1. School of Environment and Energy, State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Solid Wastes Pollution Control and Recycling, South China University of Technology, Guangzhou 510000, P. R. China
2. Department of Physics and William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong, P. R. China
3. School of Materials, Sun Yat-sen University, Guangzhou 510275 P. R. China
4. Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China
5. School of optics and photonics, Beijing Institute of Technology, Beijing 100081, P. R. China
6. Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New
Territories, 999077 Hong Kong, P. R. China