Dramatic increase in the Raman signal of functional groups on carbon nanotube surfaces

Raman spectra of octadecylamine (ODA) molecules covalently bound to the surface of single walled carbon nanotubes were obtained by varying the laser excitation energy from 1.92 to 3.81 eV. A strong dependence of the Raman intensity on the excitation energy was observed and, under specific conditions of resonance, the Raman signal from the amide linkage can become even stronger than the resonant Raman signal of the carbon nanotubes. This result is explained in terms of a chemical enhancement mechanism due to charge transfer between the ODA molecules and the nanotubes. The enhancement factor was up to 100 times higher than the value typically reported for the chemical Surface Enhanced Raman Scattering (SERS) effect. Our results show that carbon nanotubes may contribute to the still controversial understanding of the 'chemical' effects to the enhancement in SERS, extending the scope of application of this analytical tool in strategic areas such as biomedicine, catalysis and environmental analysis.