Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
DOI: https://doi.org/10.29363/nanoge.matsusspring.2025.288
Publication date: 16th December 2024
Semiconducting perovskites hold great promise for demanding applications involving light emission, such as lasers. In this respect, fully inorganic colloidal CsPbBr3 nanoplateltes (NPLs) could be of interest since somewhat similar colloidal CdSe NPLs showed exceptionally high material gain related to stimulated emission through the biexciton-exciton transition[1].
Here, we report on a spectroscopic study of CsPbBr3 2D NPLs, geared towards understanding the photo-excited states and evaluating the performance metrics related to optical gain. In line with literature reports, we show that photo-excitation of 2D CsPbBr3 NPLs leads to the formation of strongly bound excitons. Importantly, the pronounced difference between the oscillator strength of the ground-state-to-exciton transition in absorption and emission points towards considerable exciton localization, for which we estimate a coherence area of a mere 1-2 nm2. Furthermore, we show that this localization comes with the formation of an exciton-polaron, which shifts the exciton emission to the red of the exciton absorption.
Through pump-probe spectroscopy using co-polarized and cross-polarized pump and probe beams, we demonstrate that light absorption in the presence of excitons leads to bound biexcitons. These biexcitons have singlet character, and the biexciton emission exhibits an additional Stokes shift with respect to the biexciton absorption. Furthermore, the oscillator strength of this transition indicates that the biexciton area is more than 5 times smaller than the cross-sectional area of the exciton. Both elements suggest that like excitons, biexcitons couple with the CsPbBr3 lattice to form strongly localized biexciton polarons.
After fs optical pumping, CsPbBr3 exhibit a strongly redshifted, weak and short-lived band of net stimulated emission. Using the exciton and biexciton characteristics, we can account for this transient optical gain by assuming stimulated emission across the biexciton-exciton transition. We relate the limited gain performance – as compared to, for example, CdSe NPLs – to the pronounced localization of the biexciton or biexciton polaron.
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