The Nature Of A Number Of New Materials, Tio <sub> 2 </ Sub>-doped Modified Theoretical Research

Thanks to the rapid development of high-performance computers and software technology, together with the progress made in computational theories, the study of structures and properties of materials by means of computer simulation has become an interdisciplinary subject and less dependent on physical or chemical experiments nowadays. This thesis presents introduction of the development of computational material science and the fundamental of first-principles calculations in the framework of density functional theory. On the basis of this theoretical method, the investigations of structural, electronic and optical properties of materials, as well as the doping mechanisms, have been performed. The results are presented and discussed in the following parts:1. The electronic and optical properties of group IVA arsenides with the pseudo cubic structure have been studied by means of the first-principles calculations.IVA-V compounds have received much interest due to their technological application and wide use in many areas of industry. C₃N₄ is known as a hard insulator and used for its wear resistance. Si₃N₄ has attractive properties like high resistivity, heat resistance and mechanical stability. To date, researchers have made efforts to study new IVA-V compounds. There have been papers reporting theoretical predictions of the structures, electronic and optical properties of group IVA phoshpides. Recently, Hu et al. continued to find that group IVA arsenides with the 3:4 stoichiometric ratio were stable with the pseudo cubic structure. In this thesis, the band structures, density of states, dielectric functions and energy loss functions of the pseudo cubic X₃As₄ (X=Si, Ge, Sn) are calculated. The results suggest that Si₃As₄,Ge₃As₄ and Sn₃As₄ have indirect band gaps. The top of the valence bands, as well as the bottom of the conduction bands, is mostly hybridized states between X np and As 4p orbitals. The three X₃As₄ compounds all have negligible anisotropy in optical properties. The plasma frequency for X₃As₄ decreases when atom X evolves from Si to Sn, whereas the effective number of valence electrons is shown to increase with the increasing atomic numbers of atom X.2.First-principles calculations for the structural, electronic and optical properties of noble metal oxides M₂O (M=Cu, Ag, Au) with the cuprite structure are performed by using a plane wave pseudopotential method in the framework of density functional theory. For Cu₂O and Ag₂O,the correspondence between our results and the available experimental outcome is good. Au₂O has comparable lattice constant with that of Ag₂O.The direct band gaps of Cu₂O and Ag₂O are reproduced in this calculation. Au₂O can also be a semiconductor,its gap value at T point is close to zero. As DFT usually underestimates band gaps of semiconductors, the real gap of Au₂O might be more profound. The M-O bond has ionic and covalent natures meanwhile. Au-O is less ionic than Cu-O and Ag-O. According to the difference electron density, holes are observed in the originally filled M d shells, suggesting a complex intra-atomic hybridization between M d and normally empty M s, p states has happened in M₂O.In comparison with Cu₂O and Ag₂O,the depletion of Au 5d shell appears more profound than those of Cu 3d and Ag 4d orbitals. The intra-atomic hybridization is thus expected to be more evident in Au₂O.The dielectric function of Au₂O is calculated and plotted as reference for future studies.3. The origin of visible light sensitivity found in nitrogen doped anatase titanium dioxide (TiO₂) is studied by means of first-principles calculations on the basis of density functional theory. Anatase TiO₂ is an important photocatalyst with its chemical stability, high reactivity and low environmental pollution. Pristine anatase is photocatalytic active merely under the ultraviolet light illumination due to its large band gap (3.2 eV),so its solar-energy-conversion efficiency is quite low. Nitrogen doping is proved to be an effective method to enhance the visible light response of TiO₂.However, the mechanism of the visible light absorption of nitrogen doped TiO₂ is still controversial. In this thesis, the separate effects of the substitutional and interstitial nitrogen dopings on the electronic and optical properties of anatase TiO₂ are evaluated. The substitutional N introduces impurity states above the valence bands of TiO₂,which tend to delocalize with the increase of doping concentration. The optical absorption due to the electron transition from these impurity states to the conduction bands improves the absorption of TiO₂ within the visible light range. The role of intestinal N is largely determined by the interaction between the interstitial N and surrounding atoms. The NO like species, formed by interstitial N bonding with one lattice oxygen, can contribute to enhance the visibly light response of TiO₂ by producing a serious of occupied gap states above the top of the valence bands. This part of work may provide new implications for an understanding of the visible light responsive N-doped anatase.