TIO ₂ And Zns Study On Electronic Structure And Optical Properties Of Doped System

Titanium dioxide TiO2 has three different crystal phases, i.e., rutile, anatase, and brookite, among them the anatase TiO2 is widely used in photocatalysis field because of its peculiar property. On the other hand, anatase TiO2 is a wide forbidden band semiconductor with the band gap of 3.23 eV, the corresponding wavelength is 383 nm. So only the ultraviolet (UV) light can be used to excite the electrons from the valence band to the conduction band, thus the applications efficiency of the sunlight is very low. In fact, the absorption edge can be moved to the red light direction by impurity levels, so much effort have been paid to make anatase TiO2's band gap smaller by doping methods for the efficient use of sunlight and improving the photocatalysis property of the system.Zinc sulfide ZnS is aⅡ-Ⅵsemiconductor; it can emit the primary yellow light and green light. ZnS can be applied to the production of detector, sensor, and nano-semiconductor laser, which are widely used in missile early warning system, flame detector, corona detector, and space communication. ZnS is a wide forbidden band semiconductor with band gap of 3.67 eV, the corresponding wavelength is 339nm. Generally, the ultraviolet detector work in the solar-blind region, the cutoff wavelength is required to be smaller than 280 nm, so the ZnS based materials with wider band gap are desired.Using the total energy pseudo-potential approach of plane wave, we have studied the electronic structure and optical properties of the anatase TiO2 systems with Sc-doped, oxygen vacancies included, and Sc and oxygen vacancies co-existing, the Mg-doped ZnS system has also been studied. Comparisons of our results with the possible experiment results have been given.The main contents of the thesis are as follows.(1) The structure, basic physical properties, applications, and research situation of TiO2 and ZnS are introduced. In addition, the density functional theory (DFT) and our calculation tool--ABINIT package are briefly discussed.(2) The electronic structure and optical properties of the anatase TiO2 systems with Sc-doped, oxygen vacancies included, and Sc and oxygen vacancies co-existing have been studied, respectively. The obtained results show that the pure TiO2 is an indirect band gap semiconductor, the valence band is mostly contributed by O 2p electrons, and the conduction band is mostly contributed by Ti 3d electrons. The contribution by the doped Sc mainly lies in the valence band, and the light absorption in the visible region is obvious. A Mott phase transition takes place in the presence of oxygen vacancies. For the Sc and oxygen vacancies co-existing case, when oxygen vacancies far from Sc, the light absorption in the visible region is obvious, but the intensity is smaller than that of the Sc-doped system; when oxygen vacancies near Sc, the absorption in the visible region is enhanced coherently due to the influences both from doped Sc and oxygen vacancies, thus the photocatalysis property of the system can be improved greatly.(3) The electronic structure and optical properties of the Mg-doped ZnS system have been studied. The obtained results show that the pure ZnS is a direct band gap semiconductor, the valence band is mostly contributed by S 3p electrons, and the conduction band is mostly contributed by Zn 4s electrons. With Mg concentration increasing, the lattice constants change according to Vegard's law; the band gaps broaden, and the absorption spectra shift to the blue light region. When the Mg concentration x=0.5, the edge of the optical absorption is at about 275 nm. Thus, the solar-blind region ultraviolet detector's requirements can be met.