1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, 08-03, Singapore 138634
3. Center For Advanced 2D Materials and Graphene Research
Center, National University of Singapore, 6 Science Drive 2, Singapore 117546
4. Department of Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy