1. Department of Electrical and Computer Engineering, George Mason University, Fairfax, Virginia 22030, USA
2. Nanoscale Device Characterization Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, USA
3. Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
4. Chemical Sciences Division, NIST, Gaithersburg, Maryland 20899, USA
5. Microsystems and Nanotechnology Division, NIST, Gaithersburg, Maryland 20899, USA