1. Department
of Physics, South University of Science and Technology of China, Shenzhen 518055, China
2. Department
of Electronics and Tianjin Key Laboratory of Photo-Electronic Thin
Film Device and Technology, Nankai University, Tianjin 300071, China
3. Department
of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
4. Key
Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, China
5. University of Science and Technology of China, Hefei 230026, China