Mechanochromic Photoluminescent Liquid Crystals Containing 5,5′-Bis(2-phenylethynyl)-2,2′-bithiophene

5,5′-Bis(2-phenylethynyl)-2,2′-bithiophene derivative 1 containing dendritic moieties linked through amide groups was synthesized. Compound 1 exhibits a rectangular columnar phase on slow cooling from the isotropic liquid. In contrast, a cubic phase is formed as a metastable liquid-crystalline phase by rapid cooling of the isotropic melt. Mechanical shearing induces a cubic–columnar phase transition, accompanied by the change of a photoluminescent color from yellow-green to green. The infrared spectra of 1 suggest that the mechanical stimuli can induce the partial dissociation of the hydrogen bond. This may lead to more disordering of the π-stacks of chromophores.     

Russian Liquid Crystal Society awards first Freederiksz Medals

Abstract

The interest in research and development of plastic liquid crystal technology originates from the stabilization of liquid crystals with a polymer matrix for manufacturing of largearea flexible electro-optical films which can be used as electrically switchable shutters, panels and privacy windows in the building, residential and transportation sectors, as well as reflective displays. In figure 1, we show typical examples of PDLC applications as electro-optical windows. The display applications of PDLC are not within the scope of the present article.     

Selective area metal-organic vapor-phase epitaxy of InN,GaN and InGaN covering whole composition range

To investigate mask effects on InGaN selective area metal-organic vapor-phase epitaxy (SA-MOVPE), in-plane thickness profiles of InGaN were investigated with various indium contents covering between InN and GaN. A numerical simulation employing vapor-phase diffusion (VPD) model was also carried out for the quantitative analysis of VPD effect. At the growth temperatures of 650 and 700 °C, the indium content in the vapor phase and that in the solid phase exhibited almost linear relationship, suggesting that the growth of InGaN can be approximated as the superposition of the growth of InN and GaN. The effective vapor-phase diffusion length D/k_s of GaN was much smaller than InN. The profile of the InGaN growth rate in the selective area became more gradual as the solid indium content increased, and the trend seems to be the linear interpolation between the profiles of GaN and InN. However, as the indium content increases, deposition selectivity between mask and crystal surface become degenerated, and the VPD effect was almost canceled when the solid indium content exceeds 0.5. Although further improvement on the growth conditions is necessary to improve selectivity, basic information for the design of the SA-MOVPE of InGaN has been obtained, which will contribute to InGaN-based monolithically integrated multi-wavelength devices.     

Electrical elements of the optical systems based on hydrogel - electrochromic polymer composites

Abstract

The results of studies of the electrically controlled elements of optical systems based on hydrogel - electrochromic polymer with conjugated π-electron system are shown. One important feature of conjugated polymers is the ability to change their optical characteristics under an external electric field. The electrochromic effect of these polymers may find applications in optical gating and filtering systems, optical sensors, ‘‘smart windows’’, optical memories, IR-switching, and electrochromic displays. Such electrochromic polymers were embedded in the matrix of a macroporous polymer hydrogel by the oxidative chemical polymerization method. The obtained samples based on hydrogel - polyaniline and hydrogel - polyorthotoluidine composites demonstrate the electrochromic behavior. In these composites, the electrolyte, as well as the electrochromic polymer, are located directly in the volume of hydrogel.     

InAs nanowire growth on oxide-masked 〈111〉 silicon

Here we investigate the growth of InAs nanowires on 〈111〉 Si substrates masked by SiO_x using metal–organic chemical vapor deposition. We study 〈111〉 (axial) and 〈1?10〉 (radial) growth of InAs NWs by varying growth duration, temperature, group-III molar flows, V/III ratio, mask material, mask opening size, and inter-wire distance. We find that growth takes place without an In droplet and the process evolves through three successive phases: nucleation of an InAs cluster, followed by two distinct nanowire growth phases. These two growth phases have different axial and radial growth rates, which originate in a transition from having In supply dominated by the open Si area in the first phase towards an In supply from the vapor/oxide mask in the second growth phase. The linear relation found between nanowire length and diameter vs. time in the last growth phase indicates that 〈111〉 growth is not surface diffusion limited as is usually the case for catalyzed growth. A high yield of vertical nanowires is obtained if group-III flow is above and V/III ratio below threshold values, in addition to having an arsenic-terminated Si surface. Furthermore, we observe that 〈111〉 and 〈1?10〉 growth is surface kinetically limited below 520 °C and 540 °C, respectively, with activation energies of 20 and 6.5 kcal/mol. This difference in activation energies limits the selectivity of the 〈111〉 to 〈1?10〉 growth to 25:1 under optimized conditions, which must be considered when fabricating axially modulated structures. However, we find that by placing wires in large arrays it is possible to completely stop the 〈1?10〉 growth rate in favor of the 〈111〉 growth rate.     

Maskless selective growth of semi-polar (1 12¯ 2) GaN on Si (3 1 1) substrate by metal organic vapor phase epitaxy

Semi-polar (1 1 2¯ 2) GaN layers were selectively grown by metal organic chemical vapor phase epitaxy on patterned Si (3 1 1) substrates without SiO₂ amorphous mask. The (1 1 2¯ 2) GaN layers could be selectively grown only on Si (1 1 1) facets when the stripe mask width was narrower than 1 μm even without SiO₂. Inhomogeneous spatial distribution of donor bound exciton (DBE) peak in low-temperature cathodoluminescence (CL) spectra was explained by the difference of growth mode before and after the coalescence of stripes. It was found that the emission intensity related crystal defects is drastically decreased in case of selective growth without SiO₂ masks as compared to that obtained with SiO₂ masks.     

X-ray diffraction study of p -(alkoxybenzylidene)- p ′-toluidines C5H11O-C6H4-CH=N-C6H4-CH3 and C8H17O-C6H4-CH=N-C6H4-CH3

The molecular and crystal structures of two p-(alkoxybenzylidene)-p′-toluidines C₅H₁₁O-C₆H₄-CH=N-C₆H₄-CH₃ (1) and C₈H₁₇O-C₆H₄-CH=N-C₆H₄-CH₃ (2), which form the nematic phase upon melting, is determined by X-ray diffraction. The geometry of the benzylideneaniline fragments in molecules 1 and 2 is actually identical. The crystal packings of 1 and 2 are characterized by the alternation of layers formed by loosely packed aliphatic fragments of molecules and layers of closely packed aromatic fragments. The packing in the aromatic regions of 1 follows the parquet pattern. The crystal packing of 2 has a stacking structure, which is formed by π-stacking dimers superimposed on one another. The formation of the mesogenic phase upon melting of crystals 1 is due to the disturbance of the structurality of loose aliphatic layers with retention of the structure of the aromatic regions, which are stabilized by the cooperative effect of weak directed C-H ··· π-system interactions. The mesogenic phase of crystals 2 is formed upon melting as a consequence of the retention of the structure of π-stacking dimers. Original Russian Text ? L.G. Kuz’mina, N.S. Kucherepa, M.N. Rodnikova, 2008, published in Kristallografiya, 2008, Vol. 53, No. 6, pp. 1079–1085.     

Selective etching and epitaxial refilling of silicon wells in the system SiH4/HCl/H2

Wells were etched in (100) silicon by different kinds of selective etching and were selectively refilled by using different gas systems. Masking materials were SiO₂ and Si₃N₄; for the refilling the systems SiCl₄/H₂ and SiH₄/HCl/H₂ were studied. By using a combination of weak anisotropic gas etching with HCl (Si₃N₄ films as mask) and of SiH₄/HCl/H₂ as refilling system, plane surfaces without ridges at the boundary of the mask and without nuclei on the mask were reached. The results were obtained by SEM, light microscopic and profile investigations and are compared with the other kinds of etching and refilling.     

The Nematic-Isotropic Transition

Abstract

We describe simple classroom experiments that can model the nematic phase, the first order nematicisotropic phase transition, and the effect external stimuli, such as surface treatment, have on the phase transition. These experiments are suitable for the high school level but can be easily adapted to fit the middle school mathematics and science curriculum. With even more simplification, it can also be used to fit the elementary level to qualitatively demonstrate the total loss of order as a liquid crystal melts into a liquid.     

Crystal structure and phase transitions in the new crystals of [(CH3)2NH2]2CuCl4[(CH3)2NH2]Cl

Crystals grown from a solution of dimethylammonium and copper chlorides are studied using electron paramagnetic resonance (EPR) and X-ray diffraction. The dielectric properties of the crystals grown are measured. It is established that the crystals have the composition [(CH₃)₂NH₂]₂CuCl₄[(CH₃)₂NH₂]Cl and, in phase I at room temperature, are described by the orthorhombic space group Pna2₁ with the unit cell parameters a = 11.338 Å, b = 9.981 Å, and c = 15.675 Å. At temperatures of 279 K and 253 K, the crystals undergo jumpwise phase transitions into the incommensurate modulated ferroelectric phase II and commensurate modulated phase III, respectively. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 92–100. Original Russian Text Copyright ? 2004 by Kirpichnikova, Pietraszko, Bednarski, Waplak, Sheleg. Dedicated to the 80th Birthday of L.A. Shuvalov     

Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT

The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (N_D) and calamitic (N_C) nematic phases. A biaxial nematic (N_B) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The N_B phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.     

Investigation of the structure and mechanisms of thermal motion in nanostructured undecylenic acid

Abstract

We have compared the structure, phase transitions and the thermal motion mechanisms of the undecylenic acid in bulk and inside a porous matrix using powder X-ray diffraction, calorimetry and IR-spectroscopy methods. The matrix used was a porous silicon with cylindrical pores (D?=?20?nm). The interaction between the walls of the porous silicon and the nanocrystals of the undecylenic acid results in a crystalline structure, which is significantly different from the structure of bulk undecylenic acid. A phase transition in the wall-adjacent layer of the undecylenic acid nanocrystals was shown to take place. The solid-state phase transition temperatures, as well as the melting point, were shown to be lower for nanocrystals. Topological soliton motion mechanism was suggested to explain the thermal motion of the undecylenic acid molecules in the rotator phase.     

Captured Orientational Order in Polymer Network Assemblies

Abstract

The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.     

NMR and X-ray Investigations of a Plastic Crystalline Phase in MBBA

Abstract

Wideline and high resolution NMR studies have been carried out on MBBA in its isotropic, nematic and solid phases. Isotropic and nematic phase spectra correspond to what has been reported earlier. In the solid phase, contrary to expectations, very intense narrow signals similar to signals of the isotropic phase have been observed for the first time at temperatures close to the solid ? nematic phase transition temperature. This indicates rapid reorientational or translational motion in the system. The X-ray results however confirm the existence of translational order. The results are interpreted as indicative of the existence of a plastic crystalline phase in MBBA.     

Phase Diagram and Dielectric Studies in Hydrogen-Bonded Liquid Crystal System

The thermal and dielectric behaviors of mixtures were investigated for systems of p-n-hexyloxybenzoic acid (6OBA) and p-n-octyloxybenzoic acid (8OBA) by differential scanning calorimetry, polarized optical microscopy, and dielectric measurements. The T-X phase diagram was obtained for this system. Eutectic composition was calculated and received experimentally. Enthalpies of mixing were calculated to describe phase transitions physically. All mixtures show enantiotropic smectic and nematic phases. Dielectric permittivity has higher value for the mixture with fractional proportion of 30 to 70% mol for 8OBA than that for initial acid. It is assumed that co-crystallization of initial compound occurs when the ratio of components is close to equimolar ratio.     

Light Interference Study of Superionic Transition in Synthetic Metal—Solid Electrolyte Hybrid (BEDT-TTF)3AgxI8

Abstract

First order phase transition at 163–172 K is found in the (BEDT-TTF)₃Ag_xI₈ compound in close vicinity to the temperature range of superionic conductivity. This observation indicates formation of the mobile silver ions in the salt as a consequence of sublattice melting.     

Structural and Magnetic Characterisation of Ce@C82

Abstract

We report the structural and magnetic properties of the endohedral metallofullerene Ce@C₈₂. A hexagonal close packing phase [P6₃/mmc [a=11.1544Å, c=18.2256Å] is formed exclusively after vacuum annealing of the solvent precipitated compound. In contrast, sublimed Ce@C₈₂ was found to be dominantly face-centred cubic close packed [Fm-3m; a=15.766Å]. X-ray powder profile calculations revealed that the endohedral cerium atom lies close to 1.8Å from the C₈₂ cage centre in both phases. Hexagonal Ce@C₈₂ has been investigated by magnetic susceptibility measurements. Paramagnetic behaviour is maintained down to 2K attributable to Ce³⁺ ions. Towards lower temperatures, the observed paramagnetic moment falls from the free ion Ce³⁺(μ_eff =2.54μB) value, monotonically approaching 1μB at 2K.     

Observation of Second Sound Using Light Scattering from a Free Surface Smectic A

A new technique for studying the second sound mode in smectic liquid crystals has been developed. Preliminary measurements for the smectic A phase of 4-n-octyloxy-4'-cyanobiphenyl (80CB) are described. We have developed a technique to prepare well aligned samples with one free surface. This free surface was electrically driven, and the resulting surface displacement was studied using an optical heterodyne technique. The elastic constant, B, and the effective viscosity, ν_eff, were determined from the spectrum of the scattered light.     

Synthesis,Characterization and Mesomorphic Properties of New Rod-like Thiophene Based Liquid Crystals

Two new mesogenic homologous series of Schiff base esters, 2-[4-(4′-n-Alkoxy benzoyloxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4-(4′-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), comprising a thiophene moiety were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as FT-IR, ¹H-NMR and ¹³C-NMR, and mass spectrometry. The mesomorphic properties and thermal stabilities of the title compounds were studied by using differential scanning calorimetry and optical polarizing microscopy. All the derivatives are mesomorphic in nature showing the nematic phase, and the higher members of Series-A show a smectic C phase whereas Series-B exhibits only the nematic mesophase. The mesomorphic properties of the present series are compared with other structurally related compounds.     

Technology of Intercalation and New Effects in PbI2 Single Crystals Intercalated by Hydrazine

Abstract

The hydrazine intercalated lead iodide single crystal technology was developed. A new type of excitons named as interlayer excitons is described. The amplification of polariton emission (Sugakov's phenomenon) and the structure phase transition under intercalation are found and studied.