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Controlling the interparticle spacing in quantum dot (QD) solids is the most readily accessible way to control transport rates between neighboring QDs and a critical strategy for device optimization. Here, we use X-ray scattering to accurately measure the interparticle spacing in films of highly monodisperse lead sulfide (PbS) QDs that have undergone a variety of device-relevant ligand exchanges. We tabulate these values for use in simulations and to assist in data interpretation. We find that monothiol and dithiol ligand species typically result in interparticle spacings that are equal to the length of a single monothiol or dithiol ligand. Additionally, we find that spin-coating a thick film of QDs followed by a long-duration ligand exchange results in a significantly closer-packed arrangement than spin-coating many thin layers with short-duration ligand exchanges in between (layer-by-layer method). The former method preserves a remarkable degree of the superlattice order that was present in the film prior to ligand exchange, but also generates cracks due to volume loss within the solid. The similarity in interparticle spacing for many of the shortest ligands points to the importance of other factors, such as energy level matching and surface passivation in choosing the optimal ligand for a given device application. These results provide strategies for producing highly ordered QD solids with compact and functional ligands, which could lead to enhanced interdot coupling and transport phenomena.
Weidman et al. (Mon,) studied this question.