110 research outputs found

    Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes

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    Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50nm) allowed to measure current contribution from sample areas smaller than 1um^2. The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72 hours) at current intensities between 10-5A and 10-8A. Improvement of the current stability has been observed after performing long-time Joule heating conditioning to completely remove possible adsorbates on the nanotubes.Comment: 15 pages, 7 figure

    Field Emission from Self-Assembled Arrays of Lanthanum Monosulfide Nanoprotrusions

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    The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is 3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, 1 eV, a field enhancement factor of 5.8 is extracted for the nanodome emitters from Fowler-Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements

    A sequential approach for a multi-commodity two-echelon distribution problem

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    We address a Multi-Commodity two-echelon Distribution Problem (MC2DP) where three sets of stakeholders are involved: suppliers, distribution centers, and customers. Multiple commodities have to be sent from suppliers to customers, using multiple distribution centers for consolidation purposes. Commodities are collected from the suppliers and delivered to the distribution centers with direct trips, while a fleet of homogeneous vehicles distributes commodities to customers. Commodities are compatible, that is any vehicle can transport any set of commodities as long as its capacity is not exceeded. The goal is to minimize the total transportation cost from suppliers to customers. We present two sequential schemes based on the solution, in a different order, of a collection and a delivery subproblem. In both cases, the solution of the first subproblem determines the quantity of each commodity at each distribution center. The second subproblem takes this information as input. We also propose different strategies to guide the solution of the first subproblem in order to take into account the impact of its solution on the second subproblem. The proposed sequential heuristics are evaluated and compared both on randomly generated instances and on a case study related to a short and local fresh food supply chain. The results show the impact of problem characteristics on solution strategies

    Habitation

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    Dans un contexte concurrentiel accru, les industries doivent développer de nouvelles solutions pour répondre à des défis majeurs (innovation, design, fonctionnalité, consommation…). La plastronique, qui est au confluent des domaines de la plasturgie et de l’électronique, permet de répondre à ces nouvelles attentes, en proposant des objets intelligents et connectés. Elle offre la possibilité de concevoir des systèmes de forme complexe 3D. Dans ce contexte de rupture technologique, l’innovation est à l’interface du génie électrique, de la mécatronique et de la plasturgie. Elle fait également appel à des domaines connexes et indispensables tels que la physique et la chimie. La convergence de ces domaines rend indispensable l’acquisition de nouvelles connaissances et compétences transverses dans les métiers de demain. Pour répondre à ces besoins, une formation en plastronique est en émergence à l’Université de Lyon. Les modalités seront décrites dans cet article. Les compétences initiales des apprenants seront élargies pour acquérir les bases théoriques et pratiques de la conception de systèmes plastroniques. Cette formation s’appuiera sur des projets concrets en lien direct avec les acteurs variés de la filière (aéronautique, médical, robotique, etc.). Le prototypage y tiendra une place importante, en permettant de confronter les apprenants à des problématiques industrielles concrètes

    Rare-earth monosulfides as durable and efficient cold cathodes

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    In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, we first describe the successful deposition of Lanthanum Monosulfide via pulsed laser deposition on Si and MgO substrates and alumina templates. These thin films have been characterized by X-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the Scanning Anode Field Emission Microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of low work function materials having a orientation perpendicular to the surface and outcropping it are surrounded by a matrix of amorphous materials with higher work function. To date, LaS thin films have been used successfully as cold cathode emitters with measured emitted current densities as high as 50 A/cm2. Finally, we describe the successful growth of LaS thin films on InP substrates and, more recently, the production of LaS nanoballs and nanoclusters using Pulsed Laser Ablation.Comment: 61 pages, 24 figure

    Paradox of low field enhancement factor for field emission nanodiodes in relation to quantum screening effects

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    We put forward the quantum screening effect in field emission [FE] nanodiodes, explaining relatively low field enhancement factors due to the increased potential barrier that impedes the electron Fowler-Nordheim tunneling, which is usually observed in nanoscale FE experiments. We illustratively show this effect from the energy band diagram and experimentally verify it by performing the nanomanipulation FE measurement for a single P-silicon nanotip emitter (Φ = 4.94eV), with a scanning tungsten-probe anode (work function, Φ = 4.5eV) that constitutes a 75-nm vacuum nanogap. A macroscopic FE measurement for the arrays of emitters with a 17-μm vacuum microgap was also performed for a fair comparison

    Field Emission from Self-Assembled Arrays of Lanthanum Monosulfide Nanoprotrusions

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    The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is ∼3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, ∼1 eV, a field enhancement factor of ∼5.8 is extracted for the nanodome emitters from Fowler-Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements

    Field Emission from Self-Assembled Arrays of Lanthanum Monosulfide Nanoprotrusions

    Get PDF
    The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is ∼3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, ∼1 eV, a field enhancement factor of ∼5.8 is extracted for the nanodome emitters from FowlerNordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements. The promise of producing extremely high current densities in near-vacuum with no external source (such as heater, primary electron-beam, or intense light source) has generated significant interest in robust, reproducible, and fieldemission-based cold cathodes Recently, we reported the first successful deposition of lanthanum monosulfide (LaS) thin films on Si substrates using pulsed laser deposition (PLD) The nanodomes are cone-shaped LaS structures with base diameter 2r ranging from 50 to 100 nm and height h ranging from 100 to 150 nm. They were found to grow on the boundaries separating regions of the anodic alumina film that have near perfect pore ordering. Their density is ∼10 9 /cm 2 , as extracted from the field emission-scanning electron micrographs (FE-SEM) and atomic force micrographs (AFM) such as those shown in The SAFEM technique was used to measure the FE current-voltage (I-V) characteristics at different surface locations. As the nanowires are buried inside the pores, they contribute very little, if any at all, to the net FE current. Moreover, FE measurements from the arrays of nanodomes and nanodots were possible because the regions joining adjacent nanodomes and nanodots are covered with a thin percolating network of LaS over the entire array. Its presence was confirmed by measuring a low resistance value of about 2 Ohms between two electrical contacts about 1 cm apart. For each location, a full set of I-V characteristics (total measured current versus applied voltage) for different values of d, the distance between the cathode surface and the probe ball, was measured. This set of measurements was then analyzed in order to extract the apparent current density αJ versus actual applied local field γF, where γ is the local geometrical field enhancement at the surface of the cathode In order to assess the FE properties from LaS nanoprotrusions, we have compared the FE from nanodome cathodes with that from planar thin film LaS cathodes. This comparative methodology was chosen because a precise quantitative estimation of γ from topographic measurements is still subject to controversy, except for a flat surface. These experimental measurements were performed for both LaS thin films of 100 nm thickness on Si wafers and LaS nanodomes on anodic alumina films. The analysis consisted of the following steps. (1) In order to restrict the analysis of the differences in the I-V data to the surface morphology of the cathodes, both SAFEM measurements were performed with the same probe ball-to-cathode distance d = 3.65 μm. Typical I-V characteristics are shown in (2) The total FE current versus applied voltage (I-V) characteristics were measured for different values of the probe-ball-to-cathode-surface distance d. From these data, the apparent current densities (α t f J and α nd J) as a a function of the applied local field were extracted ACKNOWLEDGMEN

    Electromechanical properties of suspended Graphene Nanoribbons

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    Graphene nanoribbons present diverse electronic properties ranging from semiconducting to half-metallic, depending on their geometry, dimensions and chemical composition. Here we present a route to control these properties via externally applied mechanical deformations. Using state-of-the-art density functional theory calculations combined with classical elasticity theory considerations, we find a remarkable Young's modulus value of ~7 TPa for ultra-narrow graphene strips and a pronounced electromechanical response towards bending and torsional deformations. Given the current advances in the synthesis of nanoscale graphene derivatives, our predictions can be experimentally verified opening the way to the design and fabrication of miniature electromechanical sensors and devices based on ultra-narrow graphene nanoribbons.Comment: 12 pages, 6 figure

    The physics and chemistry of minerals and rocks, édité par R. G. J. Strens, 1976

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    Semet M. The physics and chemistry of minerals and rocks, édité par R. G. J. Strens, 1976. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 100, 2, 1977. p. 148
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