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. State Key Laboratory of Low‐Dimensional Quantum Physics Department of Physics Tsinghua University Beijing 100084 P. R. China
4. Key Laboratory of Artificial Micro‐ and Nano‐structures of Ministry of Education School of Physics and Technology Wuhan University Wuhan 430072 P. R. China