Engineering PbS-PbSe Coupled Quantum Dots via Cation Exchange: Insights into the Growth Mechanism from ZnS-CdSe Precursors

We present a direct Cd-to-Pb and Zn-to-Pb cation exchange approach for synthesizing PbS-PbSe Janus coupled quantum dots (CQDs) and PbSe quantum dots from ZnS-CdSe Janus CQDs. The choice of precursor-ligand combinations plays a crucial role in governing cation exchange while preserving nanomaterial morphology. Our study reveals that oleylamine (OLA) facilitates efficient cation exchange by solvating Cd²⁺ and Zn²⁺ions, a process that is ineffective when using oleic acid alone. Additionally, Pb-oleate without OLA fails to induce exchange due to Pb-oleate’s higher bond stability relative to Cd-oleate. Cation exchange reaction at 220°C using Pb-oleate and OLA leads to the full conversion of ZnS-CdSe CQDs into PbS-PbSe CQDs. OLA favourably solvates byproduct Cd²⁺ and Zn²⁺, which provides thermodynamic driving force to break the weaker Pb-oleate bond and evokes the cation exchange reaction with moderate kinetics. Furthermore, Cd²⁺ forms a stronger interaction with OLA through hard acid-base interactions, whereas Pb²⁺ interacts preferentially with oleic acid, influencing reaction efficiency thus, ligand-precursor deriving the cation exchange. To further explore how ligand-precursor selection affects morphology and cation exchange dynamics, we also investigated the PbCl₂-oleylamine system. At lower temperatures (150–180 °C), only partial cation exchange occurs, yielding intermediate heteronanostructures. The presence of chloride ions in PbCl₂ enhances surface reactivity by replacing surface molecules on ZnS-CdSe CQDs, accelerating cation exchange but also inducing morphological transformations. The crystal structure mismatch between wurtzite CdSe, zincblende ZnS, and rock-salt PbSe leads to unstable interfaces, driving further structural evolution. Additionally, disparities in Cd²⁺ and Pb²⁺ migration rates result in void formation within the heteronanostructures, further modifying their morphology. This study establishes ZnS-CdSe CQDs as a versatile template for cation exchange, enabling the design of diverse heterostructures with tunable compositions and morphologies. These findings provide critical insights into ligand-directed cation exchange mechanisms, paving the way for advanced heterojunction nanomaterials with tailored optoelectronic properties.