1. Center for Nanophysics and Advanced
Materials, University of Maryland, College
Park, Maryland 20742-4111, United States
2. Yale-NUS College, 6 College
Avenue East, 138614, Singapore
3. Graphene Research Centre and Department
of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore
4. Department
of Physics, The City College of New York, New York, New York 10031,
United States
5. Advanced
Materials Laboratory, National Institute for Materials Science, 1-1 Namiki,
Tsukuba, 305-0044, Japan
6. School
of Physics, Monash University, Victoria 3800, Australia