Research On The Exciton Diffusion Kinetics In A Single Lead Iodide(PbI₂) Nanoflake

Lead iodide(PbI₂)material is a typical semiconductor material used for high-performance detection at room temperature,and has been extensively and deeply explored in the fields of high-efficiency gamma-ray detection and X-ray imaging.Recently,as the emerging of the two-dimensional material family,PbI₂ has also become one member of them.PbI₂ nanoflakes not only extend the application of PbI₂ at room temperature detection to the ultraviolet field,but also greatly improve the sensitivity of X-ray detection.It even opens up a new platform for flexible photodetection.Due to its special energy band structure,PbI₂ nanoflakes have made up for the lack of 2D materials in light-emitting applications and further improved the flexibility of 2D material heterojunctions.In addition,PbI₂ nanoflakes are also critical precursors for the fabrication of quasi-2D perovskite solar cells,which have a great impact on the stability,coverage,and reproducibility of perovskites.In order to further deepen the application of PbI₂ nanoflakes in various fields,it is imperative to understand the excitonic properties and photophysical mechanisms of PbI₂ nanoflakes.In this work,a polarization-tunable femtosecond pump-probe system was used to investigate the transient optical response in a single PbI₂ nanoflake which fabricated by supersaturated solution method.From these transient spectra,polarization anisotropy of a single PbI₂ nanoflake at room temperature were also extracted and summarized.First,the morphology of PbI₂ nanoflakes was characterized by optical microscope and scanning electron microscopy(SEM),and their thickness was confirmed by atomic force microscope(AFM).PbI₂ nanoflakes with different thicknesses were selected,and detected by photoluminescence emission and Raman spectra,which obtained steady-state optical properties of samples.Secondly,we obtained the transient optical response and carrier dynamics process of a single PbI₂ nanoflake under different excitation powers in both non-resonant and resonant excitation conditions.After data fitting,the exciton annihilation radius and diffusion coefficient of a single PbI₂ nanoflake were extracted under non-resonant and resonant excitation,respectively.Based on these kinetic data,the carrier recombination mechanism of PbI₂ nanoflakes is summarized.We believed that PbI₂ nanoflakes were dominated by exciton-exciton annihilation mechanism at low excitation power.Due to the low exciton binding energy of PbI₂ nanoflakes,as the increase of excitation power,hot-excitons in PbI₂ nanoflakes were transformed into free carriers,and the recombination mechanism was gradually transformed into free carrier recombination.Therefore,the band-edge exciton dynamics of PbI₂ nanoflakes showed a trend of slowing down with the increase of excitation power.Finally,we compared the differential responses of a single PbI₂ nanoflake to different polarization conditions at room temperature.The results showed that the resonant excitation could greatly enhanced the polarization anisotropy of PbI₂nanoflakes and prolonged its polarization lifetime under both linear and circular polarized excitation.