Nematic Films Research Articles

Synthesis of optically active chiral mesoporous molybdenum carbide film

A chiral nematic mesoporous molybdenum carbide film (CN-Mo2C) was fabricated through the self-assembly of cellulose nanocrystal with peroxomolybdate. CN-Mo2C exhibits a tunable chiral nematic structure by varying the ratio of constituents, enabling a surface area of 210m2g−1. Importantly, it shows a typical chiroptical feature through polarized optical microscopy and circular dichroism spectral analysis. As it has an excellent conductivity of 2.4×10−2 S cm−1, we try to use CN-Mo2C in the electrocatalytic HER production, presenting an efficient H2 production capacity. It is a versatile synthetic strategy that can conveniently enable other large-area macroscopic chiral metal-based materials.

Hyperspectral Imaging of Photonic Cellulose Nanocrystal Films: Structure of Local Defects and Implications for Self-Assembly Pathways.

Cellulose nanocrystals (CNCs) can spontaneously assemble into chiral nematic films capable of reflecting circularly polarized light in the visible range. As many other photonic materials obtained by bottom-up approaches, CNC films often display defects that greatly impact their visual appearance. Here, we study the optical response of defects in photonic CNC films, coupling optical microscopy with hyperspectral imaging, and we compare it to optical simulations of discontinuous cholesteric structures of increasing complexity. Cross-sectional SEM observations of the film structure guided the choice of simulation parameters and showed excellent agreement with experimental optical patterns. More importantly, it strongly suggests that the last fraction of CNCs to self-assemble, upon solvent evaporation, does not undergo the typical nucleation and growth pathway, but a spinodal decomposition, an alternative self-assembly pathway so far overlooked in cast films and that can have far-reaching consequences on choices of CNC sources and assembly conditions.

Elastic constants and the formation of topological defects in hybrid nematic cells: A Monte Carlo study.

We present a Monte Carlo study of the effects of elastic anisotropy on the topological defects which can be formed in nematic films with hybrid boundary conditions. We simulate the polarized microscopy images and analyze their evolution in uniaxial systems for different values of the Frank elastic constants.

Twisting and untwisting of twisted nematic elastomers

A mechanical stretch applied to a monodomain twisted nematic elastomer thin film untwists the helical director distribution into a uniaxially aligned state. If the film thickness is less than the helical pitch, untwisting behavior is stretching direction dependent in the plane normal to the helix axis. We investigate untwisting of twisted nematic elastomers by optical, mechanical, and x-ray scattering methods. We adapt a model for cholesteric elastomers to our observations and find that the material exhibits characteristics with the combined properties of both a smectic and twisted nematic configuration.

Quantifying spatial alignment and retardation of nematic liquid crystal films by Stokes polarimetry.

Recently developed alignment techniques for liquid crystals enable the definition of arbitrary alignment patterns. We present a method to determine these two-dimensional spatial alignment distributions as well as the retardation of thin nematic liquid crystal films. The method is based on quantifying the influence of the birefringence of such a film on light with known input polarization by measuring the Stokes parameters of light. We show that we are able to distinguish arbitrary alignment patterns unambiguously. In addition, we demonstrate the ability to evaluate the homogeneity of the alignment as well as the thickness or birefringence of the film.

Uniaxial tension of a nematic elastomer with inclined mesogens

Nematic elastomer, consisting of mesogens and polymer network, has emerged as a promising material for constructing soft actuators and attracted much attention in recent years. In this paper, we conduct a systematic study of uniaxial tension of a nematic elastomer film with the loading direction forming different angles with respect to the mesogen alignment. We found that if the two ends of the film are clamped without allowing rotation during the stretch, inhomogeneous deformation in the specimen often occurs. However, if both ends of the film can freely rotate during the stretch, the film elongates and shears nearly homogeneously. Our theoretical predictions based on the free energy function of nematic elastomer agree well with experimental measurements of the elastomer undergoing homogeneous deformation with only two fitting parameters. The necking phenomena observed in nematic elastomer during the uniaxial tension have also been discussed.

Synthesis of Chiral Nematic Mesoporous Metal and Metal Oxide Nanocomposites and their Use as Heterogeneous Catalysts

We report the synthesis of chiral nematic mesoporous metal and metal oxide nanocomposites containing zinc or copper prepared through a vacuum‐assisted loading of the metal precursor into a chiral nematic mesoporous silica template followed by high‐temperature treatment. Significant metal loading into the template was achieved after a single loading cycle (up to 55 % of the pore volume) as confirmed by nitrogen sorption measurements. Growth of the metal nanoparticles was contained within the porous silica template, and subsequent removal of the template afforded free‐standing chiral nematic metal oxide films with surface areas up to 200 m2/g. To demonstrate the utility of these high surface area materials, we used the prepared CuO material as a heterogeneous catalyst for click reactions. Our prepared CuO showed excellent activity (95 % conversion after seven hours) when compared to commercially available CuO nanoparticles (20 % conversion after 24 hours) owing to its considerably higher surface area. Moreover, the prepared CuO material maintained high activity (>70 % conversion) after four cycles.

Biomimetic Mesoporous Cobalt Ferrite/Carbon Nanoflake Helices for Freestanding Lithium‐Ion Battery Anodes

AbstractStructural biomimicry is a fascinating concept to explore hierarchically organized nanomaterials for mechanical structures, catalysis, sensing, and energy storage applications. Here we report the fabrication of biomimetic mesoporous cobalt ferrite/carbon nanoflake materials with helical morphologies and evaluate their electrochemical properties as free‐standing lithium‐ion battery (LIB) anodes. Iridescent chiral nematic mesoporous chitosan films obtained from crab shells were combined with binary metallic ions to afford helical cobalt ferrite/chitosan membranes. The cobalt ferrite/chitosan composites were thermally converted to cobalt ferrite/carbon replicas with hybrid nanoflakes arranged in a twisted Bouligand‐type mesoporous network. The structure of the materials was probed by electron microscopy, powder X‐ray diffraction, and Raman spectroscopy. We directly used these freestanding cobalt ferrite/carbon films as binder‐ and additive‐free LIB anodes, where they showed a first discharge capacity of 862 mAh g−1 (at 100 mA g−1), which faded during subsequent charge‐discharge cycles. Our work demonstrates a new potential use of chiroptical chitosan membranes to develop energy storage materials, a process that may be extended to other metal‐oxide based components.

Omnidirectional transport and navigation of Janus particles through a nematic liquid crystal film

We create controllable active particles in the form of metal-dielectric Janus colloids which acquire motility through a nematic liquid crystal film by transducing the energy of an imposed perpendicular AC electric field. We achieve complete command over trajectories by varying field amplitude and frequency, piloting the colloids at will in the plane spanned by the axes of the particle and the nematic. The underlying mechanism exploits the sensitivity of electro-osmotic flow to the asymmetries of the particle surface and the liquid-crystal defect structure. We present a calculation of the dipolar force density produced by the interplay of the electric field with director anchoring and the contrasting electrostatic boundary conditions on the two hemispheres, that accounts for the dielectric-forward (metal-forward) motion of the colloids due to induced puller (pusher) force dipoles. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly and collective dynamics.

Optically Ambidextrous Reflection and Luminescence in Self-Organized Left-Handed Chiral Nematic Cellulose Nanocrystal Films

Defect-embedded chiral nematic organizations are attractive due to their light manipulation ability that is crucial for photonic applications. Self-assembly of cellulose nanocrystals (CNC) forms a ...

Light- and Humidity-Responsive Chiral Nematic Photonic Crystal Films Based on Cellulose Nanocrystals.

Light- and humidity-responsive chiral nematic photonic crystal (PC) films containing cellulose nanocrystals (CNCs) were fabricated. A photoactive polymer with hydrophilic groups, poly-(3,3'-benzophenone-4,4'-dicarboxylic acid dicarboxylate polyethylene glycol) ester, was coassembled with CNCs to form flexible iridescent films with a tunable chiral nematic order. In the coassembly process, the intermolecular hydrogen bonds of CNCs were weakened, which facilitated the fine regulation of the chiral PC nanostructure. The PC films displayed sensitive responses to both light and humidity. With increasing humidity from 30 to 100%, the chiral nematic helix pitch increased from 328 to 422 nm. The color of the PC films changed from blue to green, yellow, orange, and dark red with increasing relative humidity. Over 15 min of light irradiation, the absorption intensity of the films increased gradually. The light and humidity responses of the films were reversible. The films maintained their variable cholesteric liquid crystal texture and helical lamellar structure after light irradiation at different humidities. These PC films are expected to be useful in intelligent coatings and 3D printing.

Influence of Free Volume Determined by Positron Annihilation Lifetime Spectroscopy (PALS) on Gas Permeability of Cellulose Nanocrystal Films.

Cellulose nanocrystals (CNCs) are of increasing interest for packaging applications because of their biodegradability, low cost, high crystallinity, and high aspect ratio. The objective of this study was to use positron annihilation lifetime spectroscopy (PALS) to investigate the free volume of CNC films with different structural arrangements (chiral nematic vs shear-oriented CNC films) and relate this information to gas barrier performance. It was found that sheared CNC films with higher CNC alignment have lower free volume and hence have more tortuosity than chiral nematic self-assembled films, which lowers gas diffusion throughout the films. The overall barrier performance of the aligned CNC film obtained in this study has a higher barrier performance than high barrier polymer films like PVOH and EVOH. Furthermore, a modified model was developed for single-component CNC films to predict the gas permeability with variation of CNC alignment with validation by the data taken.

Dual Response of Photonic Films with Chiral Nematic Cellulose Nanocrystals: Humidity and Formaldehyde.

Manipulating functional stimuli-responsive materials has been a hot topic in the research of smart sensors and anticounterfeiting encryption. Here, a novel functional chiral nematic cellulose nanocrystal (CNC) film showing dual responsiveness to humidity and formaldehyde gas was fabricated. The chiral nematic CNC iridescent film could respond to environmental humidity and formaldehyde gas changes by reversible motion. Interestingly, the humidity sensitivity of the CNC iridescent film could be gated by exposing the film to formaldehyde gas. At the same time, the formaldehyde-responsive behavior is strongly affected by the relative humidity (RH), and the response range could be tuned by changing the RH over a wide range. Importantly, the formaldehyde-induced color change could be altered from invisible to visible by the naked eye when the film was exposed to a humid environment. The mechanism of this dual response of the CNC iridescent film is ascribed to the synergistic effect of cooperation and competition between water and formaldehyde molecules by constructing physical cross-linking networks by hydrogen bonds among water, formaldehyde, and CNCs. Furthermore, the "RH-concentration of formaldehyde gas-color" ternary colorimetric system was simulated, which is thought to endow the CNC iridescent film with great potential to act as a sensor in the convenient visible detection of gaseous formaldehyde. Furthermore, this work provided a promising strategy to design multi-gas-sensitive devices with convenient detection, good stability, and excellent reversibility.

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part V: Grating-coupled excitation (Erratum)

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part V: Grating-coupled excitation (Erratum)

Vortex confinement transitions in the modified Goldstone model

This paper suggests a new theoretical model, in which topological objects such as vortices, half-vortices, and solitons coexist. They strongly interact and form topological molecules, triggering a confiment/deconfinement topological phase transition. Its findings provide applications to Josephson-junction arrays of superconducting and nematic liquid crystal films

Water-Stable Flexible Nanocellulose Chiral Nematic Films through Acid Vapor Cross-Linked Glutaraldehyde for Chiral Nematic Templating.

Cellulose nanocrystals (CNCs) have drawn considerable attention for their use in optical and sensor applications due to their appealing properties of chiral nematic photonic structures. However, the flexibility and water instability of neat CNC chiral nematic films are questionable and compromise their outstanding properties. We propose a room-temperature process for fabricating flexible, water-stable chiral nematic CNC films. Aqueous glutaraldehyde (GA) was first mixed with CNCs, and then free-standing films were formed by evaporation-induced self-assembly. The chiral nematic dry films that formed were then exposed to hydrochloric acid vapor for subsequent GA cross-linking with CNCs. The GA cross-linked CNC films had a highly ordered chiral nematic organization. The enhanced water stability of the films was demonstrated by using GA cross-linked CNC films as freestanding template substrates for conducting polymers (polypyrrole) and metal oxides (iron oxide) to form flexible chiral nematic photonic hybrids.

Effects of spatially-varying substrate anchoring on instabilities and dewetting of thin nematic liquid crystal films.

Partially wetting nematic liquid crystal (NLC) films on substrates are unstable to dewetting-type instabilities due to destabilizing solid/NLC interaction forces. These instabilities are modified by the nematic nature of the films, which influences the effective solid/NLC interaction. In this work, we focus on the influence of imposed substrate anchoring on the instability development. The analysis is carried out within a long-wave formulation based on the Leslie-Ericksen description of NLC films. Linear stability analysis of the resulting equations shows that some features of the instability, such as emerging wavelengths, may not be influenced by the imposed substrate anchoring. Going further into the nonlinear regime, considered via large-scale GPU-based simulations, shows however that nonlinear effects may play an important role, in particular in the case of strong substrate anchoring anisotropy. Our simulations show that instability of the film develops in two stages: the first stage involves formation of ridges that are perpendicular to the local anchoring direction; and the second involves breakup of these ridges and formation of drops, whose final distribution is influenced by the anisotropy imposed by the substrate. Finally, we show that imposing more complex substrate anisotropy patterns allows us to reach basic understanding of the influence of substrate-imposed defects in director orientation on the instability evolution.

Tuning the cellulose nanocrystal alignments for supramolecular assembly of chiral nematic films with highly efficient UVB shielding capability

Chiral nematic CNC nanocomposite films with excellent UVB shielding capability are fabricated and supramolecularly modified by d-xylose.

Effect of ZnO dispersion in optical and structural of nematic liquid crystal

Incorporation of zinc oxide (ZnO) semiconductor particles in liquid crystal technology is advantageous due to its unique optical properties especially in ultraviolet region. In this paper, optical and film morphology of nematic liquid crystal films, 4′-(hexyloxy)-4-biphenylcarbonitrile (6OCB) were analyzed using UV-Vis absorption spectroscopy, z-scan measurement, Raman spectroscopy and X-ray powder diffraction. As expected, dramatic change of absorption in both ultraviolet and visible region were observed upon additional weightage of ZnO particles into the 6OCB mixture. Z-scan measurement obtained under 532 nm CW laser line was used to study nonlinear refractive index, n2 of the material. This ZnO particles incorporation in 6OCB mixture show self-defocusing property with thermal effects as one of its origin of nonlinear absorption occurring at 532 nm. Prominent diffraction peaks of 15°, 29°, 31°, 35° and 47° were measured from rubbed films of 6OCB doped ZnO films which indicated 6OCB and ZnO lattice crystal structures. Microscopic formations of 6OCB molecules is highly dependent with the proximity to the ZnO particle as analyzed by Raman vibrational analysis. These findings can be used to suggest optical and electrical improvement to the liquid crystal film in optoelectronics application as electro-optic modulator and optical switch.

When nanocellulose meets diffraction grating: freestanding photonic paper with programmable optical coupling.

Photonic crystals based on plasmonic or dielectric periodic structures have attracted considerable interest owing to their capabilities to control light-matter interactions with tailored precision. By using a nanocellulose derived chiral liquid crystal as a building block, here we demonstrate a bio-inspired dual photonic structure that contains the combination of microscopic periodic 1D surface grating and nanoscopic helical organization, giving rise to programmable colour mixing and polarization rotation. We show that a variation in the photonic band-gap in the bulk matrix leads to simultaneous control over the reflection and diffraction of light with controllable iridescence.