Excitons and exciton-phonon coupling in metallic single-walled carbon nanotubes: Resonance Raman spectroscopy

The first four transitions (upper and lower branches of E₁₁ ^M and E₂₂^M) for a broad diameter range (0.7-4 nm) of metallic single-walled carbon nanotubes are studied in the 1.26-2.71 eV energy range using resonance Raman spectroscopy of their radial breathing modes (RBMs). A scaling-law analysis of transition energies from 77 spectral features suggests that the transitions are excitonic in nature and that relative scaling of electron self-energies and exciton binding energies in metallic nanotubes closely matches that found in semiconductors. The previously elusive upper-branch signatures are observed at large diameters (>1.3 nm) for several chiralities for both E₁₁^M and E₂₂^M excitation. These results are discussed as a consequence of the nodal behavior of exciton-phonon coupling. Additionally, while theoretical calculations for the (n,m) -dependent matrix elements predict that the RBM intensity should decrease with increasing diameter, the opposite behavior is observed experimentally. We show that this is a consequence of an increase in the resonance Raman broadening factor Γ as diameter decreases.