1. ZJUI-UIUC Institute College of Energy Engineering Zhejiang University Hangzhou Zhejiang 31440 China
2. Department of Physics National University of Singapore 2 Science Drive 3 117542 Singapore
3. Institute of Materials Research and Engineering A*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way Innovis, Singapore 138634 Singapore
4. Department of Chemistry National University of Singapore 3 Science Drive 3 117543 Singapore
5. Center For Advanced 2D Materials and Graphene Research Center National University of Singapore 6 Science Drive 2 117546 Singapore