1. Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, Department of Electrical and Computer Engineering, Rice Quantum Institute, and Center for Nanoscale Science and Technology, Rice University, P.O. Box 1892, Houston, Texas 77251, and Fritz-Haber-Institut der Max-Plack-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany