1,721,372 research outputs found
Atomic-scale studies of oxidized Pd and Rh surfaces
No full textPd und Rh Oberflächen werden in der heterogenen Katalyse verwendet. Da oxidierte Oberflächen bessere katalytische Eigenschaften als Reinmetalloberflächen aufweisen können, befasst sich diese Arbeit mit der Oxidation dieser beiden Metalle. Diese Untersuchungen erfolgten durch eine Kombination oberflächensensitiver experimenteller (Rastertunnelmikroskopie, niederenergetische Elektronenbeugung, Photoelektronenspektroskopie) und theoretischer Methoden (Dichtefunktionaltheorie). Weiters wurde durch Reduktion mit Wasserstoff und CO die Reaktivität von ultradünnen Oxidschichten mit der einfacher Sauerstoffadsorbatstrukturen verglichen. Unter idealisierten Bedingungung (Verwendung niedrigindizierter Oberflächen) konnte die Struktur zahlreicher ultradünner Oberflächenoxide gelöst und die Reduktionskinetik detailiert beschrieben werden. Durch die Verwendung gestufter Oberflächen wurde ein erster Schritt hin zu höherer Komplexität unternommen, ohne unsere Möglichkeiten der Strukturbestimmung auf atomarem Maßstab einzuschränken.This work was motivated by the importance of Pd and Rh surfaces in catalytic reactions. As oxidized surfaces can exceed the metal in catalytic activity, the study will concentrate on the oxidation of Pd and Rh and further compare the reactivity of surface oxides with that of simple oxygen adsorption structures, using hydrogen and CO for the reduction. This study combines results from surface-sensitive experimental methods (scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy) with density functional theory calculations to allow structural determination on the atomic scale.Under idealized conditions, i.e. low-index surfaces, detailed knowledge on ultrathin surface oxide structures is obtained and the reduction kinetics are well understood. The first step in bridging the materials gap by moving from low-index to vicinal surfaces does not limit our ability of structure determination on the atomic scale
Self-assembled metal clusters on an alumina nanomesh
No full textSelbstorganisiertes Wachstum von Metallen auf geeigneten Oberflächen wurde auf Grund der bemerkenswerten katalytischen und magnetischen Eigenschaften der so erzeugten Nanopartikeln in den letzten Jahren sehr intensiv untersucht. Insbesondere die Möglichkeit, Nanopartikel mit wohl definierter Größe und Struktur, die die oben genannten Eigenschaften maßgeblich beeinflussen, zu erzeugen ist dabei vorteilhaft.Wird die Ni3Al(111) Oberfläche bei erhöhter Temperatur Sauerstoff ausgesetzt, bildet sich ein dünner Oxidfilm, der eine wohl definierte Struktur und einheitliche Dicke aufweist und die Legierungsoberfläche vollständig bedeckt. Mittels Untersuchung durch Rastertunnelelektronenmikroskopie kombiniert mit Dichtefunktionaltheorie-Berechnungen konnte die Struktur des Oxidfilms bestimmt werden. Das Oxid ist ca. 0.5 nm dick und weist eine sechszählige Symmetrie auf. An den Ecken der (Sqrt(67) x Sqrt(67))R12.2° Einheitszelle befinden sich Löcher mit einem Durchmesser von ca. 400 pm, die bis hinunter zur metallischen Substratoberfläche reichen. Die Kantenlänge der Zelle ist 4.1 nm. Die treibende Kraft für die Bildung dieses speziellen Nano-Netzes ist die Reduktion der Energie des Metall/Oxid Interface durch die Bildung von energetisch günstigeren Al-Ni Bindungen statt Al-Al Bindungen.Wegen der besseren Benetzung von Metallen auf Metalloberflächen gegenüber Oxidoberflächen, nukleieren Metallatome bevorzugt am Substrat in den Löchern statt an der Oxidoberfläche. Daher stellt das Oxid eine ideale Oberfläche für das Wachstum von selbst organisierten Metallnanopartikeln mit einem hexagonalen 4.1 nm Gitter dar. Trotzdem wachsen die meisten Metallnanopartikel jedoch nicht auf den Löchern des Oxids. Aufgrund einer Diffusionsbarriere können zum Beispiel Fe und Co nicht in die Löcher diffundieren und nukleieren daher an ihrem energetisch zweitgünstigsten Platz. Pd Atome werden jedoch in den Löchern aufgefangen und reduzieren dadurch die Diffusionsbarriere für Fe und Co Atome. Somit bilden die Pd-gefüllten Löcher einen Nukleationsplatz für Feund Co-Nanopartikel, sodass diese eine durch das Oxid vorgegebene hexagonale Anordnung aufweisen. Diese Fe- und Co-Nanopartikel wurden hauptsächlich mit Hilfe von STM und Oberflächen-Röntgenbeugung in Hinblick auf ihre Morphologie und kristallographische Eigenschaften untersucht. Fe-Nanopartikel wachsen abhängig von der Depositionstemperatur mit {100} oder {110} Orientierung auf, während Co-Nanopartikel dicht gepackte Lagen [fcc (111) bzw. hcp (0001)] mit zufälliger Stapelreihenfolge parallel zur Oxidoberfäche aufweisen. Pd-Nanopartikel wachsen in {111} Orientierung auf dem Oxid.Der Kontaktwinkel der Nanopartikel wurde durch STM Messungen bestimmt.Dieser beträgt ca. 75° bei Co-Nanopartikeln und ca. 80° bei Fe- Nanopartikeln bei einer Substrattemperatur von 470 K. Bei steigender Substrattemperatur steigt auch der Kontaktwinkel, d.h. die Nanopartikel befinden sich nicht im thermodynamischen Gleichgewicht. Die Größe der Nanopartikel auf einer idealen Oxidoberfläche ist auf ca. 1000 Atome pro Nanopartikel begrenzt.Die magnetischen Eigenschaften der Nanopartikel wurde mit Hilfe des zirkularen magnetischen Röntgendichroismus und des magnetooptischen Kerr Effekt an Oberflächen Messungen untersucht. Durch Anpassen der berechneten Magnetisierungskurven an XMCD-Messdaten konnten die magnetischen Eigenschaften der Nanopartikel bestimmt werden. Die magnetische Anisotropie-Energie der Fe-Nanopartikel ist höher als diejenige der bcc Volumenphase von Fe. Co-Nanopartikel haben im Gegensatz dazu eine geringere Anisotropie-Energie als hcp Co. Das ist durch die zufällige Stapelung von dichtgepackten Co Lagen erklärt. Die leichten Magnetisierungsrichtungen der Fe und Co Nanopartikel stehen senkrecht zur Oberfläche. Um die Messdaten vollständig durch das Modell beschreiben zu können, müssen zwei Spezies von Nanopartikel mit unterschiedlicher Kopplung an das Substrat angenommen werden; Nanopartikel die starke antiferromagnetische Kopplung aufweisen und Nanopartikel die vorwiegend ferromagnetisch gekoppelt sind und einen erheblichen biquadratischen Anteil zur Kopplungsenergie aufweisen.Daraus ergeben sich, passend zu den Messdaten, zwei unterschiedlich gekoppelte Spezies von Nanopartikeln: Nanopartikel die am reinen Oxid nukleiert sind und daher der stark antiferromagnetisch gekoppelten Spezies entsprechen und Nanopartikel auf den Atomen in den Löchern deren Kopplungsenergie sich aus der Summe der Kopplung über die Atome in den Löchern und des Oxids ergibt. Die Kopplung dieser zweiten Spezies ist aufgrund der teilweisen Aufhebung der antiferromagnetischen (reines Oxid) und ferromagnetischen Kopplung (Atome in den Löchern) schwach ferromagnetisch. Aufgrund der Variation der Kopplungsenergie an der Grenzfläche dieser Nanopartikel zum Substrat weist diese Spezies eine erhebliche biquadratische Kopplungsenergie auf.Template mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions.When the Ni3Al(111) surface is exposed to oxygen at elevated temperature a thin oxide film with a well-defined structure and uniform thickness grows and covers the alloy surface completely. The structure of the alumina film has been solved mainly by the help of scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations. The structure of the approx. 0.5 nm thick oxide film has sixfold symmetry and exhibits holes with a diameter of approx. 400 pm reaching down to the metal substrate at the corners of the (Sqrt(67) x Sqrt(67))R12.2° unit cell. The side length of the unit cell is 4.1 nm.The driving force for the formation of the oxide nanomesh is the reduction of the metal/oxide interface energy by the formation of energetically favorable Al-Ni bonds at the interface.Due to better wetting of metal on metal surfaces than on oxide surfaces, metal atoms prefer to bind to the substrate in the hole, not to the oxide. Therefore the oxide forms a template with a hexagonal 4.1 nm lattice for the growth of well-ordered metal clusters. Nevertheless, the growth of most metal clusters on top of the corner holes is not straightforward. Fe and Co atoms cannot jump into the corner holes due to a barrier for diffusion and nucleate at their second favorable adsorption site. However, Pd atoms trapped in these corner holes reduce the barrier for diffusion and create metallic nucleation sites where Fe as well as Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh.We have studied these Fe and Co clusters and applied different methods like STM and surface x-ray diffraction (SXRD) to determine the morphology and crystallography of the clusters. For Fe we found cluster growth with either bcc{110} or bcc{100} orientation, depending on the substrate temperature, and for Co we found random stacking of close-packed planes [fcc (111) and hcp (0001), respectively] on top of the clusters. Pd clusters grow with fcc{111} orientation. The contact angle of the clusters was derived from the measurements; at a deposition temperature of 470 K the contact angle of Co clusters is approx. 