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The excited state species and properties in low-dimensional semiconductors can be completely redefined by electron-lattice coupling or a polaronic effect. Here, by combining ultrafast broadband pump–probe spectroscopy and first-principles GW and Bethe-Salpeter equation calculations, we show semiconducting CrI 3 as a prototypical 2D polaronic system with characteristic Jahn-Teller exciton polaron induced by symmetry breaking. A photogenerated electron and hole in CrI 3 localize spontaneously in ∼0.9 ps and pair geminately to a Jahn-Teller exciton polaron with elongated Cr–I octahedra, large binding energy, and an unprecedentedly small exciton-exciton annihilation rate constant (∼10 –20 cm 3 s –1 ). Coherent phonon dynamics indicates the localization is mainly triggered by the coherent nuclear vibration of the I–Cr–I out-of-plane stretch mode at 128.5 ± 0.1 cm –1 . The excited state Jahn-Teller exciton polaron in CrI 3 broadens the realm of 2D polaron systems and reveals the decisive role of coupled electron-lattice motion on excited state properties and exciton physics in 2D semiconductors.
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Li et al. (2022) studied this question.
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