Nanotube-substrate interactions: distinguishing carbon nanotubes by the helical angle.

We investigate the interaction of a carbon nanotube with a graphite substrate, using an interlayer potential that explicitly treats the registry dependence of the interaction. The carbon-carbon bond lengths in nanotubes differ slightly from those in flat graphite, so that the naively commensurate angular orientations for the tube with respect to the substrate lattice are destroyed. The interaction of a one-dimensional tube with a two-dimensional substrate then leads to an unusual registry phenomenon not visible in standard layer-on-layer growth: the system develops favorable orientations which clearly are incommensurate.