# Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics
## 3 Results and Discussion  
### Characterization of the TiO\({}_{2}\)-Functionalized Mica Substrates  
The TiO\({}_{2}\)-functionalized mica substrates were characterized using XRD, with a representative diffraction pattern shown in Figure 4a. The XRD patterns of the pristine and TiO\({}_{2}\)-coated substrates, both before and after Ag activation at 2.5 J cm\({}^{-2}\), were identical. These patterns were dominated by strong reflections from the mica substrate, observed at 17.8\({}^{\circ}\), 26.9\({}^{\circ}\), 36\({}^{\circ}\), and 45.5\({}^{\circ}\) 26, corresponding to the (002), (004), (008), and (0010) planes of Muscovite 2M\({}_{1}\) (KAl\({}_{2}\)(Si,Al)\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\), PDF#00-058-2034) [33; 34]. The intense reflections from the substrate made the detection of the functional TiO\({}_{2}\) layer impossible via XRD. Therefore, Raman spectroscopy was employed to investigate the phase composition of the TiO\({}_{2}\) layer. Figure 4b displays Raman scattering spectra for the starting mica substrate, the TiO\({}_{2}\)/mica functionalized substrate, and the TiO\({}_{2}\)/mica substrate after Ag photodeposition. The Raman spectrum of the mica substrate is characterized by distinct bands in the low-frequency region at 267 cm\({}^{-1}\), 413 cm\({}^{-1}\), and 707 cm\({}^{-1}\), corresponding to the lattice O-H-O + K vibrations, the Al-O modes of AlO\({}_{6}\) octahedra, and the Si-O modes of the SiO\({}_{4}\) tetrahedra, respectively [35]. Additionally, a pronounced asymmetric peak in the high-frequency hydroxyl region centered at 3620 cm\({}^{-1}\), with a shoulder at 3650 cm\({}^{-1}\), is attributed to Si-OH modes, consistent with muscovite in agreement with the XRD evidence [36]. Figure 3: Schematic representation of the temperature-controlled heater setup used in RTD-property determination experiments. The TiO\({}_{2}\) functionalization is evident in the Raman spectrum of the TiO\({}_{2}\)/mica sample, which shows the appearance of the E\({}_{\text{g}}\) (146 cm\({}^{-1}\), 640 cm\({}^{-1}\)), B\({}_{\text{1g}}\) (398 cm\({}^{-1}\)), and A\({}_{\text{1g}}\) (519 cm\({}^{-1}\)) modes associated with anatase TiO\({}_{2}\)[37]. Although the XRD patterns did not change after Ag photodeposition, the Raman spectra did. As shown in Figure 4b, two main changes occurred. First, there was a strong suppression of the anatase A\({}_{\text{1g}}\) and B\({}_{\text{1g}}\) modes, which became broader and weaker, indicative of a significant electronic interaction between the photodeposited Ag and the TiO\({}_{2}\) surface. This effect is commonly observed in photodeposited metals [38] and is likely due to the sensitivity of these modes to surface changes and stress [39]. Additionally, the Ag/TiO\({}_{2}\)/mica spectrum exhibited new spectral features, including broad peaks centered around 750 cm\({}^{-1}\), 1300 cm\({}^{-1}\), and 1600 cm\({}^{-1}\). These peaks, which have been observed in the Raman spectra of Ag nanoparticles, can be attributed to various \(\delta\)COO\({}^{-}\), \(\
u\)C-O, and \(\
u\)COO\({}^{-}\)/\(\
u\)C=O modes of surface-bound carbonaceous species, coming from exposure to air, intensified via surface-enhanced Raman scattering (SERS) effects caused by the Ag clusters [40,41]. The optical transparency of the mica substrate, which may be advantageous for applications such as photodetector fabrication, was assessed using UV/Vis transmittance spectra (Figure 4c). The pristine muscovite substrate exhibited high transparency across the entire spectral range, with only a 10% reduction after TiO\({}_{2}\) functionalization, most pronounced at \(\lambda<\) 380 nm due to absorption below the TiO\({}_{2}\) bandgap. The Ag/TiO\({}_{2}\)/mica sample activated for Cu-ELD patterning became noticeably darker due to the presence of finely dispersed Ag particles, as seen in Figure 4c.