1. Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Republic of Singapore
2. Department of Physics, National University of Singapore, Singapore 117551, Singapore
3. Centre for Advanced Two-Dimensional Materials (CA2DM), National University of Singapore, Singapore 117546, Singapore
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
5. Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore