1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
3. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
4. Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria 3086, Australia
5. Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
6. ARC Centre of Excellence in Future Low-Energy Electronics Technologies, School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia