Measurements of tunneling conduction to carbon nanotubes and its sensitivity to oxygen gas

Abstract

We have measured the conductive properties of junctions between carbon nanotubes (CNT) and non-noble metals M (M = Al, Ti, Nb), which are separated by the native oxide (MOX) of the metal. Reproducible and asymmetric current-voltage characteristics were obtained from Ti/TiOX/CNT and Nb/NbOX/CNT junctions, while Al/AlOX/CNT exhibited no current until breakdown, which is attributed to the larger bandgap of AlOX. The conduction in the Ti- and Nb-based junctions is not due to direct tunneling since they exhibit strong temperature dependence. The presence of oxygen is shown to drastically, but reversibly, modify the current-voltage characteristics, and in particular its asymmetry. The observed phenomena are discussed in terms of an elementary model where the different work functions of M and the CNT determine the asymmetric barrier of the junction. These results are complemented with Kelvin probe microscopy measurements of the work function distribution in the vicinity of the junctions in different gas environments. Three-terminal field effect transistor devices are also demonstrated, where a M/MOX metal electrode acts as the gate electrode.