1. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, 117546 Singapore
2. Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
3. Graduate School for Integrative Sciences and Engineering (NGS), National University of Singapore, 28 Medical Drive, 117456 Singapore
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan