1. School of Mechatronics & Vehicle Engineering, Chongqing Jiaotong University, Chongqing 400074, P. R. China
2. Institute of Solid State Physics and Key Laboratory of Photovoltaic and Energy Conservation Materials, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China
3. The Key Laboratory Functional Molecular Solids Ministry of Education, Anhui Normal University, Wuhu, Anhui 241002, P. R. China