1,720,981 research outputs found
Metallization and incomplete melting of a semiconductor surface at high temperature
No full textWe present a first microscopic study of the state of a semiconductor surface, clean Ge(111), close to the bulk melting temperature. Both electronic states and ionic motion are fully treated via first-principles molecular dynamics. Results indicate a clear dynamical disordering, confined, however, to the first atomic bilayer. This region acquires a liquidlike diffusion, and is metallic. Lack of melting of the second and deeper bilayers, found to be in good quantitative agreement with recent x-ray data, indicates an incomplete wetting of the semiconducting solid by its own metallic melt. Previously conflicting data on Ge(111) are also reconciled within this picture
First-principles study of the contractive reconstruction of gold and silver monolayers on gold, silver and aluminum
No full textUsing first-principles calculations in conjunction with modeling techniques, the author has investigated the structures of Au and Ag monolayers on a number of metal surfaces. Au(100) has a c(26 {times} 68) surface unit cell and the reconstruction has been interpreted as the top layer transforming to a contracted hexagonal-close-packed layer, superimposed on the square lattice of the underlying substrate atoms. Similar reconstructions have been observed on the 5d fcc metals Ir and Pt, but not in the 4d Rh, Pd, and Ag. The author studied the energetics of a monolayer of Au and Ag using first-principles calculations. The author found that it is energetically favorable for both Au and Ag to transform from a square to hexagonal arrangement and to contract to a higher surface density, but Au gains substantially more energy than Ag. This is true both for a monolayer in isolation as well as on top of a jellium surface. The author also calculated the mismatch energy (energy loss when the top layer loses registry with the substrate) for Au and Ag, and found that Ag has a slightly higher mismatch energy. The first-principles results thus offer a strong indication that Au(100) can reconstruct but Ag will not. The reconstruction is further studied with a 2 dimensional Frenkel-Kontorowa model, with parameters extracted from the total energy calculations. The author found that it is indeed energetically favorable for the top layer of Au(100), but not for Ag, to transform to a hexagonal-close-packed structure and contract. 85 refs., 34 figs., 8 tabs
ENSEÑANZA DE LA NANOTECNOLOGÍA EN EDUCACIÓN SECUNDARIA: TEORÍA Y PRÁCTICA
Full text linkLa nanotecnología cada día tiene más presencia en nuestra vida, con una gran diversidad de aplicaciones. Por tanto, es fundamental incluirla en el currículo educativo en todos los niveles educativos. En México, su enseñanza es muy limitada, particularmente en la parte práctica, lo que influye en su entendimiento. Se desarrolló un programa para enseñar la nanotecnología a estudiantes de educación secundaria, el cual inició con actividades teórico-prácticas sobre temas de nanociencias, nanotecnología y bionanotecnología. Para terminar el programa, los estudiantes desarrollaron un proyecto de aplicación de la nanotecnología a un problema que afectase a su comunidad. El proyecto propuesto consistió en el uso de nanopartículas de plata (AgNPs) para reducir la contaminación microbiana en aguas residuales, con la finalidad de mejorar la calidad de vida. Las muestras tratadas con las AgNPs mostraron una reducción de su carga microbiana. Al finalizar el proyecto, los estudiantes comprendían conceptos relativos a la nanotecnología. Los estudiantes aprenden mejor y muestran mayor interés cuando la enseñanza en los temas de nanociencias y nanotecnología incluye experimentos y otras actividades dinámicas. Estas actividades dejan una percepción y concientización muy favorable sobre la ciencia en los estudiantes involucrados.Every day, nanotechnology has a stronger presence in our daily life, with a wide variety of applications. Therefore, it is essential to include it in the educational curriculum at all educational levels. In Mexico, science teaching is very limited, particularly in the experimental part, which influences their understanding. A program was developed to teach nanotechnology to junior high school students, which began with theoretical and practical activities on topics of nanoscience, nanotechnology and bio-nanotechnology. To complete the program, students developed a project to apply nanotechnology to a problem that affected their community. The proposed project involved the use of silver nanoparticles (AgNPs) to reduce microbial contamination in wastewater, in order to improve the quality of life. Samples treated with AgNPs showed a reduction of the microbial load. At project completion, students understood concepts of nanotechnology. The project allowed them to propose solutions to important problems in their community. Students learn better and show greater interest when teaching in the areas of nanoscience and nanotechnology includes experiments and other dynamical activities. These activities will leave a very favorable perception and awareness about science in the students involved
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
NANOSCIENCES OUTREACH ACTIVITIES IN THE INDIGINOUS COMUNITIES OF MÉXICO
Full text linkWe always say that science and technology are the tools that are needed to make progress in countries in development. We can say the same in the case of the peoples and communities: knowledge about science and technology indicate their state of development. Many of the indigenous communities are living in poverty and do not have the best conditions for a dignified human development. The fight against social marginalization of people in the indigenous communities should be one of the policy areas most relevant to the development of Mexico. That is why we believe that policies targeting the development of indigenous groups should also include as a fundamental pillar education on science and technology. Together, conservation and recovery programs of indigenous languages must be accompanied by a process of incorporation scientific terms that allow the assimilation of science in the language of the communities
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
Nanoestructuras unidimensionales autoensambladas en superficies de si (001)
Full text linkDiferentes tipos de nanoestructuras unidimensionales que pueden autoensamblarse en superficies de semiconductores se han estudiado experimental y teóricamente en los últimos años. Estos sistemas son interesantes no sólo por sus posibles aplicaciones, sino también desde un punto de vista fundamental. En la investigación básica el interés está motivado por la posibilidad de comprender interesantes fenómenos físicos que están relacionados con estos sistemas, tales como estabilidad de la superficie, el crecimiento y las propiedades de los sistemas de una sola dimensión, para mencionar sólo unos pocos. Para las aplicaciones tecnológicas, son muy importantes en la nanoelectrónica, ya que siempre hay una búsqueda de métodos para fabricar circuitos más pequeños (por debajo de las decenas de nanómetros), más allá de las capacidades de litografía normal. En este artículo haremos una revisión de algunos de estos sistemas, crecidos en la superficie Si(001).Different kinds of one dimensional nanostructures that self assemble on semiconductor surfaces have been studied experimental and theoretically in recent years. They are interesting not only because of possible applications but also from a fundamental point of view. In basic research the interest is motivated by the possibility to understand interesting physical phenomena that are related to these systems, such as surface stability, growth, and the properties of one-dimensional systems, to mention only a few. For technological applications, they are very important in nanoelectronics, since there is always a search for ways of creating smaller circuits (in the lower tens of nanometers), beyond the capabilities of normal lithography. In this article, we survey some of such systems when grown on the Si(001) surface
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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