Fe Cations Control the Plasmon Evolution in CuFeS<sub>2</sub> Nanocrystals
Yuan Yao, Anuj Bhargava, Richard D. Robinson
Abstract
Plasmonic semiconductor nanocrystals have become an appealing avenue for researching nanoscale plasmonic effects due to their wide spectral range (visible to infrared) and great tunability compared to traditional precious metal nanocrystals. CuFeS2 is an exciting semiconductor that has a prominent plasmon absorption band in the visible range (∼498 nm). Previous work has reported conflicting accounts of the existence of the plasmon band and the cation oxidation states in this material. In this work, we answer these outstanding questions and determine the origin of the plasmonic behavior in CuFeS2 by characterizing the nucleation and growth stages of the reaction through a series of ex situ and in situ probes (e.g., X-ray absorption spectroscopy and X-ray emission spectroscopy). We show that the plasmon formation is driven by the band structure modification from Fe2+ incorporation into the nanocrystals. As opposed to the pure Cu1+/Fe3+ predicted by DFT studies, a mixed oxidation state of Cu1+/Cu2+ and Fe2+/Fe3+ is observed, indicating that the chalcopyrite phase (CuFeS2) encompasses a range of oxidation states depending on the synthesis conditions. From our combined results, we propose a reaction mechanism for the CuFeS2 synthesis and outline a method to verify the phase purity of the material. Overall, this study provides a theoretical basis for tuning plasmon intensity by changing the cation oxidation state through various routes such as synthesis time and temperature.