Simulation Study Of Low-dimensional ZnO Nanostructure Based On The First-principles Calculations

Zinc oxide (ZnO) is a direct wide band-gap (Eg≈3.37eV) semiconductor of Ⅱ-Ⅵ group, with a high exciton binding energy (≈60meV) at room temperature. Compared with GaN, it has many similarities in the crystal lattice characteristics, the band structure and the photo-electric properties Moreover, it also has higher exciton binding energy and the low growth temperature. Therefore, it was considered to be a new kind of shortwave length photo-electric material as GaN. On the other hand, with the rapid development of Density Functional Theory (DFT) and relevant numerical algorithms in recent years, the First-Principles calculations based on the DFT have become the essential research tools for condensed matter physics, quantum chemistry and materials science.In this thesis, First-Principles calculations based on DFT are systematically performed to study the various ground-state properties of three-dimensional bulk ZnO, two-dimensional ZnO namofilms, one-dimensional ZnO nanowires and zero-dimensional ZnO nonoclusters. The main contents are as follows:(1)The electronic structure of the bulk ZnO was studied, through the analysis of the band structure, the total density of states and partial density of states (PDOS), we know that ZnO is a direct wide band-gap semiconductor. PDOS show that Zn3d and O2p states (especially the former) play a decisiverole in its electronic structure. In this part, we also make some discussions on the selection criteria of the key parameters used.(2)Calculates the energy band structure and the density of states of one-dimensional ZnO nanofilms, we found that present obvious metallicity. For the nanofilms with less than6Zn-O double layers, their effective elastic constants are much lower than the corresponding bulk value. While the film thickness continues increasing, the obtained result becomes almost invariable and approaches to the bulk value.(3)Size-dependent piezoelectricity of ZnO nanowires was calculated in c axis. Both the structure reconstruction and quantum confinement in ZnO nanowire are considered to be the main contributions to this size effect. Energy bands calculations show that with increasing diameter, the band gap of the ZnO nanowires decreases from1.942eV to1.421eV.(4)Through structural optimizations and energy levels analysis on zero-dimensional ZnO nanoclusters with hexagonal prism configurations, we found that the structural relaxations led to zinc atoms moving toward the center of the cluster, whereas oxygen atoms moving outward. This is important to know when attempting to passivate dangling bonds at the surfaces with surfactants, since they will then be bonded first of all to oxygen atoms.