1. a Department of Material Science and Engineering, National Key Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
2. b Department of Material Science and Engineering, National Key Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
3. c School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China
4. d School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
5. e Physics Department, Nanjing University, Nanjing 210093, P. R. China