Assignment of the fine structure in the optical absorption spectra of soluble single-walled carbon nanotubes

Raw electric-arc-produced single-walled carbon nanotubes (SWNTs) were dispersed in organic solvents via a three-step wet chemistry procedure. The soluble sample was purified by continuous dispersion-centrifugation cycles and well-separated SWNTs with high purity were successfully obtained, which enable us to test the separated tube properties and make comparisons with theoretical predictions. Optical absorption experiments on such prepared samples yield fine structures. A series of optical absorptions including some unobserved in previous works are identified. On the basis of the electronic band theory, all the spectral features are systematically assigned to the electronic transitions between pairs of van Hove singularities of semiconducting and/or metallic SWNTs. Detailed analysis of the optical absorption and Raman scattering data reveals that electric-arc-produced SWNTs are preferentially formed with seven discrete diameters and with chirality close to the armchair direction, and a chiral indices (n, m) assignment was initially made for those SWNTs dispersed in the solution phase.