1. State
Key Laboratory of Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
2. College
of Science, National University of Defense Technology, Changsha 410073, China
3. State
Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Changchun, Jilin 130022, China