Room-temperature exciton coherence and dephasing in two-dimensional nanostructures

Nature Communications. Volume 6, January 2015, Article number 6086.Electronic coherence has attracted considerable attention for its possible role in dynamical processes in molecular systems. However, its detection is challenged by inhomogeneous line broadening and interference with vibrational coherences. In particular, reports of 'persistent' coherent exciton superpositions at room temperature remain controversial, as the related transitions give typically shorter optical dephasing times of about 10-20 fs. To rationalize these reported long-lived coherences, several models have been proposed, involving strong correlation in the mechanisms of decoherence or that electronic coherences may be sustained by resonant vibrational modes. Here we report a decisive example of electronic coherence occurring in a chemical system in a 'warm and wet' (room-temperature solution) environment, colloidal semiconductor nanoplatelets, where details are not obscured by vibrational coherences nor ensemble dephasing. Comparing the exciton and optical coherence times evidences a partial correlation of fluctuations underlying dephasing and allows us to elucidate decoherence mechanisms occurring in these samples

Nanocristaux de semi-conducteurs II-VI et I-III-VI : Contrôle des propriétés optiques de structures coeur/coque

Colloidal semiconductor nanocrystals present unique optical properties which depend on their size, shape and composition. It is therefore crucial to be able to precisely control these parameters during their synthesis. Here we present the synthesis and the optical properties of two kinds of core/shell heterostructures composed of II-VI and I-III-VI semiconductors. First we show that we can grow a flat CdS shell around spherical CdSe cores to obtain dot-in-plate nanocrystals. The lattice mismatch between the two materials, together with the anisotropic shape, induces a large splitting in the fine structure of the first exciton peak. Consequently, the emission narrows and becomes 2D-polarized, in the plane of the plate. This latter property is amplified by the dielectric effect due to the anisotropic shape of the nanocrystal and is verified by polarization measurements in ensemble and at the single nanocrystal level. Moreover, the dot-in-plate nanocrystals deposit spontaneously flat on substrates which provide a fine control of the emission dipole orientation. We then present the optical properties of CuInS2 and CuInSe2 nanocrystals. Their small bandgaps allow reaching near-infrared excitation and emission wavelengths. In addition, they do not contain any toxic heavy metal ions like Cd or Pb. The growth of a small ZnS shell around these nanocrystals greatly improves their emission properties and photostability after water solubilization. We further demonstrate their use for small animal in vivo imaging.Dans cette thèse, nous nous intéressons aux propriétés de fluorescence de nanocristaux colloïdaux de semi-conducteurs composés d'un cœur entouré d'une coque. Ces hétéro-structures possèdent des propriétés optiques qui dépendent de leur taille, de leur forme et de leur composition. Il est donc essentiel de pouvoir les synthétiser en contrôlant précisément ces paramètres. Dans ce manuscrit nous présentons la synthèse et les caractérisations structurales et optiques de nanocristaux de semi-conducteurs II-VI et I-III-VI. Tout d'abord, nous montrons qu'il est possible de faire croître une coque plate de CdS sur des cœurs sphériques de CdSe. La différence de paramètre de maille entre les deux matériaux et la géométrie de la coque induisent des effets de pression anisotrope qui modifient la structure fine du premier exciton. Par conséquent, l'émission de ces nanocristaux s'affine et devient polarisée à 2D dans le plan de la coque plate. Cette dernière propriété, amplifiée par des effets diélectriques liés à la forme du nanocristal, est vérifiée par différentes mesures de polarisation. De plus, ces nanocristaux s'orientent spontanément sur substrat, ce qui permet un contrôle fin de l'orientation du dipôle d'émission. Ensuite, nous présentons la synthèse et les propriétés de nanocristaux émettant dans le proche infrarouge et composés d'éléments moins toxiques que ceux usuellement utilisés dans cette gamme : CuInS2 et CuInSe2. Ces nanocristaux à composition ternaire sont recouverts d'une fine coque de ZnS pour préserver leur fluorescence après solubilisation en milieu aqueux. Nous montrons que ces sondes fluorescentes peuvent être utilisées pour l'imagerie in vivo du petit animal

Relaxation dynamics in colloidal hybrid perovskite nanostructures: effect of confinement

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Dynamiques ultrarapides des excitons dans les nanostructures colloïdales de semiconducteurs

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Ultrafast exciton dynamics in colloidal 2D perovksite nanoplatelets

International audienceUsing femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics instrongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads todiscrete excitonic transitions and strong carrier-induced Stark features in the TA spectra. This prevents the use ofconventional relaxation analysis methods [1] extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral lineshape near the band-edge reflects the state of the system,which evolution in time can be used to probe the exciton cooling dynamics (Figure 1). The ultrafast hot excitonrelaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. Indeed, themeasured exciton cooling time suprisingly decreases for thinner NPLs. However, excitation fluence-dependentmeasurements reveal a hot phonon bottleneck effect, with a delayed relaxation when increasing the exciton density.This hot phonon bottleneck effect is found to be independent of the nature of the internal cations (organic versusinorganic), in contrast with 3D perovskites [2]. However, it is strongly affected by the ligands and/or sample surfacestate. Together, these results suggest a role of the surface ligands in the cooling process in these ultrathin colloidal 2Dnanostructures [3].References[1] Mingjie Li, Jianhui Fu, Qiang Xu and Tze Chien Sum, Slow Hot-Carrier Cooling in Halide Perovskites: Prospectsfor Hot-Carrier Solar Cells, Adv. Mater. 31, 47 (2019)[2] Thomas R. Hopper, Andrei Gorodetsky, Jarvist M. Frost, Christian Muller, Robert Lovrincic and Artem A.Bakulin, Ultrafast Intrband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites, ACS Energy Lett. 3, 9(2018)[3] Carolina Villamil Franco, Gaelle Trippée-Allard, Benoît Mahler, Christian Cornaggia, Jean-Sebastien Lauret,Thomas Gustavsson and Elsa Cassette, Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-InducedStark and Phonon Bottleneck Effects, J. Phys. Chem. Lett. 13, 1 (2022

Ultrafast exciton dynamics in colloidal perovskite nanoplatelets studied by femtosecond transient absorption

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

International audienceUsing 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]

Probing the ultrafast dynamics of excitons and charge carriers in perovskite nanostructures using transient absorption spectroscopy

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Multiexciton dynamics in colloidal 2D perovskite nanoplatelets: Auger recombination and multiple exciton generation in the strong confinement regime

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Nanocristaux de semiconducteurs II-VI et I-III-VI (contrôle des propriétés optiques de structures cœur/coque)

Dans cette thèse, nous nous intéressons aux propriétés de fluorescence de nanocristaux colloïdaux de semi-conducteurs composés d un cœur entouré d une coque. Ces hétéro-structures possèdent des propriétés optiques qui dépendent de leur taille, de leur forme et de leur composition. Il est donc essentiel de pouvoir les synthétiser en contrôlant précisément ces paramètres. Dans ce manuscrit nous présentons la synthèse et les caractérisations structurales et optiques de nanocristaux de semi-conducteurs II-VI et I-III-VI. Tout d abord, nous montrons qu il est possible de faire croître une coque plate de CdS sur des cœurs sphériques de CdSe. La différence de paramètre de maille entre les deux matériaux et la géométrie de la coque induisent des effets de pression anisotrope qui modifient la structure fine du premier exciton. Par conséquent, l émission de ces nanocristaux s affine et devient polarisée à 2D dans le plan de la coque plate. Cette dernière propriété, amplifiée par des effets diélectriques liés à la forme du nanocristal, est vérifiée par différentes mesures de polarisation. De plus, ces nanocristaux s orientent spontanément sur substrat, ce qui permet un contrôle fin de l orientation du dipôle d émission. Ensuite, nous présentons la synthèse et les propriétés de nanocristaux émettant dans le proche infrarouge et composés d éléments moins toxiques que ceux usuellement utilisés dans cette gamme : CuInS2 et CuInSe2. Ces nanocristaux à composition ternaire sont recouverts d une fine coque de ZnS pour préserver leur fluorescence après solubilisation en milieu aqueux. Nous montrons que ces sondes fluorescentes peuvent être utilisées pour l imagerie in vivo du petit animal.Colloidal semiconductor nanocrystals present unique optical properties which depend on their size, shape and composition. It is therefore crucial to be able to precisely control these parameters during their synthesis. Here we present the synthesis and the optical properties of two kinds of core/shell heterostructures composed of II-VI and I-III-VI semiconductors. First we show that we can grow a flat CdS shell around spherical CdSe cores to obtain dot-in-plate nanocrystals. The lattice mismatch between the two materials, together with the anisotropic shape, induces a large splitting in the fine structure of the first exciton peak. Consequently, the emission narrows and becomes 2D-polarized, in the plane of the plate. This latter property is amplified by the dielectric effect due to the anisotropic shape of the nanocrystal and is verified by polarization measurements in ensemble and at the single nanocrystal level. Moreover, the dot-in-plate nanocrystals deposit spontaneously flat on substrates which provide a fine control of the emission dipole orientation. We then present the optical properties of CuInS2 and CuInSe2 nanocrystals. Their small bandgaps allow reaching near-infrared excitation and emission wavelengths. In addition, they do not contain any toxic heavy metal ions like Cd or Pb. The growth of a small ZnS shell around these nanocrystals greatly improves their emission properties and photostability after water solubilization. We further demonstrate their use for small animal in vivo imagingPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF