Characterization of the Ligand Capping of Hydrophobic CdSe–ZnS Quantum Dots Using NMR Spectroscopy

Abstract

We have combined a few advanced solution phase NMR spectroscopy techniques, namely, 1H, 31P, heteronuclear single quantum coherence (HSQC), and diffusion ordered spectroscopy (DOSY), to probe the composition of the organic capping layer on colloidal CdSe–ZnS core–shell quantum dots grown via the “hot injection” route. Combining solution phase 31P and 1H NMR with DOSY, we are able to distinguish between free ligands and those coordinated on the QD surfaces. Furthermore, when those NMR measurements are complemented with matrix-assisted laser desorption ionization (MALDI) and FTIR data, we find that the organic shell of the as-prepared QDs consists of a mixture of tri-n-octylphosphine oxide (TOPO), tri-n-octylphosphine (TOP), alkyl amine, and alkyl phosphonic acid (L- and X-type ligands); the latter molecules are usually added during growth at a rather small concentration to improve the quality of the prepared nanocrystals. However, NMR data collected from QD dispersions subjected to two or three rounds of p...