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Studio dei meccanismi di trasporto di carica in film sottili a base di materiali correlati al grafene (GRM)
No full textL’avvento di tecniche economiche per produrre fogli di materiali con spessore mono-atomico, come il grafene [1], ha aperto la possibilità di progettare materiali nanostrutturati con proprietà chimico/fisiche pre-programmabili. I sistemi tecnologicamente rilevanti a base di materiali correlati al grafene (GRM) sono principalmente networks di microfogli bidimensionali distribuiti in modo casuale e altamente difettosi [2]. Sebbene il trasporto di carica sia stato ampiamente trattato per fogli bidimensionali isolati [3], uno studio esaustivo che correli le proprietà elettriche dei GRM networks con la complessità della struttura e della morfologia del materiale è ancora mancante in letteratura.
Lo scopo di questo lavoro è indagare il trasporto di carica (CT) in film sottili di GRM, con un certo grado di disordine strutturale. In particolare, abbiamo studiato i meccanismi di CT che si verificano all'interfaccia tra due fogli – spesso i meccanismi d’interfaccia sono considerati un fattore limitante – così come il ruolo della complessità geometrica del network nella conducibilità elettrica complessiva delle strutture di microfogli.
Abbiamo utilizzato fogli monoatomici di grafene ossido (GO) come materiale bidimensionale modello. Il GO consiste infatti in un reticolo di grafene conduttivo che include gruppo funzionali dell’ossigeno sia sul piano basale che ai bordi del foglio. I microfogli isolanti di GO vengono depositati su substrati di ossido di silicio e sono ridotti termicamente per ripristinare alcune delle proprietà del grafene, tra cui la conducibilità elettrica. Oltre al GO ridotto, abbiamo impiegato un GRM composto da fogli multistrato di grafene (parzialmente ossidato). Tale materiale è il GO esfoliato elettrochimicamente (eGO). Abbiamo quindi sfruttato diversi metodi di deposizione: i) spin-coating, ii) spray-coating e iii) filtrazione assistita da vuoto per fabbricare film sottili di fogli di GRM parzialmente impilati e con dimensioni laterali macroscopiche.
Le proprietà chimiche e morfologiche dei film sono state caratterizzate attraverso misurazioni di spettroscopia fotoelettronica a raggi X (XPS), microscopia a forza atomica (AFM) e diffrazione di raggi X (XRD). I meccanismi di trasporto sono stati studiati invece con misure di resistività elettrica (ρ) in funzione della temperatura, da valori ambientali fino a 5 K.
Possibili ambiguità sull'analisi quantitativa di ρ(T) sono state risolte utilizzando un metodo autoconsistente basato sull'energia di attivazione ridotta [5], ovvero la derivata logaritmica della resistività rispetto alla temperatura: W(T)=-(d lnρ)⁄(d lnT ). Questa trasformazione matematica ha permesso di analizzare il dataset ρ(T) con funzioni lineari. Abbiamo correlato i parametri caratteristici di trasporto con il grado di ordine dei nostri campioni e chiarito il ruolo dell'impacchettamento verticale dei fogli, ovvero dell'interazione π-π tra cluster aromatici sovrapposti, nel CT del film. Abbiamo inoltre evidenziato le differenze di CT tra i film basati su GO ridotto e quelli su eGO.
Il lavoro proposto pone le basi per lo sviluppo di nuovi modelli e protocolli riguardanti i meccanismi di CT in sistemi realistici di GRM, come inchiostri e compositi polimerici.
[1] Ferrari, A. C. et al. Nanoscale 7, 4598-4810, (2015).
[2] Palermo V., Chem. Comm. 49, 28, 2848-2857 (2013); Kelly A. et al, Science 356, 6333 (2017).
[3] Eda G. et al, J. Physics. Chem.C 113, 15768 (2009); Kaiser a. et al, Nano Letters 9, 1787 (2009); Joung D. and Khondaker S., Phys. Rev. B 86, 235423 (2012).
[4] Xia Z. et al, J. Physics. Chem.C 123, 15122 (2019).
[5] Zabrodskii A. G., Philos. Mag. B 81, 1131 (2001).The development of cheap techniques to produce large sheets of monoatomic thick materials, such as graphene [1], opened new avenues to design nanostructured materials with pre-programmed chemical and physical properties. Most of the technologically relevant graphene-related materials (GRMs) systems are networks composed of randomly distributed and highly defective 2D microsheets [2]. While the charge transport has been extensively studied in single nanosheets [3], a comprehensive study that correlates the electrical properties of networks composed of purely 2D graphene-based materials with the complexity of the material structure and morphology is still missing.
The aim of this work is to investigate charge transport (CT) in GRMs films, going towards structures with increasing disorder. In particular we investigated the CT mechanisms occurring at the sheet-to-sheet interface – typically the interfacial mechanisms are considered as bottlenecks – as well as the role of the geometrical complexity of the network in the overall electrical conductivity of the nanosheets assemblies.
As prototypical 2D material we used single monolayer sheets of graphene oxide (GO), which consists of a conductive graphene lattice including oxygen functionalities/ defects both on the basal plane and at the edges of the sheet. Electrical insulating GO sheets are deposited on silicon oxide substrates and thermally reduced restoring partially the conductive properties of the 2D sheets. In addition to reduced GO, we employed a GRM made of multiple staked sheets of (partially oxidised) graphene bilayers: electrochemical exfoliated GO (eGO) [4].We exploited different deposition methods: i) spin-coating, ii) spray-coating and iii) vacuum-assisted filtration to fabricate macroscopic GRMs thin films with sheets partially stacked.
Chemical and morphological properties of the films were characterized by X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) measurements. We investigated transport mechanisms measuring the temperature-dependence of the electrical resistivity (ρ) from room temperature down to 5 K.
Possible ambiguities on the quantitative analysis of ρ(T) were solved by using a robust self-consistent method based on the reduced activation energy [5], i.e. the logarithmic derivative of resistivity versus temperature: W(T)=-(d lnρ)⁄(d lnT ). This mathematical transformation allowed to analyse ρ(T) dataset with linear functions. We correlated the transport characteristic parameters with the degree of order of our samples and elucidate the role of the sheets vertical stacking, that is of the π-π interaction between overlapped aromatic clusters, in the CT in the film. We also highlighted the differences in CT between reduced GO based films and eGO ones.
The presented work could pave the way to develop new models and protocols to access the CT mechanisms in realistic GRMs, such as inks and polymer composites.
[1] Ferrari, A. C. et al. Nanoscale 7, 4598-4810, (2015).
[2] Palermo V., Chem. Comm. 49, 28, 2848-2857 (2013); Kelly A. et al, Science 356, 6333 (2017).
[3] Eda G. et al, J. Physics. Chem.C 113, 15768 (2009); Kaiser a. et al, Nano Letters 9, 1787 (2009); Joung D. and Khondaker S., Phys. Rev. B 86, 235423 (2012).
[4] Xia Z. et al, J. Physics. Chem.C 123, 15122 (2019).
[5] Zabrodskii A. G., Philos. Mag. B 81, 1131 (2001)
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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