1. State Key Laboratory of High Performance Ceramics and Superfine MicrostructureShanghai Institute of CeramicsChinese Academy of Sciences Shanghai 200050 P. R. China
2. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Materials Science and EngineeringNational University of Singapore Singapore 117575 Singapore
4. NUSNNI‐NanocoreNational University of Singapore Singapore 117411 Singapore