Simulated High-resolution Transmission Electron Microscopy Research Articles

Unexpected modulation revealed by electron diffraction in Ni-Mn-Ga-Co-Cu tetragonal martensite exhibiting giant magnetic field-induced strain

Although the modulation of martensite structure has been considered as a precondition for highly mobile twin boundary and thus a giant magnetic field induced strain, the largest strain to date has been reported in non-modulated tetragonal martensite of Ni-Mn-Ga-Co-Cu. Using high-resolution transmission electron microscopy (HRTEM), we found that nominally non-modulated tetragonal Ni46Mn24Ga22Co4Cu4 single crystal possesses a modulation. The modulation was observed in thin lamellae containing b/c twins prepared by a focused Ga-ion beam and on a sample prepared by a conventional wedge thinning method. The modulation vector q* = (0, 0.2554, 0.4642), was determined by 3D electron diffraction. The simulated HRTEM image of such a structure agrees with the HRTEM micrograph. The presence of modulation in the thin lamellae and not in bulk suggests that Ni-Mn-Ga-Co-Cu martensite is on the edge of the shear instability enabling the high mobility of twin boundaries.

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.

Dislocations in Grain Boundary Regions: The Origin of Heterogeneous Microstrains in Nanocrystalline Materials

Nanocrystalline materials reveal excellent mechanical properties but the mechanism by which they deform is still debated. X-ray line broadening indicates the presence of large heterogeneous strains even when the average grain size is smaller than 10 nm. Although the primary sources of heterogeneous strains are dislocations, their direct observation in nanocrystalline materials is challenging. In order to identify the source of heterogeneous strains in nanocrystalline materials, we prepared Pd-10 pct Au specimens by inert gas condensation and applied high-pressure torsion (HPT) up to γ ≅ 21. High-resolution transmission electron microscopy (HRTEM) and molecular dynamic (MD) simulations are used to investigate the dislocation structure in the grain interiors and in the grain boundary (GB) regions in the as-prepared and HPT-deformed specimens. Our results show that most of the GBs contain lattice dislocations with high densities. The average dislocation densities determined by HRTEM and MD simulation are in good correlation with the values provided by X-ray line profile analysis. Strain distribution determined by MD simulation is shown to follow the Krivoglaz–Wilkens strain function of dislocations. Experiments, MD simulations, and theoretical analysis all prove that the sources of strain broadening in X-ray diffraction of nanocrystalline materials are lattice dislocations in the GB region. The results are discussed in terms of misfit dislocations emanating in the GB regions reducing elastic strain compatibility. The results provide fundamental new insight for understanding the role of GBs in plastic deformation in both nanograin and coarse grain materials of any grain size.

Multiple-ellipse fitting method to precisely measure the positions of atomic columns in a transmission electron microscope image

In this paper, we propose a multiple-ellipse fitting method to accurately determine the atomic column positions in transmission electron microscopy (TEM) images. The column is enclosed by a series of ellipses fitted from contour lines at equidistant intensity levels, and each atomic column is shaped by an averaged elliptical shape to obtain its positions. In particular, the intensity profile of the atomic column can be obtained by an elliptically rotational average based on its shape; therefore, the intensities of the neighbouring atomic column can be subtracted for each atomic column during subsequent position refinement. This method can achieve precision in the picometre range, and we quantitatively measure this precision by analysing an image containing two Gaussian-shaped atoms and some simulated high-resolution transmission electron microscopy (HRTEM) images of SrTiO3.

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.

Well-Defined Silanols in the Structure of the Calcined High-Silica Zeolite SSZ-70: New Understanding of a Successful Catalytic Material.

