1. Department of Physics, Technical University of Denmark (DTU), Kgs. Lyngby 2800, Denmark
2. Centre for Nanostructured Graphene (CNG), Technical University of Denmark, Ørsteds Plads 345C, Kgs. Lyngby DK-2800, Denmark
3. DTU Nanolab - National Centre for Nano Fabrication and Characterization, Technical University of Denmark (DTU), Kgs. Lyngby 2800, Denmark
4. Department of Physics, Columbia University, New York, New York 10027, United States