1. Department of Applied Physics School of Physics and Electronics Hunan University Changsha 410082 P. R. China
2. Beijing National Laboratory for Molecular Sciences (BNLMS) Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing 100190 P. R. China
3. School of Materials Science and Engineering Hunan University Changsha 410082 P. R. China
4. State Key Laboratory of Powder Metallurgy Central South University Changsha 410083 P. R. China