Quantum dots (QDs) are semiconductor nanocrystals with outstanding color purity and size-dependent color tunability, making them attractive for various applications including displays, bioimaging, photodetectors, and photovoltaic cells. However, their vulnerability to heat, oxidation, and moisture due to surface defects limits their practical use. While inorganic shell encapsulation is effective, it can decrease quantum yield due to misfit defects. Ligand stabilization is an alternative, but traditional ligands suffer from thermal dissociation due to weak binding energies. This paper introduces a novel approach using a cross-linked block copolymer ligand to enhance QD stability.

The introduction extensively reviews existing methods to improve QD stability. It discusses the limitations of thick inorganic shells, the challenges of growing thin oxide shells like SiO2 and Al2O3, and the drawbacks of using thiolated ligands which suffer from thermal dissociation due to weak binding energies. The authors highlight the lack of successful application of cross-linked ligand networks to QDs due to harsh reaction conditions that would damage the QDs' photophysical properties. This sets the stage for their proposed novel method.

The authors synthesized thiol-terminated P(MMA-b-GMA)-SH block copolymer ligands via reversible addition-fragmentation chain transfer (RAFT) polymerization. CdSe/ZnCdS and InP/ZnSeS core/shell QDs were synthesized using standard methods. Ligand exchange replaced the native oleic acid ligands with the P(MMA-b-GMA)-SH ligands. Cross-linking of the PGMA blocks was achieved using a Lewis acid catalyst, tris(pentafluorophenyl)borane, under ambient conditions to avoid damaging the QDs. QD-PMMA nanocomposite thin films were prepared via spin casting, and white light-emitting diodes (WLEDs) were fabricated by stacking red and green QD-PMMA films on a blue-emitting LED chip. Characterization techniques included gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and time-resolved PL.

The ligand exchange increased the hydrodynamic diameter of the QDs significantly, indicating successful ligand replacement. The optical spectra of the QDs remained largely unchanged after both ligand exchange and cross-linking, showing that the photophysical properties were preserved. The PLQY showed only a slight decrease (5%) during ligand exchange. Thermal gravimetric analysis revealed a sufficient areal chain density of polymer ligands for effective cross-linking. The mild cross-linking method using a Lewis acid catalyst was crucial; other methods like UV irradiation significantly reduced PLQY. The resulting QDs showed exceptional thermal and oxidative stability in both solution and film states. The fabrication of WLEDs demonstrated the practical applicability of the method.

The study successfully demonstrates a simple yet effective method for enhancing the thermochemical stability of QDs. The use of a cross-linkable block copolymer ligand with a high areal density combined with mild cross-linking conditions preserved the QDs' photophysical properties while enhancing their stability. The resulting high stability in both solution and film states opens up new possibilities for the use of QDs in various applications, particularly in devices that require high temperatures or exposure to harsh environments. The successful fabrication of WLEDs showcases the practical potential of this technology.

This research successfully developed a novel approach to enhance the thermochemical stability of quantum dots using a cross-linkable block copolymer ligand. The mild cross-linking process preserves the photophysical properties of QDs, resulting in high stability in various conditions. The fabrication of high-performance white light-emitting diodes highlights the practical implications of this method. Future research could focus on exploring different polymer ligands and optimizing the cross-linking process for even greater stability and broader applicability.

While the study demonstrated significant improvement in QD stability, further investigation into the long-term stability under various environmental conditions is needed. The impact of specific oxidant species on the stability needs more in-depth study. The scalability of the synthesis and cross-linking process for industrial applications requires further investigation.