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 Laser action in liquid crystals: from random to periodic syatems(2007) Ferjani, Sameh; Strangi, Giuseppe; Versace, CarloItem Methane Physisorption processes on porous nanostructured materials(2001-11-03) Myrsini Kyriaki, Antoniou; Agostino, Raffaele Giuseppe; Carbone, Vincenzo; Bartolino, RobertoItem Characterization of thin transparent polymeric films obtained by plasma polymerization technique and their application to liquid crystal cells(2008-10-17) Nicastro, Gaetano; Scaramuzza, Nicola; Versace, CarloThe aim of this work Although liquid crystal displays (LCD) are quite ubiquitous in the modern world, there is still a great run for better and cheaper LCD. The main physical phenomenon that makes LCD as valuable consists in the particular way polarized light propagates through anisotropic media in general and liquid crystals in particular. Different from solid anisotropic media, the actual anisotropy of liquid crystals is imposed by surface interactions. The concepts of aligning layer and anchoring have been coined. Normally, a thin lightly dielectric polyimide film separates the liquid crystal from conducting transparent electrodes. An applied electric field between these electrodes can reorient the liquid crystal inside (the bulk) and change the transmittance of the cell. An electric field can rather easily do so such that the response time to the applied film, τon, is normally less than 1 ms. Switching off the field, liquid crystal relaxes to the initial state, the only “driving force” now, in the absence of the electric field, remains the anchoring of the liquid crystal to the surface. If this anchoring is not very strong the relaxation time, τoff, can be as large as seconds, a unsuitable value for practical purposes. Much stronger anchoring overwhelms this shortcut by the expense of using thin film transistors, difficult to insert and quite costly. Not long ago, a “fast switching response” has been observed using conducting polymers. [1] as aligning films. Conducting electro active polymers such as polypyrrole (PPyr), Polyaniline (PAn), polythiophene (PTh), or poly-o-anisidine (PoA) are complex dynamic structures that captivate the imagination of those involved in intelligent materials research [2]. Although promising response times, τoff 1. The chemical nature of the substance used, for instance polyaniline, or polypyrrole; , of 1-2 ms, even 0.5 ms, have been observed, the rate of defected samples is unacceptable high. Therefore, there is a strong incentive to carry on investigation in the field. There are many parameters that should be considered; among them we quote: 2. The way of inducing the polymerization process, either chemically, electrelectro-chemically, by DC or Rf plasma reactor; 3. the nature and number of doping or included ions; 4. Their mobility 5. Possible red-ox reactions at ITO/polymer and/or polymer/liquid crystal interfaces; 6. Thickness of the aligning films 7. Roughness or porosity of the film In this thesis we will present all the results obtained with these films about “the fast switching response”, a characterization study made with various instruments like SEM, AFM ecc. on these films and other measurements like current curves on the LC cells made with these films, all realized for better understand the properties of these films deposited via DC plasma polymerizationItem Modellization of optical radiation - Liquid crystal interaction in complex geometries(2007-11-30) Pezzi, Luigia; Blinov, Lev; Versace, CarloItem Study of transparent thin film oxide and they interaction with nematic liquid crystals(2006-11-30) Marino, Salvatore; Scaramuzza, Nicola; Longeri, M.Item Scanning probe microscopy studies and dynamic behaviour of ferroelectric domains in PbZr0.53Ti0.47O3 thin films(2009-11-09) Bruno, Emanuela; Scaramuzza, Nicola; Versace, CarloFerroelectric domains play an essential role in all ferroelectric materials applications, for example, microelectromechanical sensors systems (MEMS) and integrated optical systems. The prime interest in recent years is, however, in non-volatile random accesses memories (FRAM) based on ferroelectric thin film. This requires substantial improvement in the understanding of the basic proprieties at the nanometer length scale. Especially the domain nucleation and growth processes in ferroelectric is of key importance. As the optical methods are limited by diffraction, novel high resolution techniques are required. The scanning force methods introduced during this thesis offer the required high resolution together with high sensitivity. In this thesis experimental and theoretical evidence for the origin of the force acting on the tip, the cantilever deflection and the image contrast mechanisms is given for various SFM (Scanning Force Microscopy) operation modes. For imaging ferroelectric domains the best suited SPM technique is the piezoresponse SFM that is performed with the tip in contact with the sample. The spontaneous growth of the domains 1 nucleating in lead zirconate titanate sample is investigated in detail