Metallic and semiconducting narrow carbon nanotubes

We report local-density-functional results that show that narrow nanotubes with optimized diameters between about 0.34 and 0.5 nm can be either semiconducting or metallic, but with electron structures near the Fermi level that often cannot be understood starting from the graphene sheet model, successful in the study of larger diameter tubes. Our total-energy calculations indicate that narrow nanotubes recently observed either as the central shell of a multiwalled tube or encased in a porous zeolite, if isolated, should be stable against complete unzipping along the nanotube axis.