Showing papers in "Molecular Crystals and Liquid Crystals in 2006"
TL;DR: In this article, a nematic liquid crystal (NLC) layer with the anisotropy axis modulated at a fixed rate q in the transverse direction is considered.
Abstract: A nematic liquid crystal (NLC) layer with the anisotropy axis modulated at a fixed rate q in the transverse direction is considered. If the layer locally constitutes a half-wave plate, then the thin-screen approximation predicts 100% efficient diffraction of a normal incident wave. The possibility of implementing such a layer via anchoring at both surfaces of a cell with thickness L is studied as a function of parameter qL, and threshold values of this parameter are found for a variety of cases. Distortions of the structure of the director in comparison with the ideal profile are found via numerical modeling. Freedericksz transition is studied for this configuration. Coupled-mode theory is applied to light propagation through such a cell allowing to account for walk-off effects and effects of nematic distortion. In summary, this cell is suggested as a means for projection display; high efficiency is predicted.
85 citations
TL;DR: In this article, the synthesis and FET characteristics of perfluoropentacene and perfluorsotetracene are described, and a planar and crystalline materials that adopt π-stack structures with the short interpalnar distances of 3.27 and 3.21 are presented.
Abstract: The syntheses and FET characteristics of perfluoropentacene and perfluorotetracene are described. Both acenes are planar and crystalline materials that adopt π-stack structures with the short interpalnar distances of 3.27 A in perfluorotetracene and 3.21 A in perfluoropentacene. The oxidation and reduction peak potentials of the perfluorinated acenes shift positively compared with those of the corresponding acenes, suggesting that the HOMO and LUMO energies are diminished by fluorine substituents. Organic field-effect transistors (OFETs) with perfluoropentacene exhibit n-type semiconducting properties with high electron mobility of 0.22 cm2/V s.
71 citations
TL;DR: In this paper, the electro-optical properties of recently discovered ferroelectric particles/liquid crystal colloids were described and compared to the traditional time consuming and expensive chemical synthetic methods.
Abstract: In this article we describe electro-optical properties of recently discovered ferroelectric particles/liquid crystal colloids. We show that the presence of ferroelectric particles in a liquid crystal changes its birefringence and dielectric anisotropy. In contrast to the traditional time consuming and expensive chemical synthetic methods, this method to create liquid crystals with enhanced properties is relatively simple and has a great potential. We also demonstrate the performance of these new materials in various devices, including displays, light modulators, and beam steering devices.
67 citations
TL;DR: In this paper, the authors synthesize well-dispersed Cu nanoparticles by using thermal decomposition of Cu-oleate complex, which was prepared by the reaction with CuCl2 and sodium oleate in aqueous condition.
Abstract: In order to obtain well-dispersed Cu nanoparticles, we synthesized Cu nanoparticles by using thermal decomposition of Cu-oleate complex, which was prepared by the reaction with CuCl2 and sodium oleate in aqueous condition. The resulting well-dispersed Cu nanoparticles were synthesized by autoclave. TEM image showed well-dispersed Cu nanoparticles with diameter of 8.9±1.3 nm. EDX spectrum and XRD peaks of the nanoparticles showed the highly crystalline nature of the Cu structures. The decomposition of Cu-oleate complex was analyzed with TGA and the crystallization of Cu nanoparticles was observed with XRD. UV-Vis absorption spectrum of Cu nanoparticles is also observed.
61 citations
TL;DR: In this paper, the electronic structure of copper phthalocyanine, copper octafluorophthalocynine, and copper hexadecafluorsophthocyninine thin films was compared using ultraviolet and inverse photoemission spectroscopy.
Abstract: Ultraviolet photoemission and inverse photoemission spectroscopy was used to compare the electronic structure of copper phthalocyanine, copper octafluorophthalocyanine, and copper hexadecafluorophthalocyanine thin films. A rigid shift to lower energy was observed for both the occupied and unoccupied electronic states as the number of fluorine atoms around the phthalocyanine ring increased. The spectral features of the fluorine-substituted derivatives were otherwise very similar to copper phthalocyanine, and no change in the transport gap energy was observed. Vacuum level shifts were observed at the interface with polycrystalline gold substrates of sufficient magnitude to consistently pin the substrate Fermi level near the middle of the HOMO-LUMO gap. The calculated barrier heights for electron and hole injection across the interface were therefore equal, and no correlation with fluorine substitution was found.
55 citations
TL;DR: In this article, a conductive ink applicable to ink-jet printing was developed to fabricate conductive lines on flexible substrates and nano-sized silver particles with ∼21nm diameter were used for the direct metamodel.
Abstract: We have developed a conductive ink applicable to ink-jet printing to fabricate conductive lines on flexible substrates. Nano-sized silver particles with ∼21 nm diameter were used for the direct met...
55 citations
TL;DR: In this paper, an organic field-effect transistors (OFET) consisting of vacuum-sublimed boron-subphthalocyanine film was fabricated and evaluated in a glove box.
Abstract: We fabricated organic field-effect transistors (OFETs) consisted of vacuum-sublimed boron-subphthalocyanine film. The device characteristics were evaluated in a glove box (O2 < 1 ppm, H2O < 1 ppm) condition. The OFETs showed typical n-type characteristics and the field-effect electron mobilities were estimated to be 5.4 × 10−5 cm2/V·s for Ca source-drain electrodes and 1.2 × 10−5 cm2/V·s for Au source-drain electrodes. In the case of the OFET with Au source-drain electrodes exposed to ambient air for a few minutes, a clear conversion from n-type to p-type behavior was also observed.
52 citations
TL;DR: In this paper, a series of elongated Schiff base esters containing three aromatic rings with palmitoyl moiety as one of the terminal carbon chain and various substituents R(R=H, F, Cl, Br, OCH3, CH3, and C2H5) at the other end of molecule have been isolated, and their structures were proposed via physical measurement.
Abstract: Series of elongated Schiff base esters containing three aromatic rings with palmitoyl moiety as one of the terminal carbon chain and various substituents R(R=H, F, Cl, Br, OCH3, CH3, and C2H5) at the other end of molecule have been isolated, and their structures were proposed via physical measurement. The mesomorphic properties of these compounds were investigated via differential scanning calometry and polarizing optical microscopy. The thermal data indicate that all of these compounds exhibit mesomorphic properties. Although compounds with R=H, OCH3, CH3, and C2H5 show nematic phases, the remaining compounds containing halogen F, Cl, and Br are smectogenic in nature. The structural changes that result in the variation of transition temperature and the electronic polarizability of the respective molecules are discussed.
51 citations
TL;DR: An almost two-fold reduction of the stretching elasticity modulus of the lipid monolayer in the presence of 0.18 mol/l of sucrose in the aqueous phase is shown.
Abstract: We applied two independent methods to measure the bending elasticity of SOPC lipid membranes in the presence of different sucrose concentrations in the aqueous phase. The micropipette technique was used to study the membrane bending rigidity in the concentration range of (0.11–0.30) mol/l of sucrose, while for sucrose concentrations 0 mol/l and 0.05 mol/l the thermal fluctuation analysis of quasi-spherical vesicles was applied. Both methods revealed a strong reduction of the bending elastic modulus, when sucrose is present in the water. Using micromanipulation of emulsion droplets, we investigated the sucrose influence on the stretching elasticity of SOPC lipid monolayers at the oil-water interface. Our results showed an almost two-fold reduction of the stretching elasticity modulus of the lipid monolayer in the presence of 0.18 mol/l of sucrose in the aqueous phase. The experimental results, reported here, reveal a strong influence of sugar molecules on the elasticity of lipid mono- and bilayers.
50 citations
TL;DR: In this paper, the authors investigated the possibilities of reduction of the lasing threshold due to the anomalously strong absorption effect for a distributed feedback (DFB) lasing in chiral smectics.
Abstract: The possibilities of reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback (DFB) lasing in chiral smectics. It is shown that a minimum of the pumping wave intensity may be reached for the pumping wave propagating at an angle to the helical axes under diffraction of the pumping wave. The lowest threshold pumping wave intensity occurs for the lasing at the first band-edge lasing mode and the pumping wave propagating at an angle to the spiral axes corresponding to the first absorption maximum of the anomalously strong absorption effect at the edge of stop band. The corresponding analytical expressions are given for the case of the average dielectric constant of LC coinciding with the dielectric constant of the material limiting LC. If the pumping wave frequency is close to the lasing frequency the pumping wave propagation direction minimizing the threshold is not far from the helical axes and the pumping wave polarization m...
50 citations
TL;DR: In this article, the authors review progress on some of the major challenges of this technology, particularly the need to reduce fibre losses, and report on some recent developments including the fabrication of the first hollow core mPOF.
Abstract: Microstructured polymer optical fibres [mPOF] were first developed in 2001, and have attracted attention in part because the range of fabrication techniques possible with polymers has allowed novel structures to be made that cannot be made simply in other materials. Their material properties also offer attractive possibilities as polymers can contain a much larger variety of dopants than glass. In this article, we review progress on some of the major challenges of this technology: particularly the need to reduce fibre losses, and report on some recent developments including the fabrication of the first hollow core mPOF. Some initial investigations into changing the material properties are reviewed.
TL;DR: In this paper, it was shown that filling the nematic liquid crystal with ferroelectric nanoparticles can significantly decrease the electric Fredericksz transition threshold field, in qualitative agreement with experiment.
Abstract: A key liquid crystalline property for electro-optic applications is the Fredericksz threshold electric field There has been recent experimental interest in liquid crystal-based colloidal suspensions in which the colloidal nanoparticles both possess a permanent electric polarization and provide strong director anchoring on the particle surface Such suspensions are sometimes known as Filled Liquid Crystals Our calculations suggest, in qualitative agreement with experiment, that filling the nematic liquid crystal with ferroelectric nanoparticles can significantly decrease the electric Fredericksz transition threshold field
TL;DR: In this article, a series of azobenzene liquid crystals (azo LC) compounds were developed, both multicomponent as well as single component, that possess room temperature nematic and smectic phases.
Abstract: We have developed a series of azobenzene liquid crystals (azo LC) compounds, both multicomponent as well as single component, that possess room temperature nematic and smectic phases. Some of these materials have a wide mesophase temperature range which makes them useful for a variety of practical applications. Two main strategies were used for controlling the temperature range of these mesogenic azo LC materials: 1) chemical modification of azobenzene molecules with different terminal pendants; and 2) development of multicomponent compositions. These materials were highly photosensitive exhibiting nonlinear refraction and transmission at nanowatt/microwatt power levels. Physical, electro- and nonlinear-optical parameters of many of these materials have been characterized in detail.
TL;DR: In this article, four Schiff base esters containing the different substituents of R=CN, OH, SH, and NO2 were synthesized, and physical characterization was carried out along with spectroscopic techniques (FT-IR and NMR).
Abstract: Four Schiff base esters containing the different substituents of R=CN, OH, SH, and NO2 were synthesized, and the physical characterization was carried out along with spectroscopic techniques (FT-IR and NMR). Thermal properties were analyzed by using differential scanning calometry (DSC), and their liquid crystal textures were observed under polarized optical microscope. All compounds thus obtained exhibit mesomorphic properties of which the alignment of molecules are inclined to either nematic, N (R=SH), or smectic, SmA (for R=CN, OH, and NO2). The unidentifiable subphases in crystal region were recorded in all compounds except that with R=OH. The presence of a lateral hydroxyl group has been claimed to contribute to the enhanced molecular ordering, leading to a higher clearing temperature.
TL;DR: In this article, the authors demonstrate optical trapping and manipulation of defects and transparent microspheres in nematic liquid crystals (LCs) using fluorescence confocal polarizing microscopy.
Abstract: We demonstrate optical trapping and manipulation of defects and transparent microspheres in nematic liquid crystals (LCs). The three-dimensional director fields and positions of the particles are visualized using the Fluorescence Confocal Polarizing Microscopy. We show that the disclinations of both half-integer and integer strengths can be manipulated by either using optically trapped colloidal particles or directly by tightly-focused laser beams. We employ this effect to measure the line tensions of disclinations; the measured line tension is in a good agreement with theoretical predictions. The laser trapping of colloidal particles and defects opens new possibilities for the fundamental studies of LCs.
TL;DR: In this paper, the authors present a few examples of electrically switchable passive opto-electronic components based on composite materials obtained from initial mixtures of photocurable monomers and liquid crystalline materials.
Abstract: Soft materials exhibit very attractive mechanical, optical, and electro-optical properties and easy processing techniques. Combined with holographic techniques, they can be patterned quite easily creating many unique structures. We present here few examples, of electrically-switchable passive opto-electronic components based on composite materials obtained from initial mixtures of photocurable monomers and liquid crystalline materials. Holographic exposure results in volume gratings consisting in alternate areas of photo-crosslinked polymer and phase separated LCs, respectively. We discuss the main features of this new generation of gratings and show how most of their performances can be enhanced with respect to other kinds of composite gratings, like H-PDLCs.
TL;DR: In this article, the authors deal with photophysical characterization of some synthetic porphyrins and phthalocyanines as well as natural chlorophylls and bacteriochlorophyll's as attractive multifunctional organic materials for novel molecular photovoltaics and for photodynamic therapy or diagnosis of cancerous tissues.
Abstract: This paper deals with photophysical characterization of some synthetic porphyrins and phthalocyanines as well as natural chlorophylls and bacteriochlorophylls as attractive multifunctional organic materials for novel molecular photovoltaics and for photodynamic therapy or diagnosis of cancerous tissues. The following spectral methods were used in our investigations: electronic absorption, fluorescence, steady-state photoacoustic spectroscopy and time-resolved optoacoustic spectroscopy to get insight into the mechanisms responsible for molecular processes occurring in dyes upon light illumination. Spectral study was supported by photoelectric examination. The spectral parameters of dyes which are essential for photovoltaics and/or photodynamic therapy/diagnosis were estimated: absorption molar coefficients, fluorescence quantum yields, natural life-times, thermal deactivation parameters, quantum yields of triplet-state population, triplet-state thermal relaxation decay. It was shown that some synt...
TL;DR: In this article, 3D photonic crystal Bragg diffraction and multi-color reflections are obtained by doping the liquid crystal with gold nanowires, which can be electronically switched at millisecond speed.
Abstract: We have observed enhanced photorefractive responses in CdSe doped nematic liquid crystal cells. The effective intensity dependent refractive index coefficients are on the order of 10−2 cm2/Watt. Using polymer-dispersed mixture of these NLC, 3-D photonic crystals are fabricated by short-exposure direct holographic optical illumination. The resultant structure exhibits 3-D photonic crystal Bragg diffraction and multi-color reflections that can be electronically switched at millisecond speed. Enhanced diffraction efficiency is obtained by doping the liquid crystal with gold nanowires.
TL;DR: In this paper, a commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures, achieving a final accuracy of ± 0.0002.
Abstract: A commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures. The half leaky guided mode technique is realised for liquid crystal optical dispersion measurements. Features and problems related to experimental data interpretation and error sources in the refractive index measurements are carefully analyzed, achieving a final accuracy of ±0.0002. We present the measured optical dispersion curves in the wavelength range from 0.5 to 1.7 μm for two widely studied liquid crystals, namely E7 and 5CB.
TL;DR: In this article, synthesis, textural characterization, differential scanning calorimetry, and dilatometry studies are carried out on a new homologous series of N-(p-n-alkoxy benzylidene)-pn-pentyloxy anilines (nO.O5) where n = 1−18.
Abstract: Synthesis, textural characterization, differential scanning calorimetry, and dilatometry studies are carried out on a new homologous series of N-(p-n-alkoxy benzylidene)-p-n-pentyloxy anilines (nO.O5) where n = 1–18. The lower homologues of this series (n = 1–5) and a higher homologue (n = 16) exhibit monotropic Liquid Crystal (LC) phase. The other members of the series show enantiotropic LC phases, while 18O.O5 is a nonliquid crystal. The dilatometric studies on 7O.O5, 10O.O5, 13O.O5 and 16O.O5 reveal the first-order nature of Isotropic to Nematic Phase (IN), Isotropic to Smectic A Phase (IA), Nematic to Smectic C Phase (NC), Smectic A to Smectic B Phase (AB), Smectic C to Smectic B Phase (CB), and Smectic A to Krystal Phase (AK) transitions. The pretransition effects are estimated from dilatometry results. The results are compared with the available data on nO.m compounds.
TL;DR: In this article, photo-responsive azo polymers from novel malonic ester and silicate ester monomers having two symmetrical azobenzene groups in the side chain were synthesized.
Abstract: We synthesized photo-responsive azo polymers from novel malonic ester and silicate ester monomers having two symmetrical azobenzene groups in the side chain. The possibility of their applications as reversible data storage media was investigated utilizing optical data recording techniques. As a result, the azo polymers had very excellent stability and resolution of the stored data as well as the sensitivity to pump beam. The polymeric thin films also could be used as erasable digital data materials for a practical use by an Ar ion pulse laser (pulse width: 10 μs). In addition, the polymeric thick films (500–1000 μm) from new monomer and methyl methacrylate showed good angular selectivity for holographic memory by using optical two wave mixing technique.
TL;DR: The principles and developments of experimental methods for determining the energy parameters governing the electronic structure of organic molecules and solids are reviewed with some historical descriptions as mentioned in this paper, including the range of applicability and the merits and demerits.
Abstract: The principles and developments of experimental methods for determining the energy parameters governing the electronic structure of organic molecules and solids are reviewed with some historical descriptions. The energy parameters discussed are ionization energy of a molecule (Ig) and solid (Is), electron affinity of a molecule (Ag) and solid (As), and the energies of the vacuum level (VL) and the Fermi level (EF). The work function (Φ) can be also derived as the difference between the last two. Various types of spectroscopic and electrical measurements are discussed with typical examples, including the range of applicability and the merits and the demerits. Recent microscopic techniques for obtaining local information are also described.
TL;DR: In this paper, the effect of light induced effects in cholesterics with azoxy-nematic host materials is continued to highlight the mechanisms of the response and the dependence of the selective reflection band and the full width at half-maximum of the band on UV irradiation.
Abstract: The characteristics of cholesteric liquid crystals can be controlled by light irradiation if conformationally photo-active molecules are present. Recently, control of the selective reflection band (spiral pitch) in nemato-chiral mixtures was demonstrated when photosensitive molecules, namely nematic azoxy-based compounds, were used as the host material. In this report, the investigation of light induced effects in cholesterics with azoxy-based host materials is continued to highlight the mechanisms of the response. Different non-photosensitive chiral materials were added to different azoxy-nematic liquid crystals and the pitch change caused by UV irradiation was investigated. A change in the pitch of 50–210 nm was observed depending on the exposure time and the intensity of the light. This effect is reversible: under illumination at wavelengths greater than 410 nm, the pitch shifts in the opposite direction. The dependence of the selective reflection band and the full-width-at-half-maximum of the band on ...
TL;DR: In this paper, the authors demonstrate the feasibility of electro-optical switching, frequency filtering and negative index of refraction in 3D photonic crystals, and demonstrate that the tuning range can be as wide as 380nm, operational throughout the entire visible and far infrared spectra range.
Abstract: We present the results of experimental and theoretical studies that demonstrate the feasibility of realizing electro-optical switching, frequency filtering and negative index of refraction. In liquid crystal infiltrated TiO2 inverse opal 3-D photonic crystals, we observed a sizeable electrical tuning of the Bragg reflection peak. In frequency selective structures with a liquid crystal over-layer, the tuning range can be as wide as 380 nm, operational throughout the entire visible – far infrared spectra range. Furthermore, some of the structures can be designed to exhibit a negative index of refraction property over a predetermined frequency interval.
TL;DR: In this article, the physicochemical and electro-optical properties of introducing the pyrimidine ring into the molecular structure of calamitic liquid crystals are discussed and rationalized in terms of existent theories.
Abstract: The effect on the physicochemical and electro-optical properties of introducing the pyrimidine ring into the molecular structure of calamitic liquid crystals is discussed and rationalized in terms of existent theories; a comparison is made with the corresponding nitrogen-containing six-membered ring derivatives and 1,4-phenylene derivatives.
TL;DR: In this article, transition metal dithiolenes are used to produce liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation, which can be used for near-IR applications.
