Nanogap Sensors Decorated with SnO₂ Nanoparticles Enable Low-Temperature Detection of Volatile Organic Compounds

In this work, an experiment was carried out in order to exploit the physical properties of an electrode structure with nanometric gap to enable the operation of MOX sensors at low temperature independently from the gas sensing properties of the adopted active material. The 100 nm-gap fingers gas sensor array was fabricated by using electron beam and UV optical lithography onto 4″ silicon wafers (guaranteeing high process yield). SnO₂ nanoparticles (NPs) synthesized by sol–gel/solvothermal method were trapped between the nanogap electrodes by dielectrophoresis, and scanning electron microscopy and atomic force microscopy surface analysis were used to investigate the semiconducting NPs dispersion between the nanogap fingers. Nanogap SnO₂ NPs based-sensor responses to acetone and ethanol in dry air carrier gas at near room temperatures were reported, discussed, and compared with those obtained from 5 μm gap gas sensors (comparable to standard microgap commonly used in commercial sensors) functionalized with the same sensing material. The nanogap sensors exhibited better performance compared to the microgap ones, and larger response to ethanol than to acetone. For the lowest investigated gas concentration (10 ppm), the ethanol response (R_air/R_gas) increased with temperature from 2.56 at 50 °C to 17.91 to 100 °C, respectively from 1.56 to 3.92 for acetone. The best nanogap sensor responses were found at 100 °C with R_air/R_gas ≈ 38 for 150 ppm of ethanol, and R_air/R_gas ≈ 10 for 150 ppm of acetone. The experimental measurements confirmed the adopted theoretical model correlation between the sensor responses and the electrodes separation gap.