Determining Structural Response of CdSe:CdS Nanocrystals to High Energy Excitation

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, 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.<br/><br/>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. Transient differential atomic pair distribution function analysis corroborated that the localized disorder was likely associated with hot hole trapping on the NC surface.<br/><br/>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 in order to resolve the observed structural disorder to specific lattice planes. Both liquid jet and solid-state experimental setups were used, and current work involves analyzing this TR-XRD data in order to both determine the relationship between localized disorder and non-radiative cooling and compare the efficacy of these two methods for resolving the structural response of NCs to photoexcitation.<br/><br/>Molecular dynamics (MD) simulations and X-ray pair distribution function (PDF) experiments were performed to examine the response of CdSe:CdS NCs to heating. Combining these three methods will help to decouple the purely thermal response in the MD simulations and X-ray PDF experiments from the additional photoinduced structural disorder observed in the TR-XRD data. Results of this work will allow for a more thorough understanding of the localized disorder caused by hot hole trapping processes and improve NC engineering methods to create more efficient optoelectronic devices.