1. Department of Physics, National University of Singapore, Singapore 117551
2. Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 117557
3. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575
4. Institute of Materials Research and Engineering, Agency for Science, Technology, and Research (A*STAR), Singapore 138634
5. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546
6. NUSNNI-NanoCore, National University of Singapore, Singapore 117411