## 3 Results
### Surface Chemical Composition of the C\({}_{32}\)H\({}_{66}\) Thin Films  
XPS analysis results, both before and after cross-linking induced by bombardment, are shown in Figure 3. A comparison of XPS survey spectra of different treated C\({}_{32}\)H\({}_{66}\) samples clearly showed that the C1s signal became very small for the untreated and hexane-washed case. Here, O1s, Si2p, and Si2s came from substrates in the spectra. There was almost no change in the ratio of C1s to Si2p intensity (peak area) for the cross-linked samples. This was consistent with AFM results. The thickness of the thin film can be approximately estimated by using the C1s to Si2p peak area, following Equation (1):  
\[\frac{I_{\text{C1s}}}{I_{Si2p}}=\frac{I_{\text{C1s}}^{0}}{I_{Si2p}^{0}}\frac{1- e^{-d/\lambda_{Si2p}}}{e^{-d/\lambda_{\text{C1s}}}} \tag{1}\]  
where \(I\) is the peak intensity (peak area) of photoelectron emission, \(d\) is thickness of the thin films, and \(\lambda\) is the photoelectron mean free path in the thin films. This method was assumed one of the best methods to estimate the thickness of organic molecular layers [24; 25; 26]. Then, we obtained a relationship of the thickness of thin films as a function of C\({}_{32}\)H\({}_{66}\) concentration. Indeed, the thickness of the thin films was correlated with C\({}_{32}\)H\({}_{66}\) concentration linearly in Figure 4 for a given spin speed. It should be mentioned that the estimation may have had a larger error because Equation (1) supposed that the layer was homogenous in thickness. However, our AFM image indicated that the layer was composed of a perpendicular state (about 4.3 nm thick) and parallel state (about 0.5 nm thick). Therefore, the layer thickness estimation by XPS was an equivalent (or mass) thickness or apparent thickness in a larger analysis area (could be scaled to 100 \(\upmu\)m level). It was also noticed that C1s, in Figure 5, an asymmetric peak, showed a slight increase in the peak width after cross-linking. The asymmetric index of C1s, which had been discussed previously, was associated with carbon atomic electronic and crystal structures [27]. The  
Figure 2: AFM images of C\({}_{32}\)H\({}_{66}\) thin films prepared at the concentration of 0.13% w/v (**a**) as-deposited, (**b**) as-deposited and hexane-washed, (**c**) after hyperthermal hydrogen bombardment cross-linking, and (**d**) after hyperthermal hydrogen bombardment and hexane washing (insets: their higher-resolution images and scanning scale is 1 \(\upmu\)m \(\times\) 1 \(\upmu\)m). asymmetric index value could be estimated by peak fitting using XPS programs, such as CasaXPS (www.casaxps.com, accessed on 10 May 2022) or XPSPEAK 4.1 (free download is available from the web). More detailed information about it can be found in reference [28]. It was further interesting that the asymmetric index of C1s of the as-deposited thin film showed a decrease with the C\({}_{32}\)H\({}_{66}\) thickness (Figure 6), but it remained constant without any change after full cross-linking except for partial cross-linking and after hexane washing (using a lower bombardment energy through control of the sample position, shown in Figure 7). XPS identified that this sample was only 1 nm thick after C\({}_{32}\)H\({}_{66}\) thin-film cross-linking, confirmed by AFM morphology (see Figure S6). AFM morphology of the sample showed that the vertical molecule feature disappeared and the surface was composed of nanoparticles. The nanoparticles had about a 1 nm height estimated from Figure S6. Figure 4: Thickness of C\({}_{32}\)H\({}_{66}\) thin films estimated from the XPS intensities as a function of its spin coating concentration. Figure 3: Comparisons of XPS survey spectra of C\({}_{32}\)H\({}_{66}\) thin film prepared at the concentration of 0.3% w/v after different surface treatments.