DOI: https://doi.org/10.29363/nanoge.emlem.2022.049
Publication date: 15th July 2022
Colloidal nanoplatelets (NPLs) offer the unique opportunity to investigate at room temperature excitonic light-matter interactions in two-dimensional (2D) materials. Recently, we showed that center-of-mass localization accounts for the experimental exciton radiative lifetime at room temperature, which is constant for NPLs of different area S and far slower than expected based on the giant oscillator strength fX,tot of exciton absorption.[1] On the other hand, the biexciton oscillator strength fBX in 2D materials is uniquely linked to the biexciton extension SBX, which is the area covered by the average interdistance aBX of the interacting excitons. Since aBX is an internal coordinate, fBX should thus be intrinsically independent from S and not be diminished by center-of-mass related localizations. Here, we assess this predicted scaling on a set of 4.5 ML CdSe NPLs of increasing S using circularly-polarized transient absorption (TA) spectroscopy. We confirm that the direct exciton-to-biexciton transition yields a photo-induced absorption band in the TA spectrum. Moreover, using quantitative TA spectroscopy, we demonstrate that fBX is indeed area-independent and is approximately smaller than fX,tot by a factor aBX2/S. Interestingly, we find that SBX is comparable to the room-temperature exciton coherence area, suggesting that the radiative recombination rates of the biexciton and the localized exciton are similar. Since fBX governs absorption, emission and stimulated emission through the exciton/biexciton transition, these results underscore the relevance of this transition for the opto-electronic characteristics of 2D NPLs. Moreover, the approach to quantify the exciton/biexciton transition introduced here provides a means to rank such transitions in different 2D materials and to verify biexciton descriptors predicted from theory.
This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 714876 PHOCONA).
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