1. Walter Schottky Institut, Physik Department, and Center for Nanotechnology and Nanomaterials, Technische Universität München, Am Coulombwall 4, Garching, 85748, Germany
2. Department of Materials Science & Engineering, Northwestern University, Evanston, Illinois 60208, United States
3. Department of Chemistry, Ludwig-Maximilian-Universität München, Butenandtstraße 5-13, München, 81377, Germany