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

• Coadsorption of octanethiol (OT) and octyl thiocyanate (OTC) on Au(111). • Phase behaviors of coadsorbed self-assembled monolayers (SAMs) were examined. • OT SAMs are dominantly formed at molar concentration ratio of OT:OTC = 50:50. • Phase-separated OT and OTC SAMs are formed at molar ratio of OT:OTC = 5:95. • Adsorption of OT on Au(111) is much faster than OTC. To understand headgroup effects on phase behaviors of self-assembled monolayers (SAMs) prepared by coadsorption of octanethiol [OT, CH 3 (CH 2) 7 –SH] and octyl thiocyanate [OTC, CH 3 (CH 2) 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 °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 × 2) phase, whereas single-component 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(4 × 2) 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. [ABSTRACT FROM AUTHOR]