1. ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain
2. Thomas Young Centre and Department of Chemistry, University College London, WC1H 0AJ London, U.K.
3. Thomas Young Centre and Department of Materials, Imperial College London, SW7 2AZ London, U.K.
4. Department of Physics, Ewha Womans University, Seoul 03760, Korea
5. ICREA-Institució Catalana de Recerca i Estudis Avancats, Passeig Lluís Companys 23, 08010 Barcelona, Spain