Defect thermodynamics in spinel oxides leading to plasmonic behavior

Abstract

Many optical devices can achieve enhanced performance at small scales by exploiting the surface plasmon resonance. Such a plasmonic material must have a large free carrier concentration. Although metals meet this criterion, their plasmon resonance cannot be chemically tuned into the infrared by varying the carrier concentration. By contrast, semiconducting oxides allow this tuning, and several spinel-structured oxides have shown plasmon resonance due to the easy dopability of spinels. This work uses first-principles calculations to identify the doping mechanism of these spinels, finding that Ga2FeO4 and Cd2SnO4 can be doped by cation antisites while Fe3O4 easily forms cation vacancies. In all three cases, the defect concentration can be tuned widely by controlling the chemical potentials of the different species. The chemically related Al2FeO4 is predicted to be much less dopable, indicating the need for careful design based on cation radius and other chemical factors to achieve good plasmonic performance, even in families such as spinels which have many dopable members.