1. Department of Materials; ETH Zurich; Vladimir-Prelog-Weg 4 ,8093 Zurich Switzerland
2. Electron Microscopy Center; Empa; Swiss Federal Laboratories for Materials Science and Technology; Überlandstrasse 129 ,8600 Dübendorf Switzerland
3. Institut de Physique et Chimie des Matériaux de Strasbourg - CNRS UMR 7504; 67034 Strasbourg Cedex 2 France