1,721,097 research outputs found
Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2 : Beyond the quantum confinement effect
No full textDensity-functional theory calculations have been carried out in order to study the structural, electronic, and optical properties of oxidized silicon clusters and silicon nanocrystals embedded in SiO2 . For the isolated clusters, different Si/O bonding geometries and various levels of oxidation have been investigated, checking also the dependence of the results on the structure size. We provide strong evidences that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular we show how the multiple presence of silanonelike Si=O bonds can be a reliable model for explaining the photoluminescence redshift observed in oxidized porous silicon samples and it can be used as possible explanation also for the unexpected large photoluminescence bandwidth in single oxidized Si quantum dots
Oxygen role on the structural and optoelectronic properties of silicon nanodots
No full textThe aim of this work is to elucidate, through density functional calculations, the role of the Si–O interface bonds on the structural, electronic and optical properties of Si nanodots in order to explain the peculiar properties of aged porous Si samples and heavily oxidized Si nanoparticles. For isolated dots we show that the presence of Si=O bond results in a strong reduction of the energy gap. For Si nanodots embeddedin SiO2 we show that both the Si–O bonding and the deformation of the cage play an important role
Isolated and embedded silicon based nanodots: The role of surface oxygen
No full textWe have extensively studied the effects of oxygen on the optoelectronics properties of various types of isolated silicon nanodots, through ab initio total energy calculations within the density functional theory. Varying the cluster size we have considered different Si/O bonding geometries and different levels of oxidation. We provide strong evidences that the role of the interface region surrounding the silicon nanostructures have to be carefully taken into account in order to understand the striking optical properties of these systems. Moreover the multiple presence of silanone Si=O bonds at the nanodots surface is shown to provide a consistent interpretation of the photoluminescence red-shift observed in oxidized porous silicon samples. Finally for the first time we have performed ab initio calculations on small silicon nanodots embedded in a SiO2 matrix stressing the strong interplay between the nanodot and the surrounding host environment.We have extensively studied the effects of oxygen on the optoelectronics properties of various types of isolated silicon nanodots. through ab initio total energy calculations within the density functional theory. Varying the cluster size we have considered different Si/O bonding geometries and different levels of oxidation. We provide strong evidences that the role of the interface region surrounding the silicon nanostructures have to be carefully taken into account in order to understand the striking optical properties of these systems. Moreover the multiple presence of silanone Si=O bonds at the nanodots surface is shown to provide a consistent interpretation of the photoluminescence red-shift observed in oxidized porous silicon samples. Finally for the first time we have performed ab initio calculations on small silicon nanodots embedded in a SiO2 matrix stressing the strong interplay between the nanodot and the surrounding host environment and the active role of the interface region between them
Oxygen role on the optoelectronic properties of silicon nanodots
No full textThe optoelectronic properties of Si nanodots have been investigated using ab initio total energy calculations within the density functional theory. Structural relaxations have been considered. We have studied two types of nanodots: isolated clusters covered by H, studying the substitution of Si/H bonds with different Si/O bonds, and nanocrystals embedded in SiO2 matrix. In the first case, we find that the optoelectronic properties strongly depend on the type and the number of Si/O bonds, especially for the gap value and the arrangement of the energy levels. In the second case, the close interplay between chemical and structural effects is pointed out
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
Multiple Si=O bonds at silicon clusters surface
No full textA first-principle investigation of the effects of multiple Si=O bonds at the surface of silicon-based clusters with different sizes has been carried out. Total-energy pseudopotential calculations withindensity functional theory have been applied varying systematically the number of Si=O bonds at the clusters surface. A nonlinear reduction of the energy gap with the Si=O bond number is found. A sort of saturation limit is displayed, providing a consistent interpretation of the photoluminescence redshift observed in oxidized porous silicon samples. Moreover, our results help to clarify the very recent findings on the single silicon quantum dot photoluminescence bandwidth
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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