Impurity effects in nanotubes

We consider electronic spectra of carbon nanotubes and their perturbation by impurity adatoms over random nanotube sites within the framework of Anderson hybrid model. A special attention is given to the cases when one-dimensional Weyl (massless Dirac) modes are present in the nanotube spectrum and their hybridization with localized impurity states produces, with growing impurity concentrationc, the onset of a mobility gap near the impurity level and then, at furthercgrowth, opening of a narrow range of delocalized states within this mobility gap. That close interchange from conducting to insulating states permits very sensitive controls of nanotube conductivity by slight external factors. Such behaviors are compared for various nanotube structures, concluding in advantage of the zigzag nanotube by its highest sensitivity to conduction controls,3times higher of that for the armchair nanotube of equal width, and yet more of all the twisted nanotubes (suppressed as inverse of their longitudinal period), indicating importance of nanotube topology besides only its dimension (diameter). Some possible practical applications of such doped nanotubes are discussed.