1. Section for Surface Physics and Catalysis, Department of Physics, Technical University of Denmark, Kongens Lyngby DK-2800, Denmark
2. HPNow, Ole Maaløes Vej 3, Copenhagen N, DK-2200, Denmark
3. Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen Ø DK-2100, Denmark
4. Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdom