Liquid Crystalline Thin Films Research Articles

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

We have investigated the charge injection into a host–guest type of molecularly doped liquid crystalline thin film in liquid crystal cells. A 2-phenylnaphthalene smectic liquid crystal of 2-(4'-octylphenyl)-6-dodecyloxynaphthalene was used as a host material and a diketopyrrolopyrrole derivative of 1,4-diketo-N,N '-dimethyl-3,6-bis(4-dodecyloxyphenyl) pyrrolo[3,4-c]pyrrole was used as a dopant material: the current–voltage characteristics of the thin films with Pt, indium–tin oxide (ITO), MgAg, and Al electrodes were measured and analyzed on the basis of the Schottky mechanism. By comparing the current–voltage characteristics, we determined the majority carriers dominating the current and estimated the barrier height for electrons and holes. We found that the barrier height was smaller than the energy difference between energy levels of the host liquid crystalline material and the work functions of electrode materials. We concluded that the current in the host–guest thin film was dominated by the guest material that has a narrow energy gap and is responsible for the charge injection into the host material.

Liquid crystalline cellulosic elastomers: free standing anisotropic films under stretching

The structure and local ordering of 1,6-hexamethylenediisocyanate-(acetoxypropy1) cellulose (HDI-APC) liquid crystalline elastomer thin films are investigated by using X-ray diffraction and scattering techniques. Optical microscopy and mechanical essays are performed to complement the investigation. The study is performed in films subjected or not to an uniaxial stress. Our results indicate that the film is constituted by a bundle of helicoidal fiber-like structure, where the cellobiose block spins around the axis of the fiber, like a string-structure in a smectic-like packing, with the pitch defined by a smectic-like layer. The fibers are in average perpendicular to the smectic-like planes. Without the stretch, these bundles are warped, only with a residual orientation along the casting direction. The stretch orients the bundles along it, increasing the smectic-like and the nematic-like ordering of the fibers. Under stress, the network of molecules which connects the cellobiose blocs and forms the cellulosic matrix tends to organize their links in a hexagonal-like structure with lattice parameter commensurate to the smectic-like structure.

Molecular orientation in soft matter thin films studied by resonant soft x-ray reflectivity

We present a technique to study depth profiles of molecular orientation in soft matter thin films with nanometer resolution. The method is based on dichroism in resonant soft x-ray reflectivity using linear $s$ and $p$ polarization. It combines the chemical sensitivity of near-edge x-ray absorption fine structure spectroscopy to specific molecular bonds and their orientation relative to the polarization of the incident beam with the precise depth profiling capability of x-ray reflectivity. We demonstrate these capabilities on side chain liquid crystalline polymer thin films with soft x-ray reflectivity data at the carbon $K$ edge. Optical constants of the anisotropic refractive index ellipsoid were obtained from a quantitative analysis using the Berreman formalism. For films up to 50 nm thickness we find that the degree of orientation of the long axis exhibits no depth variation and is independent of the film thickness.

Liquid crystalline thin films as a precursor for polycrystalline thin films aimed at field effect transistors

We have investigated the availability of liquid crystalline thin films as a precursor of quality polycrystalline thin films for organic field effect transistors (OFETs) by solution process. The experiments of spin-coating at various temperatures proved that the morphology of resulting polycrystalline thin films at room temperature strongly depend on what the films immediately after spin-coating and solvent evaporation at those temperatures were, i.e., crystal, liquid crystal, or isotropic liquid. The polycrystalline films spin-coated at the temperatures for crystal and isotropic liquid phases were much roughened owing to many small crystal flakes and droplets. On the other hand, the thin films spin-coated at liquid crystal temperatures were very uniform in a whole area and showed high FET mobility having a small variation from device to device.

Novel liquid crystalline conjugated oligomers based on phenanthrene for organic thin film transistors

Three thiophene/phenanthrene hybrid oligomers were synthesized and their liquid crystalline (LC) properties were characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), temperature-dependent X-ray diffraction (XRD) and electron diffraction. Highly ordered LC phases with a broad temperature range were observed for all three oligomers probably owing to the absence of symmetry against the long axis of phenanthrene unit. Highly ordered smectic LC thin films were prepared by vacuum deposition and their organic thin film transistors (OTFTs) exhibited high mobilities up to 0.34 cm2 V−1s−1. Solution processed films showed self-healing behaviors upon thermal annealing, and OTFTs prepared by solution drop casting showed mobilities up to 0.05 cm2 V−1s−1. These results indicate that thiophene/phenanthrene hybrid oligomers are promising high mobility organic semiconductors for OTFTs.

Liquid-Crystalline Semiconducting Copolymers with Intramolecular Donor−Acceptor Building Blocks for High-Stability Polymer Transistors

The ability to control the molecular organization of electronically active liquid-crystalline polymer semiconductors on surfaces provides opportunities to develop easy-to-process yet highly ordered supramolecular systems and, in particular, to optimize their electrical and environmental reliability in applications in the field of large-area printed electronics and photovoltaics. Understanding the relationship between liquid-crystalline nanostructure and electrical stability on appropriate molecular surfaces is the key to enhancing the performance of organic field-effect transistors (OFETs) to a degree comparable to that of amorphous silicon (a-Si). Here, we report a novel donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), which contains both electron-donating quaterthiophene and electron-accepting 5,5'-bithiazole units. This copolymer exhibits excellent electrical characteristics such as field-effect mobilities as high as 0.33 cm(2)/V.s and good bias-stress stability comparable to that of amorphous silicon (a-Si). Liquid-crystalline thin films with structural anisotropy form spontaneously through self-organization of individual polymer chains as a result of intermolecular interactions in the liquid-crystalline mesophase. These thin films adopt preferential well-ordered intermolecular pi-pi stacking parallel to the substrate surface. This bottom-up assembly of the liquid-crystalline semiconducting copolymer enables facile fabrication of highly ordered channel layers with remarkable electrical stability.

Two-dimensional polarization holographic recordings in azobenzene liquid-crystalline polymer thin films

Two-dimensional （2D） polarization holographic recordings were accomplished in azobenzene side-chain liquid-crystalline polymer thin films. The 2D polarization grating makes up of two orthogonal 1D polarization gratings, which were written at the same place by two orthogonally linearly polarized 532 nm beams. It is found that the diffraction efficiency of 2D grating is lower than that of 1D grating, and the 2D grating acts as the integration of its componential 1D gratings. Furthermore, we find that the polarization dependency of diffraction efficiency and the polarization conversions arise from the collaboration of linear and circular birefringence.

Controlling the morphology of side chain liquid crystalline block copolymer thin films through variations in liquid crystalline content.

In this paper, we describe methods for manipulating the morphology of side-chain liquid crystalline block copolymers through variations in the liquid crystalline content. By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer (BCP) backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase-segregated BCP microstructures can be precisely manipulated. Increases in LC functionalization lead to stronger preferences for the anchoring of the LC mesophase relative to the substrate and the intermaterial dividing surface. By manipulating the strength of these interactions, the arrangement and ordering of the ultrathin film block copolymer nanostructures can be controlled, yielding a range of morphologies that includes perpendicular and parallel cylinders, as well as both perpendicular and parallel lamellae. Additionally, we demonstrate the utilization of selective etching to create a nanoporous liquid crystalline polymer thin film. The unique control over the orientation and order of the self-assembled morphologies with respect to the substrate will allow for the custom design of thin films for specific nanopatterning applications without manipulation of the surface chemistry or the application of external fields.

Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory

The surface-dependent anchoring and electro-optical (EO) dynamics of thin liquid crystalline films have been examined using Fourier transform infrared spectroscopy. A simple nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), is confined as 40, 50, and 390 nm thick films in nanocavities defined by gold interdigitated electrode arrays (IDEAs) patterned on polyimide-coated zinc selenide (ZnSe) substrates [Noble et al., J. Am. Chem. Soc. 124, 15020 (2002)]. New strategies for controlling the anchoring interactions and EO dynamics are explored based on coating a ZnSe surface with an organic polyimide layer in order to both planarize the substrate and induce a planar alignment of the liquid crystalline film. The polyimide layer can be further treated so as to induce a strong alignment of the nematic director along a direction parallel to the electrode digits of the IDEA. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB films. In an alternate set of experiments, uncoated ZnSe substrates were polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations. The dynamical rate constants measured for several nanoscale film thicknesses and equilibrium organizations of the director in these planar alignments show marked sensitivities. The orientation rates are found to vary strongly with both the magnitude of the applied potential and the initial anisotropy of the alignment of the director within the IDEA. The relaxation rates do not vary in this same way. The marked variations seen in EO dynamics can be accounted for by a simple coarse-grained dynamical model.

First approach to the use of liquid crystal elastomers for chemical sensors

Liquid crystalline thin films elastomers that are able to bind pesticides have been developed. The synthesis involves grafting mesogen and crosslinkable groups on a polysiloxane chain in the presence of a template molecule. The molecular imprinted material is obtained after thin film deposition, UV crosslinking and washing. Experiments of readsorption of pesticide are presented. Development of a multisensor platform based on thermal and capacitive sensors is described and tests of deposition of the polymer film are presented.

Mobility of electrons and holes in a liquid crystalline perylene diimide thin film with time of flight technique

When N, N′-diheptyl-3,4,9,10-perylenebiscarboximide (PTCDI-C7) was evaporated on an ITO glass substrate, a vapor-deposited polycrystalline film was normally obtained. It was then thermally treated on its liquid crystalline phase to get an ordered intermolecular structure. Then an ITO/PTCDI-C7/Al device was fabricated to study the drift mobility of electrons and holes by means of time-of-flight techniques. The negative charges were measured faster than the positive ones, as expected for an n-type organic semiconductor: the electron and the hole mobilities were 0.0139 and 0.0089 cm 2/(V s), respectively, at an electric field of 186 (V/cm) 1/2.

Morphology and structure of thin liquid-crystalline films at nematic-isotropic transition.

We review the main features of very thin nematic liquid-crystalline films on solid substrates, focusing on 5CB on oxidized silicon wafers. By discussing the theoretical aspects of the observed structures, we show that the phenomena at work include isotropic capillary condensation and that the coexistence of isotropic and nematic terraces in thin films is a result of the interplay of several mechanisms. Further theoretical as well as experimental efforts are needed to completely understand the wetting behavior of these systems.

Effects of Surface Morphology on the Anchoring and Electrooptical Dynamics of Confined Nanoscale Liquid Crystalline Films

The orientation and dynamics of two 40-nm thick films of 4-n-pentyl-4'-cyanobiphenyl (5CB), a nematic liquid crystal, have been studied using step-scan Fourier transform infrared spectroscopy (FTIR). The films are confined in nanocavities bounded by an interdigitated electrode array (IDA) patterned on a zinc selenide (ZnSe) substrate. The effects of the ZnSe surface morphology (specifically, two variations of nanometer-scale corrugations obtained by mechanical polishing) on the initial ordering and reorientation dynamics of the electric-field-induced Freedericksz transition are presented here. The interaction of the 5CB with ZnSe surfaces bearing a spicular corrugation induces a homeotropic (surface normal) alignment of the film confined in the cavity. Alternately, when ZnSe is polished to generate fine grooves along the surface, a planar alignment is promoted in the liquid crystalline film. Time-resolved FTIR studies that enable the direct measurement of the rate constants for the electric-field-induced orientation and thermal relaxation reveal that the dynamic transitions of the two film structures are significantly different. These measurements quantitatively demonstrate the strong effects of surface morphology on the anchoring, order, and dynamics of liquid crystalline thin films.

Controlled local organization of lyotropic liquid crystalline polymer thin films with electric fields

The ability of electric fields to induce the controlled alignment of lyotropic liquid crystalline polymer thin films on microfabricated substrates is demonstrated. These devices allow for variations in electric field geometry on the polymer alignment to be examined systematically. The polymers studied included poly(hexyl isocyanate) (PHIC) and poly(benzyl l-glutamate). The polymers were oriented by the electric field while in solution and solidified into a stable, oriented structure by solvent evaporation. Optical microscopy and transmission electron microscopy experiments make it possible to directly image the disclinations that mediate molecular alignment. The defect type and density in the solidified films are a function of the electric field geometry.

Light Propagation Through Alignment-patterned Liquid Crystal Gratings

We analysed the light propagation through alignment-patterned liquid crystal films where the alignment is planar and homeotropic for different domains, respectively. Using elastic theory the nematic director field can be analytically calculated with the method of conformal mapping in the one elastic constant approximation. A smooth change of the nematic director field is found with defect points at the substrate surfaces. In order to investigate the propagation of light passing through such a liquid crystalline thin film with spatially varying birefringence, we use a rigorous method. The use of rigorous methods allows the simulation of the time-dependent electric and magnetic fields for a region that is two-dimensional and possibly anisotropic. Simulations are made by the finite-difference time-domain method (FDTD), which is a numerical approach for the rigorous solution of the Maxwell equations. This method, in contrast to methods of geometrical and matrix optics, delivers results which include diffraction and scattering.

