Spectroscopic study of the Fermi level electronic structure of single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes (SWNTs) is investigated by transmission spectroscopy of thin films. Hole-doping of SWNTs, as a result of purification, leads to free carrier absorption in the far-IR spectral range. Thermal cycling de-dopes SWNTs, transferring spectral weight from the far-IR to the interband transitions in the near-IR spectral range. After the nanotubes are de-doped, a diameter dependent peak in the far-IR is apparent at similar to100 cm(-1), which we ascribe to a curvature induced energy gap in the DOS of nonarmchair metallic SWNTs.