Multifunctional WS ₂ –Au Field-Effect Transistors with Asymmetrical Contact for High Self-Driven Photoresponsivity and Ultrasensitive NO ₂ Sensing

Rising demand for self-driven multifunctional devices in IoT and wearable technologies necessitates efficient integration strategies, where the tunable optoelectronic properties of WS 2 offer promise. Here, we engineer asymmetric contacts in WS 2 field-effect transistors (FETs) to achieve distinct self-driven behaviors. The lower-asymmetry transistor (LAT) exhibits a nonlinear photoresponse, delivering an enhanced n-type photocurrent under 470 nm laser illumination. In contrast, the higher-asymmetry transistor (HAT) enables gate-tunable ambipolar photoresponse and achieves ultrahigh NO 2 sensitivity (94.4% at 2.8 ppm) but with a limited dynamic range. Remarkably, LAT exploits nonlinear effects to combine high sensitivity (96.6% at 8.4 ppm) with ppb-level detection (100 ppb limit) and a Langmuir-type monotonic response. Furthermore, flexible devices fabricated on PET substrates maintain robust gas response (∼20% to 2.1 ppm of NO 2 ) under 2% mechanical strain, confirming wearability. These results establish an effective framework for energy-efficient, miniaturized systems in environmental monitoring and adaptive optoelectronics.