1. Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China
2. Department of Chemistry, Capital Normal University, Beijing 100048, China
3. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
4. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing 100083, China