## 3 Results and Discussion
### Microstructure Analysis  
Raman spectroscopy serves as a non-destructive and effective method for obtaining structural information and detecting defects in N-DLC thin films. As described in [PERSON]'s article, through Raman spectroscopy, we can obtain a lot of information such as D and G peaks as well as the ratio of sp\({}^{2}\) to sp\({}^{3}\)[27]. Figure 6a displays the Raman spectra of N-DLC thin films grown under different gas flow ratios. The characteristic peaks of DLC films are observed around 1500 cm\({}^{-1}\). In typical Raman spectroscopy studies, two prominent peaks, denoted as the D peak at approximately 1350 cm\({}^{-1}\) and the G peak at around 1580 cm\({}^{-1}\)[28], are observed. We performed Gaussian-Lorentzian function deconvolution on the Raman curves [29], as depicted in Figure 6b. In this study, in order to make the fitting results more accurate, we chose four specific bands to obtain more accurate fitting results, but the discussion of N-DLC films mainly focuses on the D and G peaks. Regarding the Raman spectra of N-DLC films, the D peak is associated with the breathing mode of sp\({}^{2}\) sites, while the G peak is linked to the stretching mode of sp\({}^{2}\) sites. From the fitting results in Figure 6c, it can be observed that the width of the G peak initially decreases slightly with increasing nitrogen flow ratio. When the nitrogen flow reaches 20%, the G peak width reaches its maximum and subsequently decreases with further nitrogen flow incrementation. The broadening of the G peak is attributed to the increase in sp\({}^{3}\) content within the films [30]. The trend in G peak width variations indicates that the introduction of a certain amount of nitrogen during DLC fabrication can enhance the sp\({}^{3}\) content in the films. Previous studies have established an inverse correlation between the C sp\({}^{3}\) fraction in the films and the ID/IG ratio [17], where ID/IG represents the intensity ratio of the D peak to that of the G peak. In this study, the ratio of the fitted peak heights was utilized to reflect the peak intensity. Moreover, research by [PERSON] has demonstrated that in N-DLC films, the sp\({}^{3}\) phase not only originates from C sp\({}^{3}\) but also, within the low nitrogen doping range, the addition of nitrogen atoms leads to an increase in the N sp\({}^{3}\) fraction [28]. The nitrogen plasma's influence prompts N atoms to replace H in C-H bonds, consequently forming C-N bonds [31]. As the methane flow decreases in the introduced gas and the nitrogen flow increases, the rise in ID/IG values signifies a lower sp\({}^{3}\) content in the layers [32]. Figure 5: Variation in N-DLC film surface roughness with gas flow ratio. In order to comprehensively investigate the microstructural characteristics of the films, XPS measurements were employed in this study. As shown in Table 2, with an increase in the nitrogen-to-methane gas flow ratio, the relative content of nitrogen measured by XPS also increases, indicating that nitrogen does indeed participate in the carbon network and reacts with it. Lorentzian-Gaussian fitting was performed on the C1s spectra of non-nitrogen-doped DLC films to obtain four different peaks. As depicted in Figure 7a, the peaks at 284.8 eV and 285.08 eV are attributed to the C=C (sp\({}^{2}\)-C) and C-C (sp\({}^{3}\)-C) bonds, respectively, while the peaks at 286.47 eV and 288.23 eV correspond to C-O and C=O bonds [28], respectively. For the N-DLC films, Figure 7b-e displays the deconvoluted spectra. The peak at 287.08 eV corresponds to the C-N bond position [33], while the peak at 286.4 eV corresponds to the C-O/C=N position [34]. To gain further insights into the carbon and nitrogen bonding structure, peak fitting was carried out on the N1s spectrum of the C1N4 sample as a representative, as illustrated in Figure 7f. According to Table 2 and Figure 7f, nitrogen and carbon can form C-N and C=N bonds. The sp\({}^{3}\) phase is most abundant in the film with an elemental nitrogen content of 2.04%.