1,720,995 research outputs found
A two-body Green function technique for calculating differential cross sections for double photoionizations of atoms and molecules
No full textStructure and X-ray spectrum of crystalline poly(3-hexylthiophene) from DFT-van der Waals calculations
No full textThe minimum-energy geometrical structure of the regioregular head-to-tail poly(3-hexylthiophene) (rr-HT-P3HT) polymer has been addressed by means of density functional theory (DFT) calculations which include long-range (van der Waals) interactions. The problem of the P3HT structure has been debated in the literature in the last decades mainly for what concerns the arrangement of the alkyl side chains of the polymer and the type and content of the crystalline primitive cell. The main result of our calculations is that the energetically favored structure of the crystalline polymer at T = 0 K corresponds to polythiophene chains with slightly (∼16°) non co-planar rings and a fishbone arrangement of tilted alkyl side chains with complex internal structure. The alkyl side chains are negligibly interdigitated with those of the adjacent polymer layers; moreover the five terminal carbon atoms of each alkyl side chain are co-planar in all-trans staggered conformation. The optimized geometrical structure obtained for the rr-HT-P3HT polymer is in agreement with measured X-ray spectra of high molecular weight P3HT crystalline samples, and confirms that two non-equivalent polymer chains, mutually shifted along the backbone axis, are contained in an orthorhombic primitive cell
Strain Modulation of Band Offsets at the PCBM/P3HT Heterointerface
Full text linkImproving the efficiency of organic solar cells requires atomic insight into the interface electronic band alignment of the donor and acceptor moieties composing the device. In this article, we use ab initio calculations, with inclusion of long-range (van der Waals) interactions, to address the solid-state properties of a bulk heterojunction heterointerface between a single ordered layer of PCBM molecules adsorbed on a clean P3HT crystalline polymer. The studied interface model allowed us to focus on the basic mechanisms responsible for charge polarization and migration at the interface and to refer the energies of both moieties to the same origin. After the accurate evaluation of the relative energy positions of the near-gap electronic levels in the PCBM/P3HT complex and of the optical spectra useful for determining the nature of the electronic states, we analyzed the effect of uniaxial stress on the band alignment, and we found that both the polymer band gap and the offset between the LUMO levels of the donor and the acceptor materials decrease for compressive stress. This suggests that the donor band gap can be reduced, thus increasing solar energy harvest, and that the open-circuit voltage of the system can be tuned to improve the efficiency of PCBM/P3HT-based solar cell devices
Ab initio molecular dynamics study of the structure of emeraldine base polymers
No full textWe present a Car-Parrinello molecular-dynamics study of the three-dimensional crystalline regions of the emeraldine base II polymer. Our results are compared with the structure obtained from experimental x-ray diffraction spectra and with similar calculations performed on an isolated infinite chain of the emeraldine base polymer. Environmental interchain effects are thus highlighted
Anisotropic molecular packing of soluble C60 fullerenes in hexagonal nanocrystals obtained by solvent vapour annealing
No full textWe describe the self-assembly of soluble, chemically modified fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) into a new crystalline phase where the C60 moieties are arranged in parallel layers. Minimum C60 center-to-center distance is 10 Å within the layers, and up to 15 Å perpendicular to the layers. Highly anisotropic, mesoscopic hexagonal crystals of this material, with a lateral size of many microns and a thickness below 1 μm, are obtained from chloroform solution by solvent vapor annealing, and characterized by optical microscopy and X-ray diffraction. The crystalline structure is deduced combining experimental data with molecular modeling and ab initio calculations. The large difference in C60–C60 spacing indicates a high anisotropy in electrical and charge transport properties of this new phase
Heterointerface Electronic States and Charge Transport of Crystalline N,N′-1H,1H′-Perfluorobutil Dicyanoperylene Diimide in Field-Effect Transistor Configuration
No full textWe present a theoretical study on the nature and origin of charge carriers, charge transfer, and currents at the interface of an organic field-effect transistor (OFET) structure composed of the N,N′-1H,1H′-perfluorobutil dicyanoperylene diimide (PDIF-CN2) organic crystal adsorbed on the (001)-oriented, clean, silicon surface. Starting from the knowledge of the structural and electronic bulk properties of the organic crystal and of the surface of the dielectric substrate, we have evaluated the band structure, density of states, charge transfer, and surface currents in a model crystal made of a reconstructed silicon surface on which a single layer of PDIF-CN2 molecules was deposited in the same packing arrangement as in the bulk PDIF-CN2 organic crystal. In this way alignment of energy bands of the separate PDIF-CN2 and silicon crystals are naturally considered in the model crystal and information on the states involved in charge transport is evidenced. In particular, the analysis of the charge transfer between the dielectric substrate and the adsorbed PDIF-CN2 molecules, of the interface densities of states, and of the currents at the interface allows us to confirm existing experimental results on the distribution and origin of the currents in the FET structure. Namely, that these are essentially determined by the conducting channel made of the π orbitals of the perylene cores of the face-to-face packed PDIF-CN2 molecules on the dielectric substrate. Our results support the description of coherent bandlike transport in OFETs based on the n-type PDIF-CN2 organic crystal
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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