Use of different sensing materials and deposition techniques for thin-film sensors to increase sensitivity and selectivity

The performances of metal oxide semiconducting materials used as gas-sensing detectors depend strongly on their structural and morphological properties. The average grain size has been proved to play a prominent role and better sensor performances were found in polycrystalline films where the grain size is few tens of nm or smaller. On the other hand, thermal treatments during thin-film deposition and/or sample postprocessing could lead to a grain coalescence, thus decreasing the conductivity of the sensing film. Avoiding such a phenomenon, still keeping optimized processing conditions, will increase the sensor performances, maintaining the resistivity at acceptable values. In this work, new gas-sensing materials and new thin-film deposition procedures have been investigated. Aiming to preserve the sensitivity, to enhance selectivity and to reduce the drift, thin films of WO3 and CrTiO3 deposited by pulsed-laser ablation (PLA) and of SnO2 deposited by rheotaxial growth and thermal oxidation techniques were comparatively characterized. Three issues were mainly addressed: the variation of the conductivity as a function of RH, the sensitivity toward benzene, CO, acetone, and NO2, and the selectivity. The mid-term test has disclosed that sensors based on SnO2 show excellent response to VOC compounds; but a significant drift of the conductivity still affects their performances. In comparison, the drift of conductivity and sensor responses in WO3 and CrTiO3-based gas sensors was very low, in particular for those. sensors where some SnO2 was added during the deposition of the sensing layer. The addition of SnO2 was also found to increase the conductivity of the sensing layer and, in the case of WO3, the selectivity toward NO2.