1. MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands; Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6700 EK Wageningen, The Netherlands; FOM AMOLF Institute for Atomic and Molecular Physics, 1009 DB Amsterdam, The Netherlands; and Electron Microscopy-unit, Department of Pathology, Maastricht University, 6200 MD Maastricht, The Netherlands