1. College of Materials Science and Opto-Electronic Technology and Key Laboratory of Vacuum Physic; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
2. School of Electronic, Electrical and Communication Engineering; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
3. State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences; Institute of Chemistry Chinese Academy of Sciences; Beijing 100190 P. R. China
4. Department of Physics and Astronomy; Shanghai Jiaotong University; Shanghai 200240 P. R. China