High-Pressure O2 Annealing Enhances the Crystallinity of Ultrawide-Band-Gap Sesquioxides Combined with Graphene for Vacuum-Ultraviolet Photovoltaic Detection.

(AlxGa1-x)2O3 is emerging as a promising wide-band-gap sesquioxide for vacuum-ultraviolet (VUV, 10-200 nm) photodetectors and high-power field-effect transistors. However, how the key parameters such as the band gap and crystalline phase of the (AlxGa1-x)2O3-based device vary with stoichiometry has not been explicitly defined, which is due to the unclear underlying mechanism of the Al local coordination environment. In this work, a high-pressure O2 (20 atm) annealing (HPOA) strategy that can significantly improve the crystallinity of ß-(AlxGa1-x)2O3 and achieve a tunable optical band gap was proposed, facilitating the revelation of the local structure of Al3+ varying with Al content and the kinetic mechanism of Al3+ diffusion. By combining the as-HPOA-treated single-crystalline ß-(Al0.69Ga0.31)2O3 films with p-type graphene (p-Gr), which serves as a transparent conductor, a VUV photovoltaic detector is fabricated, showing an improved photovoltage (0.80 V) and fast temporal response (2.1 µs). All of these findings provide a rewarding and important strategy for enhancing the band-gap tunability of sesquioxides, as well as the flexibility of zero-power-consumption photodetectors.

2D van der Waals Molecular Crystal ß-HgI2 : Economical, Rapid, and Substrate-Free Liquid-Phase Synthesis and Strong In-Plane Optical Anisotropy.

2D materials have a great potential for wide-range applications due to their adjustable bandgap characteristics and special crystal structures. ß-HgI2 is a new 2D van der Waals inorganic molecular crystal material with a wide bandgap of 4.03 eV, on whose preparation and properties there are few relevant reports due to the feature of instability of molecular crystals. Here, an economical method to control the synthesis of large-size 2D ß-HgI2 single crystal by using a mineralizer-assisted solution is reported. According to angle-resolved polarization Raman spectroscopy and first-principles optical absorption calculation, 2D ß-HgI2 flake has a strong in-plane anisotropic light scattering characteristic and high optical absorption dichroism (az /ay  = 3.4), which is due to a low in-plane symmetry of the orthorhombic structure of ß-HgI2 . More importantly, due to the molecular crystal structure of ß-HgI2 , its sensitivity to temperature is less than that of 2D materials such as MoS2 , which has been confirmed by temperature-dependent Raman spectroscopy. In the work, more 2D inorganic molecular crystals are studied in the aspect of growth, which provides a theoretical basis for 2D molecular crystal optoelectronic devices' potential applications.

Experimental Evidence on Stability of N Substitution for O in ZnO Lattice.

Although the dispute remains, the N substitution for the lattice O (NO) in zinc oxide (ZnO) demonstrates the promising future in achieving the p-type ZnO-based semiconductor. In this context, a highly crystallized N-doped ZnO (ZnO:N) film is fabricated with ultralow defect density. Based on the synchrotron radiation X-ray absorption near-edge structure (XANES) and low-temperature photoluminescence (PL) spectra combined with first-principles calculations, the results demonstrate that the majority of N ions locate stably at the lattice O site to succeeding the N substitution for lattice O as the NO defects. A prototype LED device is built based on the homojunction of ZnO:N film and ZnO:Ga wafer with good electroluminescence performance. These important findings provide a rewarding avenue to the p-type ZnO semiconductor design and device fabrication, and demonstrate a prevailing guidance on the materials design and development as well.

Raman tensor of layered MoS2.

Raman tensors, one of the basic physical properties of ${{\rm MoS}_2}$MoS2, are rarely reported. Here, angle-resolved polarized Raman scatterings on basal and cross planes of layered ${{\rm MoS}_2}$MoS2 were carried out using the geometry configuration of parallel polarization, and the Raman tensors of three optical vibration modes were systematically studied. As a polar vibration mode, the differential polarizability of the ${{\rm A}_{1{\rm g}}}$A1g mode corresponding to the Raman tensor along the $c$c direction is larger than that along the $a$a direction. In addition, it is also larger than that formed by ${{\rm E}_{2{\rm g}}}$E2g and ${{\rm E}_{1{\rm g}}}$E1g modes. All the experimental results above are beneficial to the understanding of inelastic light-scattering process of ${{\rm MoS}_2}$MoS2.