1. Department of Physics; National University of Singapore; 2 Science Drive 3 117542 Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; 6 Science Drive 2 117546 Singapore
3. Department of Chemistry; National University of Singapore; 3 Science Drive 3 117543 Singapore
4. Institute for Advanced Study and Department of Physics; Nanchang University; 999 Xue Fu Da Dao Nanchang 330031 China
5. Department of Chemistry; School of Science; Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering; Tianjin 300072 China
6. Beijing National Laboratory for Molecular Sciences; Key Laboratory of Organic Solids; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 China
7. National University of Singapore (Suzhou) Research Institute; 377 Lin Quan Street, Suzhou Industrial Park Jiang Su 215123 China