Why Does CuFeS Resemble Gold?

While several potential applications of CuFeS quantum dots have already been reported, doubts regarding their optical and physical properties persist. In particular, it is unclear if the quantum dot material is metallic, a degenerately doped semiconductor, or else an intrinsic semiconductor material. Here we examine the physical properties of CuFeS quantum dots in order to address this issue. Specifically, we study the bump that is observed in the optical spectra of these quantum dots at ∼500 nm. Using a combination of structural and optical characterization methods, ultrafast spectroscopy, as well as electronic structure calculations, we ascertain that the unusual purple color of CuFeS quantum dots as well the golden luster of CuFeS films arise from the existence of a plasmon resonance in these materials. While the presence of free carriers causes this material to resemble gold, surface treatments are also described to suppress the plasmon resonance altogether.