Zinc oxide (ZnO) nanowire (NWs) are promising materials for nanoelectromechanical systems (NEMS) and piezoelectric energy harvesting due to their coupled piezoelectric, semiconducting nature and compatibility with low temperature synthesis methods. Theoretically, ultra-thin ZnO NWs are expected to show a substantial increase in the piezoelectric response, but their experimental fabrication and integration into functional devices still present difficulties. This thesis explores the effects of NW geometry, d efect chemistry and device architecture on the electromechanical response of hydrothermally grown ZnO NWs. Two device architectures are developed for advanced nanoscale characterization, including individual laterally contacted NW devices and vertically integrated Piezoelectric energy generators (PEGs). We report the first successful integration of electrically functional devices

and (PEGs) with vertically aligned ultra-thin ZnO NWs of diameter about 20 nm for the experimental study of their piezoelectric behaviour. The systematic post-growth treatments like oxygen annealing, vacuum annealing and oxygen plasma exposure are investigated by XRD, FESEM, PL, XPS, KPFM, s-MIM and PFM. An oxygen annealing at 500° C showed that the free-carrier screening is reduced and increases the effective piezoelectric coefficient from 5.3 pm/V to 6.2 pm/V, a 17% enhancement, while , preserving the NW morphology. Combining the size reduction of ZnO NWs to ultra-thin with oxygen annealing result in a synergistic enhancement of the piezoelectric performance. The oxygen annealed PEGs fabricated from 20-nm-diameter and 1-μm-long ZnO NWs showed a maximum output voltage of 134 mV under the applied force of at 1N, which is an enhancement of about 306% compared to the devices based on the thicker (~80 nm) NWs which was 33mV. The results directly link the dimensions, defect chemistry, and electromechanical response, showing that combining diameter engineering and defect engineering is an effective strategy for optimizing ZnO-basedpiezoelectric devices for future self-powered sensors and energy harvesting applications.

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