Electron energy-loss spectroscopy studies of single wall carbon nanotubes

We have carried out momentum-dependent measurements of the density response function of bulk samples of purified single wall nanotubes using electron energy-loss spectroscopy. Carbon nanotubes support both excitations between delocalized and localized electronic states. The pi-plasmon exhibits significant q-dependence, with a dispersion relation similar to that of the graphite plane, demonstrating the graphitic nature of the nanotube electron system along the tube axis. In contrast, the interband excitations observed at low energy have vanishingly small dispersion in q. These excitations between localized states are related to characteristic interband transitions between the singularities in the nanotube electronic density of states, and can thus be used to show that our samples contain significant quantities of both semiconducting and metallic single wall nanotubes. (C) 1999 Elsevier Science Ltd. All rights reserved.