First-Pinciples Study Of The Nanostructures Of Zinc Oxides

As the typicalâ…¡-â…¥compound and semi-conductive material, with a direct wide band gap of 3.37 eV, ZnO has attracted much attention because of its multi-functional behaviors, such as nanostructural diversity, unique anti-bacterial activity, and optical, electrical and piezoelectric properties. A variety of ZnO nanostructures have been successfully synthesized in recent years. Moreover, experimental and theoretical researches on their structures and properties have been reported in many well-know international journals. Therefore, in the field of nanotechnology, following carbon nanotubes, ZnO has gradually become one of the most attractive nano-materials among the potential electronic device systems. However, because of the difficulty in directly measuring the properties of nano-materials like clusters, theoretical calculations become an effective way to identify the ZnO nano-structures.As an important theoretical research method, computational chemistry plays a vital role in understanding the physical and chemical natures of nano-materials. In this work, the first-principles density-functional theory (DFT) calculations were systematically performed to study the properties of the ground-state ZnO nanoclusters as well as their low-lying isomers, such as structural features, energetic stability, adsorption with molecular oxygen, linear and nonlinear optical properties, etc. We have established, for the first time, the different and appropriate models for the investigations of the different properties mentioned above. Our study would be helpful for further studies on the ZnO antibacterial mechanisms.The geometrical and electronic structures of the ground-state and low-lying isomers have been studied with DFT calculations in NRLMOL and GAUSS03. Our calculations show that, with the increasing size, the most stable structure of ZnO cluster changes from ring to cage structures. The structural features are addressed in terms of their shapes, geometrical parameters, stability, HOMO-LUMO gaps, etc.We used finite-field (FF) method to calculate non-linear optical properties of ZnO clusters at DFT level. The effect of ring structure, cluster stability, HOMO-LUMO gap; covalent/ionic bonding, etc., on the dipole moment, polarizability, first and second hyperpolarizabilites was discussed. Both the polarizability and second hyperpolarizability exhibit distinct size dependences, indicating clearly a structural transition when cluster grows up. Our study also reveals that the linear and nonlinear optical properties of ZnO clusters vary a lot in the size range studied, as a result of their structural diversity at small sizes.The structural stability, electronic structure and adsorption energy for the adsorption of O₂ on the surfaces of ZnO clusters were studied at DFT level with the generalized gradient approximation (GGA). The most stable state of (ZnO)_N-O₂ complexes were identified and their electronic properties were discussed. The results indicate that physical adsorption is the dominate mode, and the adsorption results in favor of generating electrons under light- exciting.The geometry, density of states and band structures of pure and the Fe²⁺ doped-ZnO were studied by using the plane wave ultra-soft pseudopotential method based on the GGA. The calculations show that the doping of Fe²⁺ narrows the band gap and leads to red-shift of absorption wavelength. What is more, the Fe²⁺ doped-ZnO displays ferromagnetic.