1. Department of Engineering Physics and Center for New Materials, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Espoo, Finland, Institute of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom, and Nokia Research Center, Nanosciences, Itämerenkatu 11−13, FI-00180 Helsinki, Finland