Nanoscale Zinc Oxide and Langasite Crystal Microbalance Based Energy-efficient Composite Resonator for UV Sensing Applications

Thickness shear mode (TSM) resonators, commonly known as quartz crystal microbalances (QCM), are highly sensitive mass detectors, which can be utilized as selective sensors by coating suitable sensing layers on the crystal surface. Over the past decade, zinc oxide nanostructured materials have gained remarkable popularity as sensing media for UV sensing applications because of their excellent piezoelectric and semiconducting properties, and wide range of available synthesis techniques. However, very little research has been conducted towards integration of ZnO nanostructures on TSM resonators to fabricate highly sensitive composite sensors. Low temperature-resistance of quartz and associated challenges in synthesis of ZnO nanostructures directly on to quartz crystals are the primary hindrances responsible for the lack of research in this area. Therefore, it is essential to identify and exploit suitable piezoelectric material to replace quartz for high-temperature operations. Furthermore, potential of such sensors to detect UV radiation in real world applications, where UV intensity is very low and photocurrent is extremely small, is yet to be explored.
   
   In this thesis, we develop a low-power composite resonator based on ZnO nanostructures and langasite crystal microbalance (LCM) for UV sensing application. Firstly, various ZnO nanostructures, including well-aligned ZnO nanowire arrays, were successfully synthesized via an optimized self-seeding thermal evaporation method. Influence of purified air as an oxygen source on UV emission and detection characteristics of as-grown nanostructures was thoroughly investigated. As-grown nanowires exhibited superior UV emission and UV detection properties, compared to that of other nanostructures. Next, the influence of geometry and positioning of the sensing medium on the resonant characteristics of the composite resonator was systematically investigated using finite element model (FEM) simulation and Laser Doppler Vibrometry (LDV). Significant enhancements in sensitivity and harvested energy were observed when ZnO micro-pillars of resonant heights were placed in areas of maximum displacement on the crystal surface. Based on these findings, a novel methodology was developed for low-temperature and controlled synthesis of ZnO nanostructures on specific areas of LCM. Zinc vapor trapping and two-stage temperature ramping processes were employed to achieve a catalyst-free, self-seeding growth of ZnO nanowires and other nanostructures at growth temperatures below 600 °C. Excellent UV sensing performance was observed for each fabricated sensor, including fast response and recovery times. Lastly, a novel, low-power UV sensor instrumentation was developed using a single-shot pulse excitation, which allowed for simultaneous measurement of photoelectric and piezoelectric activities in the sensor. LDV was utilized to correlate the effect of UV illumination on the acoustoelectric properties of ZnO coated LCM sensor. An equivalent circuit model was proposed to represent the observed phenomenon under the influence of UV illumination. Findings presented in this work open new avenues for TSM based sensing applications and pave the way towards development of low-power resonant UV sensors with superior sensing performance.