Copper oxide nanocrystallites fabricated by thermal oxidation of pre-sputtered copper films at different temperatures and under oxygen and argon flows

Copper oxide nanocrystallites were synthesized from Cu thin films via controlled thermal treatment under O2 and Ar flows. The nanocrystallites were synthesized at 350, 400 and 500 °C. XRD revealed that substoichiometric copper oxides such as Cu4O3 and Cu64O were emerged with Cu and CuO for samples treated at 350 °C and 450 °C, respectively, whereas single monoclinic CuO was emerged for the samples treated at 500 °C. EDAX quantitative analysis confirmed the presence of both Cu and oxygen in the films with slightly excess oxygen for films treated at 400 and 500 °C. SEM examinations confirmed the nanocrystallite morphologies for the examined samples and that more coalescent nanocrystallites, with diameters in the range 42-75 nm, were obtained for the samples treated at 500 °C. With increasing treatment temperature from 350 to 500 °C, the films vary from highly reflecting reddish brown color to highly transparent. The estimated band gap values for the mixed copper oxide phases, samples treated at 450 °C, and pure CuO phase were 1.27 eV and 2.00 eV, respectively. High refractive index and moderate extinction coefficient values were calculated for the samples containing mixed phases which may find new application in optoelectronics. The refractive index values for pure CuO were matched with the previously reported values. The samples treated at 350 °C had very low resistivity and metallic behavior, whereas a semiconducting behavior was observed for the samples treated at 400 and 500 °C.