Unique Mixed Phases and Structures of Self-Assembled Monolayers on Au(111) Derived from Methoxy-terminated Mono(ethylene glycol)ethanethiols

Abstract

To understand the effect of the ethylene glycol (EG) substituent of alkanethiols on domain formation, surface structure, and adsorption condition of self-assembled mono layers (SAMs) on Au(111), we examined SAMs formed by 1-ethanethiol with met,hoxy-terminated mono(ethylene glycol) (EG(1)-OMe SAMs) by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy and compared the results to those of heptanethiolate-SAMs with a: normal alkyl chain of similar molecular length. STM imaging clearly revealed that the surface features of EG(1)-OMe SAMs. on Au(111) were noticeably different than those of heptanethiolate SAMs. The adsorption of, EG1-OMe molecules on Au(111) in a 1 mM ethanol solution at RT for 1 min led to the formation of SAMs containing,mixed phases: a paired-row ordered phase and poorly ordered phase containing molecular spots-with an apparent bright contrast. The paired-row ordered domain of EG(1)-OMe SAMs on Au(111) was assigned to the (2 root 5 X 5)R11 degrees, packing structure, which is comparable to a closely packed c(4 X 2) structure for heptanethiolate SAMs. After a longer immersion of 24 h, similar surface features were also observed with a different (root 3 X 7) packing structure. The formation of these unique domains for EG1-OMe SAMs on Au(111) was caused by a conformational change of the EG1-OMe backbone due to the electrostatic repulsions of oxygen-atoms-between the EG, substituents in the SAMs. We report the first STM results of EG1-OMe SAMs on Au(111) showing very unique, surface, features that have not been observed in other SAM systems derived from alkanethiols or aromatic thiols. X-ray photoelectron, spectroscopy (XPS) measurements also showed that EG1-OMe SAMs on Au(111) were formed by chemical reactions between the thiol group and Au(111) surface. The results obtained from a molecular-scale view point will provide new insight into. the effect of EG1-OMe substituent attached to ethanethiol on the formation and structure Of EG(1)-OMe SAMs on Au(111).