In Situ Monitoring of the Deposition of Flame-Made Chemoresistive Gas-Sensing Films.

Flame-deposited semiconducting nanomaterials on microelectronic circuitry exhibit exceptional performance as chemoresistive gas sensors. Current manufacturing technology, however, does not monitor in situ the formation of such nanostructured films, even though this can facilitate the controlled and economic synthesis of these sensors. Here, the resistance of such growing films is measured in situ during fabrication to monitor the creation of a semiconducting nanoparticle network for gas sensors. Upon formation of that network, the film resistance drops drastically to an asymptotic value that depends largely on the film structure or morphology rather than on its thickness and size of nanoparticle building blocks. Precursor solutions of various concentrations enable the flame deposition of Sb-doped SnO2 sensing films of different morphologies, each of which exhibit a characteristic in situ resistance pattern. Low precursor concentrations (1 mM) lead to thin (ca. 0.16 μm) films with slender columnar structures of increasing diameter (up to 25 nm) after prolonged deposition (up to 6 min) and show an oscillating in situ resistance during their fabrication. On the other extreme, high precursor concentrations (100 mM) lead to thick (up to 80 μm) dendritic and porous films consisting of nanoparticles with relatively small primary particle diameter (around 7 nm) that remain invariant of deposition duration, which is in agreement with the stable in situ resistance. Such dendritic films exhibit a sensor recovery time that is an order of magnitude longer than that of those made at lower concentrations. The above understanding enables the rapid and economic flame synthesis of thin gas sensors consisting of minimal semiconducting nanomaterial mass possessing a tuned baseline resistance and exhibiting excellent response to ethanol vapor.