1 Key Laboratory for the Physics and Chemistry of Nanodevices, Center for Carbon-Based Electronics and School of Electronics, Peking University, Beijing 100871, China.
2 Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China.
3 Key Laboratory for the Physics and Chemistry of Nanodevices, Center for Carbon-Based Electronics and School of Electronics, Peking University, Beijing 100871, China; Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China. Electronic address: youfanhu@pku.edu.cn.