Electrochemical STM investigation of 1,8-octanedithiol monolayers on Au(111). Experimental and theoretical study

M. J. ESPLANDIU; M. L. CAROT; F. P. COMETTO; V. A. MACAGNO; E. M. PATRITO

SURFACE SCIENCE

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2006 vol. 600 p. 155 - 172

0039-6028

The surface structure of Au(1 1 1) electrodes covered by 1,8-octanedithiol self-assembled monolayers (SAMs) wasstudied with in situ scanning tunnelling microscopy (STM) as a function of the electrode potential in acidic and alkalineelectrolytes. We investigated the correlation between the dynamics of the SAM and the underlying gold atoms during thereductive desorption and oxidative readsorption processes. The reductive desorption of 1,8-octanedithiol is characterizedby a transition from a compact monolayer with an upright molecular configuration to a striped phase with flat lyingdithiol molecules, whereas during the oxidative readsorption process the surface first becomes increasingly covered bythe striped phase until the transition to the compact phase occurs. We also considered under equivalent conditions1-octanethiol/Au(1 1 1) SAMs which were used as a reference to evaluate the influence of the ?SH terminal group onthe structure and stability of dithiol SAMs. The desorption and readsorption of both the dithiol and the monothiol havea considerable influence on the substrate morphology which is manifested in the dynamics of vacancy islands, gold islandsand 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 goldatoms observed experimentally is compatible with the presence of a defected layer of gold atoms. The DFT results suggestthat the formation of a monolayer may involve the diffusion and self assembly of thiolate?Au moieties rather than thediffusion of the bare thiolates across the surface. This mechanism explains the formation of a defected layer of gold atoms.