Étude ab initio des nanotubes de carbone

Le sujet de ce mémoire est l’étude ab initio des nanotubes de carbone. Premièrement, une introduction du sujet est présentée. Elle porte sur l’historique, la structure géométrique et électronique et les applications possibles des nanotubes de carbone. En deuxième lieu, la stabilité énergétique des nanotubes de carbones double parois ainsi que leur structure électronique sont étudiées. On trouve entre autres que le changement d’hybridation provoque une chute de l’énergie du dernier niveau occupé pour les petits nanotubes. Troisièmement, nous présenterons une étude sur la dépendance en diamètre et en métallicité du greffage d’unité bromophényle sur la surface des nanotubes. La principale conclusion est qu’il est plus facile de fonctionnaliser les nanotubes de petit diamètre puisque ceux-ci ont déjà une partie d’hybridation sp3 dans leur structure électronique. Finalement, le dernier chapitre aborde la combustion des nanotubes par le dioxyde de carbone. On constate que cette combustion ne peut pas débuter sur une surface intacte, ni par un pontage d’oxygène dû à la grande quantité d’énergie requise. La réaction privilégiée est alors la combustion par les extrémités du nanotube. Nous proposons une dynamique de réaction qui contient une sélectivité en diamètre.The subject of this master’s thesis is the ab initio study of carbon nanotubes. First, an introduction to the subject is presented. It covers the history, the geometric and electronic structure and potential applications of carbon nanotubes. Second, the energy stability of double-walled carbon nanotubes and their electronic structure are studied. It is found that the change of hybridization causes a lowering in the energy of the highest occupied molecular orbital’s level for small nanotubes. Thirdly, a study of the diameter and metallicity dependence for the bromophenyl bonding energy on the carbon nanotubes is presented. The main conclusion is that it is easier to functionalize the nanotubes of small diameter since they already have some sp3 hybridization in their electronic structure. Finally, the last chapter discusses the burning of carbon nanotubes with carbon dioxide. It is found that combustion can not begin on a pristine surface or by a oxygen bridge due to the large amount of energy required. The favored reaction is then burning the ends of nanotubes. We suggest a path of reaction for which a diameter selectivity is apparent

Bromophenyl functionalization of carbon nanotubes : an ab initio study

We study the thermodynamics of bromophenyl functionalization of carbon nanotubes with respect to diameter and metallic/insulating character using density-functional theory (DFT). On one hand, we show that the activation energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag nanotube ranges from 0.73 eV to 0.76 eV without any clear trend with respect to diameter within numerical accuracy. On the other hand, the binding energy of a single bromophenyl molecule shows a clear diameter dependence and ranges from 1.51 eV for a (8,0) zigzag nanotube to 0.83 eV for a (20,0) zigzag nanotube. This is in part explained by the transition from sp2 to sp3 bonding occurring to a carbon atom of a nanotube when a phenyl is grafted to it and the fact that smaller nanotubes are closer to a sp3 hybridization than larger ones due to increased curvature. Since a second bromophenyl unit can attach without energy barrier next to an isolated grafted unit, they are assumed to exist in pairs. The para configuration is found to be favored for the pairs and their binding energy decreases with increasing diameter, ranging from 4.34 eV for a (7,0) nanotube to 2.27 eV for a (29,0) nanotube. An analytic form for this radius dependence is derived using a tight binding hamiltonian and first order perturbation theory. The 1/R^2 dependance obtained (where R is the nanotube radius) is verified by our DFT results within numerical accuracy. Finally, metallic nanotubes are found to be more reactive than semiconducting nanotubes, a feature that can be explained by a non-zero density of states at the Fermi level for metallic nanotubes.Comment: 7 pages, 5 figures and 3 table

A Genetic Signature of Spina Bifida Risk from Pathway-Informed Comprehensive Gene-Variant Analysis

Despite compelling epidemiological evidence that folic acid supplements reduce the frequency of neural tube defects (NTDs) in newborns, common variant association studies with folate metabolism genes have failed to explain the majority of NTD risk. The contribution of rare alleles as well as genetic interactions within the folate pathway have not been extensively studied in the context of NTDs. Thus, we sequenced the exons in 31 folate-related genes in a 480-member NTD case-control population to identify the full spectrum of allelic variation and determine whether rare alleles or obvious genetic interactions within this pathway affect NTD risk. We constructed a pathway model, predetermined independent of the data, which grouped genes into coherent sets reflecting the distinct metabolic compartments in the folate/one-carbon pathway (purine synthesis, pyrimidine synthesis, and homocysteine recycling to methionine). By integrating multiple variants based on these groupings, we uncovered two provocative, complex genetic risk signatures. Interestingly, these signatures differed by race/ethnicity: a Hispanic risk profile pointed to alterations in purine biosynthesis, whereas that in non-Hispanic whites implicated homocysteine metabolism. In contrast, parallel analyses that focused on individual alleles, or individual genes, as the units by which to assign risk revealed no compelling associations. These results suggest that the ability to layer pathway relationships onto clinical variant data can be uniquely informative for identifying genetic risk as well as for generating mechanistic hypotheses. Furthermore, the identification of ethnic-specific risk signatures for spina bifida resonated with epidemiological data suggesting that the underlying pathogenesis may differ between Hispanic and non-Hispanic groups

