1. Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
2. Max Planck Institute for the Structure and Dynamics of Matter, CFEL, Luruper Chaussee 149, 22761 Hamburg, Germany
3. Center for Nanointegration (CENIDE), Carl-Benz-Straße 201, 47057 Duisburg, Germany