1,720,963 research outputs found
Adsorption and interaction of atoms, molecules and nanoclusters on epitaxial graphene.
Full text linkL’attività di ricerca del mio dottorato è stata dedicata allo studio di diversi aspetti dell’interazione del grafene con superfici e con adsorbati quali atomi e molecole e di come quest’ultima sia modificata da un substrato metallico sotto il grafene. Le proprietà elettroniche e meccaniche uniche del grafene lo rendono un candidato ideale per lo sviluppo di dispositivi nanoelettronici ad alta efficienza. Tuttavia, le modifiche indotte alle proprietà del grafene dalla sua interazione con tali specie sono uno dei principali problemi che impediscono un suo utilizzo su scala industriale.
Pertanto, la prima fase della mia ricerca ha riguardato l’identificazione dei fattori che governano l’intensità dell’interazione grafene-substrato. In particolare, studiando e confrontando le proprietà di diverse interfacce grafene/metallo, ho dimostrato che l’interazione fra di essi dipende dall’accoppiamento fra la banda d della superficie metallica e la banda π del grafene.
La fase successiva della mia ricerca è stata dedicata allo studio dell’interazione di piccole molecole inorganiche (principalmente CO) ed atomi (Ar) con grafene supportato su diverse superfici metalliche. I miei risultati dimostrano che le interazioni di van der Waals tra il grafene ed i suoi adsorbati sono aumentate in presenza di un substrato fortemente interagente. Grazie ad una collaborazione con ricercatori del campo della fisica computazionale, ho dimostrato che il fattore che influenza maggiormente l’energia di adsorbimento del CO sul grafene è la composizione chimica dello strato superficiale del substrato metallico, mentre la distanza tra grafene e substrato ha un effetto molto più ridotto. Questo dimostra che l’aumento dell’energia di adsorbimento è originata dalle alterazioni indotte nelle bande del grafene dal loro accoppiamento con la banda d del substrato e non, come sostenuto da diverse fonti in letteratura, da una trasparenza del grafene alle interazioni di van der Waals.
Un altro limite alla possibilità di utilizzare il grafene su scala industriale dipende dai suoi elevati costi di produzione, rispetto alla qualità ottenuta. Ho pertanto dedicato una parte del mio lavoro di tesi anche allo studio delle tecniche di crescita del grafene su superfici metalliche, con lo scopo di identificare i possibili margini di miglioramento. Nello specifico, ho dimostrato che l’energia di adsorbimento di atomi e piccoli cluster di carbonio riveste un ruolo importante già nel determinare le dinamiche di crescita del grafene, e conseguentemente la qualità e le proprietà del prodotto.
Infine, ho studiato alcune possibili applicazioni delle modifiche indotte dall’interazione del grafene con il substrato e con specie adsorbite su di esso in alcuni campi quali il magnetismo e la catalisi. In particolare, ho studiato le dinamiche del superscambio magnetico che si instaura quando il grafene è supportato su un materiale magnetico fra quest’ultimo e le specie adsorbite sul grafene. Inoltre, ho caratterizzato la struttura geometrica ed elettronica di nanoparticelle di titania supportate sul grafene, per le quali il grafene porta ad un aumento dell’attività fotocatalitica di un ordine di grandezza.
In parallelo a queste attività, mi sono dedicato anche allo sviluppo di una sorgente di nanocluster selezionati in massa, il cui fine è di permetterne lo studio con tecniche sperimentali mediate nello spazio. Per ampliare le mie conoscenze su questo tipo di strumenti, ho svolto un tirocinio di tre mesi (nell’ambito del programma Erasmus + Traineeship) presso il Dipartimento di Chimica della Technische Universität München, dove ho partecipato alla messa in funzione di uno strumento analogo.The research activity I have performed during my PhD was centred on the study of several aspects of the interaction of graphene with surfaces and adsorbates including atoms and molecules. In particular, I have investigated how the latter is modified by a metal substrate supporting graphene. The unique electronic and mechanical properties of graphene make it an ideal candidate for applications in high efficiency nanoelectronic devices. However, these properties are modified when it interacts with such species, and this represents one of the major issues still preventing its wide-spread adoption.
For this reason, in the first phase of my research I have investigated the factors which govern the intensity of the graphene-substrate interaction. In particular, by characterising and comparing the properties of several graphene/metal interfaces, I have shown that the interaction occurring between them is due to the coupling of the d-band of the substrate with the π band of graphene.
