1. School of Physics and Electronics State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Hunan University Changsha 410082 P. R. China
2. International Joint Laboratory of New Energy Materials and Devices of Henan Province School of Physics and Electronics Henan University Kaifeng 475004 P. R. China
3. College of Materials Science and Engineering Hunan University Changsha 410082 P. R. China
4. School of Materials Science and Engineering Central South University Changsha 410083 P. R. China