Phase-Controlled Synthesis and Quasi-Static Dielectric Resonances in Silver Iron Sulfide (AgFeS₂ ) Nanocrystals

Ternary metal-chalcogenide semiconductor nanocrystals are an attractive class of materials due to their tunable optoelectronic properties that result from a wide range of compositional flexibility and structural diversity. Here, the phase-controlled synthesis of colloidal silver iron sulfide (AgFeS₂ ) nanocrystals is reported and their resonant light-matter interactions are investigated. The product composition can be shifted selectively from tetragonal to orthorhombic by simply adjusting the coordinating ligand concentration, while keeping the other reaction parameters unchanged. The results show that excess ligands impact precursor reactivity, and consequently the nanocrystal growth rate, thus deterministically dictating the resulting crystal structure. Moreover, it is demonstrated that the strong ultraviolet-visible extinction peak exhibited by AgFeS₂ nanocrystals is a consequence of a quasi-static dielectric resonance (DR), analogous to the optical response observed in CuFeS₂ nanocrystals. Spectroscopic studies and computational calculations confirm that a negative permittivity at ultraviolet/visible frequencies arises due to the electronic structure of these intermediate-band (IB) semiconductor nanocrystals, resulting in a DR consisting of resonant valence-band-to-intermediate-band excitations, as opposed to the well-known localized surface plasmon resonance response typically observed in metallic nanostructures. Overall, these results expand the current library of an underexplored class of IB semiconductors with unique optical properties, and also enrich the understanding of DRs in ternary metal-iron-sulfide nanomaterials.