1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 Singapore 117543 Singapore
2. Department of Physics; Bernal Institute; University of Limerick; Limerick V94 T9PX Ireland
3. Singapore Synchrotron Light Source; National University of Singapore; 5 Research Link Singapore 117603 Singapore
4. Centre for Advanced 2D Materials, Graphene Research Centre; National University of Singapore; 6 Science Drive 2 Singapore 117546 Singapore
5. NUSNNI-Nanocore; National University of Singapore; Singapore 117411 Singapore