Changes in the Electronic States of Single-Walled Carbon Nanotubes as Followed by a Raman Spectroelectrochemical Analysis of the Radial Breathing Mode

A detailed in situ Raman spectroelectrochemical analysis of the radial breathing mode (RBM) of single-walled carbon nanotubes (SWCNTs) in bundles is presented. The intensity of the RBM feature exhibits striking changes even before the electrode potential reaches the level of the Van Hove singularity, which is involved in optical transitions causing a resonance enhancement of the Raman scattering. A difference in the behavior of semiconducting and metallic tubes has been found to reflect the different electronic structure of these two types of SWCNTs. The intensity of the RBM of metallic tubes starts to change immediately with the electrode potential which is scanned from the open Circuit value to a distinct electrode potential. On the other hand, semiconducting, tubes give a significant change of the spectral intensity at potentials deviating by ca. +/-0.5 V front the open circuit value. The resonant Raman spectra of doped carbon nanotubes are shown to reflect not only the filling of the particular electronic states by electrons or holes but also some more fundamental changes ill their energies and densities.