1,721,080 research outputs found
Formation and dynamics of small polarons on the rutile TiO2 (110) surface
No full textCharge trapping and the formation of polarons is a pervasive phenomenon in transition-metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principles techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted for in terms of a few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular-dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and a different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, due to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations, the calculated data are in excellent agreement with observations, thus validating the overall interpretation
Interplay between Adsorbates and Polarons: CO on Rutile TiO2 (110)
No full textPolaron formation plays a major role in determining the structural, electrical, and chemical properties of ionic crystals. Using a combination of first-principles calculations, scanning tunneling microscopy, and atomic force microscopy, we analyze the interaction of polarons with CO molecules adsorbed on the reduced rutile TiO2(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominate: either inside an oxygen vacancy, or at surface Ti5c sites, coupled with a surface polaron. Similar conclusions are predicted for TiO2(110) surfaces containing near-surface Ti interstitials. These results show that polarons are of primary importance for understanding the performance of polar semiconductors and transition metal oxides in catalysis and energy-related applications
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
Complex oxide surfaces studied at the atomic scale by noncontact AFM/STM and DFT
Get PDFDer größte Teil der festen Materie auf der Erde kommt natürlich in Form von Oxiden vor, die auch den größten Teil der Erdkruste ausmachen. Die meisten Elemente des Periodensystems kommen in der Natur in oxidischer Form vor, und die Gewinnung reiner Elemente erfolgt in der Regel durch chemische Reduktion des entsprechenden Oxids. Obwohl diese Materialien sehr viel komplexer sind als reine elementare Feststoffe, ist ihre weit verbreitete Verfügbarkeit und ihre Stabilität unter atmosphärischen Bedingungen der Grund für das große Forschungsinteresse an ihren Materialeigenschaften. Bei vielen Oxiden wurde bereits gezeigt, dass sie eine Fülle von faszinierenden physikalischen Phänomenen aufweisen. Es ist anzunehmen, dass in den kommenden Jahren die technische Anwendung von Oxiden in wohlkontrollierten Prozessen wie der hetero-/homogenen Katalyse und der Elektronik auf der Nanometer- oder Subnanometerskala in Umfang noch zunehmen werden. Jedes Material interagiert mit seiner Umgebung über seine exponierten Facetten - die Oberflächen, an denen Feststoffe für verschiedene Anwendungen zur Verfügung stehen. Die Oberflächenwissenschaften setzen eine Vielzahl von experimentellen und theoretischen Techniken ein, um Prozesse zu untersuchen, die an verschiedenen Facetten unterschiedlicher Festkörper ablaufen. In den letzten Jahren wurde die Untersuchung von Oberflächen mit atomarer Präzision möglich, was die Beobachtung grundlegender Phänomene auf atomarer Ebene erlaubt. In dieser Arbeit wird ein oberflächenwissenschaftlicher Ansatz zur Untersuchung verschiedener Oxidoberflächen unter gut kontrollierten Bedingungen, meist Ultrahochvakuum (UHV) und niedrigen Temperaturen, verwendet. Die eingehende Untersuchung von Eigenschaften und Mechanismen unter diesen Bedingungen bildet die Basis für das Verständnis und die Vorhersage des Verhaltens von Oberflächen unter weniger kontrollierten Umgebungen. Im Rahmen der Forschungsarbeiten, die zu dieser Dissertation geführt haben, wurden die Oberflächen mehrerer komplexer Oxide untersucht, wobei der Schwerpunkt auf zwei Modelloxiden lag: Rutil TiO2 und PerowskitSrTiO3. In Kapitel 1 werden die in dieser Arbeit verwendeten Techniken der Oberflächenphysik vorgestellt. Der Schwerpunkt liegt dabei auf den experimentellen Techniken non-contact Atomic Force Microscopy (nc-AFM) und Scanning Tunneling Microscopy (STM), die es ermöglichen, Oberflächen mit einer Auflösung von einemeinzigen Atom abzubilden, sowie auf dem theoretischen Ansatz der Dichtefunktionaltheorie (DFT), der die quantenmechanischen Eigenschaften von Oberflächenatomen und ihre Wechselwirkungen mit anderen Atomen realistisch modelliert. Die vorgestellten Techniken wurden in der Regel gemeinsam angewendet, um ergänzende Informationen über ein bestimmt physikalisches System zu erhalten. Kapitel 2 befasst sich mit der Wechselwirkung einer Modelloberflache eines binaren Übergangsmetalloxids, der TiO2(110)-Oberflache, mit kleinen Gasmolekülen mit einer Auflösung von einem einzigen Atom oder einem einzigen elektronischen Orbital. Der polaronische Grundzustand des reduzierten TiO2(110) wird vorgestellt, der sich mit Hilfe von AFM/STM und DFT in Gegenwart einzelner Moleküle verändert. Der platonische Grundzustand wird durch die Anwesenheit von schwach adsorbiertem CO geringfügig verändert, während er in Anwesenheit von stark adsorbiertem O2, welches die polaronischen Elektronen zur Bildung stärker chemischer Bindungen mit der darunter liegenden Oberflache nutzt, vollständig verschwindet. Diese Ergebnisse beleuchten und klaren die seit langem untersuchten fundamentalen Prozesse, die an dieser technologisch relevanten Oberflache ablaufen. Kapitel 3 konzentriert sich auf die Modelloberfläche eines Perowskit-Oxids mit Bulk-Terminierung, auf das SrTiO3(001). Die Oberflache wurde durch die Spaltung von Einkristallproben in situ (also unter UHV Bedingungen) dargestellt. Es wird gezeigt, dass solche Oberflächen mit Bulk-Terminierung auf atomarer Ebene der idealen Oberflache am nächsten kommen, und dass sie durch einen durch elastische Spannung unterstutzten ferroelektrischen Übergang während der Spaltung erzeugt wurden. Ein neuartige Spaltverfahren wird beschrieben und systematisch untersucht, und es zeigt sich, dass die gespaltenen Oberflächen als Folgeihrer Bulk-Terminierung mehrere faszinierende Phänomene aufweisen. Die TiO2-terminierte Oberflächen von SrTiO3(001) erfahrt während der Spaltung einen Isolator-Metall-Übergang, während die SrO-Terminierung halbleitend bleibt. Die beiden Terminierungen wechseln sich der Oberflache ab und schalten an den Domänenwänden der Polarisierung um. Je nach Beigabe einer externen Dotierung (Niobium) weisen die gespaltenen Oberflächen unterschiedliche Morphologien auf, wobei die Größe der beiden unterschiedlichen Oberflächenabschlüsse von der Nanometer- bis zur Mikrometer-/Millimeterskala reicht. Die gebrochene Symmetrie der gespaltenen Oberflächen ist nachweislich für die Ladungstrennung unter UV-Beleuchtung verantwortlich, ein Phänomen, das ansonsten durch die kubische Symmetrie des Systems verhindert wird. Diese Ergebnisse zeigen einen Weg auf,SrTiO3(001)-Oberflächen mit faszinierenden und vielversprechenden Eigenschaften zu erzeugen und zu nutzen, und ebnen den Weg für ihre Anwendung in neuen Bereichen.The largest portion of solid-state matter on Earth naturally appears in the form of oxides, which constitute the majority of its crust. Most elements in the periodic table are found throughout nature in an oxide form, and the extraction of pure elements usually proceeds through reduction from their corresponding oxide. Although much more complex than pure elemental solids, their widespread availability and stability in atmospheric conditions is responsible for a vast research interest in their bulk, powder, and surface properties. Many oxides were already shown to exhibit a plethora of intriguing physical phenomena at a wide range of thermodynamic and electromagnetic conditions. In the upcoming years, the technical application of oxides in well-controlled processes, such as hetero/homogeneous catalysis and electronics at the nanometer or sub-nanometer scale, are projected to increase in both magnitude and scope. Each material interacts with its environment through its exposed facets - the surfaces, where the solids are available for various applications. Surface Science employs a variety of experimental and theoretical techniques to investigate processes occurring at the different facets of solids. In recent years, the study of surfaces has achieved atomic precision, allowing observations of fundamental phenomena occurring at the atomic level. In this Thesis, a Surface Science approach is employed to studying several oxide surfaces in well-controlled conditions such as the ultrahigh vacuum (UHV) and low temperatures. Revealing fundamental properties and mechanisms under these conditions constitutes a strong foundation for understanding and predicting the behavior of surfaces in less controlled environments. In the research that led to this Thesis, surfaces of several complex oxide surfaces were investigated, with the largest focus on two model oxides: rutile TiO2 and the SrTiO3.Chapter 1 introduces the Surface Physics techniques used in this Thesis. The main emphasis is put on the experimental non-contact Atomic Force Microscopy (nc-AFM) and Scanning Tunneling Microscopy (STM) techniques that readily image surfaces with a resolution down to a single atom, and the theoretical Density Functional Theory (DFT) approach that realistically models quantum mechanical properties of surface atoms and their interactions. These techniques were commonly applied together, to provide complementary information regarding a given physical system. Chapter 2 focuses on the interaction of a model transition-metal binary oxide surface, the TiO2(110) surface, with small gas molecules, with a resolution of a single atom or a single electronic orbital. The polaronic ground state of the reduced TiO2(110) is introduced, which is shown using AFM/STM and DFT to change in the presence of adsorbed molecules. The polaronic ground state is slightly altered by the presence of weakly adsorbed CO, while it completely vanishes in the presence of strongly adsorbed O2, which uses the polaronic electrons to form strong chemical bonds to the underlying surface. These results illuminate and resolve many long-standing questions about the fundamental processes occurring at this technologically-relevant surface. Chapter 3 focuses on the bulk-terminated model perovskite oxide surface, SrTiO3(001). The surface was created by cleaving single crystal samples in situ, i.e., in UHV. Such bulk-terminated surfaces are shown to be the closest to pristine at the atomic level, and they were obtained through a strain-assisted ferroelectric transition during cleaving. A novel cleaving procedure is introduced and systematically studied, and the cleaved surfaces are shown to exhibit several intriguing phenomena as a consequence of their bulk-termination. TheTiO2 termination of SrTiO3(001) undergoes an insulator-to-metal transition during cleaving, while the SrO termination remains semiconducting. The two terminations are laterally alternating at the surface, and switch at polariziation domain walls. Depending on the amount of external doping (with niobium), cleaved surfaces exhibit different morphologies, with the size of two different surface terminations ranging from nanometer to micromater/millimeter scale. The broken symmetry of the cleaved surfaces is shown to be responsible for charge separation under UV illumination, a phenomenon otherwise forbidden by the cubic symmetry of the system. These results demonstrate a way to create and utilize SrTiO3(001) surfaces with intriguing and promising properties, and the pave way to their applications in many different fields previously unrelated to these surfaces. Chapter 4 demonstrates an advancement in the comparison between theoretically and experimentally obtained results on surfaces at the atomic level through simulations of the AFM contrast using DFT. It is shown that realistic AFM images can be simulated despite a series of approximations, and that chemical sensitivity can be pertained - a central advantage of the AFM technique
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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