1. National Synchrotron Radiation Laboratory; CAS Center for Excellence in Nanoscience; University of Science and Technology of China; Hefei Anhui 230029 P. R. China
2. Beijing National Laboratory for Condensed Mater Physics; Institute of Physics; Chinese Academy of Sciences; Beijing 100190 China
3. Department of Materials Science and NanoEngineering; Rice University; Houston TX 77005 USA