Strain-Induced Long-Lived Heterogeneous Luminescent States in the Outer Layers Unlock Charge Extraction in Stable Graded-Alloy Giant Quantum Dots.

Instead of a simple core-shell giant quantum dot (GQD), in graded alloy GQDs, the graded alloy shell minimises lattice-mismatch-induced strain. Here, we synthesized a series of graded alloy GQDs of CdZnSeS/ZnSeS/ZnCdS architecture y = 0-1; x = 0, 0.25, 0.5, 1, purposefully tuning the outermost shell composition to introduce a sharp strain gradient in the outer layer through controlled lattice mismatch, exhibiting long-term environmental stability, high thermal stability, and photostability. As the cadmium content in the outer layer increased, the photoluminescence became broad and red-shifted, with longer lifetimes. Microstrain analysis, together with wavelength-resolved photoluminescence excitation and time-resolved photoluminescence, confirms the emergence of strain-induced heterogeneous luminescent states. Ultrafast transient absorption and PL upconversion further demonstrated the rapid depletion of excitons and shallow traps, accompanied by the concurrent formation of long-lived photoluminescent states. Interaction with benzoquinone confirms facile electron extraction from the core via these luminescent states, with extraction efficiency scaling with the density of these states, and the immediate blueshift in photoluminescence constrains the long-lived states to surface proximity. Thus, outer layer strain engineering furnishes robust GQDs that co-optimize stability and charge extraction through strain-induced luminescent states, offering a promising design principle for photocatalytic applications.