1. School of Science, Harbin Institute of Technology, Shenzhen 518055, P. R. China
2. Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR 999077, P. R. China
3. Department of Physics, Xiamen University, Xiamen 361005, P. R. China
4. College of Physical Science and Technology, Bohai University, Jinzhou 121013, P. R. China