Mechanisms of Defect Passivation by Fluorinated Alkylthiolates on PbS Quantum Dots

Defects in the organic ligand layers on the surfaces of colloidal quantum dots (QDs) provide pathways for corrosive molecules to penetrate to the QD core. This paper describes the decrease in the permeability of the ligand shells of colloidal near-infrared-emitting PbS QDs to the molecular photo-oxidant, duroquinone (Me₄BQ), upon substituting a small fraction of their oleate ligands with either 1-dodecanethiolate (DDT) or progressively fluorinated DDT analogues (with between 1 and 10 fluorinated carbons), as measured by the yield of collisionally gated photoinduced electron transfer from the QD to Me₄BQ. The permeabilities of mixed-monolayer ligand shells of oleate and 8-16% (by surface area) DDT are 35-41% lower than those of the pure oleate monolayers. Increasing the number of fluorinated carbons in the thiolate ligands from 0 to 10 results in an additional 40-66% decrease in the permeability of the ligand shell; as few as 0.05% of collisions between the largest QDs and Me₄BQ result in electron transfer. The thiolate exchange, and fluorination of the thiolate ligands, more effectively protect the largest QDs than the smallest QDs, primarily due to the size-dependence of the types of defects in the native oleate monolayers.