Nanopopcorns: A facile, one-pot, self-regulated seed-mediated approach for the synthesis of heteronanocrystals has been developed. The mechanistic insights elucidated in this work offer a foundationa...

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.