1. Department of Electrical and Computer Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore
2. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
4. CAS Key Laboratory of Nanophotonic Materials and Devices & Key Laboratory of Nanodevices and Applications, i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
5. Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore 117543, Singapore
6. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, U.K.