On the role of extrinsic and intrinsic defects in the underpotential deposition of Cu on thiol-modified Au(111) electrodes

Underpotential deposition (UPD) of Cu on Au(111) electrodes modified by self-assembled monolayers (SAMs) of omega-(4'-methylbiphenyl-4-yl)ethanethiol (BP2) was studied in situ by electrochemical scanning tunneling microscopy. The UPD layer intercalated between SAM and Au consists of monatomic high nanoislands on top of an extended Cu film. Nucleation and growth of the Cu UPD layer are accounted for by a mechanism that is fundamentally different from the one suggested in the literature for alkanethiols. Domain boundaries, vacancy islands, or step edges do not act as nucleation sites. The electrode passivation is therefore not limited by the intrinsic structure of the SAM but by extrinsic defects, which are associated with more substantial discontinuities in the SAM. These act not only as nucleation centers for the Cu UPD but throughout the whole growth process are the only sites through which Cu penetrates. The growth proceeds by diffusion of Cu at the SAM-substrate interface until completion of the UPD layer. The implications of our observations for the generation of metal-SAM-metal structures are discussed.