Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption

This paper deals with the dependence of the electrical conductance on the presence of structural defects and of molecular oxygen adsorbates in carbon nanotube (CNT) thin films for gas molecule detection. Our results show that oxygen contamination may be responsible for the reported sensitivity of the electronic and transport properties to methane at room temperature. In particular, the sample exhibits a crossover from decreasing to increasing electrical resistance vs. methane concentration depending on the surrounding atmosphere. The obtained results show that when the nanotube walls contain topological defects, oxygen molecules become chemisorbed. We suggest that the conductivity type of the CNT can be changed from p-type to n-type by adsorption of O2 acting as an electron and donor doping the CNTs, which has p-type semiconductor character in the outgassed state. The obtained results demonstrate that nanotubes could be used as sensitive chemical gas sensor likewise indicate that intrinsic properties measured on as-grown nanotubes may be severely changed by extrinsic oxidative treatments.