Theory-Guided Identification and Development of Plasmonic Spinel Oxides*

Plasmonic materials use standing electron waves, called plasmons, to interact with incoming light. While metals can be good plasmonics, their frequency response depends on the concentration of free charge carriers which is difficult to control, so metal-based plasmonics are usually limited to the visible spectrum. To create tunable IR-active plasmonic materials, we want to find semiconductors that achieve either n- or p-type (but not both simultaneously) conduction via doping. Recent experiments show that the inverse spinels Fe₃O₄ and Ga₂FeO₄ meet these criteria, but the underlying reasons are poorly understood.
We combine first-principles calculations and experimental synthesis to understand the plasmonic activity of Fe₃O₄ and Ga₂FeO₄. For Ga₂FeO₄ we find that the dominant defect under all stable chemical potential conditions is the Ga_[Fe] antisite, which adopts +1, +0, or -1 states depending on the Fermi level. In Fe₃O₄, tunability arises from chemical-potential-dependent control of the Fe²⁺/Fe³⁺ ratio. We also explore synthetic conditions to find key correlations between structure and properties. Across different spinel oxide systems, we show that material composition, nanocrystal size, and morphology impact optical properties.