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Dislocation bending and stress evolution in Mg-doped GaN films on Si substrates
摘要： P-type doping using Mg is essential for realizing a variety of electronic and optoelectronic III-nitride devices involving hetero-epitaxial thin films that also contain a significant number of dislocations. We report on the effect of Mg incorporation on dislocation and stress evolution during the growth of GaN thin films by using in situ curvature measurements and ex situ transmission electron microscopy. A complete picture involving the interplay between three effects—dopant size effect, dislocation bending, and polarity inversion—is presented. Mg aids dislocation bending, which in turn generates tensile stresses in Mg-doped GaN films. As a result, the compressive stress expected due to the dopant size difference effect can only be discerned clearly in films with dislocation densities below 5 × 10^9 cm^?2. Polarity inversion at doping exceeding 10^19 cm^?3 is associated with a sharp drop in screw dislocation density. A kinetic stress evolution model has been developed to capture dislocation bending and size difference effects, and a match between calculated bending angle from the model and that measured from TEM images is obtained.
关键词： Mg doping,stress evolution,dislocation bending,in situ curvature measurements,transmission electron microscopy,polarity inversion,GaN
更新于2025-09-23 15:23:52
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Stoichiometric and non-stoichiometric tungsten doping effect in bismuth vanadate based photoactive material for photoelectrochemical water splitting
摘要： In photoelectrochemical (PEC) water splitting, BiVO4 has attracted attention due to its favorable band gap but it suffers low PEC performance due to poor conductivity. The vast majority of publications on this system has examined doping of stoichiometric composition of tungsten (W) on this system to increase bulk and interfacial conductivity while managing the contaminant generation of crystallographic defects and recombination sites. In this paper, a deep investigation was carried out to examine the effect of non-stoichiometric W doping in BiVO4 system. Stoichiometric and non-stoichiometric W-doped monoclinic BiVO4 (i.e. Bi1-(xtd)V1-xWxtdO4; BiV1-xWxtdO4 and BiV1-yWyO4; x ? 0.008; y ? 0.03 and d ? 0.005) were prepared using a facile dip coating technique. The stoichiometric composition contains charge balanced Bi, V and W atoms whereas non-stoichiometric compositions contain excess Bi and excess Bi and W. The non-stoichiometric composition BiV1-xWxtdO4 has shown better photoelectrochemical water splitting performance with respect to other compositions at 1.23 V vs RHE, under one sun illumination of electrode. The XRD and XPS results shows that non-stoichiometric doping with excess Bi or with excess Bi and W can possibly create an environment where V5t ions are substitutional replaced by W6t ions without generating other defects. But there was no signi?cant difference in band gap of different compositional samples observed. Further electrochemical impedance technique was used to analyze change in bulk and surface charge mobility with W-doping in BiVO4. The electrochemical impedance analysis showed the presence of low interfacial resistance, lower charge transfer resistance and high charge donor/surface state density for non-stoichiometric composition BiV1-xWxtdO4 electrode. It is evident from and cyclic voltammetry that the addition of excess Bi and W from its stoichiometric quantity ef?ciently suppressed the formation of hole-electron pair recombination sites. The electrochemical analytical results lead us to believe that the particular non-stoichiometric composition of BiV1-xWxtdO4 can signi?cantly lower trap sites and enhances kinetics of charge transfer, leading to the better photoelectrochemical water splitting performance.
关键词： Oxygen evolution,Photoelectrochemical cell,Water splitting,Bismuth vanadate,Tungsten doping
更新于2025-09-23 15:23:52
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Multi-active sites derived from a single/double perovskite hybrid for highly efficient water oxidation
摘要： The oxygen evolution reaction (OER) plays a crucial role in the application of water splitting, which is a highly competitive option for a sustainable energy future. Thus, it is vital to design highly active and durable electrocatalyst for OER. Herein a hybrid with the nominal composition of Ba2Co1.5Mo0.25Nb0.25O6-d (denoted as BC1.5MN) electrocatalyst consisting of both double perovskite and single perovskite structures is synthesized by a solid-state reaction method. When tested as an electrocatalyst for OER, the BC1.5MN electrocatalyst requires a current density of 10 mA cm-2 at an overpotential of 400 mV, an onset overpotential of 260 mV, and a Tafel slope of 70 mV dec-1, which are superior to that of precious metal oxide IrO2 catalyst. Chronoamperometric and cyclic voltammetry studies demonstrate that the BC1.5MN electrocatalyst has outstanding durability in alkaline solution. The synergistic effect between multi-active sites derived from a single/double perovskite hybrid structure results in one of the most active perovskite-based OER electrocatalysts in alkaline solution.
