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Accoppiamento di metalli con ossido di zinco: struttura, morfologia e proprietà ottiche
No full textLo sviluppo di fonti energetiche sostenibili rappresenta un tema cruciale nella ricerca scientifica contemporanea, a causa del crescente fabbisogno energetico, dell’aumento dei prezzi dei combustibili fossili e dell’impatto ambientale. Negli ultimi decenni, un crescente interesse è stato risvegliato dai sistemi metallo-ossido, i quali trovano applicazione in una grande varietà di campi. I sistemi nano strutturati, in particolare, possiedono proprietà peculiari, che possono significativamente migliorare le prestazioni dei dispositivi. In tale contesto, la comprensione dei meccanismi fisici delle superfici e delle interfacce a livello atomico è fondamentale. In questo lavoro sono studiati diversi sistemi in cui l’accoppiamento di un metallo con ossido di zinco (ZnO) promuove l’insorgere di particolari proprietà, che trovano applicazione nei dispositivi di conversione energetica. L’ossido di zinco è un materiale di interesse in numerosi campi di applicazione grazie alle sue proprietà, ai costi di produzione ridotti ed all’elevata bio-compatibilità. In questo lavoro di tesi ZnO in diverse forme associati a diversi metalli è stata studiato con varie tecniche, per ottenere una profonda comprensione dei processi fondamentali coinvolti, al fine di migliorare le prestazioni dei dispositivi.
Un film sottile di ZnO è stato depositato su un cristallo singolo di Mo(001) ed analizzato in ultra-alto vuoto, come sistema modello per investigare i primi stadi della crescita dell’ossido in un ambiente altamente controllato. Composizione e morfologia sono fortemente influenzate dalle condizioni di preparazione: l’ossidazione è favorita da trattamenti in temperatura o da un aumento del rapporto O-Zn durante la deposizione. Il substrato funge da serbatoio di ossigeno durante la crescita. I film presentano un’elevata presenza di difetti legati a deformazioni strutturali.
Se drogato con alluminio, ZnO può essere utilizzato come componente di celle solari e dispositivi opto-elettronici, come ossido conduttivo trasparente (TCO) e come materiale plasmonico alternativo ai metalli nobili. Film di ZnO drogato Al (AZO), con doping variabile fra 0 e 14% at., sono stati cresciuti su MgO tramite magnetron sputtering ed analizzati con una vasta gamma di tecniche, al fine di ottenere una descrizione completa del sistema. Le migliori prestazioni come TCO sono state ottenute per i film al 3.6% at., con resistività ridotta (6x10-4 Ohm cm) ed elevata trasmittanza (85%). Eccellenti proprietà plasmoniche del film sono state misurate al medesimo valore di drogaggio. L’origine del meccanismo di conduzione, ancora dibattuta nonostante l’elevato numero di studi effettuati, è stata univocamente associata al modello di riempimento della banda di conduzione da parte degli elettroni del drogante. Il ruolo fondamentale dei difetti nel determinare le proprietà elettroniche del materiale è stato evidenziato.
L’accoppiamento di nano particelle (NP) di argento con ZnO introduce proprietà plasmoniche, in grado di migliorare significativamente le prestazioni dei dispositivi. NP di Ag sono state depositate su diversi ossidi analizzandone la risposta ottica e come questa è legata alla morfologia del sistema. La procedura di analisi morfologica è stata sviluppata con applicazione al sistema Ag/ZrO2; un substrato costituito da un film di MgO depositato su Mo è stato scelto come sistema prototipo per lo studio dell’auto-organizzazione delle NP. Le procedure di analisi sviluppate sono state quindi applicate al sistema Ag/ZnO. La crescita delle NP è stata modellizzata a temperatura ambiente ed a 673 K e la relazione fra morfologia e proprietà plasmoniche è stata determinata confrontando i dati sperimentali con una simulazione modello. Il duplice ruolo del substrato, che condiziona la morfologia di crescita delle nano particelle e introduce un diverso accoppiamento dielettrico con il metallo, è stato analizzato in modo approfondito.The quest for sustainable and renewable energy sources is a major point in latest research, due to the increasing energy requirements and rising price of fossil-based fuels, so far as the environmental impact. The powerful connubium created in metal-oxide systems has risen more and more interest, thanks to the applications in a vaste variety of fields, including energy production, in photovoltaic, electrochemical and storage devices. Nanostructured systems, in particular, have proved to show peculiar properties, which improve the performances of various devices. In this framework, the comprehension of the physical mechanisms at the atomic level of oxide surfaces and interfaces is fundamental.
This work investigates different systems, where the coupling of a metal with zinc oxide (ZnO), promotes the rising of interesting properties with application in energy conversion devices and in other fields. ZnO has proved to be a promising material for various purposes, due to its peculiar properties, low cost and bio-compatibility. In this thesis the association of ZnO in different forms with different metals has been studied with a variety of techniques for a deeper comprehension both of the fundamental processes involved and the improvement of devices performances.
ZnO film coupling with a Mo(001) single crystal substrate has been studied. The film, prepared and analyzed in Ultra-High Vacuum represents a model system to investigate the first stages of growth of the oxide in a controlled environment. Both film composition and morphology are strongly influenced by deposition conditions: oxidation is improved by temperature treatments or increasing the oxygen-to-zinc ratio during deposition. Molibdenum substrate plays the active role of oxygen reservoir in Zn oxidation. A high presence of defects related to structural deformations has been observed.
Al-doped ZnO (AZO) is a promising component of solar cells and optoelectronic devices, both as a transparent conductive oxide (TCO) and as plasmonic material, alternative to noble metals. AZO films in a wide doping range (0-14 at.%) have been grown on MgO by magnetron sputtering and analyzed with a variety of techniques to relate the different properties and obtain a complete picture of the system. Best performances as TCO have been determined for 3.6 at. % Al content, realizing a high quality film with low resistivity (6x10-4 Ohm cm) and high transmittance (85%). High performances of the AZO films as a plasmonic material have been probed: low losses and a lower absolute value for real part of dielectric function with respect to Ag has been measured. The conduction mechanism, which is still debated despite the high number of studies performed, has been enlightened: the model of conduction band filling by donor dopant electrons has proved to give the best description of AZO behavior. The fundamental role of defects in determining the electronic properties of the material has also been highlighted.
Finally, we have analyzed the coupling of silver nanoparticles (NP) with zinc oxide, focusing on the optical response of the system and how it is related to morphology. Ag NP have been grown and analyzed on different oxides, to develop a suitable and reliable procedure to determine and inter-relate the different properties. The morphological analysis procedure is presented with application on Ag/ZrO2 as a case study. The MgO/Mo system has been chosen as a prototypical substrate for the study of Ag NP self-assembly on the oxide surface. The skills acquired have been exploited in the analysis of Ag/ZnO. Growth of silver NP on the oxide has been modeled both at room temperature and at 673 K and the relation between particle morphology and plasmonic properties has been determined by comparing experimental data with a model simulation. The role of the substrate, acting both by conditioning Ag morphology and by introducing a different dielectric coupling with the metal, has been deeply investigated
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
ZnO nanostructure formation on the Mo(001) surface
No full textIn this paper we have determined the role of oxidizing pressure and annealing temperature in the formation of ZnO nanostructures on the Mo surface. ZnO has been grown on Mo(001) by means of physical vapor deposition of Zn in molecular oxygen pressure. Combining STM, XPS, and XRD we have studied the relation between morphology, composition, and structure. ZnO grows as a film with (101Â ̄1) preferential orientation up to a thickness of 10 Ã, catalyzed by the Mo surface that provides oxygen available for oxide formation and limited by the residual polarity of the surface. Increasing the deposited amount, the role of the Mo interface becomes less effective, and part of the Zn remains metallic, forming clusters on the surface that catalyze the further growth of ZnO in the form of polycrystalline nanostructures. Surfactant Zn is removed by annealing, and peculiar oxide nanostructures are left on the surface. Increasing O pressure or reducing Zn flux allows control of nanostructure density and shape, while the continuous film underneath does not modify
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
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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