## 3 Results
### ToF-SIMS Analyses of OTMS Molecules and OTMS SAMs
These hydrocarbon ions are due to the alkyl groups of the OTMS molecules, the same as discussed for C\({}_{2}\)H\({}^{-}\) in Figure 4a,c. These ions are slightly stronger for OTMS SAMs (Figure 5c) than for the pure OTMS molecules (Figure 5a). This difference can be readily explained by that ToF-SIMS only probes the surface topmost 1-3 nm so that the alkyl groups in OTMS SAMs are all probed, while the surface of the sample of pure OTMS molecules has both headgroups and alkyl groups, which makes the probed volume contain more headgroups -Si(OCH\({}_{3}\))\({}_{3}\). By comparing the spectra in Figure 5a,c, it is clear that the two methoxy-containing ions Si(OCH\({}_{3}\))O\({}^{-}\) and Si(OCH\({}_{3}\))O\({}_{2}\)- were not present in the OTMS SAMs, confirming that the methoxy groups were no longer present in the OTMS SAMs. This is proof that the methoxy groups OCH\({}_{3}\) in OTMS headgroups -Si(OCH\({}_{3}\))\({}_{3}\) are hydrolyzed to become silanols -Si(OH)\({}_{3}\) and methanol CH\({}_{3}\)OH that evaporates, followed by condensation reaction between some of the OH groups in the silanols and the OH groups of the SiO\({}_{2}\) substrate that anchors the OTMS molecules to the substrate via Si-O-Si linkages [19]. It is also  
Figure 4: Negative secondary ion mass spectra (\(m/z\) 11–100) for (**a**) pure OTMS molecules (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\({}_{3}\))\({}_{3}\). believed that some of the OH groups of the silonols of the hydrolyzed OTMS molecules go through a condensation reaction among themselves (homocondensation) to polymerize the molecular headgroups [1; 18; 19]. This explains why organosilanes make robust SAMs on oxides. Although SiO\({}_{2}\)\({}^{-}\) and SiO\({}_{2}\)H\({}^{-}\) (61) are detected for all three samples (Figure 5), their intensities relative to other ions provide useful information in two aspects. The first one is that these two ions are less abundant than hydrocarbon ions (e.g., C\({}_{2}\)H\({}_{2}\)\({}^{-}\), C\({}_{4}\)H\({}^{-}\) and C\({}_{4}\)H\({}_{3}\)\({}^{-}\)) for the OTMS SAMs (Figure 5c), but their abundance relationship is reversed for the pure OTMS molecules (Figure 5a). This is due to the fact that Si-O linkages are below the alkyl groups in OTMS SAMs, while in the case of the pure OTMS molecules (placed on a BOPP film), the surface is mainly composed of the molecular headgroups, -Si(OCH\({}_{3}\))\({}_{3}\). The second aspect is that the relationship between the SiO\({}_{2}\)\({}^{-}\) and SiO\({}_{2}\)H\({}^{-}\) intensities is different between the SiO\({}_{2}\) substrate and the OTMS either in their pure molecules or SAMs. For the SiO\({}_{2}\) substrate, the intensity ratio between SiO\({}_{2}\)\({}^{-}\) and SiO\({}_{2}\)H\({}^{-}\) is greater than 1 (Figure 5b), while for the pure OTMS molecules (Figure 5a) and the OTMS SAMs (Figure 5c), their SiO\({}_{2}\)\({}^{-}\) and SiO\({}_{2}\)H\({}^{-}\) intensity ratios are less than 1. A possible interpretation for the observed intensity ratio SiO\({}_{2}\)\({}^{-}\)/SiO\({}_{2}\)H\({}^{-}\)\(<\) 1 for OTMS is that the abundant hydrogen species generated (from the alkyl groups) in the primary ion bombardment process are combined, in the space above the sample surface, with SiO\({}_{2}\) species to form SiO\({}_{2}\)H\({}^{-}\). Regardless of the explanation, the experimental observation of the relationship between the intensities of SiO\({}_{2}\)\({}^{-}\) and SiO\({}_{2}\)H\({}^{-}\) may serve as a ToF-SIMS criterion to differentiate between silicon oxide and organosilanes.