Colloidal semiconductor nanocrystals (NCs) are used in optoelectronic applications due to their tunable size, shape, composition, and surface chemistry. NCs reach near-unity radiative efficiencies when excited under weak conditions. However, less is known about nonradiative events under high excitation conditions. In applications such as lasers, multiexciton-harvesting solar cells, photodetectors, and electrically pumped LEDs, NCs are exposed to much higher excitation conditions, causing a rapid increase in nonradiative relaxations. Elucidating the structural origins of thermalization pathways is essential for engineering NCs that can suppress nonradiative processes to improve device efficiency. Our past work examined the effect of different energy excitation levels on localized lattice disordering in CdSe:CdS core/shell particles using femtosecond electron diffraction. At lower excitation energy (510 nm), the lattice response was dominated by the Debye-Waller effect. However, at higher excitation energy (340 nm), which is significantly higher than the band gap, a deviation from the expected Debye-Waller response was observed, suggesting additional short-range localized disordering. These deviations are likely due to formation of small surface polarons induced by hot hole trapping. Transient differential atomic pair distribution function analysis corroborated that the largest impact of the disorder was associated with small surface polarons. This work implies that the observed localized structural disorder may be associated with certain lattice planes. However, lower Q-resolution and lack of long delay time capabilities at the facilities used in earlier experiments prevented clear distinguishing of peaks, for determination of the structural geometry of the small polarons, and to measure their recovery dynamics. Additional time-resolved X-ray diffraction (TR-XRD) experiments were conducted on CdSe:CdS core-shell particles. Current work involves analyzing these TR-XRD data in order to further elucidate the specific effects of non-radiative cooling on structural lattice deformations and recovery dynamics. Results of this analysis will allow us to more thoroughly understand the localized disorder caused by hot hole trapping processes and improve NC engineering methods to create more efficient optoelectronic devices.

Elucidating structural origins of nonradiative events in CdSe:CdS nanocrystals

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