1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
2. Centre for Ion Beam Applications (CIBA), Department of Physics, National University of Singapore, Singapore 117542, Singapore
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
4. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
5. Research Centre for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan
6. International Centre for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan
7. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634, Singapore
8. Centre for Advanced 2D Materials, National University of Singapore, 117542, Singapore, Singapore