1. American Dental Association Science and Research Institute, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
2. Microsystems and Nanotechnology Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
3. Radiation Physics Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States
4. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States