Structure Dependence of the Intermediate-Frequency Raman Modes in Isolated Single-Walled Carbon Nanotubes

Abstract

Resonance Raman spectra in the intermediate- frequency mode (IFM) region have been studied for isolated single- walled carbon nanotubes (SWNTs) at the single-nanotube level. The SWNT samples with diameter in the range of 0.8−2.0 nm are first grown on quartz substrate and then transferred to silicon substrate by nanotransfer printing technique. A specific linear relation of ωRBM = 222.0/dt + 8.0 is found to best suit for our transferred isolated SWNTs. By measuring more than 80 isolated SWNTs with 36 different assigned chiralities using five different excitation lasers, the dependence of the nondispersive out-of-plane transverse optical (oTO) and the dispersive IFM − and IFM + features on the nanotube structure has been determined. It is found that the oTO, IFM − , and IFM + frequencies decrease slightly, decrease significantly, and increase slightly with decreasing nanotube diameter, respectively. The appearance of the oTO band is indifferent to SWNT chirality or type, while the IFM − feature can only be observed in metallic or MOD1 semiconducting nanotubes with small chiral angles. The IFM − feature in resonance Raman spectra thus offers a method to distinguish small θ nanotubes from large θ nanotubes and distinguish MOD1 semiconducting nanotubes from MOD2 semiconducting nanotubes.