Lattice Dynamics Of Quasi-1D Nanotube-structure Systems

Miniaturization of electronic devices is a notable character of current electronic science and technology, promoting development of the nano-technology. As typical quasi-one-dimensional nano-materials, nanotube-structure systems process the unique mechanical, thermal, electrical and optical properties, which would signify wide applications in nano-sized electronic deivces. During the past decades, nanotube-structure systems have attracted much attention in both experimental preparation and the theoretical studies. In this thesis, we have investigated the phonon dispersion relation and specific heat of born carbon nanotubes(BCNTs), carbon nanowires(CNWs) and the oxygenated single-walled carbon nanotubes(O-SWCNs), which are three typical nanotube-structure systems, based on the lattice dynamics theory. The results demonstrate the relationship between thermal properties and geometrical structures, which are in agreement with the experimental results. It mainly contains the following six chapters:In the first chapter, we give a brief introduction for the research background and the potential applications of nanotube-structure systems, as well as the problems which will be explored in this thesis.Some typical methods are introduced in chapter two for the calculation of the phonon spectrum of nanotubes.In chapter three, we have calculated the phonon spectrum of BCNTs, CNWs and O-SWCNs. The calculated results show there are also four acoustic modes at aroundΓpoint from phonon dispersion relation of BCNTs, which is similar with that of carbon nanotubes(CNTs). The number N_BCNT of distinct phonon branches of both armchair(n, n) and zigzag(n, 0) BCNTs can be expressed by N_BCNT=48+24(n-1), which is four times that of carbon nanotubes with similar tube type. However, there are only three acoustic modes in the CNWs and O-SWCNs, and the twisting mode has a non-zero frequency at the center of Brillouin zone. Moreover, some new vibrational modes are obtained in both CNWs and O-SWCNs.In chapter four, We have calculated the Raman modes of the BCNTs, CNWs and O-SWCNs. The redial breathing mode(RBM) and the lowest E_2g mode of a various BCNTs of different diameters are calculated, from which the scaling relations between the two Raman modes and the tube diameter are obtained. Some new Raman modes and obvious Raman shift are observed in CNWs. Also, a new competition mechanism is proposed for explaining the softening of RBM and stiffening of high-frequency tangential stretch modes(G modes). In addition, a distinct Raman shift is found in both RBM and G modes of CNWs, depending not only on the tube diameter and chirality but also on oxygen coverage and adsorption configurations. In chapter five, the specific heat of the BCNTs, CNWs and O-SWCNs are further studied in detail, comparing to that of pure carbon nanotubes. The specific heat(Cv) of BCNTs with various diameters have been systematically studied, and the intrinsic relationship between thermal properties of the nanotube and its geometrical structures has been obtained. From the unique behavior of low-temperature Cv, moreover, a universal quantization of thermal conductance is predicted, which may be observed in both BCNTs and CNWs. In addtion, the CNWs and O-SWCNs have been found to have larger Cv than that of pure SWCNs. The larger Cv indicates much large thermal conductivity in CNWs and O-SWCNs.In chapter six, a conclusion is simply given and a future investigation is discussed...