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Communication Dans Un Congrès Année : 2021

Ultrafast relaxation of hot excitons in colloidal 2D perovskite nanoplatelets: carrier-induced Stark and phonon bottleneck effects

Elsa Cassette

Résumé

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra (Figure 1, left panel). This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the build-up of the band-edge bleaching. Instead, we show that the TA spectral lineshape near the band-edge reflects the state of the system and thus can be used to probe the exciton cooling dynamics by a global analysis method. While the energy level separations become larger than the energy of relevant optical phonon modes, the ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs evidences the absence of intrinsic phonon bottleneck (slow cooling rate expected due to inefficient multi-LO phonon emission). We found instead a faster intrinsic cooling time in the thinner NPLs, suggesting the presence of an additional relaxation pathway, which seems more efficient with increasing confinement. To get more insight on the hot exciton relaxation process, we performed excitation fluence-dependent measurements. We reveal a hot phonon bottleneck effect, with a delayed hot exciton relaxation up to about 1 ps at the maximum fluence. The evolution of the cooling time with the excitation fluence is found independent on the nature of the internal cations when comparing formamidinium (FA)-, methylammonium (MA)- and cesium- based lead iodide NPLs (Figure 1, middle panel). However, it is strongly affected by the ligands and/or sample surface state when comparing colloidal NPLs with 2D perovskite films (Figure 1, right panel). Together, these results suggest a role of the surface ligands in the cooling process [1].
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Dates et versions

hal-04399666 , version 1 (17-01-2024)

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  • HAL Id : hal-04399666 , version 1

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Elsa Cassette. Ultrafast relaxation of hot excitons in colloidal 2D perovskite nanoplatelets: carrier-induced Stark and phonon bottleneck effects. Réunion plénière du GDR Ultrafast Phenomena (GDR UP 2021), Nov 2021, Paris, France. ⟨hal-04399666⟩
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