1. CAS Center for Excellence in Nanoscience CAS Key Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Key Laboratory of Semiconductor Materials Science Institute of Semiconductors Chinese Academy of Sciences No.A35, QingHua East Road Haidian District Beijing 100083 P. R. China
4. Department of Physics Hubei Nuclear Solid Physics Key Laboratory Wuhan University Wuhan 430072 P. R. China
5. State Key Lab Solidification Processing College of Materials Science and Engineering Northwestern Polytechnical University Xi'an Shaanxi 710072 P. R. China