1. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
2. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
3. Graphene Research Centre, Faculty of Science, National University of Singapore, 6 Science drive 2, Singapore 117546
4. Kavli Institute for Bionano Science & Technology, Harvard University, School of Engineering and Applied Sciences, Pierce Hall, 29 Oxford Street, Cambridge, Massachusetts 02138, United States