Atomic-Level Structure of the Organic-Inorganic Interface of Colloidal ZnO Nanoplatelets from Dynamic Nuclear Polarization-Enhanced NMR

Colloidal semiconductor nanoplatelets (NPLs) have emerged as a new class of nanomaterials that can exhibit substantially distinct optical properties compared to those of isotropic quantum dots, which makes them prime candidates for new-generation optoelectronic devices. Insights into the structure and anisotropic growth of NPLs can offer a blueprint for their controlled fabrication. Here, we present an atomic-level investigation of the organic-inorganic interface structure in ultrathin and stable benzamidine (bza)-supported ZnO NPLs prepared by the modified one-pot self-supporting organometallic approach. High-resolution transmission electron microscopy analysis showed a well-faceted hexagonal shape of ZnO NPLs with lateral surfaces terminated by nonpolar (101̅0) facets. The basal surfaces are flat and well-formed on one side and corrugated on the other side, which indicates that the layer-by-layer growth in the thickness of the NPLs likely occurs only in one direction via the expansion of 2D islands on the surface. The ligand coordination modes were elucidated using state-of-the-art dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy supported by density functional theory chemical shift calculations. Specifically, it was found that (101̅0) nonpolar facets are stabilized by neutral L-type bza-H ligands with hydrogen bond-supported η1-coordination mode, while polar (0001) and (0001̅) facets are covered by μ2-coordinated X-type anionic bza ligands with different conformations of aromatic rings. Moreover, the ligand packing on (101̅0) lateral facets was determined using 13C natural abundance (∼1.1%) homonuclear dipolar correlation experiments. Overall, an in-depth understanding of the growth mechanism and the unique bimodal X-type/L-type ligand coordination shell of ZnO NPLs is provided, which will facilitate further design of anisotropic nano-objects.