1. Department of Materials Science; Fudan University; Shanghai 200433 P. R. China
2. Department of Physics; Harbin Institute of Technology; Harbin 150001 P. R. China
3. State Key Laboratory of Luminescence and Applications; Changchun Institute of Optics; Fine Mechanics and Physics; Chinese Academy of Sciences; Changchun 130033 P. R. China
4. State Key Laboratory of ASIC and System; Shanghai Institute of Intelligent Electronics and Systems; School of Microelectronics; Fudan University; Shanghai 200433 P. R. China