On-Demand Charged States in Quantum Dot-ZnO Nanoparticle Assemblies toward Sub-Excitonic Optical Gain

Achieving optical gain in colloidal quantum dots (QDs) requires band-edge state filling beyond optical transparency, typically requiring multiexciton populations that are nevertheless rapidly depleted by Auger recombination. Electron charging offers a route to reduce this threshold by prefilling the lowest electron states of QDs; however, conventional photochemical charging approaches are not readily applicable to solid-state gain media because they rely on solution-phase redox chemistry. Here, we demonstrate that ZnO-QD mixed nanoparticle assemblies enable tunable electron charging and reduce the optical gain threshold. We study these assemblies in solutions and in the solid state, demonstrating a three-order-of-magnitude increase in charging efficiency in ZnO-QD films. This discrepancy originates from proximity-dependent electron transfer between ZnO nanoparticles and QDs, which we capture using a unified charging model that combines particle ratio and spatial proximity. The resulting proximity-controlled charging enables subexcitonic optical gain and amplified spontaneous emission in ZnO-QD assemblies, offering a route toward low-threshold QD gain media.