1. College of Polymer Science and Engineering and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China
2. Vacuum Electronics National Laboratory, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. Department of Physics and Electrons, Ludong University, Yantai 264026, P. R. China
4. Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400