Dipartimento di Fisica - Tesi di Dottorato
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Questa collezione raccoglie le Tesi di Dottorato afferenti al Dipartimento di Fisica dell'Università della Calabria.
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Item Synthesis and cherization of low-dimensional materials(Università della Calabria, 2020-04-16) Alessandro, Francesca; Critelli, Salvatore; Caputi, Lorenzo; Cupolillo, AnnaThe main aim of this thesis is to synthesize and study low-dimensional materials, with special focus on: silicene, PtTe2, carbon nano-onions and activated carbon. The first section of this work describes the study of the collective modes in silicene and PtTe2. Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological interest in view of the exploitation of its exotic electronic properties. This material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. Using a combined experimental (High-Resolution Electron-Energy-Loss Spectroscopy, HR-EELS) and theoretical (Time Dependent Density Functional Theory, TDDFT) approach the electronic excitations of two phases of silicene growth on silver were studied showing that silicene grown in a mixed phase on Ag(111), preserves part of the semimetallic character of its freestanding form, exhibiting an interband π-like plasmon. Recently, the PtTe2 has emerged as one of the most promising among layered materials ―beyond graphene‖. In this work, the electronic excitations of the bulk PtTe2 were investigated by means of EELS and DFT detecting a sequence of modes at 3.9, 7.5 and 19.0 eV. The comparison of the excitation spectrum with the calculated density of states (DOS) allowed to ascribe spectral features to transitions between specific electronic states. Moreover, it has been observed that, in contrast to graphene, the high-energy plasmon in PtTe2 gets red-shifted by 2.5 eV with increasing thickness. The second section of this thesis reports the synthesis of polyhedral carbon nano-onions by arc discharge in water and the electrochemical performance of activated carbon in aqueous electrolytes. CNOs, in their spherical or polyhedral forms, represent an important class of nanomaterials, due to their peculiar physical and chemical properties. In this work, polyhedral carbon nano-onions (CNOs) were obtained by underwater arc discharge of graphite electrodes using an innovative experimental arrangement. Dispersed nanomaterials and a black hard cathodic deposit were generated during the discharges and studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and thermogravimetric analysis (TGA). A model for the formation of the deposit was proposed, in which the crystallization is driven by an intense temperature gradient in the space very close to the cathode surface. Electric double layer capacitors (EDLC) are gaining increasing popularity in high power energy storage applications. Novel carbon materials with high surface area, high electrical conductivity, as well as a range of shapes, sizes and pore size distributions are being constantly developed and tested as potential supercapacitor electrodes. In this thesis, the electrochemical behavior of a highly microporous activated carbon was studied as electrode for symmetric and asymmetric capacitors in acid and neutral media. The highest capacity and energy density values were obtained in the case of the activated carbon in acid solution.Item Preparazione e caratterizzazione di nanomateriali carboniosi mediante processi idrotermali(Università della Calabria, 2020-04-21) Veltri, Francesco; Caputi, Lorenzo; Tavolaro, Adalgisa; Ciuchi, Federica; Mazzulla, AlfredoItem Synthesis and characterization of nanomaterials: graphene, silicene and carbon nano- onions(2017-10-20) Osman, Salih Mohamed; Critelli, Salvatore; Caputi, LorenzoThe electronic structure of the graphene/Ni(111) system was investigated by means of electron energy-loss spectroscopy (EELS). A single layer of graphene has been obtained on Ni(111) by dissociation of ethylene. Angle-resolved EEL spectra show a low energy plasmon dispersing up to about 2 eV, resulting from fluctuation of a charge density located around the Fermi energy, due to hybridization between Ni and graphene states. The dispersion is typical of a two-dimensional charge layer, and the calculated Fermi velocity is a factor of ~0.5 lower than in isolated graphene. The interface-π plasmon, related to interband transitions involving hybridized states at the K point of the hexagonal Brillouin zone, has been measured at different scattering geometries. The resulting dispersion curve exhibits a square root behavior, indicating also in this case a two-dimensional character of the interface charge density. As well, it has been shown that it is possible to use EELS in the reflection mode to measure the fine structure of the carbon K-edge in monolayer graphene on Ni(111), thus demonstrating that reflection EELS is a very sensitive tool, particularly useful in cases where the TEM-based ELNES cannot be applied. Clean Ag(111) surface and the two phases of silicene on Ag(111), mixed (4×4, √13×√13R19°, 2√3×2√3R30°) and 2√3×2√3R30, have been studied by XPS, LEED and EELS. EEL spectra of the Ag(111) surface covered by silicene in the (4×4, √13×√13R19°, 2√3×2√3R30°) mixed phase shows a well-defined plasmon peak whose center is located at about 1.75 eV. The 2√3×2√3R30° phase shows EEL spectra that exhibit a peak located at about 0.75 eV loss, which moves clearly towards higher energies with increasing momentum transfer. The typical parabolic dispersion relation obtained from such spectra confirms that