The structure of the calcined form of the high-silica zeolite SSZ-70 has been elucidated by combining synchrotron X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HRTEM), and two-dimensional (2D) dynamic nuclear polarization (DNP)-enhanced NMR techniques. The framework structure of SSZ-70 is a polytype of MWW and can be viewed as a disordered ABC-type stacking of MWW-layers. HRTEM and XRPD simulations show that the stacking sequence is almost random, with each layer being shifted by ±1/3 along the ⟨110⟩ direction with respect to the previous one. However, a small preponderance of ABAB stacking could be discerned. DNP-enhanced 2D 29Si{29Si} J-mediated NMR analyses of calcined Si-SSZ-70 at natural 29Si isotopic abundance (4.7%) establish the through-covalent-bond 29Si-O-29Si connectivities of distinct Si sites in the framework. The DNP-NMR results corroborate the presence of MWW layers and, more importantly, identify two distinct types of Q3 silanol species at the surfaces of the interlayer regions. In the first, an isolated silanol group protrudes into the interlayer space pointing toward the pocket in the adjacent layer. In the second, the surrounding topology is the same, but the isolated -SiOH group is missing, leaving a nest of three Si-O-H groups in place of the three Si-O-Si linkages. The analyses clarify the structure of this complicated material, including features that do not exhibit long-range order. With these insights, the novel catalytic behavior of SSZ-70 can be better understood and opportunities for enhancement recognized.

Evaluation of strain fields caused by the η′ phase in an Al-Zn-Mg-Cu alloy

Strain fields around the η′ phase precipitated in an Al-12Zn-2.4Mg-1.1Cu alloy were quantitatively measured employing high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA) technique. Afterwards, experimental strain maps were compared with the simulated strain maps obtained from a simulated HRTEM image which was created using the CrystalMaker and JEMS software. Compressive strain is found around the η′ phase, and the maximum compressive strain is about 2.8%. The simulated results are consistent with the experimental results to describe the distributions of strain fields around the η′ phase along the x direction which is parallel to [1¯1¯1 ]Al direction.

Exploring the concurrence of phase transition and grain growth in nanostructured alloy

In nanostructured alloys, the concurrence of (solid-state) grain growth and phase transition is ubiquitously observed; however, due to the lack of kinetic evidences, the underlying physics of concurrence remains scarcely understood. In this study, for nanostructured Fe alloy as a model system, macro- and micro-scale characterizations for the concurrence were carried out using in situ X-ray diffraction and in situ high-resolution transmission electron microscopy (HRTEM). Macroscopically, the grain growth occurs and ceases before the end of phase transition, and microscopically, the concurrence of grain boundary (GB) and phase boundary (PB) migrations was certified as well. These experimental results, together with the ex situ HRTEM and molecular dynamics simulation, uncovered the interaction between GB and PB migrations, i.e., both velocity and direction of the PB migration are influenced when the PB interacts with the GB. On this basis, the concurrence was utilized to produce a new kind of heterogeneous and hierarchical microstructure (i.e., dual-phase bimodal nanostructure). The present findings, yield a deep understanding of the phase transition in nanostructured alloys, and further, demonstrate the potential usage of concurrence in manipulating the nanostructures for the development of nanostructured alloys.

Thickness and rotational effects in simulated HRTEM images of graphene on hexagonal boron nitride.

Recent studies have shown that when graphene is placed on a thin hexagonal boron nitride (h-BN) substrate, unlike when it is placed on a typical SiO2 surface, it can closely approach the ideal carrier mobility observed in suspended graphene samples. This study further examines the epitaxial relationship between graphene and h-BN substrate with high-resolution transmission electron microscopy simulation. Virtual monolayer and multilayer stacks of h-BN were produced with a monolayer of graphene on top, on bottom, and in between h-BN layers, in order to study this interface. Once the simulations were performed, the phase contrast image and Moiré pattern created by this heterostack were analyzed for local and global intensity minima and maxima. In addition, h-BN substrate thickness and rotations between h-BN and graphene were probed and analyzed. The simulated images produced in this work will be used to help understand subsequent transmission electron microscopy images and electron energy-loss studies.

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.

