1. CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, People's Republic of China
2. University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
3. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics and Tsinghua–Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People's Republic of China