1. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
2. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3. Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
4. Department of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden
5. Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen, Denmark