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-wa lled carbon nanotubes (SWNT's) adsorbed on silver and gold metal island fil ms and on colloidal silver cluster substrates were investigated using diffe rent laser excitation wavelengths. The observed enhancement in the SERRS si gnal 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 "chem ical" SERS enhancement due to the interaction between the SWNT's and the me tal surfaces, and (iii) a selective resonance Raman effect between the inci dent and scattered photons and electronic transitions between the one-dimen sional van Hove singularities in the electronic density of states of metall ic and semiconducting nanotubes. We have observed changes in the relative i ntensities 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 resona nt Raman spectra due to interaction with the silver or gold surfaces are ap parent in the second-order Raman bands, especially in the dispersive featur es, such as the second-order Raman G' band, which upshifts in the SERRS spe ctra relative to the resonant Raman-scattering (RRS) spectra, providing evi dence for a significant perturbation of the electronic levels for the adsor bed nanotubes. In addition, the SERRS spectra show an additional enhancemen t of the Raman signal for specific features in the vibrational spectra of t he metallic nanotubes, in contrast to the case for the semiconducting nanot ubes for which the normal RRS and SERRS spectral profiles are very similar. These results can be explained in terms of a specific charge-transfer enha ncement effect for the metallic nanotubes.