Carbon nanotubes have recently attracted interest for their possible applications as nanoactuators and nanoswitches, as well as possible building blocks for nanoelectronics. We present ab initio calculations for uniformly charged graphene and (11,0),(9,0) and (5,5) carbon nanotubes. We also consider the effects of polaron formation in these systems. The strain-charge coeÆcient is calculated and compared for both graphene and the nanotubes under study.

We use first-principles calculations to investigate the electronic structure, atomic geometries and formation energies of point vacancies (V B , V N ), antisites (B N , N B ) and interstitials (Bi , Ni ) in cubic boron nitride (c-BN). We find that nitrogen and boron vacancies exhibit the lowest formation energies in nonstoichiometric c-BN for p-type and n-type conditions, respectively, showing intrinsic donor(V B ) and acceptor (V N ) characters. The equilibrium geometries show large outward breathing relaxations for both vacancies and for B N , with slight Jahn-Teller distortions from Td symmetry. For N B we nd an off-center distortion inducing a negative-U behavior in this center. For both interstitial centers we find stable confi- gurations in which an atomic pair occupies a nitrogen site in the lattice, N-N for Ni and N-B for Bi . These systems are stable for the different charge states investigated.