Dipartimento di Fisica - Tesi di Dottorato

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Study of the electronic and structural properties of tin dioxide and armchair graphene nanoribbons
(2016-02-02) Villamagua Conza, Luis Miguel; Pantano, Pietro; Carini, Manuela; Stashans, Arvids
This dissertation is focused on the study of the electrical and structural properties of two emerging materials, the tin dioxide (SnO2) and graphene, which have attracted the interest of the semiconductor-device community due to their extraordinary characteristics. The SnO2 has been studied by means of ab initio simulations (Vienna ab initio Simulation Package, VASP). Both n-type and p-type conductivities were investigated. The intrinsic n-type conductivity has been achieved through two schemes: the first one through the combination of oxygen deficiencies and interstitial atoms inside the SnO2 lattice, whereas in the second one, through the combination of interstitial and/or substitutional hydrogen atoms inside the SnO2 lattice. On the other hand, the p-type conductivity was achieved by codoping n-type SnO2 (from earlier configurations) with low concentrations of nitrogen and aluminum impurities. The performed theoretical studies, to a good extent, agree with the experimental results provided by our collaboration group at the National Central University, Jhong-Li (Taiwan). In prospective, these results confirmed that SnO2 is a promising candidate to replace indium in transparent conductive oxides (TCOs) used in photovoltaic, thin-film transistor, and transparent electronic applications. The theoretical study of graphene has been conducted by means of a tight-binding approach (Atomistic ToolKit simulation package, ATK): the electrical and structural properties of edge-defected armchair graphene nanoribbons (AGNRs) were studied. It was found that Stone-Wales defect (very common in carbon allotropes) placed at the edges of the AGNRs can spark an extra opening of the energy gap in graphene, in addition to that obtained through the quantum confinement of electrons. Moreover, an experimental study on the electrical properties of graphene was carried out at the Tyndall National Institute (Ireland) to understand the influence of multiple cleaning treatments on graphene field-effect transistor (GFET) devices. Debris from residual polymers that appeared during device fabrication was swept off the graphene surface without significantly degrading the electronic properties of the graphene flake. The results suggest that the unusual but extraordinary properties of these graphene allotropes can be considered as a very innovative booster for semiconductor devices, allowing the improvement of the scaling trend beyond that obtained with conventional materials.
Physical and Chemical treatments to produce graphene and their related applications
(2016-02-26) Tubon Usca, Gabriela Viviana; Pantano, Pietro; Tavolaro, Adalgisa; Caputi, Lorenzo
In 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 temperatures
Graphene synthesis by top-down and bottom-up strategies
(2016-02-26) Cid Perez, Denia marlenis; Pantano, Pietro; Caputi, Lorenzo S.; Tavolaro, Adalgisa
Graphene is a two-dimensional structure arranged in a hexagonal form (similar to a honeycomb) that has unique physicochemical properties and has generated interest in the scientific community for its properties and applications. To produce graphene, several methods are used, all of them can be divided in two approaches: the bottom-up approach and top-down approaches. In this work, both routes will be explored. In the bottom-up approach, the properties of graphene over a mono crystalline nickel surface obtained by chemical vapor deposition (CVD) will be studied. In the top-down approach, natural graphite will be used to construct graphene based materials with innovative approaches. Obtained products are characterized by Auger electron spectroscopy, Low energy electron diffraction, Scanning (SEM)and transmission (TEM) electron microscopy, Raman spectroscopy , X-ray photoelectron spectroscopy (XPS) and Energy electron loss spectroscopy (EELS). As a result, in the bottom-up approach, Cesium restored the linearity of the graphene and it behaved as a free-standing graphene, when is well known that exist a strong interaction between graphene and the metal substrate. In the top-down approach, graphene oxide free-defect layers that are very large and transparent were obtained. Preparated layers chemically seem to coordinate zeolite crystals probably to obtain the necessary energetic stabilization.

Dipartimento di Fisica - Tesi di Dottorato

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