Ligand Basicity Modulates Metal Cation Reduction Potentials at Colloidal Cadmium Chalcogenide Quantum Dot Surfaces

Reduction or oxidation reactions at colloidal semiconductor nanocrystal quantum dot (QD) surfaces are critical mechanistic steps for charge trapping or for photoinduced charge transfer. However, measuring and controlling the redox potentials of the surface is nontrivial. Here, monoanionic metal carbonyl complexes are used as electronically tunable X-type ligands for CdSe and CdS QD surfaces. IR spectroscopy of the frequencies of the C-O stretching vibrations of the anionic metal carbonyl species enable quantitative measurement of anion dissociation correlated to QD surface reduction. Spectral redox titration and spectroelectrochemical experiments show that coordination of more Lewis basic anions shifts the surface reduction potentials to more negative values, spanning a range of more than 1 V. Based on these results, complexation energies are a critical factor in controlling surface charge storage. This concept extends generally to other types of QD supporting ligands and to other QD materials. As proof-of-concept, anion exchange was used to control chemical surface reduction and photochemical electronic doping in CdSe QDs.