Ultrathin Glass-Based Flexible, Transparent, and Ultrasensitive 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, an ultrathin glass-based flexible, transparent, and ultrasensitive ZnO/glass surface acoustic wave (SAW) humidity sensor is developed using a composite sensing layer of ZnO nanowires (NWs) and graphene quantum dots (GQDs). It shows much larger effective electromechanical coupling coefficients and signal amplitudes, compared to those of flexible polymer-based SAW devices reported in the literature. Attributed to large specific surface areas of ZnO NWs, large numbers of hydrophilic functional groups of GQDs, as well as the formation of p-n heterojunctions between GQDs and ZnO NWs, the developed ZnO/glass flexible SAW sensor shows an ultrahigh humidity sensitivity of 40.16 kHz/% RH, along with its excellent stability and repeatability. This flexible and transparent SAW sensor has demonstrated insignificant deterioration of humidity sensing performance, when it is bent on a curved surface with a bending angle of 30°, revealing its potential applications for sensing on curved and complex surfaces. The humidity sensing and human breathing detection have further been demonstrated for wearable electronic applications using ultrathin glass-based devices with completely inorganic materials.