Theoretical Studies On The Functional Properties Of Carbon Nanotubes Modulated By Silicon Doping

Carbon nanotubes (CNTs) have attracted great interest due to their novel structures and properties. In this thesis, the silicon doped CNTs were studied by using cluster and periodical models based on the quantum chemistry calculations. Much attention has been paid to the structure-property relationship of the hybrid systems made of Si-doped CNTs. The properties of the carbon-based nano-materials modulated by doping were investigated, and which would provide theoretical guide for their exploitations and applications. The main work of this thesis is summarized as follows:1. The armchair CNT (n, n) n=3-14and zigzag CNT(n,0) n=7-18were calculated by means of self-consistent field molecular orbital method combined with density functional theory to study the silicon doping effects of the tubes with various diameters and edges. The geometrical structures, relative stabilities, electronic properties, nonlinear optical properties and aromaticity of the doped tubes were computed systematically and compared with those of the pure tubes. It is found that the structural distortion is occurred to each tube. The Pi-orbital vector axis angles of the silicon atoms are always obviously larger than those of carbon. The thermodynamic stability of the tubes is decreased as silicon doping according to the results of cohesive energy and Gibbs free energy. The obtained defect formation energy shows that the formations of the Si-doping defect are all endoergic, and the defect formation energy is linear-scale with the tube curvature. The silicon doping would change the energy levels as well as the space distributions of the frontier molecular orbital of CNTs, expressly for the armchair CNTs. It can be ascribed to the obvious delocalization for armchair CNTs. This is different from those of zigzag CNTs, which exhibit localized state at the edges of the tube. Hyperpolarizability for doped tubes is dramatically enhanced as silicon doping according to the results of electric field method. Additionally, the doping effect on the aromaticity of the tube is considered by using nuclear independent chemical shift. The aromaticity/anti-aromaticity transition at selected positions is occurred for the tubes as the silicon present.2. The armchair CNT (n, n) n=3-10and zigzag CNT (n,0) n=6-15with periodical models were calculated by means of self-consistent field crystal orbital method combined with density functional theory. It is found that the Pi-orbital vector axis angles of the silicon atoms are always obviously larger than those of carbon. The thermodynamic stability of the tubes is decreased as silicon doping according to the results of cohesive energy and Gibbs free energy. The formations of the Si-doping defect are all endoergic. These results are accord with those obtained with cluster models. The calculated energy band structures show that a band gap is opened, and thus they become semiconducting state from metallic state for the armchair CNTs upon silicon doping. As for zigzag tubes, the properties of semiconductor or metal are still preserved as silicon present. The young’s modulus of the doped tubes is computed by a numerical method. The results show that the young’s modulus is reduced as silicon incorporation, indicating that silicon doping would decrease the mechanical property along the tube axis. Additionally, the charge carrier mobility of the zigzag tubes is also investigated based on the deformation potential and effective mass approaches.3. The zigzag tube (12,0) with different finite lengths was calculated by means of self-consistent field molecular orbital method combined with density functional theory. The doping density and position are considered to study the silicon doping effects on the structures, relative stabilities and electronic properties. It is found that Si atoms in the doped tubes exhibit obviously larger π-orbital axis vector angles than carbon atoms, and they also tend to "pop out" from the original position. The Si-doped nanotubes exhibit lower thermodynamic stability than those of the un-doped tubes from viewpoint of both cohesive energy and Gibbs free energy. The results of reaction energy suggest that the substitutionally doping the CNTs by Si atom is more favorable to occur near the edge site. Furthermore, the energy levels of the frontier orbital vary only a little when the silicon atom is introduced into the nanotubes. However, most hybrid nanotubes present larger energy gaps than those of the pristine ones.