1. China Institute of Atomic Energy, Beijing 102413, P. R. China
2. Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629JB, Netherlands
3. School of Advanced Energy, Shenzhen Campus of Sun Yat-Sen University, Shenzhen 518107, P. R. China
4. State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China