Reinterpretation of Single-Wall Carbon Nanotubes by Raman Spectroscopy

Raman spectra of single-wall carbon nanotubes (SWNTs) exhibit a unique radial breathing mode (RBM) band (∼100-300 cm^-1) and a G^- peak (∼1570 cm^-1), along with a D band (∼1350 cm^-1). We show that the typical Raman signals for SWNTs are the signature of their helical structure determined using density functional theory simulation and structural analysis for hydrogenated and dehydrogenated SWNT samples. We demonstrate that the G^- mode at ∼1570 cm^-1 is unique to opened tubular graphene structures of ∼2 nm diameter. We also demonstrate that the D mode of ∼1350 cm^-1 is originated from edge defects of opened SWNTs, revealing strong eigenvectors, which is absent in concentric tubes. We also report a radial-tangential mode (RTM) for concentric and opened SWNTs, which appears following RBM. We also interpret the low-energy Raman signal, reported as an RBM band, to be convolution of "localized RBM" (∼170 cm^-1) and RTM (∼190 cm^-1) for helical SWNTs. We also show that the analysis of the Raman spectra of SWNTs is consistent with general understanding on Raman analysis of carbon materials.