Flexible zinc oxide surface acoustic hydrogen gas sensor based on graphene oxide sensing layer

Abstract

This thesis presents the design and development a flexible surface acoustic wave (SAW) gas sensor. Fabrication and characterization of SAW device, nanostructure material, and the gas sensing performance were examined. The flexibility of the SAW substrate is highly essential due to the uneven and curved surface. The investigated structure was based on three basic conditions of the device, which are flat, bend in, and bend out. Based on the conditions, the devices were tested for the electrical and gas sensing performances to hydrogen (H2) gas. The design of the flexible SAW gas sensor was completed using a simulation process prior to fabrication. The SAW propagation and properties were investigated using finite element method (FEM) simulation. It was observed that at bending inward radius of 1.5 μm, the total displacement and frequency shift increased by 24.5% and 89%, respectively. The simulated nanostructure sensing elements have improved the sensitivity of the gas sensor by 85.5%. For the sensing element, simulation was conducted to investigate the graphene oxide effect on bending (warping) surface towards gas. From this study, a further increase of warping angle from 180° to 270° has enhanced the binding energy. The sensor was fabricated by depositing a piezoelectric layer, interdigitated electrodes, and nanostructured material. Zinc oxide (ZnO) was deposited as the piezoelectric layer using radio frequency (RF) magnetron sputtering with different parameters and characterised using atomic force microscopic (AFM), field emission scanning electron microscopy (FESEM), and x-ray diffraction (XRD). Based on the investigation of material characteristics and surface morphology of ZnO sputtered on polyimide (PI), higher RF power increased the deposition rate at 38% from 150 to 200 W, meanwhile at 300 W, the deposition rate spiked to 67%. The S21 measurement provided insertion loss (IL) and frequency response of the SAW device. The thickness of piezoelectric thin film significantly affected the frequency response and phase velocity of the acoustic wave. The measured response of graphene nanosheet flexible SAW sensor at room temperature was taken. The radii of curvature were defined as 10 mm for bend in and bend out. The frequency shift increased in the bend in condition compared to bend out and flat conditions. The graphene oxide nanosheet sensitive element conductivity increased when electron was injected into the device surface since H2 is a reducing gas. Therefore, the centre frequency of the acoustic wave velocity decreases significantly when the sensor exposed to the H2 gas. The SAW gas sensing performance of the investigated nanostructure materials provides a way for further investigation to future commercialisation of these types of sensors for different types of flexible substrates.