1. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China
2. College of Physics and Materials Science, Tianjin Normal University, Tianjin 300074, China
3. Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong
4. Department of Information Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, Guangxi, China