GPC provides a general approach to purification of a variety of nanocrystal types, and additionally achieves ligand exchange in a continuous flow system.

Recently the addition of M Lewis acids (M = Cd, Zn) to InP quantum dots (QDs) has been shown to enhance the photoluminescence quantum yield (PL QY). Here we investigate the stability of this Lewis acid layer to postsynthetic processing such as purification and ligand exchange. We utilize gel permeation chromatography to purify the quantum-dot samples as well as to aid in the ligand-exchange reactions. The Lewis-acid-capped particles are stable to purification and maintain the enhanced luminescence properties. We demonstrate successful ligand exchange on the quantum dots by switching the native carboxylate ligands to phosphonate ligands. Changes in the optical spectra after exposure to ambient environment indicate that both carboxylate- and phosphonate-capped QDs remain air-sensitive.

We report the first comprehensive study on the purification of PbS quantum dots (QDs) using gel permeation chromatography (GPC). GPC enables the efficient and repeatable separation of unbound small molecules from QDs in a homogeneous solvent environment. This separation allows us to gain further insight into the surface capping layer of PbS QDs and measure the ligand density on QDs of different sizes. We find that small oleate-capped PbS QDs are stable in solution in toluene after purification by GPC with negligible free ligand concentrations. However, purification of larger-diameter QDs reveals two oleate populations that persist after multiple GPC purifications: one with a broad olefin resonance characteristic of strongly bound ligands in smaller QDs, and the other with a narrower peak suggesting dynamic exchange with freely diffusing species. Variable-temperature NMR indicates a reversible equilibrium between the two populations, which is also observed in samples purified through precipitation with a polar solvent. The observation of two oleate populations following GPC purification is maintained even when the large-diameter QDs are formed using a strictly anhydrous procedure that should eliminate the presence of water, hydroxide, or oleic acid in as-synthesized samples. In conducting this strictly anhydrous synthesis, we incidentally observe that water plays a role in determining the resultant size of PbS QDs prepared by traditional methods compared to their anhydrous analogues. Understanding how the surfaces of PbS QDs are terminated should aid in interpreting their behavior in subsequent ligand exchange steps as necessary for diverse applications.