Layer Reorientation Research Articles

Smectic layer reorientation by surface mode in surface stabilized ferroelectric liquid crystal

Smectic layer reorientation by surface mode in surface stabilized ferroelectric liquid crystal

Smectic layers reorientation with stripes and heliconical structure in surface stabilised ferroelectric liquid crystal

ABSTRACT High-tilt angle ferroelectric liquid crystal material in surface stabilised geometry (SSFLC) is investigated for the smectic layer reorientation and its molecular dynamics by electro-optical and dielectric spectroscopy techniques. Smectic layer reorientation (5 o to 45 o ) is obtained by controlled cooling rate and applying electric field in the vicinity of transition temperature from chiral nematic ( N ∗ ) to chiral smectic C ( Sm C ∗ ) phase. After slow cooling down to room temperature, the stripe domain textures along the rubbing are observed with quasi-bistable states. Dielectric spectroscopy has revealed the unusual increment in the dielectric permittivity. A dielectric relaxation process at lower frequency than Goldstone mode with the increase in DC bias up to 2 V is observed in the sample cooled without any bias. This increment is found to be originated by the heliconical structure within the stripes which is seen in the optical textures recorded at the time of dielectric scan. However, this increment is not observed in the sample cooled under DC bias. Thus, there exist different phenomena in the high tilt angle SSFLC sample cooled in different conditions of DC bias; stripe domains, smectic layers reorientation, heliconical stricture and twisted structure within stripes which are significant to explore deeply in high-tilt angle ferroelectric liquid crystals.

Comparative analysis of hydration layer reorientation dynamics of antifreeze protein and protein cytochrome P450

Antifreeze proteins (AFPs) inhibit ice re-crystallization by a mechanism remaining largely elusive. Dynamics of AFPs’ hydration water and its involvement in the antifreeze activity have not been identified conclusively. We herein, by simulation and theory, examined the water reorientation dynamics in the first hydration layer of an AFP from the spruce budworm, Choristoneura fumiferana, compared with a protein cytochrome P450 (CYP). The increase of potential acceptor water molecules around donor water molecules leads to the acceleration of hydrogen bond exchange between water molecules. Therefore, the jump reorientation of water molecules around the AFP active region is accelerated. Due to the mutual coupling and excitation of hydrogen bond exchange, with the acceleration of hydrogen bond exchange, the rearrangement of the hydrogen bond network and the frame reorientation of water are accelerated. Therefore, the water reorientation dynamics of AFP is faster than that of CYP. The results of this study provide a new physical image of antifreeze protein and a new understanding of the antifreeze mechanism of antifreeze proteins.

Strain‐Driven and Layer‐Number‐Dependent Crossover of Growth Mode in van der Waals Heterostructures: 2D/2D Layer‐By‐Layer Horizontal Epitaxy to 2D/3D Vertical Reorientation

AbstractHeterogeneously integrated 2D van der Waals (vdW) solids composed of compositionally distinct atomic layers are envisioned to exhibit exotic electrical/optical properties unattainable with their monocomponent counterparts. However, the underlying principle for their morphology‐controlled chemical vapor deposition (CVD) growth and its associated growth variables have not been clarified, leaving their projected technological opportunities far from being realized. Herein, by employing tungsten trioxide (WO3) nanowires as a model system that uniquely enables the detailed atomic‐scale inspections of 2D/2D interfaces, the CVD growth mechanism of 2D molybdenum/tungsten disulfide vdW vertical stacks is studied. By employing extensive transmission electron microscopy (TEM) characterization, an intriguing growth mode transition is identified in these materials, i.e., 2D/2D layer‐by‐layer horizontal epitaxy to 2D/3D vertical layer reorientation, and it is confirmed that it is driven by varying 2D layer numbers. Corroborating molecular dynamics simulations clarify that the internal strain accumulated during the course of 2D layers growth dictates the final growth mode, further supported by TEM strain map analysis. This study not only sheds a new insight on better understanding the growth principles for 2D vdW heterostructures but also offers important technical guidance on tailoring their functionalities toward exploring 2D/2D heterojunction devices.

Composite Dislocations in Smectic Liquid Crystals.

Smectic liquid crystals are characterized by layers that have a preferred uniform spacing and vanishing curvature in their ground state. Dislocations in smectics play an important role in phase nucleation, layer reorientation, and dynamics. Typically modeled as possessing one line singularity, the layer structure of a dislocation leads to a diverging compression strain as one approaches the defect center, suggesting a large, elastically determined melted core. However, it has been observed that for large charge dislocations, the defect breaks up into two disclinations [C. E. Williams, Philos. Mag. 32, 313 (1975)PHMAA40031-808610.1080/14786437508219956]. Here we investigate the topology of the composite core. Because the smectic cannot twist, transformations between different disclination geometries are highly constrained. We demonstrate the geometric route between them and show that despite enjoying precisely the topological rules of the three-dimensional nematic, the additional structure of line disclinations in three-dimensional smectics localizes transitions to higher-order point singularities.

Structural Snapshots of Metastable Intermediates Reveals Sequential Addition of Growth Units in the Formation of an Archetypal Coordination Complex: Anisotropic Layer Migration and Solid-State Thermochromic Transitions

Kinetically trapped partially preassembled metastable structures afford vital inputs on the reaction progression and the formation mechanism of technologically promising coordination assemblies. We report the structural and transformational relations in a series of coordination complexes, [Co(ad)2(H2O)4](btc-)2(H2O)2 (1), [Co(ad)2(H2O)4][Co(H2O)6](btc-)2(H2O)10 (2), [Co3(ad)2(H2O)14](btc-)2(H2O)4 (3), and [Co3(ad)2(btc-)2(H2O)8] (4), in which ad = adenine and btc = 1,3,5-benzenetricarboxylic acid, to provide insights on sequential structure evolution in an archetypal coordination assembly. Crystals of 3 undergo solid-state thermochromic transformation to a glassy phase 5 consequent to dehydration and anation reaction. With carefully optimized thermal treatment, we obtained a transient crystalline phase 4, which unambiguously proves a restructuring in the Co(II) coordination geometry from octahedral to trigonal bipyramidal. With the 3 → 4 transformation, the crystal surface undergoes drastic modification. Surface reconstruction events associated with photoreactions in the molecular crystals are noted, but analogous observations for thermally induced events are exceptional and unprecedented for coordination complexes. Correlative atomic force microscopy, nanoindentation, and structural inputs provide insights on the surface reconstruction events brought about by anisotropic long-range layer migration subsequent to 3 → 4 transition. The slip plane (011̅) that crosses the crystal face (010) at an optimal angle offers an energetically viable route for layer reorientation and migration.

The Role of Surface Charging during the Coadsorption of Mercaptohexanol to DNA Layers on Gold: Direct Observation of Desorption and Layer Reorientation

We study the coadsorption of mercaptohexanol onto preimmobilized oligonucleotide layers on gold. Monitoring the position of the DNA relative to the surface by optical means directly shows the mercaptohexanol-induced desorption of DNA and the reorientation of surface-tethered strands in situ and in real time. By simultaneously recording the electrochemical electrode potential, we are able to demonstrate that changes in the layer conformation are predominantly of electrostatic origin and can be reversed by applying external bias to the substrate.

Coexistence of polar and nonpolar domains and their photocontrol in the B7 phase of a bent-core liquid crystal containing azo dyes.

Coexisting polar and nonpolar domains have been studied by means of texture observation, x-ray analysis, optical second-harmonic generation (SHG), and SHG microscopy in the B7 phase of a bent-core mesogen doped with azo dyes. The bent-core molecules take a planar orientation and show high birefringence in the polar domain, while they take a homeotropic orientation and show low birefringence in the nonpolar domain. Good correspondence between real and SHG images was observed under a SHG microscope; the bright (high-birefringent) domain is SHG active and the dark (low-birefringent) domain is not SHG active. Photoisomerization of the azo dyes causes layer reorientation from the layer perpendicular to the substrate to that parallel to it.

The Smectic Layer Reorientation Induced by an Electric Field in a Short Pitch Ferroelectric Liquid Crystal

We studied the layer reorientation of a ferroelectric liquid crystal with a short helical pitch induced by a unipolar electric field in the vicinity of the chiral nematic-chiral smectic C phase transition. The rotation of the average layer direction in striped domains was observed by using an optical polarizing microscope as a function of the amplitude of the electric field. It was found that the angle and the associated characteristic time for layer rotation strongly depend on the frequency of the electric field.

Dynamic of Surface-Mediated Director Reorientation in a Cell with Dye Doped Liquid Crystal

We report on investigation of a light-induced anchoring of dye-doped liquid crystal(LC)in nematic phase. Contributions of light-induced dye adsorption and desorption processes as well as of the bulk-induced reorientation effect were separated by choosing specific geometries of the light irradiation. We propose a model that explains peculiarities of light-induced easy axis producing in nematic phase which differs from one in isotropic phase due to presence of anisotropic dark-adsorbed layer and effective bulk-induced director reorientation.

