On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity

ZnO-nanowire gas sensors were fabricated by a selective growth of nanowires on patterned Au catalysts thus forming nanowire air bridges or 'nanobridges' between two Pt pillar electrodes. The gas sensing properties of nanobridge gas sensors were demonstrated using a diluted NO₂. The response, as a function of temperature, was highest at 225 °C and was linearly increased with the concentration of NO₂ in the range of 0.5-3 ppm and then showed a sign of saturation. Our sensor showed higher response compared with different types of sensors including ZnO nanocrystals, Sn- and In-doped ZnO thin film, or ZnO nanowires. The enhanced response was attributed to the additional modulation of the sensor resistance due to potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire. Also nanobridge structure enabled fast recovery behavior because desorbed gas molecules can be easily swept away from the surface of ZnO nanowire without re-adsorption.