1. New Energy Technology Engineering Laboratory of Jiangsu Province & School of Science, Nanjing University of Posts and Telecommunications (NJUPT), Nanjing 210023, P. R. China
2. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. School of Physics, Southeast University, Nanjing 211189, P. R. China