1. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
2. Institute of Materials Research & Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634
3. Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor Straße 6, 12489 Berlin, Germany
4. SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, China
5. Department
of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore 117542
6. Research
Center for Applied Sciences, Academia Sinica, Taipei 10617, Taiwan
7. Physical
Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
8. Centre
for Advanced 2D Materials, National University of Singapore, Block S14, Level
6, 6 Science Drive 2, Singapore 117546