Electronic structure calculations for point defects, interfaces, and nanostructures of TiO2

Abstract

Transparent conducting oxides (TCOs) play an important role not only in optoelectronic and photovoltaic devices but also in future transparent electronics. A transparent conductor arises upon degenerately doping a semiconductor (insulator) so that the conduction becomes metallic (resistivity ~ temperature). The extra electrons occupy the conduction band (CB) states of the host and the conductivity is determined by the electron optical effective mass. Recently, anatase TiO2 films doped with Nb, i.e., Ti1-xNbxO2 (TNO), have attracted a great deal of interest as a promising candidate for TCO applications because of their low resistivity (~ 10^-4 Omega.cm) and high optical transmittance (90 % in the visible light region). A few experimental studies have been reported for the optical effective mass of electrons as a function of the carrier concentration in Nb-doped anatase, on the directions which are either orthogonal or parallel to the tetragonal axis of the crystal. In this thesis, I have determined the optical effective mass of electrons in Nb-doped anatase based on band structure calculations. The anisotropy of the crystal and the nonparabolicity of the bands have both been taken into account. I have found that in the range concentration which is relevant to transparent conductive oxide applications, the optical effective mass is determined by several branches of the conduction band, leading to a complicated dependence on the carrier concentration. The function for the optical effective mass obtained by our calculations agrees well with that obtained experimentally. In particular, the strong anisotropy of the optical effective mass has already been confirmed. Although Ta-doping of anatase TiO2 appears to be effective as well, this possibility has been not well explored. I have compared the two dopants, i.e., Nb and Ta, for doping anatase TiO2. The Ta dopant has a considerably higher solubility and a lower optical effective mass, thus acquiring more advantages than Nb. Moreover, my calculations have also explained why a reducing atmosphere is necessary for the efficient dopant incorporation, without invoking oxygen vacancies as proposed in the literature. There is no study on the effects from the quantum confinement of dopants in anatase nanowires (ANWs). Therefore, I report here the first demonstration on the role of Nb- and Ta-dopants in ANWs. The pure ANWs cut by keeping the screw axis of the original bulk structures are consistently lower in energy than the similarly oriented nanowires in which the screw symmetry is destroyed. Both Nb and Ta dopants prefer the sub-corner sites of the most stable ANWs. At the highest symmetry, the band structure of the doped ANW is similar to that of the perfect one. The increase of the photocatalytic activity upon mixing rutile and anatase powders is usually explained by assuming change separation between the two phases. There are many contradicting theories regarding the particular charge transfer between these phases. Therefore, another goal of this thesis is to study the electronic properties of the interface between anatase and rutile phases of TiO2. By calculating the band line-up of a rutile-anatase interface, I have found that both the conduction band minimum (CBM) and the valence band maximum (VBM) of the rutile phase are higher than those of the anatase phase. As a result, electrons are expected to transfer from the rutile phase to the anatase phase while holes move in the opposite direction. In my work, the optical electron effective mass is determined from the band structure of the material, which is in turn calculated by the version of density functional theory (DFT) in the generalized gradient approximation (GGA) implemented in the Vienna Ab Initio Simulation Package (VASP) package. For bulk materials, both the Perdew-Berke-Enzerhof (PBE) and the screened hybrid functional (HSE06) are used for the exchange energy. Although the HSE06 functional gives better results compared with the existing experimental measurements for Nb- and Ta-doped anatase TiO2 bulk materials, similar calculations with HSE06 for nanowires are far more expensive. Therefore, my calculations for nanowires are carried out only with the pure GGA-PBE functional. To determine the rutile-anatase interface, I have used the density functional based tight binding (DFTB) method for the molecular dynamic simulations, and then relaxed by ab initio calculations with PBE functional at 0K.