1. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China
2. New Energy Technology Engineering Laboratory of Jiangsu Provence & School of Science, Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210046, China
3. School of Physics and Nano Institute, The University of Sydney, NSW 2006, Australia
4. School of Physics, Southeast University, Nanjing 211189, China