Narrow and intense resonances in the low-frequency region of surface-enhanced Raman spectra of single-wall carbon nanotubes

Previously unexplored low-frequency Raman modes of single-wall carbon nanotubes (SWNTs) are reported. Gold deposited onto a randomly-oriented top layer of vertically aligned carbon nanotube arrays was used to induce surface-enhanced Raman scattering (SERS) "hot spots" on large-diameter SWNTs with SERS intensities up to 1900 times greater than normal Raman intensities from undecorated arrays. The linewidths of the resonances (down to 0.3 cm⊃-1) are ten times narrower than previously measured for individual SWNTs. Pairs of intense sharp resonances with identical excitation profiles were found and tentatively interpreted as the low-energy longitudinal optical and radial breathing modes of the same nanotube. SERS lines in the region of 14-30 cm⊃-1 were tentatively assigned to the ring modes of SWNTs in agreement with existing theories.