Effects of electron channeling in HAADF-STEM intensity in La2CuSnO6

Different atomic contrasts in HAADF images and EELS maps of rutile TiO2

High-angle annular dark-field (HAADF) imaging and elemental mapping at the atomic scale by scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) are widely used for material characterization, in which quantitative understanding of the contrast of the image is required. Here, we report an unexpected image contrast in the elemental mapping of rutile TiO2, where the Ti L2,3 map shows an anisotropic elliptical shape that extends along the long axis in the octahedral structure, while the atomic contrast of Ti columns in the HAADF image is almost circular. Multi-slice simulation reveals that unique electron channeling related to the rutile structure and the difference of the potentials between HAADF and EELS cause the different atomic contrasts in the two images.

Direct Visualization of Trimerized States in 1T^{'}-TaTe_{2}

Transition-metal dichalcogenides containing tellurium anions show remarkable charge-lattice modulated structures and prominent interlayer character. Using cryogenic scanning transmission electron microscopy (STEM), we map the atomic-scale structures of the high temperature (HT) and low temperature (LT) modulated phases in 1T^{'}-TaTe_{2}. At HT, we directly show in-plane metal distortions which form trimerized clusters and staggered, three-layer stacking. In the LT phase at 93 K, we visualize an additional trimerization of Ta sites and subtle distortions of Te sites by extracting structural information from contrast modulations in plan-view STEM data. Coupled with density functional theory calculations and image simulations, this approach opens the door for atomic-scale visualizations of low temperature phase transitions and complex displacements in a variety of layered systems.

Self-Epitaxial Hetero-Nanolayers and Surface Atom Reconstruction in Electrocatalytic Nickel Phosphides

Surface atomic, compositional, and electronic structures play decisive roles in governing the performance of catalysts during electrochemical reactions. Nevertheless, for efficient and cheap transition-metal phosphides used for water splitting, such atomic-scale structural information is largely missing. Despite much effort being made so far, there is still a long way to go for establishing a precise structure-activity relationship. Here, in combination with electron-beam bombardment and compositional analysis, our atomic-scale transmission electron microscopy study on Ni₅P₄ nanosheets, with a preferential (001) orientation, directly reveals the coverage of a self-epitaxial Ni₂P nanolayer on the phosphide surface. Apart from the presence of nickel vacancies in the Ni₅P₄ phase, our quantum-mechanical image simulations also suggest the existence of an additional NiP_x (0 < x < 0.5) nanolayer, characteristic of complex surface atom reconstruction, on the outermost surface of the phosphides. The surface chemical gradient and the core-shell scenario, probably responsible for the passivated catalytic activity, provide a novel insight to understand the catalytic performance of transition-metal catalysts used for electrochemical energy conversion.

Study of interfacial random strain fields in core-shell ZnO nanowires by scanning transmission electron microscopy

Imaging strain fields at the nanoscale is crucial for understanding the physical properties as well as the performance of oxide heterostructures and electronic devices. Based on scanning transmission electron microscopy (STEM) techniques, we successfully imaged the random strain field at the interface of core-shell ZnO nanowires. Combining experimental observations and image simulations, we find that the strain contrast originates from dechanneling of electrons and increased diffuse scattering induced by static atomic displacements. For a thin sample with a random strain field, a positive strain contrast appears in the low-angle annular dark-field (LAADF) image and a negative contrast in the high-angle annular dark-field (HAADF) image, but for a thick sample (> 120 nm), the positive contrast always occurs in both the LAADF and HAADF images. Through the analysis of the relationship between strain contrast and various parameters, we also discuss the optimum experimental condition for imaging random strain fields.

Unconventional Defects in a Quasi-One-Dimensional KMn6Bi5

Quasi-one-dimensional (Q1D) structures comprising a compact array of indefinitely long 1D nanowires (NWs) are scarce, especially in a bulk device-scale showing metallic and semiconducting behaviors along different axes. Unlike plentiful observations of nature of defects in three-/two-dimensional materials, there is a notable paucity of such reports in Q1D. Herein we present unconventional motific defects and their properties in a bulk Q1D KMn₆Bi₅ crystal, in which an individual NW motif acts as one body. We discovered motific inter- and intra-NW defects, such that a linear set of 1D motifs are displaced. Stress generates two domains with altered inter-NW spacings and a Bi-Mn solid solution grain, leading to a local bulk plasmon shift due to NW array reconfiguration as well as atomic rearrangement. The observation of such exotic defects and associated phenomena in this Q1D may provide guidance on overall defect mechanism in other Q1D systems and their collective anisotropic properties.

Antisite Pairs Suppress the Thermal Conductivity of BAs

BAs was predicted to have an unusually high thermal conductivity with a room temperature value of 2000  W m^{-1} K^{-1}, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is still lower than this value. To identify the origin of this large inconsistency, we investigate the lattice structure and potential defects in BAs single crystals at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration of As vacancies (V_{As}), as widely thought to dominate the thermal resistance in BAs, our STEM results show an enhanced intensity of some B columns and a reduced intensity of some As columns, suggesting the presence of antisite defects with As_{B} (As atom on a B site) and B_{As} (B atom on an As site). Additional calculations show that the antisite pair with As_{B} next to B_{As} is preferred energetically among the different types of point defects investigated and confirm that such defects lower the thermal conductivity for BAs. Using a concentration of 1.8(8)% (6.6±3.0×10^{20}  cm^{-3} in density) for the antisite pairs estimated from STEM images, the thermal conductivity is estimated to be 65-100  W m^{-1} K^{-1}, in reasonable agreement with our measured value. Our study suggests that As_{B}-B_{As} antisite pairs are the primary lattice defects suppressing thermal conductivity of BAs. Possible approaches are proposed for the growth of high-quality crystals or films with high thermal conductivity. Employing a combination of state-of-the-art synthesis, STEM characterization, theory, and physical insight, this work models a path toward identifying and understanding defect-limited material functionality.

Nature and evolution of incommensurate charge order in manganites visualized with cryogenic scanning transmission electron microscopy

Charge order is a modulation of the electron density and is associated with unconventional phenomena, including colossal magnetoresistance and metal–insulator transitions. Determining how the lattice responds provides insights into the nature and symmetry of the ordered state. Scanning transmission electron microscopy can measure lattice displacements with picometer precision, but its use has been limited to room-temperature phases only. Here, we demonstrate high-resolution imaging at cryogenic temperature and map the nature and evolution of charge order in a manganite. We uncover picometer-scale displacive modulations whose periodicity is strongly locked to the lattice and visualize temperature-dependent phase inhomogeneity in the modulations. These results pave the way to understanding the underlying structure of charge-ordered states and other complex phenomena.

Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map, at atomic resolution, the nature of incommensurate charge–lattice order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature (∼93 K). In diffraction, the ordering wave vector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the ordered state forms lattice-locked regions over a few wavelengths interspersed with phase defects and changing periodicity. The cations undergo picometer-scale (∼6 pm to 11 pm) transverse displacements, suggesting that charge–lattice coupling is strong. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative “incommensurate” order in hole-doped oxides.

Simplifying Electron Beam Channeling in Scanning Transmission Electron Microscopy (STEM)

Sub-angstrom scanning transmission electron microscopy (STEM) allows quantitative column-by-column analysis of crystalline specimens via annular dark-field images. The intensity of electrons scattered from a particular location in an atomic column depends on the intensity of the electron probe at that location. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the beam intensity incident at different depths. Understanding the parameters that control this complex behavior is critical for interpreting experimental STEM results. In this work, theoretical analysis of the STEM probe intensity reveals that intensity oscillations during specimen propagation are regulated by changes in the beam's angular distribution. Three distinct regimes of channeling behavior are observed: the high-atomic-number (Z) regime, in which atomic scattering leads to significant angular redistribution of the beam; the low-Z regime, in which the probe's initial angular distribution controls intensity oscillations; and the intermediate-Z regime, in which the behavior is mixed. These contrasting regimes are shown to exist for a wide range of probe parameters. These results provide a new understanding of the occurrence and consequences of channeling phenomena and conditions under which their influence is strengthened or weakened by characteristics of the electron probe and sample.