Structure and thermal fluctuations in thin films of smectic liquid crystals

Thermal fluctuations of the smectic layers in freely suspended thin liquid crystalline films have been studied by the methods of X-ray diffraction and reflectometry. The fluctuation dynamics is studied by the methods of photon correlation spectroscopy with the use of a source of coherent synchrotron radiation. In freely suspended smectic films, the typical relaxation times are of the order of several microseconds. In thin liquid crystalline films, the simultaneous damped and oscillatory behavior was observed for the layer undula- tions. © 2001 MAIK "Nauka/Interperiodica".

Electrostatic-field-induced chain alignment of liquid crystalline copolyether TPP thin films

A liquid crystalline (LC) copolyether has been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)propane with 1,7-dibromoheptane and 1,11-dibromoundecane with a 50/50 (both in %) equal composition of the 7- and 11-methylene monomers [coTPP-7/11(5/5)]. A mono-domain with a homeotropic alignment can be induced by a thin film surface in the LC phase. When an electrostatic field is applied to the surface-induced mono-domains parallel to the thin film surface normal, the molecular alignment undergoes a change from the homeotropic to uniaxial homogeneous arrangement. However, when the field is applied to a direction perpendicular to the thin film surface normal, the molecular alignment is about 10°-tilt with respect to the homeotropic alignment toward the a ∗-axis. This is because the permanent dipole moment of the copolyether is not right vertical to the molecular direction. The calculation of molecular dipoles indicates that the permanent dipole moment of this copolyether is about 70° away from the molecular axis, which leads to a negative dielectric anisotropy. It is speculated that the 10°- rather than 20°-tilt is due to a balance between the alignment induced by the electrostatic field and the surface. In the electrostatic field, molecules are subjected to a torque τ, which is determined by the permanent dipole moment P and the electrostatic field E: τ= P × E . The molecular realignment in both parallel and perpendicular directions to the thin film surface normal is determined by satisfying the condition of τ= P × E =0.

Bistable switching in nematic cells with a ferroelectric alignment film

In this paper, it will be presented that a nematic cell with a ferroelectric liquid crystalline thin film in senes realises a bistable switching as seen in the surface-stabilised ferroelectric liquid crystal (SSFLC) display devices. Such a bistability is found to critically depend on the squareness parameter of the FLC film as well as such matenal properties of the nematics, e.g. the elastic and the spontaneous polarization constants. It is also found that there exists an appropriate elastic constant to improve the optical transmittance. The fundamental electro-optic properties of the presently proposed nematic cells imply the advantage beyond the conventional nematic display devices with no memory effect.

Micro-probe raman studies of the dynamics of liquid crystals

We present a modified version of a Raman microspectroscope designed for dynamical studies of thin liquid-crystalline films in glass cells. This system is based on a confocal geometry and a multi-channel liquid nitrogen cooled CCD detector. The confocal arrangement makes possible the study of volumes only a few μm3 in size within a liquid-crystal cell with an axial resolution of 14 μm. The multi-channel detector allows high quality spectra to be recorded in a few seconds with less than 10 mW of laser power at the sample. Using an electrooptic modulator, dynamic studies can be done on a microsecond time scale. We present results obtained during the electrical switching of a anti-ferroelectric liquid-crystalline low molar mass organosiloxane material.

Surface Effects on the Dynamics of Liquid Crystalline Thin Films Confined in Nanoscale Cavities

The dynamics of 4-n-pentyl-4‘-cyanobiphenyl (5CB) nematic liquid crystalline thin films have been studied in real time using step-scan Fourier transform infrared spectroscopy (FTIR). In these studies, the liquid crystal was confined in a nanocavity defined and bounded by an interdigitated gold electrode array. The gold microstructures were microfabricated on a zinc selenide (IR-transparent) window. The 5CB interactions with the ZnSe substrate result in surface-induced ordering of the ultrathin layers (on the order of 40 nm). As the films increase in thickness, the nanoscale organization induced by the surface layer becomes a less significant contributor to the overall bulk structure of the sample. Time-resolved FTIR studies have enabled the measurement of rate constants for the orientation and relaxation of the thin films under an applied electric field as a direct function of confinement dimensions. Cell thicknesses ranging from 40 to 300 nm were studied. The measured rate behaviors demonstrate the stron...