1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 Singapore 117543 Singapore
2. i -Lab; Suzhou Institute of Nano-Tech and Nano-Bionics; Chinese Academy of Science; Suzhou 215123 China
3. Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 Singapore 117583 Singapore
4. Institute of Materials Research and Engineering (IMRE); Agency for Science; Technology and Research (A*STAR); 3 Research Link Singapore 117602 Singapore
5. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; 6 Science Drive 2 Singapore 117546 Singapore