Growth model for carbon nanotubes

Evidence is presented, through high-resolution electron microscopy images, for the open-end growth of carbon nanotubes. Terminations of incomplete layers of carbon, seen on the tube surfaces and cone tips, suggest that the extension and thickening of the tubes occur by the island growth of graphite basal planes on existing tube surfaces. The nucleation of positive (pentagons) and negative (heptagons) disclinations on open tube ends results in changes of growth directions, producing different morphologies. A novel structure that shows complete ``turn around growth,'' involving pentagon-heptagon pairs, of the tubular assemblies is presented.