80 research outputs found
Nano et supracristaux d'or: sur l'influence de la nanocristallinité
This thesis deals with the influence of crystal defects of gold nanocrystals, especially on the synthesis and self-assembly of gold nanocrystals whose crystal defects and size are controlled. During this work, a method have been developed to separate single and polycrystals with similar size using crystalline segregation. Then, these nanocrystals have been used as seeds for a second crystal growth in order to expand their size from 5 nm to 13 nm in diameter. The plasmonic and vibrational properties of these gold nanocrystals have also been studied. The obtained nanocrystals exhibit low size distribution that allows their self-assemblies into three dimensional ordered lattice, called supracrystals. Negative supracrystals and vicinal surfaces have been observed under specific self-assembly conditions. Moreover, it is possible to obtain supracrystals with submillimeter size containing only either single or polycrystals and study them in an individual way by X-ray diffraction. It has also been possible to correlate the translational and orientational order of gold nanocrystals within the supracrystalline lattice.Cette theÌse porte sur lâinfluence des deÌfauts cristallins des nanocristaux dâor. Elle traite de la syntheÌse et de lâassemblage de nanocristaux dâor dont la taille et le taux de deÌfauts cristallins sont controÌleÌs. Au cours de ce travail, une meÌthode permettant de seÌparer des nanocristaux de meÌme taille mais soit mono-, soit polycristallines a eÌteÌ mise au point pour ensuite les utiliser comme graines de croissance cristalline afin dâeÌtendre le controÌle de taille de 5 aÌ 13 nm. Les proprieÌteÌs plasmoniques ainsi que vibrationnelles de ces nanocristaux dâor ont pu ainsi eÌtre eÌtudieÌes en fonction de la preÌsence ou non de deÌfauts cristallins. Les nanocristaux syntheÌtiseÌs lors de cette theÌse preÌsentent une distribution de taille suffisamment faible pour permettre leur auto-assemblage en reÌseau ordonneÌ aÌ trois dimensions, appeleÌs supracristaux. Lâapparition de supracristaux inverses et de surfaces vicinales a pu eÌtre observeÌe dans certaines conditions dâassemblage. De plus, il est aussi possible dâobtenir des supracristaux de taille submillimeÌtrique ne contenant que des nanocristaux mono- ou polycristallines, et pouvant eÌtre eÌtudieÌs individuellement par diffraction de rayons X. Il a ainsi eÌteÌ possible de correÌler lâordre orientationnel et translationnel des nanocristaux dans le reÌseau supracristallin
Nano-contact microscopy of supracrystals
Background: Highly ordered three-dimensional colloidal crystals (supracrystals) comprised of 7.4 nm diameter Au nanocrystals (with a 5% size dispersion) have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy.
Results: By exploring the evolution of both the force and tunnel current with respect to tipâsample separation, we arrive at the surprising finding that single nanocrystal resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e., positive force) regime of the probeânanocrystal interaction potential. Constant height force microscopy has been used to map tipâsample interactions in this regime, revealing inhomogeneities which arise from the convolution of the tip structure with the ligand distribution at the nanocrystal surface.
Conclusion: Our combined STMâAFM measurements show that the contrast mechanism underpinning high resolution imaging of nanoparticle supracrystals involves a form of nanoscale contact imaging, rather than the through-vacuum tunnelling which underpins traditional tunnelling microscopy and spectroscopy
Road Map for Nanocrystal Based Infrared Photodetectors
Infrared (IR) sensors based on epitaxially grown semiconductors face two main challenges which are their prohibitive cost and the difficulty to rise the operating temperature. The quest for alternative technologies which will tackle these two difficulties requires the development of new IR active materials. Over the past decade, significant progresses have been achieved. In this perspective, we summarize the current state of the art relative to nanocrystal based IR sensing and stress the main materials, devices and industrial challenges which will have to be addressed over the 5 next years
Field effect transistor and photo transistor of narrow band gap nanocrystal arrays using ionic glasses
International audienceGating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolyte, thanks to a large capacitance, leads to significantly reduce operating bias but is limited to slow speed and quasi room temperature operation. None of these operating conditions are optimal for narrow band gap nanocrystal-based phototransistors for which the need of a large capacitance gate has to be combined with low temperature operation. Here we explore the use of a LaF3 ionic glass as a high capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 ”F·cm-2. We unveil the unique property of ionic glass gate to enable unprecedented tunability of both magnitude and dynamics of the photocurrent, thanks to high charge doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal to noise ratio can be improved by a factor 100 and the time response accelerated by a factor 6. Moreover, the good transparency of LaF3 substrate allows back side illumination in the infrared which is highly valuable for the design of phototransistor
Transport and Phototransport in ITO Nanocrystals with Short to Long-Wave Infrared Absorption
Nanocrystals are often described as an interesting strategy for the design of
low-cost optoelectronic devices especially in the infrared range. However the
driving materials reaching infrared absorption are generally heavy
metalcontaining (Pb and Hg) with a high toxicity. An alternative strategy to
achieve infrared transition is the use of doped semiconductors presenting
intraband or plasmonic transition in the short, mid and long-wave infrared.
This strategy may offer more flexibility regarding the range of possible
candidate materials. In particular, significant progresses have been achieved
for the synthesis of doped oxides and for the control of their doping
magnitude. Among them, tin doped indium oxide (ITO) is the one providing the
broadest spectral tunability. Here we test the potential of such ITO
nanoparticles for photoconduction in the infrared. We demonstrate that In2O3
nanoparticles presents an intraband absorption in the mid infrared range which
is transformed into a plasmonic feature as doping is introduced. We have
determined the cross section associated with the plasmonic transition to be in
the 1-3x10-13 cm2 range. We have observed that the nanocrystals can be made
conductive and photoconductive due to a ligand exchange using a short
carboxylic acid, leading to a dark conduction with n-type character. We bring
further evidence that the observed photoresponse in the infrared is the result
of a bolometric effect
Construire des pratiques participatives dans les bibliothĂšques
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Contrasted Patterns of Molecular Evolution in Dominant and Recessive Self-Incompatibility Haplotypes in Arabidopsis
Self-incompatibility has been considered by geneticists a model system for reproductive biology and balancing selection, but our understanding of the genetic basis and evolution of this molecular lock-and-key system has remained limited by the extreme level of sequence divergence among haplotypes, resulting in a lack of appropriate genomic sequences. In this study, we report and analyze the full sequence of eleven distinct haplotypes of the self-incompatibility locus (S-locus) in two closely related Arabidopsis species, obtained from individual BAC libraries. We use this extensive dataset to highlight sharply contrasted patterns of molecular evolution of each of the two genes controlling self-incompatibility themselves, as well as of the genomic region surrounding them. We find strong collinearity of the flanking regions among haplotypes on each side of the S-locus together with high levels of sequence similarity. In contrast, the S-locus region itself shows spectacularly deep gene genealogies, high variability in size and gene organization, as well as complete absence of sequence similarity in intergenic sequences and striking accumulation of transposable elements. Of particular interest, we demonstrate that dominant and recessive S-haplotypes experience sharply contrasted patterns of molecular evolution. Indeed, dominant haplotypes exhibit larger size and a much higher density of transposable elements, being matched only by that in the centromere. Overall, these properties highlight that the S-locus presents many striking similarities with other regions involved in the determination of mating-types, such as sex chromosomes in animals or in plants, or the mating-type locus in fungi and green algae
Gold nano and supracrystal : on the influence of nanocrystallinity
Cette thĂšse porte sur l'influence des dĂ©fauts cristallins des nanocristaux d'or. Elle traite de la synthĂšse et de l'assemblage de nanocristaux d'or dont la taille et le taux de dĂ©fauts cristallins sont contrĂŽlĂ©s. Au cours de ce travail, une mĂ©thode permettant de sĂ©parer des nanocristaux de mĂȘme taille mais soit mono-, soit polycristallines a Ă©tĂ© mise au point pour ensuite les utiliser comme graines de croissance cristalline afin d'Ă©tendre le contrĂŽle de taille de 5 Ă 13 nm. Les propriĂ©tĂ©s plasmoniques ainsi que vibrationnelles de ces nanocristaux d'or ont pu ainsi ĂȘtre Ă©tudiĂ©es en fonction de la prĂ©sence ou non de dĂ©fauts cristallins. Les nanocristaux synthĂ©tisĂ©s lors de cette thĂšse prĂ©sentent une distribution de taille suffisamment faible pour permettre leur auto-assemblage en rĂ©seau ordonnĂ© Ă trois dimensions, appelĂ©s supracristaux. L'apparition de supracristaux inverses et de surfaces vicinales a pu ĂȘtre observĂ©e dans certaines conditions d'assemblage. De plus, il est aussi possible d'obtenir des supracristaux de taille submillimĂ©trique ne contenant que des nanocristaux mono- ou polycristallines, et pouvant ĂȘtre Ă©tudiĂ©s individuellement par diffraction de rayons X. Il a ainsi Ă©tĂ© possible de corrĂ©ler l'ordre orientationnel et translationnel des nanocristaux dans le rĂ©seau supracristallin.This thesis deals with the influence of crystal defects of gold nanocrystals, especially on the synthesis and self-assembly of gold nanocrystals whose crystal defects and size are controlled. During this work, a method have been developed to separate single and polycrystals with similar size using crystalline segregation. Then, these nanocrystals have been used as seeds for a second crystal growth in order to expand their size from 5 nm to 13 nm in diameter. The plasmonic and vibrational properties of these gold nanocrystals have also been studied. The obtained nanocrystals exhibit low size distribution that allows their self-assemblies into three dimensional ordered lattice, called supracrystals. Negative supracrystals and vicinal surfaces have been observed under specific self-assembly conditions. Moreover, it is possible to obtain supracrystals with submillimeter size containing only either single or polycrystals and study them in an individual way by X-ray diffraction. It has also been possible to correlate the translational and orientational order of gold nanocrystals within the supracrystalline lattice
Nano et supracristaux d'or: sur l'influence de la nanocristallinité
This thesis deals with the influence of crystal defects of gold nanocrystals, especially on the synthesis and self-assembly of gold nanocrystals whose crystal defects and size are controlled. During this work, a method have been developed to separate single and polycrystals with similar size using crystalline segregation. Then, these nanocrystals have been used as seeds for a second crystal growth in order to expand their size from 5 nm to 13 nm in diameter. The plasmonic and vibrational properties of these gold nanocrystals have also been studied. The obtained nanocrystals exhibit low size distribution that allows their self-assemblies into three dimensional ordered lattice, called supracrystals. Negative supracrystals and vicinal surfaces have been observed under specific self-assembly conditions. Moreover, it is possible to obtain supracrystals with submillimeter size containing only either single or polycrystals and study them in an individual way by X-ray diffraction. It has also been possible to correlate the translational and orientational order of gold nanocrystals within the supracrystalline lattice.Cette theÌse porte sur lâinfluence des deÌfauts cristallins des nanocristaux dâor. Elle traite de la syntheÌse et de lâassemblage de nanocristaux dâor dont la taille et le taux de deÌfauts cristallins sont controÌleÌs. Au cours de ce travail, une meÌthode permettant de seÌparer des nanocristaux de meÌme taille mais soit mono-, soit polycristallines a eÌteÌ mise au point pour ensuite les utiliser comme graines de croissance cristalline afin dâeÌtendre le controÌle de taille de 5 aÌ 13 nm. Les proprieÌteÌs plasmoniques ainsi que vibrationnelles de ces nanocristaux dâor ont pu ainsi eÌtre eÌtudieÌes en fonction de la preÌsence ou non de deÌfauts cristallins. Les nanocristaux syntheÌtiseÌs lors de cette theÌse preÌsentent une distribution de taille suffisamment faible pour permettre leur auto-assemblage en reÌseau ordonneÌ aÌ trois dimensions, appeleÌs supracristaux. Lâapparition de supracristaux inverses et de surfaces vicinales a pu eÌtre observeÌe dans certaines conditions dâassemblage. De plus, il est aussi possible dâobtenir des supracristaux de taille submillimeÌtrique ne contenant que des nanocristaux mono- ou polycristallines, et pouvant eÌtre eÌtudieÌs individuellement par diffraction de rayons X. Il a ainsi eÌteÌ possible de correÌler lâordre orientationnel et translationnel des nanocristaux dans le reÌseau supracristallin
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