1. College of Materials Science and Opto-Electronic Technology & Key Laboratory of Vacuum Physics
2. University of Chinese Academy of Sciences
3. Beijing 100049
4. P. R. China
5. Department of Materials Science and Engineering
6. College of Engineering
7. Key Laboratory of Polymer Chemistry and Physics of Ministry of Education
8. Peking University
9. Beijing 100871
10. Department of Physics and Astronomy
11. Shanghai Jiaotong University