Surface Chemistry of CuInS₂ Colloidal Nanocrystals, Tight Binding of L‑Type Ligands

We analyze the surface chemistry of CuInS₂ nanocrystals synthesized in the presence of amines. Using solution NMR spectroscopy and elemental analysis, we come to the conclusion that as-synthesized CuInS₂ nanocrystals have charge neutral inorganic cores and are stabilized by a layer of tightly bound L-type amines. In situ NMR heating-up experiments show that desorption of amines can be induced by increasing the temperature, which makes the partial exchange of amines for thiols possible. On the other hand, we find that carboxylic acids are unable to bind as L-type ligands to the CuInS₂ surface. In addition, we demonstrate that the use of technical oleylamine in the synthesis of CuInS₂ nanocrystals leads to nonstoichiometric nanocrystals which have, next to oleylamine ligands, also X-type impurities on the surface that can be exchanged for carboxylic acids