Graphene induced effects on epitaxial Cr2O3 thin films grown on a Ni(111) substrate

LAUREA MAGISTRALEFilm ultra sottili di Cr2O3 sono stati cresciuti su superfici di Ni(111) e Ni(111) ricoperto da grafene. Le proprietà chimiche dei campioni sono state studiate mediante spettroscopia elettronica Auger e spettroscopia fotoelettronica a raggi x. La struttura reticolare, la topografia della superficie e le proprietà elettroniche sono state investigate rispettivamente tramite diffrazione elettronica a bassa energia, microscopia e spettroscopia a effetto tunnel. Il grafene rimane sepolto all'interfaccia Cr2O3/Ni(111), proteggendo il substrato dall'ossidazione. Quando invece il Cr2O3 viene cresciuto sul Ni pulito, uno strato di ossido (NiO) si forma all'interfaccia. L'ossidazione del substrato nel campione privo di grafene provoca lo sviluppo di una superficie rugosa, mentre nel caso di substrato ricoperto da grafene il film di Cr2O3 mantiene una morfologia piatta. Inoltre, siccome il parametro reticolare del grafene è più piccolo rispetto a quello di NiO, il film di Cr2O3 è soggetto a una diversa deformazione. Un semplice modello fenomenologico viene presentato per relazionare la deformazione del Cr2O3 alle proprietà elettroniche di Cr2O3/Ni(111) e Cr2O3/Gr/Ni(111), che sono rispettivamente isolanti e metalliche.Cr2O3 ultra-thin films have been epitaxially grown on a Ni(111) and on a graphene covered Ni(111) surface. The chemical properties of the sam- ples have been studied by means of Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy. The lattice structure, surface topography and electronic properties of the Cr2O3 films have been elucidated by means of Low Energy Electron Diffraction, Scanning Tunneling Microscopy and Spec- troscopy, respectively. The graphene layer remains buried at the Cr2O3/Ni(111) interface, pro- tecting the substrate from oxidation. On the other hand, when Cr2O3 is grown on the bare Ni substrate, a nickel oxide (NiO) film develops at the interface. The oxidation of the substrate in the graphene-free sample in- duces the development of a rough surface, while the Cr2O3 film grown on the graphene covered substrate retains a flat morphology. Furthermore, because of the smaller lattice parameter of graphene with respect to that of NiO, the Cr2O3 film experiences a different state of strain. A simple phenomenological model is proposed to relate the strain of Cr2O3 to the electronic properties of Cr2O3/Ni(111) and Cr2O3/Gr/Ni(111), which are insulating and metallic, respectively

Studio multidisciplinare finalizzato alla riqualificazione ambientale della valle del Rio della Rocca (Comune di Castellarano, Provincia di Reggio Emilia)

La valle del Rio della Rocca nel Comune di Castellarano (Reggio Emilia) è stata interessata a partire dagli anni ‘50 del secolo scorso da attività estrattive che in parte ne hanno profondamente modificato le caratteristiche ambientali, in particolare quelle geomorfologiche. In considerazione dell’interesse paesaggistico della valle e ai fini di una riqualificazione ambientale del territorio, è stato realizzato uno studio multidisciplinare volto a delineare i principali aspetti geologici, paleontologici, floristici e faunistici. Specifica attenzione è stata posta alla valutazione dell’instabilità dei versanti e alla individuazione dei geositi presenti nell’area. Sulla scorta dei risultati delle ricerche effettuate sono state elaborate proposte di riqualificazione del territorio che prevedono una valorizzazione della valle a fini geoturistici e ricreativi

Dionigi Periegeta, Oikoumhnhs Perihghsis Introduzione, Testo critico, Traduzione e commento.

Dopo l'introduzione, dedicata essenzialmente alla riproposizione delle varie ipotesi di ricostruzione della tradizione manoscritta dell'opera di Dionigi Periegeta, si presenta il testo originale della "Periegesi" con apparato e traduzione. L'ultima e più cospicua parte della tesi è dedicata al commento dell'opera, incentrato sulle problematiche testuali e interpretative. Vengono altresì affrontati problemi stilistici, di poetica, di ricerca delle fonti

Biasing crystallization in fused silica: An assessment of optimal metadynamics parameters

Metadynamics (MetaD) is a useful technique to study rare events such as crystallization. It has been only recently applied to study nucleation and crystallization in glass-forming liquids such as silicates, but the optimal set of parameters to drive crystallization and obtain converged free energy surfaces is still unexplored. In this work, we systematically investigated the effects of the simulation conditions to efficiently study the thermodynamics and mechanism of crystallization in highly viscous systems. As a prototype system, we used fused silica, which easily crystallizes to β-cristobalite through MetaD simulations, owing to its simple microstructure. We investigated the influence of the height, width, and bias factor used to define the biasing Gaussian potential, as well as the effects of the temperature and system size on the results. Among these parameters, the bias factor and temperature seem to be most effective in sampling the free energy landscape of melt to crystal transition and reaching convergence more quickly. We also demonstrate that the temperature rescaling from T > Tm is a reliable approach to recover free energy surfaces below Tm, provided that the temperature gap is below 600 K and the configurational space has been properly sampled. Finally, albeit a complete crystallization is hard to achieve with large simulation boxes, these can be reliably and effectively exploited to study the first stages of nucleation

Disclosing crystal nucleation mechanism in lithium disilicate glass through molecular dynamics simulations and free-energy calculations

Unraveling detailed mechanism of crystal nucleation from amorphous materials is challenging for both experimental and theoretical approaches. In this study, we have examined two methods to understand the initial stage of crystal precipitation from lithium disilicate glasses using molecular dynamics simulations. One of the methods is a modified exploring method to find structurally similar crystalline clusters in the glass models, enabling us to find three different embryos, such as Li2Si2O5 (LS2), Li2SiO3 (LS) and Li3PO4 (LP), in the 33Li2O·66SiO2·1P2O5 glass (LS2P1), in which P2O5 is added as a nucleating agent. Interestingly, LS2 and LP crystals were found inside the LS2P1 glass while LS crystal appeared on the glass surface, which agrees with experimental observations. The other method is free energy calculation using a subnano-scale spherical crystal embedded in the glass model. This method, which we called Free-Energy Seeding Method (FESM), allows us to evaluate free energy change as a function of crystal radius and to identify critical size of the crystal precipitation. The free energy profiles for LS and LS2 crystal nuclei in the LS2 glass models possess maximum energy at a critical radius as expected by classical nucleation theory. Furthermore, the critical radius and the energy barrier height agree well with recent experimental investigation, proving the applicability of this method to design glass–ceramics by atomistic modeling

Structural origins of the Mixed Alkali Effect in Alkali Aluminosilicate Glasses: Molecular Dynamics Study and its Assessment

The comprehension of the nonlinear effects provided by mixed alkali effect (MAE) in oxide glasses is useful to optimize glass compositions to achieve specific properties that depend on the mobility of ions, such as the chemical durability, glass transition temperature, viscosity and ionic conductivity. Although molecular dynamics (MD) simulations have already been applied to investigate the MAE on silicates, less effort has been devoted to study such phenomenon in mixed alkali aluminosilicate glasses where alkali cations can act both as modifiers, forming non-bridging oxygens and percolation channels, and as charge compensator of the AlO4− units present in the network. Moreover, the ionic conductivity has not been computed yet; thus, the accuracy of the atomistic simulations in reproducing the MAE on the property is still open to question. In this work, we have validated five major interatomic potentials for the classical MD simulations by modelling the structure, density, glass transition temperature and ionic conductivity for three aluminosilicate glasses, (25 − x)Na2O − x(K2O) − 10(Al2O3) − 65(SiO2) (x = 0, 12.5, 25). It was observed that only the core-shell (CS) polarizable force field well reproduces the experimentally measured MAE on Tg and the ionic conductivity as well as the higher conductivity of single sodium aluminosilicate glass at low temperature and the higher conductivity of single potassium aluminosilicate glass at high temperature. The MAE is related to the suppression of jump events of the alkaline ions between dissimilar sites in the percolation channels consisting of both sodium and potassium ions as in the case of alkaline silicates. The superior reproducibility of the CS potential is originated from the larger and the flexible ring structures due to the smaller Si-O-Si inter-tetrahedra angle, creating appropriate percolation channels for ion conductivity. We also report detailed assessments for using the potential models including the CS potential for investigating MAE on aluminosilicates