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Abstract Lead sulfide (PbS) quantum dots (QDs) hold great promise for solar energy conversion, yet their efficiency is compromised by a substantial Stokes shift that adversely affects their performance in photonic devices. Here, PbS QDs are integrated within single plasmonic nanocavities, significantly mitigating Stokes shifts through Purcell enhancement of their band edge emission. This approach entails bottom‐up assembly of QDs into nanoparticle‐on‐mirror structures, leading to direct emission from band‐edge excitons with radiative lifetimes suppressed below 1 ns, a drastic decrease from the 1600 ns observed in unmodified QDs. This manipulation of the Stokes shift is attributed to the increased photonic density of states within the nanocavity, which accelerates the radiative decay process and modifies exciton relaxation pathways. These results underscore the critical role of plasmonic nanocavities in modifying QD emission characteristics, offering opportunities for enhancing QD‐based device performance across a spectrum of photonic applications. 

In the synthesis of colloidal PbS nanosheets, the supernatant of the reaction solution is reused to boost the lead conversion efficiency. It doubles the conversion efficiency of the lead precursors to the PbS nanosheets. The nanosheets synthesized by reusing the supernatant have similar morphology, nearly identical thickness, and optical properties as the original ones, confirmed by transmission electron microscopy, X‐ray diffraction, and photoluminescence spectroscopy. This method reduces the toxic Pb‐containing waste during the synthesis, a step toward the green and scalable synthesis of colloidal 2D PbS nanosheets.