Proceedings of Online school on Fundamentals of Semiconductive Quantum Dots (QDsSCHOOL)
DOI: https://doi.org/10.29363/nanoge.qdsschool.2021.019
Publication date: 30th April 2021
The development of synthetic procedures for polymorphic multinary metal sulfide nanocrystals (NCs) is an important synthetic capability to achieve better synthetic control and phase purity. However, the synthesis of pure products in such systems is complicated due to the emergence of undesired unary, binary, and ternary phases. To tackle this complexity, the synthesis of intermediate binary templates and subsequent cation exchange can provide access to pure polymorphs that are otherwise thermodynamically disfavoured or synthetically challenging.
Here, we map the evolution of precursors and binary copper sulfide templates (Cu2-xS) in the formation mechanism of polymorphic Cu2ZnSnS4 (CZTS) NCs. Controlled experiments with Cu0 seeds show that selected binary phases (cubic digenite Cu1.8S, hexagonal covellite CuS, and monoclinic djurleite Cu1.94S) can be attained under comparable reaction conditions by harmoniously adjusting precursor stoichiometry and reactivity alone. We then show that the nature of the Cu2-xS template ultimately dictates the structure and polymorph of CZTS NCs (kesterite vs. wurtzite). Through digenite, the cation exchange reaction yields the kesterite (k-CZTS) phase because the unit cell parameters and anion sublattices of the two materials are highly similar. Likewise, covellite forms the k-CZTS phase although through a major, thermally induced rearrangement that transiently generates digenite. These experiments reveal the importance of in situ structural transformations during the formation of quaternary NCs. Conversely, we show that the use of djurleite nanorods directs the formation of the other CZTS polymorph; wurtzite (w-CZTS), though with prominent stacking faults in the final product. We understand this effect through the investigation of an anisotropic exchange mechanism at the different facets of the nanorods. Our results offer insight into the impact of different Cu2-xS templates in the synthesis of quaternary k-CZTS and w-CZTS NCs focusing on sulfur source effect, morphology, and phase purity. This mechanistic understanding grant synthetic handles for the careful development of tailored NCs syntheses in polymorphic systems.
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