1. Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, School of Physics, Nanjing University, No. 22 Hankou Road, Nanjing 210093, Jiangsu, P. R. China
2. Eco-materials and Renewable Energy Research Center, College of Engineering and Applied Sciences, Nanjing University, No. 22, Hankou Road, Nanjing 210093, Jiangsu, P. R. China
3. National Demonstration Center for Experimental Physics and Electronics Education, School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China
4. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, P. R. China