Cu ₂ FeS ₂ : Discovery of an Exceptional Thermoplasmonic Semiconductor via Arrested Cation Exchange

In this work, we report the discovery and cation exchange-mediated synthesis of Cu₂FeS₂, a compound predicted computationally but never observed in nature or realized in the laboratory. Our findings reveal that it possesses an anomalous electronic structure among analogous Cu-Fe-S semiconductors due to the unique valence configuration. More strikingly, this unprecedented material displays ultrahigh molar extinction coefficients (ε > 10⁷ M^-1 cm^-1) throughout the visible to near infrared (NIR) spectrum arising from remarkable localized surface plasmon resonances (LSPRs), coupled with intense electron-phonon interactions that enable ultrafast lattice heating on the 100 fs timescale. Such intrinsic attributes unequivocally designate Cu₂FeS₂ as an ideal thermoplasmonic material. It demonstrates superior photothermal conversion efficiencies (PCE) spanning both visible and NIR wavelengths, outperforming assorted well-established photothermal materials including Au nanoparticles and MXene nanosheets. As a demonstration, we leverage its prominent thermoplasmonic functionality to drive efficient photothermal dry reforming of methane under low light intensities. Beyond the results presented here, Cu₂FeS₂ is expected to provide a fertile ground for transformative investigations in many diverse fields of science.