Characterization of Carbonaceous Impurity Level in As-Produced Single-Walled Carbon Nanotubes by Using Solution-Phase Spectrophotometry

This paper describes a simple experimental procedure and a practical data analysis method for characterizing the concentration of carbonaceous impurities in the as-produced single-walled carbon nanotubes (SWNTs). This procedure starts with dispersing solid as-produced SWNT soot in an aqueous surfactant solution, namely sodium dodecyl sulfate (SDS), by using a sonicator. Then the optical absorption spectrum of this SWNT/SDS suspension is measured in the visible and the near-infrared regions (12500 cm^-1-25000 cm^-1) at a spectral resolution of 1 nm by using a spectrophotometer. The obtained spectrum features a pronounced superposition of multiple sharp peaks originating from the inter-band transitions in the electronic density of states (DOS) of the SWNTs and a broad baseline originating from the π-plasmon activities in both SWNTs and carbonaceous impurities. A practical method is proposed for use to analyze the obtained spectral data, and a numerical indicator, CIL (Carbonaceous Impurity Level), is defined as a figure of merit for comparing different as-produced SWNT samples. The validity of CIL is supported by the experimental results.