Catalytically assisted tip growth mechanism for single-wall carbon nanotubes.

The catalytic growth of single-wall carbon nanotubes is investigated at the nanotube tip using first-principles molecular dynamics and tight-binding Monte Carlo simulations. At experimental temperatures (approximately 1500 K), the catalytic atom is found to incorporate into the carbon network instead of scooting around the open edge. Consequently, the open end of SWNTs closes spontaneously into a graphitic dome, suggesting a closed-end mechanism for the catalytic growth. At 1500 K, the cobalt-carbon chemical bonds keep breaking and re-forming, providing a direct incorporation process for additional carbon, necessary for growth. The catalytic action of Co atoms is also found to play a key role in the reconstruction of the nanotube tip after carbon incorporation, by annealing defects. The present closed-end tip growth mechanism may coexist with the usual root growth mechanism.