1. Nanoscale Processes and Measurements Group, Nanoscale Device Characterization Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
2. Theiss Research, La Jolla, California 92037, USA
3. Biophysical and Biomedical Measurement Group, Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA