Reconstruction of a Cu(100) electrode in the presence of thiocyanate anions

In situ electrochemical scanning tunneling microscopy has been used to examine the surface phase behavior of specifically adsorbed thiocyanate on Cu(100) within the potential range from -950 to -600 mV vs. Ag/AgSCN. In contact with the thiocyanate-containing electrolyte a highly ordered adsorbate layer is formed at negative potentials which can be described by a rectangular c(2 x 6)-4SCN unit-mesh corresponding to a coverage of Theta = 0.33 ML. By sweeping the electrode potential into positive direction a phase transition takes place, which involves not only the adsorbate layer but also the substrate structure and thereby the entire surface morphology. As a consequence square and rectangular adislands and vacancy islands arise which lead to an enormous roughening of the copper substrate. Both, the adislands and the vacancy islands are preferentially aligned parallel to the substrate (001) directions. Ultimately the phase transition leads to a chequerboard-like arrangement of adislands stabilized by the thiocyanate adsorbate. A plausible model for the appearance of this latter phase is based on an adsorbate-induced reconstruction of the topmost copper layer with adsorbate molecules residing in "pseudo-fourfold-hollow sites" of an expanded copper layer. This phase can be described as a "pseudo-c(2 x 2)-reconstruction". (C) 2003 Elsevier B.V. All rights reserved.