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 Adsorption properties of carbon nanotubes and application of thermal desoprtion spectroscopy to ammonia and methane ices and zoisite(2010-12-14) Vasta, Roberta; Bonanno, Assunta; Falcone, GiovanniIn this work we wanted to underline the importance of Thermal Desorption Spectroscopy and its applications to several branches of Physics. Temperature-programmed desorption techniques (TPD) are important to determinate kinetic and thermodynamic parameters of desorption processes and decomposition reactions. Knowledge of the nature of the desorption process is fundamental to understand the nature of the elementary chemical processes of adsorbates, as the energetics of bonding, the specification of the chemical nature of the bound species and the nature and magnitude of interactional effect between adsorbed species. We focused our attention on the applications of Thermal Desorption Spectroscopy (TDS) to High-Energies Physics, Astrophysics and Geophysics; in fact this technique was used, respectively, to investigate the molecular hydrogen adsorption on carbon nanotubes, the effects of electron bombardment on ammonia and methane ices and changes of zoisite mineral after heating. The molecular hydrogen adsorption on carbon nanotubes was studied to find a possible solution to vacuum system problems of Large Hadron Collider (LHC); in fact, the circular path of photon beams produces synchrotron radiation which deteriorates LHC vacuum desorbing gas molecules from the ring walls. Among the desorbed species the most problematic to pump out is H2. Since LHC elements operate at low temperatures, a possible solution to vacuum problem is the installation of cryosorbent materials on the LHC walls. In this work we study the possibility to use carbon nanotubes as criosorbers in future accelerators. Our sample, furnished by Prof. Nagy group of Chemical Engineering Department of Calabria University, is constituted by MWNTs synthesized by chemical vapor deposition using C2H4 and subsequently purified. Our investigations confirm that the carbon nanotubes have a great adsorption capacity also at low temperatures both for H2 and noble gases as Kr; then we observed that H2 adsorption on CNT is described by a first kinetic-order, while Kr adsorption is characterized by a zero kinetic-order. By means of TDS we calculate the activation energy for H2 adsorption on carbon nanotubes and we found a value of about 3KJ/mol, perfectly coherent with theoretic one. Moreover, from a comparison between nanotubes and other carbon-based material (as charcoal), we noted that adsorption efficiency for CNT is almost an order of magnitude higher then charcoal. So carbon nanotubes are good candidates to cryosorbers in future accelerators. 2 As Thermal Desorption Spectroscopy application to Astrophysics we studied the effect of electron bombardment on ammonia and methane ices. The interstellar medium is composed for 99% by gas; molecules, atoms and radicals at gas state condense on dust grains surface of molecular clouds (at 10 K) creating an icy mantle with a thickness of 0.1 μm. The presence of ices is confirmed by IR spectroscopy of obscured stellar sources and in interstellar grains are localized solid mixture containing H2O, CO, CH4 and NH3. In these environments ices are subjected to chemical and physical processes, specifically to bombardment of photons and cosmic rays, with the consequent synthesis of new organic species In this work we conducted an investigation of the chemical processing of ammonia and methane ices subjected to energetic electrons. By Thermal Desorption Spectroscopy we verify the production of new organic species, after energetic irradiation in interstellar ices, as diazene (N2H2), ethane (C2H6) and acetylene (C2H2). Finally, in Geophysics and Petrology Thermal Desorption Spectroscopy can be used to study minerals chemical composition. Our interest was focused on zoisite and the sample investigated was furnished by prof. Ajò from “Institute of Inorganic Chemistry and Surfaces” of CNR, in Padova. In this work we used TDS to investigate zoisite behaviour during heating form room temperature to 650oC and to understand if its modification into tanzanite variety after heating is due to structural changes or to a dehydration mechanism.Item Investigation of dimensionality e ects on capacitorless memory and trench power