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