1. Department of Physics, Imperial College London, London SW7 2BW, U.K.
2. Nanoinstitut München, Chair in Hybrid Nanosystems, Faculty of Physics, Ludwig-Maximilians Universität München, Königinstrasse 10, 80539 München, Germany
3. Department of Materials, Imperial College London, London SW7 2AZ, U.K.
4. Thomas Young Centre for Theory and Simulation of Materials, Imperial College London, London SW7 2AZ, U.K.