Correlation between growth behavior,internal stress distribution and properties of ZnO:Zn crystals grown by carbon-assisted chemical vapor transport

Internal stress and stress-related defects are considered as the major obstacles that significantly hinder the growth of high-quality ZnO-based crystals. In this work, high-crystalline-quality ZnO:Zn bulk crystals were successfully grown by carbon-assisted chemical vapor transport (CVT). Internal stress in the crystal was directly measured by a neutron beam from a reactor, and stress distributions along the radial direction at different depths were obtained. The stress, temperature, and flow fields in the growth system were simulated by the finite element (FE) method, and the results agreed with the neutron stress analysis. The etch pit density (EPD), Hall properties, and optical transmittances of different crystal regions were studied in detail, and the distribution trend of the crystal properties was consistent with that of internal stress and stress-related defects in the crystal. It is found that the unique temperature filed in the growth system causes the crystal to bend to a slightly convex toward the growth direction and gives rise to a driving force for structural defect formation. The + c and –c faces of the crystal are subjected to tensile and compressive stress, respectively. The maximum stress values are about 280 MPa and -291 MPa near the central regions of ±c faces, while the crystal periphery is basically free of internal stress. The region near the center of +c face has an EPD of 7.5 × 10³ cm^-2 and a transmittance of 79.2% at 800 nm wavelength, while the corresponding carrier concentration and mobility are 2.27 × 10¹⁷ cm^?3 and 159 cm²/V·s, respectively. By comparison, the crystal periphery has an EPD of 10² cm^-2 with an 80.5% transmittance at 800 nm, while the carrier concentration and mobility are 1.85 × 10¹⁷ cm^?3 and 184 cm²/V·s, respectively.     

Progressive DARTS: Bridging the Optimization Gap for NAS in the Wild

International Journal of Computer Vision - With the rapid development of neural architecture search (NAS), researchers found powerful network architectures for a wide range of vision tasks. Like...     

Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration

Mn-doped BiFeO₃ (BiFe_1–xMn_xO₃, x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) polycrystalline multiferroic thin films were successfully synthesized using the facile sol-gel spin-coating method. The crystal structures, surface features, elements valences, and magnetic properties of as-prepared samples were systematically explored. X-ray diffraction and Raman spectroscopy studies revealed the substitutions of Mn into the Fe site and a rhombohedral-to-orthorhombic phase transition. The Field Emission Scanning Electron Microscopy showed a decrease in the average particle sizes and an improvement of surface morphology with increasing the concentration of the substitutes. Energy-dispersive X-ray spectroscopy confirmed the doping concentration of Mn²⁺ in the samples. X-ray photoelectron spectroscopy indicated the co-existence of Mn²⁺/Mn³⁺ ions in the doped films. The remnant magnetization value of BiFe_0.90Mn_0.10O₃ thin film was found to be approximately six times than that of pure BiFeO₃ thin film under a magnetic field of 10 kOe. The enhanced magnetic property of BiFe_0.90Mn_0.10O₃ thin film was mainly ascribed to the structural distortion of spin cycloid and the enhancement of super-exchange interaction between the Fe³⁺ (Mn²⁺) and O^2- ions.     

Synthesis of blue-emitting AlN:Eu2+ spherical phosphors by carbothermal reduction nitridation method

In this study, blue-emitting AlN:Eu²⁺ spherical phosphors were successfully synthesized for the first time by the carbothermal reduction nitridation (CRN) method, assisted with high nitrogen pressure, appropriate synthesis temperature, and the addition of CaF₂. The influence of typical experimental parameters, such as N₂ pressure, heating temperature, CaF₂ content and Eu²⁺ concentration on the morphologies and luminescence properties of AlN phosphors were comprehensively investigated. The formation mechanism of spherical morphology were significantly proffered, indicating that sufficient liquid Ca-aluminates during the AlN growth stage were essential for the spheroidization process under the action of surface tension. The synthesized AlN:Eu²⁺ spherical phosphors presented an intense blue emission band centered in the range of 427- 476 nm relative to the reaction temperature. The lifetime of AlN:Eu²⁺ phosphor was calculated to be around 1.89 μs. The temperature-dependent PL spectra suggested that the emission band did not shift until 225°C. In addition, the spectral analysis strongly suggested that the luminescence property of AlN:Eu²⁺ phosphors was significantly enhanced by the large particle size, spherical morphology, reduced impurity content, and appropriate Eu²⁺ concentration.     

Metal organic frameworks as electrocatalysts: Hydrogen evolution reactions and overall water splitting

The development of clean energy technologies to protect the environment is an important demand of the times. Electrocatalysis is emerging as a promising method for evolution of hydrogen and overall water splitting. Nowadays, metal organic frameworks (MOFs) have emerged as electrocatalysts having uniformly distributed active sites and high electrical conductivity. This review summarizes the latest advances in heterogeneous catalysis by MOFs and their composite/derivatives for efficient hydrogen evolution reaction (HER) and water splitting. Pristine MOFs with their recent development are summarized first followed by composites of MOFs with their enhanced electrocatalytic performances. Overall water splitting by using bifunctional electrocatalysts derived from MOFs with different synthetic approaches is provided and this review gives the metal-based categorisation of precursor MOFs. Different strategies to improve chemical stability, conductivity, and overall electrocatalytic properties have been discussed. In the last, perspectives on the synthesis of efficient MOF-based electrocatalyst materials are provided.