1,720,988 research outputs found
Ionothermal Syntheses and Fundamental Properties of Selenido Metalates
No full textChalcogenido (semi)metalate clusters have been in the focus of inorganic chemistry and materials chemistry owing to their structural beauty and a variety of physical and chemical properties. Structurally, the tetrahedral coordination of their metal centers allows for the formation of zeolite-type framework compounds similar to zeolites, making them attractive from syntheses to application aspects. However, given these clusters are usually highly negatively charged, it is challenging to enable the formation of discrete cluster molecules in their solid-state. Understanding the molecular subunits and how they behave in solution has therefore been a hot topic in this field over the last decade. In addition, enriching the structural diversity of the chalcogenido metalate family, whether through variations in composition, connectivity between clusters, or the formation of new cluster types, provides valuable insights into the formation of these compounds and their potential applications.
To access discrete cluster molecules, and thus study their properties in solutions, strategies including a) incorporation of tetra-valent metal cations in the reaction system, b) charge reduction by netural ligands, and c) alkylation of chalcogenido metalate clusters have been employed. In this thesis, a new cluster oligomerization strategy is presented. By oligomerizing [Ge4Se10]4− clusters into dimeric [Ge8Se19]4− and tetrameric [Ge16Se36]8− in their corresponding salts, these compounds were made soluble in DMF owing to a reduced local cation-anion electrostatic interactions; their undecomposed transfer into solutions was evidenced by mass spectrometry. The optical properties and size distribution of particles in solutions were studied by means of UV-visible spectroscopy and dynamic light scattering, respectively. Another salt comprising [Cs@GeII4(GeIV4Se10)4]7− anions was also found soluble in DMF, and adds to the few examples of chalcogenido metalates capable of selectively capturing Cs+ during crystallization, as monitored by mass spectrometry, energy-dispersive X-ray spectroscopy, and single crystal X-ray diffraction. Quantum chemical studies were also carried out to understand its exclusive preference for inclusion of Cs+ compared to the lighter alkali metal ions.
For enriching the structural diversity of chalcogenido metalates, two strategies were introduced in this thesis, a) manipulating the anionic clusters via incorporation of Cu(I) atoms; and b) modifying the structure-directing agents during the formation of new cluster anions. The importance of Cu(I) atoms for tailoring the anionic clusters owing to the diverse coordination modes of Cu+ with chalcogen atoms was highlighted by the isolation of four new compounds. All of them are narrow band-gap semiconductors. In comparison to a copper-free compound, the presence of Cu(I) allowed access to directly manipulate the chalcogenido metalate anions. The modification of the ionic liquid cations led to varied structure-directing agents, which in turn afforded the formation of two new compounds, their anisotropic conductivity along horizontal and vertical directions were also further investigated. Additionally, further structural expansions within the chalcogenido metalate family have been realized upon using different types of ionic liquids, or modifying the precursor salts.
In summary, within this thesis, a new oligomerization strategy towards isolating finite cluster oligomer-based compounds was introduced, thereby improving the solubility of chalcogenido metalates in standard solvents. In addition, the significance of the incorporation of copper ions, of the modification of ionic liquid cations and anions, and of the variation of the precursor salts for expanding the chalcogenido metalate family members was demonstrated. This thesis will help the community to better understand the controllable synthesis of new chalcogenido metalate compounds
From Ternary Zintl Solids to Molecular Nanoarchitectures: Between Match and Mismatch of p-Block Metal Atoms in Multimetallic Zintl Clusters
No full textTransformation Schwerer Chalkogenidometallat-Verbindungen in Ionischen Flüssigkeiten
No full textChalkogenidometallat-Verbindungen zeichnen sich durch eine außerordentliche strukturelle
Vielfalt sowie bemerkenswerte chemische und physikalische Eigenschaften aus. Diese machen
sie zu vielversprechenden Kandidaten für vielfältige Anwendungen. Die Eigenschaften selbst
lassen sich gezielt durch Elementvariation einhergehend mit strukturellen Änderungen
einstellen. Kenntnisse der Bildungsmechanismen liefern die entsprechenden Angriffspunkte.
Die vorliegende Arbeit widmet sich der Synthese und Charakterisierung von schweren
Telluridometallat-Verbindungen. Diese wurden sowohl in Festkörperreaktionen als auch
ionothermal in Imidazolium-basierten ionischen Flüssigkeiten (ILs) — einem etablierten
Ansatz der Arbeitsgruppe DEHNEN — dargestellt. So konnten zahlreiche bisher unbekannte
Verbindungen mit binären und ternären Anionenmotiven dargestellt werden.
Ein erster Schwerpunkt war die Erweiterung der bekannten Kx[CdyChz]-Serie (Ch = S, Se) um
Te-basierte Analoga sowie die Untersuchung des Zusammenhangs zwischen Cd:Te-Verhältnis,
struktureller Dimensionalität (0D–3D) der Anionen und optischer Bandlücke. Darüber hinaus
wurde eine neue Sn-basierte, quaternäre Verbindung synthetisiert, um das Eduktspektrum
ionothermaler Umsetzungen zu erweitern.
Ein weiterer zentraler Aspekt war die Umsetzung Hg-basierter Präkursoren in ILs mit variabler
Alkylkettenlänge und unterschiedlichen funktionellen Gruppen der Imidazoliumkationen. Dies
führte zu einer Vielzahl von Verbindungen mit komplexen Anionenstrukturen — von
Polytellurid-basierten Polymeren und isolierten Molekülen (mit und ohne organische
Substitution) bis hin zu dem bislang größten bekannten Te4-basierten Cluster. Ebenso konnten
Verbindungen mit zwei- und eindimensionalen Anionenstrukturen sowie isolierten Cluster mit
variierenden Untereinheiten und ohne Einbau von Polytellurid-Einheiten dargestellt werden.
Die Variation der Imidazoliumkationen beeinflusst nicht nur die anionische Architektur,
sondern auch die optischen Eigenschaften maßgeblich. Erste Hinweise auf erfolgreiche
ionothermale Umsetzungen zuvor festkörperchemisch synthetisierter Cd-basierter Präkursoren
wurden auch diffraktometrisch und spektroskopisch bestätigt.
Abschließend wurde der synthetische Zugang zu Verbindungen mit ternären Anionenstrukturen
durch gezielte Variation der Präkursoren, ILs und Additive untersucht. Ein Durchbruch gelang
durch die Kombination von Hg- oder Cd-basierten mit einem Sb-basierten Präkursor als Additiv
sowie einem Sn-basierten, quaternären Präkursor: Erstmals konnten unter ionothermalen
Bedingungen somit Verbindungen mit ternären Telluridometallaten dargestellt und strukturell
sowie spektroskopisch charakterisiert werden.
Die gezielte Auswahl und Kombination von Reaktanden, Imidazoliumkationen und Additiven
ist ein wirkungsvolles Instrument zur Synthese von (derivatisierbaren) Telluridometallat-
Verbindungen mit hoher struktureller Diversität. Die ermittelten Struktur-Eigenschafts-
Beziehungen, insbesondere die Korrelation zwischen Dimensionalität und optischer
Bandlücke, eröffnen neue Perspektiven für potenzielle optoelektronische Anwendungen
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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