Atomic-resolution Scanning Transmission Electron Microscopy Research Articles

High-Entropy Transition Metal Phosphorus Trichalcogenides for Rapid Sodium Ion Diffusion.

High-entropy strategies are regarded as a powerful means to enhance performance in energy storage fields. The improved properties are invariably ascribed to entropy stabilization or synergistic cocktail effect. Therefore, the manifested properties in such multicomponent materials are usually unpredictable. Elucidating the precise correlations between atomic structures and properties remains a challenge in high-entropy materials (HEMs). Herein, atomic-resolution scanning transmission electron microscopy annular dark field (STEM-ADF) imaging and four dimensions (4D)-STEM are combined to directly visualize atomic-scale structural and electric information in high-entropy FeMnNiVZnPS3. Aperiodic stacking is found in FeMnNiVZnPS3 accompanied by high-density strain soliton boundaries (SSBs). Theoretical calculation suggests that the formation of such structures is attributed to the imbalanced stress of distinct metal-sulfur bonds in FeMnNiVZnPS3. Interestingly, the electric field concentrates along the two sides of SSBs and gradually diminishes toward the two-dimensional (2D) plane to generate a unique electric field gradient, strongly promoting the ion-diffusion rate. Accordingly, high-entropy FeMnNiVZnPS3 demonstrates superior ion-diffusion coefficients of 10-9.7-10-8.3 cm2 s-1 and high-rate performance (311.5 mAh g-1 at 30 A g-1). This work provides an alternative way for the atomic-scale understanding and design of sophisticated HEMs, paving the way for property engineering in multi-component materials.

Single atoms and metal nanoclusters anchored to graphene vacancies

Fabricating dispersed single atoms and size-controlled metal nanoclusters remains a difficult challenge due to sintering. Here, we demonstrate that atoms and clusters can be immobilized using atomically clean defect-engineered graphene as the matrix. The graphene is first cleaned of surface contamination with laser heating, after which low-energy Ar irradiation is used to create spatially well-separated vacancies into it. Metal atoms are then evaporated either via thermal or ebeam evaporation onto graphene, where they diffuse until being trapped into a vacancy. The density of embedded structures can be controlled through irradiation dose, and the size of the structures through evaporation time. The resulting structures are confirmed through atomic-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. We demonstrate here incorporation of Al, Ti, Fe, Ag and Au single atoms or nanoclusters, but the method should work equally well for other elements.

New Poisson denoising method for pulse-count STEM imaging

With the recent progress in the development of detectors in electron microscopy, it has become possible to directly count the number of electrons per pixel, even with a scintillator-type detector, by incorporating a pulse-counting module. To optimize a denoising method for electron counting imaging, in this study, we propose a Poisson denoising method for atomic-resolution scanning transmission electron microscopy images. Our method is based on the Markov random field model and Bayesian inference, and we can reduce the electron dose by a factor of about 15 times or further below. Moreover, we showed that the method of reconstruction from multiple images without integrating them performs better than that from an integrated image.

Above-room-temperature chiral skyrmion lattice and Dzyaloshinskii–Moriya interaction in a van der Waals ferromagnet Fe3−xGaTe2

Skyrmions in existing 2D van der Waals (vdW) materials have primarily been limited to cryogenic temperatures, and the underlying physical mechanism of the Dzyaloshinskii–Moriya interaction (DMI), a crucial ingredient for stabilizing chiral skyrmions, remains inadequately explored. Here, we report the observation of Néel-type skyrmions in a vdW ferromagnet Fe3−xGaTe2 above room temperature. Contrary to previous assumptions of centrosymmetry in Fe3−xGaTe2, the atomic-resolution scanning transmission electron microscopy reveals that the off-centered FeΙΙ atoms break the spatial inversion symmetry, rendering it a polar metal. First-principles calculations further elucidate that the DMI primarily stems from the Te sublayers through the Fert–Lévy mechanism. Remarkably, the chiral skyrmion lattice in Fe3−xGaTe2 can persist up to 330 K at zero magnetic field, demonstrating superior thermal stability compared to other known skyrmion vdW magnets. This work provides valuable insights into skyrmionics and presents promising prospects for 2D material-based skyrmion devices operating beyond room temperature.

Synthesis and in-depth structure determination of a novel metastable high-pressure CrTe3 phase

This study reports the synthesis and crystal structure determination of a novel CrTe3 phase using various experimental and theoretical methods. The average stoichiometry and local phase separation of this quenched high-pressure phase were characterized by ex situ synchrotron powder X-ray diffraction and total scattering. Several structural models were obtained using simulated annealing, but all suffered from an imperfect Rietveld refinement, especially at higher diffraction angles. Finally, a novel stoichiometrically correct crystal structure model was proposed on the basis of electron diffraction data and refined against powder diffraction data using the Rietveld method. Scanning electron microscopy–energy-dispersive X-ray spectrometry (EDX) measurements verified the targeted 1:3 (Cr:Te) average stoichiometry for the starting compound and for the quenched high-pressure phase within experimental errors. Scanning transmission electron microscopy (STEM)–EDX was used to examine minute variations of the Cr-to-Te ratio at the nanoscale. Precession electron diffraction (PED) experiments were applied for the nanoscale structure analysis of the quenched high-pressure phase. The proposed monoclinic model from PED experiments provided an improved fit to the X-ray patterns, especially after introducing atomic anisotropic displacement parameters and partial occupancy of Cr atoms. Atomic resolution STEM and simulations were conducted to identify variations in the Cr-atom site-occupancy factor. No significant variations were observed experimentally for several zone axes. The magnetic properties of the novel CrTe3 phase were investigated through temperature- and field-dependent magnetization measurements. In order to understand these properties, auxiliary theoretical investigations have been performed by first-principles electronic structure calculations and Monte Carlo simulations. The obtained results allow the observed magnetization behavior to be interpreted as the consequence of competition between the applied magnetic field and the Cr–Cr exchange interactions, leading to a decrease of the magnetization towards T = 0 K typical for antiferromagnetic systems, as well as a field-induced enhanced magnetization around the critical temperature due to the high magnetic susceptibility in this region.

Picometer-Scale Atomic Shifts Governing Subdisordered Structures in Diamond.

Diamond is considered the most promising next-generation semiconductor material due to its excellent physical characteristics. It has been more than three decades since the discovery of a special structure named n-diamond. However, despite extensive efforts, its crystallographic structure and properties are still unclear. Here, we show that subdisordered structures in diamond provide an explanation for the structural feature of n-diamond. Monocrystalline diamond with subdisordered structures is synthesized via the chemical vapor deposition method. Atomic-resolution scanning transmission electron microscopy characterizations combined with the picometer-precision peak finder technology and diffraction simulations reveal that picometer-scale shifts of atoms within cells of diamond govern the subdisordered structures. First-principles calculations indicate that the bandgap of diamond decreases rapidly with increasing shifting distance, in accordance with experimental results. These findings clarify the crystallographic structure and electronic properties of n-diamond and provide new insights into the bandgap adjustment in diamond.

Bayesian inference of atomic column positions in scanning transmission electron microscopy images

Abstract Atomic-resolution scanning transmission electron microscopy combined with 2D Gaussian fitting enables the accurate and precise identification of atomic column positions within a few picometers. The measurement performance significantly depends on the signal-to-noise ratio of the atomic columns. In areas with low signal-to-noise ratios, such as near surfaces, the measurement performance was lower than that of the bulk. However, previous studies evaluated the accuracy and precision only in bulk areas, underscoring the need for a method that quantitatively evaluates the accuracy and precision of each atomic column position with various signal-to-noise ratios. This study introduced Bayesian inference to assess the accuracy and precision of determining individual atomic column positions under various signals. We applied this method to simulated and experimental images and demonstrated its effectiveness in identifying statistically significant displacements, particularly near surfaces with signal degradation. The use of vector maps and kernel density estimate plots obtained from Bayesian inference provided a probabilistic understanding of the atom displacement. Therefore, this study highlighted the potential benefits of Bayesian inference in high-resolution imaging to reveal material properties.

Microstructural investigation of epitaxial aluminum films grown by molecular beam epitaxy

High-quality epitaxial Al thin films with thickness in nano-to atomic-scale were grown on Si, sapphire, and GaAs substrates by molecular beam epitaxy. Microstructure of Al films as well as the interfacial bonding between Al and substrates were carefully investigated. Structural analyses from x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) shows that the Al (111) films are epitaxially grown on Si, sapphire, and GaAs substrates. Atomic-resolution STEM imaging reveals smooth and sharp interfaces without any interlayer between Al films and substrate; and the interfacial bonding are determined as Al–Si, Al–O, and Al–Ga for Al grown on Si, sapphire, and GaAs, respectively. Atomic force microscopy shows very smooth surfaces of Al films with roughness of 0.16–0.75 nm. Secondary ion mass spectrometry (SIMS) reveals the chemical composition of the deposited films, highlighting the presence of minor amounts of trapped hydroxide content.

Tuning grain boundary cation segregation with oxygen deficiency and atomic structure in a perovskite compositionally complex oxide thin film

Compositionally complex oxides (CCOs) are an emerging class of materials encompassing high entropy and entropy stabilized oxides. These promising advanced materials leverage tunable chemical bond structure, lattice distortion, and chemical disorder for unprecedented properties. Grain boundary (GB) and point defect segregation to GBs are relatively understudied in CCOs even though they can govern macroscopic material properties. For example, GB segregation can govern local chemical (dis)order and point defect distribution, playing a critical role in electrochemical reaction kinetics, and charge and mass transport in solid electrolytes. However, compared with conventional oxides, GBs in multi-cation CCO systems are expected to exhibit more complex segregation phenomena and, thus, prove more difficult to tune through GB design strategies. Here, GB segregation was studied in a model perovskite CCO LaFe0.7Ni0.1Co0.1Cu0.05Pd0.05O3−x textured thin film by (sub-)atomic-resolution scanning transmission electron microscopy imaging and spectroscopy. It is found that GB segregation is correlated with cation reducibility—predicted by an Ellingham diagram—as Pd and Cu segregate to GBs rich in oxygen vacancies (VO··). Furthermore, Pd and Cu segregation is highly sensitive to the concentration and spatial distribution of VO·· along the GB plane, as well as fluctuations in atomic structure and elastic strain induced by GB local disorder, such as dislocations. This work offers a perspective of controlling segregation concentration of CCO cations to GBs by tuning reducibility of CCO cations and oxygen deficiency, which is expected to guide GB design in CCOs.

Dependence of the Structural and Magnetic Properties on the Growth Sequence in Heterostructures Designed by YbFeO3 and BaFe12O19.

The structure and the chemical composition of individual layers as well as of interfaces belonging to the two heterostructures M1 (BaFe12O19/YbFeO3/YSZ) and M2 (YbFeO3/BaFe12O19/YSZ) grown by pulsed laser deposition on yttria-stabilized zirconia (YSZ) substrates are deeply characterized by using a combination of methods such as high-resolution X-ray diffraction, transmission electron microscopy (TEM), and atomic-resolution scanning TEM with energy-dispersive X-ray spectroscopy. The temperature-dependent magnetic properties demonstrate two distinct heterostructures with different coercivity, anisotropy fields, and first anisotropy constants, which are related to the defect concentrations within the individual layers and to the degree of intermixing at the interface. The heterostructure with the stacking order BaFe12O19/YbFeO3, i.e., M1, exhibits a distinctive interface without any chemical intermixture, while an Fe-rich crystalline phase is observed in M2 both in atomic-resolution EDX maps and in mass density profiles. Additionally, M1 shows high c-axis orientation, which induces a higher anisotropy constant K1 as well as a larger coercivity due to a high number of phase boundaries. Despite the existence of a canted antiferromagnetic/ferromagnetic combination (T < 140 K), both heterostructures M1 and M2 do not reveal any detectable exchange bias at T = 50 K. Additionally, compressive residual strain on the BaM layer is found to be suppressing the ferromagnetism, thus reducing the Curie temperature (Tc) in the case of M1. These findings suggest that M1 (BaFe12O19/YbFeO3/YSZ) is suitable for magnetic storage applications.

