1,720,972 research outputs found
N1s and C1s NEXAFS spectra of model systems for pyridine on Si(100) surface: a DFT simulation
No full textAdsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and spectroscopic measurements often in concert with theoretical calculations can assess the orientation and geometry of the molecular adsorbate. In this respect NEXAFS spectroscopy represents a powerful technique to investigate the interaction of molecules with a surface. The theoretical simulation of NEXAFS spectra of molecules adsorbed on a surface represents a significant challenge both for a proper modelling of the adsorbate system as well as for the size of system which needs theoretical methods capable to fulfill requirements of accuracy and computational economy. Here we present DFT simulations of NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models.1 The surface and the adsorbate models have been previously optimized through periodic calculations, then suitable finite clusters have been be cut out from the optimized periodic structures and used for the simulation of the angle resolved NEXAFS spectra of the adsorbed molecule. The spectra have been calculated employing a molecular DFT methodology based on the transition potential scheme in order to include the core hole relaxation effect.2 Both the N1s and C1s adsorption edges of the adsorbed pyridine have been investigated. The results show that a careful analysis of the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries; the reliability of the computational strategy is proved by the comparison of the theoretical results with the experimental data.1,3 The periodic optimization of the molecules adsorbed on the surface plays a fundamental role in the design of clusters which correctly model the adsorbed system
N1s and C1s Near-Edge X-ray Absorption Fine Structure Spectra of Model Systems for Pyridine on Si(100): A DFT Simulation
No full textThis work presents a density functional theory (DFT) based computational investigation of the NEXAFS spectra of pyridine on the Si(100) surface. The accurate modeling of the adsorbate system is tackled by a preliminary optimization of the surface and of the molecules adsorbed on it performed by a periodic slab methodology. From the optimized periodic structures suitable finite clusters are then cut out and used for the calculation of core excitation energies and oscillator strengths of the adsorbed pyridine. Various adsorption modes are considered and for each of them the polarized spectra at the N-K edge and the C K-edge of the pyridine have been simulated with the transition potential scheme in order to include the core hole relaxation effect. The careful analysis of the calculated polarized features and of the intensity trend with the change of polarization reveals important information on specific details of the adsorbtion geometries and support the comparison with the experiment. The XII, VII, IV on-dimer and cross-trench adsorption modes appear suitable to explain the experimental N1s features observed at low temperature while they cannot account for all the experimental features emerging at room temperature both in the N1s and C1s spectra. To deal with this issue the mode IX geometry is also considered for the presence of an –C=N- (imine) chemical group which appears to play an important role for the interpretation of N1s and C1s spectra at 300K. The long range effects of an extended surface model on the NEXAFS features is also analyzed showing the importance of an accurate modeling of the Si(100) reconstructed surface for a confident simulation of the NEXAFS spectra of the adsorbed pyridine
Nitrogen and Carbon K-edge NEXAFS spectra of model systems for C5H5N on Si(100): a DFT simulation
No full textAdsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Fundamental research on the covalent bonding of molecules with the surface can provide useful information on the organic/semiconductor interface. The NEXAFS spectroscopy represents a powerful technique to investigate the orientation and geometry of molecules adsorbed on surfaces and is widely used to characterize adsorbate structures, often in concert with theoretical calculations. The computational simulation of NEXAFS spectra of such systems represents a significant challenge both for a proper modelling of the adsorbate system as well as for the size of system which needs theoretical methods capable to fulfill requirements of accuracy and computational economy. Here we present a DFT-TS simulation of the NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models. The models have been previously optimized through periodic calculations performed with the Quantum Espresso code, then suitable finite clusters have been be cut out from the optimized periodic structures and used for the simulation of the angle resolved NEXAFS spectra of the adsorbed molecule, employing traditional molecular DFT techniques (through the ADF code). The results compare well with the experimental spectra and show that the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries and that the methodology employed is reliable to describe the K-shell spectra of this kind of systems
The near-edge X-ray-absorption fine-structure of O2 chemisorbed on Ag(110) surface studied by density functional theory
No full textIn this computational work a finite cluster model approach has been employed to simulate the adsorption of
O2 molecule on the Ag(110) surface by means of density functional theory. O 1s NEXAFS spectra have been
calculated with the transition potential scheme in order to include the core hole relaxation effect. From the
analysis of the density of states, experimental and calculated NEXAFS spectra found that the O2 molecule
upon adsorption on Ag(110) has completely filled π* orbitals consistent with a closed shell electronic structure
of peroxide. The only virtual valence molecular orbital still belonging to the O2 fragment is the antibonding
σ*, which will be responsible for the most intense resonance observed and calculated in the O 1s NEXAFS
spectra. The simulated polarized spectra are in good agreement with two different sets of experimental data,
with the exception of grazing incidence, for which a disagreement between theory and experiment is found.
We attributed this disagreement to a slight deviation of the adsorption geometry with respect to the ideal
case, suggesting a tilt angle of about 10°–15° of the O\O bond with respect to the surface plane
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
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
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