Scanning tunneling microscopy study on phase behavior of self-assembled monolayers formed by coadsorption of octanethiol and octyl thiocyanate on Au(111)

Abstract

To understand headgroup effects on phase behaviors of self-assembled monolayers (SAMs) prepared by coadsorption of octanethiol [OT, CH3(CH2)(7)-SH] and octyl thiocyanate [OTC, CH3(CH2)(7)-SCN] on Au(111) as a function of the OT:OTC molar concentration ratio, we investigated the surface structures of the resulting SAMs formed in 1 mM ethanol solutions at 50 degrees C for 24 h using scanning tunneling microscopy (STM). The STM study showed that the single-component OT SAMs were composed of an ordered c(4 x 2) phase, whereas singlecomponent OTC SAMs had a binary phase consisting of a disordered phase and a striped phase with a distance of 1.3 +/- 0.1 nm between rows. The fully covered c(4x2) phase on Au(111) formed by OT molecules was observed even a molar concentration ratio of OT is equal to OTC (50:50) or is much lower than OTC (10:90). The partially covered striped phase formed by OTC molecules was observed when the molar concentration ratios of OTC were much higher than OT (OT:OTC = 5:95 or OT:OTC = 1:99). Molecular-scale STM results clearly reveal the phase transitions of SAMs on Au(111) via coadsorption of OT and OTC depending on OT:OTC molar ratio. These results will help advance understanding of SAM formation and phase behavior on Au(111) by the coadsorption of alkanethiol and alkyl thiocyanates.