## 3 Results
### ToF-SIMS Analyses of OTMS Molecules and OTMS SAMs  
The results of the CA and AFM analyses verified the formation of good-quality OPA SAMs with complete coverage on the SiO\({}_{2}\) substrate. In this section, we concentrate on using ToF-SIMS to explore the interface chemistry of OTMS SAMs formed on a SiO\({}_{2}\) substrate. Condensation reactions between the hydrolyzed OTMS molecules and the SiO\({}_{2}\) substrate and among OTMS molecules themselves (homocondensation) are responsible for the robust OTMS SAMs formed on oxides. Therefore, pure OTMS molecules, which are molecules with methoxy groups free from being hydrolyzed and condensed, will be used as a reference. Since the interface involves Si-O bonding from both the substrate and the SAMs, a bare SiO\({}_{2}\) substrate, which was cleaned ultrasonically in methanol and treated by UV/ozone, will also be analyzed. Shown in Figure 4a-c are negative secondary ion mass spectra in \(m/z\) 11-100 for pure OTMS molecules placed on a biaxially oriented polypropylene (BOPP) film, a bare SiO\({}_{2}\) substrate treated by UV/ozone and complete coverage OTMS SAMs formed on such a SiO\({}_{2}\) substrate, respectively. As shown in Figure 4a, the pure OTMS molecules are characterized by two ions Si(OCH\({}_{3}\))O\({}^{-}\) (nominal \(m/z\) 75) and Si(OCH\({}_{3}\))O\({}_{2}\)\({}^{-}\) (91), which are associated with the OTMS molecular headgroups -Si(OCH\({}_{3}\))\({}_{3}\). In this article, ions associated with the headgroup, -Si(OCH\({}_{3}\))\({}_{3}\), are marked by a star in ion mass spectra for clarity. The \(m/z\) range of 11-100 was selected to illustrate the distribution of intensities of ions relative to that of the most abundant ion of C\({}_{2}\)H\({}^{-}\) (25) in Figure 4a,c as well as O\({}^{-}\) (16) in Figure 4b. The dominant C\({}_{2}\)H\({}^{-}\) is due to the methylene chains of the OTMS molecules, whether they are free molecules (Figure 4a) or in their SAMs (Figure 4c). For the same reason, the intensity of CH\({}^{-}\) (13) is also relatively strong. By contrast, as shown in Figure 4b, the SiO\({}_{2}\) substrate is dominated by O\({}^{-}\) and silicon oxide ions, such as SiO\({}_{2}\)\({}^{-}\) (60), SiO\({}_{3}\)\({}^{-}\) (76) and SiO\({}_{3}\)H\({}^{-}\) (77). The rather weak C\({}_{2}\)H\({}^{-}\) and CH\({}^{-}\) signals detected on the SiO\({}_{2}\) substrate are due to the ubiquitous adventitious hydrocarbons, which exist on almost any surface [39; 40; 41]. Because the higher \(m/z\) ions in Figure 4c are much weaker than C\({}_{2}\)H\({}^{-}\) and their comparison with those in Figure 4a,b will lead to insight into the interface chemistry  
Figure 3: Profiles (with offsets for clarity) isolated from the AFM images in Figure 2 for a bare SiO\({}_{2}\) substrate (**a**); full-coverage OTMS SAMs prepared on a UV/ozone-treated SiO\({}_{2}\) substrate (**b**); islands of OTMS SAMs formed on a UV/ozone-treated SiO\({}_{2}\) substrate (**c**); a cleaved mica (**d**). of the SAMs, the spectra of the three samples are replotted in \(m/z\) range 35-95 in Figure 5 to analyze these ions among the three samples. Figure 5a shows four strong ions of SiO\({}_{2}\)-, SiO\({}_{2}\)H\({}^{-}\), Si(OCH\({}_{3}\))O\({}^{-}\) and Si(OCH\({}_{3}\))O\({}_{2}\)-(91), in which the latter two contain a methoxy group. These ions are all associated with the OTMS headgroup, -Si(OCH\({}_{3}\))\({}_{3}\). The detection of these two methoxy-containing ions indicates that the OTMS headgroups (-Si(OCH\({}_{3}\))\({}_{3}\)) remains intact in vacuum. Figure 5b shows abundant SiO\({}_{2}\)-, SiO\({}_{3}\)- and SiO\({}_{3}\)H\({}^{-}\), which are major ions for silicon oxide. The spectrum of the OTMS SAMs on the SiO\({}_{2}\) substrate, as shown in Figure 5c, is quite different from the pure OTMS molecules and the bare SiO\({}_{2}\) substrate. First of all, relatively strong ions C\({}_{2}\)H\({}^{-}\) (38), C\({}_{4}\)H\({}^{-}\) (49), C\({}_{4}\)H\({}_{3}\)- (51) and C\({}_{6}\)H\({}^{-}\) (73) are detected from the SAMs.