1. State Key Laboratory of Surface Physics and Department of Physics and ‡Collaborative Innovation
Center of Advanced Microstructures, Fudan University, Shanghai 200433, China
2. School of Physics,
AMBER and CRANN Institute, Trinity College, Dublin 2, Ireland
3. Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
4. Graphene Research Center and Department
of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore