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

Abstract

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.

Optical properties of fully inorganic core/gradient-shell CdSe/CdZnS nanocrystals at the ensemble and single-nanocrystal levels

“…One possible explanation of this observation is that the thick (almost 6 nm) type-I gradient shell confines the exciton so efficiently as to minimize the interactions with the NC surface, which might indicate that blinking is not dominated by surface traps but rather by volume traps situated in the shell at shorter distances from the NC core [73]. Alternatively one might surmise that the surface passivation of halide-capped NCs is, at least initially, of comparable efficacy as the one provided by OA ligands, and that this passivation deteriorates subsequently due to the known tendency of halide ions to react with with ambient air and humidity [33]. The latter interpretation is supported by the fact that surface modification did seem to become relevant once the NCs had been exposed to the ambient atmosphere on glass slides for a certain time, consistent with the aging observed in the PL spectra of Fig.…”

“…In view of potential applications for highefficiency devices operating in the visible spectral range, it would therefore be interesting to incorporate high-QY CdSe-based NCs in halide perovskite materials. A number of strategies for surface-functionalization of CdSe NCs with halide ligands have already been investigated [20,[28][29][30][31][32][33][34]. Purcell-Milton et al [33] have recently published a study of the evolution of the quantum yield of CdSe-based NCs that occurs when organic ligands such as oleic acid (OA) are replaced with halide ions (Br -, Cland I -), which resulted in an increase in some cases (CdSe core-only NCs, type II CdS/CdSe) and a rather low influence for CdSe/CdS; in the case of CdSe/ZnS, however, a strong reduction of the QY was observed.…”

“…A number of strategies for surface-functionalization of CdSe NCs with halide ligands have already been investigated [20,[28][29][30][31][32][33][34]. Purcell-Milton et al [33] have recently published a study of the evolution of the quantum yield of CdSe-based NCs that occurs when organic ligands such as oleic acid (OA) are replaced with halide ions (Br -, Cland I -), which resulted in an increase in some cases (CdSe core-only NCs, type II CdS/CdSe) and a rather low influence for CdSe/CdS; in the case of CdSe/ZnS, however, a strong reduction of the QY was observed. In the present article, we focus on CdSe/CdZnS core/gradient-shell NCs [35][36][37][38] to take advantage of their hybrid band structure, which offers a combination of the advantages of, on one hand, quasi-type-II large-shell CdSe/CdS NCs, which are known be highly photostable [39], and of, on the other hand, type-I CdSe/ZnS NCs, whose electrons are well-confined in the CdSe core, which leads to short emission lifetimes.…”

Electrophoretic Deposition of Quantum Dots and Characterisation of Composites

“…All CQDs tested were synthesised in house using methods adopted from literature with details given in ESI section I, and detailed characterisation carried out in a previous publication [29].…”

“…In addition, EPD of CdSe CQDs capped in an alternative ligand shell were also tested, since these CQDs generally show higher luminescence in solution, and therefore better surface passivation, an aspect previously analysed by ourselves on these CQDs [29], and could play an important characteristic in reducing recombination when sensitising the TiO 2 electrode. In addition, we want to confirm the universality of the developed EPD approach.…”