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

Abstract

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 $({\omega }_G^{+}=1591\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}),$ while the lower-frequency peak is given by ${\omega }_G^{-}={\omega }_G^{+}-{C/d}_t^2,$ with C being different for metallic and semiconducting SWNTs ${(C}_M>C_S).\mathrm{}$ The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range $0.1<\mathrm{}{I}_{{\omega }_G^{-}}{/I}_{{\omega }_G^{+}}<0.3\mathrm{}$ for most of the isolated SWNTs $(\sim 90\%).$ Unusually high or low $I_{{\omega }_G^{-}}{/I}_{{\omega }_G^{+}}$ 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 $E_{44}^S$ interband transition and scattered photons are in resonance with $E_{33}^S.$ Since the $E_{\mathrm{ii}}$ values depend sensitively on both nanotube diameter and chirality, the $\mathrm{}(n,m)\mathrm{}$ 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 $\mathrm{}(n,m)\mathrm{}$ assignment from resonance Raman spectroscopy.