1. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology Beijing 100190 P. R. China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro‐nano Devices Renmin University of China Beijing 00872 P. R. China
4. CAS Key Laboratory for Standardization and Measurement for Nanotechnology National Center for Nanoscience and Technology Beijing 100190 P. R. China
5. Key Laboratory of Artificial Micro‐ and Nano‐structures of Ministry of Education, School of Physics and Technology Wuhan University Wuhan 430072 P. R. China