1,721,084 research outputs found
Electron Transport and Electrochemical Control of Oligo(phenyleneethynylene) Based Molecular Devices
Get PDFThe essential goals of molecular electronics are to understand the mechanism of electron transport in molecules and realise the molecular devices with controlled functionality. Many studies have shown that the mechanism of electron transport in molecular junctions is complex. The molecule itself, anchoring group, electrode material and the environment all affect the conductivity of molecular junctions. To facilitate the applications of molecular-scale electronic devices, an in-depth understanding of different factors on the electrical properties of molecular junctions is essential. This project mainly uses oligo(phenyleneethynylene) (OPE) based molecules as a model system to investigate the electrical properties of molecular devices and understand how the chemistry and electronic structure of the molecule and contacts can determine their electrical characteristics.
First, the molecular junctions of OPEs with Au/Au electrodes were formed using STM-BJ and STM-I(s) techniques. The experimental results show that the measured conductance of OPEs is different by two different techniques. The contact resistances and attenuation factors of target molecular junctions were calculated to study the length dependence of molecular junctions formed by different techniques. The molecular junctions formed using STM-BJ technique show more obvious length dependence than those formed using STM-I(s) technique. Moreover, anchoring groups used to bind the molecular backbone to the electrical contacts seem to play an important role in the conductance and length dependence of OPEs. For linking long-range OPEs, amine is a more suitable anchor to connect with electrodes than methyl sulfide, which offers a better conductivity.
Then, graphene was introduced to replace an Au electrode and formed Au/OPEs/graphene asymmetric molecular junctions. Current results show that asymmetric molecular junctions have lower conductance and attenuation factors than symmetric molecular junctions. The sharp decrease in conductance caused by the destructive quantum interference (DQI) was discussed by comparing the conductance values of para- and meta- connected OPEs with Au/Au electrodes and Au/graphene electrodes. For both Au/Au system and Au/graphene system, the DQI of meta-OPEs leads to much smaller conductance values than para- OPEs. It is worth mentioning that the DQI effect on molecular conductance varies with the electrodes and anchoring groups. For all the studied molecular junctions, the conductance values of ethynyl-anchored OPEs with Au/graphene electrodes show the most significant DQI effect when compared to amine and methyl sulfide terminated molecular junctions.
Last, the electrochemical control of methyl sulfide terminated OPEs was achieved using the electrochemical control STM (EC-STM) technique. The experiment results show that the OPEs with DQI exhibit switching characteristics. The conductance slightly increases when the positive potentials are applied to the molecule. When the negative potential is applied, the conductance of meta-connected OPE is significantly reduced. Due to the detection limit of the instrument, the molecular conductance is too small to be detectable at potentials greater than -0.4 V. For the overpotentials used in this study, the difference in conductance values of meta-connected OPEs reaches two orders of magnitude under electrochemical control.
In summary, this project shows that electron transport in OPEs is complexed and affected by many factors. The conductivity of OPEs is strongly influenced by the coupling between anchoring groups and electrodes. The conductance of OPEs with the DQI effect can be modulated by changing the electrodes or the anchoring groups. Finally, the conductance of OPEs with the DQI effect shows magnitudes changes under the electrochemical control. Our work holds future technological significance as it could generate new insight towards large switching ratio molecular devices
Études des ondes de densité de charge dans les dichalcogénures de métaux de transition
No full textDans cette thèse, nous avons exploré les propriétés et le comportement des ondes de densité de charge (ODC) dans les dichalcogénures de métaux de transition. En particulier, nous nous sommes intéressés aux différents paramètres qui pourraient influencer, modifier ou nous permettre de contrôler ces phases d'ODC. Ainsi, nous avons examiné l'adsorption atomique, l'intercalation (dopage) et les changements de phases. Grâce à une approche combinant modélisation théorique et collaboration avec des équipes expérimentales, des progrès majeurs ont été accomplis dans la compréhension de la modulabilité et du contrôle des phases d'ODC. Dans ce travail, nous nous sommes concentrés sur les composés 1T-VSe2 et 1T-VTe2, mais ces premiers résultats ouvrent des perspectives générales dans ce domaine.Une partie importante de ce travail a été consacrée au développement d'une méthodologie solide pour modéliser la configuration atomique des phases d'ODC. Cette méthodologie, fondée sur la DFPT, a permis une étude détaillée sur les propriétés énergétiques, dynamiques et structurelles des phases d'ODC. Les résultats clés de notre recherche sur le dopage à l'azote (N) dans 1T-VSe2 incluent une caractérisation détaillée des défauts observés expérimentalement. De plus, les résultats expérimentaux montrent la robustesse de la phase (4x4) d'ODC dans 1T-VSe2, car la présence de défauts ne perturbe pas la stabilité de cette phase.L'étude de l'intercalation du sodium dans 1T-VSe2 a démontré la modulabilité des phases d'ODC. Eneffet, la phase (4 × 4) d'ODC observée à la surface du 1T-VSe2 massif passe à une phase (√7 × √3) d'ODC, similaire à celle observée dans la monocouche 1T-VSe2. Nos calculs dans le cadre de la DFT ont révélé un transfert de charge significatif des atomes de sodium vers la couche supérieure de VSe2, entraînant un décalage des orbitales dz2 du vanadium vers les basses énergies.Les images STM expérimentales ont montré que la monocouche de 1T-VTe2 présente plusieurs phases d'ODC, telles que les configurations (4 × 4) et (4 × 1) avec différentes orientations. L'étude expérimentale a également démontré la possibilité de transitions réversibles entre ces phases d'ODC, induites par des impulsions de la pointe STM. Une étude théorique et numérique reposant sur des calculs NEB a révélé que les transitions rotationnelles entre les orientations (4 × 1) nécessitent moins d'énergie que les transitions entre les phases (4 × 1) et (4 × 4).Enfin, nous avons étudié les propriétés de transport électronique dans 1T-VSe2 et 1T-VTe2. Ces résultats mettent en évidence une réduction de la conductance à travers les différentes phases d'ODC par rapport à l'état non déformé, principalement en raison de modifications des distances interatomiques aux interfaces des électrodes. Cependant, aucune variation significative des propriétés de transport n'a été observée entre les différentes phases d'ODC, ce qui suggère des défis dans l'utilisation des orientations d'ODC pour un transport électronique modulable. Cette recherche pose les bases d'un domaine émergent, "l'ODC-tronique", qui vise à exploiter les propriétés uniques des ODC dans les dispositifs nanoélectroniques.Ce travail établit un lien entre les études fondamentales sur les ODC et leurs potentielles applicationstechnologiques. En développant une méthodologie pour modéliser les configurations atomiques et en approfondissant notre compréhension des propriétés des ODC, cette thèse ouvre la voie à des recherches futures visant à manipuler les ODC par l'ingénierie des défauts, l'adsorption d'atomes ou de molécules, et l'exploration de leurs effets sur le transport électronique.In this thesis, we have explored the properties and behavior of Charge Density Waves (CDWs) inTransition Metal Dichalcogenides (TMDs). In particular, we have been interested in different parameters that might influence, modify or allow us to control the CDW phases. Hence, we have considered atomic adsorption and intercalation (doping) and phases switching. Through a combination of theoretical modeling and collaboration with experimental groups, significant insights into the tunability and manipulation of CDW phases have been achieved. In this work, we have been focusing on the 1T-VSe2 and 1T-VTe2 compounds but these first results open general trends in the field.An important part of this work has been dedicated to the development of a robust methodology to model the atomic configuration of CDW phases. This methodology, grounded in Density Functional Perturbation Theory (DFPT), has enabled detailed investigations into the energetics, dynamics, and structural properties of CDW phases. As such, it contributes to advancing the theoretical tools available for the study of system exhibiting CDWs.The key findings of our research on nitrogen (N) doping in 1T-VSe2 include a detailed characterization of experimentally observed defects. Furthermore, experimental results demonstrate the robustness of the (4x4) CDW phases in 1T-VSe2, as the presence of defects does not disrupt the stability of these phases.The investigation of sodium intercalation in 1T-VSe2 has demonstrated the tunability of the CDW phases. Indeed, the (4 × 4) CDW observed on the surface of bulk 1T-VSe2 transitions into a (√7 × √3) CDW, similar to that observed in monolayer 1T-VSe2. Our Density Functional Theory (DFT) calculations have revealed significant charge transfer from sodium atoms to the top VSe2 leading to a downward energy shift of the vanadium dz2 orbitals.Experimental Scanning Tunneling Miscroscopy (STM) topography images have demonstrated that 1T-VTe2 monolayer exhibits multiple CDW phases, such as the (4 × 4) and (4 × 1) configurations with different orientations. The experimental study has demonstrated the possibility of reversible CDW phase transitions induced by external stimuli like STM tip pulses. Computational investigation using Nudged Elastic Band (NEB) calculations revealed that the rotational transitions between (4 × 1) orientations require less energy compared to transitions between (4 × 1) and (4 × 4) phases.Finally, we have investigated the electronic transport properties of 1T-VSe2 and 1T-VTe2. These results highlight a reduction in conductance across different CDW phases compared to the pristine state, primarily due to modified interatomic distances at electrode interfaces. However, no significant variation in transport properties was observed between distinct CDW phases, suggesting challenges in leveraging CDW orientation for tunable electronic transport. The transport properties are found relatively similar for both compounds. This research lays a foundation for the emerging field of "CDW-tronics", which seeks to utilize the unique properties of CDWs in nanoelectronic devices.This work bridges fundamental studies of CDWs with their potential technological applications. Bydeveloping a methodology to model atomic configurations and advancing our understanding of CDWproperties, this thesis sets the stage for future research in manipulating CDWs through defect engineering, adsorption of atoms or molecules, and exploring their effects on electronic transport. Such efforts will further unlock the potential of CDWs in practical applications, driving advancements in materials science and nanoelectronics. For instance, we can consider future transistors that operate by modulating CDW phases through an electronic gate or next generation gas sensor that utilize the interactions of CDW phases with specific molecules
Etude théorique de nouvelles nanostructures à base de graphène
No full textDepuis sa découverte, le graphène est devenu un centre de recherche et d'intérêt important en raison de ses caractéristiques mécaniques, thermiques et électriques exceptionnelles. Néanmoins, l'absence de bande interdite dans le graphène constitue un obstacle aux applications dans les domaines de l'optique, de la nanoélectronique et de la spintronique. L'ingénierie de la bande interdite impliquant la nanostructuration du graphène a été développée au fil des ans, par exemple par confinement quantique, pour surmonter cette limitation. Ce travail théorique est consacré à la modification des réponses électroniques, optiques et de microscopie/spectroscopie à effet tunnel (STM/STS) en fonction de la taille du système de nouveaux matériaux de carbone tels que les nanomeshs de graphène (GNM), les boîte quantiques de graphène de forme/taille contrôlée (GQD) et les nanorubans de graphène (GNR), afin de comparer et d'analyser les données expérimentales. Ces nouveaux matériaux carbonés sont théoriquement déposés sur des surfaces d'or Au(111) dans une simulation STM réalisée à l'aide du formalisme des fonctions de Green hors équilibre (NEGF) basé sur la méthode DFT Fireball afin d'étayer les données expérimentales. En ce qui concerne les GQD, nous simulons leur spectre d'absorption en utilisant la correction GW et les équations de Bethe-Salpeter (BSE), si possible, pour les comparer directement aux données expérimentales. Dans le cas contraire, leurs propriétés optiques sont obtenues par une approche inférieure, l'approche Tight-Binding (TB). Les impacts des agrégations et des impuretés sur leurs réponses optiques sont également explorés en étudiant la bicouche torsadée des GQDs via la méthode TB. En outre, les changements dans les propriétés électroniques de ces nouveaux matériaux de carbone en fonction de la taille de leur système sont extraits à l'aide de la méthode TB. La performance de la méthode TB est vérifiée par des simulations DFT et GW. Enfin, d'autres matériaux de faible dimension, les nouvelles structures bicouches de nitrure de bore hexagonal torsadées à près de 30° (hBN-TBLs), sont également étudiées dans cette thèse. Les méthodes DFT et TB réalisent les structures électroniques et optiques de nouveaux hBN-TBLs plus loins de 30° afin d'obtenir les paramètres d'ajustement pour le modèle TB. Ces paramètres sont ensuite utilisés pour prédire des hBN-TBL plus proches de 30°, ce qui est difficilement réalisable par DFT.Since its discovery, graphene has become a focal point of extensive research and interest because of its exceptional mechanical, thermal, and electrical characteristics. Nevertheless, the absence of a bandgap in graphene constitutes a barrier to applications in optics, nanoelectronics, and spintronics. Bandgap engineering involving the nanostructuration of graphene has been developed over the years, such as by quantum confinement, to overcome this limitation. This theoretical work is dedicated to the change of electronic, optical, and scanning tunneling microscopy/spectroscopy (STM/STS) responses as a function of system size of new carbon materials like graphene nanomeshes (GNMs), shape/size controlled graphene quantum dots (GQDs) and graphene nanoribbons (GNRs), in order to compare and analyze experimental data. These new carbon materials are theoretically deposited on gold Au(111) surfaces in STM simulation performed using the Non-equilibrium Green's function (NEGF) formalism based on the Fireball DFT method to support the experimental data. Concerning GQDs, we simulate their absorption spectrum using the GW approximation and the Bethe-Salpeter equations (BSE), if possible, to compare directly with the experiment data. Otherwise, their optical properties are achieved by a lower approach, the Tight-Binding (TB) approach. Also, the impacts of aggregations and impurities on their optical responses are explored by studying the twisted bilayer of the GQDs via the TB method. Moreover, the changes in these new carbon materials' electronic properties as a function of their system size are extracted using the TB method. The performance of the TB method is verified by DFT and GW simulations. Finally, other low-dimensional materials, new close-to 30° twisted hexagonal boron nitride bilayer structures (hBN-TBLs), are also studied in this thesis. DFT and TB methods perform the electronic and optical structures of further 30° hBN-TBLs to obtain the fit parameters for the TB model. These parameters are then used to predict closer to 30° hBN-TBLs, which are hardly to be obtained by DFT
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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