# Femtosecond Laser Textured Surfaces for Radiative Cooling: Black Metals
## 3 Results and Discussion
### Surface Chemistry on Surfaces Ablated at Tens-of-TW/cm\({}^{2}\) Fluence  
Figure 6 shows XPS spectra at the metal-specific binding energy window, which was analyzed by peak fitting and area comparison for quantitative analysis. Tables 1-3 summarize the quantitative XPS results and reveal intricate changes in chemical bonding on the surface of tungsten, aluminum, and copper, respectively, before and after laser ablation. The tungsten sample showed a notable increase in metal oxidation with WO\({}_{3}\) bonding increasing from 30.22% (W - no ablation) to 91.22% (W - 4); the oxidation is proportional to the amount of energy deposited on the surface (Table 1). This indicates that oxidation was taking place at high-temperature conditions, which is required for the inert metal W. Fs-laser pulses engraved nanostructures, such as protrusions and ripples on W-metal surfaces, which can be oxidized at elevated temperatures in air [40]. The equation-of-state (EoS) of W from cryogenic to melting temperatures has been recently proposed [41] and can predict the high-temperature and high-pressure response of W.  
The ablated aluminum surface showed an increased percentage of Al-O bonding, which is consistent with previous findings [30; 42]. The Al-O bonding increased from 61.46% to 76.80% while the Al-OH bonding dropped from 18.01% on the pristine Al surface to 8.84% on the ablated surface (Table 2). Apparently, the top layer of hydroxide was removed by ablation, and oxidation was prevalent on a nano-micro rough surface with an Al\({}_{2}\)O\({}_{3}\) layer well in line with the earlier study [30]. The oxidation might be influenced by two possible mechanisms. One is the oxidation of aluminum due to the sample's exposure to air after laser ablation. The other is the interaction between aluminum ions in plasma and oxygen with re-deposition onto the sample surface. A very fast oxidation/passivation process of Al takes place at the \(\sim\)1050-1150 \({}^{\circ}\)C window, but a distinct incubation time was required for oxidation above \(\sim\)850 \({}^{\circ}\)C as found in a recent study by differential scanning calorimetry [43]. When Al was preheated to high temperatures with oxygen, the oxidation progressed with the expansion and growth of the material, thus different granular structures were formed with thicknesses up to micrometers [43]. The evaporation of Al and initial mass loss in Ar at high temperatures was proportional to the Al vapor pressure. Figure 6: XPS analysis of pristine W, Al, Cu coupons and laser patterned at different conditions (see Figure 1 for marking). The copper sample shows a strong Cu\({}^{2+}\) satellite peak around 943 eV, indicating that the oxidation form is the cuprix Cu(II) oxide CuO. The oxidation state of copper was found to be dependent on the ablation conditions, e.g., CuO is formed at a lower beam velocity of 20 mm/s, while Cu\({}_{2}\)O tends to form at a higher velocity of 100 mm/s [36]. Similar to the Al surface, laser ablation changed the surface composition of the copper compound, with CuO increasing from 27.57% to 58.80% and Cu(OH)\({}_{2}\) decreasing from 66.50% to 39.10% (Table 3). The oxidation of Cu under fs-laser ablation can explain the color (in visible light) appearance of surfaces due to the thin oxide layer [27]. Understanding the laser processing parameters that can control Cu oxide formation on laser-ablated ripples is important for photo-cathode applications [44]. High-intensity laser pulses trigger strong thermal gradients, enhancing the oxidation process [45] and metal compounds are shifted towards metal oxides. Oxides are thermodynamically stable and dominate over the formation of hydroxides, carbides, nitrides, and oxynitrides consistent with findings in this study (Tables 1-3). Metal oxides, such as Al\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), and CuO have a lower refractive index and contribute, in part, to the reduction in reflectance.