Growth mechanisms of carbon nanotubes using controlled production in ultrahigh vacuum

We present a method for the preparation of single walled carbon nanotubes (SWNTs) on a highly oriented pyrolytic graphite (HOPG) surface in ultrahigh vacuum (UHV), for which the preparation parameters for the production of metal clusters, fixed to nanometer sized pits on the surface, and the subsequent deposition of carbon can be controlled separately. Using cobalt as the cluster metal we carried out a comprehensive study concerning the influence of the substrate temperature (up to 900 degreesC) and the effective film thickness for the carbon evaporation. With scanning tunneling microscopy in UHV at room temperature and at T=77 K we observed single, separated SWNTs of about 50 nm length, which frequently were angled or branched and included junctions between sections of different tube diameters. With a statistical evaluation of tube diameters, tube lengths, and cluster heights, we obtained new insights into the growth mechanisms. An increase of tube diameters with increasing substrate temperature and a strong catalytic activity of cobalt clusters with sizes below 4 nm is in agreement with experimental results for the gas phase growth and recent calculations for several growth mechanisms. At T=77 K the atomic structures of the SWNT were imaged together with atomic resolution on the HOPG substrate. The presence of branched SWNTs and the observed alignment of the lattice structure of the SWNT and the HOPG both indicate that the tube growth in our case probably takes place at the moving end of the SWNT and not at the fixed clusters, different from recent experiments using chemical vapor deposition for nanotube growth on substrates. (C) 2002 American Institute of Physics.