Ultra-thin Glass Based Flexible, Transparent and Ultra-sensitive Surface Acoustic Wave Humidity Sensor with ZnO Nanowires and Graphene Quantum Dots

Flexible electronic devices are normally based on organic polymer substrate. In this work, ultra-thin glass based flexi-ble, transparent and ultra-sensitive ZnO/glass surface acous-tic wave (SAW) humidity sensor is developed using a compo-site sensing layer of ZnO nanowires (NWs) and graphene quantum dots (GQDs). It shows much larger effective elec-tromechanical coupling coefficients and signal amplitudes, compared with those of flexible polymer based SAW devices reported in literature. Attributed to large specific surface are-as of ZnO NWs, large numbers of hydrophilic functional groups of GQDs, as well as the formation of p-n heterojunc-tions between GQDs and ZnO NWs, the developed ZnO/glass flexible SAW sensor shows an ultra-high humidity sensitivity of 40.16 kHz/%RH, along with its excellent stability and re-peatability. This flexible and transparent SAW sensor has demonstrated insignificant deterioration of humidity sensing performance, when it is bent on a curved surface with a bend-ing angle of 30, revealing its potential applications for sens-ing on curved and complex surfaces. The humidity sensing and human breathing detection have further been demon-strated for wearable electronic applications using ultra-thin glass based devices with completely inorganic materials.