PbS Nanocrystals Made Using Excess Lead Chloride Have a Halide-Perovskite-Like Surface

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.