Polarization dependence of radial breathing mode peaks in resonant Raman spectra of vertically aligned single-walled carbon nanotubes

Polarization-dependent resonance Raman spectra of a vertically aligned single-walled carbon nanotube (SWNT) array were measured such that the polarization of the scattered light was selected either parallel or perpendicular to that of the incident light. For the parallel configuration, radial breathing mode (RBM) peaks exhibited two different polarization angle dependencies. One group (dominated by a peak at 203 cm^-1) had a maximum scattering intensity for incident light parallel to the alignment direction whereas the other group (dominated by a peak at 181 cm^-1) had maximum intensity for excitation perpendicular to the alignment direction. This anomalous behavior has been attributed to resonance with nonvertical transitions due to perpendicularly polarized excitation. Here we systematically measure the polarization-dependent Raman spectra and compare to theoretical expectations for the two different configurations. The 203 cm^-1 group peaks are found to behave as expected for parallel-polarized dipole excitation, however the 181 cm^-1 group peaks deviate significantly from the prediction assuming perpendicularly polarized excitation. The 181 cm ^-1 group peak intensities were found to be both environment and morphology dependent, indicative of isolated SWNTs. Therefore, the sound interpretation of these results is that the apparently anomalous polarization dependence is not due to perpendicular RBM excitation but rather the parallel excitation of isolated SWNTs suspended within the array.