Liquid Crystal Molecules Research Articles

Coalescence of nematic liquid crystal droplets on freely suspended liquid crystal films

Freely suspended smectic films of a few molecular layers thick is a model system that can be used to study two dimensional phenomena. In our study, we drew a smectic C film across a few millimeter hole and stabilized it with a strong surface tension capable of suspending 5CB nematic liquid crystal droplets sprayed onto it. Thermal fluctuation of liquid crystal molecules is observed across the smectic C film and can cause the droplets to be driven closer and coalesce. The coalescence in this environment has yet to be studied more extensively. When two droplets are touching each other, connecting bridge between them forms and rapidly grows while they merge. Coalescence between them was thoroughly studied through high-speed camera under the cross-polarized observation and different regimes during coalescence process were analysed. Scaling law for various regimes will be discussed and explained. To understand the experimental result further, we also perform the study of droplet coalescence through finite element software which include inertial force, interfacial force and viscous force in the model. Comparing and analyzing the experimental and simulation results.

Optical Properties of a Composite System with Nematic LC and Colloids Quantum Dots

Background: Optical luminescence in a composite system with nematic LC 4- octyloxy-4’-cyanobuphenyl (8OCB) and semiconductor quantum dots CdSe/CdS and CdSe/CdS/ZnS has been synthesized by a water-organic method. Methods: Composites have been investigated by means of polarizing microscopy, dynamic light scattering, and measurements of dielectric properties in the frequency range from 20 Hz to 5 MHz. The non-radiative excitation energy transfer from the liquid crystal molecules to the quantum dot in the LC-QD composite is detected by using the luminescence spectroscopy method. Results: This effect as well as the shift of the luminescence band is owing to components’ intermolecular interaction. Conclusion: The optimal concentration of QD in a composite that enhanced luminescence intensity was detected.

Molecular cluster analysis using local order parameters selected by machine learning.

Accurately extracting local molecular structures is essential for understanding the mechanisms of phase and structural transitions. A promising method to characterize the local molecular structure is defining the value of the local order parameter (LOP) for each particle. This work develops the Molecular Assembly structure Learning package for Identifying Order parameters (MALIO), a machine learning package that can propose an optimal (set of) LOP(s) quickly and automatically for a huge number of LOP species and various methods of selecting neighboring particles for the calculation. We applied this package to distinguish between the nematic and smectic phases of uniaxial liquid crystal molecules, and selected candidate LOPs that could be used to precisely observe the nematic-smectic phase transition. The LOP candidates were used to observe the nucleation and subsequent percolation transition, and the effect of the choice of LOP species and neighboring particles on the statistics of local molecular structures (clusters) was examined. The procedure revealed the time evolution of the number of clusters and the dependence of the percolation curve on the number of neighboring particles for each LOP species. The LOP species with the lowest dependence on the number of neighboring particles was the best-performing LOP species in the MALIO screening strategy. These results not only show that machine learning can powerfully screen a huge number of LOP species and suggest only a few promising candidates, but also indicate that MALIO can select the best LOP species.

Polarization characteristics of polymer dispersed liquid crystal films and their effects on electro-optical properties

The electro-optical properties of polymer dispersed liquid crystal film vary with liquid crystal content and externally applied electric field, but the analysis of the film morphology cannot directly reflect the mechanism of electro-optical properties. Therefore, the polymer dispersed liquid crystal film prepared by blending liquid crystal material E7 and photopolymer NOA65 is used. Herein, the dielectric polarization regulated electro-optical properties and their related mechanisms under different liquid crystal content and electric fields are revealed. The results show that in a frequency range of 10<sup>–1</sup>–10<sup>6</sup> Hz, the film exhibits three relaxation processes respectively at low frequency, medium frequency and high frequency, which are generated by thermionic polarization, interfacial polarization and orientation polarization. According to the Arrhenius equation, the activation energy values of such polarization processes are calculated. It is found that with the increase of liquid crystal content, the activation energy of orientation polarization decreases from 0.88 eV to 0.83 eV, leading the threshold field strength and the saturation field strength of the diversion of liquid crystal molecule to decrease. Thermionic polarization under DC electric field forms an internal electric field, which causes the threshold field strength and saturation field strength to increase greatly, as compared with the scenarios under AC electric field. Such a thermionic polarization also leads the polarization relaxation time to increase, resulting in the extension of response time. This study is of guiding significance in further analyzing and improving the electro-optical properties of polymer dispersed liquid crystal films.

Analysis of morphological and electro-optical properties of silica nanoparticles induced vertically aligned liquid crystal - effect of doping and coating techniques

Conventionally, polyimide (PI) coating materials are used on the indium tin oxide (ITO) glass substrates to induce the vertical alignment (VA) of nematic liquid crystals (LCs). However, this coating technique requires a couple of steps such as spin coating, high-temperature baking and mechanical rubbing which limits their applicability to flexible LC display device applications. In the present work, VA of nematic LCs in a confined cell is induced using two different approaches: one via doping of silica nanoparticles (SNPs) as guest particles in LCs and the other with the deposition of SNPs layer on ITO glass substrates. Further, morphological and electro-optical (E-O) studies have been studied for the SNPs doped and SNPs coated vertically aligned liquid crystals (VALCs). The morphological study shows the identical degree of dark (black) and bright (white) textures of SNPs doped and coated VALC cells at OFF and ON states, respectively under the crossed polarizers. Also, the VA of nematic LC molecules in confined cells has been confirmed from the conoscopic behaviour. Consequently, morphological and E-O results revealed that the threshold (V th ) and operating (V o ) voltages required to switch the LCs from their initial VA (black state) towards minimum transition planar state and complete planar (bright) state, respectively, are reduced with better contrast ratio (CR) in SNPs coated VALC cell as compared with SNPs doped VALC cell.

Flat multifunctional liquid crystal elements through multi-dimensional information multiplexing

Flat optical elements have attracted enormous attentions and act as promising candidates for the next generation of optical components. As one of the most outstanding representatives, liquid crystal (LC) has been widely applied in flat panel display industries and inspires the wavefront modulation with the development of LC alignment techniques. However, most LC elements perform only one type of optical manipulation and are difficult to realize the multifunctionality and light integration. Here, flat multifunctional liquid crystal elements (FMLCEs), merely composed of anisotropic LC molecules with space-variant orientations, are presented for multichannel information manipulation by means of polarization, space and wavelength multiplexing. Specifically, benefiting from the unique light response with the change of the incident polarization, observation plane, and working wavelength, a series of FMLCEs are demonstrated to achieve distinct near- and far-field display functions. The proposed strategy takes full advantage of basic optical parameters as the decrypted keys to improve the information capacity and security, and we expect it to find potential applications in information encryption, optical anti-counterfeiting, virtual/augmented reality, etc.

Induction of Highly Ordered Liquid Crystalline Phase of an Azobenzene Side Chain Polymer by Contact with 4'-Pentyl-4-cyanobiphenyl: An In Situ Study.

The orientation of liquid crystal (LC) molecules is significantly governed by solid interfaces and free surfaces, and a variety of functional materials have been developed using these properties. Although LC materials are already in industrial use, particularly for LC display panels, various studies have been conducted in recent years to better grasp the interface behavior of LC molecules. In this work, we succeeded in in situ observations of induction of higher ordered LC phases at the interface between a side-chain LC azobenzene polymer film with a thickness of ∼400 nm and a low-molecular-mass nematic LC, 4'-pentyl-4-cyanobiphenyl of 35 μm thickness, using small-angle X-ray scattering measurements and polarized optical microscopy. It is revealed that the two different mesogens cooperatively form hybrid higher ordered smectic LC phases probably through weak electron transfer immediately after interfacial contact. The induction process consists of three stages in terms of dynamic structure evolutions. Upon UV irradiation, the hybrid smectic LC structure diminished. This study provides new insights into the behavior of LC molecules near the alignment film on the solid substrate.

Electro-optic response of bipolar nematic liquid crystal confined in oblate spheroid

Electro-optic response of liquid crystals (LCs) relies on the molecular reorientation of LCs under external electric field and is important for a wide spectrum of applications. Here, we uncover an interesting electro-optic response of 5CB nematic LC confined in an oblate spheroid and subjected to external electric field. Under the planar anchoring, the nematic LC spheroid adopts a bipolar structure with the bipolar axis laid in the horizontal film plane. When a threshold electric field EF, is applied, the bipolar structure reorients from the horizontal configuration (LC molecules align along long axis direction) to the vertical configuration (LC molecules align along short axis direction), involving the competition of elastic energy, surface anchoring energy and electric field energy. In contrast to bipolar nematic LC droplets, the vertical configuration does not relax to the low-energy horizontal configuration after removing E; we argue that is due to the oblate shape of the nematic LC spheroid, which traps the bipolar structure in a local energy minimum. We use continuum simulation to demonstrate the detailed response and the reorientation dynamics of bipolar nematic spheroids under E field, showing consistent results with the experiments and confirming the proposed switching mechanism. Nevertheless, the vertical configuration of the bipolar structure could relax to the low-energy horizontal configuration by thermal cycling. Our studies provide clear experimental results that show the characteristics of the electro-optic response of oblate LC spheroids, which have both fundamental and practical implications.

Cyano-4′-Pentylbipheny dopant strategy for P3HT-Based CsPbI3 perovskite solar cells with a record efficiency and preeminent stability

Poly(3-hexylthiophene) (P3HT) as hole transport layer (HTL) had been used in inorganic cesium lead triiodide (CsPbI3) perovskites solar cells (PSCs) due to its excellent hydrophobic property and cheap cost. However, its relatively low hole mobility and mismatched energy level with CsPbI3 perovskite led to large losses in fill factor (FF) and open-circuit voltage (Voc), and thus impair the photovoltaic performance of CsPbI3 PSCs. Herein, we successfully increase the hole mobility of P3HT by incorporating a nematic liquid crystal molecule 4-Cyano-4-Pentylbipheny (5CB) on account of the formation of face-on orientation and the improvement of π-π stacking of P3HT. Meanwhile, the defect density was reduced and the energy level arrangement was also optimized at CsPbI3/P3HT interface by 5CB dopant. As a result, the more balanced hole/electron mobility and suppressed non-radiative recombination are achieved, and thus Voc and FF were significantly increased. A record high-efficiency of 18.75% was obtained for P3HT-based CsPbI3 PSCs. Noteworthy, the unencapsulated CsPbI3 PSCs with P3HT:5CB HTL also shows superior moisture stability by maintaining 90% of initial PCE in ambient air with 15–30 % RH for 500 h. Our work reveals that 5CB dopant is a comprehensive strategy for improving efficiency and stability of P3HT-based PSCs.

High-resolution light field imaging based on liquid crytal microlens arrays with ZnO microstructure orientation

We proposed a liquid crystal (LC) microlens array (MLA) based on zinc oxide (ZnO) microstructure orientation and used it in a light field imaging system. The system uses convex optimization theory to jointly optimize the multi-voltage light field information and obtain a high-resolution imaging method. The core element in this system is an LCMLA, which induces the horizontal orientation of LC molecules by ZnO microstructure to effectively overcome the physical defects caused by orientation, such as mechanical friction. Combined with the proposed joint optimization high-resolution algorithm based on convex optimization theory, high-resolution imaging can be achieved using the redundant characteristics of light field images. The experiments show that the proposed method's PSNR of the reconstructed image is about 32 dB compared with the conventional method. The designed imaging system is flexible, simple, and suitable for high-resolution reconstruction.

An Evaluation of Surface-Active Agent Hexadecyltrimethylammonium Bromide with Vertical Self-Alignment Properties to Align Liquid Crystals for Various Cell Gap Conditions

We evaluated hexadecyltrimethylammonium bromide (HTAB) for liquid crystals (LCs) in layered ITO cells with various cell gap conditions. HTAB is a surfactant that can self-align vertically on the surface of indium tin oxide (ITO) substrates and induce homeotropic alignment of the LC molecules. For implementing RF devices with HTAB and LCs, we should consider limitations caused by the design conditions which are different from conventional liquid crystal displays such as cell gap. We quantified the concentration of HTAB ([HTAB]) that is necessary to form and maintain a sufficiently dense vertical alignment of 5CB (4-Cyano-4′-pentylbiphenyl). The required [HTAB] for full-homeotropic alignment was increased to the cell gap until it was too large to support the transfer of the surface alignment to the LC molecules, due to the weak anchoring nature of HTAB. We also showed the phase-change characteristic of the LC mixture related to [HTAB] for the design of RF devices driven by light or heat. This study may help to guide the development of new approaches to designing efficient RF devices that use LCs.

Optical vortex beam controlling based on fork grating stored in a dye-doped liquid crystal cell.

In this paper, we investigate the generation and controlling of the optical vortex beam using a dye-doped liquid crystal (DDLC) cell. The spatial distribution of the quasi-sinusoidal orientation of the liquid crystal molecules creates a quasi-sinusoidal phase grating (PG) in the DDLC cell. Depending on the incident light pattern, Trans to Cis photoisomerization of the dye molecules affects the orientation of the liquid crystal molecules. To do so, an amplitude fork grating (FG) is used as a mask, and its pattern is stored in the cell by a pattern printing method as the PG. One of the particular features of the stored grating in the cell is its capability in the diffraction efficiency controlled by the applied electric field. The results show, based on the central defect in the FG pattern, the diffracted probe beam in different orders is optical vortices. As a new technique, this type of stored pattern acts like an amplitude grating but according to the results, its structure is in fact a PG. This technique leads to the vortex beam switching capability by applying an electric field to the cell. The results show that by applying 22V, all the diffraction orders vanish. Meanwhile, the vortex beams reappear by removing the applied voltage. The diffraction efficiency of the vortex beams as well as its generation dependency on the polarization of the incident beam studied. The maximum efficiency of the first diffraction order for linear polarized incident beam was obtained at 0V, about 8%. Based on the presented theory, a simulation has been done which shows the Cis form of the dye molecules has been able to change the angle of LC molecules on average about 12.7°. The study of diffracted beam profiles proves that they are electrically controllable vortex beams.

Low threshold electro-optic switch and TE/TM polarization selector using liquid crystal channel waveguide

We demonstrate a Liquid Crystal (LC) core channel waveguide-based low threshold electro-optic switch by employing in-plane switching configuration and show its application for polarization selection device. The waveguide is fabricated on an indium tin oxide (ITO) coated glass substrate using the laser direct writing lithography technique. We have experimentally shown that the Freedericksz transition voltage of LC molecules is ∼ 1 V, and it relatively requires a low threshold voltage of ∼ 2 V to switch on the waveguide. The measured extinction ratio of the device is greater than 18 dB, along with a low insertion loss of ∼ 3.27 dB as compared to the conventional LC waveguides. The fabricated device is further demonstrated as a voltage-controlled polarization selector that passes either TM or TE-polarized light with a polarization extinction ratio of 28 dB and 25 dB, respectively. The proposed device exhibits the features such as low voltage control, low optical and insertion loss, high extinction ratio, and an externally controlled polarization selector, which are the key requirements in integrated photonics.

Zinc Oleate Nanorod-Induced Vertical Alignment of Nematic Liquid Crystal.

