73 research outputs found
Couplage de nanocristaux fluorescents Ă des structures photoniques et plasmoniques
Le manuscrit dĂ©crit le couplage entre un Ă©metteur fluorescent (nanocristaux colloĂŻdaux) et son environnement photonique ou plasmonique. DiffĂ©rents types d'environnements sont considĂ©rĂ©s : cristal photonique 2D (rĂ©seau de trous) ou 3D (opale), surface mĂ©tallique plane ou pĂ©riodique, nano-antenne mĂ©tallique, nano-cavitĂ© optique. DiffĂ©rents effets sont attendus du couplage dâun Ă©metteur fluorescent Ă son environnement, qui peuvent permettre dâamĂ©liorer ses qualitĂ©s de fluorescence par exemple en tant que source de photons uniques : exaltation, redirection, polarisation⊠Le contrĂŽle de l'accord (spatial, spectral, orientation) entre l'Ă©metteur et le mode optique est un aspect crucial du couplage
Light management in highly-textured perovskite solar cells: From full-device ellipsometry characterization to optical modelling for quantum efficiency optimization
While perovskite solar cells (PSCs) are now reaching high power conversion
efficiencies (PCEs), further performance improvement requires a fine management
and an optimization of the light pathway and harvesting in the cells. These go
through an accurate understanding, characterization and modelling of the
optical processes occurring in these complex, often textured, multi-layered
systems. In the present work, we have considered a typical methylammonium lead
iodide (MAPI) solar cell built on a fluorine-doped tin oxide (FTO) electrode of
high roughness (43 nm RMS). By variable-angle spectroscopic ellipsometry (VASE)
of the full PSC device, we have been able to determine the optical constants of
all the device layers. We have designed a one-dimensional (1D) optical model of
the stacked layers where the rough texture is described as layers of
effective-medium index. We have supported the model using data extracted from
scanning electron microscopy, diffuse spectroscopy and photovoltaic efficiency
measurements. We show that the 1D model, while insufficient to describe
scattering by the FTO plate alone, gives an accurate description of the full
device optical properties. By comparison with the experimental external quantum
efficiency (EQE), we estimate the internal quantum efficiency (IQE) and the
effect of the losses related to electron transfer. Based on this work, we
finally discuss the optical losses mechanisms and the possible strategies that
can be implemented to improve light management within PSC devices and further
increase their performances.Comment: 14 pages, 5 figure
SYNTHESIS AND COMPARATIVE PHOTOLUMINESCENCE OF CdZnSe / ZnS AND CdZnSe / ZnSeS ALLOY QUANTUM DOTS
In order to search for new structures and compositions of quantum dots, suppress the blinking photoluminescence (random changes between high emission state ( on) and low emission status ( off ) under continuous photo- excitation ) and serve the application purposes in bio- medical and in optoelectronic devices , we have studied the fabrication of new alloy quantum dots ( QDs ). In this paper, we present new results on alloy core / shell quantum dots, with changed alloy shell composition, that was CdZnSe/ZnSexS1-x a ML with x ( x = 0 , 0.2, 0.4, 0.5 , 0.6, 0.8) and the shell thickness in monolayer (ML) ( a = 2 , 4 , 6 ) . The emission spectra and the intensity change according to the composition of the alloy shell. The full width a half maximum ( FWHM ) of the emission spectra of quantum dots CdZnSe is 25.5 nm. Covered with a shell layer, the emission intensity of the CdZnSe core increases along with the shell thickness. For comparison purpose, two different shell materials have been used, which are ZnS and ZnSeS alloy. With the same shell thickness, the emission wavelengths and intensity of the QDs change when the shellâs composition changes.The photoluminescence (PL) decay and the PL blinking of the alloy QDs was studied. It was shown that the alloy QDs spent by the nanocrystal in the ON state ranged typically between 20 and 40 %, and was dependent on the core composition. Detailed discussions on the experiment results are presented
External quality assessment of SARS-CoV-2-sequencing: An ESGMD-SSM pilot trial across 15 European laboratories
Objective:Â This first pilot on external quality assessment (EQA) of SARS-CoV-2 whole genome sequencing, initiated by the ESCMID Study Group for Genomic and Molecular Diagnostics (ESGMD) and Swiss Society for Microbiology (SSM), aims to build a framework between laboratories in order to improve pathogen surveillance sequencing.Methods:Â Ten samples with varying viral loads were sent out to 15 clinical laboratories who had free choice of sequencing methods and bioinformatic analyses. The key aspects on which the individual centres were compared on were identification of 1) SNPs and indels, 2) Pango lineages, and 3) clusters between samples.Results:Â The participating laboratories used a wide array of methods and analysis pipelines. Most were able to generate whole genomes for all samples. Genomes were sequenced to varying depth (up to 100-fold difference across centres). There was a very good consensus regarding the majority of reporting criteria, but there were a few discrepancies in lineage and cluster assignment. Additionally, there were inconsistencies in variant calling. The main reasons for discrepancies were missing data, bioinformatic choices, and interpretation of data.Conclusions:Â The pilot EQA was an overall success. It was able to show the high quality of participating labs and provide valuable feedback in cases where problems occurred, thereby improving the sequencing setup of laboratories. A larger follow-up EQA should, however, improve on defining the variables and format of the report. Additionally, contamination and/or minority variants should be a further aspect of assessment.</p
Cohérence temporelle des photons uniques émis par un nanocristal individuel de CdSe/ZnS
This thesis describes an experiment of Michelson interferometry realized to measure the time coherence of the emission of a single core/shell CdSe/ZnS nanocrystal. The difficulty for such a measurement resides in the fast (0.1 ms) diffusion of their emission line which broadens the measured spectrum. We introduce, in a quantum framework describing the experiment, a method called photon-correlation Fourier spectroscopy (PCFS), based on the measurement of short-delays correlations between the intensities detected at the outputs of the interferometer. Contrary to standard methods, PCFS combines good time- and spectral-resolution (which is theoretically calculated). This allows us to study single nanocrystals at 300, 20 and 10 K, and to characterize both their linewidth and spectral diffusion. We report a 6,5 micro-eV linewidth (corresponding to a 200 ps coherence time), better by almost two orders of magnitude than the linewidths measured by spectroscopy of a single nanocrystal (0.1-1 meV)Cette thĂ©se dĂ©crit une expĂ©rience d'interfĂ©romĂ©trie de Michelson rĂ©alisĂ©e pour mesurer la cohĂ©rence temporelle de l'Ă©mission d'un nanocristal individuel cĆur/coquille de CdSe/ZnS. La difficultĂ© d'une telle mesure rĂ©side dans la diffusion rapide (0,1 ms) de leur raie d'Ă©mission qui Ă©largit le spectre mesurĂ© habituellement. Nous introduisons, dans une description de l'expĂ©rience par un formalisme quantique, une mĂ©thode baptisĂ©e spectroscopie de Fourier Ă corrĂ©lation de photons (PCFS), reposant sur la mesure des corrĂ©lations aux temps courts entre les intensitĂ©s dĂ©tectĂ©es aux sorties de l'interfĂ©romĂštre. Contrairement aux mĂ©thodes spectroscopiques standard, la PCFS associe une bonne rĂ©solution temporelle Ă une bonne rĂ©solution spectrale (qui est calculĂ©e thĂ©oriquement en dĂ©tail). Ceci nous permet d'Ă©tudier des nanocristaux individuels, Ă 300, 20 et 10 K, et de caractĂ©riser Ă la fois leur largeur spectrale et leur diffusion spectrale. Nous mesurons une largeur spectrale de 6,5 micro-eV (soit un temps de cohĂ©rence de 200 ps), meilleure de presque deux ordres de grandeur que les largeurs mesurĂ©es par spectroscopie d'un nanocristal individuel (0,1-1 meV)
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