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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The formation and surface structures of self-assembled monolayers (SAMs) of octyl selenocyanate (OSC, CH3(CH2)7-SeCN) on Au(111) prepared by solution and ambient-pressure vapor deposition were investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy (STM). STM observations revealed that OSC SAMs on Au(111) formed in a 1 mM ethanol solution at room temperature (RT) for 6 h were mainly composed of a disordered phase, whereas vapor-deposited OSC SAMs at RT for 6 h contained an ordered phase with many bright molecular rows, bright aggregates, and dark molecular defects. Therefore, we concluded that vapor deposition is a more effective method for fabricating SAMs of alkyl selenocyanates on Au(111) to achieve a a high degree of structural order compared with solution deposition. When vapor deposition temperatures increased to 348 K, OSC SAMs on Au(111) after deposition for 6 h were much more uniform, containing a small number of loosely packed domains. The ordered phase can be described as a (2√5 × 6)­R49° structure. Moreover, the structural quality of OSC SAMs on Au(111) formed at a higher deposition temperature of 363 K for 1 or 6 h was markedly enhanced, showing long-range ordered domains with a few structural defects. This ordered phase can be assigned to a (√3 × 2√11)­R30° structure. In this study, we report the first molecular-scale features of SAMs on Au(111) formed by adsorption of alkyl selenocyanates. We also clearly demonstrated that organic selenocyanates could be used as an excellent new precursor for the fabrication of high-quality selenolate SAMs on Au(111) using a vapor deposition method at an elevated temperature.

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Surface‐mediated reductive adsorption and self‐assembly of phenylselenyl chlorides on Au(111).

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To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at −0.818, −0.861, and −0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).