MOSFET(2011-12-19) Pierro, Silvio; Falcone, Giovanni; Pace, Calogero; Sindona, AntonelloThis thesis has dealt with two di erent problems solved with electronic device simulations (TCAD). The rst relates to the Trench power MOSFET device characterization, with particular attention to device breakdown voltage as a function of the device parameters. The second one is about the simulation of Zero capacitor DRAM devices. The trench structure analysis requires the use of a device simulator that implements the drift-di usion model with impact ionization model. For a better analysis results, the Trench structure is compared with the equivalent pn structure which shows similar trends in breakdown voltage, but slightly higher, due to the of the trench structure's absence that introduces an additional electric eld component anticipating the breakdown. For reference structure, the breakdown voltage depends only on drift region doping and length. Turning to Trench structure it can be noted that each Trench's shape parameter a ects the breakdown voltage, in particular: The distance between two structures in the trench does not a ect the calculation of the breakdown voltage. The Trench curvature radius and the oxide thickness are directly proportional to the breakdown voltage. The Trench width does not a ect the breakdown voltage. The trench length is the most important trench's parameter in order to evaluate a good model. A one-dimensional analytical model for pn structure has been presented using a maximum electrical eld value prediction as border condition for Poisson equation. The same model has been used for trench MOS devices using a correction factor for maximum electrical eld calculation based on trench penetration into drain region and on drain region length. The analytical model shows good results in comparison with simulation results for wide range simulations. For the Zero capacitor DRAM simulations, so called ZRAM devices, we have presented a simulation study aimed at understanding the operation mode, the potential performance in terms of READ sensitivity, programming windows and retention time, and the scalability of a double gate type II Z RAM cell with respect to the type I cells. We nd that the operations of a type II ZRAM cell can be implemented by changing simultaneously all electrode potentials and does not necessarily require an appropriate time sequence of bias voltages. Moreover, in the proposed operation mode, the excess charge is stored at the gate interfaces and not in the bulk body. This excess charge is a self consistent charge, created during the WRITE \1" phase by impaction ionization and BTBT at the drain side and de ned by the accumulation condition imposed by the gate bias during the HOLD phase. The independence of the stored charge on the particular WRITE \1" bias con guration allows an excellent determination and tuning of device performances by experiments and device simulations. Stored data is read by an asymmetrical bias con guration of the gate interfaces where the bottom gate interface works in a manner similar to the HOLD mode while the top gate interface works in a manner similar to the WRITE \1" mode but with a lower drain bias in order to avoid drain disturbs. The charge eventually stored at the bottom interface increases the bulk potential at the top interfaces which in order reduces the source-bulk energy barrier allowing a high READ current. Because of the exponential dependence of the READ current respect to the bulk potential a higher I1=I0 ratio is found respect to type I operation mode allowing higher READ sensitivity, programming windows and retention times. Data retention is limited by the leakage associated to the state \0" due to BTBT at the source/drain to bulk junctions. Except for device geometries with degraded SCEs (L<Item Aspects of Integrability in AdS/CFT duality(2011) Infusino, Gabriele; Rossi, Marco; Falcone, GiovanniItem Turbulence and structures in plasma astrophysics(2011) Donato, Sandro; Carbone, Vincenzo; Falcone, GiovanniItem Study of Z0 and Higgs production at ATLAS & Dream calorimetry(2010-10-21) Venturelli, Tommaso; La Rotonda, Laura; Falcone, GiovanniItem Study of SU(N) gauge theories on the lattice(2006) Falcone, Rossella; Papa, Alessandro; Falcone, GiovanniItem Proprietà elettroniche e vibrazionali di strati sottili adsorbiti su substrati metallici(2008) Politano, Antonio; Chiarello, Gennaro; Falcone, GiovanniUnderstanding the adsorption, catalytic, and electronic properties of metal/metal interfaces is one of the foremost aims of surface science. In fact, through the tailoring of the characteristics of the substrate it should be possible to obtain more selective catalysts and more effective electronic devices. This thesis is focused on the vibrational and electronic properties of various metal/metal systems, studied by high-resolution electron energy loss spectroscopy. Firstly, alkali adsorption and their coadsorption with CO, O, and OH on transition-metal and noble-metal surfaces have been investigated. Vibrational measurements demonstrated that alkalis adsorb as neutral and polarized adatoms on metal surfaces. The adsorption of CO induces alkali ionization. Short-range effects dominate in alkali coadsorption systems. Alkali oxidation is more readily achieved upon CO adsorption or dissociation than by direct exposures to oxygen. The stabilization of subsurface O was obtained on alkali-doped Cu(111), while on alkali-modified transition-metal catalysts O adatoms remain in on-surface sites. In the latter case a softening of the O-substrate bond occurs. Moreover, we studied chemical reactions at clean and alkali-doped bimetallic surfaces obtained by depositing monolayers of silver on Ni(111) and Cu(111). Important information on the metal surfaces and interfaces could be provided also by the analysis of collective electronic excitations. Surface plasmon dispersion was investigated in metal/metal systems exhibiting electron quantum confinement. Screening effects are enhanced by the presence of quantum well states and a direct correlation between the free-electron density of states of the system, the chemical reactivity, and dynamical screening processes exists. Measurements have been performed on both flat thin films and films nanostructured in islands in order to shed light on the influence of the growth mode on the electronic response of the system. The knowledge gained herein has far-reaching implications to the general understanding of processes at metal/metal interfaces. Keywords: chemisorption, alkali metals, carbon monoxide, oxygen, metallic thin films, silver, bimetallic surfaces, electronic excitations, electron energy loss spectroscopy, surface chemical reactivity, charge transfers, electron confinement.Item Scaling laws in solar wind turbulence(2009-11-27) Marino,Raffaele; Falcone, Giovanni; Carbone, Vincenzo; Noullez, Alain; Sorriso Salvo, LucaItem Periodic and aperiodic structures realized by innovative soft-metter based techniques(2014-05-19) Infusino, Melissa; Umeton, Cesare; Agostino, Raffaele; Falcone, GiovanniIl presente lavoro di tesi `e basato sullo studio e l’utilizzo di tecniche sperimentali per la realizzazione di strutture periodiche ed a-periodiche su diverse scale di grandezza. La prima tecnica oggetto di studio rappresenta un nuovo approccio rispetto alle usuali tecniche olografiche e prevede l’utilizzo delle pi`u recenti tecnologie realizzate nell’ambito dell’olografia diffrattiva, quali i modulatori di fase spaziale. I modulatori di fase spaziale consentono la produzione di distribuzioni di intensit`a sia periodiche che a-periodiche ed in generale rendendo possibili geometrie difficilmente realizzabili con le classiche tecniche di interferenza a multi-fascio. Il sistema fotosensibile di partenza `e costituito da una miscela di pre-polimeri e cristalli liquidi nematici: la separazione di fase tra le due componenti `e indotta dalla fotopolimerizzazione. L’impiego dei cristalli liquidi consente la realizzazione di strutture dinamiche le cui propriet`a ottiche sono modificabili attraverso l’applicazione di stimoli esterni, quali campi elettrici ed ottici. Le strutture relizzate hanno una periodicit`a micrometrica che le rende applicabili nel campo dell’ottica. La seconda tecnica studiata si basa su fenomeni di auto-assemblaggio nei polimeri a blocchi e consente il raggiungimento di scale di grandezza inferiori: la periodicit`a delle strutture realizzate raggiunge infatti poche decine di nanometri. Nella tecnica di autoassemblaggio la separazione di fase tra le componenti `e regolata semplicemente dalla temperatura del sistema e le strutture risultanti hanno delle caratteristiche fortemente dipendenti dalle dimensioni dei costituenti. 3Item Eletronic and morphological properties of nanometic systemsEletronic and morphological properties of nanometic systems(2014-05-19) Minniti, Marina; Antonino, Oliva; Falcone, Giovanni