1. Laboratoire de Nanostructures et Nouveaux Matériaux Electroniques (LNNME), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
2. Laboratory of Physics of Nanostructures (LPN), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
3. Department Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium
4. Center for Research on Electronically Advanced Materials, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland