1. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
2. Ecomaterials and Renewable Energy Research Center (ERERC) at School of Physics, Nanjing University, Nanjing 210093, P. R. China
3. College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, P. R. China
4. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
5. Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, P. R. China