Observation of hole doping of metallic carbon nanotubes contained in unsorted species by Raman spectroscopy

Single-walled carbon nanotubes have unique photophysical and electronic properties that can be tuned by means of redox chemistry and charge transfer. In this work the removal of hole doping from the DNA-suspended un-sorted nanotubes using reducing agent (dithiothreitol) was examined by Raman spectroscopy. Raman spectra of two nanotube:DNA aqueous suspensions with different amount of holes on the free nanotube surface obtained due to different biopolymer coverage were analyzed in the range of the radial breathing mode, tangential modes, as well as defect-induced and two-phonon modes. Noticeable transformation of intensity, spectral position and lineshape was revealed for bands assigned to metallic nanotubes contained in unsorted species while bands of semiconducting nanotubes remained unchanged. This band transformation is explained by diminished hole doping of metallic nanotubes. Tighter DNA coverage of the nanotube surface partially mitigated hole doping removal which indicates an important role of biopolymer coverage on nanotube response to reducing agent influence.