the peak is due to a collective excitation which is evidently associated to the silicene layer. These plasmons associated to silicene have never been observed in the past. Our results show that the plasmonic properties of silicene on Ag(111) are strongly dependent on the geometrical arrangement of Si atoms with respect to the substrate. Carbonaceous nanomaterials have been obtained by underwater arc discharge between graphite electrodes. TEM images showed that the resulting particles suspended in water consist of CNOs with other carbonaceous materials such as CNTs and graphene. We observed for the first time the formation of a solid agglomerate on the cathode surface. Raman and TEM studies revealed that the agglomerate is made exclusively of CNOs. The defragmentation of such agglomerate allows to obtain CNOs free of other carbonaceous materials without the complex purification procedures needed for floating nanomaterialsItem Experimental and theoretical study of polyhedral carbon Nano-Onions(2018-02-23) Basantes Valverde, Marlon Danilo; Pantano, Pietro; Caputi, Lorenzo; De Luca, GiorgioCarbon nano-onions (CNOs), in their spherical or polyhedral forms, represent an important class of nanomaterials due to their peculiar physical and electrochemical properties. Among the different methods of production, arc discharge between graphite electrodes sustained by deionized water is one of the most promising to obtain good quality CNOs. The arc discharge method is applied to optimize the production of CNOs, and the synthesized nanomaterials by TEM was studied. An innovative experimental arrangement is used to obtain CNOs dispersed in water together with other carbon nanomaterials, and a black hard cathodic deposit. A simple mechanical grinding of the deposit it allowed to obtain turbostratic polyhedral CNOs with different aspect ratios, which exhibited higher stability towards burning in air, compared to CNOs found in water. A mechanism for the formation of the CNOs contained in the deposit, different from the generally accepted mechanism responsible for the synthesis of CNOs dispersed in water, is hypothesized. These spherical or polyhedral multi-shell fullerenes are widely studied owing to their interesting electronic and mechanical proprieties; nevertheless, comparative studies on these nanoparticles remain scarce. Herein, some key electronic proprieties of single and double walled icosahedral fullerenes as function of their sizes were calculated in the frame of the Density Functional Theory. In particular, structures of icosahedral polyhedral fullerenes, previously validated, were used to get the gap between the Highest Occupied Molecular Orbital and the Lowest Unoccupied Molecular Orbital levels (H-L gap), electron affinity, first ionization potential, electronegativity as well as the Density of the electronic States. This work shows that the H-L gap of the single-wall fullerenes decreases as the nanoparticles size increases, whereas an opposite trend was obtained for the double walled fullerenes. Going from single to double wall nanoparticles; a systematic and marked decrease of the H-L gap was found although, this difference reduces increasing the size of the double walled up obtaining an inversion. The DOS structures of SW nanoparticles changes radically adding a second shell, and the extent of these changes depends on the sizes of the analyzed fullerenesItem Physical and Chemical treatments to produce graphene and their related applications(2016-02-26) Tubon Usca, Gabriela Viviana; Pantano, Pietro; Tavolaro, Adalgisa; Caputi, LorenzoIn this work Few Layers Graphene (FLG) and Graphene Oxide (GO) were produced by using physical and chemical treatments, and two types of applications were tested with GO. The first application concerns the Drug delivery in the field of nano-medical treatments, while the second regards environmental remediation for removal of pollutants from water. Few Layers Graphene (FLG) was produced from natural graphite by two methods: i) Sonication in a mixture of solvents, and ii) With the aid of an external agent (zeolite crystals) in the exfoliation process. In the first stage, the mixture was made with two types of solvents: N-methyl-2 pyrrolidone and Dimethylsulfoxide in different ratios. The exfoliation was carried out in that mixtures, then the centrifugation was applied in order to remove unexfoliated graphite. The supernatant suspensions were characterized using Ultraviolet - visible spectroscopy (UV-vis), and Raman Spectroscopy. The Uv-visible analysis and the Raman spectroscopy showed of existence of Few layers Graphene (FLG). In the second stage, the zeolite 4A (Z4A) was selected. The experiments were carried out to improve the exfoliation of graphite, after the exfoliation and centrifugation; the stability was achieved in those that were added the zeolite 4A. Supernatant solutions were characterized by Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Electron Diffration, and Raman Spectroscopy. The 3_BS suspension and the 7_F suspension showed the best results; these reached the greatest amount of days in suspension. The Electrical Characterization (EC) was carried out using 3_BS and 7_F suspensions. The drop-casting technique was used over Al2O3 substrates with gold (Au) InterDigitated Electrodes (IDE). The Current–Voltage (I-V) characterization was performed, and the results were averaged for each sample and computed; in order to obtain the 2D resistivity (ρ2D). Finally, an annealing treatment was applied on the Al2O3/Au substrates; afterwards, the resistivity improves, for 3_BS ink by a factor of 1.75 and for the 7_F ink by a factor of 1.3. Graphene Oxide was produced from natural graphite flakes. A chemical treatment was applied in order to produce graphene oxide through the Hummer’s method and Improved Hummer’s method. At the end of the process, the graphene oxide was recovered under form of colloidal suspensions. The characterization was made by Field Emission Scanning Electron Microscopy (FESEM), Ultraviolet–visible (UV-vis) spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), Energy Dispersive Spectroscopy (EDS), and Raman Spectroscopy. The results showed a good level of oxidation in the material and small flakes of graphene oxide. Concerning to the adsorption process for drug delivery, a cancer drug was used. Doxorubicin (DOX) hydrochloride was placed in contact with GO to evaluate the capacity of adsorption of the material using the depletion method. The study was carried out by using different initial concentrations of DOX and different pH values. All experiments were placed under agitation in dark conditions at room temperature and different incubation times. Once the results of final concentrations was completed, the quantity loaded onto GO were calculated. Finally, the kinetic adsorption showed a percentage of 95% at pH 3 in only 24 hours of interaction. The GO presented excellent characteristics to be used in nano-medical applications. Regarding environmental applications, an adsorption study was conducted using commercial Acridine Orange dye (AO). The adsorption process was proved using the depletion method. AO was prepared in aqueous solution at different concentrations, and these were placed under agitation and dark conditions at different contact times to evaluate the kinetic adsorption. The GO was analyzed at different weight using the highest concentration of AO. On the other hand, the temperature and the incubation time were varied, to find out the best conditions for the adsorption process. The kinetic of adsorption showed a percentage of adsorption among 75% to 95% in the first 20 min for higher concentrations and GO showed a better adsorption process to higher temperaturesItem Llow dimensional materials synthesis, characterization and applications(2017-02-24) Coello Fiallos, Diana Carolina; Pantano, Pietro; Caputi, Lorenzo; Tavolaro, AdalgisaThis thesis was carried out at the laboratory of the Surface Nanoscience in the Department of Physics of University of Calabria, and at the Institute of Membrane Technology (ITM) of CNR, dealing the study of nanostructured systems with low dimensionality. The first part of the work consisted in the synthesis and characterization of graphite oxide (GO), obtained by chemical oxidation and sonication of natural graphite. We used an innovative oxidation process with respect to those present in the literature, and the resulting material was studied by means of UV-visible and Infrared absorption spectroscopy, Raman and XPS spectroscopies, and by TEM and SEM microscopies. The sonicated graphite oxide (sGO) was tested for adsorption of dyes, with the aim to contribute to research in the field of the reduction of pollutants in the liquid phase. We used Methylene Blue, Acridine Orange and Cresyl Violet dyes, whose kinetics of adsorption on sGO was been meticulously studied by the UV-visible absorption spectroscopy. The results showed that the graphite oxide effectively absorbs the three dyes, and the chemical nature of the adsorption process was evidenced by means the kinetic simulations adopting different models. The second part of the work relates to the synthesis of carbon-based nanostructures of the type "nano-onions" (CNOs), similar to multi-layer fullerenes, of great scientific and technological interest. The CNOs can be obtained by means of arc discharge between carbon electrodes immersed in deionized water. This method was used in this thesis work, and the produced carbonaceous nanomaterials have been characterized by Raman spectroscopy and TEM microscopy. Particular attention has been paid to those nanomaterials that contain a high percentage of CNO, since the arc discharge method also generates carbon nanotubes (CNT) and amorphous carbon. The main finding of the research was the identification of a solid agglomerate on the cathode, consisting almost exclusively of polyhedral CNOs and turbostratic graphite. The genesis of such CNOs is not attributable to a crystallization process of the carbon ions expelled from the plasma zone towards the surrounding water. The evidence of the fact that during the discharge the cathode remains at a temperature certainly lower respect to the anode, has permitted to hypothesize the CNOs training process, in which the carbon ions crystallize in the presence of a temperature gradient in the immediate vicinity of the cathode surface. In the last part of this research activity, I investigated a crystal of Indium Selenium (InSe) by means of different electron spectroscopies, in ultra-high vacuum conditions. Through electron energy loss spectroscopy (EELS) the electronic properties of InSe and its reactivity towards oxygen and air have been studied. The results have allowed to identify the most relevant transitions between electronic states in good agreement with existing theoretical calculations of the electronic structure and density of states. Experimental observations have also shown that the material is very stable with respect to possible oxidizing agents.Item Proprietà elettroniche collettive del grafene supportato: influenza del substrato(2014-11-12) Ligato, Nadia; Pantano, Pietro; Caputi, Lorenzo; Cupolillo, Anna