High-Pressure Synthesis of Corundum-Type Ga2O3:Cr3+ and Application of Its Fluorescence to the Pressure Scale.

Cr3+-doped Ga2O3 crystals with a corundum structure were synthesized at high temperature and high pressure, and their excitation as well as fluorescence properties were evaluated. The crystals were green under white light illumination but deep red when exposed to ultraviolet light. This can mainly be attributed to R1 and R2 fluorescence spectra caused by the Cr3+ transition. The pressure-dependence of their fluorescence spectra is comparable with ruby (Al2O3:Cr3+), which is currently often used as a pressure scale. The excitation spectrum was shifted to the long-wavelength side compared with ruby, which enabled excitation with long-wavelength lasers, even if the pressure effect is considered. In addition, the R1 and R2 peaks were well-separated with increasing pressure, which might have advantages over the ruby scale.

Direct Imaging of Band Structure for Powdered Rhombohedral Boron Monosulfide by Microfocused ARPES.

Boron-based two-dimensional (2D) materials are an excellent platform for nanoelectronics applications. Rhombohedral boron monosulfide (r-BS) is attracting particular attention because of its unique layered crystal structure suitable for exploring various functional properties originating in the 2D nature. However, studies to elucidate its fundamental electronic states have been largely limited because only tiny powdered crystals were available, hindering a precise investigation by spectroscopy such as angle-resolved photoemission spectroscopy (ARPES). Here we report the direct mapping of the band structure with a tiny (∼20 × 20 µm2) r-BS powder crystal by utilizing microfocused ARPES. We found that r-BS is a p-type semiconductor with a band gap of >0.5 eV characterized by the anisotropic in-plane effective mass. The present results demonstrate the high applicability of micro-ARPES to tiny powder crystals and widen an opportunity to access the yet-unexplored electronic states of various novel materials.

Rhombohedral Boron Monosulfide as a p-Type Semiconductor.

Two-dimensional materials have wide ranging applications in electronic devices and catalysts owing to their unique properties. Boron-based compounds, which exhibit a polymorphic nature, are an attractive choice for developing boron-based two-dimensional materials. Among them, rhombohedral boron monosulfide (r-BS) has recently attracted considerable attention owing to its unique layered structure similar to that of transition metal dichalcogenides and a layer-dependent bandgap. However, experimental evidence that clarifies the charge carrier type in the r-BS semiconductor is lacking. In this study, we synthesized r-BS and evaluated its performance as a semiconductor by measuring the Seebeck coefficient and photo-electrochemical responses. The properties unique to p-type semiconductors were observed in both measurements, indicating that the synthesized r-BS is a p-type semiconductor. Moreover, a distinct Fano resonance was observed in Fourier transform infrared absorption spectroscopy, which was ascribed to the Fano resonance between the E(2) (TO) phonon mode and electrons in the band structures of r-BS, indicating that the p-type carrier was intrinsically doped in the synthesized r-BS. These results demonstrate the potential future application prospects of r-BS.

Inverse Perovskite Oxysilicides and Oxygermanides as Candidates for Nontoxic Infrared Semiconductor and Their Chemical Bonding Nature.

We have synthesized inverse-perovskite-type oxysilicides and oxygermanides represented by R3SiO and R3GeO (R = Ca and Sr) and studied their characteristics in the search for nontoxic narrow band gap semiconductors. These compounds exhibit a sharp absorption edge around 0.9 eV and a luminescence peak in the same energy range. These results indicate that the obtained materials have a direct-band electronic structure, which was confirmed by hybrid DFT calculations. These materials, made from earth abundant and nontoxic elements and with a relatively light electron/hole effective mass, represent strong candidates for nontoxic optoelectronic devices in the infrared range.

Structure refinement of black phosphorus under high pressure.

The structure refinement of black phosphorus was performed at pressures of up to 3.2 GPa at room temperature by powder neutron diffraction techniques. The bond lengths and bond angles between the phosphorus atoms at pressures were precisely determined and confirmed to be consistent with those of the previous single crystal x-ray analysis [A. Brown and S. Rundqvist, Acta Cryst. 19, 684 (1965)]. Although the lattice parameters exhibited an anisotropic compressibility, the covalent P1-P2 and P1-P3 bond lengths were almost independent of pressure and only the P3-P1-P2 bond angle was reduced significantly. On the basis of our results, the significant discrepancy in the bond length reported by Cartz et al. [J. Chem. Phys. 71, 1718 (1979)] has been resolved. Our structural data will contribute to the elucidation of the Dirac semimetal state of black phosphorus under high pressure.

