Influence of Polaron Occupied Surface Trap States on Photoluminescence Dynamics in CsPbBr₃ Nanocrystals

APbX₃lead halide perovskite (LHP) nanocrystals (NCs) are of interest for opto-electronic applications due to their established properties, such as wide color gamut and high photoluminescence (PL) quantum yield (QY), as well as their promise for next generation devices, such as single-photon emitters. Their photo-physical properties are heavily dependent on defect-free surface chemistry and the degree of spatial confinement, which implies a significant contribution from surface effects. Here we synthesize colloidal CsPbBr₃ NCs and use density-gradient ultracentrifugation to fractionate the ensemble into-size resolved colloidal samples ranging from 16 to 4 nm in edge length. The fractionated NCs are then post-processed with various ligand-dispersed salts to passivate surface vacancy defects created during ultracentrifugation. From photoluminescence lifetime measurements, we find a distinct crossover from single to stretched-exponential relaxation with decreasing NC size, which is indicative of a transition from an ideal 2-level emitter to a system with a distribution of emission states. To investigate the influence of surface defect states on the PL dynamics, we use <i>ab Initio </i>atomistic modeling in the strong-confinement limit, considering both fully passivated NCs and NCs with surface vacancy defects. We find that when the charge density is constrained to the lowest excited state, approximating the photo-excited state, surface electronic trap states (i.e., polarons) emerge through reorganization of nuclei . Interestingly, these polaronic trap states have bright optical features for static geometries. To test if these surface trap states remain bright in a dynamic (thermal) scenario, we implement non-adiabatic excited-state molecular dynamics simulations, which mix electronic and nuclear degrees of freedom to describe photo-induced non-radiative relaxation. It is found that the surface defected state does indeed contribute bright PL that is red-shifted from the pristine model. The atomistic simulations provide support for polaron-induced bright surface trap states as the origin of size-dependent stretched-exponential PL decays, with the implication that surface effects linked to increased spatial confinement can be detrimental to applications such as single-photon emission.