  In this paper, we study how the distribution of net charges in carbon
nanotubes can be influenced by substrate and external electric fields, using
theoretical calculations based on an extension of the atomic charge-dipole
model. We find that the charge enhancement becomes less significant when the
tube gets closer to substrate or when the dielectric constant of substrate
increases. It is demonstrated that net charges can be shifted to one side of
tube by longitudinal electric fields and the polarity of charges can be locally
changed, while transversal fields give much less influence on the charge
enhancement. These properties could be generalized for other metallic or
semiconducting nano/microwires and tubes.
