1. CAS Center for Excellence in NanoscienceCAS Key Laboratory of Nanosystem and Hierarchical FabricationNational Center for Nanoscience and Technology Beijing 100190 P. R. China
2. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. State Key Laboratory of Solidification ProcessingCollege of Materials Science and EngineeringNorthwestern Polytechnical University Xi'an 710072 P. R. China
4. College of Physics and Materials ScienceHenan Normal University Xinxiang 453007 P. R. China
5. School of Physics and TechnologyWuhan University Wuhan 430072 P. R. China