1. Institute of Optoelectronics and Nanomaterials, MIIT Key Laboratory of Advanced Display Materials and Devices, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2. School of Physics, Nanjing University of Science and Technology, Nanjing 210094, China
3. Key Laboratory of Functional Materials and Applications of Fujian Province, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, P. R. China