Functionalized alkanethiols (-CH3, -NH2, -OH, -COOH, -SH, -SO3H, -CN) of different lengths, self-assembled on Au(111), were studied with cyclic voltammetry and in situ scanning tunneling microscopy. The surface structure was determined under potential conditions at which only a dielectric behavior is expected. In general, the size of the terminal group together with the chemical interaction between neighboring thiol molecules plays a crucial role for the resulting adlayer structure. Shorter-chain thiols with bulky endgroups frequently lead to lower density coverages and more distorted packings.

We have used in-situ scanning tunneling microscopy and cyclic voltammetry to study self-assembled monolayers of ethanethiol on Au(111) electrodes. The adlayer was found to consist of domains of two different ordered structures, one corresponding to a (p × √3), well-established for other short-chain alkanethiols, and the other to an oblique primitive (4 × 3) superstructure, not previously reported for nonfunctionalized alkanethiols. At potentials slightly negative of 0 V vs SCE the adlayer undergoes a structural transformation that eventually leads to the formation of small pits and islands on the surface. Electrochemical studies in 0.1 M H2SO4 have revealed that around −0.31 V vs SCE the ethanethiol adlayer is reductively desorbed. Oxidative desorption of ethanethiol takes place at 1.15 V. The cathodic as well as the anodic desorption of the monolayer was monitored by scanning tunneling microscopy.

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Copper electrodeposition from sulfuric acid solutions onto ethanethiol-modified Au(111) electrodes was studied by in-situ scanning tunneling microscopy and cyclic voltammetry. The ethanethiol adlayer undergoes an order−disorder transition before Cu underpotential deposition starts around +0.20 V versus SCE. Five percent of a monolayer is deposited positive of the Nernst potential at a sweep rate of 10 mV s-1. At low overpotentials the Cu deposit exhibits a ramified monatomic high morphology, if the ethanethiol adlayer is dense. In all cases three-dimensional growth nucleating at large substrate defects is found in addition. Cyclic voltammetry revealed two characteristic deposition features:  firstly, a sharp cathodic peak at −0.18 V which is ascribed to the insertion of a Cu monolayer between Au and the organic adlayer and, secondly, a current loop due to Cu bulk deposition. The corresponding stripping peaks are found at 0.08 and 0.35 V, respectively.

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