Abstract: Numerous commercial and military applications exist for guest–host liquid crystal (LC) devices operating in the near- to mid-IR region. Progress in this area has been hindered by the severe lack of near-IR dyes with good solubility in the LC host, low impact on the inherent order of the LC phase, good thermal and chemical stability, and a large absorbance maximum tunable by structural modification over a broad range of the near-IR region. Transition metal complexes based on nickel, palladium, or platinum dithiolene cores show substantial promise in meeting these requirements. These new dye complexes are extraordinarily stable, possess liquid crystalline phases in their own right with the proper terminal functional groups, and can have melting points below room temperature. The latter property is especially significant for producing liquid crystal/dye mixtures with both high dye concentration and good resistance to phase separation. Because transition metal dithiolenes are zerovalent, they can exhibit high...
TL;DR: In this article, the photorefractive properties of nematic liquid crystals sandwiched between windows of cerium doped strontium barium niobate (Ce:SBN) were investigated.
Abstract: We report on the photorefractive properties of nematic liquid crystals sandwiched between windows of cerium doped strontium barium niobate (Ce:SBN). Unlike simple glass substrates, the Ce:SBN induces molecular pre-tilts that generate electric dipole moments in the liquid crystal through the flexoelectric effect. This enables the liquid crystal to become sensitive to the sign of the space-charge field and leads to an increase in the optical amplification when the cell thickness is decreased.
TL;DR: In the automotive industry, Brazil is the only country in South America to have an automotive supplier sector based on natural fibers as discussed by the authors, which can be classified into five categories: caroa, piacava, pupunha, mutum and mutum.
Abstract: Brazil is the only country in South America to have an automotive supplier sector based on natural fibers. New opportunities are arising due to an increase demand by the car makers in applying natural fibers in their parts. Several crop fibers have been developed in Brazil. Among them can be listed caroa, piacava, pupunha, mutum and others of regional application. For the automotive industry, which requires large quantities with uniform quality, the alternatives are sisal (170,000 ton/yr), curaua (150 ton/yr in 2003), malva, 200 ton/yr; Brazil is the single largest producer country of sisal, and commercially, the only one in curaua. For South America, the alternatives are fique in Colombia, abaca in equator, flax in Argentina and curaua in Venezuela. It must be understood by the target countries of drugs, is that crop fiber can be an economic alternative to coca in the Andes region, therefore an instrument of land reform and drug reduction plantations. Several companies have a strong program of a...
TL;DR: In this article, a double-heteroleptic iridium complex with three different ligands was designed to make a white phosphorescent emitter for OLED, which showed an emission peak at 495 nm in bluish green area.
Abstract: Previously, we studied the luminescence characteristics of Ir(ppy)2(F2-ppy) and Ir(ppy)2(piq-F), which are heteroleptic iridium complexes involving two kinds of ligands, where F2-ppy, ppy and piq-F represent 2-(2,4-difluoro-phenyl)-pyridine, 2-phenylpyridine and 1-(4′-fluoroyphenyl)isoquinoline, respectively. Photoluminescence (PL) spectrum of Ir(ppy)2(F2-ppy) showed an emission peak at 495 nm in bluish green area. Ir(ppy)2(piq-F) showed two peaks at 513 nm and 600 nm in the PL spectrum. In order to make a white phosphorescent emitter for OLED, we herein designed a heteroleptic iridium complex containing three different ligands. We thought that reaction of F2-ppy, ppy and piq-F ligands all together with Ir(acac)3 might lead to Ir(F2-ppy)(ppy)(piq-F) among the mixture, displaying red and bluish green emission simultaneously. Ir(F2-ppy)(ppy)(piq-F) and other double-heteroleptic complex mixture are prepared from various concentration combination of ligands. The heteroleptic iridium complex mixture displays a...
TL;DR: In this paper, mesoporous fillers were used to obtain micromechanical interlocking between the filler particles and polymer matrix, which can overcome the traditional problem of hydrolysis associated with silane coupling.
Abstract: Porous fillers can be used to obtain micromechanical interlocking between the filler particles and polymer matrix. Micromechanical interlocking in dental composites can overcome the traditional problem of hydrolysis associated with silane coupling. Such an interlocking could also enhance the overall performance of dental composites. In the current study, mesoporous silica was used as fillers to obtain experimental composites. Light curable dental resin (BisGMA/TEGDMA) was used as polymer matrix. The mechanical (compressive) and aging properties of these experimental composites were investigated. The results demonstrated that mesoporous fillers improved the mechanical and aging properties of experimental dental composites.