Structures And Electronic Properties Of Metal-Oxide

Zinc Oxide (ZnO), a wide band-gap semiconductor (3.37eV) is drawing considerable attention, because of its excellent physical and chemical properties, which find promising applications in wide range of fields, such as solar cells, sensors, photocatalysis and nanodevices. ZnO nanomaterials, such as nanoclusters, nanowires, nanotubes and nanoribbons are particularly interesting, due to the size-confinement effects which provide a way to tune the properties of these materials. The study of ZnO nanoclusters is promising because it can give useful guidance to the growth and utilization of these nanoclusters, as well as to understand the growth mechanisms of other ZnO nanomaterials.In this thesis, we performed first-principles calculations based on density functional theory (DFT) to study the geometric and electronic properties of (ZnO)n nanoclusters. The stable configurations of (ZnO)n nanoclusters are characterized in a relative wide range of sizes (n = 2 - 48). The energetic evolution of these nanoclusters as a function of cluster size n was calculated. The dependence of electronic properties of these (ZnO)n clusters on the cluster size was also studied. The main results can be summarized as follows:(1) The stable configurations of (ZnO)n clusters were obtained by optimizing the initial structures constructed through a simple approach based on "classification" and "basic elements". It was shown that the stable configurations of these (ZnO)n clusters highly depend on the cluster size and display different features at different range of size. For n = 2-7, ring-like configurations are energetically more stable and the formation energy decreases rapidly with the increase of size. When the cluster size ranges from 8 to 12, the stable configuration oscillates between ring-like structure and bubble-like structures. For n > 13, the stable configurations have bubble-like structures with the atoms being three-fold coordinated. The formation energies of these clusters decrease slightly with the increase of cluster size. These bubble-like configurations are formed by assembling rhombi, hexagons and octagons with the numbers of these polygons satisfying Euler's law. The electronic structure calculations showed that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) oscillates with the increase of cluster size. Due to the size-confinement effect, the HOMO-LUMO gap of these ZnO clusters is wider than the band gap of bulk materials (w-ZnO).(2) The stable configurations and electronic structures of some selected (ZnO)n clusters, n= 12, 16 and 24 were studied systematically, by considering the isomeric structures as more as possible. It was found that the incorporation of octagons in to the bubbles is energetically disadvantageous. The bubble-like structures consisting of rhombi and hexagons are energetically more favorable.(3) A tubular configurations of (ZnO)n nanoclusters were also studied. Quite different from the bubble-like structures, the atoms of the tubular-like configurations are four-fold coordinated, except the atoms in tube ends. The tubular clusters formed on the basis of hexagons are energetically most favorable followed by octagon tubes. The tubular clusters consisting of rhombi are energetically most unfavorable. They can convert to chain-like configurations with lower energy.