The Auger effect in semiconductor nanoparticles causes nonradiative recombination of electron-hole pairs and significantly impacts their photophysical behavior. Improving the fundamental understanding of the Auger effect is imperative and remains an active field of research. Previous works have primarily focused on nanocrystals in the strong confinement regime, while the weak confinement regime in intermediate-sized systems is less explored due to several long-standing challenges. In a recent report

Semiconductor nanocrystals, also known as quantum dots, have attracted significant attention in recent decades. Apart from the high degree of control over their composition and shape, the size of nanocrystals can be manipulated at the atomic level, giving rise to the quantum confinement of electrons and holes. This effect results in discrete energy levels and size-tunable bandgap, allowing for the absorption and emission of light across a wide range of wavelengths. Nanocrystals are thus highly attractive for applications in optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic solar cells.

Upon light absorption in a semiconductor, an electron and a hole in a bound state, known as an exciton, are generated in nanocrystals. If a third charge (electron or hole) is present, excitonic radiative recombination can be bypassed through Auger recombination, a nonradiative coulomb interaction by which an exciton recombines and yields its energy to a spectator charge. Consequently, the behavior of excitons is strongly influenced by the Auger effect, particularly in nanocrystals, due to the spatial confinement of charge carriers. For optoelectronic applications, the Auger effect is mostly detrimental because it causes nonradiative recombination processes that compete with luminescent processes, reducing the quantum yield of photoluminescence or generating intermittent fluctuations (blinking) in emission intensity. The Auger effect also impacts the charge carrier dynamics, leading to carrier relaxation, trapping, recombination, and energy transfer that compete with the desired charge extraction process. On the other hand, there are cases where the Auger effect can be favorable. A prominent example is in single-photon sources for quantum information, where the Auger effect helps suppress multi-photon emission processes by efficiently transferring the excess energy to another carrier so that photons are always emitted one by one in smaller nanoparticles.

Therefore, understanding and controlling the Auger effect in nanocrystals is pivotal for the performance of nanocrystal-based devices. One topic of debate in the literature is the size-dependence of the Auger decay time: most often, a “universal” volume scaling law is found, but several authors reported deviations from this law

In the article by Huang and co-authors

Fig 1  Illustration for the appearance of nonlocal effects.

As the nanocrystal size increases above the exciton wavefunction period λ_X, the biexciton Auger decay becomes much slower than predicted by the volume scaling law because the phase difference between the initial and final states of the spectator hole leads to an inhibition of the Auger mechanism

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Due to the suppressed Auger effect, exceptionally high biexciton efficiency reaching 80% is demonstrated in the present work. While this result may not directly promote the development of entangled photon sources, it is indeed significant for the fundamental understanding of the Auger effect. High-quality perovskite nanocrystals thus appear as a promising platform for the analysis and tuning of Auger energy transfers. Whereas the present paper compares large nanocrystals with literature data on smaller particles, future improved control on perovskite nanocrystal synthesis should provide direct comparisons between the strongly, weakly and non-confined regimes. Then a more general model will be needed to account for both $$R/a_B$$ and $$R/{\lambda }_X$$ ratios. The nanoparticle geometry provides further degrees of freedom to achieve specific confinement regimes and test theoretical predictions, as exemplified recently in CdSe nanoplatelets