Iron-Content-Dependent, Quasi-Static Dielectric Resonances and Oxidative Transitions in Bornite and Chalcopyrite Copper Iron Sulfide Nanocrystals

Manipulation of material composition can often induce unprecedented optoelectronic changes in semiconductor nanocrystals (NCs). Here, we demonstrate that bornite-phase copper iron sulfide (Cu5FeS4) NCs exhibit tunable optical characteristics from visible to near-infrared (NIR) that are strongly dependent on their iron content. These stoichiometry-dependent optical modulations confirm that the origin of the spectral response in these NCs is due to the occurrence of two fundamentally different types of resonant excitation: (i) a quasi-static dielectric resonance (DR) and (ii) a localized surface plasmon resonance (LSPR). Electronic band structure calculations by density functional theory (DFT) show that the presence of an intermediate band (IB) of states formed by empty Fe d-orbitals gives rise to the occurrence of a DR in the visible-frequency regime, analogous to the observed optical response in all-dielectric IB chalcopyrite (CuFeS2) NCs. Moreover, as the Fe content of the NCs is decreased via aerobic oxidation, the intensity of the DR diminishes, accompanied by a concomitant rise in the intensity of a hole-induced LSPR response in the NIR, reminiscent of self-doped copper sulfide NCs. Notably, the spectral position of the DR does not change as its intensity is reduced, clearly differentiating this mode of resonant excitation from the well-known LSPR response, which shifts substantially in frequency as the free-carrier concentration is modified. We further demonstrate an analogous DR-to-LSPR evolution in CuFeS2 NCs subjected to the influence of oxidizing agents and added ions. The decreased Fe content significantly reduces the contributions of Fe 3d orbitals to the IB in these ternary Cu–Fe–S NCs, resulting in a reduced DR intensity, and as the composition approaches a more Cu–S-like system, the vacancy-induced LSPR intensity becomes more pronounced. This is an example of compositionally induced DR-to-LSPR optical modulation in colloidal nanomaterials, which could be useful in dynamically responsive material applications and as a synthetic strategy for engineering desired spectral responses in semiconductor NCs.