Monoalkyl Phosphinic Acids as Ligands in Nanocrystal Synthesis

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 monoalkyl 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 oleylphosphinic 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% (without shell). Furthermore, we could synthesize CdSe and CdS nanorods using phosphinic acid ligands with high shape purity. We conclude that the reactivity toward TOP-S and TOP-Se precursors decreases in the following series: cadmium carboxylate > cadmium phosphinate > cadmium phosphonate. By introducing a third and intermediate class of surfactants, we enhance the versatility of surfactant-assisted syntheses.