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.