Molecular Dynamics Simulation of Nucleation and Growth of Single-Walled Carbon Nanotubes

Developments of large scale and high-purity generation technique of single-walled carbon nanotubes (SWNTs) [1] are desired for the practical applications of this fascinating new material. In addition to previously known laser-furnace [2] and arc-discharge [3] techniques, catalytic chemical vapor deposition (CCVD) methods [4] have been developed for the possible larger amount production with lower cost. In addition to the supported catalyst method, so-called HiPco process [5] can be regarded as an unsupported catalytic CVD process. In addition, we have developed a high-quality production technique of SWNTs at low-temperature by using alcohol as carbon source [6]. For the further optimization of the generation process, understanding of the formation mechanism is inevitable. Furthermore, the formation mechanism is really critical for the future development of synthesis techniques of SWNTs with controlled diameter and even chirality. Here, the nucleation and growth process of SWNTs were studied with classical molecular dynamics simulations. The Brenner potential [7] in its simplified form [8,9] was used for the carbon-carbon interaction. The metal-carbon potential was constructed with the covalent term based on the coordination number of metal atom and the Coulomb term due to the charge transfer from metal to carbon cluster [10]. The formation mechanism in the laser-furnace method and CCVD method should be considerably different at least at the nucleation stage of SWNTs. In the former method, carbon atoms and catalytic metal atoms are vaporized at the same time by the intense laser ablation. In the later, the catalytic metal clusters are present before the assembly of carbon atoms. Hence, 2 different simulation conditions, 'co-vaporization' and 'CCVD' are compared below. Generation conditions of SWNTs by the laser-furnace or by the arc discharge techniques are almost the same as for fullerene and endohedral metallofullerene except for the doped metal species. Hence, the same molecular dynamics simulation condition as for our previous empty fullerene [8,9] and for endohedral metallofullerene [10] except for the catalytic Ni atoms was employed. As the initial condition, the completely random vapor mixture of 2500 carbon and 25 Ni atoms were allocated in a 58.5 nm cubic fully-periodic simulation cell [11,12]. After 6 ns molecular dynamics calculation, many