One-dimensional Nanomaterials: Plasma And Chemical Vapor-Phase Synthesis And Characterizations

When the term Nanotechnology was coined in the 1980s, it heralded the beginning of a new research tide which is yet to reach its peak. Because of its revolutionary impact on the research areas including materials, communication, manufacturing, biology, medicine, and information storage and processing, nanotechnology provides an opportunity to substantially promote the core competitiveness of a nation. Thus the synthesis and processing of nanomaterials, which holds a pivotal status in nano research, is receiving increasing attention. Currently researches on nanomaterials synthesis emphasize on precise control of composition, structure, and morphology. The anisotropic nature of crystals results in different physical and chemical properties in various crystallographic directions. On the scale of nanometers, the morphology of materials will substantially influence their properties. This dissertation primarily aims at vapor phase synthesis of nanomaterials with emphasis on the control of morphology, structure and chemical composition and the characterization of the physical properties of the obtained materials. Especially, the introduction of plasma into the reaction system enables more freedom to achieve precise control. The research work presented can be categorized into three parts:(1) Synthesis and field emission properties of hexagonal AlN nanorod and nanoneedle arrays. Hexagonal AlN nanorod and nanoneedle arrays were synthesized by a chemical vapor deposition route with AlCl3 powder and NH3 gas served as the aluminum and nitrogen sources, respectively. Different morphologies induced by different AlCl3-NH3 ratios were investigated, which is attributed to the change of surface diffusion properties in Al-rich and NH3-rich conditions. The field emission measurements of the quasi-aligned AlN nanorods and nanoneedles on Si wafers were performed using a parallelplate configuration in a vacuum chamber. It was found that the apparent turn-on electric field (Eto, defined as the applied field for 10μA/cm2) of nanoneedles was 3.1 V/μm, while the value of nanorods was 15.3 V/μm. It was clear to conclude that smaller tips have smaller turn-on fields, indicating the significant improvement of field emission properties due to the large curvature geometry of the nanoneedles.(2) Morphology control of ZnO nanostructures by a chemical vapor transport deposition route. When zinc powders were heated in a tube furnace, variation in concentrations of gaseous reactants results in a supersaturation gradient. Various ZnO morphologies were selectively obtained under different levels of supersaturation and their structures were characterized in detail. Possible mechanisms were proposed to explain how the supersaturation degree influences the morphology. The experience and ideas developed in the synthesis of ZnO are also found to apply to the synthesis of CdS nanostructures very well. In this part of work, morphology is mainly controlled by supersaturation.(3) Al-O-N nanowires and nanobelts synthesized by microwave plasma enhanced chemical vapor deposition. Al2O3 is attractive as a versatile functional ceramic material and the incorporation of nitrogen element will further improve its mechanical properties. There are only few literatures on the synthesis of Al2O3 one-dimensional nanostructures and no report on the synthesis of A-O-N ternary nanowires is available. We synthesized A-O-N ternary nanowires and nanobelts by evaporation of Al powder in Ar-H2-N2 atmosphere with the assistance of microwave plasma. The plasma is critical for nitrogen incorporation into the nanowires. Without plasma, only Al2O3 nanowires can be obtained. This part of work demonstrates the effectiveness and significance of plasma in determining the chemical composition of nanostructures.