## 4 Discussion  
Basically, we have similar AFM images of the C\({}_{32}\)H\({}_{66}\) thin film as [PERSON]'s group [8,18], although they used a different preparation process: (1) a shorter step height (about 3.7 nm) of the first perpendicular layer, which is attributed to the weaker interaction of the cantilever tip to a perpendicular layer (headed by methyl groups) compared to a parallel layer (-CH), and (2) a 4.3-4.5 nm step height for the after-first perpendicular layer. However, we also found that there is a small difference: the step height of two perpendicular layers may not always be with 4.3 nm or its times. It could be 3.3 nm (Figures 1c,d and S1). The tip-molecular group's interaction mechanism obviously does not work in this case. In fact, it is difficult to believe that there is a significant interaction difference between tip and groups -CH, -CH\({}_{3}\). We propose here that the shortening of the step height of the first perpendicular layer may be the reason the first layer is composed of the co-existence of the parallel state and the perpendicular state, rather than the parallel state being underneath the perpendicular state. Then, we can reconstruct the layer structure, as shown in Figure 9, according to the line scanning profile in Figure 1. Then, all the differences in the step height of AFM images can be explained, that is, they are caused by the parallel layer thickness near the perpendicular layer. This is more in line with the model proposed by [PERSON] [8,18]. Figure 8: Insoluble thickness of the C\({}_{32}\)H\({}_{66}\) thin films prepared at the concentration of 0.3% w/v, as a function of hyperthermal bombardment fluence (**a**); effect of hydrogen bombardment on wettability of thin film surface (**b**). * represents the thickness after 100 s of H plasma treatment (plasma fluence is not measured), and \(\blacktriangle\) and \(\bullet\) represent the thin films after same treatment, with and without hexane-washing, respectively. [PERSON] et al. [1; 2; 3; 4], the recent simulation from [PERSON] et al. [15], and the experimental results of [PERSON] et al. [10]. As mentioned in the previous section, XPS only offers a mass-averaged thickness due to the height difference of co-existing parallel and perpendicular C\({}_{32}\)H\({}_{66}\) layer. We may estimate the average thickness (it should be mass thickness) of the thin films by using the AFM images with the calculated coverage of the parallel layer and perpendicular layer:  
\[d=0.5\Theta+4.3(1-\Theta) \tag{2}\]  
where \(\Theta\) is the ratio of the parallel layer (\(\sim\)0.5 nm) to the perpendicular layer (4.3 nm). For a 0.05% w/v C\({}_{32}\)H\({}_{66}\) thin film, we have an average thickness of 2.1 nm using Equation (2), which agrees well with the 2.4 nm estimated by XPS. It should be noted that the estimation is only limited to very thin layers because there is almost no single parallel layer for the C\({}_{32}\)H\({}_{66}\) concentration more than 0.13% w/v to use AFM measurement. Here, XPS will offer a better estimation result at this case. The formation of a dendrite feature or fractal characterization is generally caused by limited-diffusion aggregation of C\({}_{32}\)H\({}_{66}\) molecules during the spin-coating process. A systematical evaluation and explanation of the fractal of n-alkane on the SiO\({}_{2}\) surface was evaluated by [PERSON]'s group recently [14]. A linear increase in mass thickness (Figure 4) of the C\({}_{32}\)H\({}_{66}\) as a function of C\({}_{32}\)H\({}_{66}\) concentration indicates that the growth of the C\({}_{32}\)H\({}_{66}\) layer is associated with the concentration, and the thickness of the thin film is controlled by the concentration. This also suggests that the monolayer thickness, even the sub-monolayer-oriented molecular film, can be easily prepared by spin-coating by controlling the speed and C\({}_{32}\)H\({}_{66}\) concentration.