1. Department of Physics and Fribourg Center for Nanomaterials, Université de Fribourg, Fribourg 1700, Switzerland
2. New Technologies-Research Center, University of West Bohemia, Plzeň 301 00, Czech Republic
3. Institut Jean Lamour, UMR 7198, CNRS-Université de Lorraine, Campus ARTEM, 2 allée André Guinier, BP 50840, Nancy 54011, France
4. Institut für Halbleiter-und Festkörperphysik, Johannes Kepler Universität, Linz 4040, Austria
5. Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
6. Photon Science Division, Paul Scherrer Institut, Villigen 5232, Switzerland