Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application

Rubbing-free liquid crystal electro-optic device based on organic single-crystal rubrene

Liquid crystals (LCs) have been a vital component of modern communication and photonic technologies. However, traditional LC alignment on polyimide (PI) requires mechanically rubbing treatment to control LC orientation, suffering from dust particles, surface damage, and electrostatic charges. In this paper, LC alignment on organic single-crystal rubrene (SCR) has been studied and used to fabricate rubbing-free LC devices. A rubrene/toluene solution is spin-coated on the indium-tin-oxide (ITO) substrate and transformed thereafter to the orthorhombic SCR after annealing. Experimental result reveals that SCR-based LC cell has a homogeneous alignment geometry, the pretilt angle of LCs is low and the orientation of LCs is determined with capillary filling action of LCs. LC alignment on SCR performs a wider thermal tolerance than that on PI by virtue of the strong anchoring nature of LCs on SCR due to van der Waals and π-π electron stacking interactions between the rubrene and LCs. SCR-based LC cell performs a lower operation voltage, faster response time, and higher voltage holding ratio than the traditional PI-based LC cell. Organic SCR enables to play a role as weakly conductive alignment layer without rubbing treatment and offers versatile function to develop novel LC devices.

Insertion of the Liquid Crystal 5CB into Monovacancy Graphene

Interfacial interactions between liquid crystal (LC) and two-dimensional (2D) materials provide a platform to facilitate novel optical and electronic material properties. These interactions are uniquely sensitive to the local energy landscape of the atomically thick 2D surface, which can be strongly influenced by defects that are introduced, either by design or as a byproduct of fabrication processes. Herein, we present density functional theory (DFT) calculations of the LC mesogen 4-cyan-4′-pentylbiphenyl (5CB) on graphene in the presence of a monovacancy (MV-G). We find that the monovacancy strengthens the binding of 5CB in the planar alignment and that the structure is lower in energy than the corresponding homeotropic structure. However, if the molecule is able to approach the monovacancy homeotropically, 5CB undergoes a chemical reaction, releasing 4.5 eV in the process. This reaction follows a step-by-step process gradually adding bonds, inserting the 5CB cyano group into MV-G. We conclude that this irreversible insertion reaction is likely spontaneous, potentially providing a new avenue for controlling both LC behavior and graphene properties.

Electro-optic hybrid aligned nematic device utilizing carbon nanotube arrays and two-dimensional hexagonal boron nitride nanosheet as alignment substrates

Hybrid-aligned nematic (HAN) liquid crystal (LC) devices have both fundamental and technological importance for their applications in LC adaptive lenses, low voltage LC displays, smart windows, and many more. We report the fabrication and characterization of a nanostructure-based HAN device employing vertically aligned carbon nanotube (VA-CNT) arrays as the homeotropic alignment agent on one side and two-dimensional (2D) hexagonal boron nitride (h-BN) as the planar alignment agent on the other side of the LC cell. The LC achieves the HAN configuration in the cell, i.e., homeotropic alignment at the VA-CNT side due to the π-π stacking interaction between the LC and CNTs, and planar alignment at the h-BN side due to the π-π stacking interaction between the LC and h-BN. When an applied electric field is ramped up across this VA-CNT/h-BN HAN cell, the LC (positive anisotropic) obtains a homeotropic state, requiring no threshold voltage to start the reorientation process; this effect is similar to that of a traditional polyimide (PI)-based HAN device. This VA-CNT/h-BN HAN cell successfully demonstrates the optical, electro-optical operations and the electric field-induced dynamic response. This study reveals that two inorganic nanostructured surfaces, VA-CNT arrays and 2D h-BN, can efficiently replace the organic PI alignment agents when needed and retain the HAN device's necessary electro-optical performances. These results substantially expand the fundamental understanding and the scope of utilizing various nanostructured surfaces for LC alignment mechanisms.

Graphene as an alignment agent, an electrode, and a source of surface chirality in a smectic-A liquid crystal

A liquid crystal (LC) cell was fabricated by putting together a monolayer graphene-coated glass substrate on one side, and a rubbed planar-aligning polyimide layer on an indium tin oxide (ITO) coated glass substrate on the other side. The monolayer graphene film served as the planar-alignment agent as well as the transparent electrode on one side of the cell. The cell was filled with an achiral LC alkoxyphenylbenzoate (9OO4). The presence of the graphene film on one substrate resulted in an induced chiral signature in the otherwise achiral LC 9OO4. The induced chirality was probed utilizing the electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase. The electroclinic effect showed significant pretransitional behavior on approaching the smectic-A to smectic-C transition temperature from above. The electroclinic effect revealed a low-frequency relaxation process indicating that the chirality was induced on the LC molecules at the graphene interface and did not propagate into the bulk. A soft shear mode can break the symmetry of the hexagonal lattice of graphene on a substrate and, consequently, graphene possesses strain chirality. The noncovalent π-π interaction between the LC and the strained graphene induces molecular conformational deracemization in the LC at the graphene interface, and the LC exhibits surface-induced chirality.

Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes

A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the π−π electron stacking between the LC’s benzene rings and the h-BN’s honeycomb structure stabilizes pseudo-nematic domains surrounding the h-BN flakes. Electric field-dependent dielectric studies reveal that the LC-jacketed h-BN flakes follow the nematic director reorientation upon increasing the applied electric field. These anisotropic pseudo-nematic domains exist in the isotropic phase of the LC+h-BN system as well, and interact with the external electric field, giving rise to a nonzero dielectric anisotropy in the isotropic phase. Further investigations reveal that the presence of the h-BN flakes at a low concentration in the nematic LC enhances the elastic constants, reduces the rotation viscosity, and lowers the pre-tilt angle of the LC. However, the Fréedericksz threshold voltage stays mostly unaffected in the presence of the h-BN flakes. Additional studies show that the presence of the h-BN flakes enhances the effective polar anchoring strength in the cell. The enhanced polar anchoring strength and the reduced rotational viscosity result in faster electro-optic switching in the h-BN-doped LC cell.

Thermotropic liquid crystal (5CB) on two-dimensional materials

We present ground-state electronic properties of the liquid crystal 4-cyano-4^{'}-pentylbiphenyl (5CB) on the two-dimensional materials monolayer graphene, hexagonal boron nitride, and phosphorene. Our density functional theory results show that the physisorption is robust on all surfaces with the strongest binding of 5CB on phosphorene. All surfaces exhibit flexural distortion, especially monolayer graphene and hexagonal boron nitride. While we find type-I alignment for all three substrates, meaning the Fermi level of the system is in the HOMO-LUMO gap of 5CB, the band structures are qualitatively different. Unlike for graphene and phosphorene, the HOMO-LUMO of 5CB appear as localized states within the band gap of boron nitride. In addition, we find that the valence band for boron nitride is sensitive to the orientation of 5CB relative to the surface. The qualitatively different band structures demonstrate the importance of substrate selection for tailoring the electronic and optoelectronic properties of nematic liquid crystals on two-dimensional materials.

Enhancement of effective polar anchoring strength and accelerated electro-optic switching in a two-dimensional hexagonal boron nitride/polyimide hybrid liquid crystal device

The monolayer hexagonal boron nitride (h-BN) nanosheet is transferred onto an indium tin oxide (ITO)-coated glass substrate. This h-BN slide is placed together with a conventional planar-aligning polyimide (PI) slide to fabricate a liquid crystal (LC) cell, in which the LC achieves uniform planar alignment. The effective polar anchoring strength coefficient of this h-BN-based hybrid LC device is found to increase significantly compared to that of a standard PI/PI LC device. The presence of the monolayer h-BN nanosheet as an alignment agent increases the planar anchoring energy through the epitaxial interaction between the LC and the honeycomb structure of the two-dimensional h-BN lattice in this hybrid device. The amplified polar anchoring energy is found to accelerate the electro-optic response time of the LC in this h-BN-based hybrid device.

Two-dimensional hexagonal boron nitride nanosheet as the planar-alignment agent in a liquid crystal-based electro-optic device

The planar-alignment agent in an electro-optic liquid crystal (LC) device plays an essential role for the LC's electro-optical characteristics. Rubbed polyimide (PI) layers are conventionally used as the planar-alignment agent in traditional liquid crystal displays (LCDs). Here we experimentally demonstrate that the 2D hexagonal boron nitride (h-BN) nanosheet can serve as the planar-alignment agent in an LC cell. This h-BN has higher chemical stability and more optical transparency than the PI layer. Two h-BN-covered indium tin oxide (ITO) glass slides (without any conventional PI layers) are placed together to fabricate an LC cell. A nematic LC inside this h-BN-based cell exhibits uniform planar-alignment-which is probed by a crossed polarized optical microscope. This planar-alignment at the molecular scale is achieved due to the coherent overlay of the benzene rings of the LC molecules on the hexagonal BN lattice. This h-BN-based LC cell shows the typical electro-optical effect when an electric field is applied via ITO electrodes. The dielectric measurement across this h-BN-based electro-optic cell shows a standard Fréedericksz transition of the LC, confirming that the 2D h-BN, as the planar-alignment agent, supplies adequate anchoring energy-which can be overcome by the Fréedericksz threshold voltage. Finally, we show that the h-BN-based LC cell exhibits more optical transparency than a regular PI alignment layer-based LC cell.

Visualizing Degradation of Black Phosphorus Using Liquid Crystals

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high I_on/I_off. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.

Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography

Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufacturing and numerous emerging industrial applications with different substrate requirements. Here, we focus on ellipsometric contrast micrography (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer number, defects and contamination. We demonstrate atomic layer resolved, quantitative mapping of chemical vapor deposited graphene layers on Si/SiO₂-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. We discuss the optimization of ECM parameters for high throughput characterization. While the lateral resolution can be less than 1 μm, we particularly explore fast scanning and demonstrate imaging of a 4″ graphene wafer in 47 min at 10 μm lateral resolution, i.e., an imaging speed of 1.7 cm²/min. Furthermore, we show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2D layered structures that have previously been very challenging to characterize and thereby fills an important gap in 2DM metrology.

Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single-Crystal Graphene

Epitaxial alignment of organic liquid crystal (LC) molecules on single-crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment-layer-free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment-layer-free electro-optical LC device and LC alignment duplication, which can result in practical graphene-based flexible LC devices, are realized.

Graphene Glass Inducing Multidomain Orientations in Cholesteric Liquid Crystal Devices toward Wide Viewing Angles

The photonic reflection of a cholesteric liquid crystal (ChLC) device depends on the spatial distribution of the orientations of their helical axes, and many orientation techniques have been developed so far. In this study, we select the hybrids of graphene directly grown on quartz glass as platforms to construct ChLC-based devices. This special design makes graphene serve as both an alignment layer and a conductive layer, thus affording a more simplified device fabrication route. We reveal that multidomain structures can be evolved for ChLCs on polycrystalline monolayer graphene on quartz glass, as evidenced by polarized optical microscope characterizations. The disparate orientations of the helical axes of ChLCs and the formation of multidomain structures are proposed to be induced by the different domain orientations of graphene, leading to a wide viewing angle of the ChLC-based devices. Moreover, the pitch of ChLCs is also observed to play a key role in the relative orientations of ChLCs. A wide viewing angle of the ChLC-based device is also detected especially in the infrared spectrum region. Briefly, this work should provoke the application of graphene glass as a perfect transparent electrode in the fabrication of liquid-crystal-based devices showing broad application potentials in intelligent laser protection and energy-saving smart windows.

Enhancement of polar anchoring strength in a graphene-nematic suspension and its effect on nematic electro-optic switching

A small quantity of monolayer graphene flakes is doped in a nematic liquid crystal (LC), and the effective polar anchoring strength coefficient between the LC and the alignment substrate is found to increase by an order of magnitude. The hexagonal pattern of graphene can interact with the LC's benzene rings via π-π electron stacking, enabling the LC to anchor to the graphene surface homogeneously (i.e., planar anchoring). When the LC cell is filled with the graphene-doped LC, some graphene flakes are preferentially attached to the alignment layer and modify the substrate's anchoring property. These spontaneously deposited graphene flakes promote planar anchoring at the substrate and the polar anchoring energy at alignment layer is enhanced significantly. The enhanced anchoring energy is found to impact favorably on the electro-optic response of the LC. Additional studies reveal that the nematic electro-optic switching is significantly faster in the LC-graphene hybrid than that of the pure LC.

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

In this review article, we analyze recent progress in the application of liquid crystal-assisted advanced functional materials for sensing biological and chemical analytes. Multiple research groups demonstrate substantial interest in liquid crystal (LC) sensing platforms, generating an increasing number of scientific articles. We review trends in implementing LC sensing techniques and identify common problems related to the stability and reliability of the sensing materials as well as to experimental set-ups. Finally, we suggest possible means of bridging scientific findings to viable and attractive LC sensor platforms.

Graphene as transmissive electrodes and aligning layers for liquid-crystal-based electro-optic devices

In a conventional liquid crystal (LC) cell, polyimide layers are used to align the LC homogeneously in the cell, and transmissive indium tin oxide (ITO) electrodes are used to apply the electric field to reorient the LC along the field. It is experimentally presented here that monolayer graphene films on the two glass substrates can function concurrently as the LC aligning layers and the transparent electrodes to fabricate an LC cell, without using the conventional polyimide and ITO substrates. This replacement can effectively decrease the thickness of all the alignment layers and electrodes from about 100 nm to less than 1 nm. The interaction between LC and graphene through π-π electron stacking imposes a planar alignment on the LC in the graphene-based cell-which is verified using a crossed polarized microscope. The graphene-based LC cell exhibits an excellent nematic director reorientation process from planar to homeotropic configuration through the application of an electric field-which is probed by dielectric and electro-optic measurements. Finally, it is shown that the electro-optic switching is significantly faster in the graphene-based LC cell than in a conventional ITO-polyimide LC cell.

Study of Grains and Boundaries of Molybdenum Diselenide and Tungsten Diselenide Using Liquid Crystal

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe₂ and WSe₂. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe₂ and WSe₂ on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications.