## 3 Results  
### Formation of OTMS SAMs Verified by Water CAs and AFM Imaging  
Figure 1a is an optical image of a water droplet placed on a bare SiO\({}_{2}\) substrate ultrasonically cleaned in methanol for 15 min, showing a CA of 41\({}^{\circ}\). Depending on the cleanness of the surface, CAs of 40-50\({}^{\circ}\) were commonly observed. This level of water CAs indicates that organic contaminants are still present on the solvent-cleaned substrate because water should spread on the surface of a SiO\({}_{2}\) substrate that is free from contamination. In fact, there is always a layer of hydrocarbons absorbed on almost any type of surface, which are referred to as adventitious hydrocarbons [39; 40; 41]. We confirmed that for a SiO\({}_{2}\) substrate subjected to a UV/ozone treatment for 30-40 min, water completely spread on the surface; that is, no contact angle was measurable. This is a proof that organic contaminants, including adventitious hydrocarbons, were largely removed from the substrate, rendering the surface dominated by hydroxy groups. This surface-cleaning procedure (or other wet etching ones [42; 43]) is required to achieve high-quality organosilane SAMs on oxides. As shown in Figure 1b, hydrophobicity was achieved on OTMS SAMs formed on a SiO\({}_{2}\) substrate that was sequentially cleaned by methanol and UV/ozone treatment. The water droplet in Figure 1b shows a CA of 110\({}^{\circ}\). The average CA obtained over 10 spots on the OTMS SAM sample is 110 \(\pm\) 1\({}^{\circ}\). CAs are an excellent measure of the coverage and the crystallinity (or conformation) of the molecular chains of SAMs for alkylsilanes. More specifically, any exposure of the hydrophilic SiO\({}_{2}\) substrate (i.e., less than complete coverage of SAMs) will decrease the number of water CAs. A perfect OTMS SAM should have its surface terminated by methyl groups. Any exposure of the methylene chains in SAMs will be regarded as defective, which will in principle decrease water CAs. Therefore, the seemingly simple CA measurement is indeed sensitive to the surface chemistry of alkylsilane SAMs. There are plenty of CA data for OTMS SAMs on various oxides, which range from 105\({}^{\circ}\) to 113\({}^{\circ}\)[13; 14; 15; 16]. The CAs obtained verified the formation of high-quality OTMS SAMs on the SiO\({}_{2}\) substrate; i.e., there was complete coverage and closely packed molecular chains with their methyl groups pointing outward. In order to understand the surface morphology of the OTMS SAMs formed on the SiO\({}_{2}\) substrate, AFM was used to study the SiO\({}_{2}\) substrate and the OTMS SAMs formed on said substrate. Figure 2 shows four AFM topographic images in false color scales; that is, brighter colors represent greater height values. The scan area for AFM images in Figure 2a-c is 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m, while that for Figure 2d is 2 \(\upmu\)m \(\times\) 2 \(\upmu\)m. The broken lines inserted in the images indicate a scan line to be isolated to show a profile for each image, which are shown in Figure 3. Figure 2a shows the surface morphology for the bare SiO\({}_{2}\) substrate. When the surface of the SiO\({}_{2}\) substrate is derivatized by OTMS SAMs, as shown in Figure 2b, there are no significant differences in morphology between the two samples. This is a reflection that the surface is completely covered by a monolayer of OTMS so that the morphology of the underlying SiO\({}_{2}\) substrate is replicated. This result was confirmed on OTMS SAMs formed on multiple SiO\({}_{2}\) substrates. The average surface roughness values for the images in Figure 2a,b were 0.29 and 0.37 nm, respectively. This difference in roughness is likely due to the inhomogeneity of the morphology of the SiO\({}_{2}\) substrate captured in a scan area of 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m.  
AFM is useful in visualizing the coverage of SAMs.