1. Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Department of Physics, State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, P. R. China
4. Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States