using the EFM (Electrostatic Force Microscopy) technique. Even more interesting is the case where the domains are intentionally created switching the spontaneous polarization by means of an electric field between the EFM tip and an electrode below the sample. This allows to create any desired pattern of domains. The ability to use the same tip for domain switching and imaging is another advantage of the EFM. The domain formed in this way varies in size from few nanometers to few micrometers. Using EFM technique we demonstrate ferroelectrostatic switching in Lead Zircanate Titanate (PZT) thin film. This has important technological implication because the ferroelectric switching must be used in ferroelectric devices.Item Polarization holographic recording in polymeric and liquid crystalline materials(2006) Provenzano, Clementina; Cipparrone, Gabriella; Longeri, MarcelloOptical studies related to polarization holographic recording in photosensitive materials, as azo compounds, liquid crystals and polymeric mixtures, and dye doped polymers, were carried out. The interest in this type of holographic recording is due to the improved signal to noise ratio and the possibilities for image and signal processing, optical switch, beam steering, optical polarizers and selective erasure. We focused our investigation on the mechanisms that can possibly induce diffraction gratings in these different types of photosensitive materials, on the peculiarities of the achieved diffractive devices and on their possible applications. In particular we investigated the effects of polarization holography on azo-dye Langmuir-Blodgett films, on polymer dispersed liquid crystal (PDLC), and on liquid crystal films confined by dye-doped polymers aligning layers. In the first system, conventionally used for polarization holographic recording, we investigated the influence of the particular Langmuir-Blodgett deposition technique on the features of the recorded structures in order to obtain pure polarization gratings. The absence of surface reliefs gratings (SRG), the stability of the recorded devices and the high induced birefringence of the selected material, open up the possibility of interesting applications. In particular, we report the design and the implementation of a photopolarimeter for simultaneous measurements of Stokes parameters of light, in which the basic element is the actual polarization grating. PDLC is a non conventional system for polarization holographic recording, because no azocompounds are present in the polymeric and liquid crystalline mixtures. Polarization holographic storage produces diffraction gratings that originate mainly from the liquid crystal alignment inside the droplets of the solid polymeric matrix, created during the polymerization and phase separation processes. Polarization properties and electro-optical switching behaviour of the gratings are studied. We also report the unexpected observation of SRG in a system without azo compound, where photoisomerization and chromophore reorientation processes do not occur. In the last systems, we exploit a new method for spatially varying liquid-crystal alignment using patterned surfaces obtained by means of a polarization holographic exposure on a dye-doped polyimide. This idea is based on the fact that holographic gratings on some photosensitive material provide a periodic alignment of the nematic liquid crystals. In fact, putting in contact a Polarization holographic recording in polymeric and liquid crystalline materials ii thin film of liquid crystal with the aligning layers, we obtain a replica-grating in the bulk with the same properties of the gratings recorded on photosensitive layers. We describe the high flexibility of these replica-gratings, related to the control of the diffraction efficiency by means of an external electric field, and the very singular properties of the polarization states of the beams diffracted from this device. We also obtain two dimensional (2D) gratings consisting of a 2D array of differently twisted structures of nematic liquid crystal, achieved by a crossed assembling of polarization holograms recorded at the surfaces of the aligning substrates. These devices diffract the incident beam in several diffracted beams with various polarization states at the same time. The energy distribution can be controlled by means of the polarization state of the incident beam. Additionally, the distribution of the intensity on the diffracted beams can be completely controlled by means of a low external applied voltage. These features make the optical devices very interesting for beam steering, beam shaping and other modifications of light intensity or phase.Item Order reconstruction in nematics(2006-11-30) Ayeb, Habib; Barbieri, Riccardo; Gharbi, Abdelhafidh; Longeri, MarcelloItem Study of SU(N) gauge theories on the lattice(2006) Falcone, Rossella; Papa, Alessandro; Falcone, GiovanniItem Atomic force microscopy study of physico-chemical and nanotribological properties of hydrophobin protein(2009-11-09) Houmadi, M. Said; Giocondo, Michele; Lacaze, Emmanuelle