1. Laboratory of Molecular Materials, Department of Engineering Physics and Mathematics and Centre for New Materials, Helsinki University of Technology, P.O. Box 5100, FI-02015 TKK, Espoo, Finland, Institute of Biotechnology, University of Helsinki, P.O. Box 65, Viikinkaari 1, FI-00014 Helsinki, Finland, and Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, FI-02015 TKK, Espoo, Finland