Alignment Layer Research Articles

Surface confinement of vertically aligned liquid crystals between flexible silver nanoparticles hybridized polyimide thin layers

Alignment of liquid crystals (LCs) has emerged as an essentially fundamental approach for exploring the electro-optical performance and fabricating novel LC devices. Growing evidence suggests that the surface wettability of the alignment layer plays a key role in tuning the tilt angles of LCs. In this work, silver (Ag) nanoparticles (d ≤ 100 nm) are blended in polyimide (PI) by a wide range of concentration from 0.1% to 1.0% to tune surface wettability and align LCs. Ag nanoparticles agglomerate and result in a bumpy surface topology and somewhat reduce the transparency of hybrid thin layer to 85.1% by 1.0% doping because of their shielding effect. Nevertheless, Ag nanoparticle agglomerates change the thickness of hybrid thin layers less, and the hybrid thin layers are ∼96 nm thick like a pure PI thin layer. LCs are vertically aligned on hybrid thin layers after rubbing. Benefit from the declined anchoring energy and the outstanding surface plasmon polariton (SPP) of Ag nanoparticles, the external electric field to switch LCs is substantial boosted, and hence remarkably diminish the operating voltage and likewise notably shorten the response time. In particular, LCs that vertically aligned between PI hybrid alignment layers by 0.5% doping could be electrically switched to become planner at 3.38 V and go back to their original vertical alignment state within 73.21 ms. Ag nanoparticles agglomerates in hybrid thin layers also fairly hinder the drift of LCs when cells are mechanically bent with various radii. The hybrid alignment layers cell that blended with 0.5% Ag nanoparticles could be bent under 1500 mm without any new evident light leakages and fluctuating electro-optical characteristics. These results indicate the promising applications of PI hybrid thin layers blended with Ag nanoparticles for flexible LC devices.

Cage polyamine molecule modulating the buried interface of tin oxide/perovskite in photovoltaic devices

Molecular modulating have recently emerged as an effectual strategy for ameliorating the performance and stability of perovskite solar cells (PSCs). However, to date cage polyamine with nitrogen heterocyclic porous structure is rarely used as the modulator. Herein, a cage polyamine molecule 1,4-diazabicyclo[2.2.2]octane (DABCO) as modulator is intercalated into the buried interface between electron transport layer and perovskite layer. The situation of the perovskite bottom layer is ascertained by peeling off the buried interfaces. The intercalation of DABCO not only passivates the defect, resulting in a 54 % decrease in trap-state density; but also assists the crystal growth of perovskite, relieves lattice tensile stress; as well as optimizes the energy level alignment of the functional layers. As a result, the DABCO optimized PSC achieves a power conversion efficiency of 23.6 % along with excellent stability, compared to 20.5% for the pristine PSC. The research provides a typical molecular modulating to utilize cage polyamine in PSCs.

Nanoantenna induced liquid crystal alignment for high performance tunable metasurface.

Liquid crystal (LC) based spatial light modulators (SLMs) are a type of versatile device capable of arbitrarily reconfiguring the wavefront of light. For current commercial LC-SLM devices, the large pixel size limits their application to diffractive optics and 3D holographic displays. Pixel miniaturization of these devices is challenging due to emerging inter-pixel crosstalk, ultimately linked to the thick LC layer necessary for full phase (or amplitude) control. Integration of metasurfaces, i.e., 2D arrangements of resonant nanoantennas, with thin LC has emerged as a promising platform to boost light modulation, enabling realization of sub-wavelength pixel size SLMs with full phase (or amplitude) control. In most devices realized so far, however, the presence of an alignment layer, necessary to induce a preferential initial LC orientation, increases the voltage requirement for resonance tuning and reduces the efficiency of light modulation, something that accentuates for an ultra-thin (e.g., submicron) metasurface-LC cell. Here, we present an alternative strategy by which the LC molecular alignment is purely controlled by the periodicity and geometry of the nanoantenna without any additional alignment layer. The nanoantennas are specifically designed for the double purpose of sustaining optical resonances that are used for light modulation and to, simultaneously, induce the required LC pre-alignment. The proposed device structure allows lower voltage and reduced switching times (sub-millisecond) compared to devices including the alignment layer. This novel strategy thus helps to improve the performance of these miniaturized-pixel devices, which have emerged as one of the potential candidates for the next generation of products in a wide range of applications, from virtual/augmented reality (VR/AR) and solid-state light detection and ranging (LiDAR), to 3D holographic displays and beyond.

Energy saving dual-mode recycling of wastewater by MWCNTs-COOK/PDA/PVA assisted composite membrane

Membrane separation technology has been widely used for wastewater purification. However, the commonly used membrane filtration (including dead-end filtration and cross-flow filtration) and distillation are all supported by electric power. It causes great burden to environment, as the electric is still mainly come from thermal power generated from burning non-renewable resource. In this paper, we report a dual-mode functional composite membrane assisted by MWCNTs-COOK and PDA/PVA, where MWCNTs-COOK alignment layer acts as a framework to constructing water nanochannels and PDA/PVA co-deposition layer plays a role as a concrete which improved the integrity of the membrane. It achieved optimal pure water flux of 23.86 L m−2h−1 bar−1 with only 0.1 mg cm−2 of MWCNTs-COOK during cross-flow filtration. Under a light intensity of 100 mW cm2, the composite membrane obtained an average evaporation rate of 1.57 Kg m−2h−1 after 1 h due to the combination of two photothermal materials (MWCNTs-COOK and PDA). More importantly, even after working for 12 h, the membrane still kept an average evaporation rate of 1.52 Kg m−2h−1. And the rejection of organic foulant is up to 99.97 % during long time serving. This indicates that the fabricated composite membrane is prospective for generating clean water from waste solutions which contains foulant. Besides, the integration of electric-driven and solar-driven wastewater purification facilitates multi-application of separation membrane, contributes to electric power crisis, energy saving and multiple service scenarios.

Understanding the temperature sensitivity of the photovoltaic parameters of perovskite solar cells

Perovskite solar cells (PSCs) have promised high-efficiency and low-cost solar-to-electrical conversion that now go outdoors for practical applications; however, the elevated outdoor temperature remarkably affects the photovoltaic efficiency. To date, there has been little work about understanding the temperature sensitivity of PSCs. Here, we build an analytical model to understand the temperature sensitivity and the limiting factors of temperature coefficient (TC) in PSCs. Our model is based on diffusion-drift numerical method by involving the temperature-dependent bandgap and band edge of perovskite layers, which quantitively describe the effect of many physical factors, including carrier recombination channels, trap density, band alignment and band variation of transport layers. It was found that the built-in electrical field and minority carrier concentration at interface are closely corrected with temperature, that is responsible for the temperature sensitivity of device performance. We show that this model can be used to explain the disparate temperature sensitivity of a series of reported real PSCs, and also gives potential pathways for the design and fabrication of temperature-insensitive photovoltaics towards practical applications.

Rigid-flex stamped indium tin oxide film for convertible self-alignment

The alignment layer process, which is a key element for liquid crystal (LC) displays, was investigated for efficiency improvement and for suggesting a new mode. To produce alignment films that can be used in both twisted-nematic (TN) and vertical alignment (VA) configurations, InO solution is doped with SnO. Then, the pattern was transferred by attaching a one-dimensional soft-imprint mold, and pattern and orientation analyses were performed in accordance with the SnO ratio. The horizontal orientation performance improved as the SnO ratio increased, and the best orientation performance was observed for a film doped with 70% SnO. Further, the thermal stability improved and anchoring energy increased, which was measured to be very close to that of the rubbed polyimide film. The low transmittance of InO was overcome by doping with SnO, which has high transmittance. In the film doped with 80% SnO, the LC molecules were aligned in a vertical direction and were suitable for VA mode operation. By simple adjustment of the doping ratio, differences are observed with respect to the existing films in that the performance of the alignment layer is improved and the pretilt angles of the LC molecules can be changed. These convertible characteristics are expected to play a revolutionary role in the development of new LC driving modes.

Novel process integration flow of germanium-on-silicon FinFETs for low-power technologies

Germanium channel FinFET transistors process integration on a silicon substrate is a promising candidate to extend the complementary metal–oxide–semiconductor semiconductor roadmap. This process has utilized the legacy of state-of-art silicon fabrication process technology and can be an immediate solution to integrate beyond Si channel materials over standard Si wafers. The fabrication of such devices involves several complicated technological steps, such as strain-free epi layers over the Si substrate to limit the substrate leakage and patterning of narrow and sharp fins over germanium (Ge). To overcome these issues, the active p-type germanium layers were grown over n-type germanium and virtual substrates. The poly ((4-(methacryloyloxy) phenyl) dimethyl sulfoniumtriflate) was utilized as a polymeric negative tone e-beam resist for sub-20 nm critical dimensions with low line edge roughness, line width roughness, and high etch resistance to pattern p-Ge fins to meet these concerns. Here, the devices use the mesa architecture that will allow low bandgap materials only at the active regions and raised fins to reduce the active area interaction with the substrate to suppress leakage currents. This paper discusses the simple five-layer process flow to fabricate FinFET devices with critical optimizations like resist prerequisite optimization conditions before exposure, alignment of various layers by electron beam alignment, pattern transfer optimizations using reactive ion etching, and bilayer resist for desired lift-off. The Ge-on-Si FinFET devices are fabricated with a width and gate length of 15/90 nm, respectively. The devices exhibit the improved ION/IOFF in order of ∼105, transconductance Gm ∼86 μS/μm, and subthreshold slope close to ∼90 mV/dec.

Optical Filters with Asymmetric Transmittance Depending on the Incident Angle, Produced Using Liquid Crystalline Ink (Louver LC Filters).

In many situations in everyday life, sunlight levels need to be reduced. Optical filters with asymmetric transmittance dependent on the incident angle would be useful for sunglasses and vehicle or architectural windows, among others. Herein, we realized the production of optical filters, called "louver filters", comprising HAN-type LC film produced using liquid crystalline ink with dichroic dyes. For the formation of the HAN-type LC film, the liquid crystalline ink was aligned on a rubbed polyimide layer and polymerized by UV irradiation. Two kinds of filters are proposed: one is a filter composed of HAN-type LC film and a polarizer, and the other is composed of two HAN-LC films with a half-wave plate between them. The dependence of the asymmetric transmittance on the incident angle was confirmed for these filters. The dependence changed depending on the pretilt angle of the alignment layers. Photographs taken with the optical filters displayed their effectiveness.

Scalable Manufacturing of Large-Area Pressure Sensor Array for Sitting Posture Recognition in Real Time.

Pressure sensors are considered the key technology for potential applications in real-time health monitoring, artificial electronic skins, and human-machine interfaces. Despite the significant progress in developing novel sensitive materials and constructing unique sensor structures, it remains challenging to fabricate large-area pressure sensor arrays due to the involvement of complex procedures including photolithography, laser writing, or coating. Herein, a scalable manufacturing approach for the realization of pressure sensor arrays with substantially enlarged sensitive areas is proposed using a versatile screen-printing technique. A compensation mechanism is introduced into the printing process to ensure the precise alignment of conductive electrodes, insulation layers, and sensitive microstructures with an alignment error of less than 4 μm. The fully screen-printed sensors exhibit excellent collective sensing performance, such as a reasonable pressure sensitivity of -2.2 kPa-1, a fast response time of 40 ms, and superior durability over 3000 consecutive pressures. Additionally, an integrated 16 × 32 pressure sensor array with a sensing area of 190 × 380 mm2 is demonstrated to precisely recognize the sitting postures and the body weights, showing great potential in continuous and real-time health status monitoring.

Surface alignment of nematic liquid crystals by direct laser writing of photopolymer alignment layers

ABSTRACT We demonstrate the fabrication of good quality surface alignment layers on glass by Direct Laser Writing method using a 2-photon polymerisation technique. We use commercially available photosensitive resins to print alignment layers by scanning the focal point of a femtosecond laser near the resin-glass interface. This results in down to ~ 100 nm thin alignment layers that provide good planar anchoring of 5CB and MLC13300, with the easy axis of alignment along the scanning direction. The azimuthal anchoring strength is ~ 5 × 10−6 J/m2 and is an order of magnitude weaker compared to commercial rubbed polyimide alignment layer. The threshold voltage for Fréedericksz transition in a 90° twisted nematic cell is slightly increased compared to conventional rubbed polyimide for printed alignment layers. The turn-on switching time is longer for printed layers compared to polyimide alignment layers, whereas the turn-off time is shorter for printed alignment layers. The advantage of this new method is in its flexibility, as we demonstrate printing of complex surface alignment patterns with alignment layer thickness below 100 nm.

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

AbstractPhotodimerization of coumarin derivatives in photoactive polymers under linearly polarized UV (LPUV) light irradiation is a photochemical reaction often used to prepare the surface alignment layer that can orient liquid crystals (LCs) due to the anisotropic interfacial interactions. Herein, the synthesis of a series of novel coumarin‐containing copolymers and their use for the photoalignment surface layer of LCs is reported. These copolymers have the particular feature of bearing two coumarin side groups in each monomer unit, which is designed to have a high content of the chromophores. The alignment of LCs induced by the photoalignment layers of the coumarin‐containing copolymers is investigated. It is found that the photoalignment layers can effectively induce orientation of LC molecules along the direction of the UV light polarization used to prepare the surface layers, with an order parameter of around 0.5 achieved by adjusting the preparation conditions. Moreover, the electro‐optical behaviors of LC cells made with the photoalignment layers are investigated, confirming the alignment of LCs without the use of rubbed surfaces. Optically inscribing the photoalignment layer for nonuniform and organized LC alignment is also demonstrated.

Ionic Accumulation as a Diagnostic Tool in Perovskite Solar Cells: Characterizing Band Alignment with Rapid Voltage Pulses.

Despite record-breaking devices, interfaces in perovskite solar cells are still poorly understood, inhibiting further progress. Their mixed ionic-electronic nature results in compositional variations at the interfaces, depending on the history of externally applied biases. This makes it difficult to measure the band energy alignment of charge extraction layers accurately. As a result, the field often resorts to a trial-and-error process to optimize these interfaces. Current approaches are typically carried out in a vacuum and on incomplete cells, hence values may not reflect those found in working devices. To address this, a pulsed measurement technique characterizing the electrostatic potential energy drop across the perovskite layer in a functioning device is developed. This method reconstructs the current-voltage (JV) curve for a range of stabilization biases, holding the ion distribution "static" during subsequent rapid voltage pulses. Two different regimes are observed: at low biases, the reconstructed JV curve is "s-shaped", whereas, at high biases, typical diode-shaped curves are returned. Using drift-diffusion simulations, it is demonstrated that the intersection of the two regimes reflectsthe band offsets at the interfaces. This approach effectively allows measurements of interfacial energy level alignment in a complete device under illumination and without the need for expensive vacuum equipment.

Highly sensitive and label-free detection of biotin using a liquid crystal-based optofluidic biosensor.

A liquid crystal (LC)-based optofluidic whispering gallery mode (WGM) resonator has been applied as a biosensor to detect biotin. Immobilized streptavidin (SA) act as protein molecules and specifically bind to biotin through strong non-covalent interaction, which can interfere with the orientation of LCs by decreasing the vertical anchoring force of the alignment layer in which the WGM spectral wavelength shift is monitored as a sensing parameter. Due to the double magnification of the LC molecular orientation transition and the resonance of the WGM, the detection limit for SA can reach 1.25 fM (4.7 × 10-13 g/ml). The measurable concentration of biotin and the wavelength shift of the WGM spectrum have an excellent linearity in the range of 0 to 0.1 pg/ml, which can achieve ultra-low detection limit (0.4 fM), i.e., seven orders of magnitude improvement over conventional polarized optical microscope (POM) method. The proposed optofluidic biosensor is highly reproducible and can be used as an ultrasensitive real-time monitoring biosensor, which will open the door for applications to other receptor and ligand models.

Medication error during nonoperating room anesthesia—a case report

Despite a lot has already been done in the field of safety improvement during anesthesia, medication errors still occur during everyday practice. Syringe or ampule swaps are usually the most frequent type of medication error. Recent studies prove that nearly 80% of these errors are preventable. In our case, 30 mg (3 ml) of dopamine (Dopamin Fresenius 10 mg/ml) was injected to the patient’s intravenous line instead of 3 mg (3 ml) midazolam (Midazolam Accord 1 mg/ml) during the preparation for the carotid artery stenting procedure in nonoperating room environment. The error was realized immediately after the application. Besides temporary fulminant hypertensive reaction, tachycardia, restlessness, skin rush, troponin leak, and temporary ST-segment depression, there were no permanent consequences to the patient's health. The team was able to perform the planned procedure 30 min after the event. This medication error of ampule swap, packages of which were stored in the medication cupboard one above the other, was caused by alignment of the latent vulnerable layers of the safety system (Swiss Cheese Model of System Error and Hot Cheese Model) plus the influence of the environmental factors and active failures done by the anesthesia staff. After this event, new safety measures were established by introducing color-coded ISO 26825:2020 syringe labeling, new anesthesia trolleys with color-coded medication compartments, and color-coded medication storage cupboards. Besides this safety committee was formed for the promotion of medication safety education programs.

Fluorinated polyimide for controlling pretilt angle of liquid crystals by plasma treatment

ABSTRACT The pretilt angles of LC molecules can be controlled by tetrafluoromethane (CF4) plasma treatment. For conventional polyimide layers, the homogeneous alignment turns homeotropic owing to the invasion of fluoride atoms, which leads to the wettability transition from being hydrophilic to hydrophobic. The different treatment times generate variable pretilt angles such that the values of pretilt angles can be changed from 7.21ο to 79.53ο, and the polar anchoring energy can be tuned continuously. This technique can provide a reversible alignment layer that can be controlled by applying different plasmas of CF4 and O2. Moreover, a hybrid-aligned LC cell with patterned displays based on different pretilt angles, was proposed herein.

Tri(4-trifluoromethylphenyl)phosphine-Modified Hole-Transport Material PTAA to Improve the Performance of Perovskite Solar Cells

The energy level alignment and conductivity of hole transport layers (HTLs) are critical to the performance of perovskite solar cells (PSCs). Additive engineering is an effective approach, and we introduced tri(4-trifluoromethylphenyl)phosphine (343FP) as an additive into poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). Studies have shown that the addition of 343FP can lead to improved charge transport properties in the PTAA and can also enhance charge transport, leading to improved device efficiency of the resulting perovskite solar cell. 343FP reduces the highest occupied molecular orbital energy level of PTAA, improves the conductivity of HTLs, and passivates the interface defects, and the optimized device of 343FP shows a champion efficiency of 20.02%. In addition, due to the hydrophobicity of 343FP, compared with the traditional hygroscopic dopant lithium bis((trifluoromethyl)sulfonyl)azide (Li-TFSI)/4-tert-butylphenol (t-BP) bi-doped (Li-doped) PTAA and the undoped PTAA, the device with 343FP-doped PTAA has better environmental stability. After 1000 h of exposure to the environment, the efficiency of 78% of the original value can be maintained. This work will open up a new method for the optimization of high-efficiency and high-stability perovskite photovoltaic hole transport layers.

Autonomous alignment and healing in multilayer soft electronics using immiscible dynamic polymers.

Self-healing soft electronic and robotic devices can, like human skin, recover autonomously from damage. While current devices use a single type of dynamic polymer for all functional layers to ensure strong interlayer adhesion, this approach requires manual layer alignment. In this study, we used two dynamic polymers, which have immiscible backbones but identical dynamic bonds, to maintain interlayer adhesion while enabling autonomous realignment during healing. These dynamic polymers exhibit a weakly interpenetrating and adhesive interface, whose width is tunable. When multilayered polymer films are misaligned after damage, these structures autonomously realign during healing to minimize interfacial free energy. We fabricated devices with conductive, dielectric, and magnetic particles that functionally heal after damage, enabling thin-film pressure sensors, magnetically assembled soft robots, and underwater circuit assembly.

Polarization patterning in ferroelectric nematic liquids via flexoelectric coupling

The recently discovered ferroelectric nematic liquids incorporate to the functional combination of fluidity, processability and anisotropic optical properties of nematic liquids, an astonishing range of physical properties derived from the phase polarity. Among them, the remarkably large values of second order optical susceptibility encourage to exploit these new materials for non-linear photonic applications. Here we show that photopatterning of the alignment layer can be used to structure polarization patterns. To do so, we take advantage of the flexoelectric effect and design splay structures that geometrically define the polarization direction. We demonstrate the creation of periodic polarization structures and the possibility of guiding polarization by embedding splay structures in uniform backgrounds. The demonstrated capabilities of polarization patterning, open a promising new route for the design of ferroelectric nematic based photonic structures and their exploitation.

Manipulating the Switching of Liquid Crystals by Interfacial Nanotips Penetrating Polyimide Hybrid Alignment Layers

Liquid crystals (LCs) are an active area of interest for electro-optical devices, and the alignment of LCs has emerged as an extremely essential issue for LC devices. Evidence suggests that the surface topology of alignment layers plays a key role in both tuning the tilt angles and modulating the switching of LCs. Herein, nanotips are constructed by self-assembly of two different two-dimensional (2D) nanoflakes in PI hybrid alignment layers to modulate the switching of LCs. Both MoS2 nanotips and Ti3C2Tx MXene nanotips were observed on hybrid thin layers, but MoS2 nanotips are much thinner and taller. The hybrid thin layers are highly transparent, and the nanotips penetrating the hybrid layers roughened the surface but moderately declined the surface energy. LCs are strongly anchored on hybrid alignment layers, and the nanotips were found to topologically hinder the in-plane switching of LCs and hence accelerate the rewriting speed of optical data. Besides the accelerated optical rewriting speed, the surface plasmon–polariton (SPP) of nanoflakes also substantially boosted the external electric field to switch LCs and hence remarkably diminished the operating voltage and likewise sufficiently shortened the response time. In particular, the optical rewriting time of LCs that homogeneously aligned on hybrid alignment layers with 0.5% MoS2 nanoflakes doping has been deduced to as short as 43.36 s, which is a decrease of 37.81% compared to that of PI alignment layers cell; LCs can be electrically driven to switch optical data at 2.51 V and return to their original alignment state within 24.142 ms. These results signify the promising applications of hybrid thin layers for both traditional LC devices and optical rewritable LC devices.

Prototype Design for Grading Structures in Powder Bed Fusion Processes.

While targeted alignment in certain additive manufacturing (AM) methods such as material extrusion (MEX) and stereolithography (SLA) has been well documented in the research community, a method for targeted alignment of added fillers or fibrous materials in powder bed fusion (PBF) AM devices has yet to be successfully achieved. Similarly, incorporation of multimaterials does not work easily with any of the AM technologies. This study creates a prototype design that could be integrated into a PBF system to allow for multimaterial layer deposition and alignment of powders and powder blends. The rheological properties of polyamide powder and a range of polyamide composite blends (incorporating milled carbon fibre, graphite flakes, polytetrafluoroethylene, and glass microspheres) in different concentrations were studied, and together with the particle size distribution and particle morphology analysis were applied for the design of a prototype hopper for incorporation in the PBF system to create targeted multimaterial deposition. Different concept designs, multichambered and multi-hopper with hopper angles calculated specifically for the composite blend powders selected, were proposed. Initial deposition trials outside a PBF process were tested, and the deposited layers were measured.