Etude de la réduction du phénomène de clignotement dans les nanocristaux semi-conducteurs de CdSe/CdS à coque épaisse

Semiconductor colloidal nanocrystals exhibit a blinking phenomenon, which appears as an obstacle for many applications. In this work we were interested in the suppression of the blinking phenomenon in nanocrystals composed of a CdSe core and a thick CdS shell. We developed an easy, rapid and robust protocol for the synthesis of thick-shell CdSe/CdS non-blinking semiconductor nanocrystals. The study of the temperature dependence of the optical properties of these nanocrystals at the single particle level led us to determine the origin of the blinking suppression in these structures. We evidenced the thermal activation of the non-radiative Auger recombination, which is responsible for blinking in these nanocrystals. This thermal activation is linked to the temperature dependence of the electron localisation. At cryogenic temperature, these nanocrystals are negatively charged and exhibit excellent optical properties : they have a radiative lifetime shorter than 10 ns and a 100 % quantum yield. These remarkable features allowed the direct study of the nanocrystals magneto-optic properties. These results open the way to the design of nanocrystals with a 100 % quantum yield at room temperature.Les nanocristaux semi-conducteurs colloïdaux présentent un phénomène de clignotement qui s'avère être un obstacle pour de nombreuses applications. Dans cette thèse, nous nous sommes intéressés à la suppression du phénomène de clignotement dans les nanocristaux constitués d'un coeur de CdSe et d'une coque épaisse de CdS. Nous avons mis au point un protocole facile à mettre en oeuvre, rapide et robuste, permettant de synthétiser des nanocristaux semi-conducteurs de CdSe/CdS à coque épaisse non-clignotants. L'étude de la dépendance en température des propriétés optiques de ces nanocristaux mesurés à l'échelle individuelle nous a permis de déterminer l'origine de la réduction du clignotement. Nous avons mis en évidence l'activation thermique de la recombinaison Auger non-radiative, cette dernière étant responsable du clignotement dans ces nanocristaux. Cette activation thermique est liée à la dépendence en température de la localisation de l'électron. A basse température, les nanocristaux sont chargés négativement et présentent d'excellentes propriétés optiques : un temps de vie radiatif inférieur à 10 ns et un rendement quantique de 100 %. Ces caractéristiques remarquables ont permis l'étude directe des propriétés magnéto-optiques des nanocristaux. Ces résultats ouvrent la voie à la conception de nanocristaux ayant un rendement quantique de 100 % à température ambiante

Etude de la réduction du phénomène de clignotement dans les nanocristaux semi-conducteurs de CdSe/CdS à coque épaisse

individuelle nous a permis de déterminer l'origine de la réduction du clignotement. Nous avons mis en évidence l'activation thermique de la recombinaison Auger non-radiative, cette dernière étant responsable du clignotement dans ces nanocristaux. Cette activation Les nanocristaux semi-conducteurs colloïdaux présentent un phénomène de clignotement qui s'avère être un obstacle pour de nombreuses applications. Dans cette thèse, nous nous sommes intéressés à la suppression du phénomène de clignotement dans les nanocristaux constitués d'un coeur de CdSe et d'une coque épaisse de CdS. Nous avons mis au point un protocole facile à mettre en oeuvre, rapide et robuste, permettant de synthétiser des nanocristaux semi-conducteurs de CdSe/CdS à coque épaisse non-clignotants. L'étude de la dépendance en température des propriétés optiques de ces nanocristaux mesurés à l'échelle thermique est liée à la dépendence en température de la localisation de l'électron. A basse température, les nanocristaux sont chargés négativement et présentent d'excellentes propriétés optiques : un temps de vie radiatif inférieur à 10 ns et un rendement quantique de 100 %. Ces caractéristiques remarquables ont permis l'étude directe des propriétés magnéto-optiques des nanocristaux. Ces résultats ouvrent la voie à la conception de nanocristaux ayant un rendement quantique de 100 % à température ambianteSemiconductor colloidal nanocrystals exhibit a blinking phenomenon, which appears as an obstacle for many applications. In this work we were interested in the suppression of the blinking phenomenon in nanocrystals composed of a CdSe core and a thick CdS shell. We developed an easy, rapid and robust protocol for the synthesis of thick-shell CdSe/CdS non-blinking semiconductor nanocrystals. The study of the temperature dependence of the optical properties of these nanocrystals at the single particle level led us to determine the origin of the blinking suppression in these structures. We evidenced the thermal activation of the non-radiative Auger recombination, which is responsible for blinking in these nanocrystals. This thermal activation is linked to the temperature dependence of the electron localisation. At cryogenic temperature, these nanocrystals are negatively charged and exhibit excellent optical properties : they have a radiative lifetime shorter than 10 ns and a 100 % quantum yield. These remarkable features allowed the direct study of the nanocrystals magneto-optic properties. These results open the way to the design of nanocrystals with a 100 % quantum yield at room temperaturePARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Influence of the cluster's size of random gold nanostructures on the fluorescence of single CdSe–CdS nanocrystals

International audienceIt is well known that coupling a single emitter to metallic structures modifies drastically its fluorescence properties compared to single emitter in vacuum. Depending on various parameters such as the nature of the metal or the geometry of the metallic structure, quenching or intensity enhancement as well as radiative processes acceleration are obtained through the creation of new desexcitation channels. The use of metallic random structures gives the opportunity to magnify the effect of the coupling by strongly confined electromagnetic fields. A gold film at the perco-lation threshold is an interesting illustration of that effect. Here, we study the influence of the method used to realize these films through two different examples. First, we show that the mean size of the gold clusters constituting the film depends on the deposition method. Even if similar optical properties (in particular far-field absorption) are exhibited by the structures, crucial differences appear in the fluores-cence of single emitters when coupled to the two kinds of D. Canneson · S. Buil · X. Quélin () · J.-P. Hermier Groupe d' random gold film. Especially, we focus our attention on the creation of desexcitation channels and show that they are cluster size dependent

Enhancing the fluorescence of individual thick shell CdSe/CdS nanocrystals by coupling to gold structures

International audienceThe fluorescence properties of individual CdSe/CdS nanocrystals(NCs) with a very thick shell and deposited on metallic structures are analyzed indetail. The results obtained for two metallic structures consisting of a continuousor a semi-continuous gold film are compared. Under low pulsed excitation,a strong acceleration of radiative processes is observed. The probabilityof electron–hole pair recombinations through Auger processes dramaticallydecreases, resulting in a suppression of blinking and the appearance ofbiexcitonic cascades. An original method of photons postselection also enablesus to determine the decay rate corresponding to biexcitonic recombinations.Finally, a detailed analysis of the excitation process and the photon collectionefficiency enables us to discriminate the effect of the gold structure in termsof excitation and fluorescence acceleration. It is found that high collectionpercentages can be achieved through the modification of NC emission withplasmonic structures

Cathodoluminescence in a Scanning Transmission Electron Microscope: A Nanometer-Scale Counterpart of Photoluminescence for the Study of II–VI Quantum Dots

International audienceWe report on nanometer-scale cathodoluminescence (nanoCL) experiments in a scanning transmission electron microscope on individual core–shell CdSe/CdS quantum dots (QDs). By performing combined photoluminescence (PL) and nanoCL experiments of the same individual QDs, we first show that both spectroscopies can be used equally well to probe the spectral properties of QDs. We then demonstrate that the spatial resolution of the nanoCL is only limited by the size of the QDs themselves by performing nanoCL experiments on QDs lying side by side. Finally, we show how nanoCL can be advantageous with respect to PL as it can rapidly and efficiently characterize the optical properties of a large set of individual QDs. These results contrast with pioneering CL works on II–VI QDs and pave the way to the characterization of any II–VI quantum-confined structure at the relevant scale

Measurement of Three-Dimensional Dipole Orientation of a Single Fluorescent Nanoemitter by Emission Polarization Analysis

International audienceWe demonstrate theoretically and experimentally that the three-dimensional orientation of a single fluorescent nanoemitter can be determined by polarization analysis of the emitted light (while excitation polarization analysis provides only the in-plane orientation). The determination of the emitter orientation by polarimetry requires a theoretical description, including the objective numerical aperture, the 1D or 2D nature of the emitting dipole, and the environment close to the dipole. We develop a model covering most experimentally relevant microscopy configurations and provide analytical relations that are useful for orientation measurements. We perform polarimetric measurements on high-quality core-shell CdSe/CdS nanocrystals and demonstrate that they can be approximated by two orthogonal degenerated dipoles. Finally, we show that the orientation of a dipole can be inferred by polarimetric measurement, even for a dipole in the vicinity of a gold film, while in this case, the well-established defocused microscopy is not appropriate