1. Institute of Materials Research & Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore
2. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China
3. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
4. Department of Physics, National University of Singapore, Singapore 117551, Singapore
5. Center for Advanced Two-Dimensional Materials (CA2DM), National University of Singapore, Singapore 117546, Singapore
6. Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore