1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
2. Centre for Advanced 2D Materials, National University of Singapore, 2 Science Drive 2, Singapore 117542, Singapore
3. National Institute for Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore