1. MRT Department, Luxembourg Institute of Science and Technology (LIST), 4362 Esch-sur-Alzette, Luxembourg
2. Centre for Nanostructured Media, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, United Kingdom
3. IEE S.A., ZAE Weiergewan, 11 Rue Edmond Reuter, L-5326 Contern, Luxembourg
4. Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, The Netherlands