1. School of Physical Science and Technology, Soochow University, Suzhou 215006, P. R. China
2. School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
3. Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, P.R. China
4. Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P. R. China