Localizing Oleylamine Ligands on Amine–Halide Copassivated Indium Phosphide Nanocrystals

The surface capping of organic ligands is a full part of a colloidal nanocrystal (NC) that provides colloidal stability, helps passivating electronic trap-states, or governs charge hopping between adjacent NCs. A central aspect to understanding this ligand shell is its inherently heterogeneous nature caused by, for example, site- or facet-dependent adsorption energies. Even so, the identification of ligands bound to different surface sites remains highly challenging. In this work, we propose a case study on oleylamine/chloride copassivated InP NCs, for which we show that the heterogeneous broadening of ¹H resonances in the nuclear magnetic resonance spectrum enables ligands adsorbed to facets to be distinguished from edge- or corner-bound ligands. More specifically, we demonstrate by means of the recovery of spectral holes burned in the alkene resonance of surface-bound oleylamine that the ligand shell consists of spatially distinct pools of oleylamine characterized by a different average chemical shift. Combining this observation with classical molecular dynamics simulations, we assign the downfield and upfield side of the alkene resonance to oleylamine bound to facets, or edges and corners, respectively. Importantly, we show that this proposed spectral fingerprint of surface-localization of ligands agrees with the change in shape of the alkene resonance when increasing the NC size and when exchanging part of the oleylamine for hexadecanethiol. Given the general applicability of the methodology introduced here, we believe this work offers a starting point to assess ligand localization for a broad range of colloidal NCs and develop a better understanding of the binding and packing of ligands on NC surfaces.