Effects of Electric Fields as a Function of depth in a Smectic Film

The response of liquid crystals (LC) to applied electric fields is the basis for most display applications. Optically it is known that the bulk portion of a LC cell responds to an applied field. The remainder of the film is assumed to consist of LC molecules and layers that do not reorient due to surface interactions. The inactive interfacial layer has not been accessed directly nor characterized. We have used a 8CB smectic-A cell consisting of two ITO coated glass plates to look into the layers with grazing incidence X-ray diffraction. The plates are connected to a voltage source with a maximum output of 40.7 V DC. The changes in lattice spacing vary as a function of depth within the sample, which suggests the presence of different regions of layer reorientation within the film.

Influence of Water-Soluble Polymers on the Shear-Induced Structure Formation in Lyotropic Lamellar Phases

The shear-induced structure formation in lyotropic lamellar phases containing water-soluble polymers is investigated. The lyotropic phases consisted of sodium dodecyl sulfate/1-decanol/D2O and were mixed either with poly(n-isopropylacrylamide) (PNIPAM), hydroxyethyl starch (HES), poly(vinyl caprolactame) (PVCa), or poly(ethylenglycol)distearate (PEG-DS). Rheo-optical experiments (flow birefringence and small-angle light scattering, SALS) as well as small-angle neutron scattering (SANS) combined with a commercial rheometer were used to observe structural changes, e.g., layer reorientation or formation of multilamellar vesicles (liposomes). Equilibrium properties of the lamellar phases were investigated using quasi elastic light scattering (QELS) and static SANS, the latter was analyzed using a model proposed by Nallet et al. The polymer addition led to a viscosity increase but the flipping of aligned lamellae from parallel to perpendicular orientation was hardly affected by the polymers. The shear-induced formation of multilamellar vesicles (MLV), however, was strongly influenced by the macromolecules. The addition of small amounts of PNIPAM shifted the region where vesicles are formed to samples with higher decanol contents whereas HES, PVCa, and PEG-DS suppressed the MLV formation in all cases. Results from SANS and QELS indicate a possible correlation between the shear-induced vesicle formation and the viscoelastic properties of the surfactant bilayer.

EFFECT OF AZIMUTHAL SURFACE LAYER ANCHORING ON FIELD-INDUCED LAYER REORIENTATION IN A SHORT PITCH FERROELECTRIC LIQUID CRYSTAL

The concept of the azimuthal surface layer anchoring (ASLA) is introduced to describe the field-induced layer reorientation in a short pitch ferroelectric liquid crystal (FLC). It was found that for the larger ASLA, the smaller layer reorientation occurs and saturates in the high field regime, especially near both surfaces. The ASLA tends to renormalize the surface interactions which produce the asymmetric director switching. It is suggested that the ASLA acts as a hindered restoring force for the layer reorientation.

Smectic C* layer directional instabilities in cells with twist geometry

We have investigated the formation and structure of horizontal chevrons, as well as the reorientation dynamics of smectic layers under applied asymmetric electric fields in cells with a twist geometry. The tilted layer structure of horizontal chevron domains is found to be rotated by an angle approximately equal to the twist angle alpha, as compared with parallel rubbed substrates, alpha = 0°. The time of horizontal chevron formation decreases slightly with increasing twist angle. The smectic layer reorientation under application of time-asymmetric electric fields is found to be enhanced for reorientation into the direction of twist, while it is hindered for reorientation out of the direction of layer twist. Increasing the twist angle results in a basically linear increase/decrease in the reorientation velocity, depending on field asymmetry direction. The electro-optic behaviour of twist cells with inclined smectic layers is outlined and compared with measurements performed on cells with monostable, parallel anchoring conditions.

Theoretical model for layer reorientation in smectic A* liquid crystals subject to an oblique magnetic field

Recently, Carlsson and Osipov reported on observations of the rotation of smectic layers under certain experimental conditions, and a dynamic theory describing such rotations was presented [Carlsson, T., and Osipov, M. A., 1999, Phys. Rev. E, 60, 5619]. In the present work, the rotational motion of the smectic layers of a SmA* liquid crystal, over which an oblique magnetic field has been applied, is studied theoretically by the aid of this theory. The role of the interaction between the smectic layers and the substrates is discussed and it is shown that if this interaction is small enough, the smectic layers orient themselves in such a way that the layer normal is almost parallel to the magnetic field. The relevant material parameters governing the dynamical response of the induced tilt and the layer normal are identified, and the time dependence of the response of these two dynamical variables is calculated. From this calculation, expressions for the response times of the system are given and a numerical value of the rotational viscosity of the smectic layers is estimated.

