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 Dielectric characterization of different mesogenic substances and a mixture with non-conventional gold nanoparticles(2012-11-28) Marino, Lucia; Versace, Carlo; Bertolini, Roberto; Scaramuzza, NicolaThe study of liquid-crystalline matter and nano-structured materials is an important and vast field of research with potential implications in the development of new technologies, like sensors and displays. In this work we have analyzed and characterized different systems by dielectric spectroscopy. The first part of this thesis is devoted to the characterization of an orthoconic liquid-crystalline mixture, W-129, with ferroelectric properties. The analysis of the dielectric response of this material has revealed a plurality of ferroelectric smectic C* subphases; they represent smectic intermediate variants situated between the ferroelectric phase and the antiferroelettric one, known like antiferrielectric phases (SmCFI*, SmCFII*, ... ), whose study is still open and is collecting a lot of interest in academic environments. The same liquid crystalline mixture was then doped with unconventional gold nanoparticles. These nanoparticles are functionalized with a hydrophilic polymer which becomes hydrophobic exceeded 40°C. The nano-composite material obtained by the dispersion of the gold nanoparticles presents interesting characteristics, such as an enhancement of dielectric increments (or strengths) probably due to the molecular interactions between the ferroelectric liquid crystal and the gold nanoparticles, which translates into an increase of the order of the liquid crystal host, in a stabilization of the smectic subphases and in an enhanced memory effect already seen in the pure liquid-crystalline mixture. The third and last part is devoted to the characterization of a new "banana-shaped" liquid crystal, which exhibits some unusual physical properties during the nematic phase, in particular, the presence of regions of more ordered molecules, organized in a smectic C phase inside a nematic one. The dielectric spectra acquired during the nematic phase show the presence of a relaxation response between 10 and 20 Hz which, with the addition of relatively large values of permittivity, may suggest the presence of a ferroelectric response due to the existence of cybotactic clusters.Item 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 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.