1-octadecene is a widely used solvent for high temperature nanocrystal synthesis (120-320 °C). Here, we show that 1-octadecene spontaneously polymerizes under these conditions and the resulting poly(1-octadecene) has a comparable solubility and size to nanocrystals stabilized by hydrophobic ligands. Typical purification procedures (precipitation/redispersion cycles or size exclusion chromatography) fail to separate the poly(1-octadecene) impurity from the nanocrystal product. To avoid formation of poly(1-octadecene), we replace 1-octadecene with saturated, aliphatic solvents. Alternatively, the nanocrystals' native ligands are exchanged for polar ligands, leading to significant solubility differences between nanocrystals and poly(1octadecene), therefore allowing isolation of pure nanocrystals, free from polymer impurities. These results will help design superior syntheses and improve nanocrystal purity, an important factor in many applications.

Ligands play a crucial role in the synthesis of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, oleic acid being the most typical example. Here, we show that mono-alkyl phosphinic acids are another interesting ligand class, forming metal complexes with a reactivity that is intermediate between the traditional carboxylates and phosphonates.We first present the synthesis of n-hexyl, 2-ethylhexyl, n-tetradecyl, n-octadecyl, and oleyl phosphinic acid. These compounds are suitable ligands for high-temperature nanocrystal synthesis (240-300°C) since, in contrast to phosphonic acids, they do not form anhydride oligomers. Consequently, CdSe quantum dots synthesized with octadecylphosphinic acid are conveniently purified, and their UV-Vis spectrum is free from background scattering. The CdSe nanocrystals have a low polydispersity and a photoluminescence quantum yield up to 18%. Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands, with high shape purity. We conclude that the reactivity towards TOP-S and TOP-Se precursors decreases in the series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate.By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses.

We recently introduced monoalkyl phosphinic acids as a ligand class for nanocrystal (NC) synthesis. Their metal salts have interesting reactivity differences compared to metal carboxylates and phosphonates and provide a cleaner work-up compared to phosphonates. However, little is known about the surface chemistry of NCs with monoalkyl phosphinate ligands. Here, we probe the relative binding affinity of monoalkyl phosphinate ligands with respect to other X-type ligands. We perform competitive ligand exchange reactions with carboxylate and phosphonate ligands at the surface of hafnia, cadmium selenide, and zinc sulfide NCs. We monitor the ligand shell composition by solution 1 H and 31 P NMR spectroscopy. Using a monoalkyl phosphinic acid with an ether functionality, we gain an additional NMR signature, apart from the typical alkene resonance in oleic acid and oleylphosphonic acid. We find that carboxylate ligands are easily exchanged upon exposure to monoalkyl phosphinic acids, whereas an equilibrium is reached between monoalkyl phosphinates and phosphonates, slightly in the favor of phosphonate (K = 2). Phosphinic acids have thus an intermediate binding affinity between carboxylic acids and phosphonic acids for all the NCs studied. These results enable the sophisticated use of monoalkyl phosphinic acids for NC synthesis and for post-synthetic surface engineering.

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