We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the lineshape is consistent with an excitonic model, but not one of free-carriers. For semiconducting species, side-bands are observed at ~200 meV above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such side-bands are not apparent for metallic nanotubes,as expected from the reduced strength of excitonic interactions in these systems.
@article{arxiv.1001.4546,
title = {Excitons and high-order optical transitions in individual carbon nanotubes},
author = {Stephane Berciaud and Christophe Voisin and Hugen Yan and Bhupesh Chandra and Robert Caldwell and Yuyao Shan and Louis E. Brus and James Hone and Tony F. Heinz},
journal= {arXiv preprint arXiv:1001.4546},
year = {2010}
}