1. Research Group Electrochemical and Surface Engineering, Department of Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels B-1050, Belgium, Biological Physics Group, Department of Applied Physics, Chalmers University of Technology, Fysikgränd 3, Göteborg SE-412 96, Sweden, and Thin Film Physics Division, Department of Physics (IFM), Linköping University, Linköping SE-581 83, Sweden