1,720,974 research outputs found
Designing All Graphdiyne Materials as Graphene Derivatives: Topologically Driven Modulation of Electronic Properties
Full text linkDesigning new 2D systems with tunable properties is an important subject for science and technology. Starting from graphene, we developed an algorithm to systematically generate 2D carbon crystals belonging to the family of graphdiynes (GDYs) and having different structures and sp/sp2 carbon ratios. We analyze how structural and topological effects can tune the relative stability and the electronic behavior, to propose a rationale for the development of new systems with tailored properties. A total of 26 structures have been generated, including the already known polymorphs such as α-, β-, and γ-GDY. Periodic density functional theory calculations have been employed to optimize the 2D crystal structures and to compute the total energy, the band structure, and the density of states. Relative energies with respect to graphene have been found to increase when the values of the carbon sp/sp2 ratio increase, following however different trends based on the peculiar topologies present in the crystals. These topologies also influence the band structure, giving rise to semiconductors with a finite band gap, zero-gap semiconductors displaying Dirac cones, or metallic systems. The different trends allow identifying some topological effects as possible guidelines in the design of new 2D carbon materials beyond graphene
An Unusual Three-Dimensional Coordination Network Formed by Parallel Polycatenation of Two-Fold Interpenetrated (6,3) Layers Based on a Novel Three-Connecting Ligand
No full textThe novel tridentate ligand 1,3,5-tris(4-cyano-phenoxymethyl)-2,4,6- trimethylbenzene (1) together with silver-(I) triflate has afforded the interesting coordination polymer [Ag(1)(CF3SO3)]·0. 5H2O, containing undulated (6,3) layers of hexagonal meshes, with alternating Ag and 1 triconnected centers, entangled into 2-fold parallely interpenetrated sheets, which, furthermore, are polycatenated to generate an unique three-dimensional array
X-ray Powder Diffraction as a Tool for Facing Twins: The Case of the Monoclinic NbCoTe2 and TaCoTe2 Phases
No full textTaCoTe2 has been synthesized from the elements at 900 °C in evacuated silica tubes; the crystals of TaCoTe2 are monoclinic, space group P21/c (No. 14), with a = 8.1524(6) Å, b = 6.2649(4) Å, c = 7.7945(5) Å, β = 116.789(4)°, and Z = 4, and appear to be heavily twinned about the (100) plane. Therefore, powder diffraction data and Rietveld refinement were used to assess the correct stereochemistry of the atoms of this new compound. In addition, the already reported NbCoTe2 compound has been found to be, in the bulk, monoclinic [a = 8.1612(4) Å, b = 6.2976(3) Å, c = 7.8518(4) Å, β = 117.487(3)°], rather than orthorhombic, and isostructural to TaCoTe2. The stereochemical interpretation of this change of crystal symmetry is given in terms of a significant asymmetrical bending of one tellurium atom toward one Nb (or Ta) atom, making these metals five-, rather than six-, coordinated. © 1993, American Chemical Society. All rights reserved
The different story of π bonds
Full text linkWe revisit “classical” issues in multiply bonded systems between main groups elements, namely the structural distortions that may occur at the multiple bonds and that lead, e.g., to trans-bent and bond-length alternated structures. The focus is on the role that orbital hybridization and electron correlation play in this context, here analyzed with the help of simple models for σ-and π-bonds, numerically exact solutions of Hubbard Hamiltonians and first principles (density functional theory) investigations of an extended set of systems
Progettazione di nuovi leganti polifunzionali per la costruzione di networks di coordinazione.
No full textREACTIONS OF THE NITRIDOCARBONYL CLUSTER ANION [Rh6N(CO)15]- WITH STRONG BASES: SYNTHESIS AND CRYSTAL STRUCTURE OF THE HYDRIDIC DIANION [Rh6(μ-H)N(CO)14]2-
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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
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
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