Electronic band structure and optical properties of Cr-doped ZnO were studied using the density functional method within the generalized-gradient approximation. Three configurations with the substitution of Zn by one and two Cr atoms in different positions were considered. For the pure ZnO, the Fermi level locates at the valence band maximum, while it shifts to the conduction band and exhibits metal-like characteristic after Cr atoms are introduced into the ZnO supercell. The calculated optical properties indicate that the optical energy gap is increased after Cr doping. More importantly, strong absorption in the visible-light region is found, which originates from the intraband transition of the Cr 3d bands and the conduction bands. Our calculations provide electronic structure evidence that, in addition to usage as short-wavelength optoelectronic devices, the Cr-doped ZnO system could be a potential candidate for photoelectrochemical application due to the increase in its photocatalytic activity.

•The Cu2O/Cu/AgBr/Ag shows a much higher photocatalytic activity than the samples without Cu NPs.•The stability of photocatalyst is enhanced significantly after introducing Cu NPs.•Cu NPs assists in accelerating carrier transfer between Cu2O and AgBr.•The enhanced photocatalytic activity and stability can be attributed to the construction of the Z-scheme structure.A series of Cu2O/Cu/AgBr/Ag photocatalysts were synthesized by a redox procedure followed by photo-assisted deposition. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were utilized to characterize the structure of the system. It is found that Cu nanoparticles (NPs) could be controllable to grow in between Cu2O and AgBr. Without Cu NPs between Cu2O/AgBr, under visible light irradiation for 50 min, only a low photocatalytic degradation of methyl orange (MO) (∼51%) was observed compared with the high MO photodegradation (∼98%) in the presence of Cu NPs. Furthermore, introducing Cu NPs assists in accelerating excited carrier transfer at the interface between Cu2O and AgBr, measured by the photoluminescence spectra, photocurrent and electrochemical impedance spectra, which thus helps to increase the stability of the photocatalyst. The increased photocatalytic activity and stability can be attributed to the favorable band alignment between Cu2O and AgBr mediated by Cu NPs, which is known as the Z-scheme mechanism and confirmed by the detection of the active species. These results demonstrate that the Cu2O/Cu/AgBr/Ag is a potential visible light photocatalyst for pollutants degradation.Download high-res image (238KB)Download full-size image

The thermal stability of Ge/Al2O3 and Ge/AlN/Al2O3 stacks has been systematically studied upon post deposition annealing (PDA) at 400, 500 and 600 °C with nitrogen gas flow. X-ray photoelectron spectroscopy (XPS) with the incident photon energy of 1486.7 eV and synchrotron radiation photoemission spectroscopy (SRPES) with the incident photon energy of 720, 500 and 200 eV have been used to characterize the interface chemistry and the diffusion of Ge-oxides. More aggressive “clean-up” effect takes place with a higher substrate temperature during the atomic layer deposition (ALD) process for the growth of Al2O3 and AlN thin films. A competitive process among the Ge-oxides growth at the Ge/high-k dielectrics interface, the Ge-oxides diffusion, and GeO desorption has been suggested upon PDA treatments. The effective suppression for formation of Ge-oxides by an ultrathin AlN layer has been observed for the samples before PDA and after PDA at 400 °C. Ge-oxide diffusion in proximity to the gate oxide surface has been characterized from the Ge2p3/2 spectra by XPS after PDA at 400 °C for Ge/Al2O3 and Ge/AlN/Al2O3 stacks. The diffusion mechanism is hypothesized by diffusion of oxygen vacancy. Moreover, a significant desorption of GeO occurs after PDA at 600 °C for the AlN passivated sample.

Graphitic-like ZnO layers have been experimentally synthesized on metal substrates over the past few years. Nevertheless, the impact of metal substrates on the structural and electric properties of ZnO is still unclear. Utilizing first-principle calculations with van der Waals correction, we found that the phase transformation from graphitic-like to wurtzite structure occurs when the thickness of freestanding ZnO exceeds seven layers. With the presence of pure Ag(111) substrate, the critical transformation thickness decreases to two layers because of the depolarization effect originating from the charge transfer from Ag substrate to ZnO. Band structure analysis displays the semiconducting behaviors for the freestanding graphitic-like ZnO layers. On the pure Ag substrate, monolayer and bilayer ZnO is n-doped by the substrate and a metallic character of ZnO is observed. Importantly, the semiconducting behavior of ZnO layers is maintained when ZnO is in contact with oxidized Ag substrate because of less charge transfer between ZnO and Ag. The metal–semiconductor contact results in a Schottky barrier of 0.8 eV. The simulation findings indicate that the few-layered ZnO on oxidized Ag system possesses potential applications in optoelectronic devices.

The electrical and magnetic properties of Zn-doped Fe3O4 at different doping concentrations of Zn have been investigated using a density functional method with generalized-gradient approximation corrected for on-site Coulombic interactions. The electronic structure calculation predicts that ZnxFe3−xO4 (0 ≤ x ≤ 0.875) is half-metallic with a full spin polarization. The hopping carrier concentration of ZnxFe3−xO4 decreases with increasing x, which indicates a distinct increase in the resistivity. The saturation magnetization of ZnxFe3−xO4 increases evidently with increasing x from x = 0 to x = 0.75 (i.e. from 4.0 to 8.3 μB/f.u.) and then decreases rapidly to zero at x = 1. The robust half-metallicity, large tunability of electrical and magnetic properties of a Zn doped Fe3O4 system make it a promising functional material for spintronic applications.

Room-temperature ferromagnetism is found in polycrystalline CrxZn1−xO (0≤x≤0.0910≤x≤0.091) films prepared by magnetron sputtering. The saturated magnetization is ∼ 0.58 μB/Cr with x=0.012x=0.012, and decreases with increased Cr dopant. The Curie temperatures of the samples are above 400 K. First principles calculations based on density functional theory predict that the electrons of Cr-doped ZnO films at Fermi level are 100% spin polarized when two Zn sites are substituted by Cr atoms in the nearest neighbour configuration. The spin polarized carriers and the p–d hybridization between Cr and its four neighbouring O atoms are responsible for observed ferromagnetism.

The electrical and magnetic properties of Zn-doped Fe3O4 at different doping concentrations of Zn have been investigated using a density functional method with generalized-gradient approximation corrected for on-site Coulombic interactions. The electronic structure calculation predicts that ZnxFe3−xO4 (0 ≤ x ≤ 0.875) is half-metallic with a full spin polarization. The hopping carrier concentration of ZnxFe3−xO4 decreases with increasing x, which indicates a distinct increase in the resistivity. The saturation magnetization of ZnxFe3−xO4 increases evidently with increasing x from x = 0 to x = 0.75 (i.e. from 4.0 to 8.3 μB/f.u.) and then decreases rapidly to zero at x = 1. The robust half-metallicity, large tunability of electrical and magnetic properties of a Zn doped Fe3O4 system make it a promising functional material for spintronic applications.