Atomic Resolution Interfacial Structure of Lead-free Ferroelectric K0.5Na0.5NbO3 Thin films Deposited on SrTiO3

Oxide interface engineering has attracted considerable attention since the discovery of its exotic properties induced by lattice strain, dislocation and composition change at the interface. In this paper, the atomic resolution structure and composition of the interface between the lead-free piezoelectric (K_0.5Na_0.5)NbO₃ (KNN) thin films and single-crystalline SrTiO₃ substrate were investigated by means of scanning transmission electron microscopy (STEM) combining with electron energy loss spectroscopy (EELS). A sharp epitaxial interface was observed to be a monolayer composed of Nb and Ti cations with a ratio of 3/1. The First-Principles Calculations indicated the interface monolayer showed different electronic structure and played the vital role in the asymmetric charge distribution of KNN thin films near the interface. We also observed the gradual relaxation process for the relatively large lattice strains near the KNN/STO interface, which remarks a good structure modulation behavior of KNN thin films via strain engineering.

Homotopy-Theoretic Study &Atomic-Scale Observation of Vortex Domains in Hexagonal Manganites

Essential structural properties of the non-trivial "string-wall-bounded" topological defects in hexagonal manganites are studied through homotopy group theory and spherical aberration-corrected scanning transmission electron microscopy. The appearance of a "string-wall-bounded" configuration in RMnO3 is shown to be strongly linked with the transformation of the degeneracy space. The defect core regions (~50 Å) mainly adopt the continuous U(1) symmetry of the high-temperature phase, which is essential for the formation and proliferation of vortices. Direct visualization of vortex strings at atomic scale provides insight into the mechanisms and macro-behavior of topological defects in crystalline materials.

STEM image simulation with hybrid CPU/GPU programming

STEM image simulation is achieved via hybrid CPU/GPU programming under parallel algorithm architecture to speed up calculation on a personal computer (PC). To utilize the calculation power of a PC fully, the simulation is performed using the GPU core and multi-CPU cores at the same time to significantly improve efficiency. GaSb and an artificial GaSb/InAs interface with atom diffusion have been used to verify the computation.

Simulating realistic imaging conditions for in situ liquid microscopy

In situ transmission electron microscopy enables the imaging of biological cells, macromolecular protein complexes, nanoparticles, and other systems in a near-native environment. In order to improve interpretation of image contrast features and also predict ideal imaging conditions ahead of time, new virtual electron microscopic techniques are needed. A technique for virtual fluid-stage high-angle annular dark-field scanning transmission electron microscopy with the multislice method is presented that enables the virtual imaging of model fluid-stage systems composed of millions of atoms. The virtual technique is exemplified by simulating images of PbS nanoparticles under different imaging conditions and the results agree with previous experimental findings. General insight is obtained on the influence of the effects of fluid path length, membrane thickness, nanoparticle position, defocus and other microscope parameters on attainable image quality.

Surface channeling in aberration-corrected scanning transmission electron microscopy of nanostructures

The aberration-corrected scanning transmission electron microscope can provide information on nanostructures with sub-Angström image resolution. The relatively intuitive interpretation of high-angle annular dark-field (HAADF) imaging technique makes it a popular tool to image a variety of samples and finds broad applications to characterizing nanostructures, especially when combined with electron energy-loss spectroscopy and X-ray energy-dispersive spectroscopy techniques. To quantitatively interpret HAADF images, however, requires full understanding of the various types of signals that contribute to the HAADF image contrast. We have observed significant intensity enhancement in HAADF images, and large expansion of lattice spacings, of surface atoms of atomically flat ZnO surfaces. The surface-resonance channeling effect, one of the electron-beam channeling phenomena in crystalline nanostructures, was invoked to explain the observed image intensity enhancement. A better understanding of the effect of electron beam channeling along surfaces or interfaces on HAADF image contrast may have implications for quantifying HAADF images and may provide new routes to utilize the channeling phenomenon to study surface structures with sub-Angström spatial resolution.

HAADF-STEM analysis of layered double perovskite La(2)CuSnO(6) grown epitaxially

Structural observation of layered double perovskite oxide La(2)CuSnO(6) thin films grown epitaxially on SrTiO(3) is reported by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Particularly the transition layer at the interface was observed, and the first B site layer at the interface was found to be almost formed by the Cu atomic layer as the random structure, followed by formation of the layered structure. In addition, HAADF-STEM images indicate that the thin film is not single crystalline, but some irregular structures were observed to grow around the interface near atomic steps of the substrate of SrTiO(3). Therefore, the steps largely affect the growth process of the thin film.