1. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, United States
3. Rensselaer Polytechnic Institute, 8th Street, Troy, New York 12180, United States
4. IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, United States