Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Optically switchable and axially symmetric half-wave plate based on photoaligned liquid crystal films

We demonstrate an optically switchable half-wave plate (HWP) composed of a photoaligned and axially symmetric liquid crystal (ASLC) film containing two azobenzene derivatives, methyl red (MR) and 4-butyl-4′-methoxyazobenzene (BMAB). MR is responsible for photoalignment, and BMAB is used for optical tuning and switching the state of polarization (SOP) of probe beam (633 nm He–Ne laser) passing through the MR/BMAB doped ASLC film. The photoaligned ASLC film is first fabricated using a line-shaped laser beam (532 nm) exposure applied on a rotating LC sample. The fabricated ASLC film can passively change the linearly polarized light. Under UV light exposure, the formation of cis-BMAB (bend-like shape) within the film disrupts the LC molecules, switches the LC orientation, and further changes the SOP of the probe beam. Under laser irradiation (532 nm), the formation of trans-BMAB (rod-like shape) reverts the LC orientation back and simultaneously generates cis-MR, helping anchor the LC in the previously photoaligned orientation. The photoaligned MR/BMAB-doped LC HWP can change the linear SOP under alternating UV and visible light exposure.     

Negative giant surface potential of peeled Alq3 thin film

In order to observe negative giant surface potential (gSP), vacuum-evaporated tris(8-hydroxyquinolinolato) aluminum (Alq₃) thin film on a polytetrafluoroethylene (PTFE) substrate was peeled off by an adhesive carbon tape in dark condition. Surface potential on the backside was negative, and the absolute values were about two-thirds of the surface potential values (positive values) of the films deposited on ITO with the same thickness. Our results indicate that the gSP does not originate in interfacial or bulk charge, but in anisotropic alignment of dipole moment of Alq₃ molecules in the film. With this simple procedure, we can realize Alq₃ film with opposite molecular alignment, which can be useful in device application.     

Orientational control of liquid crystal molecules by reformed poly(dimethylsiloxane) alignment layer via ion-beam irradiation

We introduce an alternative approach to making a reformed PDMS layer as a pretilt controllable alignment layer, which can be used in the vertical alignment (VA) and twisted nematic (TN) modes, by varying the ion beam (IB) energy. Depending on different the dielectric anisotropy of surfaces, PDMS layers have been demonstrated to align LC molecules homogeneously and homeotropically. The electro-optic characteristics of aligned VA and TN-LCD based on PDMS layer were comparable to those of VA and TN-LCD based on polyimide, showing good potential of PDMS film as an alignment layer.     

Polyimide film containing ultra-fine particles

We have succedded in fabricating a polyimide film with a molecular alignment superior to that obtainable by the conventional rubbing treatment. This phenomenon is realized by means of the dispersion of ultra-fine particles such as carbon and titanium dioxide in the polyimide precursor prior to the fabrication of the polyimide film and the rubbing treatment. The molecular alignment was evaluated with birefringence and infrared dichroism measurements. Further experiments establishe that the adsorption of polyimide molecules on ultra-fine particles is a significant factor in achieving a large orientational anisotropy. A model of the alignment is presented in which the rubbing treatment induces the particles to traverse the surface of the film, causing polyimide molecules adsorbed on them to be elongated in the rubbing direction. A potential application of oriented polymide films is the homogeneous alignment of nematic and smectic liquid crystals.     

Asymmetric Fullerene Nanosurfactant: Interface Engineering for Automatic Molecular Alignments

Since the molecular self‐assembly of nanomaterials is sensitive to their surface properties, the molecular packing structure on the surface is essential to build the desired chemical and physical properties of nanomaterials. Here, a new nanosurfactant is proposed for the automatic construction of macroscopic surface alignment layer for liquid crystal (LC) molecules. An asymmetric nanosurfactant (C₆₀NS) consisted of mesogenic cyanobiphenyl moieties with flexible alkyl chains and a [60]fullerene nanoatom is newly designed and precisely synthesized. The C₆₀NS directly introduced in the anisotropic LC medium is self‐assembled into the monolayered protrusions on the surface because of its amphiphilic nature originated by asymmetrically programmed structural motif of LC‐favoring moieties and LC‐repelling groups. The monolayered protrusions constructed by the phase‐separation and self‐assembly of asymmetric C₆₀NS nanosurfactant in the anisotropic LC media amplify and transfer the molecular orientational order from surface to bulk, and finally create the automatic vertical molecular alignment on the macroscopic length scale. The asymmetric C₆₀NS nanosurfactant and its self‐assembly described herein can offer the direct guideline of interface engineering for the automatic molecular alignments.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

Quantitative methods based on twisted nematic liquid crystals for mapping surfaces patterned with bio/chemical functionality relevant to bioanalytical assays

We report methods for the acquisition and analysis of optical images formed by thin films of twisted nematic liquid crystals (LCs) placed into contact with surfaces patterned with bio/chemical functionality relevant to surface-based assays. The methods are simple to implement and are shown to provide easily interpreted maps of chemical transformations on surfaces that are widely exploited in the preparation of analytic devices. The methods involve acquisition of multiple images of the LC as a function of the orientation of a polarizer; data analysis condenses the information present in the stack of images into a spatial map of the twist angle of the LC on the analytic surface. The potential utility of the methods is illustrated by mapping (i) the displacement of a monolayer formed from one alkanethiol on a gold film by a second thiol in solution, (ii) coadsorption of mixtures of amine-terminated and ethylene glycol-terminated alkanethiols on gold films, which leads to a type of mixed monolayer that is widely exploited for immobilization of proteins on analytic surfaces, and (iii) patterns of antibodies printed onto surfaces. These results show that maps of the twist angle of the LC constructed from families of optical images can be used to reveal surface features that are not apparent in a single image of the LC film. Furthermore, the twist angles of the LC can be used to quantify the energy of interaction of the LC with the surface with a spatial resolution of <10 microm. When combined, the results described in this paper suggest nondestructive methods to monitor and validate chemical transformations on surfaces of the type that are routinely employed in the preparation of surface-based analytic technologies.     

Liquid-crystal micropolarizer array for polarization-difference imaging

Fabrication and applications are discussed for a visible-wavelength micropolarizer array consisting of a linear polarizer and a micropatterned liquid-crystal (LC) cell. LC alignment direction is controlled by means of depositing an optically transparent gold film at an oblique angle and coating the surface with an alkanethiol self-assembled monolayer. Microdomains of two perpendicular LC alignment directions are created by photolithography and etching of the gold layer, rotating the substrate 90 deg, and depositing a second oblique gold layer in the etched areas. The resulting array is used for polarization-difference imaging (PDI), a technique that enhances image contrast in the presence of scattering. Images obtained with the array require more processing than do conventional PDI images, but this method eliminates the need for an electronically activated LC filter and is especially suited to systems whose filters are closely integrated with optical sensor arrays.     

Mixed alkylsilane functionalized surfaces for simultaneous wetting and homeotropic anchoring of liquid crystals

Surfaces functionalized with a self-assembled monolayer (SAM) formed from a mixture of two alkylsilanes with different chain lengths have been designed to simultaneously improve the liquid crystal (LC) wettability and promote homeotropic anchoring of the LC. Most chemically functionalized surfaces (e.g., long alkyl chain SAMs) that promote homeotropic alignment of LC possess low surface energy and result in poor LC wettability, inhibiting LC infiltration into microstructured surfaces and sometimes resulting in LC dewetting from the surface. However, a surface modified with a mixed SAM of octadecyltriethoxysilane (C18) and ethyltriethoxysilane (C2) exhibited very low LC contact angle while providing homeotropic anchoring. Ellipsometry was used to correlate the bulk concentration of C18 in the deposition solution to the surface coverage of C18 in the mixed monolayer; these bulk and surface concentrations were found to be equal within experimental uncertainty. The LC contact angle was found to depend nonmonotically with the surface coverage density, with a minimum (14.4 ± 0.1°) at a C18 surface coverage of 0.26 ± 0.08. Homeotropic LC anchoring was achieved at a C18 surface coverage of ≥0.11 ± 0.04, in the regime where a minimum in the LC contact angle was observed. The practical application of this approach to surface modification was demonstrated using a micropillar array sensor substrate. When the array was functionalized with a conventional C18 SAM, the LC did not infiltrate the array and exhibited a contact angle of 47.4 ± 0.5°. However, the LC material successfully infiltrated and wetted the same microstructured substrate when functionalized with a C18/C2 mixed SAM, while still exhibiting the desired homeotropic anchoring.     

Surface modification using polymer Langmuir-Blodgett films

We describe a simple and effective approach to introduce a functional group into polymer film on a solid surface using reactive polymer LB films. N-dodecylacrylamide copolymers containing terminal amino groups in the side chains as the reactive moiety form a stable monolayer, and the monolayer was transferred onto a solid support to modify the solid surface using the Langmuir-Blodgett method. The transferred coatings were characterized with fluorescence, IR spectroscopies, and X-ray diffraction. The reactivity of the terminal amino group incorporated in the LB films was investigated in detail using fluorescein isothiocyanate (FITC) as a fluorescent probe. The chemical reaction between amino groups in the LB films and FITC in the bulk solution was completed within approximately 30 minutes and the chemical bond formation was confirmed by infrared spectroscopy. Furthermore, the fluorescent image of the multilayers reacted with FITC were observed with fluorescent microscopy. This method is effective for tailoring functional organic ultrathin films on solid substrates.     

Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single‐Crystal Graphene

Epitaxial alignment of organic liquid crystal (LC) molecules on single‐crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment‐layer‐free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment‐layer‐free electro‐optical LC device and LC alignment duplication, which can result in practical graphene‐based flexible LC devices, are realized.     

Magnetic-field-assisted formation of alignment polymer coatings in liquid crystal cells

We describe a new method for obtaining liquid crystal (LC) layers with planar orientation in plane-parallel cells, which is based on the technology of LC-polymer interface formation in solution under the action of an applied magnetic field. The azimuthal anchoring energy of LC at the polymer surface has been determined by measuring the angle of orientation of the nematic LC director on the substrate surface as a function of the magnetic field. The LC orientation provided by the proposed method is stable, and the anisotropy of LC anchoring is comparable with that achieved using well-known methods of alignment polymer film preparation by rubbing.     

PTCDA molecules on a KBr/InSb system: a low temperature STM study

We have used scanning tunnelling microscopy (STM) at 77?K to investigate 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules adsorbed on an ultrathin (1-2?monolayer (ML)) film of KBr grown on a c(8 × 2)InSb(001) substrate. The molecules are stabilized both at the KBr steps and on the terraces. On the 1?ML film the PTCDA molecules appear predominantly as single entities, whereas on the 2?ML film formation of molecular clusters is preferred. Differences in the adsorption configurations indicate that the interaction between the molecules and the surface differs significantly for the cases of 1 and 2?ML films. We present images of the molecules obtained with sub-molecular resolution for both filled and empty state sampling modes. We argue that the highest occupied molecular orbital (the lowest unoccupied molecular orbital) is responsible for intramolecular contrast in filled (empty) state images of the molecules, even though they are deformed due to strong interaction with the substrate.     

Mono- and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching"

Contact mode atomic force microscopy (AFM) was used to intentionally scratch a monolayer deposited on a pyrolyzed photoresist film (PPF). The force was set to completely remove the monolayer but not to damage the underlying PPF surface. A line profile determined across the scratch with tapping mode AFM permitted determination of the monolayer thickness from the depth of the scratch. A statistical process was devised to avoid user bias in determining the monolayer thickness and was used to determine the thickness as a function of derivatization parameters. PPF surfaces modified by reduction of diazonium ions of stilbene, biphenyl, nitrobiphenyl, terphenyl, and nitroazobenzene (NAB) were scratched and their modification layer thicknesses determined. For single-scan derivatizations of 1 mM diazonium ions to -0.6 V versus Ag+/Ag, the biphenyl and stilbene monolayers exhibited thicknesses close to those expected for true monolayers. However, more extensive derivatization resulted in multilayers up to 6.3 nm thick for the case of NAB. Such multilayers imply that electrons are transmitted through the growing film during diazonium reduction, despite the fact that electron tunneling would not be expected to be operative over such long distances. The results are consistent with a conductance increase in the growing film, which yields a partially conductive layer that can support further diazonium ion reduction and additional layer growth.     

