1,721,148 research outputs found
Precision Synthesis of Silicon Nanowires with Crystalline Core and Amorphous Shell
Get PDFA synthetic route to crystalline silicon (Si) nanowires with an amorphous Si shell is reported. Trisilane (Si3H8) and Sn(HMDS)(2) are decomposed in supercritical toluene at 450 degrees C. Sn(HMDS)(2) creates Sn nanoparticles that seed Si nanowire growth by the supercritical fluid-liquid-solid (SFLS) mechanism. The Si : Sn ratio in the reaction determines the growth of amorphous Si shell. No amorphous shell forms at relatively low Si : Sn ratios of 20 : 1, whereas higher Si : Sn ratio of 40 : 1 leads to significant amorphous shell. We propose that hydrogen evolved from trisilane decomposition etches away the Sn seed particles as nanowires grow, which promotes the amorphous Si shell deposition when the higher Si : Sn ratios are used.Robert A. Welch Foundation F-1464U.S. Department of Energy Office of Science, Office of Basic Energy Sciences DE-SC0001091National Defense Science and Engineering Graduate FellowshipChemistr
Light-Harvesting Antennae Based on Silicon Nanocrystals
No full textSilicon (Si) nanocrystals are relatively strong light emitters, but are weak light absorbers as a result of their indirect band gap. One way to enhance light absorption is to functionalize the nanocrystals with chromophores that are strong light absorbers. By designing systems that enable efficient energy transfer from the chromophore to the Si nanocrystal, the brightness of the nanocrystals can be significantly increased. There have now been a few experimental systems in which covalent attachment of chromophores, efficient energy transfer and significantly increased brightness have been demonstrated. This review discusses progress on these systems and the remaining challenges
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
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Assembly of colloidal nanocrystals into phospholipid structures and photothermal materials
Get PDFtextThere has been growing interest in developing colloidal metal and semiconductor nanocrystals as biomedical imaging contrast agents and therapeutics, since light excitation can cause the nanocrystals to fluoresce or heat up. Recent advances in synthetic chemistry produced fluorescent 2-4 nm diameter silicon and 1-2 nm diaemeter CuInSSe nanocrystals, as well as 16 nm diameter copper selenide (Cu₂₋[subscript x]Se) nanocrystals exhibiting strong absorbance of near infrared light suitable for biomedical applications. However, the syntheses yield nanocrystals that are stabilized by an adsorbed layer of hydrocarbons, making the nanocrystals hydrophobic and non-dispersible in aqueous solution. Encapsulating these nanocrystals in amphiphilic polymer micelles enables the nanocrystals to disperse in water. Subsequently, the Si nanocrystals were injected into tissue to demonstrate fluorescence imaging, the photothermal transduction efficiency of copper selenide nanocrystals was characterized in water, and the copper selenide nanocrystals were used enhance the photothermal destruction of cancer cells in vitro. The polymer-encapsulated copper selenide nanocrystals were found to have higher photothermal transduction efficiency than 140 nm diameter Au nanoshells, which have been widely investigated for photothermal therapy. Combining the optical properties of metal and semiconductor nanocrystals with the drug-carrying capability of lipid vesicles has received attention lately since it may create a nanomaterial capable of performing simultaneous drug delivery, optical contrast enhancement, and photo-induced therapy. Hydrophobic, dodecanethiol-coated Au nanocrystals were dispersed in water with phosphatidylcholine lipids and characterized using cryo transmission electron microscopy. 1.8 nm diameter Au nanocrystals completely load the bilayer of unsaturated lipid vesicles when the vesicles contain residual chloroform, and without chloroform the nanocrystals do not incorporate into the vesicle bilayer. 1.8 nm Au nanocrystals dispersed in water with saturated lipids to form lipid-coated nanocrystal agglomerates, which sometimes adhered to vesicles, and the shape of the agglomerates varied from linear nanocrystal chains, to flat sheets, to spherical clusters as the lipid fatty acid length was increased from 12 to 18 carbons. Including squalene formed lipid-stabilized emulsion droplets which were fully loaded with the Au nanocrystals. Results with 4.1 nm Au and 2-3 nm diameter Si nanocrystals were similar, but these nanocrystals could not completely load the bilayers of unsaturated lipids.Chemical Engineerin
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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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Topics in colloidal nanocrystals: synthesis and characterization, polymorphism, and self-assembly
Get PDFtextThe first step in the utilization of the unique and potentially technologically
impactful size- and shape-dependent properties of nanocrystals is to develop reproducible
methods to synthesize monodisperse nanocrystals. The development of the solventless
synthetic approach showed a new pathway towards the synthesis of high quality rodshaped
and triangular prism-shaped rhombohedral NiS nanocrystals. The nanorods and
nanoprims both form via the thermal decomposition of the nickel thiolate single-source
precursor. The length on the nanorods was dependent on growth conditions, varying
between 15 and 50 nm. The solventless approach was also capable of producing
triangular nanoprisms in a 1:1 ratio with the nanorods, depending on the overall
concentration of the reactant species.
We have demonstrated that synthetic pathway chosen can also dictate the shapes
and phases accessible to the nanocrystal product with our comparisons between the
solution-phase and solventless syntheses of nickel sulfide and copper sulfide. The
solution-phase approach to nickel sulfide resulted only in polydisperse cubic Ni3S4
nanocrystals, which were a reaction byproduct in the solventless synthesis, shaped either
as quasi-spheres or quasi-cubes. The stoichiometery of the copper sulfide product
synthesized in solution was dependent on the Cu:S reactant molar ratio, with either CuS
(covellite) and Cu1.8S (digenite) forming. CuS, Cu1.8S and Ni3S4 differ from the Cu2S and
NiS nanocrystals obtained by solventless decomposition of metal thiolate single source
precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and
morphology for nickel sulfide. CuS nanodisks self-assembled into well-ordered
columnar phases.
Sterically-stabilized CdS nanorods were observed to self-align into networks of
stripes several micrometers long when dropcast from dispersions at sub-monolayer
coverage. The nanorods assemble side-by-side with their long axes parallel to the stripe
direction. Nanorods 3 to 5 nm in diameter, with aspect ratios ranging from 4 to 12 were
found to form stripe patterns. We propose that interparticle attractions between the rods
drive the formation of the stripes, and that the rods are trapped in the stripe networks by
solvent evaporation dynamics.Chemical Engineerin
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