Quantum dots (QDs) have great application prospects in optoelectronic devices because of their excellent optical properties. Nevertheless, the original ligands of the long carbon chain introduced in the synthesis process are not conducive to the application of CdSe QDs in optoelectronic devices due to their insulating nature. Therefore, short-chain and aromatic ligands are employed to replace the original ligands. However, the influences of conjugation structures on ligand exchange have rarely been systematically studied from a thermodynamic perspective. In this work, two types of aromatic ligands, namely, a series of benzoic acid (BA) ligands and cinnamic acid (CA) ligands that introduced electron-donating (methoxy-) and electron-withdrawing groups (fluoro-) at the meta and para positions of the benzene ring of aromatic ligands, were chosen to investigate the ligand exchanges on the surface of QDs from oleic acid (OA) to a series of aromatic ligands by 1 H nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC). 1 H NMR showed that the average exchange ratio of CA ligands to OA was ∼1:1, which made it possible to obtain thermodynamic parameters through the determined ligand exchange model by ITC. Furthermore, thermodynamic results showed that ligand exchange with CA ligands was endothermic (ΔH > 0) and entropyincreasing (ΔS > 0), indicating that the process was entropy-driven. In addition, the equilibrium constants (K) of ligand exchange with CA ligands were larger than those with BA ligands. However, the K values of ligand exchange with BA ligands were ∼1, which resulted in ΔG ∼ 0 kJ/mol, demonstrating that ligand exchange with BA ligands was less favorable at 298.15 K. Finally, the introduction of substituents on meta and para positions of the aromatic ligand benzene ring had no significant effect on the ligand exchange process. This work not only elaborates on the process of exchange with aromatic ligands but also has fundamental significance for the application of aromatic ligand-modified QDs in optoelectronic devices.

InP quantum dots (QDs) attract growing interest in recent years, owing to their environmental advantages upon applications in display and lighting. However, compared to Cd-based QDs and Pb-based perovskites, the synthesis of InP QDs with high optical quality is relatively more difficult. Here, we established a mid-synthetic modification approach to improve the optical properties of InP-based QDs. Tris(dimethylamino)phosphine ((DMA)3P) and indium iodide were used to prepare InP QDs with a green emission (∼527 nm). By introducing zinc halides (ZnX2) during the mid-synthetic process, the photoluminescence quantum yield (PLQY) of the resulting InP/ZnSeS/ZnS core/shell/shell QDs was increased to >70%, and the full-width-at-half-maximum (FWHM) could be narrowed to ∼40 nm. Transmission electron microscopy clearly showed the improvement of the QDs particle size distribution after introducing ZnX2. It was speculated that ZnX2 was bound to the surface of QDs as a Z-type ligand, which not only passivated surface defects and suppressed the emission of defect states but also prevented Ostwald ripening. The InP cores were also activated by ZnX2, which made the growth of the ZnSeS shell more favorable. The photoluminescence properties started to be improved significantly only when the amount of ZnX2 exceeded 0.5 mmol. As the amount increased, more ZnX2 was distributed around the QDs to form a ligand layer, which prevented the shell precursor from crossing the ligand layer to the surface of the InP core, thus reducing the size of the InP/ZnSeS/ZnS QDs. This work revealed a new role of ZnX2 and found a method for InP QDs with high brightness and low FWHM by the mid-synthetic modification, which would inspire the synthesis of even better InP QDs.

Inorganic metal halide perovskites have very promising applications in light-emitting diodes (LED), solar cells, lasers, and photodetectors owing to their outstanding optoelectronic characteristics. However, halide segregation is quite a challenge in the industrialization process of pure red perovskite LEDs with stable emission. In this work, we addressed this issue by doping with KBr in the nucleation of nanocrystals (NCs) and postmodifying with quaternary ammoniums (e.g., tetrabutylammonium bromide, TBAB). Mixed halide perovskite NCs (CsPbI 3−x Br x NCs) with excellent optical performances and stabilities were obtained. The emission maximum was at ∼636 nm, and the full-width-at-half-maximum (fwhm) was 28 nm, achieving the requirement of Rec. 2020. The solution and film of KBr/TBAB-CsPbI 3−x Br x NCs could keep pure red emission after exposure in the air for 65 and 90 days, respectively. The photoluminescence quantum yield (PLQY) of the KBr/TBAB-CsPbI 3−x Br x NCs solution remained 90% and the emission peak had only a very slight shift after 240 h of irradiation at 365 nm. This study demonstrated an efficient route for highly stable pure red perovskite NCs and highlighted the critical role of surface ligands, which is expected to be applied in the future.