1. Department of Physics, National Laboratory of Solid State Microstructures & Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, Nanjing 210093, P. R. China
2. School of Mechanical and Electronic Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, P. R. China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
4. Jiangsu Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, P. R. China