1. Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States
2. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
3. Theiss Research, Inc., La Jolla, California 92037, United States
4. Nanoscale Device Characterization Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States