FIRST AND SECOND-ORDER RESONANT RAMAN SPECTRA OF SINGLE-WALLED CARBON NANOTUBES

摘要

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 enhance-ment 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 E_(laser). 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 E_(laser) range over which the metallic nanotubes contribute, and this effect is attributed to the large (hω_(phonon) ～ 0.4eV) energy of these phonons. Combination modes associated with (ω_(tang) + ωRBM) and (ωtang + 2ωRBM) have been identified. These combination modes show behaviors as a function of E_(laser) that are consistent with the behavior of their first-order constituents, namely that different nanotubes contribute to the spectra at each value of E_(laser). The behavior of the 'D-band' and G'-band features show a very large phonon frequency dependence on E_(laser), and show a resonant 2D behavior when the electron and phonon wave vectors coincide, as also occurs in other sp~2 carbons. Future Raman studies are likely to explore the relation between the Stokes and the anti-Stokes spectra as a function of E_(laser). Surface-enhanced Raman scattering (SERS) is likely to be explored as a method for achieving much higher sensitivity, allowing exploration of the vibrational spectra of a small number of nanotubes and perhaps even eventually the Raman spectrum of a single nanotube.