A Stepwise Reaction Achieves Emissive, Ultra-Small 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 e.g. binary/ternary chalcogenide and metallic impurities. Here, we develop a stepwise synthetic procedure that controls the size, morphology, and transformations of acanthite (Ag2S) and canfieldite (Ag8SnS6) intermediates. This reaction scheme improves size-dispersity and grants the production of small AZTS NCs (d: 2.1-4.0 nm) that have not been achieved through established single-injection procedures expanding the accessible quantum-confined AZTS emission to shorter wavelengths (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 <740 nm emitting AZTS NCs. We then use NMR and optical spectroscopies to demonstrate that the installation of thiol ligands improves colloidal stability and photoluminescence, while carboxylate ligands do not. Finally, facilitated by this enhanced synthetic control, we show that our AZTS NCs can act as effective and less-toxic sensitizers for red-to-blue triplet fusion upconversion. Our results highlight transferrable insights for the synthesis and post-synthetic treatment of complex less-toxic quaternary nanocrystalline systems.