关键词： hybrid,oxygen evolution reaction,double perovskite,single perovskite,electronic structure
更新于2025-09-23 15:23:52
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The PAU Survey: spectral features and galaxy clustering using simulated narrow-band photometry
摘要： We present a mock catalogue for the Physics of the Accelerating Universe Survey (PAUS) and use it to quantify the competitiveness of narrow-band imaging for measuring spectral features and galaxy clustering. The mock agrees with observed number count and redshift distribution data. We demonstrate the importance of including emission lines in the narrow-band fluxes. We show that PAUCam has sufficient resolution to measure the strength of the 4000 ? break to the nominal PAUS depth. We predict the evolution of a narrow-band luminosity function and show how this can be affected by the O II emission line. We introduce new rest-frame broad-bands (UV and blue) that can be derived directly from the narrow-band fluxes. We use these bands along with D4000 and redshift to define galaxy samples and provide predictions for galaxy clustering measurements. We show that systematic errors in the recovery of the projected clustering due to photometric redshift errors in PAUS are significantly smaller than the expected statistical errors. The galaxy clustering on two halo scales can be recovered quantitatively without correction, and all qualitative trends seen in the one halo term are recovered. In this analysis, mixing between samples reduces the expected contrast between the one halo clustering of red and blue galaxies and demonstrates the importance of a mock catalogue for interpreting galaxy clustering results. The mock catalogue is available on request at https://cosmohub.pic.es/home.
关键词： galaxies: evolution,large-scale structure of Universe,galaxies: formation,galaxies: luminosity function, mass function
更新于2025-09-23 15:23:52
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Effect of anisotropic non-Kolmogorov turbulence on the evolution behavior of Gaussian Schell-model vortex beams
摘要： The effect of anisotropic non-Kolmogorov turbulence on the evolution behavior of average intensity and coherent vortices for Gaussian Schell-model (GSM) vortex beams is investigated. Based on the extended Huygens–Fresnel principle and power spectrum of the anisotropic non-Kolmogorov turbulence, the analytical expressions for the cross-spectral density and average intensity of GSM vortex beams propagating through anisotropic non-Kolmogorov turbulence are obtained. The evolution behavior of intensity and coherent vortices for GSM vortex beams in anisotropic non-Kolmogorov turbulence has been discussed in detail by numerical simulation. The results shown that the evolution behavior of coherent vortices and intensity profile depends on the effective anisotropic factor, generalized exponent factor and structure constant of anisotropic non-Kolmogorov turbulence, as well as the beam parameters, such as beam waist, wave length and correlation length.
关键词： Gaussian Schell-model vortex beams,Anisotropic non-Kolmogorov turbulence,Evolution behavior of coherent vortices
更新于2025-09-23 15:23:52
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Operando observation of chemical transformations of iridium oxide during photoelectrochemical water oxidation
摘要： Iridium oxide is one of the few catalysts capable of catalyzing the oxygen evolution reaction (OER) in both acidic and basic conditions. Understanding the mechanism of IrOx under realistic photoelectrochemical conditions is important for the development of integrated water splitting systems. Herein, we have developed a highly efficient OER photoanode in pH 1 aqueous solutions based on a sputtered IrOx film and a p+n-Si light absorber, interfaced with sputtered Au layer. Operando high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS) was employed to monitor the oxidation state changes of IrOx during both electrochemical and photoelectrochemical (PEC) water oxidation reactions in pH 1 aqueous solutions. We observed a gradual increase of the average oxidation state of Ir with increasing anodic potential in the pre-catalytic region, followed by a reduction of Ir under O2 evolution conditions. Consistent results were obtained on dark anodes and illuminated photoanodes. However, when the thickness of IrO2 was increased to 2 and 3 nm, the spectral changes became much less pronounced and the reduction of Ir oxidation state after the OER onset was not observed. This is due to the lower surface to bulk ratio, where lattice oxygen sites in the bulk are not accessible for the formation of hydroxide. More generally, the operando method developed here can be extended to other materials, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design.
关键词： oxygen evolution reaction (OER),electrochemical and photoelectrochemical (PEC),high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS),iridium oxide,Operando method
更新于2025-09-23 15:23:52
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One-Step Solvothermal Formation of Pt Nanoparticles Decorated Pt <sup>2+</sup> -Doped α-Fe <sub/>2</sub> O <sub/>3</sub> Nanoplates with Enhanced Photocatalytic O <sub/>2</sub> Evolution
摘要： The photooxidation of water into O2 has been identified as the barrier of water-splitting, and light-driven water oxidation catalysts have been intensively explored to develop highly active water splitting materials. Despite the fascinating advantages for photocatalytic water oxidation, such as abundance in nature, inexpensiveness, low toxicity, thermo/photo-stability, and suitable electronic band structures, hematite α-Fe2O3 is a poor photocatalyst for water oxidation due to its short exciton lifetime and hole diffusion length, weak carrier mobility and shallow sunlight penetration depth. In this work, we have synthesized Pt nanoparticles decorated Pt2+-doped α-Fe2O3 nanoplates (Pt/Pt-Fe2O3 NPs) by a one-step solvothermal route, which exhibit the enhanced photoactivity and photostablity for water oxidation. The introduction of the Pt into the α-Fe2O3 by the means of elemental doping and nanoparticle decoration accounts for the enhanced performance. The doping of Pt2+ into α-Fe2O3 improves the isolation efficiency of the photo-induced carriers which remarkably increases the lifespan of hole carriers, and the adherence of metal Pt nanoparticles to the surface of α-Fe2O3 leads to formation of schottky barriers at the interface which effectively impedes the combination of photo-generated electrons and holes.
