1. School of Physics and Electronics Hunan University Changsha 410082 P. R. China
2. School of Chemistry and Materials Engineering Anhui Science and Technology University Bengbu 233030 P. R. China
3. Department of Physics and Astronomy Clemson Nanomaterials Institute Clemson University Clemson SC 29625 USA
4. School of Materials Science and Engineering Central South University Changsha 410083 P. R. China