Scanning Transmission Electron Microscopy Simulations Research Articles

STEM Structural Investigation of RE‐Au‐Si 1/1 Approximants

AbstractRE‐Au‐Si (RE=Ho, Tb) systems are 1/1 Tsai‐type quasicrystalline approximants with a cluster center decoration that can vary from a disordered tetrahedron to a rare‐earth atom. The local atomic structure of three different samples was observed by scanning transmission electron microscopy and interpreted in the light of high‐angle annular dark field simulated scanning transmission electron microscopy images. It is found that the combination of these two methods allows to identify differences in the chemical decoration of the cluster centers through quantitative analysis of line profiles.

Can we always control the thickness layer in the MBE method with atomic precision? Analysis of the problem on the MQWs GaN/AlN example

The GaN/AlN multiple-quantum-wells (MQWs) structures were studied using high resolution scanning transmission electron microscopy simulations (HR STEM) and the experimental data from HR STEM measurements. GaN/AlN MQWs were synthesized by plasma-assisted molecular beam epitaxy (PAMBE). The electron microscopy methods were used to examine both interfaces. It was shown that AlN /GaN interfaces are sharp while the GaN/AlN are diffuse over two atomic layers. The latter diffusional disorder is not related to the basic limitation of the PAMBE method, but to the chemical growth properties of GaN. The three cases were investigated: sharp interface, diffuse single monolayer (ML) and diffusive two MLs.

On the Structure of Amorphous Mesoporous Silica Nanoparticles by Aberration-Corrected STEM.

Mesoporous silica materials have demonstrated a vast spectrum of applications, stimulating an intensive field of study due to their potential use as nanocarriers. Nonetheless, when produced at the nanoscale, their structural characterization is hindered due to the re-arrangement of the pores. To address this issue, this work combines molecular dynamics simulations with electron microscopy computer simulations and experimental results to provide an insight into the structure of amorphous mesoporous silica nanoparticles. The amorphous silica model is prepared using a simple melt-quench molecular dynamics method, while the reconstruction of the mesoporous nanoparticles is carried out using a methodology to avoid false symmetry in the final model. Simulated scanning transmission electron microscopy images are compared with experimental images, revealing the existence of structural domains, created by the misalignment of the pores to compensate the surface tension of these spherical nanoparticles.

Identifying local structural states in atomic imaging by computer vision.

The availability of atomically resolved imaging modalities enables an unprecedented view into the local structural states of materials, which manifest themselves by deviations from the fundamental assumptions of periodicity and symmetry. Consequently, approaches that aim to extract these local structural states from atomic imaging data with minimal assumptions regarding the average crystallographic configuration of a material are indispensable to advances in structural and chemical investigations of materials. Here, we present an approach to identify and classify local structural states that is rooted in computer vision. This approach introduces a definition of a structural state that is composed of both local and nonlocal information extracted from atomically resolved images, and is wholly untethered from the familiar concepts of symmetry and periodicity. Instead, this approach relies on computer vision techniques such as feature detection, and concepts such as scale invariance. We present the fundamental aspects of local structural state extraction and classification by application to simulated scanning transmission electron microscopy images, and analyze the robustness of this approach in the presence of common instrumental factors such as noise, limited spatial resolution, and weak contrast. Finally, we apply this computer vision-based approach for the unsupervised detection and classification of local structural states in an experimental electron micrograph of a complex oxides interface, and a scanning tunneling micrograph of a defect-engineered multilayer graphene surface.Electronic supplementary materialThe online version of this article (doi:10.1186/s40679-016-0028-8) contains supplementary material, which is available to authorized users.

Chessboard/Diamond Nanostructures and the A-site Deficient, Li1/2–3x Nd1/2+xTiO3, Defect Perovskite Solid Solution

The crystal chemical origin of nanoscale chessboard/diamond ordering in perovskite-related solid solutions of composition Li0.5–3xNd0.5+xTiO3 (LNT, x ∼ 0.02–0.12) is investigated. Experimental and simulated scanning transmission electron microscopy (STEM) images are found to be consistent with the compositional modulation model proposed by previous authors. However, these earlier models do not satisfactorily explain other features observed in high-resolution STEM and TEM images, such as the two-dimensional {100} lattice fringes with the same periodicity, √2ap × √2ap, as the local LNT unit cell viewed along the [001] direction (where ap is the parent perovskite unit cell parameter). Based on bond valence sum calculations, we propose a new set of crystal structures for LNT in which Li ions are primarily bonded to only two O ions, and order one-dimensionally with √2ap periodicity. Bright-field STEM image simulations performed for this new model reproduced the experimentally observed √2ap lattice fringes, thus strongly suggesting that the finer features of the high-resolution (S)TEM images are the result of Li ion ordering and associated local structural relaxation. In this new model, the LNT chessboard supercell then results from the ordered combinations of two sublattices: the Li ion sublattice and its translational variants on the one hand, and the Nd0.5TiO3 sublattice and its oxygen octahedral tilt twin variants on the other. Dielectric measurements indicate the presence of long-lived polar clusters that are easily activated under an applied electric field. This suggests that these clusters consist of spatially correlated Li ions.

Artifacts in aberration-corrected ADF-STEM imaging

The introduction of an experimental black level may introduce unintended artifactual details into high-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) lattice images. This article presents the multislice simulation results of such possible situations. Three simulated scanning transmission electron microscopy (STEM) probes of sizes 0.8, 1.2 and 2.0 Å are scanned on the surface of a 〈 1 ̄ 10〉 oriented Si/Ge crystal. The simulation results suggest that high-frequency artifact peaks will appear in the power spectra when an artificial black level clips the lowest (background) signal. The lowest signal in an ADF-STEM image decreases as the incident probe shrinks in size. Therefore, care must be taken when interpreting the resolution limit of the microscope from images taken with nonzero black level setting, especially in case of sub-Å microscope. The simulation result is compared with an experimental image and they agree with each other. The analysis suggests that aberration corrected STEM provide sensitive low level detail.