Sputtered Ultrathin WO₃ for Realizing Room-Temperature High-Sensitive NO₂ Gas Sensors

In this work, we used a commercial manufacturing process to obtain a WO₃ semiconductor gas sensor to realize NO₂ detection in parts-per-billion concentration at room temperature (25-27 °C). The radio-frequency (RF) sputtering process was used to deposit an ultrathin (down to 5 nm) WO₃ sensing layer. With suitable control of the deposition ambient and the postannealing condition, the WO₃ ultrathin-film resistor with a 50-μm line width can detect 100 ppb of NO₂ without any heating setup. Notably, no nanometer process is required; hence, the production can be realized by current flat-panel display technology. By modulating the deposition condition, we investigated the influences of surface roughness, crystalline condition, and surface hydroxyl group levels on the sensing response. To achieve parts-per-billion (ppb) level detection, the ultrathin thickness is essential, and the high-level crystal together with the low-level surface hydroxyl group also enhances the sensitivity and the recovery. The humidity effect is also discussed to show an almost unchanged response in relative humidity (RH) from 30 to 70%. In summary, the WO₃ sensor shows good sensing performance, including a high sensitivity, a wide detecting concentration range (17 ppm to 100 ppb), and fast response/recovery (30 s/∼21 s) at low NO₂ concentration (0.68 ppm). Finally, the good enough selectivity, stability, and nondecayed sensing of the WO₃-based gas sensor after 14 days were demonstrated.