1. MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands
2. Advanced Materials Department, Jožef Stefan Institute, 1000 Ljubljana, Slovenia
3. Laboratory of Physics, Vinca Institute of Nuclear Sciences, University of Belgrade, 11000 Belgrade, Serbia
4. IBM Research-Zurich, 8803 Rüschlikon, Switzerland
5. Lumiphase AG, 8003 Zürich, Switzerland