1. Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, National Biomedical Imaging Center, Key Laboratory of Polymer Chemistry & Physics, Peking University, Beijing 100871, P. R. China
2. Institute of Physics, Chinese Academy of Science, Beijing 100190, P. R. China
3. School of Material Science and Engineering, Tsinghua University, Beijing 100190, P. R. China