Photoluminescence Quenching of Colloidal CdSe and CdTe Quantum Dots by Nitroxide Free Radicals

Quantum dots (QDs) and other quantum confined semiconductor nanomaterials are emerging as an important class of materials for a variety of applications involving electronic processes. Here, we investigate QD-free radical interactions, focusing on II-VI semiconductor QDs (CdSe and CdTe) and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl). Although these nitroxide free radicals have previously been shown to be efficient QD photoluminescence quenchers, we show here, using a combination of both steady-state and time-dependent photoluminescence studies, that some of these conclusions need to be revised. In particular, we show that TEMPO has no appreciable effect on the photoluminescence of II-VI QDs. Furthermore, we show that whereas 4-amino-TEMPO is indeed an efficient PL quencher for II-VI QDs (as previously reported) it is simultaneously limited by slow diffusion kinetics and low binding affinities to the nanocrystal surfaces. Our results highlight that the nature of ligand passivation of the QD surface is a very important factor in controlling the interaction between nitroxide radicals and QDs, a key aspect to consider in future applications involving such dyads.