Subbarao Krishna Prasad

This review highlights advances in integrating 2D materials with nematic liquid crystals, focusing on structure–property relationships, molecular alignment strategies, polymer networks, lyotropics, applications, and key challenges and opportunities.

We report the effect of AC electric fields in the range of 0.1-300 kHz on antiferroelectric SmZ_{A} layers of DIO in the bookshelf geometry. Significant results are (a) primary bifurcation into a quasistationary periodic instability with its voltage threshold U_{c} and wave vector q_{c} along the initial director being, respectively, quadratic and linear functions of f over 10-150 kHz, and with an azimuthal distortion of the director n which changes sign between adjacent stripes, (b) transition from the modulated state to a homogeneous state at higher voltages, and (c) third bifurcation into traveling wave periodic state on further rise in U in the region 10-40 kHz. We interpret these findings as follows. The low voltage instability is very similar to that seen in the higher temperature apolar nematic phase and is the electrohydrodynamic (EHD) instability possibly belonging to the region of dielectric inversion frequency. The azimuthal distortions of n result from an undulatory distortion of the SmZ_{A} layers in the book-shelf geometry. The intermediate homogeneous state of SmZ_{A} in which the periodic structure is absent results from a linear coupling between the layer polarization P and applied field E, giving rise to a scissoring type mutual P reorientation in adjacent layers. Finally, at even higher voltages, the medium goes over to a field-induced transition to the ferroelectric nematic, with the polarization following the AC field, and the periodic EHD instability being similar to that of the dielectric regime. The polar vector symmetry of the medium leads in general to traveling waves.

Nematic liquid crystals confined to pi cells are known to exhibit a multistable electric behavior. As previous studies show, under an increasing electric field, nematics of rodlike molecules with their elastic constants related as K_{22} (twist) <K_{11} (splay) <K_{33} (bend), undergo a continuous primary bifurcation at a critical voltage U_{c} from the bulk-planar state H to the bulk-inclined state I, followed by a discontinuous transition into the bulk-vertical state V above U_{c}. On turning the field off, the V state continuously transforms into a metastable π-twisted state T that relaxes discontinuously to the H. The present study examines the evolution and relaxation behavior of Fréedericksz's states in calamitic and bent-core nematics that differ significantly in their elastic anisotropy. Indeed, nematics with bent-shaped molecules present an anomalously low bend elastic constant, K_{33}<K_{22}<K_{11}. Our findings are the following: (a) The field-on sequence H→I→V, as also the field-off sequence V→T→H, remain common to both nematic types. (b) The two systems, however, differ in terms of the stability of the V state in a reducing field; in calamitics, the V geometry transforms discontinuously to the I above U_{c}, and a second-order I→H change follows below U_{c}; by contrast, in bent-core nematics, whether the core is flexible or rigid, the V state once formed remains stable at all voltages above U_{c}, with the V→T→H transition taking place only below U_{c}. The studies also include anisotropic propagation of fronts associated with the T→H transformation, pseudobiaxial conoscopic features of the reoriented states, periodic electric instabilities that differentiate the T and H geometries, and a comprison of computed and experimental results on cell capacitance and birefringence.

We describe the first investigations on the influence of an imposed network on the photonic band gap (PBG) structure of the liquid crystal (LC) phase through the polymer template technique. The technique consists of using a cholesteric (Ch) phase as a base for photopolymerizing a difunctional monomer, which is then removed after polymerization, leaving only the polymer scaffold template. The templated structure obtained is utilized to adjust the PBG structure of the filled LC material, exhibiting both a one-dimensional PBG (Ch phase) and a three-dimensional PBG structure (TGBC* phase with smectic C* blocks). Selective reflection measurements indicate that the imprint polymer network of the template impacts the pitch of the Ch and TGBC* phases. The absorption peaks are visible because of two distinct twisted arrangements, one originating from the template and the other from the refilled chiral substance. In the templated cell, the central wavelength (λ_min) of PBG of the refilled sample red-shifts, as well as the width of the PBG gets enhanced compared to that for the sample in the regular (nontemplated) cell. For example, in the TGBC* phase, the λ_min value red-shifts by 369 nm, and the width of the PBG enlarges by 50%. Additionally, the lattice spacing arising due to the periodicity of the SmC* helix in 2-dimension in the TGBC* phase gets enhanced. These findings demonstrate the polymer template method's effectiveness in tuning the mesophase's PBG in all three dimensions.

Conducting and alignment layers are critical for industrial scale manufacture of quality liquid crystal (LC) devices. To date, the conventional approaches utilize ITO-based electrodes as the conducting layer and rubbed polyimide as the aligning layer. However, usage of ITO has issues like scarcity of indium reserves, high cost, and toxicity, whereas employing rubbed polyimide demands high-temperature baking and mechanical rubbing that causes debris generation and electrostatic charges. Although offering a new outlook, CVD-based 2D materials provided a non-traditional route for fabricating the alignment layers. Furthermore, the previous studies are restricted to ITO-based conducting substrates and also calamitic molecules. However, from the utility point of view, the uniform alignment realized over only small areas necessitates alternative approaches to fabricate interfacial layers on non-ITO-based substrates utilizing solution-processed routes and deploying non-calamitic LCs also. Here, a generalized approach is presented to address the above-mentioned issues by considering an aluminium-doped zinc oxide (AZO) conducting layer and a hexagonal boron nitride (h-BN) alignment layer deposited via the electrophoretic deposition technique. Upon fabrication of LC devices using h-BN/AZO, large-area (>3 cm²) unidirectional alignment of LC molecules is observed. The method works equally effectively for bent-core molecules also. The alignment quality is established by azimuthal angle dependence of transmission and birefringence intensity analysis. Several critical parameters were found to be comparable between h-BN/AZO and PI/ITO devices; like operating voltage (∼1 V), switching response (in ms), image retention aspect, and ionic parameters (e.g., ionic concentration on the order of 10¹⁹ m⁻³). This study underscores the potential of h-BN/AZO as a promising candidate for display technologies.

In order to advance the scope of chiral functional materials, it is of paramount importance to engineer chiral bent-core liquid crystals (LCs) endowed with exceptional stability across a broad range of temperatures. In this context, we have devised a strategic approach, utilizing the potential of fluorinated molecules and taking advantage of the chirality of cholesterol as the pivotal component. This synergy has enabled the successful synthesis of our target LCs nF-4R. These molecules exhibit a chiral dark conglomerate (DC) phase over a broad temperature range of ∼85 °C in an enantiotropic manner. Dielectric spectroscopy measurements conducted on these materials showed persistent dielectric behavior over the entire LC range. Our study stands as a pioneering exemplar, unveiling the wide-range DC phase within the intricate landscape of chiral bent-shaped molecular systems. The mesomorphic characterization utilizing polarized optical microscopy, differential scanning calorimetry, and small-angle/wide-angle X-ray scattering measurements, revealed an optically isotropic DC phase. This phase shows domains of opposing chirality and belongs to the sponge-type DC phase. Notably, the enhancement of a specific chiral domain reveals the bias towards supramolecular chirality, attributable to molecular chirality as supported by circular dichroism measurements. This finding holds promise for chiral plasmonics, enantiomeric separation, and nano-templating applications.

A generic strategy of employing IR-transparent components to achieve prominent IR regulation via preferential placement of h-BN nanosheets in a polymer network liquid crystal with electric tunability.

The thermodynamic phase in which photopolymerisation is performed governs the morphological characteristics of a nematogen with hierarchal network feature possessing independent gelator and reactive monomer constituents.