1. Centre for Advanced 2D Materials (CA2DM) and Graphene Research Centre (GRC), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
3. Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*Star), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore
4. Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, Berkeley CA94720, United States
5. Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
6. NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands