1. CAS Center for Excellence in NanoscienceCAS Key Laboratory of Nanosystem and Hierarchical FabricationNational Center for Nanoscience and Technology Beijing 100190 China
2. Center of Materials Science and Optoelectronics EngineeringUniversity of Chinese Academy of Sciences Beijing 100049 China
3. Key Laboratory of Semiconductor Materials ScienceBeijing Key Laboratory of Low Dimensional Semiconductor Materials and DevicesInstitute of SemiconductorsChinese Academy of Sciences Beijing 100083 China
4. School of Physics and EngineeringZhengzhou University Zhengzhou 450001 China
5. School of Physics and TechnologyWuhan University Wuhan 430072 China