1. National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China
2. School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
4. Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics, Nanjing University, Nanjing 210093, China