1,721,048 research outputs found
Defects in Carbon-based Materials studied by Electron Paramagnetic Resonance Methods
Full text linkThe study of defects in carbon-based materials is of great interest, because of their elusive nature and because these defects determine some of their fundamental electronic, optical and magnetic properties.
The type of defects depends strongly on the materials structure. In ordered materials, for example, defects are due to distortions of the crystalline structure, while in amorphous materials they are related most to unsaturated bonds. In recent years, great interest has grown on graphene and graphene-like systems. The properties of these materials make them very promising for electronic and mechanical applications. Deviations from perfection can also be useful in some tecnical applications, as they enable to tailor the local properties of graphene and to achieve new functionalities.
Most of the defects that can be found in carbon-based materials are characterized by the presence of unpaired electrons and are therefore paramagnetic. In several cases, the existence of unpaired spins within a material can lead also to cooperative magnetism phenomena. A complete and accurate description of the defects distribution in a material is therefore of fundamental importance for its possible applications.
In this thesis, we identify and characterize the different types of defects that can be found in graphene-like systems by using electron paramagnetic resonance (EPR) spectroscopy and Density Functional Theory (DFT) calculations. We study materials with different types of defects and try to correlate their EPR characteristics with their structure. We also report a study on the reactivity of these materials that is determined by the presence of defects
Identification of slow relaxing spin components by pulse EPR techniques in graphene-related materials
Full text linkElectron Paramagnetic Resonance (EPR) is a powerful technique that is suitable to study graphene-related materials. The challenging ability requested to the spectroscopy is its capability to resolve the variety of structures, relatively similar, that are obtained in materials produced through
different methods, but that also coexist inside a single sample. In general, because of the intrinsic inhomogeneity of the samples, the EPR spectra are therefore a superposition of spectra coming from different structures. We show that by pulse EPR techniques (echo-detected EPR, ESEEM and Mims
ENDOR) we can identify and characterize species with slow spin relaxing properties. These species are generally called molecular states, and are likely small pieces of graphenic structures of limited dimensions, thus conveniently described by a molecular approach. We have studied commercial reduced graphene oxide and chemically exfoliated graphite, which are characterized by different EPR spectra. Hyperfine spectroscopies enabled us to characterize the molecular components of the different materials, especially in terms of the interaction of the unpaired electrons with protons (number of protons and hyperfine coupling constants). We also obtained useful precious information about extent of delocalization of the molecular states
A comparative Electron Paramagnetic Resonance study of expanded graphites and graphene
No full textGraphene, a novel electronic system with unprecedented characteristics, can be obtained using different methods, each producing materials with specific characteristics from the electronic point of view.
Among these procedures, methods based on the expansion of graphite allow to obtain graphene material in rather high quantities. We have, then, conducted a comparative study of graphene materials produced by these methods by using electron paramagnetic resonance (EPR) techniques; single-layer commercial graphene produced using the Hummers method has been used as the reference. EPR techniques enable the study of some magnetic properties of different types of electrons exhibiting paramagnetic nature. We have analysed the EPR spectra to identify the different types of paramagnetic centres contributing to the spectrum. The analysis of the temperature-dependent EPR spectra and the use of pulse techniques allowed us to separate and characterize the contribution of free conduction
electrons from the contributions of localized edge states and molecular-type paramagnetic states
Reciprocal Interaction between Waves and Turbulence within the Nocturnal Boundary-Layer
Full text linkThe presence of waves in the nocturnal boundary layer has proven to generate complex interaction with turbulence. On complex terrain environments, where turbulence is observed in a weak but continuous state of activity, waves can be a vehicle of additional production/loss of turbulence energy. The common approach based on the Reynolds decomposition is unable to disaggregate turbulence and wave motion, thus revealing impaired to explicit the terms of this additional interaction. In the current investigation, we adopt a triple-decomposition approach to separate mean, wave, and turbulence motions within near-surface boundary-layer flows, with the aim of unveiling the role of wave motion as source and/or sink of turbulence kinetic and potential energies in the respective explicit budgets. This investigation reveals that the waves contribute to the kinetic energy budget where the production is not shear-dominated and the budget equation does not reduce to a shear-dissipation balance (e.g., as it occurs close to a surface). Away from the surface, the buoyancy effects associated with the wave motion become a significant factor in generating a three-terms balance (shear-buoyancy-dissipation). Similar effects can be found in the potential energy budget, as the waves affect for instance the production associated with the vertical heat flux. On this basis, we develop a simple interpretation paradigm to distinguish two layers, namely near-ground and far-ground sublayer, estimating where the turbulence kinetic energy can significantly feed or be fed by the wave. To prove this paradigm and evaluate the explicit contributions of the wave motion on the turbulence kinetic and potential energies, we investigate a nocturnal valley flow observed under weak synoptic forcing in the Dugway Valley (Utah) during the MATERHORN Program. From this dataset, the explicit kinetic and potential energy budgets are calculated, relying on a variance-covariance analysis to further comprehend the balance of energy production/loss in each sublayer. With this investigation, we propose a simple interpretation scheme to capture and interpret the extent of the complex interaction between waves and turbulence in nocturnal stable boundary layers...
Analysis of the Jahn-Teller effect in Coronene and Corannulene ions and its effect in EPR spectroscopy
Full text linkCoronene and corannulene are popular structures that have provided the basis for the investigation of extended carbon-based structures, like graphene, fullerene and their derivatives. Here we address the Jahn-Teller (JT) effects in electron paramagnetic resonance (EPR) spectroscopy of such molecules in ionic form. By density functional theory (DFT) and a Monte Carlo based sampling of the potential energy surface, we obtain three and five symmetry-related JT equivalent conformers in coronene and in corannulene ions, respectively. The structure and the interconversion between the JT distorted forms are discussed for their implication in EPR spectroscopy. The calculation of the g-tensor by DFT methods has also been benchmarked to serve as methodological approach for the investigation of more extended graphene molecules
Resolution of EPR signals in graphene-based materials from few layers to nanographites
No full textThe focus of this chapter is set on the application of EPR methods to carbon-based materials, from nanographites to graphene-based materials, for the resolution and characterization of the different signals, related to the presence of specific species, or structures. Because of the intrinsic heterogeneity of the samples, this goal is not simple: most of the signals coming from different types of structures have similar spectroscopic features and are overlapping in the cw-EPR spectra with very different relative intensities. It is then necessary to use all possibilities that EPR offers, from the cw-EPR techniques to pulse EPR methods, to disentangle ideally all contributions. Our analysis of the EPR spectra considers the presence of three types of paramagnetic contributions: conduction electrons, edge states and molecular states. This interpretation framework has been shown to be effective for the considered materials, characterized by the presence of finite-dimension graphene layers, eventually stacked one above the other. In our analysis we investigated different experimental parameters, like the variation with the temperature of the EPR intensity, the values of the -tensors and the homogeneous lineshapes of the spectra to obtain further structural information. Pulse EPR methods were used to study and characterize species with long relaxation times (molecular states). Echo-detected EPR enabled to obtain their spectral lineshapes. Hyperfine spectroscopies, ESEEM, ENDOR and HYSCORE, determined the electron hyperfine couplings of unpaired electrons with magnetic nuclei, thus allowing the evaluation of the extent of the π-system and the presence of different types of nuclei
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
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