1. Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, Sweden, Service de Chimie des Matériaux Nouveaux, Université de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium, and Department of Science and Technology, Campus Norrköping, Linköping University, S-60174 Norrköping, Sweden