Size-dependent response of single-nanowire gas sensors

Monocrystalline tin oxide nanowires (NWs) are grown from Sn powder through thermal chemical vapour deposition. They are then dispersed onto a Si/SiO2 substrate and singularly contacted to fabricate single nanowire (NW)-resistive gas sensors. NWs with different diameter sizes are characterized as nitrogen dioxide sensors. The gas sensor response of the devices is investigated as a function of working temperature, gas concentration, and NW diameter. All the devices show optimum working performances at temperatures around 250–350 °C. Their responses were linearly related to gas concentration of up to 500 ppm, and then started to saturate up to a maximum response of 18.9 to 1000 ppm NO2 for the best device (thinnest nanowire). The limit of detection was 7.2 ppm for the same device. The gas sensing properties (response, response time, and recovery time) were studied as a function of NW diameter to investigate the depletion layer model, which is used to explain the sensing mechanism of monocrystalline metal-oxide NWs. A confirmation of such model was found, with a depletion layer depth of around 14 nm.