Trioctylphosphine- and Octanethiol-Induced Photoluminescence Recovery of CdSe/ZnS Quantum Dots after Dilution–Quenching: Implications for Quantum Dot Films

In this work, CdSe/ZnS core/shell quantum dots (QDs) were synthesized. Dilution of the as-synthesized QDs would lead to instability with photoluminescence (PL) quenching. It was found that the addition of tertiary phosphine and alkylthiol could recover the PL intensity of diluted QDs and make these QDs stable. Specifically, the recovery efficiency of tertiary phosphine was stronger than that of alkylthiol. Trioctylphosphine (TOP) and octanethiol (OT) were chosen as the representative ligands, respectively. Isothermal titration calorimetry (ITC) demonstrated that interactions between both TOP/OT and QDs were exothermic and spontaneous. The interaction between TOP and QDs was mainly driven by van der Waals forces due to the branched structure. This interaction had a negative entropy change, while the interaction between OT and QDs had a positive entropy change. There were about ∼12 TOP molecules and ∼17 OT molecules bound to a QD particle. The binding constants of TOP/OT to QDs were 1.6 × 10 3 and 12.8 × 10 3 M –1, respectively. The reaction rate between TOP and QDs was fast, and the addition of OT could slow the reaction rate constant from 0.081 to 0.024 min –1 . PL decay indicated that TOP and OT could increase the radiative recombination of excitons. Furthermore, both TOP and OT could greatly improve the brightness of the QD films, which was a prerequisite for the efficient improvement of quantum dot light-emitting diodes. This work explicitly investigated the dilution process of QDs, explored the methods for keeping diluted QDs stable and bright, and highlighted the importance of surface ligands.