1. Department of Physics, Chair for Applied Physics, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Staudtstr. 7, 91058 Erlangen, Germany
2. Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), and AMBER at CRANN, Trinity College Dublin, College Green, Dublin 2, Ireland
3. School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
4. Materials Chemistry and Analysis Group, Department of Chemistry, Tyndall Institute, University College Cork, Cork, Ireland