1. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore
2. Department of Physics, National University of Singapore, 2 Science Drive 3, 117551, Singapore
3. Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore
4. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 119077, Singapore
5. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan