1. Department of Physics, University of California, San Diego (UCSD), La Jolla, CA 92093, USA.
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Department of Physics, Boston University, Boston, MA 02215, USA.
4. Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.
5. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
7. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA.
8. Department of Physics, California State University, San Marcos, San Marcos, CA 92096, USA.
9. Graphene Research Centre and Physics Department, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore.
10. Kavli Energy NanoSciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
11. Ludwig-Maximilians-Universität and Center for Nanoscience, 80539 München, Germany.