1. Department of Applied Physics, College of Science, Nanjing University of Aeronautics and Astronautics, 211106 Nanjing, P. R. China
2. School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States
3. State Key Laboratory of Functional Material for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200250 Shanghai, P. R. China
4. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, 610054 Chengdu, P. R. China