Pretilt Angle Research Articles

Investigation of orientation behavior of nematic liquid crystals on UV-irradiated polyimide films

Abstract The orientation mechanism of liquid crystals (LCs) on surfaces remains unclear, despite several methods for controlling pretilt angles. This study investigates the relationship between the surface condition of polyimide films, whose pretilt angles can be controlled by UV dose, and LC orientation behavior. Absorbance at wavelength of 200 nm and 260 nm significantly decrease, while thickness reduces approximately 4 nm. A rubbing treatment further decreases the thickness by approximately 2 nm. Atomic force microscopy confirmed the change in molecular conformation by UV-irradiation and rubbing treatment. The dispersive and polar components of the surface free energy of UV-irradiated polyimide films are evaluated, it’s found that only the polar component changes with UV dose. Additionally, we confirm that the alignment of LCs transitions from homeotropic to planar with increased UV irradiation, demonstrating that pretilt angle distribution can be spatially controlled. These results contribute to establishing a photoalignment method for pretilt angle control.

Pretilt angle modulation of liquid crystals based on single crystal rubrene with incorporation of homeotropic polyimides

The alignment control of liquid crystals (LCs) is critical for various practical applications. The pretilt angle modulation of LCs typically requires a mechanical rubbing on substrates to orient the LCs. This study presents a contact-free approach to achieve pretilt angle modulation of LCs. Initially, a single crystal rubrene (SCR) film is deposited on the substrate. Subsequently, a mixture of nematic LCs and homeotropic polyimides (HPIs) is introduced between two SCR substrates via capillary action. During capillary filling, the synergy of capillary flow and the interaction between LCs and SCR ensures specific LC orientation. Subsequently, HPI dopants migrate toward and organize on the SCR through vertical phase separation, enhancing surface hydrophobicity and thereby increasing the pretilt angle of LCs. The pretilt angle of LCs can be continuously adjusted over a wide range from 2° to 90° by varying the HPI concentration. The contact-free process preserves against electrostatic charges, dust contamination, and surface damage typical of rubbed LC cells. This believed to be novel technique shows promise for developing no-bias-bend and bistable bend-splay LC displays.

Investigating the effect of SiO2 thin films with sculptured topographies on the alignment properties of liquid crystals

In the present work, sculptured thin films (STF) based on silicon dioxide (SiO2) induced on the indium tin oxide (ITO) coated substrates via glancing angle deposition (GLAD) technique using the physical vapor deposition (PVD) process. Our main motive to perform this work is to address the issues we have faced with the traditional alignment method and give a try to overcome those problems. Keeping these challenges in our mind, we are giving here an alternative to traditional method and explored this different alignment technique with GLAD process. We have fabricated a liquid crystal device (LCD) in a way of electrically-controlled birefringence (ECB) and placed the engineered substrates in anti-parallel alignment. Morphological, optical and electro-optic behaviour of resulting device has been studied in the ∼5 μm thick device. Effect of applied voltage on the morphology and electro-optic behaviour of liquid crystal has been established. Interestingly, LC material demonstrates electrically switchable birefringence colours viz. brown, green, pink, purple and mauve colour, under applied electric field in room temperature at 10X magnification under polarizing optical microscope. A very nice extinction and contrast has been obtained with fabricated ECB device, which elucidate the device quality with impeccable alignment without any disclination defects. Besides, the electro-optic characteristics it also shows a significant impact on the liquid crystal properties. The obtained results demonstrate nice optical retardation, a lower Frederick's threshold voltage i.e. 1.5 V and 19.03 dB higher contrast respectively. An impressive switching response time τon of 5.4 ms and τoff = 600 µs at 20 V has attained with respective device as well. Such change in parameters may correspond to the higher electrical torque experienced by the LC molecules on the application of applied field. Anchoring energy and pretilt angle of respective system has been calculated and correlated with observed behaviour. Thus, engineered device with this superior alignment method show remarkable electro-optic results which can open a new way for the development of fast LCD for advanced optical device applications.

Fast-response liquid crystal display device with trapezoidal pixel electrodes

ABSTRACT A fast response liquid crystal display (LCD) with trapezoidal pixel electrodes is proposed. The trapezoidal shape of the pixel electrode generates a slanted electric field, eliminating the need for a non-zero pre-tilt angle to prevent random domain formation in the LC. Additionally, the absence of a pre-tilt angle enhances viewing angles. During operation, LC molecules can form virtual walls, increasing anchoring energy and improving response time. At 38% transmittance, the response time is 5.1 ms, while at 60% transmittance, it increases to 8.3 ms. With a circular polariser, the device achieves a contrast ratio of 200:1 and a viewing cone of approximately 18°. Consequently, the trapezoidal pixel electrode FFS cell is suitable for fast LCDs used in automotive displays and digital signage. Its narrow viewing angles with a circular polariser also make it ideal for personal privacy displays and certain mobile displays.

The effects of side-chain flexibility on the photosensitivity of polyimide alignment film

ABSTRACT To examine the impact of side-chain flexibility on the photosensitivity of polyimide liquid crystal (LC) alignment films, two polyimides (OD-HAB-CA and OD-HAB6-CA) were synthesised. OD-HAB6-CA contains flexible hexamethylene side chains. This study demonstrates that the OD-HAB-CA alignment film can induce LC molecular alignment after 80 min of non-polarised ultraviolet light (NPUVL) irradiation. In contrast, the OD-HAB6-CA alignment film can achieve the same result after only 35 min of irradiation. This suggests that the flexibility of the side-chain can significantly improve the photosensitivity of the alignment film and shorten the required irradiation time. In addition, OD-HAB-CA and OD-HAB6-CA exhibit good thermal stability and light transmittance properties. The resulting LC cell have pretilt angles of 0.2° and 0.5°, respectively, which are suitable for in-plane switching mode LCDs.

Optimization of the properties of nematic matrix suspended with subtle quantities of the nanoparticles for improved alignment and operating voltage

ABSTRACT The current study examines and reflects the effect of minute quantity of Diamond nanoparticles (DNPs) in the Nematic liquid crystal (NLC) matrix. A well structured and strategized study is performed in order to get a deep understanding about the interactions that might have taken place between the guests and the hosts. Our study professed a significant alteration in the number of parameters of guest-host system in comparison to the pure liquid crystal (LC) matrix that could be found extremely promising for the display devices and low-power switching machines. 0.05, 0.07 and 0.1 wt/wt% concentrations of the NPs incorporated in the LC matrix and the composites are prepared with the high precision weight measurement using electrical balance. Dielectric spectrum is catalogued for both the homogeneous and vertical alignments of sample cells. Threshold voltage and operating voltage are found to be significantly decreased for 0.05 wt/wt% composite. Response time for the prepared composites is profoundly abated which is a crucial factor for the fast response devices. Careful examination of pretilt angles is executed and an improvement in the alignment is found as one more perk of this study.

P‐92: Photo‐sensitive Vertical Alignment Material with Room Temperature Dipcoating Technique

Using a room‐temperature dip‐coating technique, we propose a cinnamate‐unit‐phosphonic‐acid‐based surfactant alignment material that can be orientated by linear polarized light. Using the technique, we can treat a substrate simply and quickly, achieving a uniform pretilt angle of approximately 86° with a low dose of polarized UV light. We examined the optical and electrical properties of the alignment material under different conditions, including pretilt angles under different irradiation doses, voltage holding ratio (VHR) and residual direct circuit (RDC) when using different solvents, stability, etc. The results show that the material has acceptable performance. Since the phosphonic acid series photosensitive materials still have much room for optimization, we believe it is a potential new generation of photosensitive surfactant alignment materials.

31‐4: Curved and Fast Response Time Vertical‐alignment (VA) Liquid Crystal Gaming Display Development

We propose a fast response scheme for high definition curved VA LCD Panel. We make curved panel with different dual PI on the Array and CF substrate to obtain different pretilt angle. Fast response liquid crystal (LC) and Larger storage capacitance are used on curved panel, we can obtain fast response time without over driving (GTGavg= 6.2ms, GTGmax =9.1ms).

P‐187: Pretilt angle of liquid crystal molecules induced by photo‐aligned films of Polymerized 2‐(methacrylic acid) ethyl 3,5‐diaminobenzoic acid reaction monomer polyimide for PSVA technology

Orientation film is a key material in liquid crystal displays, and the technology of inducing liquid crystal molecular orientation through polyimide photoorientation film is widely studied. In this work, polyimide is a random copolymer synthesized from a mixture of cyclobutanetetracarboxylic dianhydride and pyromellitic dianhydride, 4,4' ‐ diaminodiphenylmethane, 3,3', 5,5'‐ tetramethylbenzidine, and Methanone, bis (4‐aminophenyl) ‐, homopolymer. In this study, we investigated the pre‐inclination of a series of photo‐crosslinked diamine structures for the polymerization of RM monomers by PSVA (Polymer Stabilized Vertical Alignment) technology, and mainly investigated the pre‐inclination of a series of light‐oriented films with different molar fraction reaction monomers polymerized on the backbone. The results show that the polymerization of 2‐(methacrylate)ethyl ester 3,5‐diaminobenzoic acid reaction monomer with 6.3% molar fraction of polyimide photooriented film achieves high pre‐tilt adjustment under low light accumulation conditions, and successfully achieves excellent optical & reliability performance.

A physicochemically reformed nanostructured graphene oxide-doped indium magnesium oxide film with enhanced electrical properties

Brush-coated graphene oxide (GO)-doped indium magnesium oxide (InMgO) films were produced in this study. The GO concentration was adjusted to 0, 5, and 15 wt%. The hybrid films were constructed using a one-step facile brush-coating process. The formed hybrid film had an oriented anisotropic morphology with a micro/nanogroove structure due to the shear stress induced by the brush hair movement. This anisotropic structure was used as a liquid crystal (LC) alignment layer. Polarized optical microscopy and pretilt angle analysis revealed the presence of a uniform and homogeneous LC alignment on the hybrid film. The film exhibited excellent light control with the LCs, as well as stable and high optical transmittance in the visible region. The GO doping of InMgO increased its polar anchoring energy and decreased its hysteresis characteristics for LC electronic device applications. These results demonstrate the potential of doping metal oxide films with GO and brush coating them for use in next-generation LC devices.

5CB molecular dynamics on TiO2-coated planar aligned ITO electrodes

ABSTRACT 5CB liquid crystal (LC) is a well-studied material for a variety of applications due to its orientational ordering and viscosity. LCs mostly suffer from ionic impurities which can be filtered using metal oxide nanoparticles. Beyond this conventional role of TiO2 nanoparticles (NPs) in ionic impurity filtration in LCs, we have employed a new methodology to improve the planar orientation of 5CB molecules by depositing a thin layer of NPs on either side of the ITO electrode. The 5CB LC exhibits a typical birefringent schlieren texture; however, the presence of TiO2 layer alters the molecular orientation. On the TiO2 surface, the 5CB LC molecules showed schlieren defect points characterised by both twofold (s = ± 1/2) and fourfold (s = ± 1) brush-like structures. The change in texture and lowering of dielectric permittivity might be attributed to the change in the pretilt angle of 5CB molecules owing to the mutual interactions between 5CB and TiO2 layer. Remarkably, the 5CB LC within the TiO2-coated cell attains the isotropic phase at lower temperature as compared to the cells coated with polyimide SE-130. The underlying mechanism for coarsening kinetics in the 5CB is elucidated using the Lifshitz-Slyozov-Wagner (LSW) theory.

Fabrication of advanced thin film for high performance display by nanoimprinting process via gallium oxide solution doped tin

Line pattern replication process through nanoimprint lithography (NIL) method has been used in numerous of research fields. NIL technology is not yet utilized for displays industry, and we propose an alignment layer of the sol-gel process using NIL. One-dimensionally nanopatterned by polydimethylsiloxane sheets cause surface changes in hybrid SnGaO thin films mixed in a 3:7 ratio, which aligns the liquid crystals (LCs) uniformly in the line pattern direction. These surface changes are confirmed through atomic force microscopy data analysis, and changes in surface shapes for different the curing temperatures in the furnace are analyzed. X-ray photoelectron spectroscopy shows that the chemical composition of the thin films changes according to curing temperatures, and the intensities of SnO and GaO increase exponentially at 200 °C compared to those at 50 °C. Through this, the van der Waals force increases between surface molecules, in the anisotropic direction to help align the LCs. Furthermore, we performed polarized optical microscopy and pre-tilt angle analysis confirm that the LCs are energized uniformly. Finally, the performance of an actual display device transmittance and electro-optical properties; the transmittance of SnGaO is 4.51p% higher than that of the currently commercialized PI-rubbing, and the voltage-transmittance curve is a perfect graph.

Micro/nanogroove-structured graphene-oxide-doped indium gallium yttrium oxide alignment layer with improved electrical characteristics

Graphene oxide (GO)-doped indium gallium yttrium oxide (InGaYO) alignment layers formed by a brush-based solution-coating process are reported in this study. The doping concentrations were adjusted to 0, 5, and 15 wt%. The alignment layers obtained thus showed anisotropic micro/nanogroove structures having induced by shear stress originating from the brush-hair movements. This structure enabled uniform and homogeneous liquid crystal (LC) alignment on the film surface with geometric constraints. The alignment state was verified by polarized optical microscopy and pretilt angle analysis. The GO-doped InGaYO alignment layer demonstrated enhanced 1.92 × 10−4 Jm−2 polar anchoring energy of the LCs and reduced hysteresis property, which aid in lower image-sticking effects. The LC device operation was verified with 1.7 V in a twisted-nematic LC system along with stable optical transmittance control. From these results, GO doping of the metal-oxide alignment layer shows the potential for functional LC device applications.

Controlled doping rates of graphene oxide in aluminum for high electrical performance and oxygen reduction reaction

The brush coating method allows the advantage of being able to produce thin films simply and quickly. This study introduces a method for thin-film production by brushing a solution containing graphene doped in Al2O3 using the sol–gel method. Graphene oxide (GO) is suitable for semiconductors with bandgap values of about 1.7 eV at room temperature, and the characteristics of the thin films were analyzed according to the doping ratio. First, X-ray photoelectron spectroscopy measurements were obtained to analyze the chemical composition of the thin-film surface. Since graphene is a carbon isomer, the characteristic of oxygen vacancies was confirmed by the increasing C–C bond intensity with increasing GO doping concentration. In addition, Raman analysis was performed to analyze the concentration of molecular groups of compounds for defects in graphene. Thereafter, through atomic force microscopy measurements, as the graphene doping ratio increased, the average roughness increased from 1.785 to 33.67, the residual DC voltage also increased by about 48.42 %, and the polar anchoring energy also increased by about 16.33 %. In addition, response-time–transmittance measurements were performed to measure the electro-optical properties of the thin film, and excellent Vth and stable response speed were obtained. Additionally, the validity of the results was supported through bandgap analysis. Finally, the degree of alignment of liquid–crystal molecules on the film surface was confirmed by polarized optical microscopy and pretilt angle measurements, and the suitability of the thin film for display devices was shown via transmittance measurements. As a result, GO:Al2O3 hybrid thin film is an excellent candidate for use as an alignment film for solar energy, secondary batteries, and next-generation liquid crystal displays.

Influence of Design Parameters of Liquid Crystal Cell on the Optical Threshold Voltage Value of Electro-Optical Effect

The influence of liquid-crystal (LC) cell design parameters on the voltage of the optical threshold of the interference effect of polarized beams controlled by an electric field has been studied by computer simulation. It is shown that under strict boundary conditions (both symmetrical and antisymmetrical), the voltage value of the optical threshold decreases rapidly depending on the cell thickness in an exponential manner and reaches a certain constant value. Under non-strict boundary conditions (both symmetrical and antisymmetrical), the voltage dependence of the optical threshold on the LC layer thickness follows a hyperbolic pattern. At the same time, under strict symmetric boundary conditions, the voltage magnitude of the optical threshold effect decreases exponentially with an increase in the pretilt angle of LC molecules on the orienting surfaces. If the boundary conditions are rigid and antisymmetric, an increase in the pretilt angle on the orienting surfaces results in a linear decrease of the optical threshold voltage of the effect. The voltage magnitude of the optical threshold effect increases linearly with an increase in the twist angle of the LC structure in the cell, for both symmetric and antisymmetric boundary conditions. A decrease in the anchoring energy of LC molecules on a substrate leads to a decrease in the voltage of the optical threshold of the effect, and this regularity has an exponential character depending on the reciprocal value of the anchoring energy of LC molecules on orienting substrates.

