Nematic Liquid Crystal Molecules Research Articles

Mechano-chemical synthesis of liquid crystal materials: an easy method towards fast, simple and almost solvent-free molecular screening

ABSTRACT This paper provides guidance for the rapid, simple, and eco-friendly synthesis of liquid crystal (LC) molecules using the hot-trend mechanochemical (MC) synthesis in the community of organic synthesis. We introduce our protocol, which may guide beginners of the MC technique to start easy and quick synthesis of LC molecules. The MC synthesis asks no need for a complex system and a large amount of solvent like the conventional solution-based synthesis. Additionally, this paper also provides some practical examples for fast screening of LC molecules including ferroelectric nematic LC (NFLC) molecules, which are subject to highly hot topics in the LC field, through multi-step MC synthesis.

Suppressing the self-discharge of MXene-based supercapacitors by liquid crystal additive

MXenes are widely used as electrodes for supercapacitors (SCs) due to their excellent conductivity and abundant redox active sites. However, the severe self-discharge of MXene-based SCs results in significant energy loss, limiting their integration with various energy-harvesting devices. Here we introduced the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) molecules into Ti3C2Tx films and LiCl aqueous electrolytes to suppress the self-discharge rate. The nematic liquid crystal molecules would arrange parallel to each other when the Ti3C2Tx films were charged, while the electrolyte viscosity would increase due to the electrorheological effect, causing the slow cross-diffusion of active ions and reducing the self-discharge. As a result, the MXene-based SCs had a low voltage drop of 35% after 4600 s and a low leakage current of 3.0 µA, respectively. The MXene-based SCs could store energy more effectively, improving their practical application potential in integrated energy harvesting devices, for example, storing the electrical energy harvested by the triboelectric nanogenerators. This work proposed a simple strategy to suppress the self-discharge of Ti3C2Tx-based SCs by liquid crystal additive, which can be extended to other MXenes, promoting the applications of MXenes in energy storage devices and integrated electronics.

Electrically controllable broadband reflective linear cross-polarization conversion based on liquid crystals.

This paper presents an electrically controllable reflective broadband linear polarization (LP) converter based on liquid crystals (LCs) for cross-polarization conversion (CPC) in the terahertz frequency range. The proposed structure achieves a high polarization conversion ratio (PCR) exceeding 0.9 within the frequency range of 236.8 - 269.6 GHz. A vital feature of this design is the dynamic control of polarization conversion by re-orienting the nematic liquid crystal molecules through voltage bias switching between 'on' and 'off' states, allowing for precise manipulation of cross-polarized and co-polarized reflected waves. Experimental results validate the simulation outcomes, demonstrating excellent agreement. In contrast to conventional reflective polarization converters with fixed frequency responses, the proposed electrically controllable polarization conversion offers significant potential in imaging and optical communications.

A Compact Optical Fiber Sagnac Loop Inserting With A Nematic Liquid Crystal Film for Highly Sensitive Temperature Sensing

Since the high temperature response characteristics of nematic liquid crystal (NLC) molecules, a compact optical fiber Sagnac loop inserted a segment of NLC film for temperature sensing was proposed and experimentally demonstrated. The NLC film was formed by filling into the micron-scale gap between the two single-mode fibers inserted into a ferrule matching sleeve. After two single-mode patch cables were rotated, the NLC film molecules were oriented due to exogenous effects. Experimental results showed that the temperature sensing performance of the designed sensor was improved by decreasing the thickness of NLC film in Sagnac loop. The thickness of NLC film in the ferrule matching sleeve was measured accurately by Fabry-Perot interference. The wide temperature measurement range of 2.2 ∼ 50.2°C and a high temperature sensitivity of -9.34 nm/°C were achieved at the NLC film thickness of 31.30 μm. Owing to the compact structure, large measurement range, and high sensitivity, the designed sensor shows a broad application prospect in high spatial resolution and high sensitivity temperature sensing.

Development of thin film quarter-waveplate for polarization-independent liquid crystal on silicon phase modulators

Liquid crystal on silicon (LCoS) phase modulators spatially modulate the phase of light across the panel. The orientation order and elongated molecules of nematic liquid crystals (LCs) means that traditional LCoS phase modulators are inherently polarization-dependent, resulting in either complicated polarization manipulation systems or high optical power loss for unpolarized incident light. We propose a polarization-independent LCoS (PI-LCoS) device that combines existing technologies, which allows for cost-effective fabrication. Our PI-LCoS phase modulator is suitable for a wide variety of applications in telecommunication, adaptive optics, and display technologies. We have demonstrated the feasibility of the proposed PI-LCoS device by fabricating polarization-independent LC cells using a thin-film quarter-wave plate composed of a photoalignment layer and a layer of LC polymer. We show experimentally that the proposed design can efficiently modulate the phase of light with arbitrary input polarization.

Eco-friendly carbon dots induced thermally stable vertical alignment in planar anchored nematic liquid crystal

We report the induction of thermally stable vertical alignment of nematic liquid crystal, 4-cyano-4′-pentylbiphenyl (5CB), in a planar anchored cell by doping with eco-friendly carbon dots (CDs, diameter ∼7–8 nm), investigated through thermal, optical, electro-optical and dielectric spectroscopic techniques. Cross-polarized optical textures and voltage-dependent optical transmission measurements demonstrate the induction of vertical alignment of CDs-5CB composites with concentration of CDs (≥0.3 wt%) in planar anchored sample cells at room temperature. Moreover, the frequency-bias-concentration dependent dielectric studies done also confirms the induced vertical alignment. For instance, the relative dielectric permittivity measured on CDs-5CB composites (CDs ≥ 0.3 wt%) in planar anchored sample cells at bias voltage of 0 V and 30 V DC are almost equal and the presence of a short axis molecular relaxation at 0 V confirms the induced vertical alignment. The concentration and bias dependence of short axis molecular relaxation frequency are also discussed. It appears that CDs are able to align 5CB molecules vertically at the substrate by overcoming the pre-existing planar anchoring. Such induced alignment would propagate through the bulk of sample cell because of the coherence length of nematic liquid crystal molecules and eventually the whole sample obtains vertical alignment. We anticipate the great utility of these composites in the production of eco-friendly liquid crystal displays and other photonic devices, sensors, etc.

Autonomous Self‐Sustained Liquid Crystal Actuators Enabling Active Photonic Applications

AbstractAdvances in biomimicry have led to the rise of advanced robotics, posing promising revolutions across a variety of fields. Programmable self‐sustained actuation in nature, such as human's heart beating, bird's wingbeats, and penguin's waddling, are intriguing and inspiring but challenging for device innovation, which hinders the emergence of autonomous self‐feedback applications, especially in optics and photonics. Herein, the design, fabrication, and operation of crosslinked liquid crystal actuators are described that combine the programming of microstructures and the engineering of macroscopic shape morphing for active optics and photonics. The actuators consist of twisted nematic liquid crystal molecules with both elastic and optical anisotropies, resulting in large bending deformations in response to heat. Programmable bending motions and self‐sustained waddling oscillations are demonstrated, further contributing to the achievements of dynamic 2D beam steering and self‐sustained light field modulation. It is envisioned that these actuators with self‐sustained performances without requiring turning the stimulus on‐off will find applications in autonomous active optical systems, photonic applications, as well as self‐governing robotics with the core feature of thermo‐mechanical‐optical transduction.

Ion-Conducting Composites of Polymers and Nematic Liquid Crystals.

In the present mini-review are discussed the findings reported in the last five years on the ion-conducting composites of polymers and molecules of nematic liquid crystals (NLCs), as well as their applications at present and in the future. Nowadays, free-standing and flexible thin films of such organic composite electrolytes synthesized from plastics and nematic soft matter are among the technically important materials and components for use in energy storage and conversion devices and in organic soft electronics, sensorics, and mechatronics. Although the physicochemical mechanisms and effects in the ion-conducting polymer/NLCs composites are well understood, the possibility to find additional ways for improving their electrical conductivity and dielectric and mechanical properties is a challenge. The efforts in this research direction are important for the development of novel ion-conductor materials and further diversification of their applications. This mini-review is focused on the key characteristics of ion-conducting polymer/NLCs composites and the new trends in their fabrication. With relevant examples, the vast research opportunities, some proposed improvements, and the creative ideas associated with these advanced materials and their intelligent use are outlined.

