1. Experimental Physics 2, Department of Physics, TU Dortmund, 44227 Dortmund, Germany
2. Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
3. Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland
4. EMPA-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf CH-8600, Switzerland