1. IBM Research Europe, Säumerstrasse 4, 8803 Rüschlikon, Switzerland.
2. Center for Soft Nanoscience, University of Münster, Busso-Peuss-Str. 10, 48149 Münster, Germany.
3. Department of Engineering of Engineering, University of Exeter, Exeter EX4 4QF, UK.
4. Department of Materials, University of Oxford, Oxford OX26HT, UK.