The traditional picture of a C nanotube as a rolled graphene sheet implies that the mechanisms of intra-layer atomic processes in the two systems should be qualitatively similar. Using density-functional theory and tight-binding methods it was shown that the mechanism of single vacancy migration in nanotubes was different from that in graphite, as the curvature of the nanotube atomic network breaks the trigonal symmetry of a perfect graphene sheet, making the diffusion anisotropic, and strongly influencing the migration barrier. It was further demonstrated that the formation energy of a double vacancy in nanotubes was lower than that for a single vacancy; a behavior which differed from that of most monatomic solids, including graphite.

Bending the Rules - Contrasting Vacancy Energetics and Migration in Graphite and Carbon Nanotubes. A.V.Krasheninnikov, P.O.Lehtinen, A.S.Foster, R.M.Nieminen: Chemical Physics Letters, 2006, 418[1-3], 132-6