Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

International audienceMulti-walled carbon nanotubes have been grown on 7 nm Ni-coated substrates consisting of 300 lm thick highly n-doped (1 0 0) sil- icon covered with a diffusion barrier layer (10 nm thick) of SiO2 or TiN, by combining hot-wire chemical vapor deposition and direct current plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas was used as carbon source and ammonia and hydrogen were used either for dilution or etching. Growth of dense aligned nanotubes could be observed only if the ammonia content was minimized (rv5%). In order to improve the electron field emission properties of the films, different geometrical factors have been taken into account: average length, length/radius ratio and spacing between nanotubes. The nanotube growth rate was controlled by the substrate temperature and the pressure in the reactor, and the nanotube height by the growth time. The nanotube diam- eter was controlled by the catalyst dot volume, and the nanotube spacing was adjusted during the patterning process of the catalyst dots. Using optical lithography, 1 lm Ni dots were obtained and several multi-walled nanotubes with diameter and length in the range 60- 120 nm and rv2.3 lm were grown on each dot. Thus, based on a two-dimensional square lattice with a lattice translation vector of 4 lm, I-V characteristics yielded an onset electric field of 16 V/lm and a maximum emission current density of 40 mA/cm2, due to the large screening effect. Using electron-beam lithography, 100 nm Ni dots were obtained and individual multi-walled nanotubes were grown on each dot. Based on a square lattice with 10 lm translation vector, I-V characteristics gave an onset field of 8 V/lm and a max- imum emission current density of 0.4 A/cm2

Sources électroniques à base de nanotubes de carbone - Application aux tubes amplificateurs hyperfréquence.

Understanding the properties of matter at the atomic scale, results considerable progress in science and technology in the late 20th century, is one of progress that has led to the development of what is now called nanoscience. Richard P. Feynman, Nobel Prize for Physics in 1965 for his work on electrodynamics Quantum had prophesied in the 19,591 range of possibilities opened up by the manipulation of material atom by atom. The laws of physics no longer apply to macroscopic this scale and become purely quantum behavior. You can change the properties of matter thus paving the way for futuristic applications. Nanotechnology - concepts and methods for nanoscience applications - have vast fields of applications in microelectronics and materials by example. And the opportunities are increasing for these specific areas but also in biotechnology, photonics and information technology suggestive of social and economic benefits énormes2. Yet it has long used nanomaterials (glass, ceramics, ...) and chemists rely on molecules of different sizes. Can we then consider this science as news? In fact, what has changed is the opportunity, through the development new tools, make, observe, analyze, assemble, understand nano-objects which are the building blocks of nanotechnology. In this manuscript, we will consider the particular case of new molecules: carbon nanotubes are studied for their remarkable properties. Their qualities electron transport are candidates for the realization of transistors, diodes or more RAM. Their mechanical properties and low weight make it any component of composite materials. Finally, their high aspect ratios due to micrometer lengths and radii are nanoscale electron sources very interesting field emission that can be used in flat screens for microscopy. This latter application of carbon nanotubes as electron source in the void that is the cause of all this work and is the subject of this brief. When the transistor was invented by Bardeen, Brattain and Shockley in the years 19,503 and the first integrated circuits were introduced in 19604 years, it seemed that the time of the electronic vacuum was counted. The emission in the vacuum thermionic effect indeed requires heating a cathode to about 1000 ° C to make electrons. Yet in 1961, the Stanford Research Shoulders Institute (SRI) has the idea of a new device, micrometer-sized, based on the tunnel effect, as a new électrons5 source. And what are the manufacturing technologies from the microelectronics which will enable the manufacture of these new electron sources say cold (because operating at room temperature) based on this principle purely quantum. Thus, when in 1968, Spindt, Shoulders hired by the IRS, published his first Results on the production of spikes in pyramidal molybdène6, it triggers an interest for electronic transmitters and a rebound in electronics in vacuum with target applications such as flat panel displays, high frequency devices, ... So the early years of the scientific community will be almost exclusively dedicated to improving this specific type of issuer. One of the finest achievements being demonstration of operation in 1993 a flat screen 6 "based on this technologie7. From 1994, however, this community will gradually turn towards new materials more robust and also easier and cheaper to produce. This is In 1995 and Rinzler8 Heer9 will demonstrate the electron emission from nanotubes carbon, a new form of carbon discovered in 1991 by Iijima10. Properties geometric, electrical, mechanical, chemical properties of these objects are indeed a material remarkable for the field emission. They are also extremely robust and processes growth used to manufacture low-cost (over large areas) cathode very performance. In addition, one of the big disadvantages of Spindt tips for use at high frequencies is the proximity between the tip and the extraction grid (~ 1μm) which is imposed by the manufacturing process and leads to higher capacity thereby limiting the frequency use. With carbon nanotubes, this issue was raised with the possibility to exclude the grid at greater distances (~ 100μm) and cutoff frequencies and more high. With amplification factors much larger than the spikes pyramidal it also maintains a flight with reasonable voltages. The study of field emission properties of carbon nanotubes will be and the bulk of this manuscript in which we will also try to show the benefits of these sources compared to other materials. The goal here is the use of these sources in microwave tube amplifiers. The basic idea that led to this study is as follows: the thermionic cathodes Conventional satisfactory and are still under development. There has currently no reason to replace the tubes. However one perceives the need a new technology for high frequencies (> 30GHz). The dream of the designers electronic tubes and is replaced by the hot cathode cold cathode which integration should allow one hand to reduce the size and weight of at least a factor of 2 and above all to significantly improve performance, particularly at high frequencies. The cutoff points such as "Spindt" being too weak to address this frequency zone (a demonstration of 199,711 in 10GHz modulation is still Today the state of the art on these devices), carbon nanotubes provide a technology interesting and challenging to replace the hot cathode. But for that must develop a source that can issue a certain density currents with certain life. This has been the object of this work: designing, developing, understand, optimize a field emission source based on carbon nanotubes. The first part will introduce the world of tubes and microwave sources electronics. We try to understand the limits of current technology and the interest presented by these new sources of carbon nanotubes. The second part will focus on the material. Firstly through its exceptional Properties and methods of manufacture. A state of the art of its use as a source electronics allow us to understand the direction we have chosen to make powerful sources. We present the main experimental achievements. The third part will be devoted to field emission in nanotubes individual carbon. In presenting specific tool that was used for these measures and main results that allowed us to understand and improve the performance. The fourth part will be when it dedicated to the field emission measurements on networks of nanotubes. We present the main results. These will coupled with individual results to show that it is possible to predict the behavior a cathode. The fifth part will demonstrate the modulation of an electron beam Microwave from a carbon nanotube cathode. First results obtained in a diode at 1.5GHz. Then those obtained in a triode 30GHz. We will end with all these results, what they bring to the understanding This new type of sources and perspectives they open when the use of cellesci in commercial devices.La compréhension des propriétés de la matière à l'échelle atomique, résultats d'avancées considérables en sciences et en technologies de la fin du 20e siècle, est l'un des progrès qui a conduit au développement de ce que l'on appelle aujourd'hui les nanosciences. Richard P. Feynman, prix Nobel de physique en 1965 pour ses travaux sur l'électrodynamique quantique, avait prophétisé en 19591 l'étendue des possibilités qu'ouvrirait la manipulation de la matière atome par atome. Les lois de la physique macroscopique ne s'appliquent plus à cette échelle et les comportements deviennent purement quantiques. On peut modifier les propriétés de la matière ouvrant ainsi la voie à des applications futuristes. Les nanotechnologies – concepts et procédés des nanosciences en vue d'applications - ont déjà de vastes champs d'applications en microélectronique et en matériaux par exemple. Et les possibilités vont en s'accroissant, pour ces domaines spécifiques mais aussi en biotechnologie, en photonique et dans les technologies de l'information laissant supposer des retombées sociales et économiques énormes2. Pourtant, on utilise depuis longtemps des nanomatériaux (verres, céramiques, ...) et les chimistes font appel à des molécules de tailles diverses. Peut-on alors considérer cette science comme nouvelle? En fait, ce qui a changé, c'est la possibilité, grâce au développement de nouveaux outils, de fabriquer, d'observer, d'analyser, d'assembler, de comprendre des nano-objets qui sont les briques de base des nanotechnologies. Dans ce manuscrit, nous nous intéresserons au cas particulier de nouvelles molécules: les nanotubes de carbone qui sont étudiés pour leurs remarquables propriétés. Leurs qualités de transport électronique en font des candidats pour la réalisation de transistors, de diodes ou encore de mémoires RAM. Leurs propriétés mécaniques et leur faible poids en font un éventuel composant de matériaux composites. Enfin leurs rapports d'aspect élevés dus à des longueurs micrométriques et des rayons nanométriques en font des sources d'électrons par émission de champ très intéressantes qui peuvent servir dans des écrans plats ou pour la microscopie électronique. C'est cette dernière application des nanotubes de carbone comme source d'électrons dans le vide qui est à l'origine de tout ce travail de thèse et fait l'objet du présent mémoire. Lorsque le transistor fut inventé par Bardeen, Brattain, et Shockley dans les années 19503 et que les premiers circuits intégrés ont fait leur apparition dans les années 19604, il semblait que le temps de l'électronique sous vide était compté. L'émission dans le vide par effet thermoïonique nécessite en effet de chauffer une cathode à environ 1000°C pour émettre des électrons. Pourtant en 1961, Shoulders du Stanford Research Institute (SRI) a l'idée d'un nouveau dispositif, de taille micrométrique, basé sur l'effet tunnel, comme une nouvelle source d'électrons5. Et ce sont les technologies de fabrication issues de la microélectronique qui vont permettre la fabrication de ces nouvelles sources d'électrons dites froides (car fonctionnant à température ambiante) basées sur ce principe purement quantique. Ainsi, lorsqu'en 1968, Spindt, engagé par Shoulders au SRI, publie ses premiers résultats sur la fabrication de pointes pyramidales en molybdène6, il déclenche un intérêt général pour ce type d'émetteurs et un regain pour l'électronique sous vide avec des applications visées telles que les écrans plats, les dispositifs hautes fréquences, ... Ainsi les premières années de cette communauté scientifique seront quasi exclusivement dédiées à l'amélioration de ce type spécifique d'émetteurs. L'une des plus belles réalisations étant la démonstration du fonctionnement en 1993 d'un écran plat 6" basé sur cette technologie7. A partir de 1994 cependant, cette communauté va petit à petit se tourner vers de nouveaux matériaux plus robustes, et également plus faciles et moins chers à produire. C'est en 1995 que Rinzler8 et de Heer9 vont démontrer l'émission d'électrons à partir de nanotubes de carbone, une nouvelle forme du carbone découverte en 1991 par Iijima10. Les propriétés géométriques, électriques, mécaniques, chimiques de ces objets en font en effet un matériau remarquable pour l'émission de champ. Ils sont de plus extrêmement robustes et les procédés de croissance permettent la fabrication à bas coût (sur de larges surfaces) de cathodes très performantes. De plus, l'un des gros inconvénients des pointes Spindt pour une utilisation à hautes fréquences est la proximité entre la pointe et la grille d'extraction (~1μm) qui est imposée par le procédé de fabrication et conduit à des capacités élevées limitant ainsi la fréquence d'utilisation. Avec les nanotubes de carbone, ce problème est levé avec la possibilité d'écarter la grille à des distances plus importantes (~100μm) et ainsi des fréquences de coupure plus élevées. Avec des facteurs d'amplification beaucoup plus grands que les pointes pyramidales, on conserve également un pilotage avec des tensions raisonnables. L'étude des propriétés d'émission de champ des nanotubes de carbone constituera ainsi l'essentiel du présent manuscrit où l'on essaiera également de montrer les avantages de ces sources par rapport à d'autres matériaux. Le but visé ici étant l'utilisation de ces sources dans des tubes amplificateurs hyperfréquence. L'idée de base qui a conduit à cette étude est la suivante: les cathodes thermoïoniques conventionnelles donnent satisfaction et font toujours l'objet de développement. Il n'y a donc actuellement aucune raison de les remplacer dans les tubes. Cependant on perçoit le besoin d'une nouvelle technologie pour fréquences élevées (>30GHz). Le rêve des concepteurs de tubes électroniques est ainsi de remplacer ces cathodes chaudes par des cathodes froides dont l'intégration devrait permettre d'une part de réduire la taille et le poids d'au moins un facteur 2 et surtout d'améliorer nettement le rendement, en particulier à hautes fréquences. La fréquence de coupure des pointes type "Spindt" étant trop faible pour adresser cette zone de fréquence (une démonstration de modulation à 10GHz en 199711 est encore aujourd'hui l'état de l'art sur ces dispositifs), les nanotubes de carbone apportent une technologie intéressante et ambitieuse pour remplacer les cathodes chaudes. Mais pour cela il faut développer une source qui puisse émettre une certaine densité de courant avec une certaine durée de vie. C'est ce qui a fait l'objet de ce travail: concevoir, développer, comprendre, optimiser une source à émission de champ à base de nanotubes de carbone. La première partie introduira le monde des tubes hyperfréquence et des sources électroniques. Nous essaierons de comprendre les limites des technologies actuelles et l'intérêt que présentent ces nouvelles sources à nanotubes de carbone. La deuxième partie se focalisera sur le matériau. Tout d'abord via ses exceptionnelles propriétés et ses méthodes de fabrication. Un état de l'art de son utilisation comme source électronique nous permettra de comprendre l'orientation que nous avons choisie pour réaliser des sources performantes. On présentera les principales réalisations expérimentales. La troisième partie sera consacrée aux mesures d'émission de champ sur nanotubes de carbone individuels. On présentera l'outil spécifique qui a servi à ces mesures et les principaux résultats obtenus qui nous ont permis de comprendre et d'améliorer les performances. La quatrième partie sera quand à elle consacrée aux mesures en émission de champ sur des réseaux de nanotubes. On présentera les principaux résultats obtenus. Ceux-ci seront couplés aux résultats individuels pour montrer qu'il est possible de prédire le comportement d'une cathode. La cinquième partie démontrera la modulation d'un faisceau électronique en hyperfréquence à partir d'une cathode à nanotubes de carbone. Tout d'abord les résultats obtenus dans une diode à 1.5GHz. Puis ceux obtenus dans une triode à 30GHz. Enfin nous conclurons sur tous ces résultats, ce qu'ils apportent sur la compréhension de ce nouveau type de sources et les perspectives qu'ils ouvrent quand à l'utilisation de cellesci dans des dispositifs commerciaux

Stable field emission from arrays of vertically aligned free-standing metallic nanowires

We present a fully elaborated process to grow arrays of metallic nanowires with controlled geometry and density, based on electrochemical filling of nanopores in track-etched templates. Nanowire growth is performed at room temperature, atmospheric pressure and is compatible with low cost fabrication and large surfaces. This technique offers an excellent control of the orientation, shape and nanowires density. It is applied to fabricate field emission arrays with a good control of the emission site density. We have prepared Co, Ni, Cu and Rh nanowires with a height of 3 mu m, a diameter of 80 nm and a density of similar to 10(7) cm(-2). The electron field emission measurements and total energy distributions show that the as-grown nanowires exhibit a complex behaviour, first with emission activation under high field, followed by unstable emission. A model taking into account the effect of an oxide layer covering the nanowire surface is developed to explain this particular field emission behaviour. Finally, we present an in situ cleaning procedure by ion bombardment that collectively removes this oxide layer, leading to a stable and reproducible emission behaviour. After treatment, the emission current density is similar to 1 mA cm(-2) for a 30 V mu m-1 applied electric field

Neo-Adjuvant Use of Sorafenib for Hepatocellular Carcinoma Awaiting Liver Transplantation

International audienceData on efficacy and safety of sorafenib in a neoadjuvant setting for HCC awaiting liver transplantation (LT) are heterogeneous and scarce. We aimed to investigate the trajectory of patients treated with sorafenib while awaiting LT. All patients listed for HCC and treated with sorafenib were included in a monocentric observational study. A clinical and biological evaluation was performed every month. Radiological tumor response evaluation was realized every 3 months on the waiting list and every 6 months after LT. Among 327 patients listed for HCC, 62 (19%) were treated with Sorafenib. Sorafenib was initiated for HCC progression after loco-regional therapy (LRT) in 50% of cases and for impossibility of LRT in 50% of cases. The mean duration of treatment was 6 months. Thirty six patients (58%) dropped-out for tumor progression and 26 (42%) patients were transplanted. The 5-year overall and recurrent-free survival after LT was 77% and 48% respectively. Patients treated for impossibility of LRT had acceptable 5-year intention-to-treat overall and post-LT survivals. Conversely, patients treated for HCC progression presented high dropout rate and low intention-to-treat survival. Our results suggest that it is very questionable in terms of utility that patients treated for HCC progression should even be kept listed once the tumor progression has been observed

Probing the energetic and structural role of amino acid/nucleobase cation-pi interactions in protein-ligand complexes.

X-ray structures of proteins bound to ligand molecules containing a nucleic acid base were systematically searched for cation-pi interactions between the base and a positively charged or partially charged side chain group located above it, using geometric criteria. Such interactions were found in 38% of the complexes and are thus even more frequent than pi-pi stacking interactions. They are moreover well conserved in families of related proteins. The overwhelming majority of cation-pi contacts involve Ade bases, as these constitute by far the most frequent ligand building block; Arg-Ade is the most frequent cation-pi pair. Ab initio energy calculations at MP2 level were performed on all recorded pairs. Though cation-pi interactions involving the net positive charge carried by Arg or Lys side chains are the most favorable energetically, those involving the partial positive charge of Asn and Gln side chain amino groups (sometimes referred to as amino-pi interactions) are favorable too, owing to the electron correlation energy contribution. Chains of cation-pi interactions with a nucleobase bound simultaneously to two charged groups or a charged group sandwiched between two aromatic moieties are found in several complexes. The systematic association of these motifs with specific ligand molecules in unrelated protein sequences raises the question of their role in protein-ligand structure, stability, and recognition.Journal ArticleResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe

SAR156497, an Exquisitely Selective Inhibitor of Aurora Kinases

The Aurora family of serine/threonine kinases is essential for mitosis. Their crucial role in cell cycle regulation and aberrant expression in a broad range of malignancies have been demonstrated and have prompted intensive search for small molecule Aurora inhibitors. Indeed, over 10 of them have reached the clinic as potential anticancer therapies. We report herein the discovery and optimization of a novel series of tricyclic molecules that has led to SAR156497, an exquisitely selective Aurora A, B, and C inhibitor with in vitro and in vivo efficacy. We also provide insights into its mode of binding to its target proteins, which could explain its selectivity