Long-range substrate effects on the stability and reactivity of thiolated self-assembled monolayers

The reactivity of the tail group of molecules absorbed in a self-assembled monolayer is affected significantly by the substrate through long-range charge redistribution occurring during the adsorption. Alkyl dithiol monolayers on GaAs are highly stable as compared to monolayers of monothiols on GaAs or dithiols on gold X-ray photoelectron spectroscopy (XPS) measurements reveal fairly weak binding of monothiol layers on GaAs, prone to rapid oxidation at the molecule - substrate interface. This is in contrast with the high stability of monothiols on gold. However, in the case of dithiols, the situation is reversed. When adsorbed on gold, the top thiol group tends to oxidize, whereas on GaAs, it does not. Furthermore, the monolayer was found to be stable in ambient for months. Contact potential difference (CPD) measurements showed a significant difference in charge distribution on the monolayers adsorbed on the two substrates, gold and GaAs. The change in reactivity and stability is attributed to the difference in the substrate-induced charge distribution across the adsorbed molecules.