To understand the effect of the ethylene glycol (EG) substituent of alkanethiols on domain formation, surface structure, and adsorption condition of self-assembled monolayers (SAMs) on Au(111), we examined SAMs formed by 1-ethanethiol with methoxy-terminated mono­(ethylene glycol) (EG1-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 EG1-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 EG1-OMe SAMs on Au(111) was assigned to the (2√5 × 5)­R11° packing structure, which is comparable to a closely packed c(4 × 2) structure for heptanethiolate SAMs. After a longer immersion of 24 h, similar surface features were also observed with a different (√3 × 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 EG1 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 viewpoint will provide new insight into the effect of EG1-OMe substituent attached to ethanethiol on the formation and structure of EG1-OMe SAMs on Au(111).

The surface structure and binding conditions of self-assembled monolayers (SAMs) on Au(111) derived from 1-acetylthio-4-[(phenyl)­ethynyl]­benzene (OPE2-SAc) without and with tetrabutylammonium cyanide (TBACN) as a deprotection reagent were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observation revealed that OPE2-S SAMs on Au(111) formed from direct adsorption of OPE2-SAc in 1 mM methanol solution at room temperature (RT) for 24 h were composed of short-range, ordered phase separated by a disordered phase. In contrast, adsorption of OPE2-SAc SAMs on Au(111) at a higher solution temperature of 50 °C for 24 h led to formation of a fully ordered phase with slightly increased domain size. The structural quality of OPE2-S SAMs on Au(111) was remarkably enhanced when TBACN was used. OPE2-S SAMs at RT had a well-ordered (√3 × √7)­R30° structure with a domain size larger than 80 nm. The SAMs deposited at 50 °C contained very uniform and highly ordered domains with a size exceeding 100 nm, which can be assigned to a (2 × 3√3)­rect structure. XPS measurements showed that OPE2-S SAMs were mainly formed via chemical interactions between sulfur and the Au(111) surface regardless of the use of a TBACN deprotection reagent. In this study, we reported the first molecular-scale features of OPE2-S SAMs on Au(111) with highly ordered domains derived from OPE2-SAc and clearly demonstrated that TBACN can be used as an effective deprotection reagent for formation of uniform and well-ordered OPE2-S SAMs with long-range domains derived from thioacetyl-protected OPE2-S molecules on Au(111).

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