1. Department of Engineering Sciences, Division of Electron Microscopy and Nanoengineering, Department of Engineering Sciences, Division of Nanotechnology and Functional Materials, and Department of Engineering Sciences, Division of Solid State Physics, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden, and Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala University, SE-751 85 Uppsala, Sweden