1. Department of Chemistry, National University of Singapore, Singapore 117543
2. Centre for Advanced 2D Materials, National University of Singapore, Singapore 117546
3. State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou, China 310027
4. Institute of Materials Research and Engineering, Agency for Science Technology and Research, 2 Fusionopolis Way, Singapore 138634
5. Department of Physics, National University of Singapore, Singapore 117542