Synchrotron x-ray study of the smectic layer directional instability

We have investigated the phenomenon of field-induced smectic layer instability, as monitored by synchrotron x-ray scattering. This instability means that, upon application of time-asymmetric electric fields to chiral smectics, the layer direction seems to "rotate" locally around an axis given by the direction of the applied field. For moderate values of field amplitude and asymmetry, domains with a favored layer inclination grow at the expense of unfavored ones, while larger fields and asymmetries generally lead to a chaotic flow behavior. At moderate amplitudes, we have followed the process of the horizontal layer folding (or horizontal chevron domain formation) and the smectic C* layer reorientation of ferroelectric liquid crystals by applying symmetric and asymmetric wave forms, respectively, and performing time resolved x-ray measurements. The studies unambiguously show the formation of a horizontal (in-plane, i.e., in a plane parallel to the cell substrates) chevron domain structure from a nonoriented sample by application of a symmetric electric field of sufficient amplitude. It is then demonstrated that a transition from the horizontal chevron domain structure to an in-plane uniform smectic layer direction takes place on application of asymmetric electric wave forms. Reversal of the field asymmetry reverses the inclination direction and selects the other layer normal direction as the uniform end state. The in-plane smectic layer reorientation process is followed here as it evolves, and analyzed directly by means of x-ray scattering.

Horizontal chevron domain formation and smectic layer reorientation in SmC* liquid crystals stabilized by polymer networks

The influence of a polymer network, stabilizing an initial texture of horizontal chevron geometry, on the in-plane smectic C* layer reorientation process is studied for different applied electric field conditions. As expected, the reorientation of smectic layers is strongly slowed down and eventually suppressed by the network, even at rather low monomer concentrations. Polymer network formation in a uniformly reoriented smectic layer state reveals that the network acts in two ways: first it gives a biased elastic torque counteracting a field of such symmetry as to cause a change from the templated layer direction; second it introduces an increased effective viscosity counteracting the reorientation in both directions. The behaviour of samples stabilized by two different kinds of polymer networks, created in between the smectic layers (intra-layer) and across them (inter-layer), is then investigated and discussed.

Dependence of the SmC* layer reorientation dynamics on enantiomeric excess

The process of the in-plane reorientation of smectic layers under application of asymmetric electric fields was investigated with respect to the enantiomeric excess. Measurements were carried out as a function of electric field asymmetry ratio, electric field amplitude and temperature. The enantiomeric excess was varied by mixing chiral antipodes of a ferroelectric smectic C* liquid crystal. We observed the general dynamic behaviour previously reported for a variation of external parameters: increasing layer reorientation velocity with increasing field asymmetry and amplitude and strongly decreasing layer reorientation velocity for decreasing temperature in the SmC* phase. The smectic layer reorientation dynamics is found to be slowed down linearly by decreasing the enantiomeric excess. The rotation vanishes on approaching the racemic mixture. The layer reorientation process changes its direction as the racemic composition of enantiomers is crossed.

Permeation flow associated with the smectic layer directional instability

The process of smectic layer reorientation (smectic layer rotation, smectic layer directional instability) under application of asymmetric electric fields is associated with a material flow along the rubbing direction. For smectic C* materials this permeation flow is located along the domain boundaries of horizontal chevrons. During the layer rotation, initially circular air bubbles inclosed within the liquid crystal, are elongated along the smectic layer plane. From this deformation we determine a linear material displacement with time and estimate a flow velocity of approximately 0.3 μm s−1 for the conditions applied.

The influence of surface treatment on the in-plane smectic layer reorientation

The dependence of the dynamic behaviour of the smectic C* layer reorientation on application of asymmetric electric fields is investigated for different surface treatment of the glass substrates, varying the alignment layer materials and rubbing conditions. It is found that already weak monostable anchoring limits the layer reorientation to twice the amount of the molecular tilt angle, while degenerate planar alignment promotes a continuous, unlimited rotation. Different layer reorientation times are observed for different alignment layer materials and can be correlated to the anchoring strength. Stronger anchoring hinders the reorientation of smectic layers. The number of buffing cycles (rubbing strength) does not have the expected pronounced effect on the layer reorientation, although increasing layer reorientation times are observed for increasing number of alignment layer rubbing cycles.