1. Department of Materials Science and Engineering, National University of Singapore 1 , Singapore 117575, Singapore
2. Department of Chemical and Biomolecular Engineering, Johns Hopkins University 2 , Baltimore, Maryland 21218, USA
3. Institute of Functional Intelligent Materials, National University of Singapore 3 , Singapore 117575, Singapore
4. Center for Advanced 2D Materials, National University of Singapore 4 , Singapore 117575, Singapore