Selective alignment of carbon nanotubes on sapphire surfaces: bond formation between nanotubes and substrates.

We present our first-principles total-energy calculations performed for carbon nanotubes (CNTs) on sapphire substrates. We find that the formation of covalent and partly ionic bonds between Al and C atoms on the Al-rich surfaces causes the selective alignment of CNTs, this being the principal reason for the CNT growth along particular crystallographic directions. We also find that the van der Waals interaction which is important on the stoichiometric surfaces produces no directional preference. The characteristic features in the electron states of the CNT on the substrate are clarified.