Optical Evaluation of the Metal-to-Semiconductor Ratio of Single-Wall Carbon Nanotubes

The metal-to-semiconductor ratio for single-wall carbon nanotubes (SWCNTs) produced by laser ablation was investigated using optical absorption spectroscopy. The SWCNTs were separated into metals and semiconductors using the density gradient centrifugation method. The optical absorption spectra of both SWCNTs were measured from the near-infrared to ultraviolet. A simple superposition analysis revealed that the pristine SWCNTs consist of 27 ± 3% metallic SWCNTs and 73 ± 3% semiconducting SWCNTs without using a theoretical correction. From the detailed oscillator fitting, it was found the absorption coefficient of the first excitonic transition of metallic SWCNTs M 11 was almost the same as that of semiconducting SWCNTs S 11 for a diameter distribution of 1.1∼1.3 nm. This result provides a simple optical evaluation method to estimate the metal-to-semiconductor ratio of various SWCNT samples produced by different methods.