The role of metal contact in the sensitivity of single-walled carbon nanotubes to NO2

The interaction of NO2 with single-walled C nanotubes and highly oriented pyrolitic graphite was studied atdifferent temperatures by using high-energy resolution core level photoemission spectroscopy. During NO2uptake below 200 K, the C1s peak exhibits a similar behavior for the two samples and correspondent NOx (x) 1-3) adspecies form on the surface of nanotubes and graphite. These results indicate the occurrence ofequivalent chemical reactions and of comparable charge transfer from the flat and curved C lattice to theoxidizing adsorbates. Starting from 200 K, no NO2 adsorption onsand therefore no interaction withsCnanotubes takes place. This excludes the possibility that any tube-adsorbate charge transfer influencing thenanotube conductivity occurs above this temperature. However, at 200 K NO2 strongly interacts with Rhnanoclusters dispersed among the C nanotube bundles. The increased work function of the oxidized Rh changesthe alignment of the nanotube bands and simulates an efficient hole doping. A similar variation of the transportproperties induced by the interaction of NO2 with metal contacts is likely responsible for the observed sensitivityof nanotube based sensor devices exposed to NO2 at room temperature.