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

• Published in: Thin solid films 2020-08, Vol.707, p.138100, Article 138100
• Main Authors: Choi, Youngsik, Seong, Sicheon, Han, Seulki, Son, Young Ji, Mondarte, Evan Angelo Quimada, Tahara, Hiroyuki, Song, Subin, Hayashi, Tomohiro, Noh, Jaegeun
• Format: Article
• Language: English
• Subjects: c(4 × 2) phase; Coadsorption; Octanethiol; Octyl thiocyanate; Phase behavior; Scanning tunneling microscopy; Self-assembled monolayers; Striped phase
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2020.138100

•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, 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 °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.