1. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing, 100871, China
3. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
4. Department of Electrical and Computer Engineering, George Mason University, Fairfax, Virginia 22030, United States