1. Technical Chemistry I and Center for Nanointegration Duisburg-Essen (CENIDE); University of Duisburg-Essen; Universitätsstraße 7 45141 Essen Germany
2. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE); University of Duisburg-Essen; 47057 Duisburg Germany