1. Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal University, 500 Dongchuan Road, Shanghai 200241, P. R. China
2. Jiangsu Laboratory of Advanced Functional Materials, School of Electronic and Information Engineering, Changshu Institute of Technology, Changshu 215500, 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