聚苯乙炔单分子膜成膜特性研究

研究了聚苯乙炔单分子膜的成膜特性及其结构，结果表明，聚苯乙炔能在较大的表面压范围内形成稳定的单分子膜，并具有不可重复压缩性，表面压力的各向异性和松弛特性，ＴＥＭ照片显示，聚苯乙炔分子链在单分子膜中是有序排列的，转移比和ＸＰＳ研究证明，单分子膜向铝片的沉积是成功的。     

Co／Pt多层膜和CoxPt1—x（x≈0.25）合金膜的垂直磁各向异性

介绍了关于Ｃｏ／Ｐｔ多层膜和ＣｏｘＰｔ１－ｘ（ｘ≈０．２５）合金膜的垂直磁各向异性研究的情况。一般认为,Ｃｏ／Ｐｔ多层膜的垂直磁各向异性主要源于界面外沿膜面法向的平移对称性中断以及Ｃｏ与Ｐｔ的ｄ电子杂化。界面处的合金化和界面不平整地垂直磁各向异性是不利的。而应力对界面各向异性的影响尚不十分清楚。在具有垂直磁各向异性的ＣｏｘＰｔ１－ｘ合金膜中观察到了Ｃｏ的聚集,Ｃｏ轨道磁矩和Ｃｏ－Ｃｏ对数中向异性。     

Monolayer to multilayer nanostructural growth transition in N-type oligothiophenes on Au(111) and implications for organic field-effect transistor performance

The evolution in growth morphology and molecular orientation of n-type semiconducting alpha,omega-diperfluorohexyl-quaterthiophene (DFH-4T) on Au(111) is investigated by scanning tunneling microscopy and scanning tunneling spectroscopy as the film thickness is increased from one monolayer to multilayers. Monolayer-thick DFH-4T films are amorphous and morphologically featureless with a large pit density, whereas multilayer films exhibit drastically different terraced structures consisting of overlapping platelets. Large changes in DFH-4T molecular orientation are observed on transitioning from two to four monolayers. Parallel electrical characterization of top-versus-bottom contact configuration DFH-4T FETs with Au source/drain electrodes reveals greatly different mobilities (mu(TOP) = 1.1 +/- 0.2 10(-2) cm(2)V(-1)s(-1) versus mu(BOTTOM) = 2.3 +/- 0.5 10(-5) cm(2)V(-1)s(-1)) and contact resistances (R(C-TOP) = 4-12 MOmegacm vs R(C-BOTTOM) > 1 GOmegacm). This study provides important information on the organic semiconductor-source\drain electrode interfaces and explains why top-contact OFET devices typically have superior performance. By direct visualization, it demonstrates that the DFH-4T film growth transition from monolayer to multilayer on Au is accompanied by dramatic morphology and molecular orientation changes, starting from an amorphous, pitted, and disordered monolayer, to crystalline and smooth bi/tetralayers but with the molecules reoriented by 90 degrees . These chemisorption-derived inhomogenities at the contact-molecule interface and the large monolayer --> multilayer --> bulk microstructural changes are in accord with the large bottom-contact device resistance and poor OFET performance.     

Planar Anchoring of C70 Liquid Crystals Using a Covalent Organic Framework Template

The surface‐induced anchoring effect is a well‐developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer‐scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF‐1) can offer an anchoring effect to template C₇₀ molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C₆₀ system, a stepwise breakdown in ordering of C₇₀ LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface‐confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.