Subdiffraction-limited far-field Raman spectroscopy of single carbon nanotubes: an unenhanced approach

We present a new approach for subdiffraction-limited far-field Raman spectroscopy of single carbon nanotubes using through-the-objective total internal reflection (TIR) excitation coupled to an atomic force microscope (AFM). By using this approach, we are able to detect spectroscopic signatures of structural changes along a single nanotube with nanometer resolution. A single multiwalled carbon nanotube is mounted on an AFM tip and imaged while tapping on the surface of a glass coverslip. As the angle of incidence of the excitation field is changed, we are able to tune the penetration depth of the evanescent field by steps as small as 2-10 nm. An increase in the ratio of the Raman D band (the disorder band) to G band (the in-plane graphitic band) of the carbon nanotube was demonstrated as the penetration depth decreased, indicating that most defects are concentrated at the end of the nanotube. We also observed frequency shifts of the G band as we changed the penetration depth. By changing the polarization of the incident beam, we are able detect the orientation and possible local curvature in the nanotubes. Coupling through-the-objective TIR with AFM is a powerful technique for studying structural and chemical properties of carbon nanotubes and can be easily extended to many other nanoscale/molecular systems.