Tesi di Dottorato
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Item Laser action in liquid crystals: from random to periodic syatems(2007) Ferjani, Sameh; Strangi, Giuseppe; Versace, CarloItem 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 Optical trapping and manipulation exploiting liquid crystalline systems(2012-11-30) Hernandez, Raul Josue; Versace, Carlo; Cipparrone, GabriellaThis thesis and all the research contained within, pretends to develop new ideas and concepts on liquid crystals (LC) and optical trapping and manipulation. The combination between optical tweezers and LC systems promises unique and exciting results. The content on the thesis is presented for those with some experience in the elds of liquid crystal and optical manipulation, and for those who are interested in begin to learn about these matters, proposing an overview of much existing work and a correlation between di erent science branches like soft matter, photonics and optical control. Two main research lines has been developed involving liquid crystalline systems and polarized optical tweezers. In the rst part, nematic LC droplets in water have been adopted to study the mechanical properties of light elds with a polarization gradient, i.e. optical tweez- ers based on polarization holographic techniques with non conventional trapping in an extended interferometric optical trap. For this purpose, LC emulsions in wa- ter were prepared, obtaining droplets with radial or bipolar director con guration, which result in optically isotropic or anisotropic particles. Exploiting the vecto- rial nature of the light and its interaction with LC droplets, an unconventional opto-hydrodynamical control and trapping has been demonstrated. The planned experiments shown that a hydrodynamic force, known as Magnus force, never con- sidered in optical micromanipulation experiments, can play an important role in the optical micromanipulation and should be considered whenever particles are forced to spin and dragged in a uid. In the second part, the study was mainly focused into developing an innovative and versatile soft matter object, namely chiral-solid microspheres. They were created by combining very simple self-assembling and photoinduced processes of the soft mat- ter, i.e. photopolymerizing cholesteric LC droplets in water emulsion. The ability to control the internal helical geometry using chemical agents in the precursor LC emulsion enables to obtain solid microspheres with radial, conical, or cylindrical con gurations of the helical structures that exhibit unique optical properties. Their exclusive capabilities were demonstrated by optical manipulation experiments in- volving optical tweezers. A unique and dichotomous behavior has been revealed by polarized circularly polarized tweezers: an attractive or repulsive optical force is ex- erted by varying the light polarization. Moreover, the application of the chiral-solid microspheres as optical microresonators for creating microlasers was also demon- strated. The high performance as well as the novel and exclusive properties make these chiral microparticles good candidates for developing new concepts in colloidal materials science, microphotonics, microlasers, optical trapping and manipulation, micro- and opto uidics and microsensors.Item Micro raman spectroscopic investigations on soft matter systems(2012-12-04) Fasanella, Angela; Versace, Carlo; Bertolini, Roberto; Cazzanelli, EnzoUniversity of CalabriaItem Novel organic optoelectronic materials(2012-11-28) Cospito, Sante; Versace, Carlo; De Simone, Bruna Carla; Bartolino, RobertoIl presente lavoro di Tesi di Dottorato di Ricerca (Scuola di Dottorato in Scienza e Tecnica "Bernardino Telesio") dal titolo "Novel Organic Optoelectronic Materials" è stato svolto all'interno dei laboratori di "Organic Optoelectronic Materials" e "New Syntheses via Organometallic Catalysis" del Dipartimento di Chimica dell'Università della Calabria. Nuovi semiconduttori organici potenzialmente impiegabili in dispositivi optoelettronici quali OLEDs, celle solari, transitors ed elettrocromici sono state progettate, sintetizzate e caratterizzate. La prima parte del lavoro ha riguardato la sintesi e la caratterizzazione di derivati di triarilammine, molecole elettron-donatrici (anodiche), da impiegare in dispositivi elettrocromici ("smart windows") per l'attenuazione della radiazione solare nel vicino infrarosso (NIR). L'intensa ed estesa banda di assorbimento nel NIR prodotta delle specie mono-ossidate ha suggerito l'impiego di queste triarilammine in dispositivi "complementari" in cui vengono utilizzate insieme a molecole elettron-accetrici (catodiche) elettrocromiche nel visibile, per un'ampia modulazione dello spettro solare. Tali sistemi sono stati dispersi all'interno di matrici polimeriche al fine di realizzare dei gel elettrocromici le cui prestazioni sono state ampiamente studiate. La risposta elettrocromica del gel è stata inoltre provata in un dispositivo plastico, dimostrandone le potenziali applicazioni campo dell'elettronica flessibile. La seconda parte del lavoro invece, ha riguardato la sintesi e lo studio delle proprietà di cristalli liquidi semiconduttori di tipo "n" (elettron-accetori) da impiegare in celle solari organiche "bulk heterojunction". Tali molecole, appartenenti alla famiglia dei viologeni estesi, hanno mostrato interessantissime proprietà mesomorfiche fortemente influenzate dalla lunghezza delle catene alchiliche ( a 9, 10 e 11 atomi di carbonio) e dell'anione utilizzato, la bis(triflimmide). Le proprietà elettrochimiche di questi composti sono state investigate sia in soluzione che nelle mesofasi. Un'efficace elettrocromismo dovuta (i) al doppio strato elettrico creato all'elettrodo degli anioni presenti e (ii) all'elevata sovrapposizione degli orbitali di frontiera dell'esteso sistema π- coniugato è stato osservato nelle fasi colonnari e smettiche in cui le molecole si sono auto-assemblate. Infine, le proprietà elettrocromiche di questi composti sono state studiate all'interno di film plastici utilizzabili in dispositivi quali i displays.Item Behavior and effects of additives in liquid crystal compounds(2011-11-03) Vivacqua, Marco; Bartolino, Roberto; Versace, Carlo; Nicoletta, Fiore P.Item Properties of biomolecules at the interfaces: studies and characterization of chromonic mesogens, from the basis to applications(2013-11-28) Tone, Caterina; Bartolino, Roberto; Versace, Carlo; Ciuchi, FedericaThe study of the interaction between molecules, in particular biological molecules and liquid crystals (LC), has experienced a huge growth in the recent years because of the development in devices engineering applied not only in photonics but also in the biomedical eld. In order to design more e cient LC devices, it is rst necessary to understand the behavior and properties of newly-synthesized liquid crystals and to garner a more indepth understanding of currently-existing LCs in order to answer pending questions about them. The aim of this thesis work, is to better understand the interactions involved at the interface between liquid crystals and other materials, whatever is their nature, i.e. polymeric or biological. We started studying the e ect of di erent con ning surfaces on the alignment of a special class of lyotropic liquid crystals, called \chromonics", which, in addition of LC properties, are biocompatible. Di erently from the most common liquid crystals, i.e. thermotropic LC, the mesogens that constitute the chromonic LC phases are not amphiphilic, but they are \plan-like" aromatic compound. This class of molecules embraces not only dyes and drugs, but also DNA and its bases. Using the knowledge acquired with chromonic mesogen, we tried to understand a more complicate system, such as the phenomena involved at the biomolecules decorated-liquid crystals lms interfaces. More speci cally, it is possible to divide the work in two macro-parts. The rst part concerns the alignment of a chromonic molecule, \disodium cromoglycate" (DSCG). The study of chromonic LC behaviour in con ned geometries and its physical properties, could be a model for more complex biological assemblies. Hence, we demonstrated the role of alignment layer's surface energy in the alignment of nematic phase of DSCG, achieving both alignments and for the rst time, a stable-in-time homeotropic anchoring of this LC solution. With the knowledge acquired from DSCG, we were able to align also DNA bases liquid crystal solutions. In particular, guanosine monophosphate in pre-cholesteric and cholesteric liquid crystals phases were perfectly aligned homeotropically without means of external elds, as was done until now, and partially planar aligned. Moreover, we observed that if ionic and/or silver doped solutions are added to the LC guanosine phases, it is possible to control the pitch of the cholesteric phase, modifying the helix structure. Instead, varying the nature of the con ning surfaces, in such conditions, it is possible to obtain guanosine vesicles. Other studies have been carried out on new chromonic complexes, synthesized at Chemistry Department of UNICAL, with possible application as anticancer drugs. A complete characterization of these compounds were done (XRD, phase diagrams, etc) and also for these compounds, we developed a\route"to drive the alignment, particularly important for future application in biophotonic devices. The second part of the work is focused on LC based biosensors. From the biotechnological and biomedical applications point of view, the studies on interactions of proteins with lipids are an area of fundamental interest, due to enormous biological importance. In fact, studies on biosensor devices are tremendously increased in recent years, focusing the attention also on nding low cost raw materials with high e ciency: liquid crystals, thanks to their high sensitivity to the external conditions, represent the best candidate. It has been demonstrated that aqueous interface of LC has an instantaneous response when exposed to phospholipids. This is a good base to study the interaction between biomolecules using LC as probe. Starting from the results found in literature, we studied the e ect of phospholipids on protein decoratede-liquid crystal interfaces by means of optical microscopy and FT-IR measurements. The rst technique allowed us to observe the response of decorated LC lm when exposed to phospholipids vesicles, while the second, gave us insight on conformational changes involved in secondary structure of the protein in function of the time of interaction between protein and LC, and the pH of the surrounding environment. The results obtained show a new methods to report speci c binding of vesicles on protein decorated interfaces.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 Modellization of optical radiation - Liquid crystal interaction in complex geometries(2007-11-30) Pezzi, Luigia; Blinov, Lev; Versace, CarloItem Turbulence and Stochastic Processes in Nematic Liquid Crystals(2010-10-22) Carbone, Francesco; Strangi, Giuseppe; Versace, CarloIn this work we studied the electrohydrodynamics instabilities (EHD) from a different point of view. We used the traditional tools, belonging to the traditional fluid turbulence framework, to the world of liquid crystals. This tools in addition, for the first time, a 3D scanning of the NLC sample driven in these turbulent regimes, give us some interesting results. As an example a fragmentation of the large scale structures (Williams Domains) whit characteristics similar to the Richardson cascade. We also studied, for the first time, the intensity fluctuation at different depth z inside the sample, and we found a strong non gaussianity in the probability density function of this fluctuation. Further information has been obtained by by studying the decorrelation processes in EHD. We found a local transition between two different regimes: the classical Kolomokorov K41 law in the y direction, and a random sweeping decorrelation in the opposite x direction. Finally we study the weak localization of light obtained during the EHD. We found a drastic reduction in the scattering mean free path `? and a strong enhancement of the backscattering cone.