# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 3 Results and Discussion
The fabrication of these composites was achieved through a solid-state reaction occurring at 450-560 \({}^{\circ}\)C between sputtered ultra-thin layers of silicon and zinc oxide. This process led to the formation of ZnO crystalline domains of different sizes within a transparent silicate matrix, which also provided self-cleaning capabilities. Table 2 reports the defined experimental conditions and wetting contact angle (WCA) of ZnO composite self-cleaning coatings. Before the reaction, the samples appeared opaque due to the stratification of the reagents. However, after undergoing the solid-state reaction (an annealing process at temperatures ranging from 450 to 560 \({}^{\circ}\)C), optical transmittance in the desired solar range was achieved. In Figure 5, the optical transmittance of sample C 1-4 is reported. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Power ZnO Target** **(W)**} & \multirow{2}{*}{**Power Si Target** **(W)**} & \multirow{2}{*}{**Pressure (Pa)**} & \multicolumn{2}{c}{**Reaction** **Temperature** **(\({}^{\circ}\)C)**} & \multirow{2}{*}{**Thickness** **[nm]**} & \multirow{2}{*}{**WCA** **(\({}^{\circ}\))**} \\  & & & & & & **[nm]** \\ \hline C 1-4 & 1000 & 400 & 0.5 & 560 & 310 & 94.4 \(\pm\) 0.7 \\ \hline C 85-4 & 850 & 400 & 0.5 & 560 & 300 & 94.4 \(\pm\) 0.7 \\ \hline C 85-45 & 850 & 450 & 2.0 & 560 & 320 & 93.4 \(\pm\) 1.2 \\ \hline C 85-45 b & 850 & 450 & 2.0 & 450 & 280 & 96.8 \(\pm\) 1.1 \\ \hline \end{tabular}
\end{table} 
Table 2: Experimental conditions and WCA of different composites. Figure 5: UV-VIS-NIR transmittance of sample C 1-4. TEM investigations, reported in Figure 6, confirm the presence of crystalline ZnO domains embedded in amorphous silicates. Alternate regions appeared in terms of different brightness, and by energy electron loss spectroscopy (EELS) we verified they are correlated to the sheet silicate layer's structure consisting of distorted tetrahedron SiO\({}_{4}\) sheets (clear regions) and distorted octahedron MO\({}_{6}\) sheets (dark regions), where M = Zn. Inside MO\({}_{6}\) layers, two crystalline ZnO nanoaggregates appeared whose interplanar spacing gave a larger average value (2.57 A) for both nanoaggregates, very close to the value (2.603 A) of the lattice parameter in the (002) direction of the ZnO crystalline structure (wurzite). Electrical characterization was performed to measure lateral and vertical resistivity in the configuration previously described and is reported in Table 3.  
Low values for lateral resistivity (samples C 1-4, C 85-4, C 85-45, and C 85-45b) could be attributed to the presence of unreacted ZnO thin layers (crystalline phases with optical band gap determined by means of [PERSON]'s plot method are in the range 3.2-3.4 eV) giving a quasi-metallic transport mechanism. Regarding vertical resistivity, all the samples show middle-high values that can be justified in terms of larger/smaller interposed silicate layer thicknesses (or in terms of larger/smaller vertical spacing between ZnO domains). Figure 6: TEM analysis of sample C1-4. (**a**) Cross-section images EFTEM, (**b**) EELS, (**c**) HRTEM, and (**d**) HRTEM ZnO nano-crystallites. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\rho\parallel[\Omega\)**cm] & \(\rho\perp[\Omega\)**cm] & **c-ZnO** \\ \hline C 1-4 & 16 & 4.7 \(\times\) 10\({}^{4}\) & present \\ \hline C 85-4 & 1.6 \(\times\) 10\({}^{8}\) & 1.4 \(\times\) 10\({}^{5}\) & not present \\ \hline C 85-45 & 2.6 \(\times\) 10\({}^{7}\) & 2.1 \(\times\) 10\({}^{4}\) & not present \\ \hline C 85-45b & 5 & 1.8 \(\times\) 10\({}^{3}\) & present \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Lateral \(\rho\parallel\) and vertical \(\rho\perp\) electrical resistivity values. In other terms, the observed electrical anisotropy is due to the presence of conductive p-type ZnO domains in the plane of the film layered with the insulating matrix in the vertical direction.