Probing the Surface Structure of Amine/Chloride-Passivated CdSe Nanocrystals Using Dynamic Nuclear Polarization-Enhanced ¹⁵ N, ³⁵ Cl, ⁷⁷ Se, and ¹¹³ Cd Solid-State NMR Spectroscopy

Ligands have been extensively used to modulate and improve the photophysical properties of nanocrystals (NCs). However, the surface structure of many NCs remains unknown. In this study, we synthesized oleate-capped CdSe NCs, exchanged them with oleylamine/CdCl₂, and used dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy to determine the structure of surface Cd atoms bound to ligands. ¹¹³Cd and ⁷⁷Se NMR experiments distinguish different types of ¹¹³Cd and ⁷⁷Se environments in the CdSe NCs. DNP-enhanced ¹¹³Cd magic angle turning (MAT) experiments enable the identification of one bulk and two surface Cd environments in CdSe NCs, while DNP-enhanced ¹⁵N{¹H} and ¹⁵N{¹¹³Cd} heteronuclear correlation experiments reveal nitrogen coordinated Cd atoms on the surface of the CdSe NCs. Indirect detection of chloride ligands was achieved using DNP-enhanced ¹¹³Cd{³⁵Cl} <i>J</i>-resolved experiments, revealing that the chlorides bridge between surface Cd atoms. Comparing the experimental NMR spectra to relativistic density functional theory calculation models establishes two distinct surface Cd environments that differ by their ligand environments: one site is coordinated by one amine and a bridging chloride, while the other is coordinated by two amines.