1. International Academy of Optoelectronics at Zhaoqing, South China Normal University, Zhaoqing, 526238 Guangdong, P. R. China
2. ARC Centre of Excellence in Exciton Science, School of Chemistry, University of Melbourne, Parkville, VIC, 3010, Australia
3. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany