Chiral index dependence of Raman spectra of single-walled carbon nanotubes analyzed by scanning-tunneling-microscopy-based tip-enhanced Raman spectroscopy

We investigated the chiral indices dependence of Raman scattering spectra of single-walled carbon nanotube (CNT) by scanning-tunneling-microscopy based tip-enhanced Raman spectroscopy (STM-TERS). Although atomic- resolution STM measurements enable the determination of the chiral indices of individual CNTs, the relationship between chiral indices and Raman spec- tral features has not yet been discussed in sufficient detail. Here, we per- formed STM-TERS measurements on 30 semiconducting CNTs adsorbed on a Ag(111) substrate. As a result, we found that, for many CNTs, the inten- sity of the G⁻ band obtained by STM-TERS significantly enhanced compared with that of CNTs placed on solid substrates or dispersed in solution under the conventional (non-TERS) condition. According to the density functional theory calculation, the polarizability of the G⁺ band is mainly parallel to the tubular axis while the G⁻ band is mainly perpendicular to the tubular axis, therefore, the G⁻ band is enhanced in the plasmon field of TERS setup. Furthermore, in the range of chiral angle of 20 deg-30 deg, the ratio of G⁻ band intensity to G⁺ band intensity was increased as chiral angle decreased. These results show that the relative intensities of the G⁺ band and the G⁻ band can also be utilized for the characterization of chiral angle of CNTs, in addition to the commonly used radial breathing mode.