1. Department of Physics, Nanoscience Center, University of Jyväskylä, FI-40014 Jyväskylä, Finland, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, Competence Center for Catalysis and Department of Applied Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden, and Department of Chemistry, Nanoscience Center, University of Jyväskylä, FI-40014 Jyväskylä, Finland