1. Department of Materials Science Fudan University Shanghai 200433 P. R. China
2. Zhangjiang Fudan International Innovation Center Shanghai 201210 P. R. China
3. Institute of Optoelectronics Fudan University Shanghai 200433 P. R. China
4. Key Laboratory of Materials Physics of Ministry of Education School of Physics and Microelectronics Zhengzhou University Daxue Road 75 Zhengzhou 450052 P. R. China