Direct Synthesis of Ultrasmall PbS Nanocrystals Passivated with a Metal-Halide-Perovskite-like Monolayer

Inorganic shelling is a key strategy to address unwanted, surface-mediated nonradiative pathways in semiconductor nanocrystals. Improved passivation particularly advances the use of the smallest nanocrystals because complex photophysics arise as nanomaterials approach the molecular limit. However, shelling strategies developed for larger/more-robust nanocrystals are not always applicable in this size regime. Herein, we introduce the direct synthesis of ultrasmall (ø < 2 nm) PbS nanocrystals shelled with a range of metal-halide-perovskite-like monolayers by employing the appropriate two-dimensional perovskite as a precursor at the point of nanocrystal growth. These passivated nanocrystals possess improved photoluminescence quantum yields, reduced dynamic spectral red-shifting after photoexcitation, and longer, more-monoexponential photoluminescence dynamics─consistent with a reduction in defect-mediated exciton–phonon coupling. We then apply these shelled, ultrasmall PbS nanocrystals as sensitizers in a triplet-fusion upconversion architecture and observe 2-fold improved triplet yields. This demonstration emphasizes the photophysical benefits realized via our 2D perovskite shelling strategy, which can be applied to other nanocrystals─especially to small particles that are similarly vulnerable to existing strategies for post-synthetic modifications.