1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Centre for Advanced 2D Materials (CA2DM) and Graphene Research Centre (GRC), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
3. Songshan Lake Materials Laboratory, Dongguan 523808, China
4. School of Electronics and Information, Northwestern Polytechnical University, Xi’an 710072, P.R. China
5. Instituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, Area Science Park, S.S. km 163.5, Trieste I-34149, Italy
6. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore
7. Singapore Synchrotron Light Source (SSLS), National University of Singapore, 5 Research Link, Singapore 117603, Singapore