Synthesis, characterization, and modeling of nanowires and nanosprings: One-dimensional quantum structures.

摘要

The focus of this work is an investigation of growth, characterization and modeling of nanowire systems. Boron carbide and silicon carbide nanowires have been successfully synthesized using plasma enhanced chemical vapor deposition (PECVD) technique. The growth conditions for nanowires have also been optimized.; Morphological structures of boron carbide and silicon carbide nanowires indicate that both have high aspect ratios (length vs. diameter). Diameters of the nanowires are in a range from 10 to 100 nm. Lengths of the nanowires are larger than 1 μm, and up to a hundred microns. Crystal structure information obtained from select area diffraction (SAD) shows that the body of a boron carbide nanowire is the rhombohedral crystal phase of B₄C, while the tip of the nanowire is composed of iron and boron. For silicon carbide nanowires, the crystal structures are in combinations of 4H-SiC and 6H-SiC or 3C-SiC. The tip of a silicon carbide nanowire is composed of nickel and silicon. The existence of tips where the composition is other than that in nanowire bodies is a clear indication that the nanowires grow via the vapor-liquid-solid (VLS) mechanism.; Electronic and vibrational properties of boron carbide and silicon carbide nanowires have been determined via near edge X-ray absorption fine structure spectroscopy (NEXAFS) and Raman spectroscopies. The electronic structure of boron carbide nanowires, as determined with NEXAFS, is very similar to that of bulk boron carbide.; Concomitant with the NEXAFS measurements and SAD pattern, the Raman spectra of boron carbide nanowires are equivalent to that of single-crystal and polycrystalline boron carbide. In addition, broadening and upward shifting of Raman modes are a manifestation of finite size effects and strain induced effects of nanowires. The Raman spectrum of silicon carbide nanowires is consistent with either 3C-SiC phase or combinations of modes of the 4H-SiC and 6H-SiC phase, which is in good agreement with SAD patterns. Broadening and downward shifting of Raman modes in silicon carbide nanowires also demonstrate the finite size effects of the nanowire geometry.; Selective area deposition demonstrates that the distribution of the nanowires on the substrate surface is pre-determined by arranging the catalyst seeds location on the substrate surface. Meanwhile, the diameters of the nanowires are determined by the sizes of metallic catalyst seeds based upon VLS growth modes. This suggests that more complicated patterns and fine nanowire distribution can be achieved using such a method.; A novel nanospring structure has been discovered in this work. Boron carbide and silicon carbide nanosprings have been synthesized using PECVD technique. The structures of the boron carbide and silicon carbide nanosprings have been determined to be amorphous, rather than crystalline. In order to understand the amorphous nanospring growth mechanism, a nanospring growth model has been established, which is based on VLS growth mode.; In this model, the orientation of the catalyst relative to nanowire and contact angle anisotropy play key roles in driving nanospring growth. This growth model successfully describes nanospring growth and can be universally used to describe the growth of all types of nano- and micro-sized amorphous springs, regardless of their composition. In order to determine the nanospring mechanical properties, I proposed a measurement method and established a theoretical model to find out the impacts of spring geometry and material shear modulus on their mechanical properties.