1. Anhui Province Key Laboratory of Optoelectric Materials Science and Technology College of Physics and Electronic Information Anhui Normal University Wuhu 241002 P. R. China
2. Department of Physics Shanghai Normal University Shanghai 200234 P. R. China
3. Institute for Clean Energy and Advanced Materials School of Materials and Energy Southwest University Chongqing 400715 P. R. China
4. Shanghai Aerospace Electronics Technology Institute Shanghai 201108 P. R. China
5. National Laboratory of Solid State Microstructures Nanjing University Nanjing 210093 P. R. China