1. Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8th St., Troy, New York 12180, USA
2. Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, USA
3. Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, Maryland 20899, USA