1. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China
2. Henan Key Laboratory of Rare Earth Functional Materials, Zhoukou Normal University, Zhoukou 466001, P. R. China
3. State Key Laboratory for Environment-friendly Energy Materials, School of Materials and Chemistry, Southwest University of Science and Technology, Mianyang 621010, P. R. China
4. School of Geography and Biological Information, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China
5. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States
6. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States