Zhenxing Fang

Density functional theory study on the electronic and optical properties of three crystalline phases of BiVO4

The differences in the photocatalytic activity of bismuth vanadate in three crystalline phases have been investigated through calculating their electronic structures and optical properties based on density functional theory. Our results indicate that zircon-tetragonal BiVO4 has a direct and wide band gap, while monoclinic BiVO4 and scheelite-tetragonal BiVO4 has indirect and narrow band gaps. The density of states and atom populations of monoclinic and scheelite-tetragonal phases are similar, but slightly different from those of zircon-tetragonal phase. Among three phases, the monoclinic BiVO4 possesses the largest dipole moment and the lightest effective mass of carriers, which can promote the generation and separation of photo-induced carriers, and subsequently may improve photocatalytic activity.

Computational design of inorganic nonlinear optical crystals based on a genetic algorithm

Based on the results of density functional theory calculations, a theoretical method to design inorganic nonlinear optical (NLO) crystals for second harmonic generation (SHG) is presented. In this method, a specialized genetic algorithm (GA) is developed to search the stable structures of the inorganic crystal with known compositions and study the noncentrosymmetric stable structures and the second-order nonlinear optical properties by calculating the corresponding SHG coefficients. Unlike normal GA techniques, the main feature of the present method is that the coordination fashions of the building units are introduced to construct the structures of individuals during the GA procedure, which can obviously improve the efficiency and success rate of obtaining the stable structure of the inorganic crystals. As typical examples, two ternary compounds, AgGaS2 and LiAsSe2 crystals are considered, and besides the structures observed experimentally, the geometries and optical performance of other metastable (or more stable) phases have been explored. Our results clearly demonstrate that the present method can provide a feasible way to design and optimize new inorganic NLO crystals.

Deposition of (WO3)3 nanoclusters on the MgO(001) surface: A possible way to identify the charge states of the defect centers

Periodic density functional theory calculations have been performed to study the most stable structure of the (WO(3))(3) nanocluster deposited on the MgO(001) surface with three kinds of F(S) centers (F(S)(0), F(S)(+), and F(S)(2+)). Our results indicate that the configuration of (WO(3))(3) cluster, including the cyclic conformation and the heights of three W atoms, and the oxidation states are sensitive to the charge state of the F(S) center. It is interesting that the electron-riched F(S) (0) vacancy on the MgO(001) surface can act as a promoting site to enhance the W-W interaction and the W(3)O(3) cyclic conformation is maintained, while the skeleton of cluster becomes flexible when (WO(3))(3) is adsorbed on the electron-deficient vacancy (F(S)(+) and F(S)(2+)). Accordingly, three F(S)-centers exhibit different arrangements of X-ray photoelectron spectra, the scanning tunneling microscopy images, and the vibrational spectra after depositing (WO(3))(3) cluster. Present results reveal that the (WO(3))(3) cluster may be used as a probe to identify the different F(S) centers on the MgO(001) surface.

Different Atomic Terminations Affect the Photocatalytic Nitrogen Fixation of Bismuth Oxybromide: A First Principles Study

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O-terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high-energy N2 intermediates. Interestingly, the Bi-terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N2 molecules comparing with O-terminated BiOBr {001} facets with OVs. Moreover, the Bi-terminated BiOBr {001} facets may tend to yield NH3 instead of N2 H4 .

Pressure-tuning the nonlinear-optical properties of AgGaS 2 crystal: a first-principle study

A specialized genetic algorithm approach in combination with first-principles calculations is employed to predict the stable structures of AgGaS2 crystal at different pressures. The results show that the chalcopyrite structure first transforms to the monoclinic Cc phase, and then to a centrosymmetric structure that the second-harmonic generation (SHG) response of AgGaS2 is disappeared. The effects of external pressures, up to 7 GPa, on the linear and second-order nonlinear optical properties of AgGaS2 are explored systematically. Our work reveals that the resistance to laser-induced damage, the transparency range, and the phase matchability can be improved by the pressure-induced deformation of AgGaS2 crystal. Moreover, the feature of the strong SHG response of AgGaS2 crystal is still preserved in the whole IR region even under pressure up to 7 GPa.

Indium selenide monolayer: a two-dimensional material with strong second harmonic generation

Because of their unique physical properties, two-dimensional (2D) materials have attracted extensive interest as potential candidates for next generation electrical and optoelectronic applications; in particular, large optical second harmonic generation (SHG) has been recently discovered in a number of 2D materials, which allows them to have valuable applications in electro-optic modulators and switches, frequency conversion, and so forth. Here, we performed a detailed first-principles study on the geometries, electronic structures and SHG properties of a recently prepared In2Se3 monolayer. Our results reveal that In2Se3 with a single-layered structure may be a material with the strongest SHG response among IIIA–VIA semiconductors reported to date, and the maximum magnitude of the static SHG coefficient is predicted to be 208.8 pm V−1. The extraordinary SHG is strongly correlated with a special arrangement of two kinds of In–Se polyhedra, namely, [InSe6]9− octahedra and [InSe4]5− tetrahedra, on the opposite sides of the In2Se3 monolayer. Further investigations on other In–Se compounds with different structures and stoichiometries indicate that besides the configuration of the In–Se building block, the SHG response can be significantly influenced by the interlayer interactions, and the magnitude of the SHG coefficient of the In2Se3 monolayer decreases gradually as the interlayer distance decreases until it approaches that of In2Se3 bulk in the R3m phase. Our present findings provide a deep understanding of the SHG properties in IIIA–VIA 2D semiconductors and are also helpful for designing new 2D materials with tunable SHG susceptibility.

A DFT study of (WO3)3 nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

The structures and electronic properties of (WO3)3 nanoclusters adsorption on defective MgO ultrathin films on Ag(001) have been investigated by means of density functional theory (DFT) calculations including dispersion interactions. Our results show that, after deposition, the oxygen vacancy on the defective MgO/Ag(001) films is healed by one terminal oxygen atom of the (WO3)3 clusters through forming four O–Mg bonds. The conformation of the (WO3)3 nanoclusters is distorted slightly and the W3O3 cyclic conformation of adsorbed (WO3)3 nanoclusters is still maintained. The defective MgO/Ag(001) 2D films lead to enhancement of the adsorption energy between the (WO3)3 clusters and the substrates, compared to that on defect-free MgO/Ag(001) 2D films and a defective MgO(001) surface. It is interesting that obvious charge transfer (2.74e) occurs from the defective MgO/Ag(001) films to the 5d empty state of the (WO3)3 clusters, which mainly originated from spontaneous electron tunneling through the thin MgO dielectric barrier, and less from the surface defective state as the consequence of the formation of O–Mg adsorption dative bonds at the interface. In addition, compared with (WO3)3 nanoclusters in the gas phase, on the defect-free MgO/Ag(001) 2D films and the defective MgO(001) surface, different scanning tunneling microscopy images and vibrational spectra for depositing (WO3)3 nanoclusters are observed, which could help in the identification of (WO3)3 nanocluster adsorption on the defective MgO/Ag(001) ultrathin films in future experiments. As a consequence, our results reveal that (WO3)3 nanoclusters adsorption on defective MgO/Ag(001) ultrathin films provide a new avenue to tune and modify the charge state and chemical reactivity of tungsten oxide nanoclusters.