Understanding the Surface Properties of Halide Exchanged Cesium Lead Halide Nanoparticles.

This report describes a characterization study of the surfaces of CsPbBr and CsPbBrI perovskite nanoparticles (NPs) obtained via a simultaneous purification and halide exchange (HE) postsynthetic processing technique. We studied composition-dependent NP-ligand interactions via diffusion ordered NMR (DOSY) and quantified resulting photoluminescence quantum yield (QY) as a function of halide exchange as well as ligand exchange. Importantly, ligand binding strength and QY were found to decrease when successive purification and/or halide/ligand exchange steps were taken without careful concurrent additions of acid and base ligands. This suggests that ligands added during postsynthetic processing steps are localized at the surface of the NP, passivating open surface sites. Further, we show that CsPbBrI with increasing CsPbI character, obtained via the same method, have decreasing ligand density, from 6.4 to 1.4 to 0.2 nm, indicating the composition-dependence of surface ligand binding, which also has consequences on the QY of the resulting mixed-halide NPs. These results shed further light on the importance of ion-ligand moiety additions during purification and halide exchange of highly emissive CsPbBr NPs to maintain their as-synthesized properties, as well as the intrinsic differences in surfaces binding and photostability between near-unity QY CsPbBr and mixed-halide CsPbBrI NPs.