1. Adolphe Merkle Institute University of Fribourg Chemin des Verdiers 4 Fribourg CH‐1700 Switzerland
2. Département de Physique and Fribourg Center for Nanomaterials Université de Fribourg Fribourg CH‐1700 Switzerland
3. Department of Materials and Earth Sciences Surface Science Laboratory Technical University of Darmstadt Otto‐Berndt‐Strasse 3 64287 Darmstadt Germany
4. Department of Materials Science and Engineering Cornell University 330 Bard Hall Ithaca NY 14853‐1501 USA