Electronic properties of double-walled carbon nanotubes (DWNT) and the effect of functionalization

Although promising for many electronic applications, further understanding of carbon nanotubes systems are required for practical designs. A difficulty currently hindering further development in this field is the considerable degradation of transport properties in a single-wall carbon nanotube (SWCNT) when it is subjected to ambient conditions or functionalized. Double-wall carbon nanotubes (DWCNT) could solve this problem, by allowing the outer tube to be functionalized while the inner tube would retain a pristine structure and it's promising electronic properties. However, our understanding of interactions between the tubes and their consequences on the system's electronic properties is still incomplete. In this presentation, we investigate those interactions using density-functional theory (DFT) calculations. In particular, we investigate separately the effects of structural deformations, Fermi energy realignment and electronic orbital overlap on the band structure of DWCNT. The effects of functionalization will also be addressed

Ab initio study of the dependence of the reactivity upon carbon nanotube diameter

One of the main research efforts of the recent years has been the development of an efficient way to select desired carbon nanotubes according to their size and their electronic properties. This selectivity would allow easier fabrication of field effect transistor and light-emitting diode devices with appropriate nanotubes. An appealing approach to assess this problem is to use the dependence of chemical functionalization thermodynamics on the material’s properties. In this talk, ab initio studies of carbon nanotubes functionalized with bromophenyl will be presented. The radius dependence of the binding and activation energies of this functionalization will be reported. The purpose of this presentation is also to demonstrate the performance of linear-scaling density-functional theory code ONETEP, which provides the possibility carrying out large system simulations (up to several tens of thousands of atoms). Furthermore, the diameter dependence of the oxidation of carbon nanotubes by carbon dioxide will be presented

Electronic properties of double-wall carbon nanotubes and the effect of functionalization

Although promising for many electronic applications, further understanding of carbon nanotubes systems are required for practical designs. A difficulty currently hindering further development in this field is the considerable degradation of transport properties in a single-wall carbon nanotube (SWCNT) when it is subjected to ambient conditions or functionalized. Double-wall carbon nanotubes (DWCNT) could solve this problem, by allowing the outer tube to be functionalized while the inner tube would retain a pristine structure and it's promising electronic properties. However, our understanding of interactions between the tubes and their consequences on the system's electronic properties is still incomplete. In this presentation, we investigate those interactions using density-functional theory (DFT) calculations. In particular, we investigate separately the effects of structural deformations, Fermi energy realignment and electronic orbital overlap on the band structure of DWCNT. The effects of functionalization will also be addressed

The PlaceLab: A live-in laboratory for pervasive computing research (video

In this video, we introduce the PlaceLab, a live-in laboratory for the study of ubiquitous computing technologies in the home. The PlaceLab is a real home where the routine activities and interactions of everyday home life can be observed, recorded for later analysis, and experimentally manipulated. Volunteer research participants individually live in the PlaceLab for days or weeks at a time, treating it as a temporary home. Meanwhile, sensing devices integrated into the fabric of the architecture record a detailed description of their activities. The facility generates sensor and observational datasets on typical domestic activity that can be used for research in pervasive computing, preventive healthcare, user interface design, and other fields where domestic contexts impact behavior. Mobile and ubiquitous computing devices are transforming the way that people interact with digital information. Ideally, emerging human-computer interfaces will be able to automatically detect context and present and gather information without unduly disrupting the complex activity inherent to any real and dynamic environment such as the home. Studying behavior in naturalistic livin

Computing Research

Abstract. Ubiquitous computing researchers are increasingly turning to sensorenabled “living laboratories ” for the study of people and technologies in settings more natural than a typical laboratory. We describe the design and operation of the PlaceLab, a new live-in laboratory for the study of ubiquitous technologies in home settings. Volunteer research participants individually live in the PlaceLab for days or weeks at a time, treating it as a temporary home. Meanwhile, sensing devices integrated into the fabric of the architecture record a detailed description of their activities. The facility generates sensor and observational datasets that can be used for research in ubiquitous computing and other fields where domestic contexts impact behavior. We describe some of our experiences constructing and operating the living laboratory, and we detail a recently generated sample dataset, available online to researchers.

CD8 Follicular T Cells Promote B Cell Antibody Class Switch in Autoimmune Disease

CD8 T cells can play both a protective and pathogenic role in inflammation and autoimmune development. Recent studies have highlighted the ability of CD8 T cells to function as T follicular helper (Tfh) cells in the germinal center in the context of infection. However, whether this phenomenon occurs in autoimmunity and contributes to autoimmune pathogenesis is largely unexplored. In this study, we show that CD8 T cells acquire a CD4 Tfh profile in the absence of functional regulatory T cells in both the IL-2-deficient and scurfy mouse models. Depletion of CD8 T cells mitigates autoimmune pathogenesis in IL-2-deficient mice. CD8 T cells express the B cell follicle-localizing chemokine receptor CXCR5, a principal Tfh transcription factor Bcl6, and the Tfh effector cytokine IL-21. CD8 T cells localize to the B cell follicle, express B cell costimulatory proteins, and promote B cell differentiation and Ab isotype class switching. These data reveal a novel contribution of autoreactive CD8 T cells to autoimmune disease, in part, through CD4 follicular-like differentiation and functionality