Semiconductor quantum dots (QDs) are prime candidates as harvesters of light and donors of excited charge carriers for solar energy conversion. Our group’s recent efforts have established the validity of utilizing carbodiimide-mediated coupling chemistry to selectively tether two QDs through the formation of an amide bond between the terminal functional groups of capping ligands. The unique properties of QDs can be exploited to tune energetic offsets to promote interfacial charge transfer between QDs. We previously reported on excited-state hole transfer in dispersed CdS/CdSe QD heterostructures, which exhibit quasi-type-I energetic offsets. Unfortunately, type-I energetic offsets rely on the excitation of one QD component, which promotes recombination as a result of charge transfer occurring in only one direction. To address this issue, we synthesized and characterized CdSe/CdTe QD heterostructures, which exhibit type-II energetics. Type-II energetics promote interfacial charge separation, irrespective of which constituent QD is initially excited. Moreover, the CdSe/CdTe QD heterostructures afford enhanced control over the thermodynamic driving forces for charge transfer. Within these systems photogenerated holes are transferred from CdSe to CdTe on timescales of 10-8 s. In an effort to further extend charge separation and to slow charge recombination, we have prepared ternary heterostructures. A covalently-linked bilayer of QDs was immobilized on a metal oxide substrate to promote an additional stepwise excited-state charge transfer process, facilitating extended spatial separation of charge carriers and longer charge-separated state lifetimes. Dynamic quenching of emission was observed in heterostructure-modified thin films, consistent with excited-state charge transfer. Rate constants for photoinduced hole transfer between QDs are on the order of 107 s-1. CdSe/CdTe heterostructures exhibit bidirectional charge transfer, in which the rate constant for photoinduced electron transfer from CdTe to CdSe was estimated to be on the order of 108 s-1. These results demonstrate that carbodiimide-mediated coupling chemistry can be used to immobilize QD multilayers on metal oxidesubstrates, and that such assemblies undergo rapid charge transfer to facilitate extended charge separation. This presentation will highlight these recent results as well as our ongoing time-resolved spectroscopic characterization of photoinduced charge transfer in CdSe/CdTe systems.

Assembly of covalently-linked quantum dot heterostructures: Characterization of excited-state charge-transfer dynamics in dispersed and multilayered systems

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.