1. Department of Electrical Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom
2. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
3. Center for Nanostructured Graphene, Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
4. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
5. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, 02138, USA
6. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA