Liquid Crystalline Thin Films Research Articles

Nanoscale View of Alignment and Domain Growth in a Hexagonal Columnar Liquid Crystal.

Highly ordered liquid crystalline (LC) phases have important potential for organic electronics. We studied the molecular alignment and domain structure in a columnar LC thin film with nanometer resolution during in situ heating using four-dimensional scanning transmission electron microscopy (4D STEM). The initial disordered vapor-deposited LC glass thin film rapidly ordered at its glass transition temperature into a hexagonal columnar phase with small (<10 nm), well-aligned, planar domains (columns oriented parallel to the surface). Upon further heating, the domains coarsen via bulk diffusion, then the film crystallizes, then finally transforms back to an LC phase at an even higher temperature. The LC phase at high temperature shows straight columns of molecules, which we attribute to structure inherited from the intermediate crystalline phase. Nanoscale 4D STEM offers direct insight into the mechanisms of domain reorganization, and intermediate crystallization is a potential approach to manipulate orientational order and texture at the nano- to mesoscale in LC thin films.

Multiphoton‐And SHG‐Active Pyrimidine‐Based Liquid Crystalline Thin Films Toward 3D Optical Data Storage

AbstractAn original asymmetric achiral pyrimidine core molecule, with long carbon chains on one side and a coumarin fragment on the other, is designed. This advanced molecular structure enabled the spontaneous emergence of crystalline and liquid crystalline thin films with strong 2nd harmonic generation (SHG) without any corona polishing process or tedious deposition techniques. Moreover, pin‐pointed multiphoton absorption in the liquid crystalline phase effectively switches off the SHG signal locally and permanently, underlining its interest in 3D data storage. This prototypical material, which can be fully addressed by NLO processes, multiphoton writing, and SHG reading, paves the way for the development of efficient flexible materials for 3D optical data storage.

Unrestricted Chiral Patterning by Laser Writing in Liquid Crystalline and Plasmonic Nanocomposite Thin Films.

Obtaining hierarchical structures with arbitrarily controlled chirality remains a challenge. Here, thin films featuring chiroptically bipolar patterns are produced by a device utilizing microscale photothermal re-melting of materials exhibiting chirality synchronization. This device operates autonomously, guided by an algorithm that facilitates the homochiral growth of supramolecular organic helices through controlling their re-melting. The chirality synchronization phenomena of constitutionally achiral molecules grants availability of both handednesses of the helices, enabling unrestricted chiral writing in the film. The collective chiroptical response of assembled molecules is utilised to guide the patterning process, creating a foundation for optically secured information. The established methodology enables achieving dissymmetry factor values for circular dichroism (CD) a magnitude higher than previously reported, as confirmed with state-of-the-art, synchrotron-based Mueller matrix polarimetry (MMP). Moreover, the developed method is extended to nanocomposites comprising gold nanoparticles, providing the opportunity to tune the CD toward the plasmonic region. This strategic application of photothermal processing, specifically laser-directed melting, uncovers the potential to broaden the selection of nanostructured materials with precisely designed functionalities for photonic applications.

Preparation of High-quality Vapor-deposited Crystalline Organic Thin Films through Crystallization from Their Liquid Crystal Phase

High-quality crystalline organic thin films with large crystalline domain size were fabricated by vacuum deposition, followed by a post-annealing and cooling crystallization from their respective smectic A (SmA) liquid crystal precursor states. Crystallinity, surface smoothness and air exposure stability of ionic liquid crystalline thin films were much improved by this process, and the electric double layer action was for the first time observed at the SmA phase. Moreover, a seeded temperature gradient cooling process from the SmA phase was proposed and its capability was demonstrated in the fabrication of benzothienobenzothiophene derivative thin films, achieving their directional lateral crystal growth with high precision controllability.

Hydrogen‐Bonded Cholesteric Liquid Crystals—A Modular Approach Toward Responsive Photonic Materials

A supramolecular approach for photonic materials based on hydrogen‐bonded cholesteric liquid crystals is presented. The modular toolbox of low‐molecular‐weight hydrogen‐bond donors and acceptors provides a simple route toward liquid crystalline materials with tailor‐made thermal and photonic properties. Initial studies reveal broad application potential of the liquid crystalline thin films for chemo‐ and thermosensing. The chemosensing performance is based on the interruption of the intermolecular forces between the donor and acceptor moieties by interference with halogen‐bond donors. Future studies will expand the scope of analytes and sensing in aqueous media. In addition, the implementation of the reported materials in additive manufacturing and printed photonic devices is planned.

Orientation and Morphology Control of the Liquid Crystalline Block Copolymer Thin Film by Liquid Crystalline Solvent.

The critical challenge to engineer the morphological structures in the strongly phase-segregated block copolymer thin film is to overcome the preferential wetting of the blocks at the interface and direct the self-assembly process. Herein, we utilize surface activity and selective solvation of a nematic (N) liquid crystalline (LC) solvent, 5CB, to facilely alter the LC anchoring and the orientation of the nanophase separated structures of the smectic-nematic (S-N) LC block copolymer thin film. For the neat S-N diblock copolymer thin film, the nanostructures are parallel aligned. In contrast, with continuous introduction of 5CB into the system, the orientations of the characteristic nanostructures and the morphologies of the LC thin film can be consequently changed, yielding the perpendicularly oriented lamellar or cylindrical structures with the feature size below 10 nm. The homeotropic alignment of the 5CB nematics near the air interface plays a critical role to induce this unique behavior in the S-N/5CB systems, which offers an opportunity to fine-tune the interfacial structures and the morphological patterning in the block copolymer thin film.

Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films: A new design space for organic light-emitting diodes

Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.

Beam shaping diffractive wave plates [Invited

We present and discuss opportunities opened up by a new generation of beam shaping optical elements that combine capabilities of digital spatial light polarization converters and diffractive properties of thin liquid crystalline films with patterned orientation of anisotropy axis (diffractive wave plates). Several functions of the new generation beam shapers are demonstrated, among them converting a laser beam of a Gaussian profile into a ring profile in the far field, a flattop profile, and into complex images. We also describe electrically controlled beam shaping optical elements which can be turned off and on within milliseconds by applying a low external voltage. Optical, morphological, and electro-optical properties of the components are characterized.

Highly Efficient Virus Rejection with Self-Organized Membranes Based on a Crosslinked Bicontinuous Cubic Liquid Crystal.

To remove viruses from water, the use of self-assembling liquid crystals is presented as a novel method for the synthesis of membranes with a regular pore size (below 1 nm) and controlled pore structures. Nanostructured bicontinuous cubic liquid-crystalline (LC) thin films are photopolymerized onto a polysulfone support layer. It is found that these membranes reject the virus, Qβ bacteriophage (≈20 nm diameter) by >99.9999%. Prepressurization of the membrane appears to enhance their virus rejection properties. This is the first example of nanostructured LC membranes that are used for virus rejection, for which they show great potential.

Photoinduced charge recombination in dipolar D-A-A photonic liquid crystal polymorphs.

A hexylalkoxy dipolar D-A-A molecule [7-(4-N,N-(bis(4-hexyloxyphenyl)amino)phenyl)-2,1,3-(benzothia-diazol-4-yl)methylene]propane-dinitrile, (C6-TPA-BT-CN) has been synthesized and the photophysics studied via femtosecond transient absorption spectroscopy (FsTA) in toluene and in amorphous and liquid crystalline spherulite thin films. Two spherulite macromolecular crystalline phases (banded, and non-banded) were observed through concentration dependent, solution processing techniques and are birefringent with a negative sign of elongation. A dramatic change in the electronic absorption from blue in amorphous films to green in spherulites was observed, and the molecular orientation was determined through the combined analysis of polarized light microscopy, X-ray diffraction, and scanning electron microscopy. FsTA was performed on amorphous films and show complex charge recombination dynamics, and a Stark effect, characterized from the combined TPA+˙ and [BT-CN]-˙ spectroscopic signatures at 450 nm and 510 nm and identified through spectroelectrochemistry. Radical cation dynamics of TPA+˙ was observed selectively at 750 nm with >503.3 ps (18%) recombination kinetics resulting in a rather significant yield of free charge carriers in amorphous films and consistent with previous reports on energetically disordered blend films. However, photoexcitation on large, non-banded spherulites areas (>250 μm) reveal average monoexponential charge recombination lifetimes of 169.2 ps from delocalized states similar to those observed in amorphous films and are 5× longer-lived than previous reports [Chang et al., J. Am. Chem. Soc., 2013, 135, 8790] of a related methyl-DPAT-BT-CN whose amorphous thin films were prepared through vapor deposition. Thus, the correlation between the microstructure of the blend film and the photoinduced radical pair dynamics described here is critical for developing a fundamental understanding of how dipolar states contribute to the charge carrier yield in a disordered energy system.

Competitive concurrence of surface wrinkling and dewetting of liquid crystalline polymer films on non-wettable substrates

Polymeric thin films coated on non-wettable substrates undergo film-instabilities, which are usually manifested as surface deformation in the form of dewetting or wrinkling. The former takes place in fluidic films, whereas the latter occurs in solid films. Therefore, there have rarely been reports of systems involving simultaneous deformations of dewetting and wrinkling. In this study, we propose polymeric thin films of liquid crystalline (LC) mesogens prepared on a non-wettable Si substrate and apply a treatment of plasma irradiation to form a thin polymerized layer at the surface. The resulting compressive stress generated in the surface region drives the formation of wrinkles, while at the same time, dipolar attraction between LC molecules induces competitive cohesive dewetting. Intriguing surface structures were obtained whereby dewetting-like hole arrays are nested inside the randomly propagated wrinkles. The structural features are readily controlled by the degree of surface cross-linking, hydrophilicity of the substrates, and the LC film thickness. In particular, dewetting of LC mesogens is observed to be restricted to occur at the trough regions of wrinkles, exhibiting the typical behavior of geometrically confined dewetting. Finally, wrinkling-dewetting mixed structures are separated from the substrate in the form of free standing films to demonstrate the potential applicability as membranes.

Phase Control of Lyotropic Liquid Crystals Using Amino Acid Ionic Liquids As Sovlents

Liquid crystals have attracted considerable attention as functional soft materials because they form well-ordered nanostructures. They have been applied in a variety of fields. In particular, they are used for the development of nanochannel materials transporting substances, such as ion, electron, and small molecules. Previously, we have developed ion-transporting liquid crystals by using ionic liquid. For example, columnar (Col) liquid-crystalline (LC) phosphonium salts give one-dimensional ion conductive pathways while smectic (Sm) LC imidazolium salts form two-dimensional ion conductive layers. In the course of these studies, we have focused on the use of bicontinuous cubic (Cubbi) phases having three-dimentionally interconnected nanochannel networks. For the construction of Cubbi LC assembles, we have used lyotropic LC systems containing ionic liquids (ILs) as solvents. Previously, we reported that lyotropic LC properties of amphiphiles can be controlled by the selection of anion of ILs. In these systems, induction of Cubbi phases has been successfully achieved. However, for the construction of LC system forming Cubbi phases in a wide temperature region, further insight how IL design influence self-organization of amphiphiles is still required. In the present study, we newly synthesized some ionic liquids. As cations, 1-(2-methoxyethyl)-3-methylimidazolium ([Im]), and cholinium ([Ch]) were used. As anions, we selected amino acids ([AA]). L-alanine ([Ala]), L-phenylalanine ([Phe]), and 3-(2-naphthyl)-L-alanine ([Nap]) were employed. We mixed these ILs with amphiphiles1 in various ratios and examined the LC properties of the obtained mixtures (1/IL) by varying temperature and component ratio and then constructed lyotropic LC phase diagrams. Comparing these phase diagrams, we examined the effect of physicochemical properties of ILs on the lyotropic LC behavior of the amphiphile. 1/ILs mixtures exhibit lyotropic LC behavior. Comparing the lyotropic LC phase diagrams, it was found that the LC region in the phase diagrams consists of Col, Cubbi, Sm, and so on. Induction of Cubbi phases having 3D ionic nanochannels is observed for the phase diagrams of 1/[Im][AA] and 1/[Ch][AA] while there are differences in their appearance. When examining the effect of anion species, it was found that the use of [Ala] is more effective for the expansion of Cubbi LC area than [Phe]. When examining the effect of cation species, 1/[Ch][AA] keeps LC phases in a wider temperature range than 1/[Im][AA]. For understanding factors of ILs that govern lyotropic LC behavior, we examined physicochemical properties of ILs. The thermal properties of the ILs were examined by differential scanning calorimetry measurements. All ILs showed glass transition temperature (Tg ). Tg increased as the increase of bulkiness of the side chain group in the anion; -73 °C ([Im][Ala]), -50 °C ([Im][Phe]), and -32°C ([Im][Nap]). It is probably attributed to the increase of the van der Waals force and some other interactions between the component ions. To examine the hydrophilicity of ILs, contact angle measurements were performed. We used two substrates, one has a surface bearing hydroxyl groups and the other has a surface bearing alkyl chain groups. On the hydrophilic substrate, the contact angle of [Im][Ala] is smaller than that of [Im][Phe]. In contrast, contact angle of [Im][Ala] is larger than that of [Im][Phe] on the hydrophobic substrate. It is well known that contact angle becomes smaller as the increase of affinity between liquids and substrates. It was strongly suggested that the hydrophilicity of [Im][Ala] is higher than that of [Im][Phe]. Considering these physicochemical properties of ILs, we found that bulkiness of anions and hydrophilicity of ILs are significant factors that govern the self-organization behavior of amphiphiles. One of interesting applications of these LC phases is for ion conductive pathways. For the ion conduction measurements for LC thin film samples, we employed two types of cells; one equips comb-shaped gold electrodes and the other equips indium tin oxide electrodes. The former cell provides ionic conductivity parallel to the substrate (σ//). The latter one provides ionic conductivity perpendicular to the substrate (σ⊥). By comparingσ// and σ⊥, we examined the relationships between the macroscopic continuity of ionic nanochannels and dimensional-order of LC phases. The ionic conductivity of the present materials would also be discussed in the presentation.

Using dislocations to probe surface reconstruction in thick freely suspended liquid crystalline films.

Surface interactions can cause freely suspended thin liquid crystalline films to form phases different from the bulk material, but it is not known what happens at the surface of thick films. Edge dislocations can be used as a marker for the boundary between the bulk center and the reconstructed surface. We use noncontact mode atomic force microscopy to determine the depth of edge dislocations below the surface of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline B phase. Here, 3.0±0.1 nm high steps are found with a width that varies with temperature between 56 and 59°C. Using a strain model for the profile of liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we find that the number of reconstructed surface layers increases from 4 to 50 layers as the temperature decreases from 59 to 56°C. This trend tracks the behavior of the phase boundary in the thickness dependent phase diagram of freely suspended films of 7O.7, suggesting that the surface may be reconstructed into a smectic F region.

Odd–Even Effects in the Dynamics of Liquid Crystalline Thin Films on the Surface of Single Walled Carbon and Silicon Carbide Nanotubes: Computer Simulation Study

Translational and reorientational dynamics of monolayer films of n-cyanobiphenyl (with n = 5–8) series of mesogens on the surface of single walled carbon nanotubes and heterogeneous silicon carbide nanotubes has been studied using molecular dynamics computer simulation. Activation characteristic of diffusion of the mesogens on the surface of the heterogeneous nanotube is selective with respect to the number of carbon atoms in the aliphatic tail of the mesogen, opposite to the case of the mesogens on the surface of the homogeneous carbon nanotube, which reveals nonselective activation. The orientational order parameter ⟨P2⟩ and thermal activation energy of dipolar relaxation of the studied systems also exhibit a characteristic odd–even pattern with respect to the length of the hydrocarbon chain of the mesogen molecule.

Effects of confinement, surface-induced orientations and strain on dynamical behaviors of bacteria in thin liquid crystalline films.

We report on the organization and dynamics of bacteria (Proteus mirabilis) dispersed within lyotropic liquid crystal (LC) films confined by pairs of surfaces that induce homeotropic (perpendicular) or hybrid (homeotropic and parallel orientations at each surface) anchoring of the LC. By using motile vegetative bacteria (3 µm in length) and homeotropically aligned LC films with thicknesses that exceed the length of the rod-shaped cells, a key finding reported in this paper is that elastic torques generated by the LC are sufficiently large to overcome wall-induced hydrodynamic torques acting on the cells, thus leading to LC-guided bacterial motion near surfaces that orient LCs. This result extends to bacteria within LC films with hybrid anchoring, and leads to the observation that asymmetric strain within a hybrid aligned LC rectifies motions of motile cells. In contrast, when the LC film thickness is sufficiently small that confinement prevents alignment of the bacteria cells along a homeotropically aligned LC director (achieved using swarm cells of length 10-60 µm), the bacterial cells propel in directions orthogonal to the director, generating transient distortions in the LC that have striking "comet-like" optical signatures. In this limit, for hybrid LC films, we find LC elastic stresses deform the bodies of swarm cells into bent configurations that follow the LC director, thus unmasking a coupling between bacterial shape and LC strain. Overall, these results provide new insight into the influence of surface-oriented LCs on dynamical bacterial behaviors and hint at novel ways to manipulate bacteria using confined LC phases that are not possible in isotropic solutions.

Thermotropic, side-chain ordered polymeric coatings: gas permeability switching via a thermal stimulus

Rapid access to large areas of stimuli-responsive materials is attractive in the field of membrane science for purification systems and molecular valves. Thermotropic liquid crystalline (LC) systems show sharp property changes through the smectic–LC transition induced by temperature. Comparison of similar amorphous and liquid crystalline systems allows for the elucidation of the characteristics of the LC phase. Lightly crosslinked C6F13 and C8F17 perfluorinated side-chain acrylate networks were UV cured as thin films resulting in amorphous and thermotropic liquid crystalline thin films, respectively. Thermal and morphological characterization indicates a restructuring of the liquid crystalline phase through the isotropic transition giving rise to an increase in transport and free volume properties. The amorphous film showed no dramatic change in transport or free volume properties.

Electrical and NO2 sensing properties of liquid crystalline phthalocyanine thin films

Abstract In this study, the electrical and nitrogen dioxide (NO2) sensing properties of a series liquid-crystalline octakisalkylthiophthalocyanine (C6S)8PcM [M = 2H, Ni, Cu, Zn] thin film were investigated. The thin films were coated on to the Interdigital Transducer (IDT) with gold electrodes, using jet-spray technique in an inert ambient from a solution of phthalocyanines in chloroform. The electrical properties of liquid-crystalline phthalocyanines were studied within wide temperature range from 293 to 423 K under the high purity nitrogen (N2) flow and in the frequency range from 500 Hz to 1 MHz for ac electrical measurements. It was observed that the I–V curves of all phthalocyanines showed a significant hysteresis at low temperature and the maximum dc conductivity obtained for (C6S)8PcZn thin film. The ac conduction mechanism of all thin films could be explained with quantum-mechanical tunneling (QMT) model. The NO2 gas sensing measurements were done in the concentration range of 1–10 ppm at the temperatures of 100 °C and 150 °C. The maximum sensor response (%) was obtained as approximately 10,000 and the response time was observed to be a few seconds for (C6S)8PcH2 thin film at 150 °C.

Self‐assembly and structure formation in liquid crystalline phthalocyanine thin films studied by Raman spectroscopy and AFM

This work presents an investigation of films prepared by doctor blade casting, the formation of self‐assembled microstructures of a liquid crystalline phthalocyanine with highly oriented molecules. Raman Spectroscopy in combination with atomic force microscopy is applied to study the structures within the films. By keeping the substrate at room temperature or at 353 K during coating, different geometric structures namely rods and islands form. Rod‐like structures are growing in coating direction, whereas directional growth of the islands is not observed. The distribution of the rod lengths varies widely, whereas the width appears more uniform. Annealing of the samples shows a different behavior of the two textures. Islands tend to melt, and rods smooth their structural form, which is extracted from Raman imaging in combination with atomic force microscopy. Additionally, Raman imaging gives insight into laterally different relative crystallinity. These observations are discussed in the context of the molecular orientation as probed by polarized Raman spectroscopy. These polarized Raman spectra indicate azimuthal alignment of the molecules within the rods (edge on alignment). This alignment occurs along and also perpendicular to the growth direction. In contrast to the alignment in the rods, the molecules inside the islands occurring at higher temperature do not show preferential molecular orientation. After annealing, no preferential molecular orientation is observed in rods because of the loss of anisotropy, too. Copyright © 2012 John Wiley &amp; Sons, Ltd.

High electron mobility in a columnar phase of liquid-crystalline perylene tetracarboxylic bisimide bearing oligosiloxane chains

A liquid-crystalline (LC) semiconductor based on perylene tetracarboxylic bisimide (PTCBI) bearing four 1,1,1,3,3-pentamethyldisiloxane chains was synthesized. This LC PTCBI derivative exhibits ordered columnar phases and the low temperature columnar phase is retained below room temperature. The electron mobility in the ordered columnar phase of this compound at room temperature exceeds 0.1 cm2 V−1 s−1. Moreover, this compound is soluble in various organic solvents, except alcohols, and thin films in which the columnar aggregates are aligned parallel to the substrates are produced by spin-coating. In particular, LC thin films in which the columnar stacks are uniaxially oriented can be produced by a friction transfer method.

Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films

Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films