Vapor-phase Synthesis, Characterization And Photoluminescence Properties Of Quasi-one-dimensional Oxide Nanomaterials

Quasi-one-dimensional (1D) nanomaterials, including nanowires(rods), nanobelts, nanotubes, nanocables, heterojunction and superlattice nanowires, are ideal systems for investigating the dependence of electrical transport, optical properties, magnetic properties, and mechanical properties on size and dimensionality. In addition, they are expected to play an important role as interconnects and functional components in the "bottom up" design and fabrication of new electronic and optoelectronic devices. Now the research of the 1D nanomaterials has already become one of the important subjects of nanomaterial science.Vapor-phase synthesis of quasi-one-dimensional nanomaterials is a promising method to prepare nanomaterials, because it is convenient, well-controlled, wide-used as well as able to prepare high-quality nanocrystals. By adjusting the experimental parameters such as time, temperature, pressure and so on, we can get various nanostructures and even to get the heteroj unctions and superlattice nanowires. Therefore, we choose vapor-phase synthesis to prepare our samples. By direct thermal evaporation and oxidation of the pure metallic magnesium and zinc powders, we successfully synthesized bulk-quantity MgO and ZnO nanomaterials. The morphology, structure, chemical composition and physical properties of these quasi-one-dimensional nanostructures are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) as well as photoluminescence (PL) spectroscopy. The growth mechanism of various nanostructures/microstructures as well as their photoluminescence properties is further investigated. The main results in our experiments are as follow:1. Growth mechanism and ultraviolet-light emission of the self-assembled complex of MgO nanostructuresBulk-quantity net-like nanodendrites and. four-fold hierarchical nanostructures are synthesized by direct thermal evaporation and oxidation ofthe metallic Mg powders. Their growth mechanism is explained using the self-catalytic vapor-liquid-solid (VLS) mechanism together with dendritic-crystal epitaxial growth mechanism. Four-branch and eight-branch nanodendrites are also detected. The photoluminescence (PL) spectrum reveals that the peak with the maximum intensity is centered at about 3.16 eV (392 nm). Through Gaussian fitting, a strong and narrow ultraviolet-light emission peak centered at 3.16 eV (392 nm) and a relatively weak but broad blue-light emission band centered at 2.74 eV (453 nm) are observed in the PL emission spectrum, which are respectively attributed to the recombination luminescence of the F+ and F centers (belonging to oxygen-vacancy related defect levels) in the MgO nanostructures. In addition, another very weak and broad red-infrared emission band can also be detected, which is probably dueto the relaxation luminescence of impurity levels in the MgO nanostructures. 2. Growth mechanism and characterization of ZnO microbelts and self-assembled microcombsWe report the structural characterization and growth mechanism of ZnO microbelts and microcombs synthesized by direct thermal evaporation and post-oxidation of the pure Zn powders. The synthesized ZnO products are hexagonal wurtzite structured and in the range of micrometer size. The formation of the microcombs follows a two-step process: the comb-stems are formed by a fast vapor-solid (VS) growth along [1120] direction, and the comb-teeth are grown by a subsequent faceted epitaxial growth parallel to the (0001) polar surface. A small quantity of enantiomorphous twin-microcombs are also detected in the synthesized products. Photoluminescence (PL) spectrum shows that the prepared products are green emission, centered at about 507 nm, which is caused by the oxygen vacancies. These ZnO microcombs, having nanorods with the uniform diameter, length as well as spacing along the comb-stem. They probably pave a simple and direct way to produce diffraction gratings.