1. Department of Physics; National University of Singapore; Singapore 117542 Singapore
2. Department of Chemistry; National University of Singapore; Singapore 117543 Singapore
3. Center for Advanced 2D Materials; National University of Singapore; Singapore 117546 Singapore
4. School of Physics; Southeast University; Nanjing 211189 P. R. China
5. Institute of Materials Research and Engineering; 2 Fusionopolis Way, Innovis, #08-03 Singapore 138634 Singapore
6. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
7. International Center for Materials Nanoarchitectonics; National Institute for Materials Science; Tsukuba 305-0044 Japan
8. School of Physics and Electronics; Central South University; Changsha 410083 China
9. Institute for Advanced Study and Department of Physics; Nanchang University; Nanchang 330031 P. R. China
10. National University of Singapore (Suzhou) Research Institute; Su Zhou 215123 P. R. China