High-quality solution-based Co and Cu co-doped ZnO nanocrystalline thin films: Comparison of the effects of air and argon annealing environments

Abstract

Solution-based high-quality Co and Cu co-doped ZnO thin films that were annealed in air and argon were investigated in terms of their structural, optical and magnetic properties. Compared with argon-annealed thin films, the air-annealed thin films exhibited a single phase ZnO-like wurtzite structure without any secondary phases and higher crystalline quality. Surface analyses revealed denser, smoother and much more homogeneous film surfaces with larger grain size in samples that were sintered in air as compared to those sintered in argon. The incorporation of Co2+ and Cu2+ ions into the ZnO lattice at the Zn2+ site and the presence of Zn, Co, Cu and O atoms were confirmed for both sintering atmospheres. Optical measurements indicated that increased Cu doping levels decreased the band gap energy of the Co-doped ZnO thin films for both the annealing environments. Ultra-violet emission bands at approximately 389–398 nm (i), violet emission bands at approximately 417–424 nm (ii), blue emission bands that are centred at 456 nm (iii) and green emission bands that are centred at 523 nm (iv) were observed. In addition, sharply decreased UV emission and upsurged green emission were detected with increased Cu doping level in Co-doped ZnO, as well as with shifting annealing atmosphere from air to argon. Magnetic measurements indicated that the ferromagnetic response of the Co and Cu co-doped ZnO thin films was stronger in samples that were sintered in argon those than in air due to the enhanced oxygen vacancies and increased Cu doping level of up to 3%.