1. School
of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
2. School
of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China
3. Division
of Functional Materials, Central Iron and Steel Research Institute, Beijing 100081, China
4. Key
Laboratory for the Physics and Chemistry of Nanodevices and Department
of Electronics, Peking University, Beijing 100871, China