1. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China
2. School of Physics, Henan Normal University, Xinxiang, Henan 453007, P. R. China
3. School of Materials Science and Engineering, Institute of New Energy, China University of Petroleum (East China), Qingdao, Shandong 266580, P. R. China
4. ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia