Distance Dependence of the Energy Transfer Rate From a Single Semiconductor Nanostructure to Graphene

The near-field Coulomb interaction between a nano-emitter and a graphene monolayer results in strong F\"orster-type resonant energy transfer and subsequent fluorescence quenching. Here, we investigate the distance dependence of the energy transfer rate from individual, i) zero-dimensional CdSe/CdS nanocrystals and ii) two-dimensional CdSe/CdS/ZnS nanoplatelets to a graphene monolayer. For increasing distances $d$, the energy transfer rate from individual nanocrystals to graphene decays as $1/d^4$. In contrast, the distance dependence of the energy transfer rate from a two-dimensional nanoplatelet to graphene deviates from a simple power law, but is well described by a theoretical model, which considers a thermal distribution of free excitons in a two-dimensional quantum well. Our results show that accurate distance measurements can be performed at the single particle level using graphene-based molecular rulers and that energy transfer allows probing dimensionality effects at the nanoscale.Comment: Main text (+ 5 figures) and Supporting Information (+ 7 figures

Крещение Крыма

Цель статьи - определить место и значение фестивной культуры в условиях социальных трансформаций

Synthèse et propriétés optiques de nanocristaux colloïdaux du groupe II-VI : homo- et hétèro-structures

Ces travaux de thèse ont porté sur une nouvelle classe de semi-conducteurs colloïdaux sous forme de nanoplaquettes composées de chalcogénures de cadmium. Ces nanocristaux, comparables à des puits quantiques, présentent un confinement excitonique dans une seule direction, l’épaisseur, qui est contrôlée au niveau atomique. Les nanoplaquettes sont caractérisées par une excellente résolution spectrale et de bons rendements quantiques. Par conséquence, elles représentent de potentiels candidats pour développer des dispositifs optoélectroniques comme des diodes électroluminescentes ou bien des photo-détecteurs. Toutefois, dans ce but, il est nécessaire d’élargir la gamme de longueurs d’ondes d’absorption et d’émission et d’augmenter leur rendement quantique. Pour cela, nous avons étudié la synthèse colloïdale de nanoplaquettes à base d’homo- et d’hétèro-nanoplaquetts des groupes II-VI. Les nanocristaux fabriqués ont été caractérisé par spectroscopie UV-visible et de fluorescence, par diffraction à rayons X et par microscopie électronique. Dans un premier temps, nous avons optimisé la préparation de nanoplaquettes de CdTe en utilisant des procédés de synthèse colloïdale par injection de précurseurs à hautes températures. Ensuite, des structures plus complexes ont été investiguées. Par exemple, nous avons synthétisé nanoplaquettes cœur/couronne de CdSe/CdTe qui possèdent une structure électronique de type-II. Nous avons également étudié la croissance de couches d’un deuxième semi-conducteur dans la direction de l’épaisseur de plaquettes cœur pour la fabrication de structures type cœur/coque. Grâce au contrôle de la composition chimique du cœur et de la coque, l’alignement de bande a été modulé pour obtenir structures électroniques de type-I, quasi type-II et type-II.This thesis project is based on the development of a novel class of colloidal two-dimensional nanocrystals, i.e. nanoplatelets (NPLs), composed of cadmium chalcogenides. These nanocrystals, in analogy to quantum wells, are characterized by an exciton confinement along one direction, i.e. the thickness, which can be controlled at atomic level. Nanoplatelets possess unique optical features as an excellent spectral resolution and good quantum yields. As consequence these nanocrystals are potential candidates for the fabrication of optoelectronic devices such as electroluminescent diodes or photo-detectors. However, for this aim it is necessary to enlarge the range of the absorption and emission wavelengths and to increase their quantum yield. For this reason, we investigated the colloidal synthesis of II-VI homo- and hetero-nanoplatelets which have been characterized by UV-Vis and photoluminescence spectroscopy, by X-ray diffraction and by electronic microscopy. First, we optimized the synthesis of CdTe NPLs using colloidal synthesis based on precursors injection at high temperatures. Then, we focused on more complexes hetero-structures. For example, through lateral extension reactions we obtained CdSe/CdTe core/crown NPLs which possess a type-II electronic structure. Successively, we studied the synthesis of core/shell NPLs by the growth of a second semiconductor layer along the thickness of NPLs cores. Depending on the core and shell chemical composition we could engineer the band gap of the nanoplatelets between type-I, quasi type-II and type-II electronic structures

A remarkably simple α-oximation of aldehydes via organo-SOMO catalysis

A novel α-oximation reaction of unactivated aldehydes has been achieved in excellent yields by reaction with NaNO 2-FeCl 3 couple and in the presence of pyrrolidine as organocatalyst. © 2012 The Royal Society of Chemistry

Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets

We have synthesized atomically flat CdSe/CdTe core/crown nanoplatelets (NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control of the crown lateral dimensions. In these type-II NPLs, the charges separate spatially, and the electron wave function is localized in the CdSe core while the hole wave function is confined in the CdTe crown. The exciton’s recombination occurs across the heterointerface, and as a result of their spatially indirect band gap, an important emission red shift up to the near-infrared region (730 nm) is observed with long fluorescence lifetimes that range from 30 to 860 ns, depending on the type of interface between the core and the crown. These type-II NPLs have a high quantum yield of 50% that can be further improved to 70% with a gradient interface. We have characterized these novel CdSe/CdTe core/crown NPLs using UV–vis, emission, and excitation spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy

Etude pluridisciplinaire de chapelles funéraires thébaines de l'époque ramesside

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From dilute isovalent substitution to alloying in CdSeTe nanoplatelets

International audienceCadmium chalcogenide nanoplatelet (NPL) synthesis has recently witnessed a significant advance in the production of more elaborate structures such as core/shell and core/crown NPLs. However, controlled doping in these structures has proved difficult because of the restrictive synthetic conditions required for 2D anisotropic growth. Here, we explore the incorporation of tellurium (Te) within CdSe NPLs with Te concentrations ranging from doping to alloying. For Te concentrations higher than ∼30%, the CdSexTe(1−x) NPLs show emission properties characteristic of an alloyed material with a bowing of the band gap for increased concentrations of Te. This behavior is in line with observations in bulk samples and can be put in the context of the transition from a pure material to an alloy. In the dilute doping regime, CdSe:Te NPLs, in comparison to CdSe NPLs, show a distinct photoluminescence (PL) red shift and prolonged emission lifetimes (LTs) associated with Te hole traps which are much deeper than in bulk samples. Furthermore, single particle spectroscopy reveals dramatic modifications in PL properties. In particular, doped NPLs exhibit photon antibunching and emission dynamics significantly modified compared to undoped or alloyed NPLs

Etude pluridisciplinaire de chapelles funéraires thébaines de l'époque ramesside

International audienc

Etude pluridisciplinaire de chapelles funéraires thébaines de l'époque ramesside

International audienc