1. Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, P. R. China
2. Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, P. R. China
3. School of Physics, Henan Key Laboratory of Photovoltaic Materials, Henan Normal University, Xinxiang, Henan 453007, P. R. China
4. School of Physics and Optoelectronics Engineering, Anhui University, Hefei 230601, China