1. National Laboratory of Solid State Microstructures and Ecomaterials and Renewable Energy Research Center (ERERC) at Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People's Republic of China
3. Jiangsu Key Laboratory for Nano Technology, Nanjing 210093, People's Republic of China
4. College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, People's Republic of China