A Stepwise Reaction Achieves Ultrasmall Ag₂ZnSnS₄ Nanocrystals

Pirquitasite Ag₂ZnSnS₄ (AZTS) nanocrystals (NCs) are emergent, lead-free emissive materials in the coinage chalcogenide family with applications in optoelectronic technologies. Like many multinary nanomaterials, their phase-pure synthesis is complicated by the generation of impurities, e.g., binary/ternary chalcogenides and reducedmetals. Here, we develop a stepwise synthetic procedure that controls the size, morphology, and transformations of acanthite-like (Ag₂S) and canfieldite-like (Ag₈SnS₆) intermediates. This reaction scheme grants the production of small AZTS NCs (diameter: 2.1-4.0 nm) that we cannot achieve through established single-injection procedures─expanding the accessible range of quantum-confined AZTS emission to shorter wavelengths (λ: 650-740 nm). We show that the initial sulfur stoichiometry is the key handle for template-size tunability and reveal that temporally separating transformation steps is crucial to obtaining phase-pure AZTS NCs with emission λ < 740 nm. We then use NMR and optical spectroscopies to demonstrate that the installation of thiol ligands improves colloidal stability, while exposure to carboxylic acids does not. Finally, facilitated by this enhanced synthetic control, we show that our ultrasmall AZTS NCs can act as effective, less-toxic sensitizers for red-to-blue triplet fusion upconversion. Our results highlight transferrable insights for the synthesis and postsynthetic treatment of complex, less-toxic quaternary nanocrystalline systems.