1. School of Microelectronics Shandong Technology Center of Nanodevices and Integration State Key Laboratory of Crystal Materials Shandong University Jinan 250100 P. R. China
2. School of Physics Shandong University Jinan 250100 P. R. China
3. State Key Laboratory of Crystal Materials Institute of Crystal Materials Shandong University Jinan 250100 P. R. China
4. School of Physics Sun Yat‐Sen University Guangzhou 510275 P. R. China
5. Vacuum Interconnected Nanotech Workstation Suzhou Institute of Nano‐Tech and Nano‐Bionics Chinese Academy of Sciences Suzhou 215123 P. R. China