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 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).

Formation of a Highly‐ordered Thiophene Monolayer on Au(111) via Vapor Phase Deposition

“…Thiophene was obtained from Tokyo Chemical Industry Co., Ltd. (Japan). The atomically‐flat Au(111) substrates were fabricated by thermal evaporation, as described in previous literature . The Au(111) substrates were thermally annealed in furnace at 420 °C for 4 h to clean the gold surface and then used immediately for SAM preparation.…”

“…The ordered domains were divided by domain boundaries indicated by the white lines. Tri‐directional ordered domains with an angle of 60° or 120° for thiophene SAMs were observed, as indicated by the white arrows, reflecting that the growth of SAMs is driven by the three‐fold symmetry of the Au(111) lattice, as is the case with alkanethiol SAMs . The sizes of these ordered domains usually fall in the range of 20–100 nm, which is much larger than those observed for thiophene SAMs on Au(111) with lateral dimensions of several nm to about 12 nm prepared by solution deposition at room temperature .…”

“…Numerous studies are devoted to the preparation of molecular thin films on metal surfaces and the characterization of their physical and structural properties . The adsorption of sulfur‐containing organic molecules on gold is often applied in the formation of self‐assembled monolayers (SAMs) with controllable orientation and thickness . The growth mechanism of SAM formation from thiophene or alkylthiophene in solution was monitored by surface plasmon resonance spectroscopy and infrared reflection adsorption spectroscopy, suggesting that their adsorption geometry changed from lying‐down to an upright orientation with increasing surface coverage.…”

Phase‐separated Nanodomains Formed by Coadsorption of Alicyclic and Aromatic Thiols on Au(111)

“…The adsorption of thiols onto metals creates self‐assembled monolayers (SAMs) that can provide a versatile route for fabrication of nanoscale structures and functionalized monomolecular thin films . Two‐component mixed SAMs are especially promising due to their strong potential to tune the physical and chemical properties of surfaces by changing the tail group, length, and chemical structure of the thiol molecular backbone.…”

“…Two‐component mixed SAMs are especially promising due to their strong potential to tune the physical and chemical properties of surfaces by changing the tail group, length, and chemical structure of the thiol molecular backbone. Therefore, SAMs have been utilized for various practical applications including pH‐sensitive supramolecular switches, protein adsorption, organic field‐effect transistors, and molecular electronic devices . Mixed SAMs on gold can be readily prepared using three methods: coadsorption of two different thiols, adsorption of asymmetric disulfides, and sequential adsorption .…”