New Structure and Facile Displacement of Cyclohexanethiol Self-Assembled Monolayers by Alkanethiols on Au(111)

Abstract

Self-assembled monolayers (SAMs) derived from organic molecules containing thiol moiety on metal surfaces have attracted much attention because of their fundamental interests in surface science as well as the high possibility of their practical applications [1]. As one of new classes of SAMs, we investigated the structure of cyclic thiol SAMs and found that cyclohexanethiol (CHT) SAMs formed at room temperature on Au(111) have a (5 2 10)R48 superstructure resulting from the existence of two energetically stable conformers, i.e., the equatorial and axial chair conformers [2]. We found that CHT SAMs formed at -20 C have a different superstructure with higher surface overage, which can be assigned to (5 × 2√3) structure, suggesting that the flexibility of CHT ring was remarkably influenced by deposition temperature. On the other hand, it is expected that CHT molecules in SAMs with a low coverage can be easily exchanged by other incoming molecules. Molecular-scale exchange processes of CHT self-assembled monolayers (SAMs) by alkanethiols were examined by scanning tunneling microscopy (STM). In this study, it was elucidated that CHT molecules in SAMs are easily displaced by octanethiols or decanethiols even though these thiols have shorter alkyl chains. Complete replacement of CHT SAMs by alkanethiols occurred within 1 h, resulting in the formation of close-packed (√3 × √3) R30˚ domains for alknethiol SAMs. From these results, we demonstrated that these transient CHT SAMs can be used as temporal layers in soft lithography for protection of reactive surfaces.

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