1. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
2. Department of Mechanical & Materials Engineering, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United States
3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
4. Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
5. Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
6. Department of Electrical & Computer Engineering, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United States