A density functional study of adsorption of sodium-chloride overlayers on a stepped and a flat copper surface

The adsorption properties of sodium-chloride monolayers and bilayers on the flat (1 0 0) surface and the stepped (3 1 1) surface of copper have been investigated using density functional calculations. We have identified both electrostatic and covalent contributions to the bonding between the overlayers and the substrate. The larger corrugation of the electrostatic potential on the stepped surface than on the flat surface makes the adsorption stronger on the stepped surface than on the flat surface and favours the adsorption of the chlorine atom on top of a copper atom in the steps of the stepped surface. A further stabilisation of this bonding geometry is obtained from the formation of weak covalent bond between these two atoms. A simple “image charge” model for the bonding is found to break down in this case. The large geometric buckling of the monolayer on the stepped surface is predicted to give rise to a large difference between the work functions of the monolayer and the bilayer.