Experimental High-resolution Transmission Electron Microscopy Research Articles

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures

WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider $\mathrm{C}\ensuremath{\leftrightarrow}\mathrm{vacancy}$ and $\mathrm{W}\ensuremath{\leftrightarrow}\mathrm{Ti}$ substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Co materials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.

Atomic column heights detection in metallic nanoparticles using deep convolutional learning

The physico-chemical and mechanical properties of metallic (e.g., Au, Pt) nanoparticle complexes highly depend on the order and the distribution of their atomic structure. While transmission electron microscopy (TEM) provides the highest spatial resolution to an image of the nanoparticle’s atomic structure, it is time consuming and cumbersome to estimate the atomic column height from the two-dimensional projected TEM images. With continued progress of in-situ or operando TEM techniques in discovery of nanoscale science, it is paramount to develop artificial intelligence approaches that can be integrated with real-time TEM imaging. In this work, we present a modeling framework based upon deep learning approach (i.e., convolutional neural network – CNN) for the detection of the atomic column heights in the experimental high-resolution transmission electron microscopy (HRTEM) images of gold nanoparticles of different sizes. For this purpose, we propose a method for the generation of the training dataset based on the Wulff construction, in order to bring a physically realistic treatment to the network’s learning process. Moreover, we introduce a model based on the regression scheme, as a valid alternative to a classification approach reported in the prior literature. In addition to counting atoms in verity of columns and nanoparticles, the model also provides insights concerning the experimental conditions suitable for the appropriate identification of atomic column heights by the neural network. Thus, the developed modeling approach establishes a basis for accelerated or ‘on-the-fly’ analysis of nanoparticles as well as a framework for extending deep learning models to broad applications in nanoscience.

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.

Large-scale experimental and theoretical study of graphene grain boundary structures

We have characterized the structure of 176 different single-layer graphene grain boundaries grown with chemical vapor deposition using $g1000$ experimental high-resolution transmission electron microscopy images using a semiautomated structure processing routine. We introduce an algorithm for generating grain boundary structures for a class of hexagonal two-dimensional materials and use this algorithm and molecular dynamics to simulate the structure of $g79\phantom{\rule{0.16em}{0ex}}000$ linear graphene grain boundaries covering 4122 unique orientations distributed over the entire parameter space. The dislocation content and structural properties are extracted from all experimental and simulated boundaries, and various trends are explored. We find excellent agreement between the simulated and experimentally observed grain boundaries. Our analysis demonstrates the power of a statistically significant number of measurements as opposed to a small number of observations in atomic science.

Effects of non-rotationally symmetric aberrations on the quantitative measurement of lattice positions in a graphene monolayer using high-resolution transmission electron microscopy.

It is crucial to determine the position of lattice atoms in a monolayer specimen with high precision using high-resolution transmission electron microscopy (HRTEM). Image simulations indicate that the intensity centers of periodic lattice atoms in graphene may deviate from their intrinsic positions if non-rotationally symmetric aberrations (except for three-fold and six-fold aberrations) exist in the HRTEM imaging system. In this letter, we quantitatively compared the deviations caused by non-rotationally symmetric aberrations, which are equivalent to individually produce a π/4 phase shift in the wave aberration function at a given frequency of 7.2 nm(-1). A two-fold aberration caused a maximum shift of 0.3 Å, and in the images affected by the axial coma, graphene still maintained its hexagonal structure while all of the measured atomic positions deviated. Furthermore, we discovered that atoms on each sublattice tended to shift by similar distances in the image. Based on this rule, we retrieved the intrinsic bond length between neighboring carbon atoms by 'shifting' the measured atom positions in an experimental HRTEM image affected by residual aberrations.

Numerical correction of anti-symmetric aberrations in single HRTEM images of weakly scattering 2D-objects

Here, we present a numerical post-processing method for removing the effect of anti-symmetric residual aberrations in high-resolution transmission electron microscopy (HRTEM) images of weakly scattering 2D-objects. The method is based on applying the same aberrations with the opposite phase to the Fourier transform of the recorded image intensity and subsequently inverting the Fourier transform. We present the theoretical justification of the method, and its verification based on simulated images in the case of low-order anti-symmetric aberrations. Ultimately the method is applied to experimental hardware aberration-corrected HRTEM images of single-layer graphene and MoSe2 resulting in images with strongly reduced residual low-order aberrations, and consequently improved interpretability. Alternatively, this method can be used to estimate by trial and error the residual anti-symmetric aberrations in HRTEM images of weakly scattering objects.

