Temperature dependence of iron-catalyzed continued single-walled carbon nanotube growth rates: Density functional tight-binding molecular dynamics simulations

The temperature dependence of continued single-walled carbon nanotube (SWNT) growth on an iron cluster is investigated using quantum chemical molecular dynamics simulations based on the density functional tightbinding method. As a model system for continued SWNT growth, a (5,5) armchair-type SWNT seed attached to an iron Fe38 cluster was used. Continuous and rapid supply of C atoms was provided in the vicinity of the nanotube-metal interface area. The simulations were performed at temperatures of 1000, 1500, and 2000 K. The simulations reveal fastest growth at 1500 K, although the differences are moderate. In the observed growth process, formation of polyyne chains at the rim of the nanotube-metal interface efficiently initiates pentagon/hexagon/heptagon ring formations in the carbon sidewall, leading to "lift-off" of the nanotube from the metal cluster. At 1000 K, the SWNT lift-off is suppressed despite the fact that the total number of created rings in the nanotube is comparable to that at 1500 K. In addition, relatively long polyyne chains tend to form extensions from the carbon sidewall to the metal cluster at 1000 K, whereas at 2000 K, deformation of the nanotube becomes more pronounced and diameter narrowing sets in, and polyyne chains at the rim of the nanotube easily dissociate at this high temperature. These physical and chemical events at 1000 and 2000 K can be considered inhibiting factors preventing efficient growth of the nanotube.