Metallization of single-wall carbon nanotube thin films induced by gas phase iodination

In this work we report on modification of electrical and optical properties of extended thin films of single-wall carbon nanotubes induced by iodination from gaseous phase. High resolution transmission electron microscopy and Raman data have revealed formation of different types of polyiodide species or one-dimensional iodine crystals (depending on pristine nanotube geometry) inside nanotubes. UV-vis-NIR optical absorption spectra of iodinated nanotubes demonstrated a clear suppression of the optical absorption band corresponding to the first electron transition for semiconducting nanotubes (with the factor determined by the nanotube geometry). It was interpreted as a result of charge transfer from nanotubes to polyiodide species formed inside them. Because of this effect the Fermi level shifted down into the valence band, and the nanotube conductivity type was changed. For filled nanotube films the metallic type of temperature-dependent electrical resistance behavior was observed at elevated temperatures. A reduction of the electrical resistance of pristine films by an order of magnitude (from 1550 Ohm/sq to 270 Ohm/sq; from 865 Ohm/sq to 150 Ohm/sq; from 700 Ohm/sq to 70 Ohm/sq at 300 K) has been observed. The obtained sheet resistances are comparable with those for the most popular today material for transparent conductive electrodes - indium tin oxide.