Spontaneously homogeneous alignment of liquid crystals on self-assembly organic rubrene

A novel spontaneously homogeneous liquid crystal (LC) alignment with self-assembled organic rubrene layer on the substrate surface has been reported. The spontaneous alignment technique without surface pre-treatment results in low cost, short processing time, and the room-temperature processing extends application to plastic device. The rubrene-nematic LC mixture is used to fabricate the LC cell. During the capillary filling, the LC molecules align in the flow direction. Meanwhile, the flow-induced LC alignment enforces the tetracene of the rubrene dopants along the flow direction because of the van der Waals and π–π interactions between the rubrene and LC molecules. Afterwards, the rubrene dopants spontaneously migrate onto substrate surface and self-assemble as the triclinic crystalline rubrene (TCR) with ribbon morphology. The LC alignment on TCR has a low pretilt angle, thereby providing a homogeneous LC alignment spontaneously. LC alignment on TCR shows a higher thermal stability compared to conventional polyimide due to the strong interactions of the rubrene and LC molecules. Moreover, the rubrene molecules remain in the bulk suppress the free-ions, decrease the rotational viscosity of LCs and hence shortening the response time of cell. The suppressed free-ions also decrease the screening effect on the substrate surfaces and hence the operation voltage of cell. The TCR structure on the substrate surface and the residual rubrene molecules dispersed in the bulk simultaneously contribute to the enhancement in electro-optical performance of the LC cell.

Bismuth-magnesium-oxide-based graphene oxide hybrid film for liquid crystal device application

Hybrid films containing bismuth magnesium oxide (BiMgO) and graphene oxide (GO) are investigated in this study. Solution-processed brush coating is used to fabricate the hybrid films, with the graphene oxide weight ratios adjusted to 0, 5, and 15%. The hybrid films show more stable and higher optical transmittances than the pure BiMgO film. A directional micro/nanogroove structure is observed on the hybrid film surface, which is derived from the shear stress during the brush-coating process; this anisotropic structure is used as a uniform liquid crystal (LC) alignment layer. Uniform and homogeneous LC alignment is then demonstrated via polarized optical microscopy and pretilt angle analyses, along with perfect light-control performance. Enhanced LC polar anchoring energy and hysteresis-free characteristics are observed for the brush-coated hybrid film as the GO-doping concentration is increased. Based on this perspective, the BiMgO-based GO hybrid film achieved via brush coating has potential for applications in next-generation LC systems.

Novel properties of high-performance multi-component mixture for Vertically Aligned mode LCDs

ABSTRACT Vertically Aligned mode Liquid Crystal Displays (VALCDs) which have generated huge research interest due to their excellent contrast and wide viewing angle require materials with negative dielectric anisotropy. This work highlights the systematic development of a fifteen-component mixture from pure liquid crystalline compounds comprising laterally fluorinated bi-, tri- phenyl, tolane, bicyclohexane compounds and non-mesogenic compounds exhibiting negative dielectric anisotropy. The temperature dependence of birefringence (Δn), dielectric anisotropy (Δε), threshold voltage (Vth), bend elastic constant (K33), relaxation time (τ0) and rotational viscosity (γ1) as a function of temperature of this fifteen-component nematic mixture have been reported. This mixture exhibits a high Figure of Merit (FoM) which is desirable for reducing the device response time. Additionally, the orientational order parameter (OOP) values, activation energy (Ea), visco-elastic co-efficient (γ1/K33) and material parameters (A 0 ) have also been reported. The effect of pretilt angle in the alignment layers on the threshold voltage (Vth) and switching time (τ0) to ascertain the applicability of this material in Vertically Aligned mode Liquid Crystal Displays (VALCDs) have also been discussed.

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

Graphene-oxide-doped oriented nickel oxide film achieved through brush coating for liquid crystal system

Hybrid films containing nickel oxide and graphene oxide are introduced in this work. The hybrid films are fabricated through a one-step brush-coating process, and the weight ratios of graphene oxide are controlled to 0, 5, and 15 %. The fabricated hybrid films show oriented nano/microgroove structures on their surfaces by directional shear stress, which originates from the brush hair movements during the solution-processed brush-coating process. The hybrid film formation is confirmed using X-ray photoelectron spectroscopy. The films also demonstrate high optical transmittance with amorphous structures. The hybrid films are then used to form liquid crystal (LC) alignment layers, and uniform as well as homogeneous LC alignment is confirmed via polarized optical microscopy and pretilt angle analyses. Compared to the pure nickel-oxide film, the hybrid film with graphene oxide enhances LC alignment stability by increasing the polar anchoring energy of the surface LC molecules. From these observations, the hybrid film containing metal oxide and graphene oxide is expected to be a promising alternative for functional LC systems.