1. nanotech@surfaces Laboratory Empa—Swiss Federal Laboratories for Materials Science and Technology Überlandstrasse 129 8600 Dübendorf Switzerland
2. Institute of Physics École Polytechnique Fédérale de Lausanne 1015 Lausanne Switzerland
3. Present address: Laboratory of Molecular Simulation (LSMO) Ecole Polytechnique Fédérale de Lausanne (EPFL) Valais Rue de l'Industrie 17 1951 Sion Switzerland
4. Present address: Department of Materials Science and Engineering Cornell University Ithaca NY 14853 USA