Influence of donor-to-acceptor ratio on excited-state electron transfer within covalently tethered CdSe/CdTe quantum dot colloidal heterostructures

Abstract

Carbodiimide-mediated coupling chemistry was used to synthesize heterostructures of CdSe and CdTe quantum dots (QDs) with varying ratios of electron-donating CdTe QDs and electron-accepting CdSe QDs. Heterostructures were assembled via the formation of amide bonds between the terminal functional groups of CdTe-adsorbed 4-aminothiophenol (4-ATP) ligands and CdSe-adsorbed N-hydroxysuccinimide (NHS) ligands. The number of charge acceptors on the surfaces of QDs can greatly influence the rate constant of excited-state charge transfer with QDs capable of accommodating far more acceptors than molecular chromophores. We report here on excited-state electron transfer within heterostructure-forming mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs with varying molar ratios of CdTe to CdSe. Photophysical properties and charge transfer were characterized using UV-vis absorption, steady-state emission, and time-resolved emission spectroscopy. As the relative concentration of electron-accepting CdSe QDs within mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs increased, the rate and efficiency of electron transfer increased by 100-fold and 7.4-fold, respectively, as evidenced by dynamic quenching of band-edge emission from CdTe QDs. In contrast, for non-interacting mixtures of thiophenol capped CdTe QDs and NHS-capped CdSe QDs, which served as control samples, photophysical properties of the constituent QDs were unperturbed and excited-state charge transfer between the QDs was negligible. Our results reveal that carbodiimide-mediated coupling chemistry can be used to control the relative number of donor and acceptor QDs within heterostructures, which, in turn, enables fine-tuning of charge-transfer dynamics and yields. These amide-bridged dual-QD heterostructures are, thus, intriguing for light harvesting, charge transfer, and photocatalysis.