1. Department of Physics; National University of Singapore; Singapore 117551 Singapore
2. Centre for BioImaging Sciences and Department of Biological Sciences; National University of Singapore; Singapore 117557 Singapore
3. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; Singapore 117546 Singapore
4. Department of Chemistry; University of Illinois at Chicago; Chicago IL 60607 USA
5. Department of Chemistry; National University of Singapore; Singapore 117543 Singapore
6. NUSNNI-NanoCore; National University of Singapore; Singapore 117411 Singapore
7. Departments of Physics and Biopharmaceutical Sciences; University of Illinois at Chicago; Chicago IL 60607 USA