1. Department of Physics, Faculty of Science, National University of Singapore, Singapore 117542, Singapore
2. Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603, Singapore
3. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore
4. NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore
5. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore
6. Computational Condensed Matter Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
7. Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, Singapore