1. Department of Physics, National University of Singapore, Singapore 117542, Singapore
2. Centre for Advanced 2D Materials and Graphene Research Center, National University of Singapore, Singapore 117542, Singapore
3. Advanced Material Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Chemistry
Department, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
5. Yale-NUS College, 6 College Avenue East, Singapore 138614, Singapore
6. NanoCore, National University of Singapore, Singapore 117576, Singapore