Besides the theoretical discussion, experimental results are also proposed. "In 2D graphite, the so-called G band around 1582cm - is the only first-order Raman peak". In the single-walled nanotubes, this band is "split into many features around 1580 cm -1 ". Besides this band, we have another strong first-order Raman mode, which is the radial breathing mode (RBM) . "The RBM is a unique phonon mode, appearing only in carbon nanotubes and its observation in the Raman spectrum provides direct evidence that a sample contains SWNTs. The RBM is a bond-stretching out-of-plane phonon mode for which all the carbon atoms move coherently in the radial direction, and whose frequency is about 100-500 cm -1 . The RBM frequency is inversely proportional to the tube diameter and is expressed as ωRBM=C/dt" . The constant is C=248 cm -1 nm in the case of isolated SWNTs on a substrate made of SiO2. This relationship between diameter and angular frequency is due to "the mass of all the carbon atoms along the circumferential direction, [which is] proportional to the diameter" .