Comparison of intensity and absolute contrast of simulated and experimental high-resolution transmission electron microscopy images for different multislice simulation methods

The Stobbs factor problem, a major difference of absolute contrast between experimental and simulated high resolution transmission electron microscopy images has been reported frequently. In this respect, some modifications to the multislice simulation techniques were proposed to improve the correspondence to the experiment. The influence of different suggestions on the simulated contrast is investigated by numerical simulations. The agreement between experiment and simulations is then checked by comparison with high-resolution micrographs of crystalline gold. The experimental data is therefore compared to simulated intensities computed for the thickness and orientation of the specimen measured by refinements of diffraction patterns. The agreement of both intensity and contrast is investigated and the remaining contrast discrepancy is determined. The results show a good agreement for a small objective aperture, while for a larger aperture a difference of contrast by a factor of 1.2 can still be observed. Without any aperture, the deviation between experiment and simulations is largest.

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.

From Sphere to Multipod: Thermally Induced Transitions of CdSe Nanocrystals Studied by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations are used to show that a spherical zinc blende (ZB) nanocrystal (NC) can transform into a tetrapod or an octapod as a result of heating, by a local zincblende-to-wurtzite phase transformation taking place in the NC. The partial sphere-to-tetrapod or sphere-to-octapod transition occurs within simulation times of 30 ns and depends on both temperature and NC size. Surprisingly, the wurtzite (WZ) subdomains are not formed through a slip mechanism but are mediated by the formation of highly mobile Cd vacancies on the ZB{111} Cd atomic planes. The total potential energy of a tetrapod is found to be lower than that of a ZB sphere at the same numbers of atoms. The simulation results are in good agreement with experimental high-resolution transmission electron microscopy (HR-TEM) data obtained on heated colloidal CdSe NCs.

Inclusion of radiation damage dynamics in high-resolution transmission electron microscopy image simulations: The example of graphene

Computer image simulations provide a crucial aid to high-resolution transmission electron microscopy (HRTEM) in gaining fundamental understanding of the structure of materials. Interpretation of HRTEM images is, however, complicated due to continuous structure deformation caused by the imaging electron beam. A computational methodology has been implemented that takes into account the effects of the electron beam on deformation of sample structure during observation and imaging in HRTEM. The evolution of the sample structure is described as a sequence of externally initiated discrete damage events with a frequency determined by the cross section, which depends on the energy of the electron beam. A series of images showing structure evolution with time is obtained by coupling molecular dynamics simulations with the image simulation. These simulation parts are linked by two experimental parameters: the energy of the electron beam and the electron dose rate. As the energy of the electron beam also determines resolution and contrast of the obtained HRTEM image, a careful selection of its value is required to achieve a fine balance between reduction of the sample damage caused by the electron beam and the quality of the acquired image. The proposed computational approach is used to simulate the recently observed process of structural transformation of a small graphene flake into a fullerene cage. The simulated series of images showing the evolution of a graphene flake under the 80 keV electron beam closely reproduces experimental HRTEM images with regard to the structure evolution route, evolution rate, and signal-to-noise ratio. We show that under the increased electron beam energy of 200 keV a similar observation will be obscured by high damage rate or low signal-to-noise ratio.

Atomic interface structure of bixbyite rare-earth sesquioxides grown epitaxially on Si(1 1 1)

High-resolution transmission electron microscopy (HRTEM) is applied to thin crystalline films of (Gd1−xLux)2O3 and (La1−xLux)2O3 on Si(1 1 1). Electron micrographs confirm that an atomically smooth and chemically abrupt interface of epitaxially grown rare-earth thin films is feasible. This work demonstrates an unambiguous recognition of cubic bixbyite, which is the common cubic phase of rare-earth sesquioxides at room temperature except for the light lanthanoids. This serves as prerequisite to analyse the atomic configuration of the cubic oxide lattice at the interface to the Si(1 1 1) surface. Two new configurations which have not been considered yet, are proposed and proved by comparison of experimental HRTEM images and contrast simulations.

Atomic crystal structure of ordered In3Sb1Te2 ternary alloy studied by high-resolution transmission electron microscopy

The atomic structure of In3Sb1Te2 was investigated by high-resolution transmission electron microscopy (HRTEM) and the National Center for Electron Microscopy Simulation System. In3Sb1Te2 has a disordered NaCl structure; In atoms are positioned in a face-centered cubic lattice, and Sb and Te atoms are located at randomly occupied octahedral sites. However, the observed HRTEM images of the ordered In3Sb1Te2 phase show that Sb and Te atoms are located in specific positions, that is, at the center and edge of the unit cell, respectively. The atomic model of the ordered In3Sb1Te2 is confirmed by simulated images, which match the experimental HRTEM images.

Quantitative analysis of layering and in-plane structural ordering at an alumina–aluminum solid–liquid interface

Real-time observations of Al–Al 2O 3 dynamic liquid–solid interfaces on the atomic scale indicate the presence of structural ordering in the liquid at the solid–liquid interface. The main problem with direct high resolution transmission electron microscopy (HRTEM) interpretation is that the imaging conditions and aberrations in the imaging system have a significant influence on the contrast in the image, and may lead to inaccurate conclusions about the structure examined. New quantitative results based on using a single image iterative wave function reconstruction are presented. This technique requires only a single experimental image, and allows extraction of reliable and aberration-free structural information from experimental HRTEM micrographs. This numerical phase retrieval method was successful in analysis of the experimental data and allowed, for the first time, direct extraction of quantitative information regarding the degree of ordering (parallel and perpendicular to the interface) at liquid–solid interfaces. The degree of ordering at the Al 2O 3–Al interface at 750 °C was quantified and both layering and in-plane ordering were found. The layering in the liquid extends to about four to five layers (about 1 nm from the edge of the crystal). The in-plane ordering, which was observed only in the first three layers of the liquid, decays faster than the layering. In addition, the interlayer spacings measured in the liquid indicate that the liquid atoms at the interface are influenced by the structure of the crystal, while further away the ordering of the liquid atoms gradually disappears, until they adopt the characteristics of the bulk liquid.

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.

Odd electron diffraction patterns in silicon nanowires and silicon thin films explained by microtwins and nanotwins

Odd electron diffraction patterns (EDPs) have been obtained by transmission electron microscopy (TEM) on silicon nanowires grown via the vapour-liquid-solid method and on silicon thin films deposited by electron beam evaporation. Many explanations have been given in the past, without consensus among the scientific community: size artifacts, twinning artifacts or, more widely accepted, the existence of new hexagonal Si phases. In order to resolve this issue, the microstructures of Si nanowires and Si thin films have been characterized by TEM, high-resolution transmission electron microscopy (HRTEM) and high-resolution scanning transmission electron microscopy. Despite the differences in the geometries and elaboration processes, the EDPs of the materials show great similarities. The different hypotheses reported in the literature have been investigated. It was found that the positions of the diffraction spots in the EDPs could be reproduced by simulating a hexagonal structure with c/a = 12(2/3)(1/2), but the intensities in many EDPs remained unexplained. Finally, it was established that all the experimental data, i.e. EDPs and HRTEM images, agree with a classical cubic silicon structure containing two microstructural defects: (i) overlapping Σ3 microtwins which induce extra spots by double diffraction, and (ii) nanotwins which induce extra spots as a result of streaking effects. It is concluded that there is no hexagonal phase in the Si nanowires and the Si thin films presented in this work.

A transmission electron microscopy study on the crystal structure and atomic arrangement of Al–Nd thin films deposited on glass substrates

The crystal structure and atomic arrangement of Al–2 at.% Nd thin films were investigated by a transmission electron microscopy (TEM) study. As a result, we have revealed that Nd atoms exist in Al matrix as solid solutions in the state of as-deposited and annealed at low temperature, while the intermetallic compounds with the composition of Al 4Nd are precipitated as the annealing temperature increased above 300 °C. The precipitated Al 4Nd has a tetragonal structure with nine cyclic layers, and the atomic arrangement was identified by the fast Fourier transform (FFT) and Fourier filtered images as well as the experimental high-resolution transmission electron microscopy (HRTEM) images. Consequently, Nd atoms, forms of solid solutions or precipitates, effectively suppress the grain growth of Al.

Shape effect of microtwins on high‐resolution transmission electron microscope images

AbstractThe shape effect of microtwins on high‐resolution transmission electron microscopy (HRTEM) images is demonstrated with the help of a detailed comparison between experimental HRTEM images of recrystallized silicon (including many microtwins) and corresponding dynamical simulations. The analysis points to an important fact: The shape effect of microtwins leads to artificial contrasts. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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

REW– exit-wave reconstruction and alignments for focus-variation high-resolution transmission electron microscopy images

A Windows-based computer program has been developed for exit-wave reconstruction and experimental high-resolution transmission electron microscopy image alignment. While the exit-wave reconstruction is performed mainly using the maximum-likelihood method, image alignments may be carried out using several algorithms, including the time-consuming but robust genetic algorithm.