1. State Key Laboratory of High Performance Ceramics and Superfine MicrostructuresShanghai Institute of CeramicsChinese Academy of Sciences 1295 Dingxi Road Shanghai 200050 P. R. China
2. University of Chinese Academy of Sciences No.19(A) Yuquan Road Beijing 100049 P. R. China
3. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
4. National Synchrotron Radiation LaboratoryUniversity of Science and Technology of China Hefei 230026 P. R. China
5. Beijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of Sciences Beijing 100190 P. R. China
6. South China University of Technology Guangzhou 510640 Guangdong P. R. China
7. Department of ChemistryThe City College of New York 160 Convent Avenue New York NY 10031 USA