Ligand-exchange procedures are ubiquitous in the functionalization of colloidal nanocrystals for applications in biological imaging, photocatalysis, and photonic/optoelectronic devices. However, the rich interactions between functional ligands and the nanocrystal surface offer a vast opportunity to achieve emergent self-assembled structures. Here, using 1 H NMR as a probe and L-type-promoted Z-type ligand displacement as a tool to study PbS nanocrystals, we demonstrate that 9-anthracene carboxylic acid (9-ACA) ligands strongly segregate to the highestenergy binding sites at the conclusion of X-for-X exchanges. These weaker sites are associated with nanocrystal facet-edges, and linewidth analysis corroborates that 9-ACA replaces the most conformationally dynamic native ligands. The templated assembly of this bulky model fluorophore at the nanocrystal surface is an opportunity to enhance energy transport and contrasts sharply with conventional aliphatic ligands, where we find that exchanges are isotropic. Our results indicate that ligand−ligand interactions and the spatial correlation of nanocrystal binding-site heterogeneity can be leveraged to produce functionalized particles with tailored, anisotropic ligand morphologies. This opportunity to promote clustering could influence the design of photoactive ligands for multiexcitation processes such as incoherent photon conversion.

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A clear understanding of the surface of nanocrystals informs our views of nucleation and growth, and allows for tailored ligand exchanges to meet target applications. PbS colloidal quantum dots are attractive for infrared optoelectronic devices, but PbS nanocrystals made using excess PbCl2 (PbS-eCl NCs) have found limited use, despite showing advantageous ensemble properties. Here, we use 1D and 2D 1H NMR to determine that the native passivation of PbS-eCl NCs involves bound oleylammonium. Then, by mapping the set of permissible ligand exchanges, we uncover that the surface of these nanocrystals matches the behavior of lead halide perovskites. Building on this insight, we infer the ligand binding motif and perovskite-like atomic structure that forms a thin, intrinsic shell on the PbS core. Indeed, we show that two-dimensional L2PbCl4 (L = oleylammonium) sheets are readily formed in the reaction mixture prior to the nucleation of PbS-eCl NCs. Our structural model for the surface then allowed us to develop techniques to improve nanocrystal purification, colloidal stability, and the postsynthetic installation of X-type ligands. In all, we show that the synthesis and surface of PbS-eCl NCs should be treated differently compared to traditional PbS NCs prepared from lead oleate, and highlight instead that ligand exchanges developed for lead halide perovskites can translate to this infrared material. The framework that we present for the manipulation of PbS-eCl NCs in solution can advance their wider use in optoelectronic devices.

Kesterite Cu 2 ZnSnS 4 (k-CZTS) nanocrystals have received attention for their tunable optoelectronic properties, as well as the earth abundance of their constituent atoms. However, the phase-pure synthesis of these quaternary NCs is challenging due to their polymorphism, as well as the undesired formation of related binary and ternary impurities. A general synthetic route to tackle this complexity is to pass through intermediate template nanocrystals that direct subsequent cation exchange toward the desired quaternary crystalline phase, particularly those that are thermodynamically disfavored or otherwise synthetically challenging. Here, working within this model multinary system, we achieve control over the formation of three binary copper sulfide polymorphs, cubic digenite (Cu 1.8 S), hexagonal covellite (CuS), and monoclinic djurleite (Cu 1.94 S). Controlled experiments with Cu 0 seeds show that selected binary phases can be favored by the identity and stoichiometry of the sulfur precursor alone under otherwise comparable reaction conditions. We then demonstrate that the nature of the Cu 2−x S template dictates the final polymorph of the CZTS nanocrystal products. Through digenite, the cation exchange reaction readily yields the k-CZTS phase due to its highly similar anion sublattice. Covellite nanocrystals template the k-CZTS phase but via major structural rearrangement to digenite that requires elevated temperatures in the absence of a strong reducing agent. In contrast, we show that independently synthesized djurleite nanorods template the formation of the wurtzite polymorph (w-CZTS) but with prominent stacking faults in the final product. Applying this refined understanding to the standard one-pot syntheses of k-and w-CZTS nanocrystals, we identify that these reactions are each effectively templated by binary intermediates formed in situ, harnessing their properties to guide the overall synthesis of phase-pure quaternary materials. Our results provide tools for the careful development of tailored nanocrystal syntheses in complex polymorphic systems.