Investigation of Quantum Dot–Metal Halide Interactions and Their Effects on Optical Properties

Quantum dots (QDs) are a class of important light-emitting nanomaterials, which have shown considerable potential for a range of applications. Here, we report detailed studies of the interactions between cadmium halide salts and II–VI-based quantum dots affecting their optical properties. A specific set of experiments have been utilized to better understand these effects using a range of core and core–shell quantum dots as model systems, examining CdSe, CdS, CdS/CdSe, CdTe/CdSe, CdSe/CdS, CdSe/ZnS QDs, and CdSe/CdS dots in rod nanostructures. In our studies, we have demonstrated that significant increases in photoluminescent (PL) and photoluminescent quantum yields (PLQY) can be achieved, producing a 1.5–4-fold increase for CdSe QDs and 1.4–1.8-fold increase for a number of other Cd-based core–shell nanostructures. To explain these phenomena, the interaction’s efficiency for three alternative ligand-capped CdSe QDs have been examined, with results showing a weak dependency on capping ligand. By contrast, we have demonstrated that variation of the halide anion of the cadmium salt shows a strong dependence, with decreasing effectiveness found when comparing Cl– to Br– and I–. In addition, we have been able to show a large increase of PLQY for reverse type I (CdS/CdSe) and type II (CdTe/CdSe) QDs, both nanostructures which display strong surface-sensitive PL properties, while type I (CdSe/CdS) nanostructures showed a weaker effect, with an inverse relationship relative to shell thickness. Finally, it was also found that ZnS or ZnS-shelled QDs show the onset of cation exchange, causing PL red shifting and a significant reduction of PLQY. Therefore, the culmination of these results can be best explained using standard covalent ligand classification, and points to this treatment working via a three-pronged approach, in which surface passivation takes place through the presence of the L-type ligand oleylamine, the Z-type ligand, Cd(oleylamine), and the X-type ligand Cl– anion, the combination of which produces the total optimal effect observed. Overall, this study presents important approaches to increase quantum yields in a range of widely utilized QDs and provides important insights into the underlying interactions of this type of surface treatments as means to improve the resulting optical properties.