1. Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, United Kingdom
2. Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe,
Germany
3. Institut für
Materialwissenschaft, Technische Universität Darmstadt, 64287 Darmstadt,
Germany
4. IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, United
States