1. Advanced Research Center for Nanolithography, Science Park 106, Amsterdam 1098XG, The Netherlands
2. Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, Amsterdam 1090 GD, The Netherlands
3. Laboratory of Micro and Nanotechnology, Paul Scherrer Institute, Forschungsstrasse 111, Villigen 5232, Switzerland