1. Department of Physics, National University of Singapore, Singapore 117551, Singapore
2. College of Physics and Space Science, China West Normal University, Nanchong 637002, China
3. Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore
4. Centre for Advanced Two-Dimensional Materials, National University of Singapore, Singapore 117546, Singapore
5. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis 138634, Singapore