## 3 Results
### ToF-SIMS Analyses of OTMS Molecules and OTMS SAMs
It is clear that Si\({}_{2}\)O\({}_{8}\)H\({}^{-}\), Si\({}_{3}\)O\({}_{7}\)H\({}^{-}\), Si\({}_{4}\)O\({}_{9}\)H\({}^{-}\) and Si\({}_{5}\)O\({}_{11}\)H\({}^{-}\) are much stronger for the SiO\({}_{2}\) substrate (Figure 6b) than for the OTMS SAMs (Figure 6c). This can be explained by the fact that the presence of the OTMS SAMs (whose thickness is -2 nm) reduces the generation of these ions (because of the 1-3 nm probing depth of ToF-SIMS). The fact that the ratios of the intensities of Si\({}_{3}\)O\({}_{7}\)H\({}^{-}\), Si\({}_{4}\)O\({}_{9}\)H\({}^{-}\) and Si\({}_{5}\)O\({}_{11}\)H\({}^{-}\) relative to that of Si\({}_{2}\)O\({}_{5}\)H\({}^{-}\) are similar for both samples suggests that either these ions from the OTMS SAMs also originate from their underlying SiO\({}_{2}\) substrate or the -Si-O-Si- network of the OTMS SAMs has a similar ion fragmentation pattern as the SiO\({}_{2}\) substrate for these four ions, or a mixture of both. On the other hand, it is interesting to note that the OTMS SAMs on a SiO\({}_{2}\) substrate (Figure 6c) show other silicon oxide ions that are rather weak from the bare SiO\({}_{2}\) substrate (Figure 6b), which are Si\({}_{2}\)O\({}_{4}\)H\({}^{-}\), Si\({}_{3}\)O\({}_{5}\)H\({}^{-}\) (165) and Si\({}_{3}\)O\({}_{6}\)H\({}^{-}\) (181). Therefore, it is plausible that these are mainly, if not solely, associated with the -Si-O-Si- network resulting from the polymerization of the islands generated from the hydrolyzation of the methoxy groups of the OTMS molecules. Shown in Figure 7 are the positive secondary ion mass spectra (\(m/z\) 20-150) for the three samples. The spectrum of the pure OTMS molecules (Figure 7a) is dominated by Si(OCH\({}_{3}\))\({}^{+}\) (121) and Si(OCH\({}_{3}\))\({}_{2}\)H\({}^{+}\) (91). These two ions are much more abundant than hydrocarbon ions, C\({}_{x}\)H\({}_{y}\)+, such as C\({}_{3}\)H\({}_{5}\)+ (41), C\({}_{3}\)H\({}_{7}\)+ (43) and C\({}_{4}\)H\({}_{7}\)+ (55). Other three less abundant ions shown in Figure 7a are Si(OCH\({}_{3}\))\({}^{+}\) (59), Si(OCH\({}_{3}\))H\({}_{2}\)+ (61) and Si(OCH\({}_{3}\))OH\({}_{2}\)+ (77). It is clear that the positive ion mass spectra of the pure OTMS molecules are dominated by ions fragmented from their headgroups, -Si(OCH\({}_{3}\))\({}_{3}\). As shown in Figure 7b, the bare SiO\({}_{2}\) substrate, which was UV/ozone treated, shows abundant Si\({}^{+}\) and SiOH\({}^{+}\) signals. The hydrocarbon ions C\({}_{2}\)H\({}_{3}\)+ (27), C\({}_{2}\)H\({}_{5}\)+, (29), C\({}_{3}\)H\({}_{5}\)+,  
Figure 7: Positive secondary ion mass spectra (\(m/z\) 25–150) 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\({}_{3}\))\({}_{3}\). C3H7+ and C4H7+ detected on the SiO2 substrate are due to residual hydrocarbons or adventitious hydrocarbons on the oxide surface. Also detected on the SiO2 substrate are CH3O+ (31) and C3H7O+ (59), which are apparently introduced by the UV/ozone treatment because their intensities increased in comparison to a pristine SiO2 substrate (not shown) prior to the UV/ozone treatment. Si(OCH3)+ from the pure OTMS molecules and C3H7O+ from the bare SiO2 substrate have the same nominal \(m/z\) 59, while their exact \(m/z\) values are 58.995 and 59.050, making them well separated. It is confirmed that neither of these two ions was detected from the OTMS SAMs (Figure 7c). Figure 7c shows that the spectrum of the OTMS SAMs is dominated by the hydrocarbon ions C2H3+, C2H5+, C3H5+, C3H7+ and C4H7+, which is due to the alkyl groups of the OTMS SAMs. These alkyl groups are also responsible for the drastically reduced intensities of Si+ and SiOH+ in comparison with those of the bare SiO2 substrate (Figure 7b). The ions associated with the headgroups, -Si(OCH3)3, of the pure OTMS molecules (Figure 7a) disappear in the spectrum of the OTMS SAMs (Figure 7c).