Diameter-dependent combination modes in individual single-walled carbon nanotubes

Resonant Raman spectra of individual single-walled carbon nanotubes (SWNTs) and thin SWNT bundles have been acquired by scanning confocal Raman microscopy. The position of several Raman modes was followed in dependence of the diameter- dependent radial-breathing modes (RBMs) and the excitation energy. For semiconducting nanotubes a systematic shift of one of the bands between 1650 and 2100 cm(-1) is observed with decreasing nanotube diameter and attributed to a combination mode of the G-band and the RBM