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

ABSTRACT In this work, we report the discovery and cation exchange‐mediated synthesis of Cu ₂ FeS ₂ , 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 > 10 ⁷ 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 Cu ₂ FeS ₂ 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, Cu ₂ FeS ₂ is expected to provide a fertile ground for transformative investigations in many diverse fields of science.