Zinc Oxide Based Nanostructures:Controllable Synthesis, Growth Mechanism And Optical Properties

Wide band gap (-3.37eV). large exciton binding energy (-60meV). low toxicity, good thermal stability, large specific surface area and high electron mobility make ZnO nanostructures as a good candidate upon which to develop high-temperature ultraviolet (UV) optoelectronic devices and ultraviolet lasers, as well as to construct functional devices. Recently, extensive efforts have been devoted to developing UV light-emitting devices and lasers using ZnO nanowires (NWs). However, due to the high surface-to-volume ratio, ZnO nanowires generally have much more surface defects, such as oxygen vacancy and hanging bond, than thick ZnO rods or bulk ZnO. These surface defects weaken and can even quench UV emission. Considerable enhancement (～3--6times) of UV emission intensity has been obtained by coating ZnO NWs with ZnS, SnO2, Al2O3and amorphous MgO, respectively. The study on epitaxial growth of highly crystalline MgO shells on surfaces of ZnO NWs will open a new way to the further improvement on UV emission.In recent years, hierarchical nanostructures with complex morphology and high dimensionality have received great research interest because of the novel properties provided by their advanced geometric structures and the special atom arrangements on the specific facets of these nanostructures. Considerable efforts have been devoted to the synthesis of hierarchical ZnO nanostructures. The reported synthesis strategies can be mainly classified into two categories:(1) two-stage hydrothermal synthesis route combined with the reseeding process;(2) high-temperature (850-1000℃) thermal evaporation of precursor powers to synthesis nanowire (NW) and subsequently secondary growth of branched nanostrucures via hydrothermal approach accompanied by the reseeding treatments.In this dissertation, we first study the effects of synthesis conditions on the morphologies of ZnO nanostructures. We prepare aligned ZnO nanowire arrays and ZnO nanostructures with different morphologies. Then we mainly study novel highly regular ZnO hierarchical nanostructures with high surface-to-volume ratio and ZnO/MgO core-shell structure nanowires with single crystal MgO shells. Based on the observations on a series of morphological evolutions and characterizations of crystal structures, the corresponding growth mechanisms for both ZnO hierarchical nanostructures and ZnO/MgO core-shell structure nanowires are proposed. The main points in this dissertation can be summarized as follows:(1) The aligned ZnO nanowire arrays with different diameters, nanocomb, nanobolt, star fish-like and seenpod of the lotus-like ZnO structures have been synthesized by CVD method with proper technological parameters.The corresponding growth mechanisms are discussed.(2) We successfully fabricated a novel highly regular ZnO hierarchical nanostructure with six rows of single crystal ZnO nanowire arrays in double3-fold-symmetry, using a two-stage synthesis scheme without any catalyst and chemical reagent. Our synthesis employs a simple thermal evaporation of Zn powder CVD followed by a post-annealing. In the first stage. Zn-rich ZnO nanotubes are formed; then in the annealing stage, the resulting Zn-rich ZnO nanotubes provide the backbones and the supply of Zn to grow ZnO NWs. A growth model for ZnO hierarchical nanostructure based on dense arrangement principle is proposed. The blue-shift emission can be clearly observed from the PL spectra for annealing samples, and the green emission disappears but violet emission is significantly enhanced, revealing Vo decrease significantly and some EX-Zn, transforms to a relative higher Zni level after annealing.(3) The core-shell ZnO/MgO nanowires with single crystal MgO shells were achieved by using a two-step CVD method. Based on HRTEM revealed the relation of ZnO [0002] and MgO [200] directions, and the geometry relation of cube, we find that the [111] direction of grown cubic MgO crystal is parallel to [0002] direction of hexagonal ZnO NW crystal. The epitaxial growth of cubic MgO shells on hexagonal ZnO NWs surfaces benefits from the close interplanar distances of MgO (111) and ZnO (0002) planes. The formation of high crystal quality MgO shells not only improved the crystal quality of ZnO NWs surfaces but also passivated the remained surface defects of ZnO NWs, and consequently greatly enhanced UV emission. Comparing with bare ZnO NWs, more than10times enhancement of ultraviolet (UV) emission is obtained from ZnO/MgO NWs. The related crystal growth model is proposed.(4) The lichee-like Zn-rich ZnO nanoclusters (80nm) composed of superfine ZnO particle (10nm) and densely accumulated superfine ZnO particle (13nm) were synthesized on the surfaces of ZnO nanowires to form new ZnO hierarchical nanostructures. which were located at different distances from Zn source. by Zn powder thermal evaporation route. These new ZnO hierarchical nanostructures with high surface-to-volume ratio have potential applications in photocatalysis, sensor devises and solar cells.