Studying Phase Transition in Nanocarbon Structures

摘要

We investigate phase transitions in C_(60) and present a novel theoretical approach for the description of its fragmentation and formation. This theoretical approach consists of a statistical mechanics model combined with a topologically-constrained forcefield which was developed to describe the formation and fragmentation of C_(60) within a specific C_(60) ? 30C_2 channel. Based on this forcefield, we conduct molecular dynamics simulations where we demonstrate that at the phase transition temperature, both the cage and gaseous phases were found to coexist and the system continuously oscillates between the two phases, i.e. the fullerene repeats its fragmentation and reassembly within a single molecular dynamics trajectory. Combining the results of the molecular dynamics simulations and the statistical mechanics approach, we obtain a phase transition temperature of 3800-4200 K at pressures of 10-100 kPa, in good correspondence with carbon-arc discharge experiments. Furthermore, we also conduct molecular dynamics simulations using the Tersoff potential to investigate the effect of lifting the C_(60) ? 30C_2 constraint on the phase transition of C_(60). Finally, we investigate phase transitions for the following systems consisting of 240 carbon atoms: fullerene, buckybowl, nanocarbon, graphene and carbon onion. We demonstrate that the C_(240) fullerene is the most stable of the 5 phases, while the uncapped (10,10) nanotube is the least stable. We also show that the carbon onion, nanotube and buckybowl all transform into a fullerene-like structure before total decomposition. In particular, the C_(60) of the C_(60)@C_(180) carbon onion fully fragments and its 60 atoms are incorporated into the C_(180) shell to form a C_(240) fullerene, while both the nanotube and buckybowl evaporate a few C atoms before forming a cage-like structure.