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. Department of Physics
7. The University of Hong Kong
8. China
9. Department of Information Materials Science and Engineering
10. Guilin University of Electronic Technology
11. Guilin 541004
12. College of Physics and Material Science
13. Tianjin Normal University