Kohei Aso

Electron beam irradiation offers a route for carrier-type control in two-dimensional semiconductors. We fabricated a suspended p -type gallium selenide (GaSe) device and measured its electrical transport using in-situ transmission electron microscopy. Focused electron beam irradiation of the GaSe channel increased current from the nanoampere to the microampere range. Gate-dependent measurements revealed conversion from p -type to n -type conduction, accompanied by an increase in electron density. The increased electron density is expected to narrow the depletion region at the Au/GaSe interfaces, producing nearly linear current–voltage characteristics. These results demonstrate local control of carrier type and density in GaSe devices.

Effective interfacial contacts and conductive pathways within composite electrodes are critical for enhancing the performance of all‐solid‐state batteries (ASSBs). Herein, we fabricated LiCoO ₂ –Li ₁₀ GeP ₂ S ₁₂ (LCO–LGPS) composites with uniform and nonuniform microstructures by controlling composite particle sizes and mixing conditions. The microstructured LCO–LGPS composites were characterized by scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (SEM‐EDX), scanning spreading resistance microscopy (SSRM), and pico‐current conductive atomic force microscopy (C‐AFM). Results reveal that uniform microstructures produce large contact areas between LCO and LGPS with uniform 3‐dimensional (3D) e ^– /Li ⁺ ‐conductive pathways. However, in uniform microstructures, e ^– (LCO)‐ and Li ⁺ (LGPS)‐conductive pathways are interrupted by LGPS and LCO, respectively. In contrast, nonuniform microstructures provide small contact areas between LCO and LGPS, and both well‐ and poorly connected 3D conductive pathways are present. Increasing the microstructural uniformity decreases the overvoltage at low C‐rates but increases the overvoltage with increasing C‐rates because of the interrupted conductive pathways, even though they are uniformly distributed. These findings suggest that “well‐mixing” does not absolutely guarantee the enhanced performance of composite electrodes. It is important to construct e ^– /Li ⁺ ‐conductive pathways well‐connected and uniformly distributed like a 3D network. Our findings provide new insights for controlling the microstructures of composite electrodes.

Layered metal oxides are used as cathode materials in lithium‐ion batteries due to their high energy density. However, these materials undergo degradation due to structural changes at high charging voltages (>4.2 V vs. Li/Li ⁺ ). One approach to addressing this issue involves coating the cathode with metal oxides, but understanding the effects of such a coating remains limited. In this study, we investigated the effect of a Zr‐based oxide coating on an epitaxial LiCoO ₂ (LCO) model cathode using atomic‐resolution scanning transmission electron microscopy (STEM), focusing on the cathode–electrolyte interfacial structure under partially coated conditions. Electrochemical measurements showed that LCO with Zr‐based oxide coating exhibited reduced discharge voltage decay during 4.5 V cycling under our conditions. STEM observations indicated that the Zr‐based oxide coating is spatially non‐uniform, enabling a direct comparison between adjacent coated and uncoated regions. The layered LCO cathode is preserved at the coated regions, while a spinel Co ₃ O ₄ ‐like structure approximately 2 nm thick forms at the adjacent uncoated regions. These observations provide direct structural evidence of a spatial correspondence between local coating presence and preservation of layered LCO at the interface.

Intrinsic one-dimensional (1D) moiré superlattices provide a state-of-the-art platform for exploring 1D physics. They emerge when one of the two long-range moiré periodicities collapses, which requires a mirror symmetry in the parent 2D materials. Planar-chiral 2D materials such as ReSe₂, which lack mirror symmetry, have been excluded from hosting intrinsic 1D moiré superlattices, severely limiting the available material platforms. Here, we propose a racemic van der Waals assembly strategy to overcome this symmetry requirement by compensating the planar chirality through stacking opposite enantiomers, which cannot be achieved by conventional enantiomer-pure assembly. Using the racemic assembly, we fabricated planar-achiral bilayer ReSe₂, which was confirmed by circularly polarized Raman spectroscopy, and identified two 1D moiré superlattices near 58 and 61° via transmission electron microscopy. Geometrical analysis identifies six discrete critical twist angles that satisfy the intrinsic 1D condition. Our results establish racemic assembly as a general route to achieve intrinsic 1D moiré superlattices in planar-chiral 2D materials.

Layered cathodes are essential for achieving a high energy density in rechargeable lithium-ion batteries, but they suffer from capacity fade due to local crystal phase transitions during charge-discharge cycles. Understanding these phase transitions is essential to minimizing their effects, but their nanometer size and sensitivity to experimental conditions make observation difficult. Here, we visualized nanometer-scale crystal phases within an epitaxial LiCoO₂ cathode after 100 charge-discharge cycles by matching experimental and simulated cepstra derived from scanning nanobeam electron diffraction. While the LiCoO₂ bulk remained a layered structure, spinel- and rocksalt-type phases were observed within 3 nm of the cathode-electrolyte interface. The developed method achieved a spatial resolution of 1.5 nm across a field of view of several hundred nanometers with minimal electron beam damage. The cepstral matching analysis offers valuable insights into the interfacial phase transitions and will aid in the design of high-performance lithium-ion batteries.

Dichalcogenides, such as molybdenum disulfide (MoS ₂ ), are being studied extensively due to their 2D feature and various material properties. Although crystal structures are critical for applications, conventional atomic structure analyses have a limited field of view. In this study, the crystal domains of monolayer MoS ₂ synthesized by metal–organic chemical vapor deposition (MOCVD) are analyzed using 4D scanning transmission electron microscopy (STEM) and unsupervised machine learning. Twist domains (±11°) are identified through the nonnegative matrix factorization (NMF) and hierarchical clustering of numerous (>22k) diffraction patterns from a wide field of view. Preprocessing for detecting noncentrosymmetry effectively visualizes the polarities of distinct MoS ₂ domains by highlighting the violation of Friedel's law in diffraction physics. Analyses reveal that the specimen deposited on Al ₂ O ₃ (0001) at 850 °C consists of domains measuring ≈100 nm in size and featuring many mirror‐twin boundaries. The findings provide valuable insights into optimizing the MOCVD process and elucidating crystal growth mechanisms.

Molybdenum disulfide (MoS₂) is a layered material known to show various phases. Most studies on it have focused on its semiconductor phase, but it is known to also have a metallic 1T' phase. This 1T' phase has also drawn attention as a quantum spin Hall phase, but the 1T' phase is metastable, and methods for transforming or stabilizing it are still challenging. This Communication demonstrates a method for effectively transforming the monolayer or the topmost layer of multilayer semiconductor MoS₂ (the 1H or 2H phase) into the 1T' phase via ultraviolet-ozone (UVO) treatment, followed by polymer enwrapment of the MoS₂ surface. UVO induces the transformation of the 1H (2H) phase into the 1T' phase, but the generated phase is unstable. The enwrapment procedure with the polymer poly-l-lysine was found to be effective in transforming the 1H (2H) phase into the 1T' phase and stabilizing it. Moreover, this procedure transformed only the topmost layer and generated a vertical 1T'/2H heterostructure in multilayer cases. This study shows the high potential of surface organic chemical procedures to control the phases in 2D transition metal dichalcogenides.

Metal oxyhydroxides are essential nanomaterials for recent technologies because of their diverse applications, such as catalysis, adsorbents, and precursors of metal oxides. These applications rely on the controlled crystal structures of metal oxyhydroxides formed via hydrolyzed metal monomers’ condensation. However, characterizing the atomic-scale structures of the metal oxyhydroxides has still been challenging due to their diverse structural types, nanometer-scale sizes, and beam sensitivity. Here, we developed a data-driven analysis approach for atom-resolved transmission electron microscopy images of titanium oxyhydroxide (metatitanic acid) nanoparticles. Lattice spacings and angles were measured for each of the 1300 nanoparticles with random crystal orientations, providing three-dimensional structural information. Our findings reveal their anatase-like structure with alternating layers of titanium dioxide (TiO₂) and titanium hydroxide (Ti(OH)₄) planes. The revealed structure is key to understanding their role as a precursor for metastable anatase TiO₂. Our approach unveils the three-dimensional structure of metal oxyhydroxides with high statistical reliability and low electron dose, paving the way for property understanding and application design.

Moiré effects in two-dimensional (2D) twisted van der Waals structures are attracting great interest owing to the emergence of intriguing physical properties. Despite growing interests, moiré effects with large twist angles remain unexplored because the increase of twist angle is thought to quickly suppress the size of moiré patterns. In this study, we focused on large-angle twisted tungsten ditelluride and discovered two orthogonal one-dimensional (1D) moiré patterns at twist angles around 62° and 58° using transmission electron microscopy. Their diffraction patterns exhibited pair formation features supporting the one-dimensionality of these moiré patterns. We also succeeded in revealing the origin of large-angle moiré patterns as well as the intrinsic nature of 1D moiré patterns. The observed moiré patterns are purely 1D in the sense that the long-range periodicity exists only along one direction, being a promising platform to study 1D phenomena. Our results propose a universal concept for studying large-angle twisted structures.