1. Group of Nanotechnology, USAL‐NANOLAB University of Salamanca Salamanca 37008 Spain
2. Departamento de Química Analítica, Química Física e Ingeniería Química University of Alcalá Madrid Madrid 28871 Spain
3. Departamento de Física de Materiales Universidad Complutense de Madrid Madrid 28040 Spain
4. Instituto de Micro y Nanotecnología (IMN‐CNM‐CSIC) Tres Cantos Madrid 28760 Spain
5. Physics Department and INFN University of Pavia Pavia 27100 Italy
6. Department of Physics University of Warwick Coventry CV4 7AL UK