## 4 Discussion
We also find that, the C1s features like the peak width and symmetrychanges during the film deposition and cross-linking, although there are only C-C/C-H bonds, which supposes that they are indistinguishable in the C 1s spectrum. The asymmetric feature of the C 1s peak in n-alkane has been noted previously [29], for example, in hexatriacontane (n-C\({}_{36}\)H\({}_{74}\)) carbohydrate, but it has not been completely understood for the peak tail in the high-binding-energy side. Our previous works [27; 28; 30; 31] and others' [32] have shown that the asymmetric change in the C1s peak in HOPG and carbon nanotubes [28] induced by low-energy ions bombardment can be attributed to the electron states localization of broken C-C bonds and fragments (for example, formation of nanoparticles of a-C [27; 28; 30; 31]) of extensive delocalization of the electron state. Herein, the decrease in asymmetric index and peak width (or full-width at half-maximum, FWHM) upon the thickness of C\({}_{32}\)H\({}_{66}\) may partially be attributed to the decrease in the localized electron state in an individual molecule or more molecules in each layer (both perpendicular and parallel state). Although we know that the crystalline of C\({}_{32}\)H\({}_{66}\) is only involved in the van de Waals interaction with the increase in film thickness, the oriented structure of the C\({}_{32}\)H\({}_{66}\) may share their electrons in the lamellae to delocalize the electron state. Indeed, it is consistent with our AFM result: the partial cross-linking of C\({}_{32}\)H\({}_{66}\), as seen in Figure S6, and the clustering of C\({}_{32}\)H\({}_{66}\) result in the increase in asymmetric index and peak width. This is in good agreement with the XPS results in Figure 7. However, both fully cross-linked and partially cross-linked thin films show a smaller asymmetric index, suggesting that the delocalized electron state both in cross-linked and crystalline C\({}_{32}\)H\({}_{66}\) is similar. A slight increase in  
Figure 9: A proposed C\({}_{32}\)H\({}_{66}\) layer structure of two oriented molecules based on AFM line scanning profile of Figure 1. The model fit for low C\({}_{32}\)H\({}_{66}\) concentration deposition. asymmetric index (Figure 7) of the film after cross-linking may suggest the formation of amorphous carbon or clustering [27; 28; 30; 31; 32], consistent with AFM images where the clustering (Figure 2\(c\),d) of C\({}_{32}\)H\({}_{66}\) is due to cross-linking. The formation of nanoparticles of the cross-linked C\({}_{32}\)H\({}_{66}\) films is in accordance with STM results [33; 34] of the cross-linking of SAMs of dodecanethiol on Au(111) induced by hyperthermal proton bombardment. Although we observe a decrease in the films (in fact, it is average step height of perpendicular lamellae in AFM image) after cross-linking induced by hyperthermal hydrogen bombardment in the AFM image (see Figure S5), we do not note any decrease in the average thickness of the film after fully cross-linking by XPS estimation. It may be ascribed to the decrease in mean free path of the photoelectron in the cross-linked films compared to that in the films without cross-linking, due to the increase in the density of cross-linked films. In addition, some residual solvent is absorbed by the films after using solvent to wash. The slight decrease in WCA for treated samples may indicate the loss of hydrogen in CH\({}_{3}\) to CH or C-C in the top of the molecules due to the cross-linking induced by HHIC. The increase in WCA with bombardment time for the washed treatment within 200 s suggests that the cross-linking induced by HHIC is incomplete during that time. This incomplete cross-linking of the films can be partly washed away by hexane and can leave part of the Si wafer (SiO\({}_{x}\)) surface exposed to air, which will result in the decrease in the WCA. Moreover, wettability of films after HHIC treatment further supplies more information of the cross-linking process. We note that there is an essential difference in WCA between surfaces treated after hydrogen plasma and HHIC.