1. Department of Physics, Chair of Experimental Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstraße 7, 91058 Erlangen, Germany
2. Institute of Solid State Physics, Technische Universität Berlin, 10623 Berlin, Germany
3. Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
4. Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel