1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.
2. Centre for Advanced 2D Materials (CA2DM), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore.
3. Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan.
4. Institute of Physics, The Czech Academy of Sciences, Prague 16200, Czech Republic.
5. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456, Singapore.
6. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
7. Regional Centre of Advanced Technologies and Materials, Palacký University, Olomouc 78371, Czech Republic.