## 3 Results
### ToF-SIMS Analyses of OTMS Molecules and OTMS SAMs
Another important group of ions for understanding the interface chemistry of OTMS SAMs on SiO\({}_{2}\) substrate are SiO\({}_{3}\)\({}^{-}\) and SiO\({}_{3}\)H\({}^{-}\), which are characteristic of silicon oxide  
Figure 5: Negative secondary ion mass spectra (_m_/_z_ 35–95) for (**a**) pure OTMS (placed on a polypropylene film), (**b**) a UV/ozone-treated SiO\({}_{2}\) substrate and (**c**) OTMS SAMs prepared on a UV/ozone-treated SiO\({}_{2}\) substrate. Ions indicated by a star (*) are associated with the OTMS headgroup, -Si(OCH\({}_{3}\))\({}_{3}\). (Figure 5b). There are few SiO\({}_{3}\)\({}^{-}\) signals detected for both the pure OTMS molecules (Figure 5a) and the OTMS SAMs (Figure 5c), while their SiO\({}_{3}\)H\({}^{-}\) intensities are greatly reduced in comparison with those for the SiO\({}_{2}\) substrate (Figure 5b). Figure 6 shows the negative secondary ion mass spectra for the three samples from \(m/z\) 100 to 400. As shown in Figure 6a, for the pure OTMS molecules, an abundant pseudo-molecular ion [M-CH\({}_{3}\)]\({}^{-}\) (359) was detected, where M = C\({}_{18}\)H\({}_{37}\)Si(OCH\({}_{3}\))\({}_{3}\) represents the molecular formula of OTMS and the methyl group removed is from one of the three methoxy groups. Though much weaker in intensity in comparison to [M-CH\({}_{3}\)]\({}^{-}\), a deprotonated molecular ion [M-H]\({}^{-}\) (373) was also detected. Also shown in Figure 6a are two OTMS molecular headgroup-related ions, Si(OCH\({}_{3}\))\({}_{2}\)OH\({}^{-}\) (107) and Si(OCH\({}_{3}\))\({}_{3}\)\({}^{-}\) (121). As shown in Figure 6b, the SiO\({}_{2}\) substrate sample's spectrum is characterized by silicon oxide cluster ions, Si\({}_{2}\)O\({}_{4}\)\({}^{-}\) (120), Si\({}_{2}\)O\({}_{4}\)H\({}^{-}\) (121), Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\) (137), Si\({}_{2}\)O\({}_{7}\)H\({}^{-}\) (197), Si\({}_{4}\)O\({}_{9}\)H\({}^{-}\) (257) and Si\({}_{5}\)O\({}_{11}\)H\({}^{-}\) (317), with Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\) being the most abundant and Si\({}_{5}\)O\({}_{7}\)H\({}^{-}\) (197) being the second most. When zoomed in (not shown), the nominal \(m/z\) values for Si(OCH\({}_{3}\))\({}_{3}\)\({}^{-}\) (Figure 6a) and Si\({}_{2}\)O\({}_{4}\)H\({}^{-}\) (Figure 6b) are both 121, but their exact \(m/z\) values are 121.033 and 120.941, respectively, and they are two totally separated peaks in the spectrum of Figure 6a. At \(m/z\) 137, it is also confirmed that the intensity of Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\) is extremely weak for the pure OTMS molecules (Figure 6a). Instead, there is a stronger ion Si(OCH\({}_{3}\))\({}_{3}\)O\({}^{-}\), which is related to the OTMS molecular headgroup, -Si(OCH\({}_{3}\))\({}_{3}\), and whose exact \(m/z\) value is 137.028, while that of Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\) is 136.937. It is clear that ions from the pure OTMS molecules (Figure 6a) are not detected from the OTMS SAMs (Figure 6c). As discussed before (for the two lower mass ions associated with the OTMS headgroups, -Si(OCH\({}_{3}\))\({}_{3}\) shown in Figure 5a), this is a reflection that the methoxy groups no longer exist in the OTMS SAMs due to their hydrolysis and condensation/homocondensation reaction. The higher mass silicon oxide cluster ions (i.e., Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\), Si\({}_{2}\)O\({}_{7}\)H\({}^{-}\), Si\({}_{4}\)O\({}_{9}\)H\({}^{-}\) and Si\({}_{5}\)O\({}_{11}\)H\({}^{-}\)) shown in Figure 6b, which are characteristic  
Figure 6: Negative secondary ion mass spectra (\(m/z\) 100–400) for (**a**) pure OTMS (placed on a polypropylene film); (**b**) a UV/ozone-treated SiO\({}_{2}\) substrate; (**c**) OTMS SAMs prepared on a UV/ozone treated SiO\({}_{2}\) substrate. Ions indicated by a star (*) are associated with the OTMS headgroup, -Si(OCH\({}_{3}\))\({}_{3}\). of the SiO\({}_{2}\) substrate, are not detected from the pure OTMS molecules (Figure 6a). This observation suggests that the methoxy groups in the pure OTMS molecules are intact, thus preventing the formation of -Si-O-Si- network.