1. London Centre for Nanotechnology, University College London 1 , London WC1H 0AH, United Kingdom
2. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen 2 , 47057 Duisburg, Germany
3. Department of Solid State Sciences, Ghent University 3 , 9000 Ghent, Belgium
4. Institut für Physik, Johannes Gutenberg-Universität Mainz 4 , Staudingerweg 7, 55128 Mainz, Germany
5. Department of Electronic and Electrical Engineering, University College London 5 , London WC1E 7JE, United Kingdom
6. WPI Advanced Institute for Materials Research, Tohoku University 6 , 2-1-1, Katahira, Sendai 980-8577, Japan