Several transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick-slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × √7 superstructure on the copper monolayer. Stick-slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick-slip resolution persists at all normal forces investigated.
Keywords: AFM; friction; friction force microscopy; nanotribology; underpotential deposition.