Modification of surface-enhanced Raman scattering spectra of single-walled carbon nanotubes as a function of nanotube film thickness

In this paper we report modifications of the surface enhanced Raman scattering (SERS) spectra with film thickness of carbon single-walled nanotubes (SWNTs). The decrease of film thickness reveals an increased degree of disorder, which is interpreted in terms of degradation of the single-walled nanotubes and the formation of particles similar to highly oriented pyrolytic graphite (HOPG), C₆₀ and amorphous carbon. The degradation of the nanotubes, dependent on the metal substrate type and its morphology, is explained on the basis of interactions between the nanotubes and the metal substrate. The D band, which is an indicator of the degree of disorder, increases its intensity when the film thickness is decreased. SERS spectra recorded at different excitation wavelengths reveal that metallic nanotubes are primarily affected. A mutual correlation was found between the variations in the relative intensity of the D band and the Breit-Wigner-Fano component due to metallic tubes displayed by Raman bands situated in the 1500--1600 cm^-1 range. A direct interaction between C₆₀ and nanotubes is revealed by comparative SERS spectra on SWNTs and (SWNTs+C₆₀) films. At λexc=676.4 nm the narrowing in the lower energy side of the broad band associated to tangential vibration modes indicates that the metallic tubes are involved in the formation of a [(SWNT)⁺C₆₀^-] complex type.