Optical properties of semiconducting and metallic single wall carbon nanotubes: effects of doping and high pressure

We demonstrated that optical absorption spectra of single wall carbon nanotubes (SWNT) changed drastically by the doping of halogens and alkali metals. From disappearance of absorption peaks attributable to interband optical transitions, it was established that the semiconducting phase of SWNT can be doped amphoterically. Emergence of new absorption peaks induced by heavy doping are explained by invoking the involvement of low-lying valence states (or high-lying conduction states) in the optical transition. We also found that the interband absorption peaks considerably and reversibly broadened and shifted under high pressure up to 4.1 GPa. Good correspondence of the present results to recent theoretical works suggests that this change may reflect a semiconductor–metal transition induced by intertube interactions and/or by symmetry breaking.