1. School of Materials and Chemical Engineering, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China
2. Department of Electric Power, North China University of Water Resources and Electric Power, Zhengzhou 450000, P. R. China
3. Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, P. R. China