1. a IFF, Center of Nanoelectronic Systems for Information Technology, Research Center Juelich , D-52428 , Juelich , Germany
2. b Institute of Crystallography, Russian Academy of Sciences , 119333 , Moscow , Russia
3. c Institute of Bio- and Nanosystems, Center of Nanoelectronic Systems for Information Technology, Research Center Juelich , D-52428 , Juelich , Germany