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