Exploiting Ultrafast Auger Recombination in Colloidal ZnSe Quantum Dots for Photoemission and Hydrated Electron Generation.

Colloidal ZnSe quantum dots (QDs) have recently emerged as a promising toxic-element-free semiconductor platform for a variety of applications. A particularly exciting opportunity is associated with the highly potent reduction power of the photogenerated electrons in ZnSe QDs, owing to their large bandgap and high-energy conduction-band electrons. These properties enabled efficient hydrated electron generation in Mn2+-doped ZnSe QDs through spin-exchange Auger recombination mechanism. Here, we report for the first time that hydrated electrons can be also generated in undoped plain ZnSe QDs, originating from their exceptionally rapid multiexciton Auger recombination. We study size-dependent Auger recombination in monodisperse ZnSe QDs using transient absorption spectroscopy and identify biexciton lifetimes (τXX) shorter than those of comparable-size CdSe QDs by almost 10-fold. Importantly, we find the volume-scaling law of τXX that holds in hexane phase breaks down in aqueous phase for the small-size regime, highlighting the crucial role of the dielectric confinement effect in Auger processes. Transient absorption spectra also reveal size-dependent thresholds for photoemission of free electrons, with smaller QDs requiring fewer excitons due to their faster Auger recombination. In water, photoemission generates hydrated electrons with quantum yield exceeding 3%.