1. Department of Engineering Physics and Mathematics, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland; Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014, Helsinki, Finland; PANIPOL Ltd., P.O. Box 163, FIN-06101, Porvoo, Finland; and Institute of Biotechnology, Electron Microscopy, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki, Finland