1. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, Barcelona, Catalonia, 08193, Spain
2. Laboratory of Semiconductor Materials, Institute of Materials, Faculty of Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
3. School of Chemistry, Cardiff University, Main Building, Park Place, CF10 3AT Cardiff, UK
4. Department of Energy and Mineral Engineering, Pennsylvania State University, University Park, PA 16802, USA
5. Institute of Physics, Faculty of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
6. ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Catalonia, Spain