The Theoretical Study On The Stability And Related Properties Of Silicon And Group Ⅲ Nitride Naotubes

Carbon nanotube can be viewed as a seamless, hollow tubular structure rolled from infinite graphite. It can be regarded as a one dimensional material with fascinating properties and wide potential applications. Latest experimental and theoretical studies demonstrate that it is quite possible that the nanotube structures can be prepared from other layered materials. For example, h-BN, MX₂ (M=Ti, Nb, Mo; X= S, Se) nanoparticles with layered structures can form closed-caged structures with polyhedral nanotubular shapes. Under the influences of the success in the other inorganic nanotubes, the structural stability and electronic properties of the silicon and group-Ill nitride nanotubes were investigated by the conventional carbon nanotubes morphologies. And some significant conclusions have already been obtained from these researches.To date, no experimental research about the structure of single-walled silicon nanotubes (SWSiNTs) has been reported. In addition, there exists much conflict among the data from different research groups. However, the relationship between the electronic and physical properties and their structures was verified. The situation for group-Ill nitride nanotubes is the same. A1N and GaN nanotubes are based on sp³ hybridization; In contrast, BN nanotubes are based on sp² hybridization in accord with the conventional carbon nanotubes. As mentioned above, three kinds of possible SWSiNTs are simulated with the strict density functional theory B3LYP method at 6-31G (d) basis set level. Furthermore, their electronic spectra are calculated by INDO/S method using localized density matrix (LDM) method. Then the apparent distinction of the morphology among carbon, BN, A1N and GaN nanotubes are explained theoretically. And bending benzene analogues are very useful models in exploring the stability of the nanotube morphology based on sp² hybridization for the compounds with Ï€-conjugated six-membered ring structures. Our work will focus on the following three aspects in this thesis:1. This study predicts that the possible structures of stable tube-shaped silicon nanotubes. The polygonal nanotubes are the most stable tubular morphologies due to their sp³ hybridization. Compared with zigzag nanotubes, armchair nanotubes are the more reasonable structure due to the efficient overlap of p_z orbitals and delocalization of Ï€ bonds. The Ï€_σ-Ï€_σ orbital interaction between the parallel rings stabilizes the polygonal nanotubes and makes their ⊿Ehomo-lumo decrease compared with the σ-σ* energy gap. With the increasing of chiral angle 6, the efficient orbital's overlap and electron's delocalization of chiral structure (n,m) will decrease. The narrow ⊿Ehomo-lumo indicates that the small diameter Si nanotubes are metallic.2. The absorption spectra of these three kinds of SWSiNTs are calculated with LDM method using INDO/S semiempirical parameters. The following conclusions are deduced from the calculated spectra. The overall spectra red shift and the relative amplitudes increase...