An innovative approach to gas sensing using carbon nanotubes thin films: sensitivity, selectivity and stability response analysis

We report that carbon nanotubes (CNTs) thin films have been successfully prepared as new NO/sub 2/ and H/sub 2/ gas sensors in the NO/sub 2/ concentration range 7 ppb - 7 ppm and H2 8.5ppm - 850 ppm respectively. 150 nm thick CNTs thin films, have been prepared by rf-PECVD on Si/Si/sub 3/N/sub 4/ substrates, provided with Pt interdigital sputtered electrodes, and post-annealed under controlled conditions. The electrical response has been investigated at different operating temperatures and gases over a period of 6 months. Sensitivity response analysis has demonstrated that CNTs are more responsive to NO/sub 2/ than H/sub 2/. By selecting suitable annealing conditions, the microstructure significantly changes enabling: to adjust the base line resistance of the film (i.e. the resistance in dry air) from 100 Ohms to several mega-Ohms, and to improve gas sensitivity. Selectivity test have highlighted negligible cross sensitivity effects to the detection of NO/sub 2/, when H/sub 2/ is the interfering gas. Long term stability test have shown a neglegible increase of the base line resistance and an improvement of the NO/sub 2/ sensitivity.