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 $203{\text{cm}}^{-1}$) had a maximum scattering intensity for incident light parallel to the alignment direction whereas the other group (dominated by a peak at $181{\text{cm}}^{-1}$) 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 [Y. Murakami et al., Phys. Rev. B 71, 085403 (2005)]. Here we systematically measure the polarization-dependent Raman spectra and compare to theoretical expectations for the two different configurations. The $203{\text{cm}}^{-1}$ group peaks are found to behave as expected for parallel-polarized dipole excitation, however the $181{\text{cm}}^{-1}$ group peaks deviate significantly from the prediction assuming perpendicularly polarized excitation. The $181{\text{cm}}^{-1}$ 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.

• Received 24 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.165442