1. i3N—Institute for Nanostructures, Nanomodelling and Nanofabrication, Department of Physics, University of Aveiro, 3810-193 Aveiro, Portugal
2. NanoElectronics Group, MESA+ Institute for Nanotechnology, University of Twente, 7522 NB Enschede, The Netherlands
3. Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
4. Department of Chemical and Biomolecular Engineering, New York University, Brooklyn, 11201 New York, United States
5. Departamento de Fı́sica e Astronomia, IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Universidade do Porto, 4169-007 Porto, Portugal
6. Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany