1. National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, P. R. China
2. Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, P. R. China
3. School of Materials Science and Engineering, Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China
4. Zhejiang Institute of Photonelectronics, Jinhua, Zhejiang 321004, P. R. China