1. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Ecomaterials and Renewable Energy Research Center, Nanjing University 22 Hankou Road, Nanjing 210093, P. R. China, http://ererc.nju.edu.cn
2. Department of Physics, Nanjing University 22 Hankou Road, Nanjing 210093, P. R. China