Enhancement in Photoluminescence of Nematic Liquid Crystals Doped with CsPbBr3 Quantum Dots

We report the enhancement in photoluminescence (PL) of nematic liquid crystals (NLCs) doped with CsPbBr3 (perovskite) quantum dots (QDs). QDs play a vital role to enhance the PL intensity, which has been attributed to the increase in the planar alignment of NLC molecules by reducing light leakage centres and intrinsic defects. QDs were synthesized by the hot injection chemical method. The structural properties of QDs were examined by X-ray Diffraction (XRD) and Transmission Electron Microscopic (TEM) measurements. Orthorhombic crystallite structure of CsPbBr3 QDs without secondary phase formation was confirmed from XRD pattern. The particle size of QDs was calculated by TEM and found to be 7.3 (± 1.1) nm. The optical properties of QDs were studied by recording UV-Visible absorption and PL spectra at room temperature. The absorption spectrum reveals the excitonic peak at 509 nm and PL emission is observed at 514 nm. The chemically prepared QDs were further used to enhance the emission of NLC. Firstly, liquid crystal sample cells were fabricated by conventional polyimide technique. NLC doped with CsPbBr3 QDs were inserted into the sample cell via the capillary action. The filled sample cells were characterized by polarising optical microscope (POM) images and PL spectra measurements. The dark and bright state images of POM demonstrate the uniform alignment and homogeneous texture of liquid crystal molecules in both cells. The molecular alignment is further improved while the light leakage centres and defects were reduced after doping QDs in the host NLC material. This leads to a significant enhancement in absorption and hence the emission of NLC. We found the enhancement in PL intensity by 11% compared to pure NLC. The improved emission of NLC is attributed to modified molecular alignment in presence of QDs. Results based on present work will help to fabricate QDs based display devices with improved the optical contrast and lower the operating voltage

CdSe quantum dots enhancing blue emission of nematic liquid crystals

Herein, we report the enhancement in blue emission of nematic liquid crystals (NLCs) doped with CdSe quantum dots (QDs) at room temperature. CdSe QDs were synthesized by a high temperature wet chemical method. X-ray diffraction pattern suggest zinc blend crystal structure of CdSe QDs without impurity phase formation. The absorption peak and PL emission of QDs is observed at 503 nm and 526 nm respectively. The QDs size is found to be 2.3 nm calculated by excitonic peak. The polarization states of the QD-based NLCs were studied by using polarising optical microscopic (POM) images under crossed geometry of polarizer and analyser. The uniform colour distribution throughout the cell indicates uniform cell thickness with a planar alignment of liquid crystal molecules. Moreover, dark, and bright states of POM images are used to analyse the defects in alignment of NLC molecules. It is worthwhile to note here that the incorporation of QDs in NLCs helps in reducing defects and light leakage centres which further increases the absorption and hence emission of NLCs. The significant enhancement in the PL intensity of the NLC of about 60% upon CdSe QDs doping is observed at room temperature which attributed to the increase in molecular alignment of NLCs composite.

Chemically Functionalized Gold Nanosphere-Blended Nematic Liquid Crystals for Photonic Applications.

A demand for functional materials that are capable of tailoring light-emissive properties has apparently been rising nowadays substantially for their utilization in organic optoelectronic devices. Motivated by such promising characteristics, we present highly emissive as well as aggregation-induced emission (AIE) electroluminescent composite systems composed of a nematic liquid crystals (NLC) blended with polyethylene-functionalized gold nanospheres (GNSs). The major findings of this study include superior electro-optical properties such as threshold voltage reduction by around 24%. The fall time is reduced by 11.50, 30.33, 49.33, and 63.17% respectively, and rotational viscosity is reduced by 13.86, 32.77, 36.97, and 49.58% for 5.0 × 1011, 5.0 × 1012, 2.5 × 1013, and 5.0 × 1013 number of GNS-blended liquid crystal (LC) cells. The increased UV absorbance and greatly enhanced luminescence properties have been attributed to surface plasmon resonance near the surface of GNSs and AIE effect risen due to agglomeration of the capping agent with the NLC molecules respectively, and these characteristics make them suitable for new-age display applications.

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.

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.

Tunable dielectric and memory features of ferroelectric layered perovskite Bi4Ti3O12 nanoparticles doped nematic liquid crystal composite

Herein, we report the synthesis of ferroelectric layered perovskite Bi4Ti3O12 (BT4) nanoparticles (NPs) and the temperature-dependent dielectric and electro-optical (especially memory effect) properties of 4-pentyl-4′-cyanobiphenyl (5CB) nematic liquid crystal (NLC) doped with 1 wt% BT4 NPs (i.e. 5CB-BT4 composite) using polarising optical microscopy and frequency-dependent dielectric spectroscopy techniques. BT4 NPs were synthesised via a microwave-assisted chemical method and characterised using various instrumental techniques, which confirmed the formation of a non-stoichiometric and oxygen-deficient orthorhombic crystal phase. The agglomeration-free and uniform dispersion of BT4 NPs in the 5CB matrix was confirmed by optical textures. The optical memory studied by bias voltage-dependent (ON-OFF) optical textures is decreased by ∼ 2.6 times in the 5CB-BT4 composite compared with 5CB. Moreover, dielectric parameters such as dielectric permittivity, dielectric loss, loss tangent, conductivity, and activation energy of 5CB and composite (5CB-BT4) are estimated using dielectric spectroscopy. The dielectric anisotropy is decreased, whereas no shift in the clearing temperature is observed in the 5CB-BT4 composite compared to the 5CB sample. Also, the DC conductivity of 5CB-BT4 composite is found to be increased by approximately four times compared to the 5CB. Our studies clearly demonstrate the tunability of the dielectric and optical memory features of NLC (5CB) matrix by dopant BT4 NPs, without significantly affecting the molecular alignment of the NLC molecules. Such composites would certainly be useful in the fabrication of NLC based tunable devices such as optical memory and conductivity switches.

Contribution of Multiwalled carbon nanotubes to dielectric and elastic properties in E7 nematic liquid crystal

In this article, electrical parameters, and elastic constants of nematic liquid crystal (NLC) contributed multiwalled carbon nanotubes (MCN) are presented by dielectric spectroscopy method. The effects of reorientation on the nematic liquid crystal by doping of multi-walled carbon nanotubes were investigated to reveal. Analyzes of dielectric strength Δε′(ω), absorption coefficient α, relaxation time τ, dielectric anisotropy Δε′V, dielectric anisotropy ratio γ, splay elastic constant K11, and cross-over angular frequency ωco of MCN contribution to E7 nematic liquid crystal with DC supply voltage are presented. Reorientation of elastic dipolar liquid crystal molecules was demonstrated by reducing the positive type (p type) dielectric anisotropy value in the broadband angular frequency range with the MCN contribution to the E7 coded nematic liquid crystal. The supply voltage applied to the NLC samples was observed to have nearly Debye-type relaxation mechanism in the host E7 LC structure, tending to switch to a Debye-type relaxation mechanism by MCN doping. Dipole effects were enhanced by doping of multi-walled carbon nanotubes in the orientation of elastic NLC molecules, and the relaxation time of host E7 NLC and doped MCN/E7 NLC decreased with increasing bias voltages. Cole-Cole analysis with different supply voltages, it has been seen that it turns into a new equivalent circuit depending on the MCN contribution.

Electrically Induced Hydrodynamic Effect in Nematics Caused by Volume Reduction

A pressure gradient caused by the local field-induced reduction of the effective molecular volume results in a flow of the nematic liquid crystal (NLC). Here, the hydrodynamics of homeotropically aligned NLC molecules under the influence of this pressure gradient was studied theoretically. The equations describing the system were written and solved in the steady-state case using analytical methods, and the stationary velocity of the observed flow was found. We discussed the obtained results and compared them with existing experimental results.

THE INFLUENCE OF SIZE EFFECTS ON THE ORDERING OF LIQUID CRYSTALS LOCATED ON NANORIBBON GRAPHENE

Experiments on computer simulation of the behavior of polar nematic phenylpropargyl ethers of parachlorophyllens on the surface of graphene nanoribbon made it possible to reveal a number of regularities. The dynamics of moleculesis studied when the dimensions of graphene change under the action of an electric field and temperature. As a research method, the method of molecular dynamics was used in the approximation of a liquid state of aggregation. The simulation was carried out in the atomistic approach. It was shown that the graphene nanoribbon and electric filed defines the self-assembly of the nematic liquid crystals molecules in the biaxial state. These states appear at the variation of the graphene nanoribbon ratio of width to length. It was found that in the clearing area the ordering of the parachlorophyllens starts to grow when the value of the ratio of width to length is 3: 1.

Slave-master mechanism of thermotropic liquid crystal phase transitional behavior

We introduce the slave-master mechanism in the phase behavior of two-component systems where both components could exhibit some kind of (quasi) long-range order. The key feature of the mechanism is that one component strongly influences the other while the reverse influence is negligible. To illustrate the effect, we consider systems where each isolated component exhibits an order-disorder phase transition on varying a control parameter (e.g., temperature) to which we refer as the generalized temperature. We present a minimal model that shows robust slave-master behavior. As exemplary systems, we present thermotropic liquid crystals (LCs) exhibiting nematic and smectic A phase order, focusing on the interaction strength between orientational and translational order. Mixtures of nematic LC molecules and anisotropic nanoparticles are also presented. We show that qualitatively different behaviors could emerge via the specific nature of the inter-component interaction. Mastering this mechanism could lead to various applications based on phase tunable properties.

Thickness Dependent Electro-Optical Behaviour of Homeotropically Aligned Nematic Liquid Crystal

The homeotropically aligned liquid crystal (LC) display devices are of great interest in recent time. Thus, in the present work, the thickness dependent electro-optical behaviour of nematic liquid crystal (NLC) in homeotropic alignment (HA) has been studied. To achieve the HA of NLC, polyimide (PI) coated ITO glass substrates were used. Consequently, cleaned ITO glass substrates (with thickness of 0.7 mm) were used for the fabrication of the LC cells. The pure NLC material (LC-BYVA) was filled into PI coated cells by the capillary injection method. Subsequently, the filled cells were examined with the help of polarized optical microscope (POM), which show the good dark and bright orthoscopic textures of NLC in field ON and OFF states, respectively under cross-polarizers. The conoscopic study of the images also confirmed the perfect HA of NLC molecules. The dark states change to good bright state with the change in orientation of NLC director from homeotropic to planar at operating voltages as seen under cross-polarizers. The electro-optical (E-O) study also shows the good contrast ratio (CR) of display cells.

Impact of strontium titanate nanoparticles on the dielectric, electro-optical and electrical response of a nematic liquid crystal

The impact of ferroelectric nanoparticles (FNPs), namely strontium titanate (SrTiO3) on the dielectric, electro-optical and electrical response of a room temperature nematic liquid crystal (NLC), 5CB (4-pentyl-4′-cyanobiphenyl) is investigated through dielectric, electro-optical and electrical techniques. The dielectric studies clearly show that low frequency dielectric relaxation shifts towards higher frequency as concentration of FNPs increases but decreases to the value even lower than pure sample at 2 wt% concentration. Afterwards, the short axis molecular relaxation observed in homeotropic cell obtained from homogeneous aligned cell by applying bias voltage higher than threshold shifts towards higher frequency by ∼ 28 % in composites but remains independent of concentration. Interestingly, after the removal of bias voltage, dielectric spectra does not return to its original state even after ∼ 22 min in the composites unlike pure sample and could be considered as apparent dielectric memory (i.e. memory effect). However, the voltage dependent optical textures confirm the short term memory in the composites. The electrical response recorded as a function of frequency in the composite is also significantly different than pure sample which further confirms the impact of FNPs on NLC. We believe that the observed effects of FNPs on dielectric, electro-optical and electrical behavior could be due to the plausible favorable interaction among FNPs and NLC molecules. Such investigations would certainly be pivotal in tuning the dielectric and electro-optical properties of NLCs using FNPs and helpful in designing the next-generation tunable electro-optical and soft memory based devices.