SSZ-52, a Zeolite with an 18-Layer Aluminosilicate Framework Structure Related to That of the DeNOx Catalyst Cu-SSZ-13

A new zeolite (SSZ-52, |(C14H28N)6Na6(H2O)18|[Al12Si96O216]), related to the DeNOx catalyst Cu-SSZ-13 (CHA framework type), has been synthesized using an unusual polycyclic quaternary ammonium cation as the structure-directing agent. By combining X-ray powder diffraction (XPD), high-resolution transmission electron microscopy (HRTEM) and molecular modeling techniques, its porous aluminosilicate framework structure (R3m, a = 13.6373(1) Å, c = 44.7311(4) Å), which can be viewed as an 18-layer stacking sequence of hexagonally arranged (Si,Al)6O6 rings (6-rings), has been elucidated. The structure has a three-dimensional 8-ring channel system and is a member of the ABC-6 family of zeolites (those that can be described in terms of 6-ring stacking sequences) like SSZ-13, but it has cavities that are twice as large. The code SFW has been assigned to this new framework type. The large cavities contain pairs of the bulky organic cations. HRTEM and XPD simulations show that stacking faults do occur, but only at the 5-10% level. SSZ-52 has considerable potential as a catalyst in the areas of gas conversion and sequestration.

Complex Defect in Pyrite and Its Structure Model Derived from Geometric Phase Analysis

New methods for defect analysis can lead to improved interpretation of experimental data and thus better understanding of material properties. Although transmission electronmicroscopy (TEM) has been used to study defects for many decades, interpretive ambiguities can arise for cases that seem simple or even trivial.Using geometric phase analysis (GPA), an image processing procedure, we show that an apparent simple line defect in pyrite has an entirely different character. It appears to be a b = ½[100] edge dislocation as viewed in a [001] high-resolution TEM (HRTEM) image, but the measured u(x) and u(y) displacements are asymmetric, which is inconsistent with a simple line dislocation. Instead, the defect is best understood as a terminating {101} marcasite slab in pyrite. The simulated HRTEM image based on this model reproduces the defect contrast and illustrates the power of GPA analysis for (1) avoiding potential pitfalls of misinterpreting apparently simple defects in HRTEM images, (2) detecting differences in elastic properties at the atomic scale, and (3) providing data for the positions of atom columns, thereby facilitating the construction of structure models for complex defects.

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.

Correlation of the atomic and electronic structures and the optical properties of the Σ5(2 1 0)/[ 0 01] symmetric tilt grain boundary in yttrium aluminum garnet

A series of atomic models of the Σ5(210)/[001] symmetric tilt grain boundary in yttrium aluminum garnet (YAG) are constructed according to the coincident site lattice theory. Calculations performed using empirical potentials show that the O-termination configurations, namely G(1) and G(2), are the most energetically favorable boundary structures. First-principles density functional theory calculations have been further performed to understand the atomic and electronic structures, effective charge, potential distributions and optical properties of G(1) and G(2) grain boundaries. The simulated high-resolution transmission electron microscopy images of G(1) and G(2) are generally in good agreement with the experimental micrographs of the Σ5(210)/[001] grain boundaries. Results show that the effective charges of the atoms in the grain boundary region are related to their coordination numbers and bonding lengths. Moreover, the overall total density of states of G(1) and G(2) has been found to have similar features with the bulk YAG, with the exception of some defect states being introduced at the top of the valence band, resulting in the reduction of the band gap. The calculated optical properties show that the refractive indices of both grain boundary models are slightly larger than those of the bulk YAG. This is in agreement with the experimental observation that the refractive index of the polycrystalline YAG is higher than that of its single crystalline counterpart.

A unique arrangement of atoms for the homologous compounds InMO3(ZnO)m (M = Al, Fe, Ga, and In)

In this paper, a 2m×2m supercell with a unique arrangement of the atoms is taken as the unit cell of the InMO3(ZnO)m systems. A new formation rule of the atomic structure for these systems is suggested. The In atoms occupy the octahedral sites to form InO2− layer. The M atoms in the MO(ZnO)m+ (M/Zn-O) slab located at the tetrahedral or trigonal bipyramidal sites are aligned along the [1¯10] direction of the hexagonal lattice, forming a stepped configuration with a bilateral symmetry. The zigzag boundary can be clearly identified in our simulated high-resolution transmission electron microscopy (HRTEM) images based on this structure, which shows a good agreement with the experimental results. This structure has the minimum formation energy compared with other reported models based on the total-energy calculations. All of the HRTEM experimental data can be well explained based on the model. It gives us a new point of view to understand and study these materials.