In metal-oxide gas sensors the relationship between nano-size and thermal stability is very challenging. Miniaturization of the sensitive material down to nano-scale and doping with noble metals has led to successful detection of analytes in ppb concentrations, but long term stability, which may be related to crystal growth, is still unresolved. The sensitivity of SnO₂, the most widely used metal-oxide for gas sensors, increases drastically with decreasing grain size from 20 to 5 nm. Preserving these sizes during sensor fabrication and operation is difficult since high process and operation temperatures (200 – 900 °C) are applied. Here one-step synthesis of highly sensitive films of SnO₂-SiO₂ nanostructures is explored by FSP direct deposition and in-situ annealing resulting thermally stable and, well-adhering cauliflower-like deposits (1) and transparent sensors (2). Control and stabilization of the SnO₂ particle and neck size is obtained by varying the SiO₂ content during synthesis and direct deposition of these SnO₂-based films. Detailed film characterization is carried out to understand the role of SiO₂ for inhibiting SnO₂ grain and crystal growth as well as the role of primary particle and neck size on film sensitivity of EtOH, a common analyte in sensor development (3).

1. A. Tricoli, M. Graf, F. Mayer, S. Kühne, A. Hierlemann, S.E. Pratsinis “Micropatterning Layers by Flame Aerosol Deposition - Annealing”, Adv. Mater., in press, (2008).

2. S. Kühne, M. Graf, A. Tricoli, F. Mayer, S.E. Pratsinis, A. Hierlemann, “Wafer-level flame-spray-pyrolysis deposition of gas-sensitive layers on microsensors”, J. Micromechanics and Microengineering, 18, 035040 (2008).

3. A. Tricoli, M. Graf, S.E. Pratsinis “Optimal doping for enhanced SnO₂ Sensitivity and Thermal Stability”, Adv. Funct. Mater., in press, (2008).