c-ALD-Grown Metal Oxide Shell Enables Distance-Independent Triplet Energy Transfer from Quantum Dots to Molecular Dyes

Hybrid nanocomposites including quantum dots (QDs) and molecular dyes offer tunability of charge and energy transfer processes, which are attractive for applications spanning from light-emitting devices to photocatalysis and photon upconversion. Core@shell QDs have garnered attention for their ability to modulate charge and energy transfer from the core to the dye via the shell. Recent studies suggested that the core@shell QDs are less demanding in terms of the QD-dye electronic coupling requirements and that the distance dependence of transfer efficiencies may vary from what is traditionally observed in molecular systems. However, these studies remain only a few, and the mechanistic role of the shell is often unclear. Here, we synthesize QD-dye nanocomposites including CdSe@AlOx core@shell QDs, and we report the observation of a nearly distance-independent triplet energy transfer (TEnT) to polyaromatic hydrocarbon (PAH) dyes from the CdSe core of up to almost 2 nm. The use of colloidal atomic layer deposition (c-ALD) for the growth of the metal oxide shell enables tunability of the system, which is crucial to elucidate the role of the shell and the thickness dependence of the TEnT. We propose a plausible mechanism where a hole-transfer-mediated TEnT takes place and defects in the metal oxide act as intermediate states, which enable a long-range, distance-independent TEnT. The versatility of the c-ALD methodology, along with the ability of oxides to preserve the optoelectronic properties of QDs, showcases the potential of oxide shells to optimize the QD-dye interaction to achieve long-range TEnT to molecular dyes, opening new avenues for applying QDs as sensitizers in light-harvesting, emission, and conversion applications.