1. Department of Chemistry, National University of Singapore, 3 Science Drive 3 117543, Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore 117546, Singapore
3. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, People’s Republic of China
4. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore 117456, Singapore