1. Institute of Microelectronics, Chinese Academy of Science, Beijing 100029, China
2. Key Laboratory for the Physics and Chemistry of Nanodevices and Center for Carbon-based Electronics, School of Electronics, Peking University, Beijing 100871, China
3. School of Integrated Circuits, Nanjing University, Qixia District, Nanjing, Jiangsu 210023, China
4. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China