The dependence of the electrical conductance upon the presence of structural defects and of molecular oxygen adsorbates in carbon nanotube thin films for gas-molecule detection was studied. The results showed that oxygen contamination could be responsible for the reported sensitivity of the electronic and transport properties to methane at room temperature. In particular, the sample exhibited a cross-over from decreasing to increasing electrical resistance as a function of methane concentration; depending upon the surrounding atmosphere. The results showed that, when the nanotube walls contained topological defects, oxygen molecules became chemisorbed. It was suggested that the conductivity type of the carbon nanotube could be changed from p-type to n-type by the adsorption of O2 which acted as an electron and donor doping the carbon nanotubes which had a p-type semiconducting nature in the outgassed state. The results demonstrated that nanotubes could be used as sensitive chemical gas sensors, and indicated that intrinsic properties measured on as-grown nanotubes could be markedly changed by extrinsic oxidative treatments.

Interaction of Methane with Carbon Nanotube Thin Films: Role of Defects and Oxygen Adsorption. L.Valentini, I.Armentano, L.Lozzi, S.Santucci, J.M.Kenny: Materials Science and Engineering C, 2004, 24[4], 527-33