Optical properties of single-wall carbon nanotubes

Four kinds of single-wall carbon nanotubes (SWNTs) with different diameter distribution have been synthesized and optical absorption spectra have been measured. Three large absorption bands due to the optical transitions between spike-like density of states, characteristics of SWNTs, were observed from infrared to visible region. Comparing with the calculated energy band, it has been concluded that the first and the second lowest absorption bands are due to the optical transitions between spikes in semiconductor phases and the third one is due to that in metallic phases. Absorption Peaks sensitively shifted to higher energy side with decreasing tube diameters as the band calculation predicted. Resonance Raman spectra were also measured using various laser lines. When the excitation is in an energy region corresponding to the absorption band of metallic phase, spectra have shown Breit-Wigner-Fano line shape, which is a sign of metallic phase. Using these results, we can easily characterize SWNTs from the optical absorption spectra without Raman measurements and transmission electron microscope observations.