Surface-enhanced resonant Raman spectroscopy of single-wall carbon nanotubes adsorbed on silver and gold surfaces

The surface-enhanced resonant Raman-scattering (SERRS) spectra of single-walled carbon nanotubes (SWNT's) adsorbed on silver and gold metal island films and on colloidal silver cluster substrates were investigated using different laser excitation wavelengths. The observed enhancement in the SERRS signal of the SWNT's results from: (i) an ``electromagnetic'' surface-enhanced Raman spectral (SERS) enhancement due to resonances between optical fields and the electronic excitations in the metallic nanostructures, (ii) a ``chemical'' SERS enhancement due to the interaction between the SWNT's and the metal surfaces, and (iii) a selective resonance Raman effect between the incident and scattered photons and electronic transitions between the one-dimensional van Hove singularities in the electronic density of states of metallic and semiconducting nanotubes. We have observed changes in the relative intensities and shifts in the peak frequencies of several vibrational modes of the SWNT's upon adsorption on a metal surface, which indicate a specific interaction of the nanotubes with the metal surface. Changes in the resonant Raman spectra due to interaction with the silver or gold surfaces are apparent in the second-order Raman bands, especially in the dispersive features, such as the second-order Raman ${G}^{\ensuremath{'}}$ band, which upshifts in the SERRS spectra relative to the resonant Raman-scattering (RRS) spectra, providing evidence for a significant perturbation of the electronic levels for the adsorbed nanotubes. In addition, the SERRS spectra show an additional enhancement of the Raman signal for specific features in the vibrational spectra of the metallic nanotubes, in contrast to the case for the semiconducting nanotubes for which the normal RRS and SERRS spectral profiles are very similar. These results can be explained in terms of a specific charge-transfer enhancement effect for the metallic nanotubes.