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Title: Synthesis and characterization of II-IV group and silicon related nanomaterials
Other Titles: Yu gui xiang guan de er zhi si zu na mi cai liao de he cheng yu te xing miao shu
Authors: Ismathullakhan, Shafiq
Department: Department of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2008
Publisher: City University of Hong Kong
Subjects: Nanostructured materials.
Notes: CityU Call Number: TA418.9.N35 I86 2008
xii, 94 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2008.
Includes bibliographical references.
Type: thesis
Abstract: Despite considerable efforts, rational synthesis of ZnO nanostructures with tunable n-type conductivity is a challenging issue. On the other hand, as-synthesized ZnO nanostructures are often randomly oriented, and thus have limited applications in optoelectronic devices. Herein, we report a controlled growth and doping process of well-aligned zinc oxide (ZnO) nanowire (NW) arrays via thermal evaporation. Influence of Gallium (Ga) dopant on the growth direction of ZnO NWs was examined. The growth direction of ZnO NWs was found to depend on the dopant content. Electrical transport properties of ZnO NWs were studied by fabricating and characterizing single nanowire field-effect transistors (FETs). It is shown that the ZnO NW conductivity can be tuned by two orders of magnitude, through the way of doping. Doping is a widely used method to tailor the electrical and optical properties of semiconductors by introducing discrete energy states in the band gap. In this regard, the role of Indium (In) as a luminescence activator and as a compensator of n-type materials is of considerable significance for II–VI compound semiconductors. In this work, high quality n-type Indium doped cadmium sulphide (CdS) nanomaterials were fabricated by doping through a simple thermal evaporation method. Photoluminescence studies on the intrinsic and doped nanoribbons reveal the presence of discrete exciton emission bands in doped samples. Studies on field emission properties of doped CdS nanopens and nanopencils were also carried out. It is found that the nanopens with sharp tip has less turn on field compared to the nanopencil samples. Photoconductive response characteristics of single CdS nanoribbon (NR) to various wavelengths were also investigated. It is shown that a single CdS NR photoconductor can be used as one switch in optoelectronic applications, because of its reversible switching ability between high and low conductivities. Manipulation of nanomaterials remains as another major challenge in the field of nanotechnology, despite significant progress. The fabrication of integrated systems using nanomaterial requires the site-specific growth or placement on relevant device platforms. In addition, the formation of complex and multi-component structures are needed for low-dimensional structures and electronic devices. In this dissertation, heteroepitaxial growth of single-crystalline ZnSxSe1-x nanowire arrays on ZnS nanoribbon substrates by the metal-catalyzed vapor-liquid-solid growth method were carried out. ZnSxSe1-x nanowire arrays were aligned crosswise to the top surface and vertically grown on side surfaces of ZnS nanoribbon substrates with variable compositions making it having tunable optical properties. Photoluminescence spectroscopy of the nanostructures reveals the lasing emission from the nanowires beyond threshold excitation intensity and exciton emissions below threshold and at low temperatures. Control of channel diameter and branching of a hierarchical tubular nanostructure is important for developing nano-channels or nano-containers for various applications. A simple thermo-evaporation synthesis and two-step method for epitaxial growth of branched silicon oxide (SiO) nanotubes from ZnS/SiO core-shell nanowire heterostructures using zinc sulfide (ZnS) and SiO as sources were studied. ZnS nanowires were synthesized by Au-catalyzed vapor-liquid-solid growth, and served as templates to form amorphous SiO nanotubes via evaporation of the ZnS core. Successive SiO coating and ZnS wire removal was found to graft new branches to the original tube and the diameter of the prepared porous SiO nanotube could be post-processed by electron beam irradiation. The resulting core-shell structures were found to have uniform diameters, which are suitable for heterostructure nanodevices fabrication. Low-temperature photoluminescence studies on the SiO nanotubes sample reveal the visible-light emission centered at 612 nm. The present growth template approach may be extended to assemble nano-fluidic channel network for bioanalytical and chemical separations and branched field effect transistors.
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