Ternary chalcopyrite quantum dots, such as AgInS₂ (AIS) or CuInS₂ (CIS), as well as quaternary kesterite quantum dots (QDs), reveal unique properties with no analogs in the realm of conventional binary A^IIB^VI QDs, in particular, high tolerance to compositional non-stoichiometry, doping and alloying, as well as remarkable composition- and size-dependent spectral characteristics, photoluminescence (PL) mechanisms, charge carrier dynamics, etc. The chalcopyrite CIS and AIS QDs revealed volcano-shaped dependencies of the efficiency of PL emission and interfacial charge on their composition, with the highest values reached far from stoichiometry, at In:Ag(Cu) ratios of 4-5, highlighting a unique potential of such non-stoichiometric QDs. The realization of this potential crucially depends on the availability and versatility of synthetic approaches, allowing the QD composition, size, surface chemistry, and ligand shell structure to be tailored to specific light harvesting/conversion applications. The present talk shows that conventional heating-up/hot injection syntheses in high-boiling-point coordinating solvents can be successfully rivaled by mild and relatively “green” approaches of direct synthesis and size-tuning of ternary/quaternary QDs in aqueous solutions.

Aqueous approaches proved to be universal and applicable to many ternary chalcopyrite QDs, such as CIS, AIS, and CAIS, as well as to quaternary kesterite QDs, in particular Cu(Ag)₂ZnSnS₄. These approaches allow the QD composition to be varied in a broad range (for example, In(Sn):Ag(Cu) ratio variable between 1 and 20), protecting QDs with various ligands (mercaptocarboxylic acids, multi-functional ligands such as glutathione or polymers, such as polyethylene imine), and covering QDs with various protective shells (ZnS, CdS, In₂S₃, etc.). When combined with fine size-selective precipitation and post-synthesis solvent transfer, the aqueous approaches deliver unprecedented synthetic flexibility and variability of QD composition and size, allowing the aqueous synthesis to be upscaled to a combinatorial high-throughput robot-assisted regime. The latter is capable of yielding hundreds of samples with different spectral characteristics and PL dynamics within the same synthetic protocol, exemplified in the present talk by a high-throughput PL study of core AIS and core/shell AIS/ZnS QDs.

The combinatorial potential of the aqueous synthesis of ternary/quaternary QDs can be further expanded by their unique tendency to spontaneous alloying, which results in more complex multinary chalcogenide QDs and allows more sophisticated combinatorial synthesis to be performed with several types of ternary QDs used as sacrificial precursors. This approach is exemplified by a high-throughput robot-assisted synthesis of non-stoichiometric chalcopyrite CuAg-In-SSe (CAISSe) QDs from individual CIS, AIS, CISe, and AISe QDs as precursors. The CAISSe QDs revealed unexpected volcano-shaped dependences of PL intensity and lifetime on both Cu:Ag and S:Se ratios, advocating a large potential of the high-throughput screening of aqueous QDs for the discovery of new materials with advanced functionalities.

The broad range of instruments for composition and size variation of multinary QDs available in aqueous syntheses, including precise size selection, spontaneous alloying of QDs, as well as various cation, ligand, and solvent exchanges are currently incorporated into a unified concept of high-throughput robot-assisted screening capable of delivering tens of thousands of different QD species to be tested as light emitters and photovoltaic absorber materials. The present talk highlights our developing strategy of combining high-throughput aqueous syntheses with accelerated characterization and machine-learning-assisted analysis of QD properties as functions of their composition and size. The latter is expected to provide meaningful feedback to select new compositions and steer the synthesis toward the desired QD properties, closing the “synthesis-characterization-analysis” loop and enabling automated high-throughput material discovery within the domain of multinary metal-chalcogenide QDs.