Screening of water dipoles inside finite-length armchair carbon nanotubes

Short single-walled carbon nanotubes (SWNTs) have been proposed as good candidates for artificial nano-scale channels due to their small size and stable structure. Unlike many insulating biological channels in nature, the nanotubes respond to external charges and electric potentials effectively due their delocalized /spl pi/-electrons. We study the electronic structure and dielectric properties of finite-length armchair SWNTs within a self-consistent /spl pi/-orbital tight binding (TB) method. By including up to the third nearest neighbor interactions, we have successfully reproduced the periodic oscillation of the finite band gap as a function of the nanotube length and the HOMO/LUMO orbitals predicted by ab initio calculations. We also look into the screening ability of SWNTs by applying uniform electric fields along or across the nanotubes. We show that as the length increases, the parallel screening constant /spl epsiv//sub /spl par// grows almost linearly to infinity while the perpendicular screening constant /spl epsiv//sub /spl perp// converges to its bulk value when the nanotube length exceeds ten times the radius. /spl epsiv//sub /spl par// and /spl epsiv//sub /spl perp// are found to have weak and strong dependence on the magnitude of the band gap respectively, which can be explained by the symmetry of the involved subbands.