1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
2. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
3. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
4. National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou 215123, P. R. China
5. Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, P. R. China
6. National Synchrotron Radiation Laboratory, Department of Chemical Physics, University of Science and Technology of China, Hefei 230029, P. R. China
7. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore