1. Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
2. Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
3. Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
4. Department of Physics, Jiangsu Second Normal University, Nanjing 210013, China