Steric hindrance and exchange in the coadsorption of octanethiol and decyl thiocyanate on Au(111)

Abstract

Organic thiocyanates (R–SCN) form thiolate self-assembled monolayers (SAMs) on metal surfaces through the cleavage of the S–CN bond. Comparison of thiocyanate and thiol monolayers prepared using the same deposition conditions demonstrate substantial differences in the formation of the respective monolayers. These studies directly compare the self-assembly of thiocyanates and thiols. Mixed SAMs were formed through the simultaneous exposure of an Au(111) surface to octanethiol and decyl thiocyanate at different concentrations and ratios and were characterized under ambient conditions using scanning tunneling microscopy. The resulting SAMs consist primarily of octanethiolates, including when the concentration of decyl thiocyanate in solution is in excess by two orders of magnitude. Reducing the solution concentration of both octanethiol and decyl thiocyanate while maintaining the same relative ratio enhances the persistence and surface coverage of decyl thiocyanate on the surface. Thiocyanate displacement is likely facilitated by thiols, which is lessened when the octanethiol concentration is reduced. Furthermore, thiocyanate adsorption is limited by the bulkier head group. Here, we demonstrate how the deposition conditions of two compounds can be used to guide the formation of the mixed self-assembled monolayers from components with differing adsorption kinetics. This can provide greater control of the surface composition which would be useful in the patterning of surfaces.