Abstract Three-dimensional (3D) integrated one-selector one-resistor (1S1R) cross-point arrays are emerging as a leading next-generation memory technology. Achieving both nonvolatile memory switching and volatile threshold switching within a single material system offers tremendous potential for simplifying fabrication. This study demonstrates that the memory and threshold switching behavior of germanium-telluride (Ge x Te 1-x ) devices can be precisely tuned by controlling their atomic ratio. In the Te-deficient Ge x Te 1-x system, the device exhibits nonvolatile memory switching with fast operating speed. Conversely, the Te-rich system functions as an excellent bidirectional selector for high-density memory applications, exhibiting high selectivity (> 10 4 ) and a short delay time (< 55 ns). Material analyses using X-ray photoelectron spectroscopy and Raman spectroscopy reveal that these distinct electrical behaviors are driven by composition-dependent changes in the atomic structure. Furthermore, the fully integrated 1S1R stack demonstrates robust SET/RESET switching characteristics, maintaining a stable memory window of 0.2 V and an on/off current ratio exceeding two orders of magnitude (~ 1.12 × 10 2 ). Notably, this integrated configuration provides superior operational reliability and enhanced endurance (~ 8 × 10 6 cycles) compared to standalone phase-change memory devices. By elucidating these underlying mechanisms, this work reduces the fabrication complexity of stackable devices and opens up wide-ranging applications for unified memory and selector systems.
Abstract Focused ion beam–scanning electron microscopy (FIB–SEM) was used to investigate the effects of acceleration voltage, beam current, and cryogenic conditions on the cross-sectional preparation of proton exchange membrane fuel cell (PEMFC) cathode catalyst layers. Milling at 30 kV and 0.26 nA effectively minimized curtaining artifacts and produced an apparent cross-sectional pore morphology similar to that obtained by cryogenic milling, indicating that optimized room-temperature milling can provide cross sections suitable for consistent two-dimensional (2D) pore-area analysis under the conditions examined. Representative FIB–SEM cross-sectional images of PEMFC cathode catalyst layers prepared under different FIB milling conditions: (a–c) 5 kV at room temperature, (d–f) 30 kV at room temperature, and (g–i) 30 kV under cryogenic conditions (− 150 °C). The indicated voltage and current values represent the FIB acceleration voltage and ion beam current used for milling, respectively. The red dashed box in (f) highlights the optimized room-temperature milling condition (30 kV, 0.26 nA) used for comparison with the corresponding cryogenic condition in (i).
Abstract Optoelectronic characterization of local bandgap, dielectric response, and plasmon at the nanometer scale has become increasingly important for understanding and optimizing modern electronic, optoelectronic, and quantum materials. Conventional optical techniques generally provide spatially averaged information and are therefore limited in their ability to resolve localized electronic variations arising from defects, interfaces, strain fields, and quantum confinement. To overcome these limits, monochromated scanning transmission electron microscopy–valence electron energy loss spectroscopy (STEM-VEELS) has evolved from a qualitative low-loss spectroscopy technique into a quantitative nanoscale bandgap metrology platform. By analyzing low-loss inelastic scattering signals with a sub-nanometer electron probe and sub-100 meV energy resolution, STEM-VEELS enables the direct investigation of bandgap onsets, local density of states (LDOS), and interband transitions at the nanoscale, although the effective spatial resolution of these measurements can be substantially broader than the probe size because of inelastic-scattering delocalization. This review summarizes the fundamental principles and recent advances in monochromated STEM-VEELS for local bandgap measurement. Particular emphasis is placed on the physical origin of low-loss excitations, addressing critical challenges such as momentum transfer and spatial delocalization. Practical experimental strategies for rigorous zero-loss peak (ZLP) subtraction and bandgap onset determination are systematically discussed. Furthermore, major analytical artifacts—such as Cherenkov radiation, guided optical modes, and beam-induced damage—are critically reviewed alongside state-of-the-art mitigation strategies. Recent developments in off-axis and Bessel-aperture STEM-VEELS geometries, machine-learning-assisted spectral analysis, automated onset extraction, and uncertainty-aware bandgap mapping are highlighted. Representative applications involving quantum dots, wide-bandgap oxides, heterointerfaces, and two-dimensional materials demonstrate the unparalleled capability of STEM-VEELS to resolve spatially varying electronic structures. Finally, future perspectives, including operando spectroscopy, ultrafast VEELS, artificial intelligence-driven analysis, and SEM-REELS, are discussed, providing a robust framework for standardization and quantitative nanoscale bandgap metrology.
Abstract Cryogenic-temperature friction stir processing (CT-FSP) was performed on AA6xxx sheets at 600 RPM and 500 mm/min using in-process liquid N2 cooling to investigate microstructure, precipitate evolution, and the resulting mechanical response. Multiscale characterization EBSD, SEM/EDS and microhardness tests were conducted. CT-FSP produced a highly refined predominantly equiaxed stir zone (SZ) with average grain sizes of approximately 1.6–1.8 µm at the surface and mid-thickness, and slightly coarser grains (~ 4.4 µm) near the bottom and within the thermomechanically affected zone (TMAZ). Dynamic recrystallization, evidenced by a high fraction of high-angle grain boundaries and low KAM, dominates the SZ. The recrystallization fraction varies laterally and through the thickness due to heterogeneous strain and thermal fields. Second phase particles experience mechanical fragmentation, partial solvus dissolution, and rapid reprecipitation during CT-FSP, yielding a finer and more uniformly dispersed distribution in the upper SZ, while coarser Al–Fe–Si and Mg2Si particles persist near the bottom/TMAZ. Deep-learning-based image segmentation reveals that the mean equivalent-circle diameters of Al–Fe–Si and Mg2Si phases decrease from ~ 0.97 µm and 0.80 µm in the as-received sheet to ~ 0.6–0.8 µm locally after CT-FSP, accompanied by reduced nearest-neighbor spacing in the SZ. This localized particles refinement and redistribution, together with extensive DRX, accounts for the combined SZ softening and TMAZ hardening, establishing a clear microstructure-property linkage in cryogenic FSPed AA6xxx sheet.
In this study, urchin-like CuO/Cu₂O mixed-phase nanostructures were synthesized via a simple surfactant-assisted approach, where the surfactant regulated the nucleation and anisotropic growth, leading to a hierarchical morphology with radially arranged nanoneedles. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses verified the coexistence of CuO and Cu₂O phases within individual microspheres. High-resolution transmission electron microscopy (HRTEM) revealed well-defined lattice fringes assigned to Cu₂O, while XRD, XPS, SAED, and EDS analyses collectively suggest the coexistence of CuO and Cu₂O phases within the hierarchical microspheres. Energy-dispersive spectroscopy (EDS) mapping and line-scan profiles indicated a gradual compositional variation of copper and oxygen from the core to the surface region. Furthermore, in situ electron-beam irradiation demonstrated structural densification of the urchin-like architecture, providing insight into its structural stability. These results offer fundamental understanding of mixed-phase copper oxide formation and highlight their potential for high-surface-area functional materials.
