1. Department
of Physics, University of California, San Diego, La Jolla, California 92093, United States
2. Graphene Research Centre, National University of Singapore, 117542, Singapore
3. Department of Physics, Boston University, Boston, Massachusetts 02215, United States
4. Department of Physics and Astronomy, University of California, Riverside, California 92521, United States
5. Department
of Physics, Stony Brook University, Stony Brook, New York 11794, United States
6. Department
of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States
7. Ludwig-Maximilians-Universität and Center for Nanoscience, 80539 München, Germany