1. National Laboratory of Solid-State Microstructures and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China
2. College of Engineering and Applied Sciences and Jiangsu Key Laboratory of Artificial Functional Materials, Nanjing University, Nanjing210093, China
3. Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland20899, United States
4. Maryland NanoCenter, University of Maryland, College Park, Maryland20742, United States