1. Department of Advanced Materials Catalonia Institute for Energy Research (IREC) Barcelona 08930 Spain
2. Micro and Nano Systems Department Barcelona Institute of Microelectronics ‐ National Centre of Microelectronics (IMB‐CNM) Barcelona 08193 Spain
3. Department of Physics Basel University Basel CH4056 Switzerland
4. Catalan Institution for Research and Advanced Studies (ICREA) Barcelona Spain