1,720,974 research outputs found
Synthesis, magnetism and reactivity of graphene nanoribbons
Get PDF294 p.En esta tesis profundizaremos se enfoca desde un ángulo fundamental la ciencia de las cintas de grafenoPrimero se definirán sus propiedades basándose en ideas bien fundamentadas como la aromaticidad yla topología. Una vez entrados en materia nos centraremos en los 5-armchair y las 3,1-quirales. Se mues-tra un estudio sobre el crecimiento de los primeros que dio lugar al descubrimiento de un nuevo meca-mismo de polimerización. A continuación se estudian sus propiedades para demostrar cómo los 5-aGNR(por sus siglas en inglés) son materiales topológicos que presentan electrones desapareados con actividadMagnética en sus extremos. Este magnetismo está relacionado con una alta reactividad que también se halló en los 3,1-quirales, a pesar de que estos últimos no presentan magnetismo. Se observó una reactividadextrema frente al agua y el oxígeno. La degradación frente a estos elementos generó radicales magnéticoscon capacidad para acoplarse, lo cual presentó una oportunidad de cuantificar y modelizar este acopla-miento en diferentes configuraciones. Por último, se desarrollaron dos estrategias exitosas a la hora deproteger las cintas del ataque ambiental, las cuales pueden ser extrapoladas a otros derivados del grafenoque muestren la recién descubierta reactividad, lo cual facilitaría su transferencia a la industria
Addressing the instability of zigzag graphene nanoribbon edges
No full textResumen del trabajo presentado al Transpirenian Nanotechnology Workshop (TrNano), celebrado online del 7 al 9 de abril de 2021.Graphene nanoribbons (GNR) zigzag edges are closely related with the hosting of spin-polarized states. However, the unpaired π-orbitals hosting the uncompensated spin, whilst being one of their most interesting characteristics, may also be their weakest point when it comes to surviving in ambient conditions. In this study, we have performed STM/AFM experiments that demonstrate the characteristic reactivity of zigzag edges, even for ribbons with a limited radicalary character. Their exposure to ambient conditions rendered oxidized ribbons with dramatically affected electronic properties. Further experiments were performed with reference oxidizing and reducing agents at low pressures as idealized models of the GNR degradation. Different chemical structures are proposed for these defects, supported by HR-STM measurements and theoretical calculations.Peer reviewe
Síntesis de hidrosilanos funcionalizados. Estudio de su actividad en hidrosililación de CO2
Get PDFEn los últimos cinco años, el conocimiento sobre los procesos de hidrosililación catalítica de CO2 se ha desarrollado significativamente. La contribución de nuestro equipo de investigación a dicho campo se ha centrado en el diseño de sistemas catalíticos basados en complejos de iridio con ligandos monoanionicos tridentados del tipo NSiN. En particular, el complejo [Ir(H)(RFCO2)(NSiN)(coe)] (1) (coe = cis-cicloocteno; NSiN = fac-bis(4-metilpiridin-2-iloxi)metilsililo; RFCO2 = CF3(CF2)2CO2, heptafluorobutirato) ha demostrado ser un precursor catalítico de gran actividad y selectividad en procesos de hidrosililación de CO2. En el presente trabajo, se han estudiado procesos de hidrosililación de CO2 catalizados por el compuesto 1, en los que el silano contiene sustituyentes con grupos NMe2. En primer lugar, se realizó la síntesis y caracterización de dos nuevos silanos, HSiMe2(C6H5NMe2) y HSiMe(C6H5NMe2)2. A continuación, se realizaron estudios, mediante RMN de 1H y 13C, de la reacción de dichos silanos con CO2 (y 13CO2) en presencia del compuesto 1 como precursor catalítico. Se ha observado que la naturaleza del silano tiene un clara influencia sobre la actividad y selectividad en este tipo de procesos, siendo el compuesto HSiMe2(C6H5NMe2) mucho más activo que la especie HSiMe(C6H5NMe2)2. Además, se ha podido concluir que la presencia del grupo amino inhibe la reactividad del silano resultante en comparación con lo observado para HSiMe2Ph y favorece procesos de hidrólisis
Control de las propiedades electrónicas de nanotiras quirales de grafeno mediante la superficie de soporte
No full textGraphene is a material of great versatility. For instance, the sp2 covalent bonds linking the C atoms in extensive 2D sheets confers great mechanical strength3. Moreover, it exhibits huge electronic mobility and sensitivity of conductivity to external parameters, which makes it interesting towards microelectronics, as well as in the energy sector and data storage. All these properties add to the prospect of numerous applications in the field of materials science. Notably, the electronic properties of graphene are tunable8. When graphene shrinks in one of its dimensions to form ribbons, its conductivity decreases and transforms from metallic to semiconductor below a critical width9, while still preserving the appealing properties of its extended form. Thus, it is of interest to the field of molecular electronics in order to build nanotransistors. The conductivity dependence of graphene nanoribbons on the width has a characteristic length scale in the range of Angstroms8. Therefore, an extremely precise manufacturing method is required. On Surface synthesis is a bottom-up approach that allows graphene manufacturing with atomic precision featuring exact width and edge geometry10. This is nowadays far better than the top-down methods, whose size limit is currently on the scale of tens of nanometers11 and are prone to produce spurious effects due to sample damage. Nanotechnology with such precision requires cutting-edge experimental infrastructures. Based on state of the art Scanning Tunnelling Microscope (STM) of the Laboratorio de Microscopías Avanzadas (LMA), the electronic structure of the different parts of the nanoribbons has been locally studied, facilitating a deeper comprehension of the quantum nature responsible for their superior properties. The aim of this work seeks the experimental accomplishment of these points: - Synthesize chiral (3,1)-GNR from the selected precursor on Au(111). - Study their morphology and electronic structure, focusing on the identification of expected edge states. - To develop new methods to decouple the GNRs from the substrate by addition of NaCl and determine its effectiveness. Decoupling the molecular structures would be used both to study the ribbons with higher resolution, and to release them from the substrate. A new type of chiral nanoribbon, the (3,1)-cGNR, has been successfully synthesised from dBQA precursors. This molecular precursors, the subsequent polimerization steps and the nanoribbons were structurally confirmed by topographic STM images. Specifically, the period and internal structures have been confirmed using topography imaging with normal metallic tips and functionalized ones, which verified to match the theoretical calculations. Furthermore, its electronic properties have been successfully studied, and the higher part of its valence ban, as well as the presence of edge states in the vicinity of the Fermi level have been confirmed. In addition, the NaCl growth and response to temperature after deposition onto Au(111) showed different density agglomeration into islands. This effect, in presence of cGNR caused a generalized displacement of the molecular structures, affecting its orientation with respect to the substrate and causing some of them to stack on top of other cGNRs. This resulted in an effective ribbon decoupling from the metallic substrate, which increased the energy and spatial resolution when studying the ribbons, both on the STM images and the STS spectra. This allowed us to investigate its internal electronic structure without the need for functionalising the tip. Indeed, the performance of a regular metal tip on stacked ribbons parallels the one of functionalized tips on a metal surface, making the burdensome process of tip functionalization unnecessary. In the STS spectra, the decoupling caused a sharpening of the bands that allows its proper identification. As outlook, we propose to use this decoupling method as a simple technique to study the electronic structure of the edge states in cGNR, or as a tool to spontaneously detach the ribbons from the substrate for a possible transferring method to Si-based substrates GNRs on a larger scale, bridging the gap between on-surface synthesis and electronic application
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
Emergence of π-magnetism in fused aza-triangulenes: Symmetry and charge transfer effects
No full textOn-surface synthesis has paved the way toward the fabrication and characterization of conjugated carbon-based molecular materials that exhibit π-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative, was recently shown to display rich on-surface chemistry, offering an ideal platform to investigate structure–property relations regarding spin-selective charge transfer and magnetic fingerprints. Herein, we study electronic changes upon fusion of single molecules into larger dimeric derivatives. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell dimers covalently coupled through sterically accessible carbon atoms. Meanwhile, its open-shell structure on Au(111) leads to coupling via atoms displaying a high spin density, resulting in symmetric or asymmetric products. Interestingly, whereas all dimers on Au(111) exhibit similar charge transfer properties, only asymmetric ones show magnetic fingerprints due to spin-selective charge transfer. These results expose clear relationships among molecular symmetry, charge transfer, and spin states of π-conjugated carbon-based nanostructures.This work was funded by the Spanish MCIN/AEI/10.13039/501100011033 (PID2019-107338RB-C62, PID2019-107338RB-C63, PID2019-109555GB-I00, PID2022-140845OB-C64, PID2022-140845OB-C65) and the European Union “NextGenerationEU”/PRTR (TED2021-132388B-C42, TED2021-132388B–C43), by the Basque Government (IT1591-22) and by the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019–2022, ED431G2019/03) European Regional Development Fund (ERDF). F.A.-G. thanks Ministerio de Universidades, Plan de Recuperación, Transformación y Resiliencia and UAM for the Programa de Recualificacion (CA5/RSUE/2022-00234).Peer reviewe
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
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