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. State Key Laboratory for Artificial Microstructure and Mesoscopic Physics; School of Physics; Peking University; Beijing 100871 P.R. China
3. Materials Sciences Division; Lawrence Berkeley National Laboratory; Berkeley CA 94720 USA