Ligand-Assisted Colloidal Synthesis of Alkali Metal-Based Ternary Chalcogenide: Nanostructuring and Phase Control in Na–Cu–S System

The development of sustainable and tunable materials is crucial for advancing modern technologies. We present a controlled synthesis of colloidal Na–Cu–S nanostructures. To overcome the reactivity difference between Na and Cu precursors toward chalcogens in a colloidal synthesis and to achieve metastable phase formation, we designed a single-source precursor for Cu and S. The decomposition of this precursor creates a Cu–S template into which Na ions diffuse to form metastable Na–Cu–S. By leveraging the reactivity of sulfur precursors, we synthesized Na₃Cu₄S₄ (orthorhombic) and Na₂Cu₄S₃ (monoclinic) nanocrystals with distinct properties. A mechanistic investigation reveals a predictive pathway previously unobserved in alkali-metal-based ternary chalcogenide systems. Further, computational DFT calculations demonstrate that Na₃Cu₄S₄ exhibits metallic characteristics while Na₂Cu₄S₃ is semiconducting, with an optimal band gap for photovoltaic applications. This research advances our understanding of ternary chalcogenide systems and establishes a framework for the rational design of complex nanomaterials.