1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
2. Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, USA
3. Weinberg Medical Physics LLC, Bethesda, Maryland 20817, USA
4. Materials Science and Engineering Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
5. Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA