5 years ago

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

Unique Mixed Phases and Structures of Self-Assembled Monolayers on Au(111) Derived from Methoxy-terminated Mono(ethylene glycol)ethanethiols
Hungu Kang, Sicheon Seong, Seulki Han, Jaegeun Noh, Masahiko Hara, Eisuke Ito, Young Ji Son
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).

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05817

DOI: 10.1021/acs.jpcc.7b05817

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