## 3 Results and Discussion  
Figure 1 depicts the C1s XPS spectra of the GO layer on the SiO\({}_{2}\)/Pt structure. The peaks of sp\({}^{2}\) (aliphatic, C-C), sp\({}^{3}\) (aromatic, C-C), hydroxyl (C-OH)/epoxy (C-O-C), carbonyl (C=O), and carboxyl (O=C-OH) were observed at 284.52, 285.08, 286.39, 287.06, and 288.57 eV, respectively [21]. Therefore, a GO layer with many oxygen-related groups was deposited effectively. Figure 2a depicts the XPS depth profile of the GO sample. The carbon peak indicates that the GO layer was between the Cu and SiO\({}_{2}\) layers. Figure 2b depicts the C1s XPS spectra of the GO layer between the Cu and SiO\({}_{2}\) layers. The proportions of C-containing components calculated from the XPS spectra of Figures 1 and 2b are listed in Table 1. After the Cu electrode deposition, the number of oxygen functional groups within the GO layer decreased. The reduction process of reduced graphene oxide can be achieved through chemical, thermal, or electrochemical reduction [21] because the oxygen functional groups within the GO layer are unstable. Therefore, the oxygen functional groups within the GO layer would react with the Cu electrode due to chemical reduction or thermal reduction during the Cu deposition process. Hence, some oxygen functional groups reacted with the Cu electrode to form the Cu\({}_{x}\)O interface. Figure 1: X-ray photoelectron spectroscopy (XPS) spectra of the graphene oxide (GO) layer on the SiO\({}_{2}\)/Pt structure. Figure 2: (a) XPS depth profile of the Cu/GO/SiO\({}_{2}\)/Pt structure. (b) C1s XPS spectra of the GO layer between the Cu and SiO\({}_{2}\) layers. The RS properties of the Cu/SiO\({}_{2}\)/Pt structure in the atmosphere were thoroughly investigated in our previous study [9], and the resistance of the Cu/SiO\({}_{2}\)/Pt structure can effectively be reversibly switched between an LRS and an HRS by bipolar applied voltages. However, Figure 3a indicates that the control sample cannot be reversibly switched by applied voltages in the N\({}_{2}\) ambient. Initially, the current of the device increased as the applied voltage increased. When the applied voltage was increased to 6 V, the current increased abruptly and the resistance state was switched from the initial-resistance state (IRS) to the LRS, known as the forming process. To prevent permanent damage, a current compliance of 1 mA was used. The applied voltage was then swept in the negative polarity to switch the resistance state to an HRS. However, resistance states cannot be switched back to the HRS. Figure 3b shows that RS properties of the GO sample can be found in the N\({}_{2}\) ambient. The RS was initialized by the forming process, and the resistance state switched from the IRS to the LRS. The resistance state was then switched from the LRS to the HRS using a negative voltage, and this was called the \"Reset\" process. Subsequently, the resistance state could be switched from the HRS to the LRS by a positive voltage, which was called the \"Set\" process. The resistance state could be reversibly switched between the HRS and the LRS by the Reset and the Set processes. For the two samples, the forming processes were quite different. Due to the dissolution of Cu ions into the SiO\({}_{2}\) layer of the GO sample, more defects within the SiO\({}_{2}\) layer caused a larger leakage current. Therefore, the IRS resistance of the GO sample was lower than that of the control sample. In addition, the forming voltage of the control sample was approximately 6 V, whereas that of the GO sample was approximately 2.2 V. For the GO sample, Cu ions were dissolved into the SiO\({}_{2}\) layer, and thus only 2.2 V was initially required to form Cu conducting filaments. For the control sample, the abruptly increased current of 6 V was due to the dielectric breakdown and not to the connection between the top and bottom electrodes by Cu conducting paths. Therefore, after the forming process, the resistance state of the control sample could not be switched back to the HRS, which was unsuitable for RRAM applications.