## 3 Results
The PTFE RF thin film exhibited excellent water repellency owing to its low surface energy. Figure 4 shows the water contact angle value of each sample for transmittance at 500 nm. The pure PTFE, Cu 20 RF, and Cu 50 RF thin films showed almost similar transmittance, while their water contact angle decreased proportionally with increasing Cu content. This was because the amount of Cu contained in the thin film was so small that it did not affect the transmittance; however, the Cu was distributed on the surface and therefore affected the surface energy. For different power sources, the water contact angle decreased in the order Cu 80 RF > Cu 80 MF > Cu 80 DC, in the same order as their transmittance. Table 1 summarizes the water contact angle, sheet resistance, transmittance at 500 nm, and b\({}^{\star}\) (yellow index) value of each sample. Figure 3: (**a**) Transmittance spectra in the wavelength range 300–2400 nm according to Cu content, (**b**) transmittance spectra according to power sources using the Cu 80 target, (**c**) absorption spectra according to Cu content and power sources, and (**d**) photographs of the Cu/PPFC nanocomposite thin films placed on a LED lamp. Figure 5a shows the XRD patterns of the Cu/PPFC nanocomposite thin films prepared by RF sputtering. The XRD pattern of the PTFE RF thin film shows a broad peak around 20 = 23\({}^{\circ}\), implying that the thin film is amorphous. Cu 20 RF also exhibited an amorphous state, indicating that the Cu content is randomly distributed in the polymer matrix without forming clusters. The XRD patterns of the Cu 50 RF thin films showed Bragg peaks attributed to the (200) crystal face of Cu at 20 \(\approx\) 50.6\({}^{\circ}\). These results are clear evidence that the Cu/PPFC nanocomposite is well formed. The Cu 80 RF thin films showed Bragg peaks due to CuF\({}_{2}\) and Cu\({}_{2}\)O crystals. Considering that the Cu/PPFC nanocomposite thin film was fabricated in an argon atmosphere under vacuum, it is considered that the Cu\({}_{2}\)O crystal peaks were due to the oxidation of Cu when the thin film was exposed to air. Unexpectedly, Cu crystalline peaks were not observed in the Cu 80 RF thin film. Figure 5b shows the XRD patterns of the Cu 80 RF, MF, and DC nanocomposite thin films. Unlike the Cu 80 RF thin films, which only exhibit CuF\({}_{2}\) and Cu\({}_{2}\)O crystal phases, the XRD patterns of the Cu/PPFC nanocomposite thin films prepared by DC and MF sputtering show a Cu cubic crystal phase at 20 \(\approx\) 40.5\({}^{\circ}\), Cu\({}_{2}\)O cubic crystal phase at 20 \(\approx\) 36.6\({}^{\circ}\), and CuCO\({}_{3}\) monoclinic crystal phase at 20 \(\approx\) 25.8\({}^{\circ}\)[32, 33]. The fraction of the Cu crystal phase calculated from the XRD patterns of the Cu 80 MF and Cu 80 DC thin films was 14.2% and 21.1%, respectively, whereas that in the Cu 80 DC thin films showed larger values. The crystal size of the Cu 80 MF and Cu 80 DC thin films, as calculated by the Debye-Scherrer equation (D = 0.916\(\AA/\beta\)cos \(\theta\), D: Crystalline size, \(\lambda\): X-ray wavelength, \(\beta\): Full width half-maximum (FWHM) of Bragg peak, \(\theta\): Scattering angle), was about 5 nm [32].