Enhanced NO₂ gas sensing of a single-layer MoS₂ by photogating and piezo-phototronic effects

NO₂ sensors with ultrahigh sensitivity are demanded for future electronic sensing systems. However, traditional sensors are considerably limited by the relative low sensitivity, high cost and complicated process. Here, we report a simply and reliable flexible NO₂ sensor based on single-layer MoS₂. The flexible sensor exhibits high sensitivity to NO₂ gas due to ultra-large specific surface area and the nature of two-dimensional (2D) semiconductor. When the NO₂ is 400 ppb (parts per billion), compared with the dark and strain-free conditions, the sensitivity of the single-layer sensor is enhanced to 671% with a 625 nm red light-emitting diode (LED) illumination of 4 mW/cm² power under 0.67% tensile strain. More important, the response time is dramatically reduced to ∼16 s and it only needs ∼65 s to complete 90% recovery. A theoretical model is proposed to discuss the microscopic mechanisms. We find that the remarkable sensing characteristics are the result of coupling among piezoelectricity, photoelectricity and adsorption-desorption induced charges transfer in the single-layer MoS₂ Schottky junction based device. Our work opens up the way to further enhancements in the sensitivity of gas sensor based on single-layer MoS₂ by introducing photogating and piezo-phototronic effects in mesoscopic systems.