The Structure And Properties Of Nanotubes And Hydrogenated Nanotubes: A First-principle Study

As one of the most promising nanomaterials, nanotubes have attracted much attention for researches because of their special structural, fantastic mechanical, electrical and thermal properties. In this thesis, we investigate the structural, mechanical, electrical and thermal properties of the nanotubes and hydrogenated nanotubes by SIESTA software based on the first-principles. The calculation includes two parts:in the first part, we study the elastic and electrical properties of stable hydrogenated zigzag carbon nanotubes configurations with the hydrogen atoms adsorbed on the outerwall of the nanotube; in the second part, we calculate the armchair and zigzag boron nanotubes. The stability is the focus of the research compared with the carbon nanotube. We perform the molecular dynamics simulations and predict the thermal stability and analyze the thermal expansion property of the boron nanotubes.For the research of the hydrogenated carbon nanotubes, firstly, we analyze three typical adsorbed patterns and study their relativistic stabilities. It was found that the double-dimer pattern is the most stable structure concluded from the energy calculations. The hydrogenated tubes (7,0) to (11,0) are calculated considering the all configurations in the form of two double-dimer pattern. We found that there are two types of transitions at X=n-8/n×100% coverage, where n is the carbon number of the CNTs. Type A only occur on (7,0) and (8,0) CNTs, while type B exist on (8,0), (9,0), (10,0) and (11,0) CNTs. These two types of transitions configurations are more stable than the other configurations at the same coverage. The enhancement of sp3 hybridization was found for all C-C bonds in the type-B case, whereas only enhancement of sp3 hybridization for C-C bonds with hydrogen adsorption in the type-A case only in the axial direction was found. As type A occurs, the energy structure of the hydrogenated carbon nanotube is changed from semiconductive behavior to metallic behavior. While, type B is a semiconductive transition, which changes from metallic to semiconductive behavior or the band gap is enlarged. Moreover, the band gap of the type B configurations is larger than that of the configuration at 100% coverage. The variety of charge density mainly focuses on the C-C bond without hydrogen adsorption, and has a close relationship with the C-C bond length. The reduction of the C-C bond length will result in an increase of the charge density. Otherwise, the charge density decreases.The Young’s moduli of hydrogenated carbon nanotubes decrease gradually with increasing of the adsorption coverage. The Young’s modulus value reduces to half and one third of pristine carbon nanotube at 50% and 100% coverages, respectively. The configurations with type A have larger Young’s modulus than other ones at the same adsorption level. It means that the type A transition may improve the Young’s modulus of adsorption configurations. Generally speaking, the Young’s modulus of the configuration with type B changes slightly compared with others at same adsorption level. For example, the Young’s modulus of the type A and type B configurations on (8,0) carbon nanotube is 400.63 and 358.53GPa, while that of the other configuration at the same adsorption level is about 370 GPa.We investigate the boron nanotubes which are rolled up from the most stable boron planar sheet at present, a-sheet. We found that the strain energy decreases as the diameter increases. Under the axial stretching, the strain energy increases for increasing stretching along the Z axis. The armchair boron nanotubes are more stable than zigzag ones. The Young’s moduli of boron nanotubes with the diameter from 0.65 to 1.96 nm lie between 0.52 to 0.63TPa calculated as the shell thickness is adopted as 0.244 nm. The values of zigzag ones are 15% larger than that of armchair ones. The Young’s moduli values of the two types boron nanotubes increase firstly and decrease afterward with the increase of the diameter. The optimum Young’s moduli of the two types lie in the diameter range from 1.14 to 1.40 nm. The Poisson ratios are from 0.16 to 0.20.We perform molecular dynamics simulations to estimate thermal stability of boron nanotube. The boron nanotubes are stable below 1000K and the small diameter one has high thermal stability. The thermal expansion coefficient of the two types boron nanotubes are negative at low temperature and positive at moderate temperature. The trend is in agreement with carbon nanotubes. Take boron nanotube (4,0) for example, the thermal expansion coefficient reaches a minimum value of-2.09×10-5K-1 at about 200K, then at 440K, the thermal expansion coefficient becomes zero and the length of nanotube is minimum. As the temperature reaches 770 K, the length of the nanotube returns its initial length at 0 K, and then the nanotube expands with the increase of the temperature. The transverse vibration mode of the boron atoms in the center of boron hexagon is obvious at low temperature, which contributes to the thermal expansion coefficient of boron nanotubes. The small diameter boron nanotube has excellent negative expansion behavior and have highly potential applications prospect.