1. Central Facility of Electron Microscopy, Ulm University, Albert Einstein Allee 11, 89069 Ulm, Germany
2. Physics/Department of Natural Sciences, Åbo Akademi, FI-20500 Turku, Finland
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany
4. Department of Applied Physics, Aalto University, PO Box 14100, 00076 Aalto, Finland