Effect of Z-Type Metal Halide Ligand Exchanges on Photoluminescence Quantum Yields in Quantum Dots

Surfaces play a critical role in dictating nanocrystal’s physical and chemical properties due to their high surface-to-volume ratio. Colloidal quantum dots (QDs) are generally synthesized with insulating organic ligands, which can be post-synthetically exchanged with inorganic ligands to enhance their integration into devices. Metal halides, which function as Z-type electron acceptor ligands, are a popular choice, with reports of improved optoelectronic properties across a variety of II-VI and III–V material systems. For example, near-unity photoluminescent quantum yields (PLQY) have been observed in InP quantum dots after treatment with InF₃, while the carrier transport of CdSe thin films has significantly improved after CdCl₂ ligand exchange. However, the choice of the specific identity of metal and halide remains largely empirical, leading to conflicting results. These discrepancies highlight the lack of mechanistic understanding regarding how the identity of Z-type ligands affects PLQY, necessitating the development of a more robust and cleaner experimental system for investigation.

The surface and the optical properties can be simultaneously monitored by combining ¹H NMR and steady-state photoluminescence (PL) spectroscopies. Varying the metal, halide, and material system will allow for a holistic picture of dynamic QD surfaces concerning whether chemical passivation always translates to effective electronic passivation. Displacement of cadmium oleate from the surface of QDs will be facilitated by dispersing in a coordinating solvent, tetrahydrofuran, which allows for a 1-for-1 Z-type exchange. Ultimately, an understanding of QD surface interactions with Z-type ligands will pave the way for the rational design of surface treatments that offer both chemical and electronic passivation, furnishing a suite of core-only QDs with high PLQYs without the need for shell growth, which is highly beneficial for devices requiring efficient emission and fast carrier transport.