Surface Termination of CsPbBr Perovskite Quantum Dots Determined by Solid-State NMR Spectroscopy.

Cesium lead halide perovskite quantum dots (QDs) have gained significant attention as next-generation optoelectronic materials; however, their properties are highly dependent on their surface chemistry. The surfaces of cuboidal CsPbBr QDs have been intensively studied by both theoretical and experimental techniques, but fundamental questions still remain about the atomic termination of the QDs. The binding sites and modes of ligands at the surface remain unproven. Herein, we demonstrate that solid-state NMR spectroscopy allows the unambiguous assignment of organic surface ligands via H, C, and P NMR. Surface-selective Cs solid-state NMR spectra show the presence of an additional Cs NMR signal with a unique chemical shift that is attributed to Cs atoms terminating the surface of the particle and which are likely coordinated by carboxylate ligands. Dipolar dephasing curves that report on the distance between the surface ammonium ligands and Cs and Pb were recorded using double resonance H{Cs} RESPDOR and H{Pb} S-REDOR experiments. Model QD surface slabs with different possible surface terminations were generated from the CsPbBr crystal structure, and theoretical dipolar dephasing curves considering all possible H-Cs/Pb spin pairs were then calculated. Comparison of the calculated and experimental dephasing curves indicates the particles are CsBr terminated (not PbBr terminated) with alkylammonium ligands substituting into some surface Cs sites, consistent with the surface-selective Cs NMR experiments. These results highlight the utility of high-resolution solid-state NMR spectroscopy for studying ligand binding and the surface structure of nanomaterials.