1 Microsystems and Nanotechnology Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
2 Institute for Research in Electronics and Applied Physics and Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA
3 Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
4 Joint Quantum Institute, NIST/University of Maryland, College Park, Maryland 20742, USA
5 e-mail: kartik.srinivasan@nist.gov