Visualizing Polaron Formation in Lead Halide Perovskite Quantum Dots by Time-Resolved Serial Crystallography

Excitonic polaronic fields in metal halide perovskites (MHPs) are of intense research interest as a possible mechanism to explain their unique light harvesting and light emission properties. Quantum dots (QDs) of such materials exhibit additional size-dependent variations in their optoelectronic properties like the Stokes shift. We report the observation of signatures of polaronic deformation fields in CsPbBr3 QDs upon single resonant excitation by adapting the technique of time-resolved serial femtosecond crystallography at X-ray free electron lasers (XFELs). This method allows one to observe changes to the three dimensional shape of diffracted Bragg peaks from these quantum dots upon optical excitation which are, in turn, exquisitely sensitive to inhomogeneous lattice deformations in the particle.<br/><br/>Due to this sensitivity we were able to observe statistically significant changes in the average deformation field after resonant excitation with, on average, one photon per particle. Initial modelling shows evidence for optical-phonon-like deformations and indicates a larger extent of the electron polaron compared to the hole polaron, as well as an anisotropic spatial distribution of the deformation field due to confinement effects.