First and Second-Order Resonant Raman Spectra of Single-Walled Carbon Nanotubes

In summary, overtones and combination modes have been identified in the second-order spectra for the two dominant features in the first-order spectra (the radial breathing mode and the tangential mode) that are associated with the resonant Raman enhancement process arising from the 1D electronic density of states. Just as for the case of the first-order spectra, the resonant contributions to the second-order spectra also involve a different set of (n, m) nanotubes at each laser excitation energy Elaser. A second-order analog is observed for the broad spectral band identified with contributions from metallic nanotubes to the first-order tangential mode spectra. The unique feature of the second-order tangential overtone band shows a larger Elaser range over which the metallic nanotubes contribute, and this effect is attributed to the large (ħωphononc04eV) energy of these phonons. Combination modes associated with (ωtang+ωRBM) and (ωtang+2ωRBM) have been identified. These combination modes show behaviors as a function ofElaser that are consistent with the behavior of their first-order constituents, namely that different nanotubes contribute to the spectra at each value of Elaser. The behavior of the ‘D-band’ and G-band features show a very large phonon frequency dependence on Elaser and show a resonant 2D behavior when the electron and phonon wave vectors coincide, as also occurs in other sp2 carbons.