1. Department of Materials Science and Engineering, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, INRS-EMT, Université du Québec, J3X 1S2 Varennes, Québec, Canada, Department of Applied Physics and Physico-Informatics, Keio University, Kouhoku-Ku, Yokohama 223-8522, Japan, CNR-INFM, Laboratorio Nazionale TASC, Basovizza, 34012 Trieste, Italy, and Sincrotrone Trieste S.C.p.A., Basovizza, 34012 Trieste, Italy