75° and for Fe clusters approx. 80°. With increasing deposition temperature the contact angle increases, i.e., the clusters are not in thermodynamic equilibrium. The size of the clusters grown on top of an ideal defect-free oxide is limited to approx. 1000 atoms/cluster. For larger clusters coalescence happens and a continuous film forms.The magnetic properties of the clusters and the Ni3Al(111) substrate have been studied by means of x-ray magnetic circular dichroism (XMCD) and surface magneto-optic Kerr effect (SMOKE). SMOKE measurements show that the Curie temperature of the substrate surface highly depends on the stoichiometry and thereby on the preparation history of the sample.By fitting calculated magnetization curves to the data measured by XMCD the magnetic properties of the clusters could be determined.The anisotropy of Co clusters is less than for hcp bulk Co. This is probably a consequence of random stacking of close-packed Co planes. The anisotropy of Fe clusters is enhanced compared to bulk bcc Fe, as expected for nanoparticles. The easy axis of the clusters is perpendicular to the surface.In order to describe the experimental data by the model two types of clusters with different coupling to the substrate have to be taken into account: clusters with strong AF coupling and predominantly FM coupled clusters which also show a considerable biquadratic contribution to the coupling energy. Basic considerations show that the atoms inside the corner holes mediate FM coupling of the clusters to the substrate. Most probably the coupling energy depends on the atoms inside the holes, which can be filled not only by Pd atoms but also Co atoms. The oxide mediates AF coupling. Therefore, we find two different cluster species in agreement to our measured data: clusters nucleated on top of the clean oxide which correspond to the strong AF coupled species and clusters nucleated on top of the corner holes with a coupling energy that corresponds to the sum of the coupling energies mediated by the oxide and the atoms inside the holes. The second species is predominately FM and only weakly coupled due to partial cancellation of FM and AF coupling mediated by the atoms inside holes and the oxide, respectively. These clusters also show considerable biquadratic coupling which we explain by the variations of the coupling energy at the cluster/substrate interface.<br /
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
In situ Röntgenstreuuntersuchungen der Elektrooxidation von Platin Einkristallelektroden
Full text linkPt electrooxidation and reduction is linked to the degradation of Pt-based electrocatalysts for electrochemical energy conversion. The atomic-scale mechanisms of Pt electrooxidation, dissolution and restructuring of Pt single crystal electrodes are studied with surface X-ray scattering techniques. High energy surface X-ray diffraction (HESXRD) was employed for the first time to study the atomic-scale structure of single crystal surfaces in electrochemical environment. A study of the initial electrooxidation of Pt(111) and Pt(100) using HESXRD revealed different surface stability of both electrodes. This difference was explained on the basis of the differing location of the extracted Pt atoms in the oxides. A specific stripe oxide is formed on Pt(100), which produces unstable atoms at the stripe ends. At higher potentials a second amorphous Pt (hydr-)oxide was found, which is located further apart from the surface. Pt dissolution during oxide formation was attributed to the nucleation & growth of the stripe oxide, while dissolution during oxide reduction is linked to the Pt (hydr-)oxide. Fast simultaneous measurements of the electrochemical charge transfer and the coverage of extracted Pt atoms on Pt(111) revealed that Pt extraction is a fast, potential-driven process, whereas formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. The formation of nanostructures upon repeated oxidation and reduction was studied with grazing incidence small angle X-ray scattering. The structural evolution of the lateral and vertical nanostructure size is similar on Pt(111), Pt(100) and Pt(110). Rapid nanostructure steepening was followed by a growth mode where the ratio of vertical and horizontal feature size remains constant. This was attributed to a dynamic equilibrium of up- and downhill currents of Pt (hydr-)oxide atoms, which are detached from a step edge oxide
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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