Surface modification and defect engineering of semiconductor zinc oxide nanocrystals

Abstract

Semiconductor zinc oxide (ZnO) nanocrystals have shown great potential in the fields of optoelectronics, piezoelectronics, photocatalysis, chemical sensors, and biomedical applications due to their exceptional electrical and optical properties. The electronic and plasmonic behaviors of ZnO nanocrystals are significantly influenced by the large amounts of surface states originating from defects, attached molecules, and organic groups on the nanocrystal surface. Regulating these surface states through surface modifications provides another way to tune the properties of ZnO nanocrystals. Apart from surface states, ZnO nanocrystals possess plenty of native defects such as oxygen vacancies, which makes them a natural n-type semiconductor. Defect engineering is capable of controlling the performance of ZnO nanocrystals as well. ZnO nanocrystals synthesized by non-thermal plasmas have monodisperse size distribution, controllable defects and doping, and network morphology, making them promising model systems for both fundamental and application studies. In this thesis, band gap and surface depletion layer tuning by surface modifications, improvement of photocatalytic activity by defect engineering, and N-doping in ZnO nanocrystals are studied. Specifically, a tunable band gap range from 3.38 eV to 3.53 eV was achieved in ZnO nanocrystals through surface modifications. The wider band gap allows higher transmission of photons in the ultraviolet range and improves ZnO performance as a transparent conductive oxide (TCO). The surface depletion layer of ZnO nanocrystals was shown to be fully reduced by applying UV irradiation and alumina coating deposited by atomic layer deposition (ALD). Also, by changing the oxygen flow rate during non-thermal plasma synthesis, whitish and yellowish ZnO was produced. The photocatalytic activity of whitish ZnO was found to be 1.9 times larger than that of yellowish ZnO, and this result was connected to defects in the ZnO. Lastly, by injecting nitrogen (N2) gas into plasma, N impurity was also introduced into ZnO nanocrystals, which may lead to new strategies for the production of p-type doped ZnO. The surface modification and defect engineering techniques explored here have promise for application to other semiconductor metal oxide nanocrystals.