1. Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02141, United States
2. Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
3. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 117546 Singapore
4. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, 117575 Singapore
5. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
6. Yale-NUS College, 16 College Avenue West, 138527 Singapore