1. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology
2. School of Science
3. Tianjin University
4. Tianjin 300072
5. P. R. China
6. College of Physics and Material Science
7. Tianjin Normal University
8. Tianjin 300074
9. China
10. Department of Physics
11. The University of Hong Kong
12. Hong Kong
13. Department of Information Materials Science and Engineering
14. Guilin University of Electronic Technology
15. Guilin 541004