1. Sargent Joint Research Center, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
3. School of Science, Hainan University, Haikou 570228, P. R. China