Nitrogen plasma irradiation of Fe doped ZnO nanowire arrays for improved optical properties

Abstract

This work reports on the ability to improve the optical properties of hydrothermally grown, vertically-aligned arrays of wurzite ZnO and Fe doped ZnO nanowires by means of nitrogen doping during high-powered plasma irradiation. The morphology, structural and optical properties of ZnO, Fe:ZnO NWs and N2 – Fe:ZnO NWs are studied by scanning electron microscopy, grazing incidence X-ray diffraction and UV–visible spectroscopy, respectively, whereas elemental doping of the nanostructures are investigated by means of X-ray photo-electron spectroscopy. SEM micrographs show no discernible morphological differences between the ZnO, Fe:ZnO NWs and N2-Fe:ZnO NWs, however a clear increase in light transmission over the visible to UV range, accompanied by a decrease in optical band gap of 3.261 eV for ZnO NWs to 3.255 eV for Fe:ZnO NWs and ultimately 3.250 eV for N2-Fe:ZnO nanowires is observed. This band gap reduction is ascribed to the presence of a high density of shallow donor states in the bandgap of the ZnO lattice, subsequently causing an overall reshape of the conduction band minimum, caused by filling of oxygen vacancies and substitutional replacement of lattice oxygen by the highly kinetic α-N species during oxynitride, O-Zn-N bond formation. The results suggest that nitrogen doping during plasma irradiation provides a controlled method of deep lattice nitrogen doping that is favorable for improved photo-electrochemical, photovoltaic and photo-response applications of ZnO nanowires.