Localizing Ligands on Nanocrystal Surfaces

Colloidal nanocrystals synthesized in apolar media are organic/inorganic hybrids that consist of a crystalline core and a surface capping of organic ligands. While the physical properties of individual nanocrystals are mostly determined by the inorganic core, the role of the organic ligand shell cannot be underestimated. Examples range from synthetic aspects to properties of single nanocrystals and nanocrystal assemblies, such as precursor conversion and nucleation and growth kinetics, the passivation of electronic trap states or the realization of high charge carrier mobility. As a result, the question as to how ligands bind and pack to the nanocrystal surface has been central to nanocrystal research for many years. Extensive insight was obtained by introducing innovative experimental methods to identify surface-bound ligands and, more recently, computational approaches to calculate properties of these ligands. An experimental method that stands out for the analysis of ligand binding is nuclear magnetic resonance (NMR) spectroscopy, due to its capability of identifying and quantifying nanocrystal-bound moieties through diffusion-ordered spectroscopy (DOSY) or nuclear Overhauser effect spectroscopy (NOESY). However, while in-situ NMR analysis of ligand exchange gives experimental insight in ligand binding - often formalized through the L-, X- or Z-type classification borrowed from coordination chemistry - understanding ligand packing from NMR data proofed more difficult.<br/><br/>In this talk, we discuss recent progress on the interpretation of the NMR lineshape in terms of ligand packing on nanocrystal surfaces. We take InP nanocrystals synthesized by reacting InCl₃ and tris-diethylaminophosphine in oleylamine as a model system. After showing that the resulting nanocrystals have a mixed ligand shell consisting of chloride and oleylamine, we use spectral hole burning to show that the oleylamine ¹H NMR resonances are heterogeneously broadened. Moreover, by recording the relaxation dynamics of spectral holes, we show that the heterogeneous set of bound ligands consists of at least 2 pools that are not in close proximity, are not involved in chemical exchange and feature a different solvation. In practice, when using aromatic solvents, the more solvated pool is retrieved at the upfield side of the resonance, while the less solvated pool at the downfield side. Using molecular dynamics simulations based on a realistic, atomically precise InP model nanocrystal, we identify the most and least solvated pool as ligands bound to edges and facets, respectively. Interestingly, changes of NMR lineshape following partial ligand exchange reactions indicate that the pool of edge-localized ligands can be selectively removed, a result in line with the predicted weaker binding of ligands at those sites.<br/><br/>To conclude, we highlight that ligands may pack in different, static or dynamic ways to nanocrystal surfaces. Since each of these modes will have a different effect on the heterogeneous broadening of NMR resonances of bound ligands, more is to be learned from spectral hole burning and spectral hole burning relaxation than discussed in this talk.