1. Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 11529, Taiwan
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
4. SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Shenzhen University, Shenzhen 518060, China
5. Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
6. Department of Physics, Chemistry, Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, 104 Davey Lab., University Park, Pennsylvania 16802, United States
7. Departament of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States