Phonon contribution to electrical resistance of acceptor-doped single-wall carbon nanotubes assembled into transparent films

The electrical resistance of pristine and acceptor-doped single-wall carbon nanotubes assembled into transparent films was measured in the temperature range of 5 to 300 K. The doping was accomplished by filling the nanotubes with iodine or CuCl from the gas phase. After doping the films resistance appeared to drop down by one order of magnitude, to change the nonmonotonic temperature behavior, and to reduce the crossover temperature. The experimental data have been perfectly fitted in frames of the known heterogeneous model with two contributions: from the nanotube bundles (with quasi-one-dimensional conductivity) and from the interbundle electron tunneling. The doping was observed to decrease the magnitudes of both contributions. In this paper we have revealed the main reason of changes in the nanotube part. It is considered to be connected with the involvement of low-energy phonons, which start to participate in the intravalley scattering due to the shift of the Fermi level after doping. The values of the Fermi level shift into the valence band are estimated to be equal to -0.6 eV in the case of iodine doping and -0.9 eV in the case of CuCl doping. These values are in qualitative agreement with the optical absorption data.