A Stepwise Reaction\nAchieves Ultrasmall Ag<sub>2</sub>ZnSnS<sub>4</sub> Nanocrystals

Pirquitasite Ag<sub>2</sub>ZnSnS<sub>4</sub> (AZTS) nanocrystals\n(NCs) are emergent, lead-free emissive materials in the coinage chalcogenide\nfamily with applications in optoelectronic technologies. Like many\nmultinary nanomaterials, their phase-pure synthesis is complicated\nby the generation of impurities, e.g., binary/ternary chalcogenides\nand reducedmetals. Here, we develop a stepwise synthetic procedure\nthat controls the size, morphology, and transformations of acanthite-like\n(Ag<sub>2</sub>S) and canfieldite-like (Ag<sub>8</sub>SnS<sub>6</sub>) intermediates. This reaction scheme grants the production of small\nAZTS NCs (diameter: 2.1–4.0 nm) that we cannot achieve through\nestablished single-injection proceduresexpanding the accessible\nrange of quantum-confined AZTS emission to shorter wavelengths (λ:\n650–740 nm). We show that the initial sulfur stoichiometry\nis the key handle for template-size tunability and reveal that temporally\nseparating transformation steps is crucial to obtaining phase-pure\nAZTS NCs with emission λ < 740 nm. We then use NMR and optical\nspectroscopies to demonstrate that the installation of thiol ligands\nimproves colloidal stability, while exposure to carboxylic acids does\nnot. Finally, facilitated by this enhanced synthetic control, we show\nthat our ultrasmall AZTS NCs can act as effective, less-toxic sensitizers\nfor red-to-blue triplet fusion upconversion. Our results highlight\ntransferrable insights for the synthesis and postsynthetic treatment\nof complex, less-toxic quaternary nanocrystalline systems.