1. Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
2. Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
3. Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States