1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
2. SZU-NUS Collaborative Center and International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technology, Engineering Technology Research Center for 2D Materials Information Functional Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shen Zhen, 518060, China
3. School of Materials Science and Engineering, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Sun Yat-Sen University, Guangzhou 510275, P. R. China
4. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
5. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 2 Science Drive 3, Singapore 117546
6. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634
7. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456
8. Department of Physics & State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, P. R. China