Probing the organosilane SAM-SiO₂ interface by ToF-SIMS: inter-ion intensity relationships as indicators of the...

The interfacial chemistry of octadecyltrimethoxysilane (OTMS) self-assembled monolayers (SAMs), approximately 2 nm thick, formed on silicon oxide surfaces reflects contributions from both the interfacial siloxane network and the underlying SiO₂ substrate. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) enables molecular-level probing of this interface through silicon oxide cluster ions generated during primary ion bombardment. However, interpretation is hindered by the lack of unambiguous diagnostic ions that uniquely distinguish between contributions from the siloxane network and the underlying SiO₂ substrate. Here, we demonstrate that this limitation can be overcome by analyzing the relative intensities of commonly observed cluster ions, rather than relying on uniquely identifiable species. In particular, the ion pairs Si₂O₄H^-/Si₂O₅H^- and Si₃O₆H^-/Si₃O₇H^- are identified as sensitive probes of the siloxane network, with the relative intensities of the oxygen-deficient and fully oxygenated ions reflecting the degree of oxygen-deficient (undercoordinated) silicon environments. By leveraging these inter-ion relationships, this approach provides a robust and generalizable framework for elucidating interfacial chemistry in organosilane SAM systems. Principal component analysis (PCA) of the ToF-SIMS data independently corroborates these findings, confirming that chemically meaningful information is embedded in variations in silicon oxide cluster ion emission and enables clear discrimination between siloxane-derived and substrate-derived contributions.