1. MESA+ Institute for Nanotechnology, MESA+ Strategic Research Orientation “Nanofabrication”, Molecular Nanofabrication, and Solid State Physics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, and Philips Research, High Tech Campus Eindhoven 11, 5656 AE Eindhoven, The Netherlands