1. Department of Engineering Physics and Mathematics, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland; Institute of Biotechnology, Electron Microscopy, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki, Finland; and Neste Chemicals Ltd., P.O. Box 310, FIN-06101 Porvoo, Finland