1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA.
2. Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA.
3. Physical Measurement Laboratory, NIST, Gaithersburg, MD 20899, USA.
4. Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA.