1. Surface Physics Laboratory (National Key Laboratory), Department of Physics, Fudan University, Shanghai 200433, China
2. Department of Physics, the Chinese University of Hong Kong, Shatin, Hong Kong, China
3. Department of Microbiology and Microbial Engineering, School of Life Sciences, Fudan University, Shanghai 200433, China
4. Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China