1. Institute for Manufacturing, Department of Engineering, University of Cambridge, Cambridge CB3 0FS, United Kingdom
2. Institute for Manufacturing, Department of Engineering, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0FS, United Kingdom
3. Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom
4. Walter Schottky Institut, Fakultät für Physik, Technische Universität München, Am Coulombwall 4, 85748 Garching bei München, Germany