Polaronic-Distortion-Driven Enhancement of Excitonic Auger Recombination in ZnO Nanoparticles

Surface effects and quantum confinement render nanomaterials' optoelectronic properties more susceptible to nonradiative processes than their bulk counterparts. These nonradiative processes usually contain a series of interwoven and competing subprocesses, which are challenging to disentangle. Here, we investigate the structural origin of Auger recombination in ZnO nanoparticles using transient absorption spectroscopy and ultrafast electron diffraction. The photogenerated hot holes are captured by oxygen vacancies via an excitonic Auger process, inducing significant local polaronic distortions around the oxygen vacancy and its neighboring zinc tetrahedron on a subpicosecond time scale. The recombination of trapped holes accelerates lattice thermalization and stabilizes the formed small hole polarons. Subsequently, the distorted lattice captures additional electrons in a conduction band, forming a long-lived (>6 ns) exciton-polaron complex that may account for the visible luminescence. Our findings are potentially applicable to other transition metal oxide nanomaterials, bringing insights for optimizing their functional properties.