Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.