1. Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Physical Measurement Laboratory, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, United States
3. Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, United States
4. Materials Science and Engineering Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, United States