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The excited state species and properties in lowdimensional 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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Polaron is a composite quasiparticle derived from an excess carrier trapped by local lattice distortion, and it has been studied extensively for decades both theoretically and experimentally. However, atomic-scale creation and manipulation of single-polarons in real space have still not been achieved so far, which precludes the atomistic understanding of the properties of polarons as well as their applications. Herein, using scanning tunneling microscopy, we succeeded to create single polarons in a monolayer two-dimensional (2D) semiconductor, CoCl2. Combined with first-principles calculations, two stable polaron configurations, centered at atop and hollow sites, respectively, have been revealed. Remarkably, a series of manipulation progresses — from creation, erasure, to transition — can be accurately implemented on individual polarons. Our results pave the way to understand the physics of polaron at atomic level, and the easy control of single polarons in 2D semiconductor may open the door to 2D polaronics including the data storage.

Photogenerated charge carrier dynamics at the WSe2/hBN van der Waals interface play an important role in optical device applications. The carrier behavior has been argued to be related to the interlayer phonon–phonon interaction in the heterostructure. However, the effect of the interlayer coupling on the electron–hole recombination dynamics is still unclear. Using the ab initio nonadiabatic molecular dynamics approach, we investigate the photoexcited electron dynamics at the interface, which has a type I energy alignment. The out-of-plane phonon of hBN is found to strongly couple with the WSe2 out-of-plane A′1 phonon, enhancing the electron–phonon interaction and accelerating the electron–hole recombination compared to pristine WSe2. Our work provides valuable guidance on the design of novel two-dimensional optoelectronic and opto-phononic devices.