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 at different temperatures by using high-energy resolution core level photoemission spectroscopy. During NO2 uptake 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 of equivalent chemical reactions and of comparable charge transfer from the flat and curved C lattice to the oxidizing adsorbates. Starting from 200 K, no NO2 adsorption onsand therefore no interaction withsC nanotubes takes place. This excludes the possibility that any tube-adsorbate charge transfer influencing the nanotube conductivity occurs above this temperature. However, at 200 K NO2 strongly interacts with Rh nanoclusters dispersed among the C nanotube bundles. The increased work function of the oxidized Rh changes the alignment of the nanotube bands and simulates an efficient hole doping. A similar variation of the transport properties induced by the interaction of NO2 with metal contacts is likely responsible for the observed sensitivity of nanotube based sensor devices exposed to NO2 at room temperature.