering must be taken into account. Here, we study the impact of these effects by applying OAEH and MR-STEM to the characterization of a model SrTiO3 specimen. The different illumination setups that are required for the two techniques are found to lead to different responses of the recorded signal to dynamical diffraction. We show that the interpretation of the results in terms of electrostatic potential depends on the choice of experimental parameters, such as defocus, convergence semi-angle and objective aperture size. Finally, we discuss practical aspects of the applicability of each technique.

Resume : Intermediate-temperature solid oxide fuel cells (SOFCs) have shown excellent performance making them an important part in future energy systems. However, understanding the relationship between materials, processing and performance is challenging. Alternative design concepts like co-sintering of inert substrate supported cells offer great opportunities to study the interplay between different cell components and improve methodological aspects which can be transferred to different cell concepts. In order to correlate manufacturing parameters, potential aging mechanisms and cell performance a comprehensive understanding of the underlying 3D microstructure is necessary. Energy dispersive X-ray spectroscopy - scanning transmission electron microscopy (EDXS-STEM) was utilized to identify performance-critical regions which were 3D reconstructed using focused ion beam ? scanning electron microscopy (FIB-SEM) tomography. Within this study, we used a state-of-the-art Thermo Scientific Helios G4 FX instrument equipped with numerous detectors allowing simultaneous readout of up to four detectors. Imaging parameters and the combination of multiple detector signals were systematically investigated to improve segmentation accuracy. Pore size, tortuosity and distribution of secondary phases were extracted from 3D reconstructions to model the impact on cell performance. [1] We acknowledge funding by the German Ministry for Economic Affairs and Energy (BMWi)

Resume : Y-doped Ba(Zr,Ce)O3 is a promising electrolyte material for protonic ceramic fuel cells1. We studied the atomic-scale composition and structure of both the bulk and grain boundaries (GBs) of Ba1.015Zr0.8-xCe0.2YxO3 (x=0.05 and 0.136) samples by aberration-corrected analytical TEM techniques. For both samples, GBs showed decreased density for all elements, and relative concentration increases for Ba and Y, decreases for Zr and O. Two types of Ba rich anti-phase boundaries (APBs) were observed in the bulk region by HAADF, which we denote as APB-1 and APB-2. APB-1 has a phase shift of half a {100} plane distance along [100] direction with BaO planes as the connecting planes. APB-2 has a phase shift between 0 to half a {100} plane distance also along [100] direction. Different to APB-1, (Zr, Ce,Y)O planes dominated by Ce and Y are present between the neighboring BaO planes, where Ce shows reduced valence. APB-1s are more frequent for x=0.05 samples, while x=0.136 samples show more APB-2s. The importance of APBs to accomodate cation nonstoichiometries will be discussed. More details and their correlation to proton conductivity will be presented. We acknowledge BMBF for funding (ProtoMem, No. 03SF0537C) 1 K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333-359

Resume : Low voltage transmission electron microscopy offers the possibility of damage-free characterization of certain fragile systems, including many 2D materials. Here, we present a modified Zeiss Gemini 500 SEM with a homemade transmission diffraction stage that can perform measurements with a pixelated detector (4D-STEM) down to 5 keV and potentially even lower. This system includes a piezo stage with 6 degrees of freedom and nm precision. The absence of post specimen lenses allows collecting diffraction patterns with very high scattering angles while avoiding any distortions. We demonstrate the capabilities of this system by mapping layer thickness, orientation, phase, strain and piezo-electric fields of different kinds of materials including bulk-like samples, nanowires and 2D materials. The very low electron energies allow not only for damage-free investigation of certain systems as a bulk crystal, but also in the presence of surfaces or defects. Another advantage of the system is that it can map areas much larger than is possible in a conventional STEM, which allows bridging the gap to mesoscopic scales that are often relevant for real world applications.

Resume : The flourishing developments of high entropy alloys (HEAs) provide the new path to the next generation engineering alloys for diverse potential applications in recent years. Precipitation hardening is one of the strategies to fabricate the strong alloys for the applications at room temperature. Among precipitation phases, the intermetallic sigma (?) phase with complicated structure leads to relatively hard and brittle property of alloys. This work aims to understand the elemental distribution and lattice distortion of intermetallic ? phase in CoCrFeNiMo HEA. First, the X-Ray diffractometry (XRD) was applied to confirm the existence of lattice distortion of HEA at macroscopic scale. Then, the lattice distortion of ? precipitates in CoCrFeNiMo HEA was demonstrated visually by annular dark field (ADF) imaging in STEM. Compared to the ? phase of FeMo alloy, the atomic arrangements of ? phase in HEA show the significant lattice distortion. The statistic study is also provided for discussing the lattice distortion. Moreover, the atomic scale EELS mappings exhibit the random compositional distribution of and low degree of ordering of ? precipitates. Briefly, we found that the ? precipitates in CoCrFeNiMo HEA have the lattice distortion and random compositional distributions at atomic scale.

Resume : The quest for advanced functional materials towards higher performance remains a key area in which complex oxides are a promising class of materials across a large spectrum of functionalities. Transition-metal oxides exhibit a strong interplay between their charge, lattice, orbital, and spin degrees of freedom that are usually further activated under external stimuli. These phenomena originate from subtle but critical local modulations of their atomic and/or electronic structures that strongly govern their physical properties at the macroscopic scale. Hence the requirement to probe the nanostructure’s local properties in their native environments using in situ ultra-high resolution electron spectromicroscopy. Here we present recent variable-temperature STEM/EELS experiments performed on a Cs-corrector monochromated NION microscope equipped with a HennyZ liquid nitrogen stage holder and MEMS. Archetypal V2O3 system was investigated to evaluate how deep their electronic structures can be revealed at the local scale under tuneable temperature stimuli. New insights into nanostructured V2O3 systems were investigated by probing its temperature-driven metal-to-insulator (MIT) transition through both low-loss and core-loss spectroscopic signatures [1] down to the nm-scale upon low-temperature thermal cycling. These findings are of main importance for a deeper understanding of the electronic states’ mechanisms governing MIT toward improving MIT switching efficiency at the macroscopic scale. [1] H. Abe, et.al., Jpn. J. Appl. Phys. 38 (1999) 1403-1407

Resume : Solid oxide fuel cell (SOFC) systems are expected to play a major role in future energy conversion systems. A design for an SOFC using thin functional layers on an inert substrate and a single co-sintering step was used to investigate material interactions and formed secondary phases in the SOFC. As a result of co-sintering, a manganese oxide secondary phase with particle sizes down to 10 nm has been found in the Ni/YSZ anode. Depending on their location in the anode, these particles reduce the triple phase boundary (TPB) length and impair the anode performance. In this study, scanning transmission electron microscopy ? energy dispersive X-ray spectroscopy (STEM-EDXS) tomography was systematically investigated to study the three-dimensional distribution of manganese oxide particles in a SOFC anode. All measurements were performed in a FEI Osiris 200 kV ChemiSTEM with high EDXS count rates and reduced detector shadowing. Tomography artefacts in the reconstructions were investigated to identify limiting factors for the tomogram resolution. The reconstructions were used to model the manganese oxide positioning and estimate their impact on the effective TPB length. The secondary phase particles were found to be preferentially located inside or at the boundaries of nickel grains. However, large particles were also found to be located at TPBs, potentially impairing the anode performance.

Resume : The series of alloys with nominal compositions of Fe80Co3Si2B12Cu1Nb2 and Fe78Co4Si2B14Cu1Nb1 (at.omic %), were prepared by arc melting of high-purity elements (Fe, Co, Si, Cu, Nb) as well as Fe-B prealloy, in the atmosphere of Ar. The rapidly quenched ribbons were prepared by single roller melt spinning method. Transmission electron microscopy was used for acquisition of atomic structure images, chemical composition, bonding information, internal electromagnetic fields and optical information. The wealth of information on physical, chemical, electronic, optical and magnetic properties on the atomic scale is a key to materials research and future technological developments. The magnetic properties of the alloys were studied at room temperature using hysteresis loop tracer and vibrating sample magnetometer. Optimization of composition of the alloys was carried out to obtain high saturation induction (Bs) and low coercivity (Hc). An attempt to correlate the alloy composition, ribbon thickness, structure and magnetic properties was made. The thickness of the ribbons was between 20 and 36 μm. To obtain nanocrystalline ribbons, a series of 1 hour annealings were carried out at temperature ranging from 380 ºC up to 550 ºC. The temperature of crystallization stages depends on chemical composition, as expected. The optimum annealing temperature (Ta) of the alloys studied is between 475 °C and 575 °C. The microstructure of the obtained samples was studied by transmission electron microscopy. All samples consist of bcc Fe(Si) crystalline phase and the remaining amorphous matrix. After annealing at the optimum temperature (T=525ºC), in the Fe80Co3Si2B12Cu1N2 alloy shown bright field image (BF), dark field image (DF) from reflex (110) and circular electron diffraction (SADP) revealed that the crystalline phase was bcc-Fe, with the grain size of about 10 nm. Measured interplanar distances (d) were of: d1 = 2.02 Å, d2 = 1.42 Å, d3 = 1.17 Å, d4 = 1.02 Å, d5 = 0.9 Å. They correspond to theoretical distances in Fe (Im-3m): d1 = 2.027 Å, d2 = 1.433 Å, d3 = 1.17 A, d4 = 1.013 Å, d5 = 0.906 Å. Slightly lower values obtained for the studied alloy indicated that there was a small amount of Si substitutionally dissolved in bcc-Fe. On the FFT, observe the reflections from the 110 Fe  planes, which are visible on the reverse transform in a grain size of about 10 nm. In the Fe78Co4Si2 B14Cu1Nb1 alloy, after annealing at the optimum temperature (T=500ºC) nanocrystals were also found. After analyzing the microstructure of the samples, the small grain size and the nanocrystal structure of the samples were confirmed.

Resume : Acceptor doped ceria polycrystal is an important class of electroceramic that can be used as an oxygen-ion-conducting solid electrolyte, it is able to operate at low temperature (below 700 oC) with high performance. The macroscopic electrical properties of ceria polycrystals have a strong association with the dopant concentration at grain boundary areas. Therefore it is essential to study the grain boundaries at atomic level, which can be realized by means of advanced transmission electron microscopy (TEM) techniques. In this contribution, general grain boundaries from CeO2 polycrystals with 0.1 mol % and 10 mol % Y2O3 doping were chosen as study object; Prior to the TEM specimen preparation with Focus Ion Beam (FIB), Electron Backscattered Diffraction (EBSD) was performed to determine the grain orientations, in order to find grains with desired relative orientation. With an emphasis on the morphological observation, mapping of chemical composition at grain boundary regions and valence state variation of Ce across the grain boundary interfaces, High-Resolution TEM (HRTEM), Energy-dispersive X-ray spectroscopy (EDXS) and Electron energy loss spectroscopy (EELS) measurements were carried out.

Resume : We use NanoMAX, a modified transmission electron microscope that combines atomic resolution and precursor sources, to observe in situ the growth of Ge nanowires (NWs) by the vapour-liquid-solid (VLS) method. The Ge atoms are provided either by a molecular beam epitaxy (MBE) effusion source or by a Ge2H6:H2 (10:90) collimated gas source for chemical vapour deposition (CVD). Au droplets, evaporated on a heating SiC membrane, are used as catalysts. The use of the gaseous mixture allows us to observe the effect of hydrogen on the growth. By yielding pure Ge, the MBE source eliminates the influence of H, known to strongly affect the surface energy of crystalline Ge [1]. It provides a reference for growth conditions. It additionally allows sending precursor atoms one by one, slowing down kinetics and making in situ observations easier. We show that, at a given MBE flux, the growth of Ge NWs in the < 111> direction is not a monotonous process. It can reverse indeed, while the incoming flux is maintained: atomic layers close to the catalyst-NW interface can dissolved back into the liquid catalyst. Those dissolved atoms then diffuse out of the catalyst towards the NW?s base. Real-time recordings show their precipitation at steps on the walls. This mechanism corresponds to the reversion of the VLS growth and leads to NW thickening. We analyse the causes of such fluctuations in terms of the presence of Au and H atoms at the surface. [1] A. D. Gamalski et al, Nano Lett. 15, 8211-8216 (2015)

Resume : The several properties of amorphous and crystalline titania (TiO2) make it appealing in a broad range of fields. Then, deeply understanding how diverse parameters control the outcomes of amorphous-to-crystalline transition could have a broad impact for both fundamental and applicative sciences. In this framework, anodically grown TiO2 amorphous nanotubes (TANs) represent a typical case since, despite their many proposed applications, the thermally-driven crystallization has been mainly studied on large-sized populations under ambient pressure, leading to averaged collective responses. Studying the crystallization of TANs by High Resolution Transmission Electron Microscopy (HRTEM), we were able to observe these transitions in situ at a single nanotube level upon different environmental and electron beam energy conditions, thus gaining an unprecedented level of detail on the crystallization process and its local evolution. Applying this approach to different beam energies in high vacuum regime and in inert (N2) and reducing (H2) atmosphere, the crystallization temperature was varied and diverse TiO2 phases were favored. The structural evolution of TANs studied by HRTEM was combined with pre- and post- crystallization compositional investigations by Electron Energy Loss Spectroscopy (EELS) aiming to observe the variation of Ti/O stoichiometry, providing new insights about the influence of pressure, electron beam energy and chemical environment on TANs’ evolution.

Resume : All-solid-state lithium ion batteries are a promising technology, capable of overcoming the problems of conventional lithium-ion batteries using liquid electrolytes. Garnet-type Li7La3Zr2O12 (LLZO) is a promising solid electrolyte candidate due to its high Li+ conductivity, its chemical stability against metallic lithium and its compatibility with other electrode materials [1]. Characterization of LLZO by scanning electron microscopy (SEM) and (scanning) transmission electron microscopy ((S)TEM) reveals the microstructure characteristics of polycrystalline bulk samples and of electrode/LLZO interfaces. However, preparation and investigation of samples is challenging, as the crystalline LLZO phase quickly becomes amorphous when exposed to high-energy electrons. In this study we use dry inert gas to avoid sample degradation during handling and transfer. Electron-beam damage was minimized for SEM and (S)TEM imaging by selecting optimum electron energies. This allows the investigation of the interface between LLZO and a metallic lithium anode. The interface microstructures are correlated with conductivity measurements obtained by electrochemical impedance spectroscopy to facilitate targeted optimization. We have analysed the microstructures of different LLZO surface modifications (polishing and metallic interlayers) aiming at low interfacial resistance. [1] Ramakumar, S., et al., Progress in Materials Science 88 (2017): 325-411

Resume : Nanowires NWs based on strained ternary semiconductors, are grown by molecular beam epitaxy (MBE) on Si or GaAs substrate using Vapor-Liquid-Solid (VLS) mechanism. Due to the high mismatch of the lattice parameters of basic compounds: GaAs, ZnTe, CdTe, MgTe, MnTe, InAs, CdSe which can exceed 7%, the growth of NWs is challenging and their final shapes and morphologies are complex. The NW heterostructures, investigated by TEM are coherently strained but the elastic relaxation (by bending, twisting or morphological transformations) is clearly visible. The residual strain field is studied for individual nanowires by TEM methods. The Geometric Phase Analysis (GPA) is performed with the use of HR-TEM and HR-STEM images obtained from different zone axis axes in planar view or/and using perpendicular FIB cross-section of the NWs. Due to the significant deformations of the NWs (bends and twists of the atomic columns) the high-resolution images in zone axis can be obtained only for several tens of nanometers long fragments of the NWs, which themselves are up to a few microrometers long.. Therefore we use Scanning Electron Diffraction (SED) to map bending and twisting of the lattice, as well as chemical lattice distortions related to the changes in the elemental composition. The series of electron diffraction patterns obtained in the STEM mode is analyzed to receive the individual maps of the lattice distortions components, via combination of simulated diffraction patterns and local cross-correlation algorithms. The initial model of plastic relaxation is proposed. The research was partially supported by National Science Centre (Poland) by grant No. 2016/21/B/ST5/03411

Resume : Exotic thin crystals with unexpected transrotational micro-, nanostructures [1] have been discovered by transmission electron microscopy (TEM) for crystal growth in thin (10-100 nm) amorphous films of different chemical nature (growing number of pnictogenide- and chalcogenide-based materials, oxides, metals and alloys) prepared by various methods. Here we present the studies of the unusual phenomenon (including reversible transformation from/to amorphous state) performed in situ in TEM under the influence of e-beam. By means of TEM bend-contour method regular internal bending of crystal lattice planes in a growing crystal is analyzed. We call it transrotation: crystal unit cell translation is complicated by small rotation realized round an axis lying in the film plane. It can result in strong regular lattice orientation gradients (up to 300 degrees per µm) of different geometries: cylindrical, ellipsoidal, toroidal, saddle, etc. Transrotational microcrystal resembles ideal single crystal enclosed in a curved space. For some geometries they have bending of atom/lattice planes similar (but much lower) to that of nanotubes and nanonions. Complex skyrmion-like lattice orientation texture is observed in some spherulite crystals. Transrotation is strongly increasing as the film gets thinner. Transrotational micro crystals have been recognized for some vital thin film materials, i.e. phase-change materials of memory devices, silicides, ferroelectrics, etc. The transrotation is the basis for prospective lattice-rotation, topological and strain nanoengineering of functional and smart thin-film materials. [1] V.Yu. Kolosov and A.R.Tholen, Acta Mater., 48 (2000) 1829.

Resume : Localized Surface Plasmons (LSPs) of metal nanoparticles confine the electromagnetic fields on the nanometer length scale, which enables the potential applications including surface-enhanced Raman scattering (SERS), biosensing, and nano-optics. Not only the LSP of metal nanoparticles itself, but the combination with semiconductor nanostructures has attracted researcher due to its unique energy dispersion relation.[1] Understanding the physical nature of the phenomena is crucial for tailoring them to specific needs, which can be achieved by correlating experiments with theoretical simulations. Theoretical simulations for plasmons are conducted by some different approximations based on Classical Electrodynamics including Boundary Element Method (BEM)[2] and Discrete Dipole Approximation (DDA)[3], which are compared with measurements like cathodoluminescence and electron energy-loss spectra (EELS). However, there has been little effort to simulate the plexciton, interaction between a LSP and an exciton in a semiconductor. Here, we examine the reliability of simulating plexcitons with the similar approach within Classical Electrodynamics as plasmons. Using both BEM(MNPBEM)[2] and DDA(DDEELS)[3], the low-loss EELS of the Ag/ZnO nanowire system are simulated and compared with experimental results [1]. The validity of these two simulation results are discussed. [1] Wei, Jiake, et al. Nano letters 15.9 (2015): 5926-5931. [2] Hohenester, Ulrich. Computer Physics Communications 185.3 (2014): 1177-1187. [3] Geuquet, Nicolas, and Luc Henrard. Ultramicroscopy 110.8 (2010): 1075-1080.

Resume : We present a MEMS-based microfluidic cell allowing the study of chemical reactions controlled by electrical biasing in a liquid environment, inside a Transmission Electron Microscope (TEM). Extra focus was placed on minimizing liquid thickness to enable high resolution TEM imaging, which impedes diffusion of species to the microelectrode. We can overcome the diffusion barrier and increase mass transport with a liquid pump in order to study faster electron transfer rates. One common challenge when trying to determine electron transfer kinetics at the liquid-metal interface is to reproducibly control and determine mass transport to the electrode. We focus on characterizing the mass transport in our electrochemical cell in order to quantitatively characterize an oxidation-reduction (redox) reaction. By combining a finite element model for mass transport and experimental current-voltage measurements of a simple redox molecule, we can extract a value for the electron transfer rate at the liquid-metal interface. This is a key parameter in a Butler-Volmer model for electrode kinetics. This is possible thanks to precise flow control with a pressure-based liquid pump combined with our MEMS-based electrochemical cell, with on-chip inlet and outlet. Well-characterized mass transport opens the doors to study more complex phenomena: we show experimental in situ TEM liquid-biasing results of reversible metal growth and its dependence on mass transport.

Resume : Defects of decagonal quasicrystal approximants generally exhibit structural characteristics that are peculiar compared with traditional crystals with small lattice parameters. We will report several kinds of defects of decagonal approximant with giant unit cells in Al-Cr-Fe-Si alloys, studied by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The structural features of these defects at the atomic level were revealed and the corresponding projected structural models along the pseudo-tenfold axis were proposed and discussed.

Resume : The band structure of single-walled carbon nanotubes (SWCNTs) is entirely determined by the cutting direction and the dimensions of the graphene slab the tubule is constructed from. Synthesizing nanotubes with a desired configuration has proven immensely difficult and alternative routes to tune the properties are still being sought for. One promising approach is substitutional doping by impurity elements. Here, by simultaneous plasma and laser irradiation of a SWCNT-graphene van der Waals heterostructure, we incorporate silicon heteroatoms in both materials. The vacancies thus created are primarily healed by thermally diffusing carbon and Si atoms, the latter of which we directly identify in the lattice by using atomic resolution scanning transmission electron microscopy. These atoms are found in 3- coordinated, 4-coordinated and not identifiable configurations in SWCNTs with a relative frequency of ~0.69, ~0.20 and ~0.11, respectively and thus in good agreement with earlier theoretical predictions for single and double vacancies. These 1D-nanostructures doped with Si heteroatoms may show up as a promising candidate for water splitting, gas sensing and drug delivery applications.

Resume : Thermoelectric materials based on the ternary Mg2Si1-xSnx system have attracted increased attention, due to enhanced performance and potential in applications at the intermediate temperature range (500?900 K). Band structure engineering and nanostructuring approaches have resulted in high figure of merit (ZT) values, reaching up to 1.4 in Bi-doped materials. In this study, a detailed structural characterization of improved performance Bi-doped Mg2Si1-x-ySnxGey materials is presented, using high resolution transmission electron microscopy (HRTEM) selected area diffraction (SAD) and post-experimental HRTEM image processing (Geometric Phase Analysis, GPA). The materials were grown either by two-step synthesis or mechanical alloying, followed by hot pressing. Microscopy results reveal strong phase separation for the two step synthesis samples, in contrast to uniformity observed for the mechanical alloying materials. This is confirmed both by more average SAD pattern analysis, as well as by atomic level imaging (HRTEM and GPA). The differences in lattice constants compared to nominal values are even up to 1.3% -depending on phase stoichiometry- for the sample prepared with two step synthesis and only 0.6% for the one synthesized by mechanical alloying. These findings are consequently discussed in relation to thermoelectric measurements results, where both materials exhibit excellent ZT for their Si-rich counterparts, albeit their distinct synthesis approaches and nanostructure.

Resume : Recently, nanoporous materials have been pointed out as a promising candidate for the development of highly sensitive sensors due to their high specific surface area. The precise control of the pore and ligament size is a key parameter for the development of such sensors. The most used technique in order to create a nanoporous structure is the dealloying process. It consists in removing the less noble metal from an alloy in order to create the nanoporous structure. The working model of porosity evolution during dealloying has been well reported. Briefly, three steps are highlight, first the dissolution of the less noble metal, then the diffusion of the more noble element and finally the coarsening of the ligament. Among them, the latest step is not fully understood. However, it is the key step leading to the fine control of the pore and ligament size. In this work, we report an in situ TEM study of the coarsening. In order to perform this analysis, nanoporous nanowires with different pore and ligament sizes were analyzed. It is well reported that the coarsening is observed either during dealloying than during annealing. Indeed, the evolution of the morphology during annealing was recorded by TEM. We highlight the two coarsening behavior reported on the literature for nanoporous samples depending on the pore and ligament size.

Resume : Quantum information technology is considered to overcome the fundamental limits of today's computer hardware. A promising candidate for the key component of a quantum computer, the quantum bit (qubit), is resembled by four Majorana zero modes. Such qubits are topologically protected and hence promise to compute fault-tolerantly. The elusive Majorana modes are found at the interface between topological matter and superconductors. Both, the development of suitable topological matter of high crystalline perfection and a perfect interface to a superconductor is the focus of our research. (Bi,Sb)-Te compounds, which we grow by molecular beam epitaxy on Si(111) consist of quintuple layers e.g. Te-Bi-Te-Bi-Te layers and possibly Bi-Bi layers, which are bound by van der Waals forces. Depending on the growth conditions various stoichiometries can be realized, which result in a different stacking and hence different topological properties. For instance Bi2Te3 is a three-dimensional topological onsulator, while in Bi1Te1. a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. Onto these layers superconducting metals such as Al or Nb are deposited. Advanced scanning transmission electron microscopy and energy dispersive X-ray analyses are performed to study structure and composition across the topological interface/superconductor interface on the atomic scale.

Resume : High carbon micro-alloyed steel with a duplex structure of austenite as well as martensite has found large applications in the comminution process where strain rates vary from low to high. During such process austenite to martensite transformation occurs which needs proper understanding. The formation mechanism and the quantification of deformation-induced martensitic (DIM) transformation as a function of strain at various strain rates have been investigated. The specimens were compressively deformed at four different loading condition of 100, 200, 275 kN as well as to complete failure at varying strain rates (2.56x10-4- 2.56x10-1 /s). The subsequent change in the volume fraction of the transformed phases was measured through the Rietveld refinement technique. The amount of transformed martensite showed an increasing trend as a function of strain at different strain rates and was found to have a significant effect on the mechanical behaviour of the material. In addition, the effects of volume fraction on the stress-strain partitioning behaviour of the constituent phases during compressive deformation were also investigated. Localised, as well as inhomogeneous deformation among the constituent phases, was observed due to the strain incompatibility which further leads to the development of stress triaxiality in the material. The strain partitioning coefficient was found to increase with the increase in the martensitic content as a function of strain at various strain rates.

Resume : FULL TITLE (was cut off in field above): Towards an in situ TEM based correlation of texture controlled thin film growth and switching characteristics in filamentary type HfO2-RRAM ABSTRACT Hafnia based resistive memory devices are promising candidates as next generation non-volatile memory due to their performance and proven compatibility to CMOS technology [1]. Current models for the resistive switching behavior rely on the electric field driven formation and dissolution of oxygen deficient nanoscale conducting paths often discussed as "filaments" [2]. The physical mechanisms responsible are governed by the motion of oxygen ions or vacancies, Joule heating induced by localized currents and interfacial oxygen exchange processes [3]. By quasi-epitaxial growth and texture transfer, distinct sets of growth orientations have been achieved for the active hafnia layer in c-Al2O3/TiN/HfO2/Pt stacks. Characterization by atomic-resolution Z-contrast imaging and high-resolution automated crystal orientation mapping (ACOM) yield detailed multidimensional datasets. Most strikingly, grain boundaries, which play a crucial role in the electroforming process due to their localized inter-bandgap states were found in all of our crystalline hafnia thin films. They interconnect the electrodes in the metal-insulator-metal (MIM) structure and represent highly probable sites for initial dielectric breakdown/electroforming [4]. In situ TEM experiments on these devices [5] bridge the gap between the atomistic, macroscopic structural properties and the corresponding electrical characteristics of each device. Spectroscopic as well as structural investigations will focus on the most susceptible components of the RRAM device. REFERENCES: [1] S. U. Sharath et al., ?Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices,? Adv. Funct. Mater., vol. 27, no. 31, Jul. 2017. [2] R. Waser, R. Dittmann, G. Staikov, and K. Szot, ?Redox-Based Resistive Switching Memories ? Nanoionic Mechanisms, Prospects, and Challenges,? Adv. Mater., vol. 21, no. 25?26, pp. 2632?2663, Jul. 2009. [3] G. Niu et al., ?Operando diagnostic detection of interfacial oxygen ?breathing? of resistive random access memory by bulk-sensitive hard X-ray photoelectron spectroscopy,? Mater. Res. Lett., vol. 7, no. 3, pp. 117?123, Mar. 2019. [4] K.-H. Xue et al., ?Grain boundary composition and conduction in HfO2: An ab initio study,? Appl. Phys. Lett., vol. 102, no. 20, p. 201908, May 2013. [5] A. Zintler et al., ?FIB based fabrication of an operative Pt/HfO2/TiN device for resistive switching inside a transmission electron microscope,? Ultramicroscopy, vol. 181, pp. 144?149, Oct. 2017. [6] The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) under research grant MO 3010/3-1, the European Research Council (ERC) FOXON (805359) and WAKeMeUP under H2020-EU.2.1.1.7.

Resume : Two general types of bulk (massive, volumetric) metallic glasses are formed by suppressing either nucleation or growth process of a competing crystalline phase(s). Nanocrystallization of a Ti50Ni23Cu22Sn5 bulk metallic glass, a representative of the former glass family, which formation is crystal nucleation-controlled was studied within the supercooled liquid region by using a state-of-the-art experimental technique with elemental mapping at near-atomic resolution [1]. A especial focus was made on the nature of incubation period at constant temperature and pressure which is still poorly understood from both theoretical and experimental viewpoints. Molecular dynamics simulation performed for the Ti55Ni45 liquid acting as a simplified model system of the (Ti,Sn)55(Ni,Cu)45 alloy illustrated the solute partitioning process in the liquid state. Both experimental and simulation data indicate formation of nanometer-range chemical rearrangements which are supposed to reduce the energy barrier in the complex energy landscape. This process leads to a high density of homogeneously nucleating crystallites after the completion of a macroscopically observed incubation period. Two Fe-based alloys with good glass-forming ability: Fe48Cr15Mo14C15B6Tm2 and Fe48Cr15Mo14C15B6Y2 represent the latter glass family [2]. The reasons for significantly enhanced glass-forming ability of these alloys are connected with low growth rate of the -Fe36Cr12Mo10 phase which nuclei exist in the glassy phase. The low growth rate is connected with large inhomogeneous strain in the growing nanoparticles, while nucleation of eutectic colonies is hampered by slow diffusion of a rare-earth alloying element. [1]. Z. Wang, C.L. Chen, S. V. Ketov, K. Akagi, A. A. Tsarkov, Y. Ikuhara and D. V. Louzguine-Luzgin, Materials & Design, 156, (2018), 504-513. [2]. D.V. Louzguine-Luzgin, A.I. Bazlov, S.V. Ketov and A. Inoue, Materials Chemistry and Physics, 162, (2015), 197–206.

Resume : The excellent chemical and thermal stability of transition metal perovskite related oxides that belong to the Ruddlesden?Popper (R?P) 1,2 homologous series with the general formula An+1BnO3n+1 ((ABO3)nAO) (A=Rare and/or alkaline earth elements and B= transition metal), in which the multiple perovskite layers that are n octahedra thick alternating with single AO rock salt layers, make them very attractive materials. Electronic conduction may occur in perovskite-like blocks, built up from corner-sharing BO6 octahedra, where B-cations occupying the center of the polyhedra can present multiple non-localized oxidation states. Besides, AO rock-salt layers may present anionic vacancies and/or interstitial oxygen ions, both defects resulting in ionic conduction. Among the R?P series, the interest has been mainly focused on the n=1 intergrowth oxides, where one-octahedron-thick layer alternate with single AO rock salt layer along packing axis, though there are some articles reporting the properties of n=2 and 3 members. In this communication, a new members3,4 with n=1, 2 and ? of Ruddlesden?Popper series, has been prepared and a detailed structural characterization has been carried out by High Resolution Transmission Electron Microscopy (HRTEM) by using a JEOL JEM-3000F electron microscope and Scanning Transmission Electron Microscopy (STEM) performed on a JEOL JEM-ARM200cF microscope (Cold Field Emission Gun). Interestingly, interstitial oxygen were clearly observed. Such fact has been associated to high ionic conductivity. REFERENCES 1. S. N. Ruddlesden and P. Popper, Acta Crystallographica, 1957, 10, 538-540. 2. S. N. Ruddlesden and P. Popper, Acta Crystallographica, 1958, 11, 54-55. 3. K. Boulahya, M. Hassan, D. Munoz Gil, J. Romero, A. Gomez Herrero, S. Garcia Martin and U. Amador, Journal of Materials Chemistry A, 2015, 3, 22931-22939. 4. K. Boulahya, D. Munoz Gil, M. Hassan, S. Garcia Martin and U. Amador, Dalton Trans., 2017, 46, 1283-1289.

Resume : A thorough understanding of the emergence of magnetic behavior from nanoscale volumes of ferromagnetic material requires both a quantification of magnetic moments as well as the ability to correlate them to the spatial features in question. Recent advances with the Electron Magnetic Circular Dichroism (EMCD) technique using Scanning Transmission Electron Microscopy (STEM) indicate that real space maps of magnetic transitions can be produced with a spatial resolution of less than one nanometer. Such maps require the ferromagnetic material to be crystalline and oriented in a low symmetry direction, limiting the view to atomic planes at best. However, our recent theoretical work demonstrates that, not only can the orientation dependence of an EMCD signal can be understood and modelled, but that STEM-EMCD can even be performed on the zone-axis of ferromagnetic crystals, provided a proper separation of magnetically-sensitive electron transfer momenta can be achieved post-specimen. In this contribution, we present our recent attempts to realize such an experimental design. We begin by reviewing our work relating the sample orientation to the EMCD signal strength as well as presenting a detailed assessment of experimental errors in application of the sum rules. We conclude with some preliminary results from the installation of a patterned aperture in the TEM projector system.

Resume : With the increasing demand in renewable energy sources, the development of more efficient solar cells is crucial. Cu(In,Ga)Se2 thin film solar cells are among the most promising absorbers for the fabrication of a thin-film solar cells. Here, grain boundaries are often considered for recombination activity inside the cell. Although EBIC measurements show that most ?3 twin boundaries have no change in carrier collection, some show enhanced or decreased carrier collection and thus a beneficial or detrimental effect on the cell efficiency [1]. Atom probe tomography results show no change in composition at these interfaces and therefore suggest that the reason for the change in carrier collection is due to structural properties at the boundaries. By combining EBSD with EBIC we are able to identify and analyze these grain boundaries. Afterwards they are characterized using correlative APT/TEM directly on prepared atom probe tips. This gives us not only precise composition information in three dimensions at the interface but also a detailed structural characterization on the same region of interest. We can combine these results with the previously collected EBIC results and are thus able to correlate electrical properties with chemical and structural properties to understand the reasons for the change in carrier collection. To realize this, we use a specially designed TEM holder that not only allows for easy sample transfer between the atom probe and the TEM but also enables high tilting angles of up to 70° for alpha tilt and 8° beta tilt. References: [1] Mohit Raghuwanshi, Bo Thöner, Purvesh Soni, Matthias Wuttig, Roland Wuerz, and Oana Cojocaru-Mirédin, Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se2 Grain Boundaries: Correlation with Microstructure, ACS Applied Materials & Interfaces 2018 10 (17), 14759-14766

Resume : Energy dispersive X-ray spectroscopy (EDS) for in-situ analysis in the electron microscope is explored. One certain request is speed. Fast efficient EDS is achieved using large solid and high take-off angles for photon collection, good collimation and suitable sample holders. Holders should not shadow the detector(s). Geometric optimization in combination with high brightness electron sources and aberration correction allows EDS identification and tracking of single atoms at low accelerating voltages in STEM within a few seconds [1]. Reaction cell geometry and materials as well as window materials and e.g. reaction species sticking to them must be considered. Constantly changing data streams demand new ways of data acquisition and processing. EDS experiments at temperatures up to the range of 1000°C in STEM and SEM will be used to discuss challenges. Agglomeration and evaporation of material in a Pd-Au-nanoparticle test structure during heating in a conventional STEM were analyzed and evaluated using phase analysis and statistical methods. The stability of the holder in Z-direction with changing temperature was important [2]. Heat radiation changes the low energy part of EDS spectra. Still, low energy element ID and chemical phase analysis are available within certain limits. [1] R. M. Stroud et al. Appl. Phys. Lett. 108 (2016) 163101 [2] T. T. van Omme et al., Ultramicroscopy 129 (2018) 14

Resume : Tin antimony sulfide (SnSb4S7) is one of the most promising compounds for the next generation of optoelectronic and thin film photovoltaic devices. SnSb4S7 material was synthesized by a solid-state reaction using earth-abundant tin, antimony and sulfur elements. The structural properties of the TAS powder were investigated by transmission electron microscopy (TEM). X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selected electron diffraction (SAED) were employed to establish the crystalline nature of the powder. The XRD spectrum confirms that the powder is polycrystalline in nature and all identified peaks belong to the SnSb4S7 phase. The TEM observations demonstrated that the powder was polycrystalline in nature with rod-shaped structure. The SAED pattern shows bright spots indicating the highly crystalline nature of the TAS material.

Resume : One of the main challenges in conventional STEM techniques is the difficulty to fully image and interpret a lattice consisting of both high and low Z elements. While only high Z elements can be imaged in HAADF-STEM, (A)BF-STEM can display low Z elements, though atomic positions in the images are difficult to interpret as there is no clear contrast variation between different atoms (1). Novel integrated differential phase contrast (iDPC) STEM imaging is instrumental in showing both low Z and high Z elements with clear contrast variation (2). iDPC STEM is based on center of mass (COM) based imaging (3), and its strength in simultaneous high and low Z detection has been shown experimentally in GaN (2) and Ɣ-TiH (4) domains. Moreover, iDPC STEM images have also been shown to exhibit improved contrast in beam sensitive materials, exemplified with graphene (2). In materials science, we are seeing an increasing particular interest in structures that are comprised of low Z elements and beam sensitive materials (i.e. 2D materials, zeolites, MOFs, etc.) requiring low dose imaging techniques. With an enhanced contrast as well as wide Z-range detection capability, iDPC STEM should be considered as the key sub-angstrom imaging method in these research areas. (1) Bosch, E.G.T. et al. Ultramicroscopy, 2015, 156, 59-72. (2) Lazić, I. et al. Ultramicroscopy, 2016, 160, 265-280. (3) Müller, K. et al. Nat. Commun., 2014, 5, 5653. (4) de Graaf, S. et al. arXiv:1812.09118.

Resume : Core-shell, core-frame, and heterostructured nanocrystals are particularly intriguing because their metal-metal interfaces allow for charge transfer, the build-up of compressive and tensile strains, and the integration of materials with distinct physicochemical properties into a single architecture. While it is apparent that heteroepitaxy has a profound influence on nanostructure growth modes and the resulting structure-property relationships, an in-depth understanding is still lacking. Our group has devised a strategy that is unique in that it results in the formation of intricately shaped nanostructures that form a heteroepitaxial relationship with the underlying substrate material. It is this feature that presents numerous opportunities in terms of utilizing substrate-induced epitaxy as an auxiliary control in nanostructure synthesis. Here, we demonstrate this concept through the synthesis of core-shell nanocubes on oxide substrates with various crystallographic orientations, where for each case the nanostructure architecture and alignment is dictated by the heteroepitaxial relationship established. A combination of High Resolution TEM and STEM imaging as well as Selected Area Electron Diffraction performed by FEI Titan 80-300 Transmission Electron Microscope has been employed for structural and compositional characterization of the samples. TEM cross-sectional samples were prepared using an FEI Helios SEM/FIB dual beam tool. The work provides an early indication that substrate-induced epitaxy could emerge as an important synthetic lever in the rational design of substrate-based bimetallic nanostructures.

Resume : The rapidly growing field of van der Waals materials and 2D heterostructures continues to offer surprises to researchers. Here we will describe the use of high resolution annular dark field scanning transmission electron microscopy to uncover new structural features in this exciting class of material. For example, we will present recent work looking at interfaces between transition metal dichalcogenides and boron nitride. These are crucial in many optoelectronic device architectures but we find that unless the selenides (MoSe2, W Se2 NbSe2) are processed in an inert environment protruding defects prevent the realisation of pristine interfaces.[1] We will also discuss recent work describing the microstructures produced when 2D van der Waals materials are subjected to mechanical deformation. Using graphite, BN and MoSe2 as examples, we show that the types of defects produced can be predicted based on knowledge of the bend angle and thickness of the material.[2] We find that above a critical thickness the materials exhibit numerous twin boundaries and that for large bend angles these can contain nanoscale regions of local delamination. We show that the induced changes in local stacking order could be used to locally tune the optoelectronic behaviour and propose that such features are important in determining how easily the material can be thinned by mechanical or liquid exfoliation. [1] AP Rooney et al. Nano letters 17, 5222, (2018) [2] AP Rooney et al. Nature communications 9, 3597, (2018)

Resume : Identifying the three-dimensional (3D) position of every atom is the ultimate goal of structural characterization. Because transmission electron microscopy (TEM) only provides two-dimensional (2D) projections of the sample, electron tomography is used to retrieve the 3D structure from a set of projections. However, this approach is problematic for electron-beam sensitive samples such as 2D materials. We demonstrate new insights into the three-dimensional structure of defects in graphene, in particular grain boundaries and silicon impurities, obtained in a new approach from only two atomically resolved TEM images recorded at different angles. The structure is obtained through an optimization where both the atomic positions as well as the simulated imaging parameters are iteratively changed until the best possible match to the experimental images is found. We applied the method to a set of grain boundary structures with misorientation angles nearly spanning the entire range (2.6-29.8°). The measured height variations at the boundaries reveal a strong correlation with the misorientation angle, with lower angles resulting in stronger corrugation and larger kinks. Our approach also allows to visualize electron-beam induced dynamics perpendicular to the graphene plane, which would not be observable when the sample is not tilted. Our results allow for the first time a direct comparison with theoretical predictions for corrugations as well as access to out-of-plane dynamics.

Resume : Understanding order-disorder transitions and the role played by defects in strongly correlated materials will contribute towards applications as well as understanding fundamental physics. Charge density waves (CDW) refer to the modulation of electron charge density in low-dimensional metals. CDW can also be considered to be electronic crystals showing an elastic response to defects in crystal structure [1]. The presence of crystalline defects has been shown to pin the collective motion of CDW condensates under an electric field [1]. Charge density wave (CDW) materials therefore provide a suitable model system where effects of disorder on transport and ordering behavior can be investigated [1]. We have investigated the influence of electron beam-generated lattice defects on the structure of periodic lattice distortion (PLD) which accompanies CDW modulation in 1T-TaS2 and 1T-TaSe2. Lattice defects were generated through high-energy electron beam irradiation in a transmission electron microscope (TEM). Using atomic resolved high-resolution TEM (HRTEM) imaging, we directly visualize the CDW/PLD structure. We observe formation of dislocation-like defects as well loss of long range order in the CDW/PLD structure with increased exposure to the electron beam [2]. These results are further discussed and compared to density functional theory calculations. [1]. P. Monceau, Adv. Phys. 61, 325 (2012). [2]. M. K. Kinyanjui, T. Björkman, T. Lehnert, J. Köster, A. Krasheninnikov , and U. Kaiser, Phys. Rev. B 99, 024101(2019)

Resume : Quantitative electron microscopy techniques continue to evolve and further drive materials innovation. Nevertheless, some key aspects of material analysis remain difficult to realise with established electron microscopy (EM) methods which require electron transparent specimens. This limits the chemical analysis of beam sensitive materials (e.g. soft/carbon-based materials) and novel EM capabilities would be of benefit. Here, we present the capabilities, applications and underlying processes of secondary electron spectroscopy in conjunction with Secondary Electron Hyperspectral Imaging (SEHI) [1]. The analysis is performed in a low-voltage scanning electron microscope (SEM), where the voltage and dose can be tailored to beam-sensitive materials and there is no need for the preparation of electron transparent samples. We show that our method allows the association of localised secondary electron spectral signatures with materials and their electron affinities [2,3]. Thus, 4D nanoscale resolved spectral information can be correlated to macroscale properties of different materials with varied applications [4,5]. We use computational and modelling approaches to give us a more fundamental understanding of the mechanism by which vital material properties such as crystallinity and molecular orientation are encoded in SEHI. The potential of SEHI to not only image materials but also map local properties holds significant promise to access information which has previously been inaccessible to EM. 1. N. Stehling et al., MRS Commun. 8, 226?240 (2018) 2. M. Dapor et al., J. Electron Spectros. Relat. Phenomena 222, 95?105 (2018) 3. R.C. Masters et al., Adv. Sci. (2019). 4. N.A. Stehling et al., Front. Mater. 5, 84 (2018). 5. K.J. Abrams et al., Phys. Status Solidi 14, 1700153 (2017).

Resume : We employ in situ high resolution transmission electron microscopy (HRTEM) to look into the pyrolysis of free-standing nanocrystalline graphene films. A two-step growth mechanism was identified where the intermediate temperature growth (600-1000 ºC) occurs by consuming the amorphous carbon around the crystallites and the high temperature growth (1000-1200 ºC) proceeds by the merging of crystallites. This catalyst-free transformation forms carbon nanostructures with varying size, shape and mobility[1]. Time-resolved HRTEM studies on these nanostructures at high temperatures provide insights into the different mechanisms responsible for the growth of graphitic domains. The study shows that the growth of the domains is mainly caused by the migration and merging of the graphitic subunits. In addition to this, strong structural and size fluctuations of individual graphitic subunits at high temperatures are observed. Graphene nanoflakes are highly unstable and tend to lose atoms or groups of atoms to adjacent larger domains indicating an Ostwald-like ripening as an additional growth mechanism in these 2D materials. Beam-off experiments confirm that the observed dynamics are inherent temperature-driven and atomistic simulations, carried out to estimate the activation energy for the different processes, indicate that defects in the substrate play a critical role for the observed dynamics. Reference 1. Shyam Kumar, C. N. et al.Nanoscale 9, 12835?12842 (2017).

Resume : Synthetizing III-V semiconductor nanowires (NWs) using the vapour-liquid-solid (VLS) method can result in the growth of crystal structures different from their bulk phase. In GaAs NWs, for example, the stable phase zinc-blende (ZB) coexists with the metastable wurtzite (WZ) structure. Remarkably, the valence and conduction bands are misaligned in the two phases, so that small sections of one phase within the other effectively confine charge carriers along the wire axis. Therefore, the possibility of controllably switching between phases opens up opportunities to create novel heterostructures commonly identified as crystal phase quantum dots. In contrast to compositional heterojunctions, crystal phase heterostructures have the intrinsic property of minimizing residual strain and alloy intermixing at interfaces. Here, we present experimental observations of the growth of self-catalyzed Ga-As nanowires using a modified environmental transmission electron microscope (ETEM) equipped with molecular-beam-epitaxy (MBE) sources. NWs are grown directly inside the microscope and their growth is monitored in situ and in real time with high spatial and temporal resolution. First, we show that by changing the growth conditions and in particular the ratio between fluxes of Ga and As, we modify the volume of the catalyst droplet and hence the contact angle between the droplet and the crystal. A change in contact angle alters the balance of capillary forces at the triple phase line (TPL). Thus, it renders the nucleation site more favorable for one phase with respect to the other. We find that the phase change takes place at specific contact angles for both Au-catalyzed and self-catalyzed growth modes. In the light of these results, we discuss the existing models and we propose a new model to describe the phase selection in III-V nanowires.

Resume : For particle-assisted growth of III-V semiconductor nanowires, understanding of the seed particle dynamics is key to understand the growth kinetics of the resulting nanowire. To asses the state of the droplet during growth, in-situ measurement of a growing nanowire is needed. Here, we use aberration-corrected environmental transmission electron microscopy (ETEM) equipped with x-ray energy dispersive spectroscopy (EDS) to do both atomic resolution imaging with frame rate >20fps and compositional analysis of the same nanowire during growth. For growth of Au-seeded GaAs nanowires, we show that the average growth rate is depending on two processes – bilayer completion time and incubation time between layers. By correlating EDS measurement on seed particle composition for different precursor ratios, we identify a Ga-limited and a Ga-rich regime where one or the other process is limiting the average growth rate. We will also show recent results from other III-V material combinations, where scanning TEM together with EDS mapping is performed on growing nanowires. By recording EDS maps with short dwell times together with particle tracking, we follow change in seed particle composition during transient growth conditions. Using principal components analysis, improved spatial and temporal resolution is achieved.

Resume : In our previous studies [1], we reported that it is possible to obtain (Ga,Mn)As dilute magnetic semiconductor in hexagonal – wurtzite (WZ) structure (unusual for GaAs) as a shell deposited on the sidewalls of WZ (Ga,In)As NWs by low temperature MBE growth. Ex-situ post-annealing of such NWs revealed that above 450⁰C the (Ga,Mn)As shells decompose to MnAs nanoprecipitates embedded coherently in the WZ GaAs matrix. Here we report on the effect of in-situ annealing of NWs with WZ (Ga,Mn)As shells in transmission electron microscopy (TEM). This approach enables investigations of MnAs nanocrystals formation via collecting large number of images registered at subsequent stages of the annealing process. Such experiment can be done with the use of a dedicated TEM sample holder designed for high temperature treatment and compensation of thermal drifts inside the microscope. These studies reveal that structural changes in WZ NWs starting from Mn atoms segregation in WZ lattice at temperatures around 300⁰C followed by phase transition to hexagonal MnAs nanoprecipitates above 400⁰C. At higher temperatures, we observe growth of such precipitates with visible Moiré pattern in high-resolution TEM images. Evolution of magnetic properties from single-phase, non-annealed (Ga,Mn)As to decomposed, granular GaAs:MnAs material has been investigated with SQUID. These investigations have been supported by National Science Centre (Poland) through grants No. 2017/25/N/ST5/02942 and 2018/28/T/ST5/00503. References: [1] J. Sadowski et al. Nanoscale 9, 2129 (2017)

Resume : The investigation of the physics of nanoscale systems ideally requires atomic spatial resolution and femtosecond time-resolution. Ultrafast Transmission Electron Microscopes (UTEM) have recently emerged as unique tools with unprecedented spatio-temporal resolutions. However, the performances of the first UTEMs were limited by the brightness of the photocathodes used as ultrafast electron source. In this context, it was soon realized that UTEMs relying on laser-driven electron sources based on nanoscale emitters would overcome this limitation. This was confirmed by the results obtained on the UTEM based on a laser-driven Schottky type electron source developed in Göttingen. We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source, which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses, which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The measured brightness is the largest reported so far for UTEMs. Combining this new high brightness source with an injection/Cathodoluminescence system, composed of a parabolic mirror placed above the sample holder, the UTEM can be used to perform time-resolved ultrafast pump-probe TEM experiments. We will show the possibilities of such an instrument for ultrafast imaging, diffraction, electron holography and spectroscopy.

Resume : An ultrafast transmission electron microscope (UTEM), which has been developed at the IPCMS in Strasbourg (France), is able to obtain images, electron diffraction patterns, and electron energy-loss spectra (EELS) with nanometer and pico/nanosecond space?time resolution. The microscope is designed to study ultrafast transformations in nanomaterials. After excitation of the object with a laser pulse, its behavior is studied by an ultrashort electron pulse which traverses the object. In this pump-probe approach, electron microscopy images, diffraction patterns, and EELS are acquired to study the structural and chemical evolution of reversible and irreversible reactions. In this contribution, we present the UTEM setup and its two operating modes, namely the single-shot mode, where one intense nanosecond electron pulse is used for studying irreversible phenomena, and the stroboscopic mode that uses trains of weak electron pulses, dedicated to the study of reversible processes. We show our first experimental results in the single-shot mode about irreversible chemical reactions in nanoparticles at the nanosecond scale. The fast kinetics of transformations in nanoparticles is studied, combining ultrafast imaging, EELS, and diffraction.

Resume : A cation exchange (CE) reaction occurs when a cationic species in a crystalline structure is partially or completely replaced, leaving the anionic lattice unmodified. CE is usually performed as a fast reaction occurring in liquid on inorganic colloidal nanoparticles (NPs), thus hindering the possibility of directly imaging the process. We show that an in situ Transmission Electron Microscopy (TEM)/Scanning TEM (STEM) approach at solid state gives a way to overcome these limitations. In fact, when heated, spherical Cu2Se NPs expel free Cu species that can be exploited to perform in situ CE reactions into CdSe nanowires (NWs) deposited on a common heated substrate. When reached by the free Cu species, CdSe NWs undergo the CE of Cd with Cu. We show how this phenomenon occurs at different temperatures, depending on the crystalline phase of the CdSe NWs and then on the energy possibly requested to allow the anion lattice changing its structure. Moreover, further insights were found by studying via in situ STEM-Energy Dispersive Spectrometry (EDS) this phenomenon: first, the free copper, before entering the CdSe NWs, surrounds the wires; second, once entered into the NWs, it substitutes the Cd moving along their axis with different speeds in the two opposite directions. Then, the application of the in situ STEM heating approach, combined with the slower kinetics of solid state CE reactions, allows to direct image the transient states leading to the final products of CE.

Resume : Uncovering kinetic pathway of non-classical multistep nucleation at atomic scale is critical to understanding complex microscopic mechanism of heterogeneous nucleation and crystallization. However, due to the intricacies in tackling such challenging topics experimentally, the structure of intermediate states and the temperature dependent effect on the multistep nucleation at atomic scale are still not fully understand. In this study, we report the direct in-situ atomic-scale observations of kinetic processes of interfacial multistep nucleation pathways using an aberration-corrected transmission electron microscope. We provide direct evidence to temperature-dependent multistep nucleation pathways in supported bismuth system at atomic scale: droplet-crystal nucleus two-step nucleation pathway at high temperature close to the melting point of bulk bismuth, droplet-local ordered structure-crystal nucleus three-step nucleation pathway at medium temperature between a critical temperature Tc1 and the lowest size-dependent melting temperature of bismuth nanoparticle, and a cluster-crystal nucleus two-step nucleation pathway at temperature, lower than the lowest melting point (Tl). The related critical temperature Tc1 and the lowest melting in the three pathways are confirmed based on the size-dependent melting phase diagram of Bi nanoparticles obtained by both experimental observation and theoretical prediction. Statistical analyses imply that the nucleation pathway plays an important role in deciding the nucleation and crystallinity kinetics. The understanding of multiple intermediate steps in nuclei and the temperature effect on nucleation pathways has important implications for the synthesis and growth of cluster, amorphous and crystalline materials; and thus our findings further enrich the nucleation theory.

Resume : Any dielectric material under a strain gradient presents flexoelectricity. Here, we synthesized 0.75 sodium bismuth titanate ?0.25 strontium titanate (NBT-25ST) core?shell nanoparticles via a solid-state chemical reaction directly inside a transmission electron microscope (TEM) and observed domain-like nanoregions (DLNRs) up to an extreme temperature of 800?°C.1-3 We attribute this abnormal phenomenon to a chemically induced lattice strain gradient present in the core?shell nanoparticle. The strain gradient was generated by controlling the diffusion of strontium cations. By combining electrical biasing and temperature-dependent in situ TEM with phase field simulations, we analyzed the resulting strain gradient and local polarization distribution within a single nanoparticle. The analysis confirms that a local symmetry breaking, occurring due to a strain gradient (i.e. flexoelectricity), accounts for switchable polarization beyond the conventional temperature range of existing polar materials. We demonstrate that polar nanomaterials can be obtained through flexoelectricity at extreme temperature by tuning the cation diffusion. The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) under research grant MO 3010/3-1 and the European Research Council (ERC) "Horizon 2020" Program under Grant No. 805359-FOXON. References: [1] L. Molina-Luna et al. Nature Communications, volume 9, Article number: 4445 (2018). [2] M. Acosta et. al., J. Am. Ceram. Soc. 98 (2015) 3405?3422. [3] J. Koruza et. al., J. Eur. Ceram. Soc. 36 (2016).

Resume : The technological potential of the group-III sesquioxides Ga2O3, In2O3 and Al2O3 as transparent semi-conducting oxides could be greatly increased by forming solid solutions of them to tune their physical properties. However, since these compounds exhibit different thermodynamically stable crystal structures and also different cation coordination environments (4- and 6-fold), the theoretically predicted (InxGa1-x)2O3 phase diagram shows only rather narrow stability windows for the monoclinic and cubic phases at the Ga- and In-rich edges, respectively. In this contribution we crystallize amorphous (InxGa1-x)2O3 films deposited on sapphire substrates in-situ in an environmental holder in the transmission electron microscopy. The samples were heated up to 1000 °C in different annealing atmospheres. To obtain a clean and damage free electron transparent lamella, an alternative preparation method based on wedge polishing of the specimen was used. For all studied samples we observed initially crystallization in a metastable phase before at higher temperatures the material either transformed into the expected thermodynamically stable phase or decomposed into grains of different composition. Our calculations show that for (InxGa1-x)2O3 alloys entropy associated with the cation configuration present in the initial amorphous material stabilizes the metastable phase. This finding is interesting from application point of view since the accessible composition range is largely extended.

Resume : Supported metallic nanoparticles (NPs) used in heterogeneous catalysis have generally to be calcined and reduced when preparing and conditioning catalytic systems. During these heat treatments, NPs growth by Ostwald Ripening or coalescence (sintering) has to be controlled if not avoided since it generally leads to a degradation of the catalytic properties. It is today possible to perform such treatments in situ in an environmental Transmission Electron Microscope (ETEM) under gas (air, oxygen or hydrogen) at high temperature and to study the growth mechanisms in pseudo-real time (see e.g. [1]). We have developed here an advanced software to track accurately the trajectories (and events such as the NPs disappearance or fusion) of hundreds or more individuals from time-lapse series of STEM (Scanning TEM) images in the case of metal-based NPs diffusing on transition alumina as a supporting phase during in situ ETEM experiments. It intends to provide statistically representative data describing the evolution of the system and to allow a quantitative evaluation of the effect of electron beam exposure conditions on the diffusion behavior of the NPs population [2]. [1] A.T. DeLaRiva et al., J. of Catalysis, 308 (2013) 291. [2] Thanks are due to CLYM (www.clym.fr) for access to the FEI-Titan ETEM microscope. This study is financially supported by the ANR through the 3DCLEAN project n°15-CE09-0009-01 and by the IngeLyse federation (FR 3411 CNRS, www.ingelyse.com) at University of Lyon.

Resume : A comparison of the catalytic activity of two phases (? and ?) of MnO2 for CO oxidation was studied. It was observed that due to higher surface reducibility of ?-phase, bare ?-MnO2 nanorods converted CO completely at a lower temperature compared to the ?-MnO2 nanorods. The rationale behind this was given using XPS and TPR experiments. The TPR experiments show a striking difference in the CO consumption plots over reducibility over ?- and ?-MnO2 nanorods. This difference might indicate a variation in the way these nanostructures opt to reduce. To study this in detail, we explore and compare the shape and structural transformations under a reducing atmosphere. A surface transformation is expected when the material tries to accommodate a higher concentration of oxygen vacancies. In this chapter, we report a study of the morphological and structural transformation under (a) CO atmosphere (ex-situ heating) and (b) reducing conditions inside TEM (in-situ heating). We discuss the changes in structure and morphology for both the phases and attempt to see if any reverse transformation of the reduced materials occurs under O2 treatment. It is observed that ?- and ?-MnO2 behave very differently under redox conditions. After the post-reduction materials are O2-treated, it is observed that ?-MnO2 tends to completely revert to its original structure while ?-MnO2 irreversibly converts to Mn2O3. Using these results, we comment on the regeneration and reusability of the catalysts for applications that involve reducing conditions at higher temperatures.

Resume : Micro- and nano-structure particularities of new materials for specific applications, such as additive manufacturing, are influenced by atomic scale effects. In order to understand and exploit these effects, advanced ex- and in-situ characterization methods are necessary. This paper presents advanced microscopic methods correlated with theoretical calculations that were used to understand the mechanisms of twin formation, spinodal decomposition, pipe diffusion and nucleation in different alloy types. Two- and three-dimensional STEM acquisition of high angular annular dark field images (HAADF) and spectrum image data sets of both dispersive X-ray (EDX) and electron energy loss spectrometry (EELS) are key tools for the localization and identification of the alloying elements with low concentration as well as their influence on stable and metastable phases. [1-6] Information regarding the recrystallization of grains, diffusion of alloying elements and nucleation of new phases at different temperatures was obtained by in-situ high resolution STEM observations. 1. Li JH., Albu M., Hofer F., Schumacher P., Acta Mater. 83 (2015), 187?202. 2. Albu M., et al, Nature Scientific Reports 6 (2016), 31635 3. Gariboldi E., et al, Acta Mater. 125 (2017), 50-57 4. Li JH., et al, Adv. Eng. Mat. 19 (2017), 1600705 5. Haberfelner G., Orthacker A., Albu M., Li JH., Kothleitner G., Nanoscale 6 (2014), 14563-14569 6. Orthacker A, et al, Nature Materials 17 (2018), 1101-1107 7. Part of this research has received funding from the FFG under project no. SP2018-003-006

Resume : Significant advances in the development of functional nanomaterials can be achieved by using in-situ transmission electron microscopy in liquid environment (liquid TEM), that allows dynamic investigations of nanomaterials in their native state at the highest spatial and temporal resolution. This ground breaking approach opens wide range of possibilities in high-resolution in situ dynamic studies where case-by-case specialized experiments can be performed inside liquid chambers. On the other hand, water radiolysis intimately interrelated with the irradiation of high-energy electron beam during specimen observation is posing serious challenges in the resulting data interpretation. Namely, water radiolysis will produce radicals, which are especially strong oxidizers or reducers, implying that the system cannot be freely observed without affecting it. In view of this, results of dynamic processes inside liquid TEM, which control nucleation, early growth and dissolution of gold will be presented. Special emphasize will be given to the modelling of chemical environment inside these cells as a consequence of water radiolysis. It will be shown that only by establishment of precise quantitative model for radiolysis, experimental results can be put in the proper context. For example, by including both, the radiolysis products and analyite (Au) in the whole experimental reaction sequence. We will correlate the outcomes of these models with the experimental data to predict the growth and dissolution regions of gold nanoparticles at various electron doses and temperatures of solution.