Inelastic X-ray Scattering Studies of Plasmons 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 depend exclusively on the number of walls and not on the radius or chiral vector. This shift indicates a change of strength of screening and the effective interaction at inter-atomic distance, and thus suggests an alternative mechanism of tuning the properties of the nanotube in addition to the well-known control provided by chirality and tube diameter.