1. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, People's Republic of China
2. Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong, People's Republic of China
3. School of Physics and Electronic Sciences, Changsha University of Science and Technology, Changsha 410004, China
4. ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia