Doping controls plasmonics, electrical conductivity, and carrier-mediated magnetic coupling in Fe and Sn codoped In₂O₃ nanocrystals: Local structure is the key

Multifunctional Fe-Sn codoped In₂O₃ colloidal nanocrystals simultaneously exhibiting localized surface plasmon resonance band, high electrical conductivity, and charge mediated magnetic coupling have been developed. Interactions between Sn and Fe dopant ions have been found critical to control all these properties. Sn doping slowly releases free electrons in the colloidal nanocrystals, after reduction of active complex between Sn⁴⁺ and interstitial O^2-. Unexpectedly, Fe codoping reduces the free electron concentration. Our X-ray absorption fine structure spectroscopy (XAFS) results show that Fe³⁺ and Sn⁴⁺ substitutes In³⁺ in the In₂O₃ lattice for all Fe-doped In₂O₃ NCs and Sn-doped In₂O₃ NCs. Interestingly, for Fe-Sn codoped NCs, a smaller fraction of Fe³⁺ gets reduced to Fe²⁺ by consuming free electrons produced by Sn doping. Therefore, Fe doping can manipulate free electron concentration in Fe-Sn codoped In₂O₃ nanocrystals, controlling both plasmonic band and electrical conductivity. Free electrons, on the other hand, facilitate magnetic coupling between distant Fe³⁺ ions. Such charge mediated magnetic coupling is useful for spin-based applications.