1. Department of Chemistry and Graphene Research Centre; National University of Singapore; 3 Science Drive 3 117543 Singapore
2. 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 TechnologySoochow University; Suzhou 215123 P.R. China
3. Department of Electrical and Computer Engineering; National University of Singapore; 117576 Singapore
4. NUSNNI-NanoCore; National University of Singapore; 117576 Singapore
5. Department of Physics; National University of Singapore; 117576 Singapore