Quantifying the thickness of WTe2 using atomic-resolution STEM simulations and supervised machine learning.

For two-dimensional (2D) materials, the exact thickness of the material often dictates its physical and chemical properties. The 2D quantum material WTe2 possesses properties that vary significantly from a single layer to multiple layers, yet it has a complicated crystal structure that makes it difficult to differentiate thicknesses in atomic-resolution images. Furthermore, its air sensitivity and susceptibility to electron beam-induced damage heighten the need for direct ways to determine the thickness and atomic structure without acquiring multiple measurements or transferring samples in ambient atmosphere. Here, we demonstrate a new method to identify the thickness up to ten van der Waals layers in Td-WTe2 using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy image simulation. Our approach is based on analyzing the intensity line profiles of overlapping atomic columns and building a standard neural network model from the line profile features. We observe that it is possible to clearly distinguish between even and odd thicknesses (up to seven layers), without using machine learning, by comparing the deconvoluted peak intensity ratios or the area ratios. The standard neural network model trained on the line profile features allows thicknesses to be distinguished up to ten layers and exhibits an accuracy of up to 94% in the presence of Gaussian and Poisson noise. This method efficiently quantifies thicknesses in Td-WTe2, can be extended to related 2D materials, and provides a pathway to characterize precise atomic structures, including local thickness variations and atomic defects, for few-layer 2D materials with overlapping atomic column positions.

Local defect and mid-gap state analysis of GaN using monochromated EELS combined with nanodiffraction and atomic-resolution imaging

Defects in semiconductor materials significantly impact their inherent properties, making the evaluation of local defects and their energy levels crucial for controlling device performance. With advancements in monochromators, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) has emerged as a promising approach for assessing crystal and band structures of semiconductor materials at the nanoscale. However, there is limited experimental research investigating the relationship between actual defective structures in local regions and mid-gap states. In this study, we conducted high-energy resolution EELS (HR-EELS) measurements with an energy resolution of less than 100 meV to detect the mid-gap states of GaN. Various defects were intentionally induced through Ga-ion implantation, and the defect concentration varied in the depth direction. To understand the origin of the mid-gap states, we performed 4D-STEM analysis and atomic-resolution STEM observations. The HR-EELS measurements provided insights into the depth-dependent valence-loss spectra, revealing that the intensities corresponding to mid-gap states gradually increased toward the surface, whereas the slope at the onsets corresponding to interband transition decreased. Furthermore, local structural analysis unveiled the presence of structural disorder and defective structures, indicating the existence of extended defects such as stacking faults and domain boundaries. Observably, these defective structures were abundant near the surface and less pronounced in deeper regions. Based on these experimental findings, we concluded that the variations in valence-loss spectra can be utilized to qualitatively evaluate the crystal imperfections at the nanoscale.

Novel Polytype of III-VI Metal Chalcogenides Nano Crystals Realized in Epitaxially Grown InTe.

III-VI metal chalcogenides have garnered considerable research attention as a novel group of layered van der Waals materials because of their exceptional physical properties and potential technological applications. Here, the epitaxial growth and stacking sequences of InTe is reported, an essential and intriguing material from III-VI metal chalcogenides. Aberration-corrected scanning transmission electron microscopy (STEM) is utilized to directly reveal the interlayer stacking modes and atomic structure, leading to a discussion of a new polytype. Furthermore, correlations between the stacking sequences and interlayer distances are substantiated by atomic-resolution STEM analysis, which offers evidence for strong interlayer coupling of the new polytype. It is proposed that layer-by-layer deposition is responsible for the formation of the unconventional stacking order, which is supported by ab initio density functional theory calculations. The results thus establish molecular beam epitaxy as a viable approach for synthesizing novel polytypes. The experimental validation of the InTe polytype here expands the family of materials in the III-VI metal chalcogenides while suggesting the possibility of new stacking sequences for known materials in this system.

Depth sectioning using environmental and atomic-resolution STEM.

(Scanning) transmission electron microscopy (TEM) images of samples in gas and liquid media are acquired with an environmental cell (EC) via silicon nitride membranes. The ratio of sample signal against the background is a significant factor for resolution. Depth-sectioning scanning TEM (STEM) is a promising technique that enhances the signal for a sample embedded in a matrix. It can increase the resolution to the atomic level, thereby enabling EC-STEM applications in important areas. This review introduces depth-sectioning STEM and its applications to high-resolution EC-STEM imaging of samples in gases and in liquids.

Atomic-scale investigation of γ-Ga2O3 deposited on MgAl2O4 and its relationship with β-Ga2O3

Nominally phase-pure γ-Ga2O3 was deposited on (100) MgAl2O4 within a narrow temperature window centered at ∼470 °C using metal-organic chemical vapor deposition. The film deposited at 440 °C exhibited either poor crystallization or an amorphous structure; the film grown at 500 °C contained both β-Ga2O3 and γ-Ga2O3. A nominally phase-pure β-Ga2O3 film was obtained at 530 °C. Atomic-resolution scanning transmission electron microscopy (STEM) investigations of the γ-Ga2O3 film grown at 470 °C revealed a high density of antiphase boundaries. A planar defect model developed for γ-Al2O3 was extended to explain the stacking sequences of the Ga sublattice observed in the STEM images of γ-Ga2O3. The presence of the 180° rotational domains and 90° rotational domains of β-Ga2O3 inclusions within the γ-Ga2O3 matrix is discussed within the context of a comprehensive investigation of the epitaxial relationship between those two phases in the as-grown film at 470 °C and the same film annealed at 600 °C. The results led to the hypotheses that (i) incorporation of certain dopants, including Si, Ge, Sn, Mg, Al, and Sc, into β-Ga2O3 locally stabilizes the “γ-phase” and (ii) the site preference(s) for these dopants promotes the formation of “γ-phase” and/or γ-Ga2O3 solid solutions. However, in the absence of such dopants, pure γ-Ga2O3 remains the least stable Ga2O3 polymorph, as indicated by its very narrow growth window, lower growth temperatures relative to other Ga2O3 polymorphs, and the largest calculated difference in Helmholtz free energy per formula unit between γ-Ga2O3 and β-Ga2O3 than all other polymorphs.

Denary High-Entropy Oxide Nanoparticles Synthesized by a Continuous Supercritical Hydrothermal Flow Process.

High-entropy oxide nanoparticles (HEO NPs) have been intensively studied because of their attractive properties, such as high stability and enhanced catalytic activity. In this work, for the first time, denary HEO NPs were successfully synthesized using a continuous supercritical hydrothermal flow process without calcination. Interestingly, this process allows the formation of HEO NPs on the order of seconds at a relatively lower temperature. The synthesized HEO NPs contained 10 metal elements, La, Ca, Sr, Ba, Fe, Mn, Co, Ru, Pd, and Ir, and had a perovskite-type structure. Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements revealed homogeneous dispersion of the 10 metal elements. The obtained HEO NPs also exhibited a higher catalytic activity for the CO oxidation reaction than that of the LaFeO3 NPs.

Atomic Diffusion-Induced Polarization and Superconductivity in Topological Insulator-Based Heterostructures.

The proximity effect at a highly transparent interface of an s-wave superconductor (S) and a topological insulator (TI) provides a promising platform to create Majorana zero modes in artificially designed heterostructures. However, structural and chemical issues pertinent to such interfaces have been poorly explored so far. Here, we report the discovery of Pd diffusion-induced polarization at interfaces between superconductive Pd1+x(Bi0.4Te0.6)2 (xPBT, 0 ≤ x ≤ 1) and Pd-intercalated Bi2Te3 by using atomic-resolution scanning transmission electron microscopy. Our quantitative image analysis reveals that nanoscale lattice strain and QL polarity synergistically suppress and promote Pd diffusion at the normal and parallel interfaces, formed between Te-Pd-Bi triple layers (TLs) and Te-Bi-Te-Bi-Te quintuple layers (QLs), respectively. Further, our first-principles calculations unveil that the superconductivity of the xPBT phase and topological nature of the Pd-intercalated Bi2Te3 phase are robust against the broken inversion symmetry. These findings point out the necessity of considering the coexistence of electric polarization with superconductivity and topology in such S-TI systems.

Atomically Unraveling Highly Crystalline Self-Intercalated Tantalum Sulfide with Correlated Stacking Registry-Dependent Magnetism.

In self-intercalated two-dimensional (ic-2D) materials, understanding the local chemical environment and the topology of the filling site remains elusive, and the subsequent correlation with the macroscopically manifested physical properties has rarely been investigated. Herein, highly crystalline gram-scale ic-2D Ta1.33S2 crystals were successfully grown by the high-pressure high-temperature method. Employing combined atomic-resolution scanning transmission electron microscopy annular dark field imaging and density functional theory calculations, we systematically unveiled the atomic structures of an atlas of stacking registries in a well-defined √3(a) × √3(a) Ta1.33S2 superlattice. Ferromagnetic order was observed in the AC' stacking registry, and it evolves into an antiferromagnetic state in AA/AB/AB' stacking registries; the AA' stacking registry shows ferrimagnetic ordering. Therefore, we present a novel approach for fabricating large-scale highly crystalline ic-2D crystals and shed light on a powerful means of modulating the magnetic order of ic-2D systems via stacking engineering, i.e., stackingtronics.

Enantiomorph conversion in single crystals of the Weyl semimetal CoSi

Chiral intermetallic phases may show unusual chemical and physical properties with nontrivial structure-property relationship. It is therefore of particular interest to study the structural conversion between domains of different handedness. Here, the atomic decoration of the enantiomorph exchange area within single crystal of the Weyl semimetal CoSi is determined by a combination of atomic-resolution scanning transmission electron microscopy imaging, single crystal X-ray diffraction and quantum chemical analysis of atomic interactions. Two-atomic [CoSi] units are shown to be the bonding base for the FeSi-type structure and may be considered as ‘pseudo-molecules’, thinking of molecular organic crystals. Tiny reorganisation of atomic interactions within these units results in the appearance of sequence ‘faults’ in the structure pattern i.e. in a different structural motif in the enantiomorph exchange area, which – contrary to the A and B enantiomorphs of CoSi – contains an inversion centre and allows a local enantiomorph ‘conversion’. Due to the special features of atomic interactions, the reorganisation of multi-atomic bonds leads to slightly higher total energy. This appears within one and the same grain which is prepared by the short distance chemical vapor transport.

La-Al Intermetallic Alloy Anode for Realizing High-Energy Fluoride-Ion Battery

An all-solid-state fluoride-ion battery is one of the promising candidates for the next-generation high-energy batteries owing to the high theoretical energy density. However, the practical capacities of anodes are significantly low compared with cathodes, and therefore it is an urgent task to develop new anode materials for fluoride-ion batteries. Here, we show that the LaAl3 alloy anode delivers a reversible high capacity of 298 mAh g−1 with only 0.66% capacity fading per cycle. By using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy, we investigate the structural and chemical evolution of LaAl3. We find that LaAl3 is firstly decomposed into LaF3 and AlF3 nanocrystals, forming the nanoscale network of the F– ion conduction path owing to the high ionic conductivity of LaF3. In the subsequent cycles, the redox reaction of Al/AlF3 nanocrystals solely proceeds, contributing to the reversible high capacity. Our findings should open new avenues for realizing high-energy fluoride-ion batteries.