1. Key Laboratory of Polar Materials and Devices (MOE) Department of Electronics East China Normal University Shanghai 200241 P. R. China
2. Jiangsu Laboratory of Advanced Functional Materials School of Electronic and Information Engineering Changshu Institute of Technology Changshu 215500 P. R. China
3. Department of Physics Department of Materials Science and Engineering and Department of Biomedical Engineering City University of Hong Kong Tat Chee Avenue Kowloon Hong Kong China
4. School of Physics and Electronic Engineering Hanshan Normal University Chaozhou 521041 P. R. China