# Tunable Coloring via Post-Thermal Annealing of Laser-Processed Metal Surface
## 3 Results and Discussion
### Surface Chemical Compositions  
Table 1 shows the change in chemical element compositions of OL1 and OL2 before and after the annealing process at 500 \({}^{\circ}\)C for 1.5 h. Specifically for OL1, the atomic percentage of O rises from 24.0% to 70.0%, while that of Ti falls from 70.9% to 25.6%, indicating a significant increase in O content after annealing OL1, despite an evident change in color. Actually, the color hue of OL1 was not significantly changed after annealing. It is found that the increased ratio of O and Ti after annealing could indicate further oxidation as a result of thermal annealing. For OL2, the atomic percentage of O rises from 14.6% to 66.1%, while that of Ti falls from 79.3% to 29.4%. Before annealing, the ratios of O:Ti in both oxide layers are significantly less than 2, which means that the Ti surface has not been fully oxidized. Because the energy dispersive X-ray (EDX) detecting depth is around 100-1000 nm, while the thickness of the oxide is under 200 nm, this result is probably due to incomplete oxidization of the titanium surface or detection of the titanium substrate. However, the ratio increases above 2 after post-annealing, which means that the surface has been further oxidized and probably polluted with other oxides. Figure 3a,b shows the XPS spectra of OL1 and OL2 before and after thermal annealing. The Shirley function is applied for background correction. The peaks fit with Lorentzian-Gaussian. Figure 3a shows the Ti 2p\({}^{3/2}\) and Ti 2p\({}^{1/2}\) peaks at binding energies of 456.9 eV and 462.4 eV, respectively, for the Ti substrate. OL1\({}_{\text{Ref}}\) shows the Ti 2p\({}^{3/2}\) and Ti 2p\({}^{1/2}\) peaks at binding energies 459.1 and 464.8 eV, respectively, both of which are attributed to TiO\({}_{2}\)[21]. Although the XPS spectra of OL1\({}_{500^{\circ}\text{C,1.5h}}\) show the same main peaks at that of OL1\({}_{\text{Ref}}\), they also contain several sub-peaks of Ti 2p\({}^{3/2}\) and Ti 2p\({}^{1/2}\) at binding energies of 457.4 eV and 464.1 eV, respectively. These are attributed to Ti\({}_{2}\)O\({}_{3}\), as referenced by other works [22]. Figure 3b shows the Ti 2p\({}^{3/2}\) and Ti 2p\({}^{1/2}\) peaks of OL2\({}_{\text{Ref}}\) at binding energies 459.0 eV and 464.7 eV, respectively. After the annealing, the spectra display main peaks of Ti 2p\({}^{3/2}\) and Ti 2p\({}^{1/2}\) at 459.2 eV and 464.9 eV, respectively, as well as sub-peaks at binding energies of 457.5 eV and 463.2 eV, respectively, that are attributed to Ti\({}_{2}\)O\({}_{3}\)[22]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Element** & **OL1\({}_{\text{Ref}}\)** & **OL1\({}_{\text{500^{\circ}}\text{C,1.5h}}\)** & **OL2\({}_{\text{Ref}}\)** & **OL2\({}_{\text{500^{\circ}}\text{C,1.5h}}\)** \\ \hline \multirow{4}{*}{Atomic \%} & O & 24.0 & 70.0 & 14.6 & 66.1 \\  & Ti & 70.9 & 25.6 & 79.3 & 29.4 \\ \cline{1-1}  & C & 3.4 & 4.4 & 0.0 & 4.5 \\ \cline{1-1}  & Cu & 1.7 & 0.0 & 6.1 & 0.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Element compositions of OL1 (oxide layer 1) and OL2 (oxide layer 2) before and after thermal annealing. The detective depth of XPS is usually below 10 nm thinner than the thickness of the oxide; thus, the results only represent the oxide surface. More details analyzed with XPS spectrum are collected in Table 2. The peak areas are determined by fitting a Lorentzian-Gaussian to the data. By comparing the peak areas of Ti\({}^{3+}\) and Ti\({}^{4+}\), the portions of TiO\({}_{2}\) and Ti\({}_{2}\)O\({}_{3}\) can be calculated, and the atomic ratio of O 1s: Ti 2p is given directly by XPS analysis. For OL1\({}_{\rm Ref}\) and OL2\({}_{\rm Ref}\), the ratios of O 1s: Ti 2p are both above 2. For OL1\({}_{\rm 500^{\circ}C,1.5h}\), the ratio of Ti\({}_{2}\)O\({}_{3}\):TiO\({}_{2}\) is 0.23, indicating that the ratio of O 1s: Ti 2p should be 1.89, while the experimental result is 2.19.