1. Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom
2. Cavendish Laboratory, University of Cambridge, Cambridge CB2 1TN, United Kingdom
3. Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore
4. Division of Material and Nano Physics, Department of Applied Physics, KTH Royal Institute of Technology, Stockholm, SE-106 91, Sweden
5. Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom
6. Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
7. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117542, Singapore