Rhombohedral boron monosulfide as a metal-free photocatalyst.

Most of previous photocatalysts contain metal species, thus exploring a metal-free photocatalyst is still challenging. A metal-free photocatalyst has an advantage for the development of economical and non-toxic artificial photosynthesis system and/or environmental purification applications. In this study, rhombohedral boron monosulfide (r-BS) was synthesized by a high-pressure solid-state reaction, and its photocatalytic properties were investigated. r-BS absorbed visible light, and its photocurrent action spectrum also exhibited visible light responsivity. The r-BS evolved hydrogen (H2) from water under ultraviolet (UV) as well as under visible light irradiation, and its internal quantum efficiency reached 1.8% under UV light irradiation. In addition to the H2 evolution reaction, the r-BS photocatalyst drove carbon dioxide (CO2) reduction and dye oxidation reactions under UV irradiation. Although bare r-BS was not so stable under strong light irradiation in water, cocatalyst modification improved its stability. These results indicate that r-BS is a new class of non-metal photocatalyst applicable for H2 production, CO2 reduction, and environmental purification reactions.

Simple and reliable direct patterning method for carbon-free solution-processed metal oxide TFTs.

Metal oxide TFT fabrication based on a solution-processing method is considered a promising alternative to conventional vacuum processing and has a number of advantages such as low cost, large-area fabrication, and process simplicity. A simple and reliable, direct patterning method for obtaining a carbon-free aqueous metal oxide film is presented herein. Patterning, which is achieved by selective photoreaction of water molecules under ultraviolet irradiation and by a safe, environment-friendly chemical etching process using a non-toxic organic acid, is followed by an annealing process at a temperature of 350 °C to obtain carbon-free metal oxide TFTs. In-Ga-Zn oxide (IGZO), TFTs on SiO2 dielectrics that were fabricated with a direct patterning method exhibited an average mobility of 4.3 cm2/V·s with good uniformity, which is comparable to TFTs formed by conventional photolithography. The TFTs exhibited stable performance with small (within 0.5 V) shifts in switch-on voltage under positive and negative bias stress. Fabrication of flexible IGZO TFTs by direct patterning was also achieved.

Simple and efficient fabrication of room temperature stable electride: melt-solidification and glass ceramics.

Electrides are ionic compounds in which electrons act as anions. These compounds are expected to have interesting properties arising from their exotic structure. The fatal drawbacks of the thermal and chemical instability of organic electrides were resolved by the synthesis of a room temperature (RT) stable electride using single crystalline 12CaO.7Al2O3 (C12A7) with a nanoporous structure and the chemical treatments for a long duration. However, an innovative fabrication method is obviously required for practical applications such as cold electron-emitter and thermionic devices. Herein we report a simple synthesis for polycrystalline C12A7 electrides with a moderate electronic conductivity via a strongly reducing C12A7 "melt", i.e., direct solidification of the melt or crystallization of the transparent glass. Generation of carrier electrons and precipitation of the C12A7 phase from the strongly reducing melt and glass are likely associated with the incorporation of carbon-related anions for stabilizing the C12A7 phase and keeping the mobile electrons in C12A7. These findings will be broadly utilized for applications by mass production in a desired shape and dimension, facilitating the research of electrides.

High-density electron anions in a nanoporous single crystal: [Ca24Al28O64]4+(4e-).

We removed approximately 100% of clathrated oxygen ions from the crystallographic cages in a single crystal of 12CaO.7Al2O3, leading to the formation of high-density (approximately 2 x 10(21) cm-3) electrons highly localized in the cages. The resulting electron forms a structure that we interpret as an F+ center and migrates throughout the crystal by hopping to a neighboring cage with conductivity approximately 100 siemens per centimeter, demonstrating that the encaged electron behaves as an anion. The electron anions couple antiferromagnetically with each other, forming a diamagnetic pair or singlet bipolaron. The resulting [Ca24Al28O64]4+(4e-) may be regarded as a thermally and chemically stable single crystalline "electride."