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

Ligands have been extensively used to modulate and improve the photophysical properties of CdSe nanocrystals (NCs). However, the surface structure of many nanocrystals remain 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 that bind to the ligands. 113Cd and 77Se NMR experiments distinguish the 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} J-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.