Effects of solvent on the formation of octanethiol self-assembled monolayers on Au(111) at high temperatures in a closed vessel: A scanning tunneling microscopy and X-ray photoelectron spectroscopy study

Self-assembled monolayers of 1-octanethiol (OT-SAMs) on Au(111) substrates prepared using ethanol, N,N′-dimethylformamide (DMF), toluene, or hexane were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to investigate the effects of the solvent on the molecular packing at high solution temperatures (90 °C) and high pressures (21 psi) in a closed vessel. STM imaging revealed the formation of ordered OT-SAMs with a (3 × √7)R11° overlayer structure on Au(111) surfaces in ethanol, whereas OT-SAMs prepared in DMF revealed large ordered domains with a (√3 × √3)R30° phase and a c(4 × 2) superstructure. OT-SAMs prepared in toluene produced a mixture of ordered and striped phases, while no clear ordered structures were observed for OT-SAMs prepared in hexane. The disposition of OT bound to the Au(111) surfaces depended on the solvent, which also affected whether the OT-SAMs formed ordered or disordered phases. The intensities of XPS peaks at 161 and 162.3 eV increased with domain disorder, possibly due to changes in the adsorption configurations of the OT molecules.