1. Hefei National Research Center for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, USA
3. School of Science, Beijing University of Posts and Telecommunications (BUPT), Beijing 100876, P. R. China
4. Department of Physics, University of Miami, Coral Gables, Florida 33146, USA