1. Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany
2. Center for Nanostructured Graphene, Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
3. Department of Applied Mathematics and Computer Science, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark