Halide-Driven Halogen-Hydrogen Bonding versus Chelation in Perovskite Nanocrystals: A Concept of Charge Transfer Bridging

The choice of surface functionalized ligands to encapsulate semiconductor nanocrystals (NCs) is important for tailoring their optoelectronic properties. We use a small bidentate 8-hydroxyquinoline (HQ) molecule to surface functionalize CsPbX3 perovskite NCs (X = Cl, Br, I), along with traditional long-chain monodentate ligands. Our experimental results using optical and ultrafast spectroscopy depict a halogen-hydrogen bonding formation in the HQ functionalized CsPbCl3 and CsPbBr3 NCs, which act as a charge transfer (CT) bridging for the interfacial hole transfer from the NCs to the HQ molecule as fast as 540 fs. In contrast, weak chelation is observed for HQ-coupled CsPbI3 NCs without an active CT process. We explain two distinct surface coupling mechanisms via the polarizability of halides and larger PbI64- octahedral cage size. Control of two contrasting halide-dependent surface coupling phenomena of a small molecule that further regulate the CT process may have significant implications in their development in optoelectronics.

Automated summary

The choice of surface functionalized ligands is crucial in tailoring the optoelectronic properties of semiconductor nanocrystals. In this study, we used a small bidentate molecule called 8-hydroxyquinoline (HQ) to functionalize CsPbX3 perovskite nanocrystals (X = Cl, Br, I). Our experimental results revealed a halogen-hydrogen bonding formation in HQ-functionalized CsPbCl3 and CsPbBr3 nanocrystals, enabling fast charge transfer from the nanocrystals to the HQ molecule within 540 fs. However, weak chelation was observed in HQ-coupled CsPbI3 nanocrystals without an active charge transfer process. The understanding of these distinct surface coupling mechanisms has significant implications in the development of optoelectronics.