Effect of Number of Walls on Plasmon Behavior in Carbon Nanotubes
  We investigate the physical parameters controlling the low energy screening in carbon nanotubes via electron energy loss spectroscopy and inelastic x-ray scattering. Two plasmon-like features are observed, one near 9 eV (the so-called pi plasmon) and one near 20 eV (the so-called pi+sigma plasmon). At large nanotube diameters, the pi+sigma plasmon energies are found to depend exclusively on the number of walls and not on the radius or chiral vector. The observed shift indicates a change in the strength of the screening and in the effective interaction at inter-atomic distances, and thus this result suggests a mechanism for tuning the properties of the nanotube. 
M. H. Upton, R. F. Klie, J. P. Hill, T. Gog, D. Casa, W. Ku, Y. Zhu,   M. Y. Sfeir, J. Misewich, G. Eres and D. Lowndes