1. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China
2. Beijing Synchrotron Radiation Facility,
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Shanghai Institute Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China
4. Center of Electron Microscopy, State Key Laboratory of Silicon Materials,
Department of Materials Science and Engineering, Zhejiang University, Zheda Road 38, Hangzhou, Zhejiang 310027, China