1. ICFO Institut de Ciències Fotòniques The Barcelona Institute of Science and Technology Castelldefels Barcelona 08860 Spain
2. Laboratory of Ultrafast Science Department of Physics University of Cyprus Nicosia 1678 Cyprus
3. Inorganic Nanocrystals Laboratory Department of Chemistry University of Cyprus Nicosia 1678 Cyprus
4. ICREA Institució Catalana de Recerca i Estudis Avançats Barcelona 08010 Spain