ZnO nanowires for energy harvesting and gas sensing applications: a quantum mechanical study

The research activity related to my PhD project is focused on providing a better understanding on energy harvesting capabilities and gas sensing mechanism of ZnO nanowires. Nanowires made of materials with non-centrosymmetric crystal structures are expected to be ideal building blocks for self-powered nanodevices due to their piezoelectric properties, yet a controversial explanation of the effective operational mechanisms and size effects still delays their real exploitation. To solve this controversy, we propose a methodology based on Density Functional Theory (DFT) calculations of the response of nanostructures to external deformations that allows us to distinguish between the different (bulk and surface) contributions: we apply this scheme to evaluate the piezoelectric properties of ZnO [0001] nanowires, with a diameter up to 2.3 nm. Our unified approach allows for a proper definition of piezoelectric coefficients for nanostructures, and explains in a rigorous way the reason why nanowires are found to be more sensitive to mechanical deformation than the corresponding bulk material. Gas-sensing mechanism of ZnO nanowire is investigated using ethanol as our prototype gas. In particular, we show that in the case of ethanol, it has larger binding energy to the ZnO surface compared to oxygen gas, hence able to remove pre-adsorbed oxygen molecules on the surface, and leads to release of trapped electrons to conduction band. Therefore, ZnO sensing is strongly linked to oxygen removal from the surface. Furthermore, in this work oxygen vacancies distribution and concentration in ZnO nanostructures, which is still a question of debate is investigated using combined DFT and Climbing-Image Nudged Elastic Bands (CI-NEB) approach. This work has successfully addressed some of the unanswered questions related to application of ZnO nanowires in field of energy harvesting and gas sensing, and may invaluable in fine-tuning nano-devices to attain enhance performance.