1. Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
2. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742, USA.
3. Nanoscale Devices Characterization Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
4. Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria.
5. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.