Electrochemical STM investigation of 1,8-octanedithiol monolayers on Au(1 1 1).: Experimental and theoretical study

The surface structure of Au(1 1 1) electrodes covered by 1,8-octanedithiol self-assembled monolayers (SAMs) was studied with in situ scanning tunnelling microscopy (STM) as a function of the electrode potential in acidic and alkaline electrolytes. We investigated the correlation between the dynamics of the SAM and the underlying gold atoms during the reductive desorption and oxidative readsorption processes. The reductive desorption of 1,8-octanedithiol is characterized by a transition from a compact monolayer with an upright molecular configuration to a striped phase with flat lying dithiol molecules, whereas during the oxidative readsorption process the surface first becomes increasingly covered by the striped phase until the transition to the compact phase occurs. We also considered under equivalent conditions 1-octanethiol/Au(1 1 1) SAMs which were used as a reference to evaluate the influence of the -SH terminal group on the structure and stability of dithiol SAMs. The desorption and readsorption of both the dithiol and the monothiol have a considerable influence on the substrate morphology which is manifested in the dynamics of vacancy islands, gold islands and indentation of step edges. Quantum mechanical calculations in the framework of density functional theory (DFT) show that adsorbed thiols greatly facilitate the detachment of gold atoms from step edges. The high mobility of gold atoms observed experimentally is compatible with the presence of a defected layer of gold atoms. The DFT results suggest that the formation of a monolayer may involve the diffusion and self assembly of thiolate-Au moieties rather than the diffusion of the bare thiolates across the surface. This mechanism explains the formation of a defected layer of gold atoms.