1. Department of Materials Science and Engineering, Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 United States
2. Center for Nanoscale Science and Technology, NIST, Gaithersburg, Maryland 20899 United States
3. Maryland Nanocenter at the University of Maryland, College Park, Maryland 20742 United States
4. Sandia National Laboratories, Livermore, California 94550 United States