Thiolate-Bonded Self-Assembled Mono layers on Ni(111): Bonding Strength, Structure, and Stability

Self-assembled monolayers (SAMs) grown on surfaces of ferromagnetic metals have attracted increasing attention as they can act as corrosion inhibitors on easily oxidizable transition metals and are potentially relevant for application in spintronics. We have performed a model study of aromatic thiol SAMs grown on atomically flat Ni(111) by means of synchrotron-based X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and density functional theory. Our analysis demonstrates that well-defined bonding. through the sulfur headgroup of the molecules (thiolate bonding) can be established at 200 K. However, the bonding configuration is metastable: breaking of the C-S bond and subsequent chemisorption of both fragments on the Ni surface decreases the total energy. The low activation barrier for C-S dissociation hampers the formation of room-temperature-stable monolayers. In addition, we show that end groups with a strong affinity to the nickel substrate can severely modify the global pattern of interaction of the thiol molecules with the surface upon adsorption.