Rajratan Basu

Graphene exhibits optical chirality only when its in-plane mirror symmetries are broken—by strain corrugation, buckling, interlayer twist, chemical corrugation, or planar patterning—yet the microscopic route by which such interfacial handedness is transferred to adjacent otherwise achiral soft matter needs to be understood in full theoretical depth. We present a symmetry-based, predictive model showing that non-collinear corrugations of a graphene sheet generate a geometric pseudoscalar that acts as a chiral field at the graphene–liquid crystal (LC) interface. This field deracemizes configurationally achiral smectic-A LC molecules aligned by π–π stacking on the graphene surface and, under an out-of-plane electric field, drives a surface electroclinic effect (ECE)—a measure of chirality in the LC. The framework yields clear predictions: a single-pole frequency response; a rise-then-saturate behavior of the electroclinic coefficient with increasing graphene corrugation amplitude; and a monotonic increase of the cutoff frequency with roughness. The predictions are consistent with previously reported experimental observations, including graphene circular-dichroism studies and our companion LC measurements. The model explains why ultraflat graphene is optically achiral, yet strained graphene induces a robust interfacial ECE, and it provides practical design knobs for engineering chiral responses in 2D/soft-matter hybrids, opening a broadly applicable route to interfacial chirality control.

The impact of ionic impurities on the fundamental properties of nematic liquid crystals (LC) has long been observed; however, their influence on fundamental material parameters remains quantitatively unclear. Here we develop and validate a predictive theoretical framework that integrates ionic Coulomb self-energy into the Landau-de Gennes formalism, revealing how free ions nonlinearly suppress the dielectric anisotropy. Additionally, we propose a model that clarifies how electrostatic ion drag contributes to an increase in the rotational viscosity of the nematic phase. We experimentally verify both the dielectric anisotropy model and rotational viscosity model using a dual-frequency nematic LC doped with controlled concentrations of monolayer graphene flakes, which effectively modulates the ionic environment. The experimental results exhibit strong agreement with the theoretical predictions across both positive and negative dielectric regimes, as well as for the ion-modulated rotational viscosity of the LC. Dynamic optical switching measurements further corroborate the rotational viscosity results. This work establishes a self-consistent link between microscopic ionic screening and macroscopic rheo-optic behavior in soft anisotropic media, advancing the fundamental understanding of ion-related electrostatics in complex fluids.

Free ion impurities in liquid crystals significantly impact the dynamic electro-optic performance of liquid crystal displays, leading to slow switching times, short-term flickering, and long-term image sticking. These ionic contaminants originate from various sources, including LC cell fabrication, electrode degradation, and organic alignment layers. This study demonstrates that doping LCs with a small concentration of helical carbon nanotubes effectively reduces free ion concentrations by approximately 70%. The resulting reduction in ionic impurities lowers the rotational viscosity of the LC, facilitating faster electro-optic switching. Additionally, the purified LC exhibits enhanced dielectric anisotropy, further improving its performance in display applications. These findings suggest that helical carbon nanotubes doping offers a promising approach for mitigating ion-related issues in liquid crystals without the need for additional chemical treatments, paving the way for an efficient liquid crystal display technology.

Free-ion impurities in liquid crystals (LCs) significantly impact the dynamic electro-optic performance of liquid crystal displays (LCDs), leading to slow switching times, short-term flickering, and long-term image sticking. These ionic contaminants originate from various sources, including LC cell fabrication, electrode degradation, and organic alignment layers. This study demonstrates that doping LCs with a small concentration of helical carbon nanotubes (hCNTs) effectively reduces free-ion concentrations by approximately 70%. The resulting reduction in ionic impurities lowers the rotational viscosity of the LC, facilitating faster electro-optic switching. Additionally, the purified LC exhibits enhanced dielectric anisotropy, further improving its performance in display applications. These findings suggest that hCNT doping offers a promising approach for mitigating ion-related issues in LCs without the need for additional chemical treatments, paving the way for an efficient LCD technology.

The presence of excess free-ion impurities in ferroelectric liquid crystals (FLCs) results in several issues in electro-optical liquid-crystal displays (LCDs), such as slow electro-optical responses and image-sticking effects. This study experimentally demonstrated that two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheets can function as both planar alignment and ion-capturing agents in an electro-optic FLC device. The 2D h-BN nanosheet was used as a planar alignment agent on one side of an LC cell, while a standard planar-aligning polyimide (PI) layer was used on the other side of the cell. The FLC exhibited uniform planar alignment in this h-BN-PI cell configuration. The results showed that free-ion impurities in the FLC were significantly suppressed in the h-BN-PI cell compared to a standard PI-PI LC cell. This reduction in free-ion density in the h-BN-PI cell was attributed to the ion-capturing capability of the 2D h-BN nanosheets. Consequently, the reduction of ionic impurities led to enhanced effective spontaneous polarization and accelerated electro-optic response of the FLC in the h-BN-based cell.

The alignment agent plays a crucial role in the electro-optical characteristics of an electro-optic liquid crystal (LC) device. Conventionally, rubbed polyimide (PI) layers have been utilized as alignment agents in LC devices. However, these organic PI layers are sensitive to UV rays and high temperatures. The rubbing procedure involved in preparing PI layers also introduces drawbacks, including non-uniform brightness due to uneven rubbing mechanisms and the presence of residual fiber dust. In our recent research, we demonstrated that various hexagonal 2D (inorganic) materials, such as monolayer graphene, monolayer tungsten diselenide (WSe2), and monolayer hexagonal boron nitride (h-BN) nanosheets, can effectively serve as planar-alignment agents in electro-optic LC devices. In this Perspective, we review the rubbing-free LC alignment process, which relies on the coherent overlay of the benzene rings of LC molecules on the hexagonal lattice structure of 2D materials. We review our recent results on LC devices utilizing 2D materials as alignment agents, highlighting their potential advantages. Furthermore, we discuss the challenges associated with this research field. Finally, we provide our perspectives on the future directions and opportunities for advancing this area of study, aiming to overcome challenges and extend the benefits of employing 2D materials as planar-alignment agents in electro-optic LC devices.

The free-ion concentration in a nematic liquid crystal (LC) is found to be significantly reduced when gold nano-urchins (AuNUs) of 50-nm diameter are dispersed in the LC in dilute concentrations. The nano-urchins on AuNUs trap a significant amount of mobile ions, reducing the free-ion concentration in the LC media. The reduction of free ions results in a decreased rotational viscosity and accelerated electro-optic response of the LC. The study is carried out with several AuNUs concentrations in the LC, and the experimental results consistently suggest that there exists an optimal concentration of AuNUs, above which they tend to aggregate. At the optimal concentration, the ion trapping is maximum, rotational viscosity is at its lowest, and the electro-optic response is the fastest. Above this optimal AuNUs concentration, the rotational viscosity is found to increase, and consequently, the LC no longer exhibits an accelerated electro-optic response.

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.