1. Department of Physics, East China University of Science and Technology, Shanghai 200237, China
2. Department of Electrical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. School of Opto-Electronic and Communication Engineering, Xiamen University of Technology, Xiamen 361024, China
4. Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China