The Structure of Oleate-Capped CdSe Nanocrystals Determined by DNP-Enhanced Dipolar Dephasing Solid-State NMR Spectroscopy

Semiconductor nanocrystals (NCs) are of interest due to their tunable electronic and optical properties. However, the atomic-level characterization of surface structures of NCs remains an unresolved challenge. Here, we demonstrate that rotational-echo double-resonance (REDOR) experiments can be used to determine structural models of surfaces of CdSe NCs. An atom-efficient ligand-exchange scheme is demonstrated to introduce partially 13C-enriched oleate ligands onto CdSe NCs and enable 13C-dephased REDOR experiments. DNP-enhanced 13C{77Se}, 77Se{13C}, 13C{111Cd}, and 111Cd{13C} REDOR NMR experiments were performed on the 13C-enriched oleate-capped zinc blende CdSe NCs. Despite the modest isotope abundance of 77Se and 111Cd, DNP-enhanced 13C{77Se} and 13C{111Cd} REDOR NMR experiments show significant dephasing due to dipolar coupling between 13C atoms and multiple 77Se or 111Cd atoms that are present in the underlying NC lattice. Multispin simulations of 13C{77Se}, 77Se{13C}, 13C{111Cd}, and 111Cd{13C} REDOR curves are performed for different surface structure models, accounting for the structure of the underlying CdSe NC lattice. Based upon these simulations, we propose a structural model of oleate-capped CdSe NCs where chelating carboxylate ligands bind to Cd atoms on {100} zinc blende CdSe surfaces. The combination of REDOR NMR experiments and multispin simulations is a powerful and general method for determining the atomic-level structures of semiconductor NCs and other inorganic materials.