1,721,001 research outputs found
Tight-binding formulation of the dielectric response in semiconductor nanocrystals
No full textWe report on a theoretical derivation of the electronic dielectric response of semiconductor nanocrystals using a tight binding framework. Extending to the nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed for bulk semiconductors, we show how local field effects can be included in the study of confined systems. A great advantage of this scheme is that of being formulated in terms of localized orbitals and thus it requires very few computational resources and times. Applications to the optical and screening properties of semiconductor nanocrystals are presented here and discussed. Results concerning the absorption cross section, the static polarizability, and the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals compare well to both first principles results and experimental data. We also show that the present scheme allows us to easily go beyond the continuum dielectric model, based on the Clausius-Mossotti equation, which is frequently used to include the nanocrystal surface polarization. Our calculations indicate that the continuum dielectric model, used in conjunction with a size dependent dielectric constant, underestimates the nanocrystal polarizability, leading to exceedingly strong surface polarization fields
Role of local fields in the optical properties of silicon nanocrystals using the tight binding approach
No full textThe role of local fields in the optical response of silicon nanocrystals is analyzed using a tight binding approach. Our calculations show that, at variance with bulk silicon, local field effects dramatically modify the silicon nanocrystal optical response. An explanation is given in terms of surface electronic polarization and confirmed by the fair agreement between the tight binding results and that of a classical dielectric model. From such a comparison, it emerges that the classical model works not only for large but also for very small nanocrystals. Moreover, the dependence on size of the optical response on the nanocrystal size is discussed, in particular treating the limit of large size nanocrystals
The reaction of pyrrole with dimethyl carbonate under phosphazene catalysis: N-methoxycarbonylation vs N-methylation
No full textPhosphazene (t-butylimino-tris(dimethylamino)phosphorane (P1-t-Bu), t-butylimino-tris(pyrrolidino)-
phosphorane (BTPP)) and amidine (1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)) superbases have been
investigated as catalysts in the direct reaction of pyrrole with dimethyl carbonate (DMC). The system
phosphazene/pyrrole/DMC proves to be a flexible synthetic tool, as it offers a solution not only for the
direct and selective phosgene-free synthesis of 1-methoxycarbonyl pyrrole (1), but also for the
straightforward and selective synthesis of 1-methylpyrrole (2) through a safer way which avoids the use
of harmful methylating agents such as methyl halides and (MeO)2SO2. The influence of factors
(temperature, catalyst loading, reaction time, etc.) affecting yields and selectivities has been
investigated. We have also ascertained that a major reaction pathway to the formation of 2 involves
the decarboxylation of the primary product 1. Also this process was catalytically promoted by the
phosphazene catalyst. Co-generated CO2 opened a way to the deactivation of phosphazene catalyst,
which converted mainly into catalytically inactive OP(NR2)3 (NR2 = NMe2, NC4H8)
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
CONDUCTION-BAND ANISOTROPY EFFECTS IN SPHERICAL SEMICONDUCTOR NANOCRYSTALS: A THEORETICAL STUDY
No full textTight-binding calculation of the optical absorption cross section of spherical and ellipsoidal silicon nanocrystals
No full textElectronic and optical properties of silicon nanocrystals are calculated and discussed within a semiempirical tight-binding approach, which allows to study systems composed of thousands of atoms. Oscillator strengths, frequency-dependent optical absorption cross sections, and static dielectric constants are investigated for both spherical and ellipsoidal nanocrystals, with the aim of pointing out their size- and shape-dependent features. We show that the anisotropy of the optical functions follows the nanocrystal shape, and a comparison is discussed between very elongated structures and quantum wires
Polaron effects and boundary conditions in cylindrical wires
No full textIn this work we show that the polaron effects in cylindrical quantum wires are function of the cylinder radius R-0 through the boundary conditions for both the ionic and the electronic motion and through the size dependence of the static and high frequency dielectric constants. We find that the dielectric constants are increasing functions of R-0. This fact and the different boundary conditions for the ions and the electrons have the final consequence that polaron self-energy can either be an increasing or a decreasing function of R-0
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
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