CdSe/ZnS Quantum Dots with Enhanced Stability and Conductivity by Thiolphenyl Ligands for Displays

Thiol ligands are widely used to modify quantum dots (QDs) due to their strong affinity. Aromatic structures can be introduced to improve the charge transport of the QDs. In this work, the as-synthesized CdSe/ZnS core/shell QDs were modified with thiolphenyl (TP) derivatives by ligand exchange. Photoluminescence (PL), ultraviolet-visible (UV-vis) absorption, and time-resolved photoluminescence (TRPL) spectroscopy were used to investigate the optical properties of QDs before and after ligand exchange. The elemental quantification by energy-dispersive X-ray (EDX) spectroscopy proved that TP ligands were adsorbed. The PL intensities of QDs were enhanced after ligand exchange with TP ligands (except for pentafluorothiophenol, PF-TP) through passivation. Besides, the QDs exhibited better stability and electrical conductivity. Combined with 1H nuclear magnetic resonance (1H NMR), fluorescence spectroscopy, and isothermal titration calorimetry (ITC), the effects of electron-donating/-withdrawing ability and steric hindrance of the substituents on the ligand exchanges were analyzed. The reactions of QDs with TP ligands containing electron-donating substituents were exothermic and entropy-decreasing. However, the reactions with those ligands containing electron-withdrawing substituents were exothermic and entropy-increasing. Interestingly, the reaction of PF-TP with QDs could be fitted to a two-stage model. In general, we found that the stronger electron-donating ability of substituent would lead to less desorption of OA ligands, thus fewer trap states, and better passivation, stability, and conductivity of QDs. Good stability and conductivity of QDs are prerequisites for high performance in displays. This work will greatly contribute to the selection of aromatic ligands for QDs.