1. NanoMaterials Group, Laboratory of Physics and Center for New Materials, Helsinki University of Technology, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland, VTT Processes, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland, and Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland