G-band resonant Raman study of 62 isolated single-wall carbon nanotubes

We report G-band resonance Raman spectra of single-wall carbon nanotubes (SWNTs) at the single-nanotube level, By measuring 62 different isolated SWNTs resonant with the incident laser, and having diameters d_t ranging between 0.95 nm and 2.62 nm, we have conclusively determined the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diameter dependent (ω_G ⁺ = 1591 cm^-1), while the lower-frequency peak is given by ω_G ^- = ω_G ⁺ - C/d_- ², with C being different for metallic and semiconducting SWNTs (C_M>C_S). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1<1_ωG ^-/ 1_ωG ⁺<0.3 for most of the isolated SWNTs (∼90%). Unusually high or low 1ωG/1 ωG+ ratios are observed for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E₄₄ ^S interband transition and scattered photons are in resonance with E₃₃ ^S. Since the E_ii values depend sensitively on both nanotube diameter and chirality, the (n, m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theoretical predictions and experimental observations about these special G-band phenomena gives additional support for the (n, m) assignment from resonance Raman spectroscopy.