Nanocomposite zinc oxide nanorods capped with oleic acid (ZOR) with positive dielectric anisotropy liquid crystal (LC) 4'-octyl-4-biphenylcarbonitrile (8CB) filled in unaligned cells exhibit homeotropic alignment of host LC molecules. Further, systematic investigation of the textural, dielectric, and conductivity properties of nanocomposites filled in planar cells is performed with increasing concentration of nanorods. At a nanorod concentration ≤0.2 wt % in 8CB, the order parameter of nanocomposite samples is found to be increasing and ionic conductivity is found to be decreasing as compared to pure LC. Beyond 0.3 wt % concentration of nanorods in 8CB, vertical alignment (VA) of host LC is observed even in a planar aligned cell. The VA of LC molecules in ZOR nanocomposites is confirmed through attenuated total reflection-Fourier transform infrared absorption spectra studies.

Asymmetrically anchored liquid crystal cell for display and photonics applications

ABSTRACTAsymmetrically anchored liquid crystal (LC) cells have different anchoring energies between the top and bottom substrates. A substrate with patterned electrodes has comparatively weak anchoring energy. Therefore, asymmetrically anchored LC cells exhibit more twisted deformation in the on-state than conventional in-plane switching or fringe-field switching cells. In this study, we introduced asymmetrically anchored LC cells for display and photonic applications. We first introduced asymmetrically anchored LC cells to achieve high transmittance for LC displays. The LC molecules can rotate between the two substrates more easily because one of the two substrates has weaker anchoring energy. This resulted in a significant increase in transmittance. Three-terminal electrodes can be used to provide a vertical trigger pulse to an LC cell, thereby overcoming the delayed turn-off switching of an asymmetrically anchored cell without reducing transmittance. Next, we introduced an adjustable polarisation rotator with an asymmetrically anchored LC cell. To offer optical waveguiding, the arrangement of LC molecules can be controlled into a twisted configuration. During the cover of the wavelength range of 400–800 μm, we confirmed that the proposed LC cell can rotate the polarisation axis of linearly polarised light by up to 90°.

Recent Progress in Liquid Crystal‐Based Smart Windows: Materials, Structures, and Design

AbstractAs the vital elements for building environments, windows are utilized to bring light as well as thermal irradiation into the interior places and afford the medium for the communication between the interior and exterior of buildings. Liquid crystal (LC)‐based smart windows, which are mainly made of LC materials or polymer/LC composites, can dynamically modulate light transmittance and regulate the heat radiation entering buildings from the sun via the orientation variation of the LC molecules. Such smart windows play an important role in intelligent buildings and building energy conservation, which contributes to providing a more comfortable and low‐energy‐consumption living environment for mankind. In this review, recent progress in LC‐based smart windows of each category is overviewed. New materials, new ideas, and new methods to improve the performance of conventional LC‐based smart windows and create novel smart window technologies are outlined. Different advanced LC‐based smart window technologies, including electrochromism, thermochromism, photochromism, humidity‐triggered chromism, mechanical self‐powered technique, and dynamic infrared regulating mechanisms, are discussed systematically. The applications of LC‐based smart windows in indoor illumination/privacy protection switching, human‐machine interaction, and thermal regulation for intelligent buildings are highlighted. Last, the challenges and perspectives of LC‐based smart windows are also provided.

End Group Effect of Asymmetric Benzodithiophene-Based Donor with Liquid-Crystal State for Small-Molecule Binary Solar Cell.

Liquid-crystal small molecule donor (LC-SMD) is a new type organic semiconductor, which is attractive not only for the easy synthesis and purification, well-defined chemical structures, etc., but also for the LC state that makes the crystallinity and aggregation state of molecules adjustable. Here, one new LC-SMD (a-BTR-H4) is synthesized with 1D alkoxyl and 2D thiophene-alkylthiol side-chained benzo[1,2-b:4,5-b']dithiophene core, trithiophene π-bridge, and 3-(2-ethylhexyl) rhodanine end group. a-BTR-H4 shows low LC transition temperature, 117 °C, however, counterpart material (a-BTR-H5) with the same main structure but 3-ethyl rhodanine terminal group does not show LC properties. Although a-BTR-H4/H5 show similar Ultraviolet-visible absorption spectrum and energy levels, a-BTR-H4 affords relatively high photovoltaic performances due to favorable blend morphology produced by the consistent annealing temperature of Y6-based accepters and liquid crystal temperature of donors. Preliminary results indicate that a-BTR-H4 gains a power conversion efficiency (PCE) of 11.36% for Y6-based devices, which is ascribed to better light harvest as well as balanced carrier generation and transport, while a-BTR-H5 obtains 7.57% PCE. Therefore, some materials with unique nematic LC phase have great application potential in organic electronics, and further work to utilize a-BTR-H4 for high-performance device is underway.

