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

Abstract

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.

Improved photocatalytic and free radical scavenging studies of synthesized polymer PVP/Azadirachta indica leave extract-mediated Ni-Zn doped hematite nanoparticles

“…Notably, the UV-vis absorption spectrum of magnetite displayed an absorption band spanning 340-430 nm. Consistently, the band gap of magnetite NPs was identified at approximately 2.2 eV, corroborating findings from earlier research [29]. Dopant atoms (Ni and Zn) in the crystal lattice of a material can significantly influence its electronic structure, including the band gap.…”

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“…6,10 Hence, the plasmon-like features in the transient absorption spectra actually originate from the unique IB band structure of the Cu 3 VSe 4 NCs which enables Frölich resonance and leads to a quasi-static dielectric resonance in the visible region. 43,44 IB semiconductors such as CuFeS 2 , CuAgS 2 , and Cu 3 VS 4 have been reported to possess dielectric resonance. Gaspari et al proposed that IB semiconductors do not show divergences in the dielectric response, and are therefore natural epsilon-near-zero materials over a wide spectral region.…”

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“…[55] Third, Yao et al reported that the SPR can be controlled by controlling the stoichiometry of iron atoms. [62] Finally, Lee et al have observed a very similar SPR-like feature for AgFeS 2 NCs, [71] but both these effects and the variation of iron stoichiometry are not currently fully understood.…”

“…[ 62 ] Finally, Lee et al. have observed a very similar SPR‐like feature for AgFeS 2 NCs, [ 71 ] but both these effects and the variation of iron stoichiometry are not currently fully understood.…”