1. Department of Physics, Nanoscience Center, FIN-40014 University of Jyväskylä, Finland, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, Department of Chemistry, Nanoscience Center, FIN-40014 University of Jyväskylä, Finland, and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden