Ⅱ-Ⅵ Group Semiconductor Nanomaterials:Research On Synthesis, Microstructure And Property

The growth of novel and high quality semiconductor nanomaterials in a controllable manner is foundationally important in nanotechnology and has been investigated widely. As building blocks for nanodevices, the semiconductor nanomaterials have to be specific in terms of optical, electronic and chemical properties. In view of that, researchers turned their attention to the control synthesis of nanomaterials. The goal is to tailor-make nanomaterials of specific properties (architecture, size, morphology, and growth pattern of products) for the fabrication of nanodevices. In the world of semiconductor nanomaterials, core-shell nanostructures, nanoporous materials, and one-dimensional nanomaterials are popular. Making use of the facilities in our laboratory, I systemically investigated the controlled growth of nanomaterials prepared by hydrothermal/solvothermal and thermal evaporation methods. The growth mechanisms as well as the optical and electronic properties of the as-prepared materials were investigated. The main achievements are as follows:Part I:Core-shell Nanostructures(1) Magnetic Fe/ZnO composites showing the shape of "sea urchin" were hydrothermally synthesized. The synthesis was conducted at low temperature (80℃), and there was no need of using a surfactant. The Fe nanoparticles were encapsulated inside a shell of self-assembled ZnO nanospikes. Because of the encapsulation of the Fe nanoparticles, the Fe/ZnO composites were stable in air and high in magnetization. With the incorporation of Fe3+ions inside the ZnO lattice, the UV emission band of ZnO disappeared, and the defect-related peak became strong in intensity. The results reveal that the PL properties of ZnO can be tuned by doping ZnO with Fe3+ions.(2) Novel core-shell nanostructures comprised of cubic sphalerite and hexagonal wurtzite ZnS were synthesized by a simple hydrothermal method. The metastable wurtzite ZnS was encapsulated by a shell of sphalerite ZnS and the wurtzite ZnS core was protected from the outside environment. A possible mechanism for the fabrication of the core-shell nanostructures was presented. By simple control of Zn:S ratio, one can tune the optical properties of ZnS nanomaterials such as excitonic absorption and PL emission.Part II:Nanoporous Materials(3) Using Cd(NO3)2·4H2O as precursor and ethanol/water as solvent, I synthesized Cd compounds with various kinds of shape and size. Unlike the conventional oil-water surfactant approach, the adopted method is biologically safe, simple and environment-benign. The approach is not expensive and requires no special equipment or organic surfactants. By varying experimental parameters, one can achieve selective growth of nanocrystals or microcrystals of Cd compounds, such as single-crystalline CdO, Cd(OH)2nanowires, Cd5(OH)8(NO3)2(H2O)2nanowires, nanobelts, microflowers, microblock and micromat. Through the calcination of Cds(OH)8(NO3)2(H2O)2in air, nanoporous CdO materials can be produced. According to the photocatalytic activity results, the prepared CdO samples are effective for the photocatalytic degradation of methylene blue. It is envisaged that the method is also suitable for the synthesis of nanostructures of other oxides such as MgO, CuO.(4) By means of the above described solvothermal approach that is biologically safe and environment-friendly, Zn5(OH)8(NO3)2(H2O)2and Ni3(NO3)2(OH)4microflowers were synthesized. The precursor was nickel nitrate (Ni(NO3)2·6H2O) or zinc nitrate (Zn(NO3)2·6H2O) and absolute ethanol was used as solvent. Through the calcination of the Ni3(NO3)2(OH)4and Zn5(OH)8(NO3)2(H20)2compounds at an appropriate temperature, nanoporous NiO and ZnO microcrystals were produced. It is meaningful to point out that except for preparing good precursors for the generation of NiO and ZnO nanomaterials, ZnO micromaterials could also be synthesized using this kind of environment-friendly solvothermal approach. The work would start a new chapter for the preparation and assembly of new single-crystalline materials with a reasonable and environment-friendly method.Part III:One-dimensional Nanomaterials(5) By means of thermal evaporation, I produced morphology-tunable nanostructures of single-crystalline CdS in a controlled manner. They were in the form of cuspidated double headed nanocomb, corrugated nanoribbons, nanobelts, nanorods and nanowires. The approach is low-cost and the generation of product highly reproducible. A mechanism for the growth of the CdS nanomaterias was described. The unique optical properties of the as-prepared CdS nanomaterials suggest that they could be utilized in the industry of optoelectronic nanodevices.(6) Being successful in the controlled growth of CdS nanostructures in terms of morphology and size, it was desirable to expand the applications of CdS; much effort was devoted to explore the band-gap tunability of CdS-based nanomaterials. I synthesized high-quality CdxZn1-xS nanocrystals with tunable morphologies and superior optical properties. Compared to the method of two-step thermal evaporation, the adopted one-step thermal evaporation method was simple, low-cost and shorter in time. The as-prepared samples were in the forms of nanoswords, super-long nanowires, cubic nanopillars, heterogeneous nanobelts, branched nanorods, nanocombs, and hollow microspheres. It is worth pointing out that to the best of our knowledge, the fabrication of these kinds of ZnxCd1-xS hollow microspheres by means of one-step thermal evaporation has never been reported before. The work would start a new way for the growth and application of hollow microspheres on Si substrates. The unique properties of the obtained ZnxCd1-xS nanomaterials suggest that they could be utilized in the industry of optoelectronic nanodevices. It is worth pointing out that the low-cost and environment-benign approach adopted in the present study could be applied to synthesize nanostructures of other compounds such as ternary MgxZn1-xS, CuxCd1-xS and quaternary CuCdMgS semiconductors.