1,721,028 research outputs found
ytterbium nanoparticles: properties, recent synthesis, developments and prespectives
No full textMaterial science interest is more and more directed to the synthesis and characterization of nano-sized particles. This interest is justified by the fact that the reduction of the particle size of a given material to the nanometer scale causes the rising up of exotic properties that are not present in the bulk state due to size-dependent quantum effects and huge increase of the interfacial species. Further striking features can be gained by controlling nanoparticle shape or through the building up of spatially ordered mono-, bi- and three-dimensional nanoparticle arrays.
Then, the wise selection of the nanoparticle synthetic strategy offers the possibility of preparing a wide class of novel materials and devices for specialised applications. In this ambit, much effort has been directed to set up efficient protocols for nanoparticle synthesis and spatial organization. In particular, since they tend to spontaneously grow and are highly reactive, an useful synthetic route should allow fine size and shape control and nanoparticle passivation.
Among all the materials which can benefit from their reduction to the nanoparticle state, Yb is of interest due to its electronic configuration ([Xe] 4f145d06s2) in the ground state (1S0) and the related physico-chemical properties such as low work function (2.63 eV), high electron injection performance in electroluminescent devices, long range magnetic order, valence fluctuations and superconductivity of some of ytterbium-based intermetallics.
However, an effective synthetic procedure of Yb-based nanostructures should take into account its chemistry. In fact, it has a strong tendency to be in oxidised states and its powders can easily enflame in contact with air. This implies that its preparation through chemical reduction processes requires very drastic conditions and explains why investigations on the synthesis and characterization of nanosized Yb are almost absent in the literature.
On the other hand, top-down approaches as laser ablation of metal target in liquid environment, although successfully employed for the preparation of noble metal nanocolloids, revealed not efficient in the case of Yb due to a weak nanoparticle stabilization and post-ablation coalescence/aggregation processes. To face this problem, we set up a novel preparation by laser ablation of bulk Yb target immersed in some selected reverse micellar solutions. This method, combining the advantages of top-down (high purity, no side products) and bottom-up (compartimentalising/stabilizing media) strategies, proved to be efficient in the controlled synthesis of nanosized Yb allowing, after evaporation of the organic solvent, spatial organization of nanoparticles in the surfactant matrix. Moreover, the proposed method allows, by changing the experimental conditions (surfactant nature and concentration, laser fluence, presence of nanoparticles on which Yb can be deposited, etc), the preparation of different varieties of Yb-based nanostructures.
The aim of this contribution is to furnish to the reader a panoramic view of this research field and to highlight its potentialities for the preparation of Yb-based nanostructures
Self-assembly in surfactant-based mixtures driven by acid–base reactions: bis(2-ethylhexyl) phosphoric acid– n-octylamine systems
Full text linkStructural and dynamic features of bis(2-ethylhexyl) phosphoric acid (HDEHP)–n-octylamine (NOA)
mixtures as a function of the NOA mole fraction (XNOA) have been investigated by SAXS, WAXS, IR,
dielectric spectroscopy and polarized optical microscopy. In the 0 ¡ XNOA , 0.5 range, mixtures are
transparent liquids, while the abrupt formation of a waxy solid characterized by an hexagonal
bidimensional structure occurs at XNOA = 0.5. Such a composition-induced phase transition results from
the synergetic effect of the progressive increase in number density of ordered HDEHP–NOA nanodomains
with XNOA. Mainly driven by an HDEHP to NOA proton transfer, the increase of structural order with XNOA
arises from the progressive substitution of loosely hydrogen bonded HDEHP–HDEHP aggregates with
strongly bonded NOA–HDEHP ones. Analysis of SAXS patterns at temperatures in the 10–70 uC range
emphasized that these local structures are scarcely impacted by an increase of thermal fluctuations. Effects
due to the steric compatibility between HDEHP and NOA apolar moieties have been highlighted. Overall,
the results allow us to emphasize the role of specific polar and apolar interactions joined to steric effects in
regulating the molecular organization in surfactant mixtures and can be used to design novel materials
with planned physico-chemical properties
Complex permittivity of FeCl3/AOT/CCl4 microemulsions probed by AC impedance spectroscopy
No full textThe complex permittivity of FeCl3/AOT/CCl4 microemulsions in the 1–105 Hz frequency range has been
measured by the conventional AC complex impedance technique. Measurements as a function of the volume
fraction of the dispersed phase (FeCl3 + AOT) and temperature at fixed salt-to-AOT molar ratio (R,
R = 0.5) show that the entrapment of FeCl3 clusters significantly enhances the local permittivity of the
AOT reverse micelles and the number density of charge carriers resulting from the peculiar state of the
confined inorganic salt. An estimate of the apparent static permittivity of the FeCl3 ionic clusters
entrapped in the core of AOT reverse micelles gives the very high and quite surprisingly value of about
237.
Moreover, a thorough analysis of conductivity data and of their temperature dependence strongly supports
the hypothesis that the charge transport in these systems is mainly sustained by a mechanism of
hopping consisting in the continuous jumping of charged species within supra-micellar aggregates of
AOT reverse micelles whose aggregation is driven by fluctuating opposite charges on contacting micelles
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
How self-assembly of amphiphilic molecules can generate complexityin the nanoscale
No full texttGiven the importance of nanomaterials and nanostructures in modern technology, in the past decadesmuch effort has been directed to set up efficient bottom up protocols for the piloted self-assembly ofmolecules. However, molecules are generally disinclined to adopt the desired structural organizationbecause they behave according to their own specific intermolecular interactions. Thus, only some selectedclasses of chemical compounds are capable to lead to useful self-assembled structures. Amphiphiles,simultaneously possessing polar and apolar moieties within their molecular architecture, can give awide scenario of possible intermolecular interactions: polar–polar, polar–apolar, apolar–apolar interac-tions, eventual directional H-bonds, steric hindrance and so on. This peculiarity efficiently triggers thepossibility of originating complex behavior, i.e. the formation of interacting structures at hierarchicallength-scales characterized by emerging and specific properties and functions. However, if one places ina becher the molecules constituting a living cell, he does not observe the formation of a living cell evenafter vigorous and prolonged stirring and/or heating. This consideration suggests that the building up ofcomplex structures is not only an affair of molecular structure, system composition and self-assemblingprocesses but additional subtle features can contribute to the overall process. The aim of this contributionis to furnish to the reader a panoramic view of this exciting problematic clarifying what is meant withthe concept of complexity and how the rich world of amphiphilic molecules are employed for obtain-ing complex nanostructure-based systems with novel characteristics for applications in nanotechnology.While the literature is full of contributions focusing on specific aspects of self-assembly and complexity,the present work constitutes an easy to read critical point of view which tries to interact with the imagi-nation of readers hopefully leading to the discovery of novel aspects and interconnections and ultimatelystimulating new ideas and research
Physicochemical investigation of surfactant-coated gold nanoparticles synthesized in the confined space of dry reversed micelles
No full textAbstract
Gold nanoparticle/surfactant composites have been synthesized by a novel reduction reaction in the confined space of dry sodium bis(2-
ethylhexyl)sulfosuccinate (AOT) or lecithin reversed micelles dispersed in n-heptane and cyclohexane, respectively. The reaction was carried out
by adding an opportune amount of anhydrous hydrazine/tetrahydrofuran solution to a suspension of HAuCl4-containing dry reversed micelles
dispersed in organic solvent. UV–vis investigation ascertained the formation of stable metal gold nanoparticles and the analysis of FT-IR spectra
highlighted the formation of an oriented surfactant monolayer at the nanoparticle surface. Simple evaporation under vacuum of the organic solvent
leads to the preparation of very interesting gold nanoparticle/surfactant liquid crystal composites. Such nanocomposites, characterized by WAXS
and SAXS, showed the presence of small (radius∼14 A° ) and stable gold nanoparticles dispersed in the surfactant matrix. It has been also emphasized
that a simple washing process with ethanol is a suitable method to eliminate the excess of surfactant leaving a novel nanomaterial constituted by
gold nanoparticles coated by a monolayer of opportunely oriented surfactant molecules.
© 2005 Elsevier B.V. All rights reserved
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