# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 2 Materials and Methods  
Aluminum nitrides were fabricated by means of sputtering deposition, using the proprietary multi-cathode sputtering custom built apparatus, named ENEA2, equipped with a process chamber and a load lock chamber. ENEA2 is the apparatus by which key technologies in the field of selective thin-film solar coatings for receiver tubes have been developed, patented, and transferred to the international industry [20]. The mirror substrate was positioned in front of the target in a planar configuration, and the fabrication process was reactive sputtering at an Ar + N\({}_{2}\) pressure of 3 Pa with power supplied to the Al cathode of 1800 W for the auxetic phase and a pressure of 0.3 Pa with power supplied to the target of 3600 W for the piezoelectric phase. Zinc oxide-based nanocomposites were fabricated by means of a solid-state reaction between thin films of ZnO and silicon obtained by means of a planar magnetron multi-cathodesputtering apparatus by Material Research Corporation Mod. MRC 643 (Orangeburg, NY, USA). Boron-doped Si and undoped ZnO sputter targets (dimensions: H = 38 cm and L = 12 cm) were used coupled with a bi-polar pulsed DC and an RF generator, respectively. Sputtering processes were carried out at pressures of 2.0 Pa, starting from a base vacuum of about 2 \(\times\) 10\({}^{-4}\) Pa. The solid-state reaction was carried out in a vacuum at temperatures between 450 \({}^{\circ}\)C and 560 \({}^{\circ}\)C, with reaction times ranging from 3 to 6 h for conductive samples containing ZnO micro- or nano-domains, respectively. Solar mirrors substrates were purchased by Societa Vetraria Biancaldese Spa, Biancalde, Italy. UV-VIS-NIR analysis was performed using a double-beam PerkinElmer mod (PerkinElmer, Springfield, IL, USA). Lambda 900 instrument, equipped with a 15 cm integrating sphere to measure global spectral reflectance and transmittance. The optical bandgap of phases constituting different composites was defined by UV-VIS-NIR reflectance and transmittance measurements, by means of [PERSON]'s plot method [21], assuming a direct transition for ZnO and silicates. The assumption of a direct transition for ZnO and zinc silicates is based on the known electronic structure and optical properties of these materials. ZnO, being a wide-bandgap semiconductor, exhibits a direct bandgap, which allows for the efficient absorption of light and direct electronic transitions. Similarly, zinc silicates are considered to follow a direct bandgap transition due to their crystalline structure and similar electronic characteristics to ZnO. This assumption is supported by previous studies, where direct transitions have been observed in these materials through optical characterization techniques [22,23]. The thickness of each sample was measured mechanically using a Tencor P-10 surface profiler (Tencor, Milpitas, CA, USA). The Static Water Contact Angle (WCA) was measured with the direct optical method of drop-shape analysis, by means of the contact angle meter KRUSS DSA-100 (KRUSS, Hamburg, Germany). For testing electrical properties of different materials, two main configurations were utilized: planar and vertical. The first is achieved by means of electrical aluminum contact evaporation onto the film (with a known and regular geometry) and the second is achieved by interposing the film between a metallic sputtered thin W layer and an evaporated metallic contact, as shown in Figure 1.  
For W deposition, the chamber was filled with 200 sccm of Argon at a pressure of 0.4 Pa. The DC power applied was 2500 W, the speed of translation was 500 mm/min, and the number of scans was 4.