Absorption spectra of high purity metallic and semiconducting single-walled carbon nanotube thin films in a wide energy region

Absorption spectra of high purity metallic and semiconducting single-walled carbon nanotubes separated by the density-gradient ultracentrifugation method have been measured in the wide energy region from 1 meV to 5 eV. In the high purity metallic nanotube sample, a strong and broad absorption band has been observed at 0.06 eV. This observation suggests that the optical properties of even high purity metallic nanotube bundles cannot be explained by the simple Drude conduction model. We discuss the origin of these absorption bands for metallic and semiconducting nanotube samples by considering the existence of a small energy gap in metallic nanotube bundles and plasmon resonance.

► High purity semiconducting and metallic single-walled carbon nanotube (SWNT) films are studied.
► Absorption spectra of these films are measured from the THz to the UV energy region.
► Broad absorption bands are observed at 0.06 eV for both samples.
► The intrinsic small energy gap for metallic SWNTs is one of possible origins of the absorption band.
► For semiconducting SWNTs, the defect states may cause such mid-gap absorption.