1. National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China
2. Department of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China
3. Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 201800, China
4. Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States