Investigation on the Fabrication of Nanostructures and Their Characterization

Abstract

Along with the development of the nanoscience and nanotechnology, various nanostructures have been synthesized because of their enormous potentials for improving the performance of devices. Nanostructured materials, fabricated by bottom-up approach, have homogeneous chemical composition, few defects, and single crystalline nature. This thesis is devoted to the investigation on growth and characterization of nanostructures which include 0-D magnetite nanoparticles, 1-D GaN, ZnO, silicon oxide nanowires, and 2-D GaN and FeOCl nanosheets. Firstly, the growth of Cr- and Mn- doped GaN nanowires are discussed. Cr-doped GaN nanowires has been proposed to be ferromagnetic, resulting from a band hybridization between the N 2p and Cr 3d states and is to be more robust than Mn-doped GaN nanowires. We demonstrated the successful fabrication of single-crystalline GaN nanowires doped with Cr, which shows ferromagnetism at room temperature. The structural study shows that these nanowires have diameters of several tens nm to a few hundred nm and are of single crystalline and the doping concentration of Cr is about 2.06 at %. The ferromagnetic and electrical characteristics of in situ Mn-doped GaN nanowires fabricated in the absence of any catalyst are reported. The nanowires are of single-crystal hexagonal structure, containing Mn up to 2.5 atomic %. Magnetism measurements indicate that the nanowires have room temperature ferromagnetism with Curie temperature above 350 K. Magnetic force microscopy verifies that the ferromagnetism of the individual nanowire is uniform along the nanowire. An electrical transport measurement reveals that the nanowire has a weak gating effect and is of the n-type. Secondly, the growth of ZnO and silicon oxide nanowires is discussed. Zinc oxide bicrystal nanobelts were fabricated via a vapor phase transport of a powder mixture of Zn, BiI3, and MnCl2&#8226

H2O at temperatures as low as 300 °C. The bicrystal nanobelts have the widths of 40&#8211

150 nm and lengths of tens of microns. The energy dispersive x-ray spectroscopy result verifies that the bicrystal nanobelts contain higher concentration of both Bi and Mn along the grain boundary. The investigation of the growth mechanism proposes that Mn-Bi alloy may induce the formation of bicrystal nanobelts. Photoluminescence spectra show that the ultraviolet emission of the bicrystal nanobelts has a blueshift of 18 meV as compared to Bi&#8211

ZnO nanowires at 10 K. The bicrystal nanobelts also exhibit ferromagnetism at room temperature. Silicon oxide nanowires which contains Au nanoparticles or an Au nanowire were fabricated by thermal evaporation chemical vapor deposition method using Au as catalyst. Silicon oxide wafers were used as the collector. The diameters of silicon oxide nanowires range from 20 to 150 nm. The larger the diameter of Si nanowire is, the larger the diameter of embedded Au nanoparticles. The separation between Au nanoparticles increases with the diameter. Different forms of silicon oxide nanowires were observed at different growth temperature: silicon oxide nanowires embedded with Au-containing nanoparticles at 1250 °C and Au/silicon oxide coaxial nanocables at 1425 °C. By using KCN solution, the nanoparticles or the nanocables inside silicon oxide nanowires were extracted, thus hollow silicon oxide nanotubes was created. Thirdly, a facile growth of GaN and FeOCl hierarchical nanostructures by conventional chemical vapor deposition (CVD) method are discussed. Morphology of the GaN and FeOCl nanostructures are classified to (i) bulk structures, (ii) porous nanosheets, and (iii) nanosheets and microspheres, which were synthesized at high, mid, and low substrate temperature zones, respectively. Positions of the substrates decide the substrate temperatures. The high, mid, and low substrate temperature correspond to 800 ℃, 700 ℃, and 600 ℃ in GaN growth and 600 ℃, 500 ℃, and 400 ℃ in FeOCl growth, respectively. Even though real growth temperatures of both materials are different, morphologies of each nanostructures is quite similar. Through an investigation of growth mechanism, vapor-solid (VS) mechanism may be a key to explain the similarity in morphology of GaN and FeOCl nanostructures. Finally, synthesis of magnetite nanoparticles by modified Massart’s method with poly (vinyl alcohol) (PVA) solution is discussed. Although the average sizes of the nanoparticles were different with and without PVA solution in fabrication, the crystal structures between them were same magnetite structures. However, physical properties measurement system (PPMS) results revealed the size difference from the existence of blocking temperature and the relative difference of the saturation magnetization. The compaction of the nanoparticles was conducted by magnetic pulsed compaction (MPC) method. The difference of the PPMS results between the nanoparticles with PVA solution and those compact is scarce. MPC is considerable method for the compaction of the magnetite nanoparticles because of maintaining of superparamagnetism before and after compaction.