1. Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, United States
2. National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, and Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
3. Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States
4. Department of Physics, Beijing Normal University, Beijing 100875, China
5. School of Physics, Nanjing University, Nanjing 210093, China