Controlled Synthesis and Exploration of Cu_xFeS₄ Bornite Nanocrystals

Plasmonic semiconductor nanocrystals (NCs) are a new and exciting class of materials that enable higher control of their localized surface plasmon resonance (LSPR) than metallic counterparts. Additionally, earth-abundant and non-toxic materials such as copper iron sulfides are gaining interest as alternatives to heavy metal-based semiconductor materials. Colloidal bornite (Cu₅FeS₄) is an interesting but underexplored example of a heavy metal-free plasmonic semiconductor. This report details the hot-injection synthesis of bornite yielding NCs ranging from 2.7 to 6.1 nm in diameter with stoichiometric control of the copper and iron content. The absorbance spectra of bornite NCs with different Cu:Fe ratios change at different rates as the particles oxidize and develop LSPR in the near-infrared region. X-ray photoelectron spectroscopy results indicate that oxidation produces sulfates rather than metal oxides as well as a decrease in the iron content within the NCs. Additionally, increasing iron content leads to decreases in carrier density and effective mass of the carrier, as determined by the Drude model. This controlled synthesis, combined with a further understanding of the relationship between the particle structure and optical properties, will enable the continued development and application of these fascinating heavy metal-free plasmonic semiconductor nanoparticles.