Polarization dependence of resonant Raman scattering from vertically aligned single-walled carbon nanotube films

We present experimental evidence of drastic changes in low-frequency Raman scattering spectra depending on the polarization of the incident laser with respect to the single-walled carbon nanotube (SWNT) axis. Employing recently developed vertically aligned SWNT films, which have a high density $(1.0\times {10}^{17}{\mathrm{m}}^{-2})$ and a thickness of $5\mu \mathrm{m}$, enabled us to obtain sufficient Raman scattering intensity from the film cross section where bundles of SWNTs are aligned along the same direction, in addition to from the top surface of the film. The measured peaks of the radial breathing mode (RBM) by 1.96, 2.41, and $2.54\mathrm{eV}$ incident lasers are clearly distinguished into 2 groups. One group of peaks is dominant for perpendicular polarization while the other group of peaks is dominant in the case of light polarized parallel to the SWNT axis. The selective vanishing of the perpendicular peaks by adsorption of molecules to the SWNTs along with the resultant change in optical absorption spectrum evidences that the parallel and perpendicular peaks originate from $\Delta \mu =0$ and $\Delta \mu =\pm 1$ excitations of electrons, respectively. The grouping behavior of RBM peaks also causes the drastic spectral variation caused by a change in incident laser power. The unambiguous classification of each RBM peak’s nature presented in this study will allow sounder characterization of SWNTs by the resonant Raman scattering analysis.