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.