## 3 Results and Discussion
All DLC thin films in this study exhibited high contact angle values, which is highly likely to be applied to a functional coating for surface protection and self-cleaning. The results of the Raman spectrum are presented in Figure 4, including the G-peak position and the ratio of the integrated areas under the D- and G-peaks (I\({}_{D}\)/I\({}_{G}\)) in the RF-PECVD DLC thin films as measured with a change in thickness. The Raman spectrum shows the typical spectrum of the DLC thin films with an almost symmetrical carbon peak  
Figure 3: Contact angle (**a**) images and (**b**) values of the RF-PECVD DLC thin films with various film thicknesses: 20, 40, 60, and 80 nm. Figure 2: (**a**) Hardness and elastic modulus, and (**b**) friction coefficient in the RF-PECVD DLC thin films with various film thicknesses: 20, 40, 60, and 80 nm. at approximately 1560 cm\({}^{-1}\) and a silicon substrate peak at approximately 950 cm\({}^{-1}\), with a weak shoulder at approximately 1370 cm\({}^{-1}\) related to the disorder. The strength of the Raman peak increased slightly with an increase in the thickness of the DLC thin films, and Gaussian curve fitting for the Raman spectrum resulted in D (disordered) and G (graphitic) bands near 1370 and 1560 cm\({}^{-1}\), respectively. As shown by the results of Gaussian curve fitting for the Raman spectrum (Figure S1), the G-peak shifted from 1558 to 1564 cm\({}^{-1}\), and the I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio decreased from 0.290 to 0.148, with an increase in thickness. The shift of the G-peak toward a relatively higher wavenumber and decrease in the I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio indicates a decrease in the \(sp^{3}\) bonds in the DLC thin films, which is related to the density of the DLC thin film caused by the collision effect of carbon and hydrogen ions on the substrate surface with an increase in deposition time, a decrease in carbon \(sp^{3}\) bonds due to a decrease in hydrogen ions, and an increase in the surface temperature of the thin-film surface [6, 7]. In other words, the increase in deposition time brought the \(sp^{3}\) bonding fraction in the DLC thin films to further reduce content of hydrogen as the surface temperature increases due to active binding on the thin-film surface and activation of the increased ions [6, 7, 8]. [PERSON] et al. reported that the disorder in the thin film increases as the energy of carbon ions exposed to plasma increases, and the hydrogen content in the thin film decreases the number of \(sp^{3}\) bonds with hydrogen in the thin film; these affected the tribological and optical properties of the DLC thin films with increases in both intensity and full width at half maximum (FWHM) of the G-peak in the Raman analysis. Figure 5 shows the optical properties, such as transmittance, reflectance, refractive index, and absorption coefficient, measured with an increase in the thickness of the RF-PECVD DLC thin films. The transmittance of the DLC thin films tended to decrease as the thickness increased, and the transmittance of the 550 nm visible light decreased from 69% to 58%. The refractive index and absorption coefficient of the DLC thin films increased from 1.400 to 2.124 (633 nm) and from 0.010 to 0.014, respectively, with an increase in thickness.