1. Department of Physics, National University of Singapore, Singapore 117542, Singapore
2. Singapore-MIT Alliance for Research and Technology Centre, Singapore 138602, Singapore
3. Centre for Advanced 2D Materials, National University of Singapore, Singapore 117546, Singapore
4. Department of Biomedical Engineering, National University of Singapore, Singapore 117583, Singapore
5. Department of Physics, City University of Hong Kong, Kowloon, Hong Kong, P. R. China
6. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, United States