1. Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112–0830, USA.
2. Department of Chemistry, University of Chicago, Chicago, IL 60637, USA.
3. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA.
4. Institut für Experimentelle und Angewandte Physik, Universität Regensburg, D-93040 Regensburg, Germany.