1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 117543 Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; 6 Science Drive 2 117546 Singapore
3. Institute of Functional Nano and Soft Materials (FUNSOM); Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology; Soochow University; Suzhou 215123 P. R. China
4. Institute of Materials Research and Engineering; A*STAR (Agency for Science Technology and Research); 2 Fusionopolis Way 138634 Singapore
5. Department of Materials Science and Engineering; National University of Singapore; 9 Engineering Drive 1 117575 Singapore
6. Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 117583 Singapore
7. Department of Materials Science and Engineering; Faculty of Engineering; Monash University; Wellington road Clayton 3800 Victoria Australia