Three-Dimensional Tetrapodal ZnO Microstructured Network Based Flexible Surface Acoustic Wave Device for Ultraviolet and Respiration Monitoring Applications

Sensitivity of flexible surface acoustic wave (SAW) sensors to ultraviolet (UV) light is enhanced by the application of ZnO micro- and nanostructured networks as sensing layers. In this study, the influence of three-dimensional (3D) tetrapodal ZnO microstructured networks (T-ZnO MNs) spin coated onto Al foil/ZnO piezoelectric film SAW devices on the sensor performance under a stimulus of UV-light irradiation and the influence of relative humidity was investigated. The UV-light sensitivity is increased from -3.03 × 10 ^-6 to -5.25 × 10 ^-6 cm ² mW ^-1 with the application of 3D T-ZnO MNs. Likewise, the humidity sensitivity is enhanced by a factor of 2.9 at 90% relative humidity, which is explained by the porous structure of T-ZnO MNs. In addition, the measured sensitivity to the UV-light intensity is demonstrated to be significantly modified under bent conditions because the surface of the ZnO piezoelectric film is relatively denser in the case of bending. This study also demonstrates that the sensing performance of the respiratory characteristics is increased by nearly a factor of 1.7 under bent conditions after utilization of the T-ZnO MNs, showing its capability for breath and respiration monitoring applications.