1. Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193 Bellaterra, Barcelona, Spain
2. Consejo Superior de Investigaciones Científicas, ICN2 Building, Campus UAB, 08193 Bellaterra, Barcelona, Spain
3. Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain