Structural and electronic properties of oxide surfaces and interfaces: an ab-initio study

Metal oxide low dimensional structures are an important class of nanomaterials with unique properties and useful functionalities that are attractive for a variety of applications ranging from electronics to biomedicine and energy conversion. In this thesis DFT calculations are performed for two different classes of metal oxides low dimensional structures. The first class of systems which will be considered concerns the clean and defected zinc oxide non polar surfaces. The study is aimed to elucidate the thermodynamic and kinetic stability of the clean surface against the formation and diffusion of oxygen vacancies. At variance with other oxide materials and ZnO surfaces with different orientation, we show that, under exposure to molecular oxygen in the gas phase, no significant amounts of oxygen vacancies can be sustained by the surface, in agreement with recent Scanning Tunnelling Microscope (STM) observations. However, our calculations show also that under ultrahigh vacuum and high temperature conditions, the observation of oxygen vacancies might be possible. We characterize the defected surfaces electronic and structural properties as a function of the position of the defect with respect to the surface and discuss the diffusion paths of such defects both parallel and across the surface. The second class of metal oxides nanostructures that we have investigated in this thesis is represented by STO-anatase interface. We report on first principles calculations of the properties of the epitaxial STO-anatase heterojunction, with an emphasis on the electronic band profile and lineup at the interface. The valence and conduction band offsets are calculated as a function of the number of anatase layers deposited onto the STO, as well as of the position of an oxygen vacancy with respect to the interface. It is shown that the presence of oxygen vacancies in the STO is a way to effectively lower the barrier heights at the interface. Our results are in agreement with recent experiments reporting nearly zero band offsets.