Following this, I have investigated the interaction between small inorganic molecules (such as CO) and atoms (Ar) with metal-supported graphene. My results show that these interactions, which are based on van der Waals forces, increase when graphene is supported on a strongly interacting surface. In collaboration with computational scientists, I have shown that the main factor influencing the adsorption energy of CO on graphene is the chemical composition of the substrate’s topmost layer, while the distance between graphene and the substrate plays a negligible role. This proves that this increased adsorption energy is due to the modifications induced by the substrate on the bands of graphene by their coupling with the substrate’s d-band. A contribution due to the transparency of graphene to van der Waals interaction, which had been proposed by some works in literature, could instead be refuted.
Another issue related to the wide-scale application of graphene which I have addressed here is the cost-effectiveness of its synthesis. In particular, part of my research work was dedicated to the identification of the factors which can be address to improve the existing synthesis techniques. More in detail, I have demonstrated the important role of the adsorption energy of atoms and small clusters of carbon on graphene on the dynamics of its growth, and therefore on the quality of the final product.
Finally, I have studied some possible applications which exploit the modifications induced by the interaction of graphene with its substrate and with species adsorbed on it on the properties of the latter, in fields such as magnetism and catalysis. More in detail, I have studied the dynamics leading to a magnetic superexchange between graphene supported on a magnetic material and molecular adsorbates. Furthermore, I have characterised the geometric and electronic structure of titania nanoparticles supported on different graphene/substrate interfaces, which have shown a 20-fold increase in their photocatalytic activity with respect to a graphene-less control system.
In parallel to these activities, I have worked on the development of a mass-selected nanocluster source, whose final aim is to allow the characterisation of the properties of nanoclusters as a function of their mass with space-averaging experimental techniques. To improve my knowledge on this kind of apparatus, I have made a 3-month traineeship (within the Erasmus + project) at the Chemistry Department of Technische Universität München, where I have worked on the commissioning of an analogous device
Determining the atomic coordination number in the structure of β12 borophene on Ag(111) via X-ray photoelectron diffraction analysis
Full text linkThis study investigates the electronic properties of the borophene β12 phase on Ag(111) and correlates them with specific structural features by combining high-resolution core-level photoelectron spectroscopy, X-ray photoelectron diffraction, and density functional theory-based calculations. We establish a link between the atomic coordination number of the non-equivalent B atoms in the β12 unit cell and the observed spectroscopic signatures in the B 1s spectrum. This finding is conclusively proven by photoelectron diffraction, which confirms that this polymorph exhibits minimal corrugation on Ag(111). These results contribute to a deeper understanding of the properties of various borophene structures on metallic substrates and may stimulate further studies in realizing nanoscaled structures where the atomic coordination number plays a central role
Superexchange pathways stabilize the magnetic coupling of MnPc with Co in a spin interface mediated by graphene
Full text linkWe investigate the magnetic response of a spin interface constituted by MnPc molecules adsorbed on graphene/Co and its robustness against thermal fluctuations by x-ray magnetic circular dichroism. Element-selective hysteresis loops reveal a remarkable antiferromagnetic coupling between MnPc and Co that is strong both in perpendicular and in-plane magnetic orientations, thanks to the magnetic anisotropy properties and electronic configuration of MnPc. The magnetic interaction between MnPc and Co is mediated by the molecular states and the graphene π orbitals in a superexchange mechanism that allows a strong exchange coupling while the molecular orbitals symmetries are preserved by the graphene decoupling layer. Our results show that the strength and stability of the magnetic coupling between MnPc molecules and Co layer(s), intercalated at the graphene/Ir(111) interface, is further optimized by the open
3d shell of the central Mn ion. The magnetic properties are compared with analogous molecular spin interfaces with high thermal stability, paradigmatic examples to exploit in surface-supported molecular spin electronics
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
Metal phthalocyanines interaction with Co mediated by a moiré graphene superlattice
No full textThe assembling of metal phthalocyanines on the rippled moiré superlattice of graphene/Ir(111) intercalated with one Co layer is driven by the site-dependent polarization field induced by the incommensurate graphene-Co interface. We have performed an X-ray absorption and photoemission study to unveil the role of the metallic centers and of the organic ligands in the molecule-Co interaction process mediated by graphene. Notably, we consider different electronic molecular orbitals, i.e. phthalocyanines with Cu and Mn metallic ions. The spectroscopic response suggests almost unaltered CuPc molecular states upon adsorption, and the rippled graphene carpet decouples completely the electronic interaction between the molecules and the Co layer, while a slight hybridization is present for MnPcs. MnPc molecules, trapped in the valleys of the moiré graphene superlattice, slightly intermix, through the orbitals protruding out of the molecular plane, with the underlying Co, while the organic ligands are almost unaltered. Graphene acts as an interlayer and mediates the interaction between metal phthalocyanines and the metallic substrate, prevent- ing a strong chemical intermixing and enabling the assembly of almost unaltered molecules, preserving their electronic/magnetic state
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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