1,720,992 research outputs found
Temperature-induced valence instability in the charge-transfer crystal TMB-TCNQ
No full textThe occurrence of so-called temperature-induced neutral-ionic transitions (TINIT) in mixed-stack charge-transfer crystals is quite rare. Here we reinvestigate one of the crystals which has been claimed to undergo such a transition, 3,3',5,5'-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). Extensive optical data allow us to conclude that the transition should be classified as a valence instability, and not as a “true” TINIT, as the ∼0.5 neutral-ionic borderline is not crossed. The ionicity rho, or average charge at the molecular sites, indeed changes very little at the transition, from about 0.3 to about 0.4, and is accompanied by stack dimerization. The transition is first order with large hysteresis, and the crystal may crack or break. For this reason we have been unable to collect x-ray structural data on the low-temperature phase, but with the help of semiempirical calculations we are able to assess a plausible scenario for this peculiar phase transition and its mechanism
Ferrocene Molecular Architectures Grafted on Si(111): A Theoretical Calculation of the Standard Oxidation Potentials and Electron Transfer Rate Constant
Full text linkThe standard oxidation potential and the electron transfer (ET) rate constants of two silicon-based hybrid interfaces, Si(111)/organic-spacer/Ferrocene, are theoretically calculated and assessed. The dynamics of the electrochemical driven ET process is modeled in terms of the classical donor/acceptor scheme within the framework of Marcus theory. The ET rate constants, k(ET), are determined following calculation of the electron transfer matrix element, V-RP, together with the knowledge of the energy of the neutral and charge separated systems. The recently introduced Constrained Density Functional Theory (CDFT) method is exploited to optimize the structure and determine the energy of the charge separated species. Calculated ET rate constants are k(ET) = 77.8s(-1) and k(ET) = 1.3 x 10(-9) s(-1), in the case of the short and long organic-spacer, respectively
An Integrated Experimental/Theoretical Study of Structurally Related Poly-Thiophenes Used in Photovoltaic Systems
Full text linkIn this work, a series of eight thiophene-based polymers (exploited as “donors” in bulk
heterojunction photovoltaics cells), whose structures were designed to be suitably tuned with the
electronic characteristics of the [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), is considered,.
The electronic properties of the mono-, di-, trimeric oligomers are reckoned (at the Hartree-Fock
and DFT level of the theory) and compared to experimental spectroscopic and electrochemical
results. Indeed, electrochemical and spectroscopic results show a systematic difference whose
physical nature is assessed and related to the exciton (electron-hole) binding energy (Je,h). The
critical comparison of the experimental and theoretical band gaps, i.e., the HOMO-LUMO energy
difference, suggests that electrochemical and DFT values are the most suited to being used in the
design of a polythiophene-based p-n junction for photovoltaics
First-Principles Estimation of Core Level Shifts for Hf, Ta, W, and Re
Full text link[Image: see text] A simple first-principles approach is used to estimate the core level shifts observed in X-ray photoelectron spectroscopy for the 4f electrons of Hf, Ta, W, and Re; these elements were selected because their 4f levels are relatively close to the Fermi energy. The approach is first tested by modeling the surface core level shifts of low-index surfaces of the four elemental metals, followed by its application to the well-studied material TaSe(2) in the commensurate charge density wave (CDW) phase, where agreement with experimental data is found to be good, showing that this approach can yield insights into modifications of the CDW. Finally, unterminated surface core level shifts in the hypothetical MXene Ta(3)C(2) are modeled, and the potential of XPS for the investigation of the surface termination of MXenes is demonstrated
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
Enhanced photogeneration of polaron pairs in neat semicrystalline donor-acceptor copolymer films via direct excitation of interchain aggregates
Full text linkWe investigate the photogeneration
of polaron pairs (PPs) in neat
films of the semicrystalline donor–acceptor semiconducting
copolymer PCPDTBT. Carefully selecting the solution-processing procedures,
we obtain films with different amounts of crystallinity and interchain
aggregation. We compare the photogeneration of PPs between the films
by monitoring their photoinduced absorption in ultrafast pump–probe
experiments, selectively exciting nonaggregated or aggregated polymer
chains. The direct photoexcitation of interchain π-aggregates
results in prompt (<100 fs) charge generation. Compared to the
case where nonaggregated chains are excited, we find an 8-fold increase
in the prompt PP to singlet-exciton ratio. We also show that highly
crystalline lamellar nanostructures not containing π-stacked
or any light-absorbing aggregates do not improve the efficiency of
PP photogeneration. Our results show that light absorption from interchain
aggregates is highly beneficial for charge photogeneration in semiconducting
polymers and should be taken into account when optimizing film morphologies
for photovoltaic devices
How intermolecular geometrical disorder affects the molecular doping of donor-acceptor copolymers
Full text linkMolecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor–acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor–acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor–acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping
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