Quantifying ligand exchange on quantum dots

The intense colors emitted by lead halide semiconductor nanocrystals such as cesium lead bromide, CsPbBr₃, make these materials promising candidates for application in television displays, solar cells, and light-emitting diodes. Standard synthesis procedures leave these nanocrystals, or quantum dots (QDs), covered with oleic acid and oleylamine ligands, which chemically stabilize the particles. But these so-called native ligands don’t stay put. They adsorb and desorb from these QDs more readily than they do from less ionic QDs such as cadmium selenide. Because the ligands affect QD optoelectronic properties—and because they often need to be replaced or altered after synthesis for applications—researchers need to quantify the ligand-exchange chemistry on these lead halide QDs. That’s exactly what Sara R. Smock, Travis J. Williams, and Richard L. Brutchey of the University of Southern California have done. Using NMR spectroscopy and other methods, the team showed that the native ligands undergo reversible exchange with long-chain carboxylic acid and alkylamine ligands and also measured those equilibrium constants and other parameters. They also showed that phosphonic acid ligands, in contrast, displace native ligands and bind irreversibly (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201806916).