## 3 Results and Discussion
For further clarification, Raman mapping was performed over the interfacial positions of the copper-graphite interface equivalent to the graphite oxide, reduced graphite powder, and copper-graphite sintered body as clearly marked in Figure 5a. Five representative regions indicate \(X_{1}\) (\(\sim\)1 \(\upmu\)m), \(X_{2}\) (\(\sim\)6 \(\upmu\)m), \(X_{3}\) (\(\sim\)15 \(\upmu\)m), and \(X_{4}\) (-28 \(\upmu\)m), which corresponded to the distance from the composite interface, respectively. The \(D\) band indicates the presence of disordered carbons, whereas the \(G\) band indicates the graphitized carbon. Therefore, peak changes in \(D\) and \(G\) bands over five mapping regions reflect the existence probability of the oxygenic functional groups and disordered carbons on the CGC interface after the SPS process. Particularly, Raman spectra measured at \(X_{3}\) (\(\sim\)15 \(\upmu\)m) meant a lower presence probability of the oxygenic functional groups and disordered carbons. The \(D\) band indicates the presence of disordered carbons, whereas the \(G\) band indicates the graphitized carbon. Therefore, peak changes in \(D\) and \(G\) bands over five mapping regions reflect the existence probability of the oxygenic functional groups and disordered carbons on the CGC interface after the SPS process. Particularly, Raman spectra measured at \(X_{3}\) (\(\sim\)15 \(\upmu\)m) meant a lower presence probability of the oxygenic functional groups and disordered carbons on the CGC interface after the SPS process. Particularly, Raman spectra measured at \(X_{3}\) (\(\sim\)15 \(\upmu\)m) meant a lower presence probability of the oxygenic functional groups and disordered carbons on the CGC interface after the SPS process. Figure 4: Comparison of Raman spectra for the Cu-coated graphite powders, copper–graphite composite interface, and Cu surface. Figure 3: X-ray photoelectron spectroscopy of the interface between the copper and graphite composite. carbons compared with other three regions. This assumption could be reasonably rationalized by the fact that GO tends to be readily reduced to graphene-like sheets by getting rid of the oxygenic functional groups with the restoration of a \(\pi\)-conjugated structure. The Raman shift (cm\({}^{-1}\)) and intensity ratio (\(I_{\text{D}}\)/\(I_{\text{G1}}\)) were introduced to firmly confirm the presence probability of rGO at the composite interface as plotted in Figure 6. As a result, we concluded that rGO was formed at the copper-graphite interface [37; 38]. In the case of \(I_{\text{D}}\)/\(I_{\text{G1}}\), analysis of rGO in the copper-graphite interface could not be easily handled because of inconsistent of the values for graphite, GO, and rGO with the mapping results. Characteristic peaks in the Raman spectra varied as the GO underwent chemical reduction. The peak around 1581 cm\({}^{-1}\) was attributed to first-order scattering of phonons of \(sp2\) carbon atoms, which is generally labeled as the \(G\) peak; similarly,  
Figure 5: Raman mapping was performed over the interfacial positions of the copper–graphite interface equivalent to the graphite oxide, reduced graphite powder, and copper–graphite sintered body. (**a**) A cross-sectional view of the copper-graphite composite visualized by optical microscopy. Raman mapping results for five representative spots of the Cu-graphite composites interface (**b**) 1, (**c**) 6, (**d**) 15, and (**e**) 28 \(\upmu\)m from the composite interface. a breathing mode of k-point photons of symmetry around 1348 cm\({}^{-1}\) is referred to as the \(D\) peak. The \(I_{\rm D}\)/\(I_{\rm G1}\) ratio is designated as the degree of disorder, such as that due to defects, ripples, and edges. Figure 6 shows typical analytical data for graphite, GO, and the chemically rGO surface. The \(I_{\rm D}\)/\(I_{\rm G1}\) ratios were found to be 0.02, 0.7, and 0.87, respectively. The higher ratio of \(D\) to \(G\) bands strongly imply the higher presence probability of oxygenic functional groups and disordered carbons.