1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P.R. China
2. College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P.R. China
3. School of Physics & Engineering, Zhengzhou University, Zhengzhou 450001, P.R. China
4. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P.R. China
5. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States