Unveiling Electronic Structure, Band Alignment, and Charge Carrier Kinetics of Copper Sulfide

In this work, we synthesize pure covellite copper sulfide (CuS) nanostructures using a hydrothermal approach, adjusting the copper stoichiometry to investigate their structural, optical, and electrochemical properties. Time-resolved photoluminescence reveals nanosecond-range excited state lifetimes, while femtosecond transient absorption spectroscopy shows that hole absorption dominates over photoinduced electrons with relaxation kinetics influenced by copper stoichiometry. We examine the impact on band edges and stability through cyclic voltammetry, finding that the optical band gap of the CuS nanostructures aligns closely with electrochemical values, approximately 1.9 to 1.8 eV. Ultraviolet photoelectron spectroscopy confirms changes in the valence band maxima, with varying stoichiometry elevating the valence band edge, indicating a near-metallic nature for CuS. Our study enables precise engineering of these nanostructures, positioning CuS as a promising low-cost material for energy harvesting and optoelectronic applications, including photovoltaics, photocatalysis, and energy storage.