1. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
2. Department of Mathematics, Jilin University, Changchun 130012, P. R. China
3. Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
4. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States