1. School of Electronic Science and Engineering, Southeast University, Nanjing, Jiangsu 210096, China
2. Center for Nanoscale Systems, Harvard University, Cambridge, Massachusetts 02138, United States
3. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States
4. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
5. John A. Paulson School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 02138, United States