1. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics and Center for Carbon-based Electronics, Peking University, Beijing 100871, P. R. China
2. State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P. R. China
3. Department of Chemistry, The University of Hong Kong, Hong Kong 999077, P. R. China
4. Collaborative Innovation Center of Quantum Matter, Beijing 100871, P. R. China
5. Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKL-MEMD), Beijing 100871, P. R. China