Wide field of view chiral imaging with a liquid crystal planar lens enabled by digitalized nanogratings.

To compensate for the inability for polarization imaging by conventional methods, metasurface optics with compactness and multi-function emerge as an approach to provide images with different linear and circular polarizations. Here, we propose a liquid crystal (LC) geometric phase-based chiral imaging lens (CIL) that simultaneously forms images of objects with opposite helicity. The CIL (Diameter 2.3 cm) was optimized by a spatial multiplexing algorithm and realized using the digital holography technique, where the LC domains were regulated by pixelated nanogratings with varied orientation. We investigated the potential of the patterning technique toward high order LC alignment by balancing the periodicity and depth of the nanogratings. The CIL exhibited a wide field of view of ±20°, which is attributed to the self- assembling effects of LC molecules. The compactness, lightness, and ability to produce chiral images of the LC CIL even at large angles have significant potential for practical polarization imaging.

End-of-Life Liquid Crystal Displays Recycling: Physico-Chemical Properties of Recovered Liquid Crystals

This report focuses particularly on liquid crystals display (LCD) panels because they represent a significant amount of all WEEE collected. Technologies involving liquid crystals (LCs) have enjoyed considerable success since the 1970s in all fields of LC displays (LCDs). This currently provokes the problem of waste generated by such equipment. Based on current statistical data, the LC amount represents approximately 1.3 g for a 35-inch diameter LCD panel unit possessing a total weight of 15 kg. In France, a recent study revealed LCD waste to represent an average of 5.6 panels per household. This represents an important quantity of LCs, which are generally destroyed by incineration or washed out with detergents during the recycling processes of end-of-life (EOL) LCDs. Hence, the aim of this study is to show that it is possible to remove LC molecules from EOL-LCD panels with the goal of valorizing them in new sectors. EOL-LCD panels have undergone various stages of dismantling, chemical treatments and characterization. The first stage of manual dismantling enables the elimination of the remaining physical components of the panels to process LC molecules only, sandwiched between the two glass plates. Mechanical treatment by scraping allows us to obtain a concentrate of LCs. The results obtained from chemical and physical techniques show that these molecules retain the characteristics essential for their operation in the field of optical and electro-optical devices. As the use of LCD surfaces continues to rise significantly, the amounts and economic stakes are huge, fully justifying the development of an LC recovery process for used panels. Many potential uses have been identified for these LC molecules: in new flat LCD panels after purification of the LCs concentrate, in PDLC systems, as lubricants or in thermal applications.

A label-free liquid crystal-assisted aptasensor for trace level detection of tobramycin in milk and chicken egg samples

Developed herein is an aptasensing array based on liquid crystal (LC) for monitoring of tobramycin (TOB) antibiotic. The direction of LC molecules from vertical to a random status was induced by the conformational changes of the specific aptamer due to its selective interaction with the target. The dark view of the aptasensing platform changed to colorful through observation by a polarized light microscope that clarifies the TOB presence. The aptasensor is especially able to determine TOB in the linear concentration range of 0.005–600 pM with a limit of detection (LOD) as 0.0021 pM. The TOB values can be determined successfully in the milk and chicken egg samples that highlights the potential applicability of the designed aptasensor. The proposed sensing approach is facile, operator-independent, label-free, and ultra-sensitive, making it novel for developing real-time portable sensing devices for future.