## 3 Results and Discussion  
### Modified Adsorbed Water on Various FS Surfaces  
Figure 1a shows the water contact angle of FS surfaces after treatment with various H2O2 solutions for 3 h. It was found that the water contact angle of pristine FS surface  
is \(\sim\)66\({}^{\circ}\), which is close to the value reported in previous publication [18]. Moreover, the water contact angle of FS surface does not change when treated with pure water, but it decreases with the concentration of the H\({}_{2}\)O\({}_{2}\) solution, implying the FS surface becomes more hydrophilic when it is treated with higher concentration H\({}_{2}\)O\({}_{2}\) solutions [19]. When treated with H\({}_{2}\)O\({}_{2}\) solutions, the under-coordinated Si and Si-O\({}^{-}\) groups on FS surface can react with H\({}_{2}\)O\({}_{2}\); as a result, more Si-OH groups are formed on the FS surface [15; 20]. This can explain why the hydrophilicity of FS surface increases with the concentration of the H\({}_{2}\)O\({}_{2}\) solution (Figure 1a). Although the adhesion force (\(F_{\text{a}}\)) between the diamond tip and various FS surfaces is relatively low (tens of nN) compared to the applied force for nanowear tests (tens of nN), the adhesion force on the FS interface increases with the concentration of the H\({}_{2}\)O\({}_{2}\) solution (Figure 1b). Generally, the adhesion force between a diamond tip and the FS surface involves the capillary force (\(F_{\text{c}}\)) and van der Waals forces (\(F_{\text{vdw}}\)), as well as the electrostatic forces and chemical bonding force [21]. Among these surface forces, only the capillary force and van der Waals forces vary with the thickness of the adsorbed water film in humid air, which can be a function of the water contact angle of the diamond tip and the FS surface [21]. Based on the proposed theory, with the decrease in the water contact angle of the FS surface, the thickness of the adsorbed water film on the FS surface increases from \(\sim\)0.42 nm to \(\sim\)0.52 nm; as a result, the capillary force increases and van der Waals forces decrease (Table 1). Since the adhesion force on the FS surface increases after treatment in H\({}_{2}\)O\({}_{2}\) solution, the increased hydrophilicity and capillary force on the FS surface should be the main reason for the increased adhesion force (Figure 1b). Figure 1: (**a**) Water contact angle measured for various FS surfaces. (**b**) Adhesion force between the diamond tip and various FS surfaces in humid air. (**c**) Raman spectra and (**d**) SFG spectra of various FS surfaces. Figure 1c displays the Raman spectra of various FS surfaces. It was found that there are two main peaks centered at \(\sim\)437 cm\({}^{-1}\) and \(\sim\)784 cm\({}^{-1}\), which originate from the coupled vibrational modes of the silica random network [22]. The two relatively sharp bands centered at \(\sim\)496 cm\({}^{-1}\) (D\({}_{1}\)) and \(\sim\)606 cm\({}^{-1}\) (D\({}_{2}\)) are the typical bands of in-phase breathing motions of oxygen atoms in puckered four-membered and planar three-membered ring structures, respectively [23]. It can be seen that after treatment with various H\({}_{2}\)O\({}_{2}\) solutions, there is no obvious structure evolution of the FS surface (Figure 1c). The peak from 3050 to 3600 cm\({}^{-1}\) is a typical O-H stretch region [24], and the accurate peak position depends on the strength of hydrogen bonding. The peak at \(\sim\)3700 cm\({}^{-1}\) could be assigned to the free OH groups on the FS surface. Since there is no significant change in the range of 3100-3800 cm\({}^{-1}\), no significant change in the adsorbed water structure on the FS surface occurs [24]. This finding seems to be inconsistent with the results in Figure 1a,b, which is due to the much deeper information (hundreds of nanometers) provided by Raman spectra [25]. Figure 1d shows the SFG spectra of various FS surfaces in 50% RH conditions.