We demonstrate the use of ultra-small PbS quantum dots as endothermic sensitizers for red-to-blue triplet-fusion upconversion, achieving nanocrystal-sensitized upconversion photochemistry.

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.

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.

Given their nanoscale dimensions, colloidal semiconductor nanocrystals provide unique systems for investigating the dynamics controlling surface chemistry and fundamental issues regarding lattice reorganization upon changes in electron distribution. These systems are particularly amenable to ultrafast electron probes, offering an atomic level picture of the lattice reorganization involved following photoexcitation. Here, we study lead sulfide (PbS) quantum dots with ultrafast electron diffraction to characterize the atomic motions following high-intensity photoexcitation. Short-range nonthermal lattice distortions and increased atomic disorder were observed in PbS colloidal quantum dots ranging from 2.4 to 8.7 nm in size. These effects scaled inversely with size and were less pronounced in nanocrystals with a chloride-containing surface rather than only organic ligands, which is consistent with an effect arising at the surface. The anisotropic, nonthermal lattice disordering occurs preferentially along the (100) crystallographic directions, which could indicate an anisotropic distribution of localized charge between the differing lattice terminations of the {111} and {100} crystal facets. This is consistent with the larger anharmonicity for the lattice potential at lattice sites with reduced ligand coordination relative to the bulk, which has been shown to cause accelerated relaxation into dynamic and static surface trap sites. Through an exploration of quantum dot size and variation in surface termination, this work provides the missing structural details to advance our understanding and control of charge-carrier formation, trapping, and recombination processes in nanoscale semiconductor systems.

III‐V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared (IR). However, the covalent nature of III‐V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: this work shows herein that ligand exchanges, previously developed in II‐VI and IV‐VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, this work develops a co‐passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X‐type methyl ammonium acetate (MaAc) and Z‐type ligands InBr3. This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a fourfold reduction in the rate of first‐exciton absorption linewidth broadening over time‐under‐stress, and leads to a doubling in photoluminescence (PL) lifetime. The resulting devices show 37% external quantum efficiency (EQE) at 950 nm, the highest value reported for InAs CQD photodetectors.