Potential-induced structure transitions in self-assembled monolayers: ethanethiol on Au(100)

We have characterized the structural behaviour of ethanethiol self-assemble d monolayers (SAMs) on Au(1 0 0) in 0.1 M H2SO4 as a function of electrode potential, using in-situ scanning tunneling microscopy (STM). After modific ation of the Au(1 0 0) electrode in an ethanolic solution of ethanethiol, S TM images in air reveal a disordered thiol adlayer and a surface that is co vered by 25% of monoatomic high gold islands, which originate from lifting of the (hex) reconstruction during thiol adsorption. In contrast to alkanet hiol SAMs on Au(1 1 1), no vacancy islands are seen on the Au(1 0 0) surfac e. After contact of the SAM-covered Au(1 0 0) electrode with 0.1 M H2SO4 un der potential control, two different structures are observed, depending on the potential range positive or negative of +0.3 V vs. SCE. In both cases t he emerging ordered structures are quadratic, their unit cells being rotate d by 45 degrees with respect to the main crystallographic axes of the subst rate. However, the ordered structure at negative potentials is more densely packed than the one at positive potentials, and in addition the surface re veals an almost 50% coverage of monoatomic high gold islands. The structure of the SAM changes reversibly with the electrode potential, the long range order gradually decreasing with each transition. Concomittant with this st ructure transition monoatomic deep holes are created when the potential is stepped from the cathodic to the anodic region. The experimental observatio ns are rationalized by a high mobility of the gold thiolate moiety, causing the surface density of the SAM-covered gold to change drastically with pot ential. (C) 2001 Elsevier Science B.V. All rights reserved.