1. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China
2. School of Optoelectronic Engineering, Guangdong Polytechnic Normal University, Guangzhou, 510665, China
3. Department of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong 999077, China