Simulated High-resolution Transmission Electron Microscopy Images Research Articles

Reconstructing the exit wave of 2D materials in high-resolution transmission electron microscopy using machine learning

Reconstruction of the exit wave function is an important route to interpreting high-resolution transmission electron microscopy (HRTEM) images. Here we demonstrate that convolutional neural networks can be used to reconstruct the exit wave from a short focal series of HRTEM images, with a fidelity comparable to conventional exit wave reconstruction. We use a fully convolutional neural network based on the U-Net architecture, and demonstrate that we can train it on simulated exit waves and simulated HRTEM images of graphene-supported molybdenum disulphide (an industrial desulfurization catalyst). We then apply the trained network to analyse experimentally obtained images from similar samples, and obtain exit waves that clearly show the atomically resolved structure of both the MoS2 nanoparticles and the graphene support. We also show that it is possible to successfully train the neural networks to reconstruct exit waves for 3400 different two-dimensional materials taken from the Computational 2D Materials Database of known and proposed two-dimensional materials.

Recombination of Shockley partial dislocations by electron beam irradiation in wurtzite GaN

Dissociated a-type screw dislocations in gallium nitride, comprising pairs of 30° Shockley partial dislocations separated by I2 basal stacking faults, were observed by aberration-corrected high resolution transmission electron microscopy (HRTEM). HRTEM image simulations, in conjunction with density functional theory calculations, led to the identification of the core structures of the Shockley partials. Both partials were found to belong to the glide set rather than the shuffle one, while the core with gallium polarity is reconstructed, but the one with nitrogen polarity is not. During in situ irradiation by the electron beam, the I2 stacking fault ribbon was found to shrink, ultimately leading to a remerging of the two partials. This reversal of the dissociation reaction was attributed to recombination enhanced dislocation glide, whereby the Shockley partial with nitrogen polarity was identified to be the mobile one. A possible model explaining this mobility is proposed comprising a local modification of the dislocation's electronic structure due to the presence of nitrogen vacancies at its core.

Irradiation damage in nuclear graphite at the atomic scale

Microstructures of nuclear graphite have been investigated based on the analysis of both X-ray diffraction patterns and High Resolution Transmission Electron Microscopy images (HRTEM) from experiments and from molecular dynamics simulations. One filler particle – composed of almost aligned crystallites separated by Mrozowski cracks – and the whole sequence of single crystal graphite irradiated from its virgin state up to highly damaged (64.5% of defects) have been simulated. Simulations are assessed against experiments based on the analysis of its microstructures extracted with Bragg's law and Scherrer's equation applied to simulated X-ray diffraction patterns. In particular, the evolution of cell parameters with defect concentration generated by irradiation agrees well with experimental observations. Simulated HRTEM images show many features observed in experimental images in both virgin and irradiated French nuclear graphite. Some of these features can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleading interpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues of graphene layers in highly damaged graphite.

Identification of Extended Defect Atomic Configurations in Silicon Through Transmission Electron Microscopy Image Simulation

We used atomistic simulation tools to correlate experimental transmission electron microscopy images of extended defects in crystalline silicon with their structures at an atomic level. Reliable atomic configurations of extended defects were generated using classical molecular dynamics simulations. Simulated high-resolution transmission electron microscopy (HRTEM) images of obtained defects were compared to experimental images reported in the literature. We validated the developed procedure with the configurations proposed in the literature for {113} and {111} rod-like defects. We also proposed from our procedure configurations for {111} and {001} dislocation loops with simulated HRTEM images in excellent agreement with experimental images.

Determination of atomic-scale chemical composition at semiconductor heteroepitaxial interfaces by high-resolution transmission electron microscopy

The determination of atomic structures and further quantitative information such as chemical compositions at atomic scale for semiconductor defects or heteroepitaxial interfaces can provide direct evidence to understand their formation, modification, and/or effects on the properties of semiconductor films. The commonly used method, high-resolution transmission electron microscopy (HRTEM), suffers from difficulty in acquiring images that correctly show the crystal structure at atomic resolution, because of the limitation in microscope resolution or deviation from the Scherzer-defocus conditions. In this study, an image processing method, image deconvolution, was used to achieve atomic-resolution (∼1.0 Å) structure images of small lattice-mismatch (∼1.0%) AlN/6H-SiC (0001) and large lattice-mismatch (∼8.5%) AlSb/GaAs (001) heteroepitaxial interfaces using simulated HRTEM images of a conventional 300-kV field-emission-gun transmission electron microscope under non-Scherzer-defocus conditions. Then, atomic-scale chemical compositions at the interface were determined for the atomic intermixing and Lomer dislocation with an atomic step by analyzing the deconvoluted image contrast. Furthermore, the effect of dynamical scattering on contrast analysis was also evaluated for differently weighted atomic columns in the compositions.

Atomistic modelling and high resolution electron microscopy simulations of CeO2 nanoparticles

In cerium oxide nanoparticles, the facets exposed control their reactivity and catalytic behavior and hence their performance in applications such as three-way catalysis, gas sensors, or solid fuel cells. Thus, the precise characterization of the cerium oxide facets exposed is crucial. In the present work, the geometry of branched cerium oxide nanoparticles, obtained by means of ligand-controlled overgrowth, is determined through the comparison between High Resolution Transmission Electron Microscopy (HRTEM) experimental and simulated images. Two possible 3D structures, corresponding to octapod and tetrapod geometries, were suggested from a preliminary examination of the experimental HRTEM images. A comparative analysis of the gray scale intensity profiles of the simulated and experimental HRTEM images was then performed. This method proved capable of discriminating between the two proposed geometries, showing CeO2 nanoparticles to have an octapod geometry. The obtained results were further confirmed by comparing High Angular Dark Field experimental and simulated images of the nanoparticles. This method can be suitable whenever 3D-Tomographic reconstruction is not feasible in the TEM, for instance, in the case of highly beam sensitive materials.

Electron Microscopy Study of Binary Nanocolloidal Crystals with ico-AB13 Structure Made of Monodisperse Silica Nanoparticles

Binary nanocolloidal crystals (BNCCs) made of two different nanoparticles are a promising new class of advanced functional materials with synergetic and collective properties. These materials have been shown to exhibit excellent structural diversity and a wide range of different stoichiometries. In this study, BNCCs with an ico-AB13 structure were prepared as inorganic bulk materials by simple solvent evaporation of silica nanocolloidal aqueous solutions. Systematic analysis of Fourier diffractograms of high-resolution transmission electron microscopy (HRTEM) images shows the reflection conditions of the space group Fm3̅c without any ambiguities, which is indicative of the ico-AB13 structure. Simulated HRTEM images based on the ideal ico-AB13 structure were also calculated under the kinematical scattering approximation. These images showed the characteristic fine structure of the ico-AB13 structure along the high-symmetry axes. The agreement between the observed and simulated HRTEM images indicates the validity of the ideal ico-AB13 model structure. This result shows that HRTEM together with a quantitative image simulation is a versatile method for the characterization of the BNCCs at the nanoscale and can be routinely applied for any nanocolloidal crystals, even for complex structures.

Two new superstructures Ba4Ti5O10 and Ba4Ti4O11 in epitaxial barium titanate nanodomains determined by nanobeam electron diffraction and high-resolution transmission electron microscopy

Barium titanate thin films deposited on MgO (0 0 1) by RF magnetron sputtering are composed of two new superstructures Ba4Ti5O10 and Ba4Ti4O11 formed as epitaxial nanodomain structures. Due to the nanometre scale size of domains and the complexity of the structure in the film, the single-crystal X-ray diffraction technique does not have the capability for the crystallographic structure determination of the two new superstructures. We used nanobeam electron diffraction to reconstruct the three-dimensional diffraction space and hence the symmetry of the new superstructures. Both Ba4Ti5O10 and Ba4Ti4O11 are monoclinic structures with a space group of Cm (b-unique axis). Ba4Ti5O10 has a lattice parameter a = 16.49 Å, b = 3.94 Å, c = 8.94 Å and β = 103°, while Ba4Ti4O11 has a = 17.88 Å, b = 3.94 Å, c = 7.21 Å and β = 98°. Atomic structural models for the two new superstructures were established by reconstructing the high-resolution transmission electron microscopy (HRTEM) images taken from the three major axes and refined by matching the simulated HRTEM images and calculated electron diffraction patterns with the experimental results. The two superstructures are epitaxially grown on MgO with their b-axis parallel to the growth direction.

A new modulated structure in α-Fe2O3 nanowires

A new modulated structure consisting of periodic (112̄0) stacking faults (SFs) in the α-Fe2O3 nanowires (NWs) formed by the thermal oxidation of Fe foils is reported, using a combination of high-resolution transmission electron microscopy (HRTEM) observations and HRTEM image simulations. The periodicity of the modulated structure is 1.53 nm, which is ten times (33̄00) interplanar spacing and can be described by a shift of every ten (33̄00) planes with 1/2 the interplanar spacing of the (112̄0) plane. An atomic model for the Fe2O3 structure is proposed to simulate the modulated structure. HRTEM simulation results confirm that the modulated structure in α-Fe2O3 NWs is caused by SFs.

High-resolution transmission electron microscopy study of twinned ZnS nanoparticles

The microstructural properties of twinned ZnS nanoparticles were investigated by high-resolution transmission electron microscopy (HRTEM) and the National Center for Electron Microscopy Simulation System. ZnS nanoparticles have a zinc blende structure with {111} twin boundaries. However, the HRTEM images that did not correspond to the lattice fringe of zinc blende were observed in certain directions. Interestingly, the HRTEM images matched with periodically twinned structures such as wurtzite 8H and 10H. To understand this phenomenon, an atomic model of the twinned zinc blende containing a (111) twin boundary was simulated. The simulated HRTEM images matched well with the experimental HRTEM images.

Optimal accelerating voltage for HRTEM imaging of zeolite

The electron irradiation damage of MFI-type zeolite was estimated under various accelerating voltages of 100, 200 and 300 kV from successively captured high-resolution transmission electron microscopy (HRTEM) images. To determine the optimal accelerating voltage for HRTEM imaging of electron-sensitive MFI zeolite, the critical dose was estimated from the disappearance of a specific fast Fourier transform spot calculated from experimental images. Based only on the electron dose, a higher voltage was more advantageous. However, taking into account the minimum dose for imaging with a CCD camera, the optimal accelerating voltage for imaging MFI zeolite was 200 kV. The minimum dose for image detection with a CCD camera was surmised from the output/input signal ratio dependence on the accelerating voltage and the contrast range in simulated HRTEM images.

Atomistic modeling and HRTEM analysis of misfit dislocations in InN/GaN heterostructures

The enhanced structural mismatch of InN and GaN binary alloys leads in almost spontaneous formation of misfit dislocations (MDs) at the corresponding interfaces, thereby accommodating plastic relaxation. The open issue of the MD array in-plane configuration is addressed through a combination of high resolution transmission electron microscopy (HRTEM) observations with energetic mapping and HRTEM image simulation of InN/GaN interfaces extracted by atomistic modeling. Energetic mapping on the interfacial area of InN/GaN supercells relaxed by the Tersoff interatomic potential, designates that the MD arrays adopt 〈1 1 2¯ 0〉 line directions and their Burgers vectors are b=1/3〈2 1¯ 1¯ 0〉. HRTEM image simulations further reveal that the local arrangement of Moiré fringes observed in these interfaces depends strongly on the thickness of the TEM foil, thus resolving contradictory experimental reports. Geometric Phase Analysis on the simulated images justifies the results obtained by energetic mapping.

A-site occupancy in the lead-free (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 piezoceramic: Combining first-principles study and TEM

The crystal structure of the lead-free piezoelectric ceramic (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 was investigated by first-principles calculations and high-resolution transmission electron microscopy (HRTEM) imaging. Structures with different A-site occupation were relaxed by total energy calculations within density functional theory and then used for simulating the corresponding HRTEM images. Simulated and experimental HRTEM images were compared and the closest match selected for structure interpretation. By combining these techniques, we have identified the Bi(Ba)/Na distribution on the A-site to be homogeneous. We exclude the possibility that regions visible in HRTEM images within one grain can be attributed to different ordering but to a slight tilting of the structure with respect to the electron beam.

Expansion of interatomic distances in platinum catalyst nanoparticles

Abstract We study the atomistic structure of Pt catalyst nanoparticles using HRTEM (high-resolution transmission electron microscopy). The particles exhibit a faceted, cubo-octahedral shape, extended planar defects, and mono-atomic surface steps. HRTEM imaging with negative spherical aberration yielded atomic-resolution images with a minimum of artifacts. Combining digital image processing, quantitative image analysis, and HRTEM image simulations to determine local variations of the spacing between neighboring Pt atom columns, we have found an expansion of the lattice parameter in the particle core and even larger, locally varying expansion of Pt–Pt next-neighbor distances at the particle surface. The latter likely originates from an amorphous oxide on the nanoparticle surface and/or dissolution of oxygen on subsurface sites. These structural features may significantly impact the catalytic activity of Pt nanoparticles.

Octahedral tilt twinning and compositional modulation in NaLaMgWO6

A combination of selected-area electron diffraction (SAED), neutron powder diffraction (NPD) and high-resolution transmission electron microscopy (HRTEM) reveals a complex superstructure in the ordered perovskite NaLaMgWO(6). Through indexing of SAED patterns the unit-cell dimensions are found to be 46.8 x 7.8 x 7.9 A, which corresponds to a 12a(p) x 2a(p) x 2a(p) superstructure of the simple Pm3m perovskite unit cell. HRTEM images reveal the formation of an unmistakable stripe contrast that repeats with the same periodicity. Doubling of the b and c axes is brought about by a combination of layered ordering of Na and La, rock-salt ordering of Mg and W, and octahedral tilting. The a axis repeat distance results from a one-dimensional twinning of the octahedral tilts in combination with a compositional modulation. Modeling of the NPD pattern shows that the underlying tilt system is a(-)a(-)c(0) with tilt angles of approximately 8 degrees about the a and b axes. The octahedral tilt-twin boundaries run perpendicular to the a axis and are separated by 6a(p). Simulated HRTEM images show that octahedral tilt twinning alone cannot explain the stripes seen in the HRTEM images, rather a compositional modulation involving the A-site cations is necessary to explain the experimental images.

A series of aluminum tungsten oxides crystallizing in a new ReO 3-related structure type

A series of new aluminum tungsten oxides with the general formula Al 4W 2 n O 6 n +2 ( n=4–7) was found and structurally characterized by electron diffraction, high-resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The structural model for Al 4W 10O 32 ( I4/ mmm (space group no. 139); a≈0.375, c≈3.95 nm) consists of slabs of [5×∞×∞] corner-sharing WO 6 octahedra that are connected via edge-sharing to AlO 6 octahedra. Simulated HRTEM images agree well with the experimental ones and thus support the proposed structural model. The connection between adjacent slabs of WO 3 via AlO 6 octahedra represents a novel variant of crystallographic shear operation for ReO 3-type structures. The crystallites display a wide range of stacking sequences that are frequently intergrown with each other.

Compositional analysis of mixed–cation‐anion III–V semiconductor interfaces using phase retrieval high‐resolution transmission electron microscopy

Employing exit-plane wave function (EPWF) reconstruction in high-resolution transmission electron microscopy (HRTEM), we have developed an approach to atomic scale compositional analysis of III-V semiconductor interfaces, especially suitable for analyzing quaternary heterostructures with intermixing in both cation and anion sub-lattices. Specifically, we use the focal-series reconstruction technique, which retrieves the complex-valued EPWF from a thru-focus series of HRTEM images. A study of interfaces in Al(0.4)Ga(0.6)As-GaAs and In(0.25)Ga(0.75)Sb-InAs heterostructures using focal-series reconstruction shows that change in chemical composition along individual atomic columns across an interface is discernible in the phase image of the reconstructed EPWF. To extract the interface composition profiles along the cation and anion sub-lattices, quantitative analysis of the phase image is performed using factorial analysis of correspondence. This enabled independent quantification of changes in the In-Ga and As-Sb contents across ultra-thin interfacial regions (approximately 0.6 nm wide) with true atomic resolution, in the In(0.25)Ga(0.75)Sb-InAs heterostructure. The validity of the method is demonstrated by analyzing simulated HRTEM images of an InAs-GaSb-InAs model structure with abrupt and graded interfaces. Our approach is general, permitting atomic-level compositional analysis of heterostructures with two species per sub-lattice, hitherto unfeasible with existing HRTEM methods.

Microstructures of ZnO films deposited on (0001) and r-cut α-Al 2O 3 using metal organic chemical vapor deposition

Zinc oxide films were deposited on (0001) and r-cut α-Al 2O 3 under identical conditions using metal organic chemical vapor deposition. Microstructures of the ZnO films were studied in detail using conventional and high-resolution transmission electron microscopy (HRTEM), electron diffraction, and HRTEM image simulations. The films deposited on these two substrates show distinctive structural differences. The film grown on r-cut α-Al 2O 3 shows a high-quality single crystal with an orientation relationship of α-Al 2O 3[− 101–1]//ZnO[0001] and α-Al 2O 3(10–1–2)//ZnO(2–1–10). The interface between the film and the substrate was abrupt and decorated with high density of misfit dislocations. Film grown on α-Al 2O 3 (0001) shows several orientation domains. Typically, one domain correspond to the classic growth model such that α-Al 2O 3 (0001)//ZnO(0001) and α-Al 2O 3 [11–20]//ZnO[10–10]. Another domain corresponds to the growth mode such that α-Al 2O 3 [11–20]//ZnO[10–10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of α-Al 2O 3 such that ZnO(0001) is almost parallel to the α-Al 2O 3 (− 1104) plane. This orientation reduces the extent of lattice mismatch as compared with the classic growth mode. The interface between ZnO and α-Al 2O 3 is abrupt and possesses periodic dislocations.

Imaging of dioctahedral 2:1 layers by high-resolution transmission electron microscopy (HRTEM): Possibility of recording the dehydroxylate

High-resolution transmission electron microscopy (HRTEM) images of dioctahedral 2:1 phyllosilicates (muscovite, paragonite, pyrophyllite, etc.) acquired by intense electron radiation may not record the natural state but rather the dehydroxylate phase, e.g., NaAl2Si3AlO11 in the case of paragonite, NaAl2Si3AlO10(OH)2. Intense electron radiation on paragonite changes its electron diffraction pattern by a small increase in cell edges and a considerable decrease of the β angle, which are consistent with dehydroxylation. Comparison between experimental and simulated HRTEM images also indicates that the experimental image contrast is in better agreement with that for the dehydroxylate structure than the natural state. Thus, special care is necessary when analyzing the structures of dioctahedral 2:1 phyllosilicates from their HRTEM images, e.g., positions of octahedral cations that were proposed to change by migration during the dehydroxylation process

Series of modulated structures inSrxCoO2and the modulation rule

The nominal composition of Sr{sub 0.39}CoO{sub 2}, prepared by a low-temperature ion exchange technique, has been systematically studied by transmission electron microscopy, energy dispersive x-ray spectroscopy (EDX), and electron-energy-loss spectroscopy (EELS). Besides a well-defined {radical}(3)a{sub p}x{radical}(3)a{sub p} modulated structure, a series of long periodic modulated structures (n{radical}(3)a{sub p}x3a{sub p}) was revealed by high-resolution transmission electron microscopy (HRTEM) in some microzones, which are induced by alternate position ordering of strontium ions. The results of EDX analyses confirm the variation of Sr content in corresponding modulated phases. Moreover, the L{sub 3}/L{sub 2} intensity ratio of Co-L{sub 2,3} edges in the EELS spectra indicates the variation of Co ionic valence state induced by the different contents of Sr cations. Based on the {radical}(3)a{sub p}x{radical}(3)a{sub p} superstructural model, we propose the possible corresponding structural models in this paper. The simulated HRTEM images, based on those models, are consistent with experimental ones. In addition, several phase separations and some structural defects were observed in the sample.