关键词： photocatalytic O2 evolution,Pt nanoparticles decoration,Pt2+ doping,schottky barrier,α-Fe2O3 nanoplates
更新于2025-09-23 15:23:52
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Synergistic Effect Between WO3/Activated Carbon and BiVO4 Nanoparticles for Improved Photocatalytic Hydrogen Evolution
摘要： Modified composite of the pure monoclinic tungstun oxide with 2.0% of activated carbon photocatalyst with BiVO4 as coupling content is synthezed via facile hydrothermal route. The composite is fabricated with coupling ratio of 0.5%,1.0%,1.5% and 2.0% dopant BiVO4. These composite were characterized by the XRD, SEM, UV–Vis, PL and BET to investigate the various properties (particle size, structural, morphological, purity and optical) and the energy band of the photocatalytic material. It is commonly examined that the C-WO3 showed the extraneous results for the evolution of the hydrogen energy by increasing the coupling contents upto 1.5% of BiVO4 and gave extraordinary photocatalytic activity towards the hydrogen energy production. The formation of the orthorhombic phases from the monoclinic and hexagonal at 2.0% of doping content indicated the increase of size of the particles and energy band gap. The average grain size of the composite is ranging from 30 to 50 nm. The increment of the BiVO4 content in the C-WO3 composite causes the reduction of photocatalytic activity because of the increase in the grain size and the forbidden gap of the photocatalytic composite.
关键词： BiVO4,WO3,Photocatalyst,H2 evolution
更新于2025-09-23 15:23:52
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Weak metal lines in optical high-resolution Very Large Telescope and Keck spectra of “cool” PG?1159 stars
摘要： PG 1159 stars are very hot (effective temperatures Teff = 75 000–200 000 K), hydrogen-deficient (pre-) white dwarfs. They probably are the result of a late helium-shell flash that laid bare the He, C, and O rich intershell matter of the progenitor Asymptotic Giant Branch (AGB) star. Their chemical surface composition thus allows to conclude on details of AGB-star nucleosynthesis. Due to their very high effective temperatures, detailed spectral analyses are usually completely reliant on ultraviolet observations, except for some species in the hottest PG 1159 stars (Teff >～ 130 000 K), which do exhibit highly excited lines from the CNO elements and neon (C iv, N v, O vi, Ne vii-viii) in optical spectra. Particularly problematic are, however, the coolest members of the PG 1159 class that exclusively show C iv lines in the optical. Access to the nitrogen abundance is important to decide which of the late-thermal pulse evolutionary scenarios was experienced by a particular star, while a high oxygen abundance is an important marker that the star could pulsate. In the present paper, we investigate high-resolution high signal-to-noise optical spectra of three “cool” PG 1159 stars (PG 0122+200, PG 2131+066, MCT 0130?1937, Teff = 80 000–95 000 K). With the help of non-LTE model atmospheres and synthetic spectra, we are able to identify a large number of weak CNO lines (C iii, N iv, O iii-v) that were not detected before in these stars. They allow abundance determinations and enable us to constrain the effective temperature to high precision through ionization equilibria without the requirement to access the ultraviolet spectral range.
关键词： stars: abundances,white dwarfs,stars: evolution,stars: AGB and post-AGB,stars: atmospheres
更新于2025-09-23 15:23:52
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Visible Light Driven Hydrogen Evolution by Molecular Nickel Catalysts with Time-Resolved Spectroscopic and DFT Insights
摘要： Hydrogen (H2) is a clean fuel that can potentially be a future solution for the storage of intermittent renewable energy. However, current H2 production is mainly dominated by the energy intensive steam reforming reaction, which consumes a fossil fuel, methane, and emits copious amounts of carbon dioxide as one of the byproducts. To address this challenge, we report a molecular catalyst that produces H2 from aqueous solutions, is composed of affordable, earth-abundant elements such as nickel, and has been incorporated into a system driven by visible light. Under optimized conditions, we observe a turnover number of 3880, among the best for photocatalytic H2 evolution with nickel complexes from water?methanol solutions. Through nanosecond transient absorption, electron paramagnetic resonance, and UV?vis spectroscopic measurements, and supported by density functional theory calculations, we report a detailed study of this photocatalytic H2 evolution cycle. We demonstrate that a one-electron reduced, predominantly ligand-centered, reactive Ni intermediate can be accessed under visible light irradiation using triethylamine as the sacrificial electron donor and reductive quencher of the initial photosensitizer excited state. In addition, the computational calculations suggest that the second coordination sphere ether arms can enhance the catalytic activity by promoting proton relay, similar to the mechanism among [FeFe] hydrogenases in nature. Our study can form the basis for future development of H2 evolution molecular catalysts that incorporate both ligand redox noninnocence and alternative second coordination sphere effects in artificial photosynthetic systems driven by visible light.
关键词： Proton relay,Second coordination sphere,DFT calculations,Time-resolved spectroscopy,Visible light photocatalysis,Hydrogen evolution,Molecular nickel catalysts
更新于2025-09-23 15:23:52