1. School of Materials 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. School of Physics and MicroelectronicsZhengzhou University Zhengzhou 450052 China
4. Department of Applied Physics and Materials Research CenterThe Hong Kong Polytechnic University Kowloon Hung Hom Hong Kong 99077 China