||In this thesis, the fabrication and characterization of one-dimensional nanostructures have been studied systematically to understand the growth mechanism and structure transformation of one-dimensional nanostructures. The growth behavior of the ultrathin ZnSe nanowires with diameter less than 60 nm was found to be different from classical vapor-liquid-solid (VLS) process. The growth rate increases when the diameter of nanowires decreases, in contrast to the classical VLS process in which the growth rate increases with the diameter. The nucleation, initial growth, growth rates, defects, interface structures and growth direction of the nanowires were investigated by high resolution transmission electron microscopy (HRTEM). We found the structure and growth direction of ultra-thin nanowires are highly sensitive to growth temperatures and diameters of nanowires. At a low growth temperature (380°C), the growth direction for most nanowires is along <111>. Planar defects were found throughout the nanowires. At a high growth temperature (530°C), uniform nanowires with diameters around 10nm were grown along <110> and <112> directions, and the nanowires with diameters larger than 20nm were mainly grown along <111> direction. The possible growth mechanism of ultrathin nanowires was proposed by combining the solid catalytic growth with the interface diffusion theory, in order to explain how the growth temperature and the size of the catalysts influent the morphology, growth direction and growth rate of ultrathin nanowires. Structural and phase transformation of a nickel coated Si nanowire to NiSi2/SiC core-shell nanowire heterostructures has been investigated by the in-situ Transmission Electron Microscope (TEM). The phase transformation is a single-site nucleation process and therefore a single crystalline NiSi2 core resulted in the core-shell nanowire heterostructures. The transformation of the Si nanowire to NiSi2/SiC core-shell nanowire heterostructures was extremely fast and completed instantly due to high temperature annealing. Furthermore, the phase transformation preferred to begin at the defect and bending region of the nanowires, as the nickel can easily diffuse through the native oxide on the Si nanowires rather than the other regions. By removing the native oxide on the Si nanowires using HF, the temperature required for the phase transformation was decreased significantly. However, without the native oxide, the phase transformation became a multi-site nucleation process, and the nanowire became a polycrystalline and multiphase nickel silicide nanowires after the reaction. A simple and effective method is developed for fabricating high-quality vertically aligned ZnO nanowire arrays using carbonized photoresists. ZnO nanowires fabricated by this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. We further fabricated vertically aligned ZnO/a-Si core-shell heterojunction nanowire arrays through direct chemical vapor deposition (CVD) of amorphous silicon on ZnO nanowire surfaces. The thickness of the a-Si shells linearly increases with deposition time and the deposition rate was about 5nm/min at 530 °C. Since the Si shell is p-type and the ZnO core is intrinsic n-type semiconductors, the ZnO/ amorphous silicon core-shell nanowires naturally formed hetero p-n junctions. The antireflection property of the ZnO/amorphous silicon core-shell nanowires is dramatically enhanced due to the rough interface between the ZnO and amorphous silicon. Additionally, the intensity of the Photoluminescence spectrum of ZnO/amorphous silicon core-shell structures is decreasing with the thickness of the amorphous silicon shell increases.