Figure 1. XRD patterns for the standard values of JCPDS [Nos. 05–0667 and 45–0937] and granular copper oxide films obtained by sputtering Cu at room temperature with different oxygen flow rates of 20% and 30%.

Afterwards, the copper oxides prepared at the 20% and 30% oxygen flow rates are referred to as Cu₂O and CuO, respectively. The accurate chemical composition of the films is summarized in Supplementary Table 1.