1,720,960 research outputs found
Photodynamics and time resolved fluorescence of azobenzene in solution: A mixed quantum-classical simulation
No full textWe have simulated the photodynamics of azobenzene
by means of the Surface Hopping method. We have considered both
the trans --> cis and the cis --> trans processes, caused by excitation in the
n -> π* band (S1 state). To bring out the solvent effects on the excited
state dynamics, we have run simulations in four different environments: in vacuo, in n-hexane, in methanol, and in ethylene glycol. Our
simulations reproduce very well the measured quantum yields and the
time dependence of the intensity and anisotropy of the transient
fluorescence. Both the photoisomerization and the S1 f S0 internal
conversion require the torsion of the N=N double bond, but the N—C bond rotations and the NNC bending vibrations also play a
role. In the trans f cis photoconversion the N=N torsional motion and the excited state decay are delayed by increasing the solvent
viscosity, while the cis f trans processes are less affected. The analysis of the simulation results allows the experimental observations
to be explained in detail, and in particular the counterintuitive increase of the trans f cis quantum yield with viscosity, as well as the
relationship between the excited state dynamics and the solvent effects on the fluorescence lifetimes and depolarization
Theoretical Analysis of a 2D Metallic/Semiconducting Transition-Metal Dichalcogenide NbS2//WSe2 Hybrid Interface
Full text linkA first-principles theoretical study of a monolayer-thick lateral heterostructure (LH) joining two different transition metal dichalcogenides, NbS2 and WSe2, is reported. The NbS2//WSe2 LH can be considered a prototypical example of a metal (NbS2)/semiconductor (WSe2) 2D hybrid heterojunction. First, realistic atomistic models of the NbS2//WSe2 LH are generated and validated, its band structure is derived, and it is subjected to a fragment decomposition and electrostatic potential analysis to extract a simple but quantitative model of this interfacial system. Stoichiometric fluctuations models are also investigated and found not to alter the qualitative picture. Then, electron transport simulations are conducted and they are analyzed via band alignment analysis. It is concluded that the NbS2//WSe2 LH appears as a robust seamless in-plane 2D modular junction for potential use in optoelectronic devices going beyond the present miniaturization technology
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
Electronic Transport in 2D-Based Printed FETs from a Multiscale Perspective
No full textAs 2D materials (2DMs) gain the research limelight as a technological option for obtaining on-demand printed low-cost integrated circuits with reduced environmental impact, theoretical methods able to provide the necessary fabrication guidelines acquire fundamental importance. Here, a multiscale modeling technique is exploited to study electronic transport in devices consisting of a printed 2DM network of flakes. The approach implements a Monte Carlo scheme to generate the flake distribution. By means of ab initio density functional theory calculations together with non equilibrium Green's functions formalism, detailed physical insights on flake-to-flake transport mechanisms are provided. This later feeds a 3D drift-diffusion and Poisson solution to compute self-consistently transport and electrostatics in the device. The method is applied to MoS2 and graphene-based dielectrically gated FETs, highlighting the impact of the structure density and variability on the mobility and sheet resistance. The prediction capability of the proposed approach is validated against electrical measurements of in-house printed graphene conductive lines as a function of film thickness, demonstrating its strong potential as a guide for future experimental activity in the field
Semiempirical hamiltonian for simulation of azobenzene photochemistry
No full textWe present a semiempirical Hamiltonian that provides an accurate description of the first singlet and triplet
potential energy surfaces of azobenzene for use in direct
simulations of the excited-state dynamics. The parameterization made use of spectroscopic and thermochemical data and the
best ab initio results available to date. Two-dimensional potential
energy surfaces based on constrained geometry optimizations
are presented for the states that are most relevant for the
photochemistry of azobenzene, namely, S0, S1, and S2. In order to run simulations of the photodynamics of azobenzene in hydrocarbons or hydroxylic solvents, we determined the interactions of methane and methanol with the azo group by ab initio
calculations and fitted the interactions with a QM/MM interaction Hamiltonian
Exact decoupling of two dipole-dipole interacting dimers
No full textIt is today possible to test many quantum mechanical predictions, even the most puzzling ones, setting up sophisticated experiments on exemplary "textbook" physical systems like a single atom or molecule or a single material quantum harmonic oscillator. It is therefore conceptually highly exciting to conceive simple but not trivial physical situations representable by exactly solvable hamiltonian models, in the grasp of the experimentalists. In this paper we study a physical system consisting of two coupled identical dimers. Each molecule possesses both fermionic and bosonic degrees of freedom and its internal non adiabatic dynamics is governed by a bilinear term conserving the total excitation number. The two molecules are assumed to interact by a dipole like term. Our main result is that the hamiltonian model representing such a composite system may be unitarily put in a form describing two fictitious uncoupled JC dimers provided the initial excitation number is less than two. The advantage of these canonical transformations is that it makes the restricted dynamical problem exactly tractable. In this way we may successfully study the time evolution of quantum correlation effects get established in the dimer-dimer system due to dipolar like coupling
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
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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