1. School of Material Science and Engineering and Anhui Provincial Key Laboratory of Advanced Functional Materials and DevicesHefei University of Technology Hefei 230009 China
2. School of Electronic Science and Applied PhysicsHefei University of Technology Hefei 230009 China
3. Key Laboratory of Materials Physics of Ministry of EducationDepartment of Physics and EngineeringZhengzhou University Zhengzhou 450052 China