1. National Laboratory of Solid‐State Microstructures Jiangsu Key Laboratory of Artificial Functional Materials College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China
2. Collaborative Innovation Center of Advanced Microstructures Nanjing 210093 China
3. Physical Measurement Laboratory National Institute of Standards and Technology Gaithersburg MD 20899 USA
4. Maryland NanoCenter University of Maryland College Park MD 20899 USA