Electronic structures and energetics of [5,5] and [9,0] single-walled carbon nanotubes

Standard enthalpies of formation, ionization potentials, electron affinities, and band gaps of finite-length 5,5] armchair and 9,0] zigzag single-walled carbon nanotubes (SWNTs) capped with C(30) hemispheres obtained by halving the C(60) fullerene have been computed at the B3LYP/6-311G* level of theory. Properties of SWNTs are found to depend strongly on the tube length and, in the case of the 9,0] zigzag species, on the relative orientation of the caps. The metallic character of an uncapped infinite-length 5,5] armchair SWNT manifests itself in the oscillatory dependence of the properties of capped finite-length tubes on their size. An infinite-length 9,0] zigzag SWNT is predicted to be a semiconductor rather than a metal irrespective of the presence of caps. The present results underscore the slow convergence of SWNT properties with respect to the tube length and uncover small but significant radial distortions along the long axes of SWNTs.