1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
2. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA.
3. Department of Mechanical and Materials Engineering, University of Nebraska–Lincoln, Lincoln, NE 68588, USA.
4. Department of Physics and Astronomy, University of Nebraska–Lincoln, Lincoln, NE 68588, USA.