1. Department of ChemistryNational University of Singapore 3 Science Drive 3 Singapore 117543 Singapore
2. Centre for Advanced 2D Materials and Graphene Research CentreNational University of Singapore Singapore 117546 Singapore
3. State Key Laboratory for Mesoscopic Physics and Department of PhysicsPeking University Beijing 100871 P. R. China
4. Department of Materials Science & EngineeringNational University of Singapore 9 Engineering Drive 1 Singapore 117575 Singapore
5. Department of PhysicsNational University of Singapore 2 Science Drive 3 Singapore 117542 Singapore
6. Institut für Kristallzüchtung Max‐Born‐Str. 2 Berlin 12489 Germany
7. School of PhysicsSun Yat‐sen University Guangzhou 510275 P. R. China