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 CsPbX 3 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 CsPbCl 3 and CsPbBr 3 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 CsPbI 3 NCs without an active CT process. We explain two distinct surface coupling mechanisms via the polarizability of halides and larger PbI 6 4- 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.