Nanostructural Research On Two Types Of ZnO Based Compound

Nanomaterials have attracted much attention in recent years because of their unique properties and potential applications.The properties,such as surface reconstructions and discrete energy levels,are significantly different from that of the bulk,when the size approaches to the exciton Bohr radius.In the thesis,the structural and electronic properties of ZnO nanoclusters and Zn₂SiO₄ nanowires are focused on,using an ab initio method which based on density-functional theory.The Zn_mO_n clusters of different size were constructed from wurtzite ZnO.The relaxation of a Zn₄O₄ cluster in different vacuum regions indicates that the interaction of clusters influences its final structure.The shapes of the relaxed Zn_mO_n clusters incline to round because of the interaction of Zn and O ions while keep the original three-fold symmetric axis due to the repulsions of same ions in the horizontal plane.The nonstoichiometric clusters have three Zn atoms in angular positions which make the clusters unstable.In all relaxed clusters,it is interesting to note that the O atoms are at the peripheral positions,the Zn atoms move inside,and the distance between O atoms is longer than that between Zn atoms, indicating that the Coulombic repulsions play an important role in the cluster reconstructions. The hybridization at the surface leads to sp³ pyramidal configurations around the anions in compound clusters,while the cations tend to become sp²-like planar bonded at the surface. The average bond length in all relaxed clusters increases with the size and gradually approaches to that of the bulk,which means the smaller clusters have stronger ionicity.In the nonstoichiometric clusters,the Coulombic repulsions of the redundant Zn ions lead to the increase of the Zn-O bond lengths.Furthermore,by comparing the E_b of different sizes and stoichiometric proportions of the clusters,it is concluded that the larger clusters with stoichiometric proportions are more stable.On the other hand,hexagonal Zn₂SiO₄ nanowires were grown by chemical vapor deposition.The CL spectra of the nanowires under 14 K exhibited three distinct peaks:one strong peak at 470 nm and another two weak bands at about 675 nm and 290 nm,respectively. To understand the mechanisms,the structural and electronic properties of Zn₂SiO₄ surface were studied using ab initio method.A super cell of the surface was constructed based on the hexagonal Zn₂SiO₄.After relaxed,the symmetry of the nanowires is lowered and the calculated band gap is 2.55 eV which is smaller than that of the bulk.It is suggested that the band gap transition from the atoms far away from that of the Zn₂SiO₄ nanowire surface is responsible for the emission at 290 nm.And the electron transition from the surface of the nanowires can attribute to the 470 nm emission.The further discussion about densities of states shows that the 470 nm emission is probably induced by the transition from the s states of Zn to p states of Si,or s states of Si to d states of Zn.