Humidity tolerant ultra-thin NiO gas sensing films.

When the gas sensor active layer film thickness is decreased increased sensitivity to changes in the adsorbate concentration are expected when measuring the resistance of the layer, in particular when this thickness is on the order of the Debye length of the material (one to tens of nanometers), however this is demonstrated only for a limited number of materials. Herein, ultra-thin NiO films of different thickness (8 to 21 nm) have been deposited via chemical vapour deposition to fabricate gas sensor devices. These NiO sensors were exposed to different relative levels over a wide range of operating temperatures and found to display humidity tolerance far superior to previous report for SnO2 materials. Sensor performance toward a range of NO2 concentration (800 part-per-billion to 7 part-per-million) was evaluated and an optimum operating temperature of 125°C determined. The dependence of the potential relative changes with respect to the NO2 concentration and of the sensor signal with respect to the geometrical parameters were qualitatively evaluated in order to derive a transduction model capable to fit the experimental results. The selective sensitivity towards NO2 was confirmed by the limited response for different reducing gases, CO, CH4, NH3 and SO2 under optimum operating conditions.