Vapor-phase Synthesis And Photoluminescence Properties Of One-dimensional Metal Oxide Nanomaterials

One dimensional (1D) nanomaterials, including nanowires (rods), nanotubes, nanobelts, nanocables, heterojunction and superlatice 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 "botom up" design and fabrication of new electronic and optoelectronic devices. Now the research of the 1D nanomaterials has already become one of the centre subjects of nanomaterial science. Though the research of 1D nanomaterials already has been got considerable progresses, it still remains a significant challenge ro achieve controlled synthesis of 1D nanomaterials with desired morphologies, arrangement, components, structures and properties, which is a foundation and prerequisite for the applications of the nanomaterials. Focusing on the research of controlled synthesis of 1D nanomaterials and relative physical properties, we have done a series of work, and main contents and conclusion can be summarized as following:1. Self-Assembly and Hierarchical Organization of Ga₂O₃/In₂O₃ NanostructuresWe report on the realization of novel 3-D hierarchical heterostructures with 6-and 4-fold symmetries by a transport and condensation technique. It was found that the major core nanowires or nanobelts are singlecrystalline In₂O₃, and the secondary nanorods are single-crystalline monoclinicβ-Ga₂O₃ and grow either perpendicular on or slanted to all the facets of the core In₂O₃ nanobelts. Depending on the diameter of the core In₂O₃ nanostructures, the secondary Ga₂O₃ nanorods grow either as a single row or multiple rows. As-grown Ga₂O₃/In₂O₃ hierarchical heteronanostructures exhibit an intense PL in the blue-light emission, with a peak at about 494 nm (2.51 eV).2. synthesis and photoluminescence properties of chain-like In₂Ge₂O₇ and GeO₂/ZnGeO₃ crystal structures.Bulk-quantity of uniform structures such as In₂Ge₂O₇ nanobelts, chain-like In₂Ge₂O₇/ GeO₂ nanocables and chain-like GeO₂/ZnGeO₃ were successfully synthesized by the simple thermal evaporation method without the presence of catalyst. The morphology and structure of the as-synthesized the product are characterized by using X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The growth process of the ternary In₂Ge₂O₇ nanobelts is based on vapor-solid growth mechanism. The as-synthesized In₂Ge₂O₇ nanobelts are single crystals with a monoclinic thortveitite-type structure growing along the [210] direction. A strong and broad violet emission peak at about 410 nm was observed in the room-temperature photoluminescence measurements. The growth process of the chain-like In₂Ge₂O₇/amorphous GeO₂ core/shell nanocables is based on vapor-solid growth mechanism. Studies indicate that typical chain-like nanocables consist of single crystalline In₂Ge₂O₇ nanowires (core) with diameter of about 30 nm and amorphous GeO₂ chainlike nanostructures (shell). Four emission peaks, namely 401 nm, 448.5 nm, 466.5 nm and 491 nm, were observed in the room-temperature photoluminescence measurements. X-ray diffraction analysis indicates the product is mainly composed of GeO₂ mixed with a bit of ZnGeO₃. Scanning electron microscopy characterization shows that the structures consist of particle chains with an almost uniform distance between each particle. The chain-like structures consist of two parts: the ellipsoidal part and the linear part. Energy-dispersive spectroscopy reveals that different parts of the structure have different compositions. Room-temperature photoluminescence measurement shows that the synthesized structures have a strong emission band at a wavelength of 488.5 nm.3. Synthesis and Characterization of Indium-Doped ZnO Nanowires and In doped Ga₂O₃ NanowiresThe In-doped ZnO (IZO) nanowires has been synthesized by a thermal evaporation method. The morphology and microstructure of the IZO nanowires has been extensively investigated using scanning electron microscopy (SEM) and x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The products in general contain several kinds of nanowires. In the present work, a remarkable type of IZO zigzag nanowires with a periodical twinning structure has been investigated by transmission electron microscopy (TEM). HRTEM observation reveals that this type of IZO nanowires has an uncommonly observed zinc blende crystal structure. These nanowires, with a diameter about 100 nm, grow along [111] direction with well-defined twinning relationship and the well-coherent lattice cross the boundary. In addition, IZO nanodendrities structure was also observed in our work. A growth model based on the vapor-liquid-solid mechanism is proposed for interpreting the growth of zigzag nanowires in our work. Due to the heavy doping of In, the emission peak in photoluminescence spectra has red-shifted as well as broadened seriously. In doped Ga₂O₃ zigzag nanowires and undoped Ga₂O₃ nanowires were successfully synthesized by the simple thermal evaporation method without the presence of catalyst at 1000℃. The growth process of the nanowires is based on vapor-solid (VS) growth mechanism. Its morphology and microstructures were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. Studies indicate that the diameter of the well-proportioned nanowires is about 100nm. But the zigzag shaped nanowires have a diameter of about several hundreds of nanometers. A emission peak centered 457 nm was observed in the room-temperature photoluminescence measurements. The blue bands centered 457 nm can be resulted from oxygen vacancies and oxygen-gallium vacancies centers.4. Self-Catalyst selective growth and optical properties of single-crystalline ZnGa₂O₄ and Zn₂SnO₄ nanowiresUniform zinc gallate (ZnGa₂O₄) nanowires are successfully synthesized by one-step simple thermal evaporation of a mixture of ZnO and Ga₂O₃ powders under controlled conditions. Its morphology and microstructures were determined by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. The observations reveal that the as-synthesized ZnGa₂O₄ nanowires are single-crystalline with cubic spinel structure, usually several tens of microns in length. The typical diameters of ZnGa₂O₄ nanowires range from100 to200 nm. The room-temperature photoluminescence spectrum of ZnGa₂O₄ nanowires features four luminescence peaks around 400, 503, and 686 nm, which are attributed to self-activation center of the octahedral Ga-O, electronic transitions of localized Ga3+ ion in the octahedral Ga-O, single oxygen vacancies (V_O^*), respectively. A self-catalytic vapor-liquid-solid (VLS) formation mechanism is also proposed to interpret the growth of ZnGa₂O₄ nanowires in our work.Mass production of transparent semiconducting ternary oxide Zn₂SnO₄ nanowires is successfully synthesized by the thermal evaporation method without any catalyst. The as-synthesized products are characterized with field-emission scanning electron microscope (FE-SEM), X-ray powder diffraction (XRD), energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SEAD). A formation of Zn₂SnO₄ nanowires based on a self-catalytic VLS growth mechanism is discussed. The photoluminescence spectrum (PL) of the nanowires shows a broad blue-green emission around the 300–600 nm wavelengths with a maximum center at 580 nm under room temperature.5. Synthesis and Characterization of Al₂O₃/SiO₂ and ZnS/SiO₂ coaxial nanowire heterostructuresAl₂O₃/SiO₂ coaxial nanowire heterostructures and the ZnS/SiO₂ coaxial core/shell nanocables were synthesized by a simple thermal evaporation method. The growth process of the Al₂O₃/SiO₂ coaxial nanowire heterostructures is based on vapor-solid (VS) growth mechanism. The diameter of the nanowires is well-proportioned and generally about 100-150nm. Studies indicate that typical nanostructure consists of single twinning-crystalline Al₂O₃ nanowires (core) with diameter of about 50 nm and amorphous SiO₂ shell. Three emission peaks, namely 364 nm, 398 nm and 442 nm, were observed in the room-temperature photoluminescence measurements. The ZnS core has a cubic sphalerite structure with the coexistence of periodically alternating twins along the [111] growth direction and stacking fault. The growth mechanism of the product follows a VS process. The diameter of the nanostructures was about 20-250nm. The room-temperature PL spectrum of our product show four emission band centering at about 548nm, 614nm, 649nm and 670nm, respectively, which may originate to the impure ZnS, existence of Si and oxide related defects.