Abstract Submitted to the    NANOTUBE'04 Conference:

Purified single wall carbon nanotubes have been treated by SOCl₂. We observed that SOCl₂ reacts readily with the nanotube sample, forming a homogenous dispersion. Entangled networks obtained after filtration show an increase in the electrical conductivity up to factor of 5. EDX and XPS identify presence of sulphur after the reaction with SOCl₂. Thus, we assume that S containing groups, such as -SO-, -SOCl, -SO₂Cl or -SO₃H are attached to the nanotubes. The charge transfer due to the doping reaction was investigated by XPS. The carbon 1s peak is shifted by 0.4 eV towards lower binding energy, suggesting p-type doping. This is consistent with ab initio calculations of SOCl₂ interacting with nanotube fragments, where we observe a down-shift of the Fermi level into the next van Hove singularity in the nanotube density of states. Clear evidence for the electronic structure modification after SOCl₂ exposure is also found in optical absorption spectra. The absorption band at 0.75 eV, related to semiconducting tubes, vanishes in the modified SWNT sample. In agreement with the calculated electronic structure, this finding suggests a shift of the Fermi level into the valence band, thus converting semiconducting tubes into conductors. The difference in the conductivity between the pristine and functionalized sample is even more pronounced at low temperatures. The pristine sample shows a semiconducting character in the temperature dependent conductivity, whereas metallic behavior is observed in the functionalized material. The positive temperature dependence of the thermopower confirms holes as majority carriers in the chemically doped system.