Jagdish K. Vij

Dielectric spectroscopy is known to be one of the most powerful techniques for studying the ferroelectric and other polar fluids. Since the discovery of ferroelectricity in Liquid Crystals, the technique has successfully been employed for characterization of the ferro-, antiferro- and ferri-electric liquid crystalline phases. However, more recently, interesting issues have been raised in the literature on the question of the applicability of dielectric spectroscopy to the study of ferroelectric nematics and to the parasitic effects arising from the insulating alignment layers in determining the dielectric properties of the nematic ferroelectric (NF) layer. The issues addressed in the manuscript are (i) whether the real values of dielectric permittivity of N_F are lower or higher than the measured ones (ii) whether the real values of dielectric permittivity and the dielectric loss of the materials in the cell with the alignment layers be recovered from the dielectric measurements? We also present an example of the effect of the insulating alignment layers on the dielectric properties of a non-ferroelectric nematic phase.

The electro-optical (EO) switching properties of a rod-shaped liquid crystal (LC) molecule, WJ-16, dipole moment of 10.8 D and superparaelectric (SPE) behaviour in its nematic and smectic-A (SmA) phases.

Binary mixtures developed as thresholdless antiferroelectric (TLAF) materials, MC881-MC815 and MC881-MC452, are studied using optical rotatory power (ORP), resonant soft x-ray scattering (RSoXS), and dynamic light scattering (DLS). A single subphase is confirmed to exist in both mixtures by ORP, stably over a wide temperature range (around 80^{∘}C below RT) but in a narrow concentration range (1-3 wt.%), with curved boundaries; between SmC_{A}^{*} and the subphase, the ratio of synclinic and anticlinic orderings changes continuously. The antiferroelectric four-layer superlattice structure of the subphase is now firmly established by RSoXS using free-standing films, although a two-layer SmC_{A}^{*} structure is observed in silicon nitride membrane cells often used in RSoXS, highlighting the influence of surface conditions. The TLAF properties of the subphase are confirmed in homogeneously aligned cells by observing apparent tilt angles induced by an applied electric field. The relaxation times responsible for the TLAF properties are studied by DLS, indicating the smooth flip-flopping of a group of molecules in a single layer, which must also be responsible for the continuous change among SmC_{A}^{*}, the subphase, and SmC^{*}. Each smectic layer appears to play an important role as a mesoscopic entity.

The discovery of the ferroelectric nematic (N_F) phase has attracted significant attention from the soft matter community, both for exploring the scientific basis of its origin and for advancing its potential applications in devices such as optical modulators and high-energy-density storage supercapacitors. Unlike rigid solid-state ferroelectrics, the fluidic nature of N_F materials offers additional flexibility in device fabrication. In the N_F phase, these materials exhibit remarkably large saturated spontaneous polarization (≳6 μC/cm²), which is comparable to that of the solid-state ferroelectrics, and consequently, these materials will exhibit exceptionally strong nonlinear electro-optic effects. In this paper, we report the synthesis of three new N_F phase compounds with an extended temperature range of up to 103 °C. Two of these compounds show a direct Iso-to-N_F phase transition, and in two cases, a monotropic N_F phase was observed below 30 °C. The design and synthesis of the materials are described. The compounds are characterized by analysis of textures using polarizing optical microscopy. X-ray measurements show that the effective length of the molecule is smaller in the N_F phase than in the N/Iso phase seemingly arising from a better orientational ordering of the polar molecules. Dielectric measurements confirm the presence of a ferroelectric mode of large dielectric strength in the N_F phase. Temperature-dependent birefringence data show a large increase in the order parameter at the Iso–N_F or N–N_F transition temperature. The fundamental properties determined for these materials will support their future applications.

Binary mixtures of ferroelectric nematic DIO and the superparaelectric compound WJ-16. Mixtures display the usual ferroelectric and new hyper-dielectric mode. Both phases exhibit giant dielectric permittivity.

Printed nanosheet networks are important for various applications in electronics, sensing, and energy storage. Understanding charge transport in such systems requires separately assessing the contributions of both the nanosheets themselves as well as the inter‐nanosheet junctions. However, achieving this using standard electrical characterization methods can be challenging. Here, a broadly applicable method is presented for the separation and extraction of the temperature‐dependent junction resistance ( R _J ) and nanosheet resistivity ( ρ _NS ) by combining a simple theoretical model with temperature‐dependent resistivity measurements on networks fabricated using different nanosheet sizes. R _J is found to be the bottleneck for transport in networks composed of large, thick nanosheets, whereas ρ _NS begins to dominate in networks composed of smaller, thinner nanosheets. The extracted ρ _NS shows weak temperature dependence consistent with semiconducting behavior arising from a mixture of Bernal and rhombohedral layer stacking. R _J exhibits a power–law dependence which is best described by inter‐sheet hopping via delocalized states just above a disorder‐induced mobility edge. This approach enables simultaneous quantification of junction and nanosheet transport in any nanomaterial network. It provides a simple yet powerful method to achieve an extensive understanding of the transport mechanisms, facilitating the design of optimized printed electronic devices.

Liquid crystalline (LC) materials integrating polar order and mirror symmetry-broken helical superstructures are of growing interest for advanced applications. We report a new series of achiral bent-core LCs based on 4-cyanoresorcinol bis-terephthalate cores with end-chains ranging from OC₂H₅ to OC₂₀H₂₃. For chains ≥OC₆H₁₃, all compounds form smectic phases which exhibit a paraelectric-to-(anti)ferroelectric transition with Curie–Weiss type divergence, accompanied by an onset of molecular tilt leading to a SmA–SmC phase transition. Increasing chain length induces a tilt correlation crossover from anticlinic (alternating, SmC_a) to synclinic (uniform, SmC_s) ordering. Notably, a stable heliconical smectic phase with a helical axis perpendicular to the polar layers (Sm(CP)^hel) emerges near this crossover. Application of an alternating electric field further expands the Sm(CP)^hel stability range, replacing the SmC_s phase and vanishing near SmC_a. The helical superstructure – spanning C₁₀ to C₂₀ chains and a temperature range up to 80 K – is attributed to synergistic effects of the polar cyano apex, weak molecular bending, and transient helicity. These transversely polarized heliconical phases complement the recently reported longitudinally polarized analogues, offering new pathways for designing chiral LCs from achiral molecules.

Binary mixtures of the ferronematic compound DIO with recently reported non-ferroelectric material WJ-16 which shows Colossal Permittivity (CP) ~5000 and superparaelectricity (SPE) were studied by POM, electrical switching studies, and dielectric spectroscopy. Three mixtures with different contents of WJ-16 as 10, 25 and 50% in DIO as host were prepared. Our original expectation was the development of new nematic materials with both ferroelectric nematic (NF) and non-ferroelectric CP phases. The non-ferroelectric phase in mixtures exhibits a CP mode originally observed in pure WJ-16 and was termed as superparaelectric. However, the dielectric spectroscopy of mixtures shows two distinct relaxation processes: the typical paraelectric response and the CP mode. Therefore, this CP mode cannot be called superparaelectric and is redefined it as Hyper Dielectric mode. This is the first direct demonstration of materials with both ferroelectric and Hyper-dielectric phases in liquid crystalline materials. The hyper dielectric phase has a good potential as a working media for supercapacitors industry.

The growing demand for advanced photonic and electro-optical devices necessitates the rational design of novel functional materials. Liquid crystals (LCs) are particularly promising due to their highly tunable electro-optical properties. Building on this potential, we synthesized a series of polar bent-core LCs, F4-na (dipole moment ∼9.4 D), featuring a tetrafluorinated terminal motif and varying terminal chains. Distinct structure–property relationships are observed in this series of compounds, with the shorter chain homologues forming polar cybotactic clusters (N_cyb phase) alongside nematic and tilted smectic phases. Dielectric spectroscopy reveals non-trivial dipolar ordering, attributed to short-range polar order within cybotactic clusters, notably present without net macroscopic polarization. Under an AC field, the materials form electroconvection patterns, suggesting potential for optical modulation devices. Furthermore, the F4-na materials, particularly the lower homologues showing cybotactic clusters, stabilize the otherwise unstable blue phase (BP) at room temperature when doped with a chiral additive, achieving a maximum BP range of 22.9 °C. This overcomes the challenges in achieving room-temperature BP with our easily synthesizable materials, holding strong potential for 3D photonic applications. Overall, our findings offer promising opportunities for advancing room-temperature photonic and electro-optical devices while enhancing the understanding of self-assembly in soft functional materials.

Printed networks of nanoparticles (e.g., nanodots, nanowires, nanosheets) are important for a range of electronic, sensing and energy storage applications. Characterizing the temperature dependence of both the nanoparticle resistivity (ρ_NW) and interparticle junction resistance (R _J) in such networks is crucial for understanding the conduction mechanism and so for optimizing network properties. However, it is challenging to extract both ρ_NW and R _J from standard electrical measurements. Here, using silver nanowires (AgNWs) as a model system, we describe a broadly applicable method to extract both parameters from resistivity measurements on nanowire networks. We achieve this by combining a simple theoretical model with temperature-dependent resistivity measurements on sets of networks fabricated from nanowires of different lengths. As expected, our results demonstrate that R _J is the predominant bottleneck for charge transport within these networks, with R _NW/R _J in the range 0.03-0.7. We demonstrate that the temperature dependence of ρ_NW exhibits characteristic Bloch-Grüneisen behavior, yielding a Debye temperature between 133-181 K, which aligns with reported values for individual nanowires. Likewise, our findings for residual resistivity and electron-phonon coupling constants closely match published values measured on individual nanowires. The junction resistance also follows Bloch-Grüneisen behavior with similar parameters, indicating the junctions consist of metallic silver. These findings confirm the validity of our method and provide a deeper insight into the conduction mechanisms in AgNW networks. They also pave the way toward simultaneous measurement of ρ_NW and R _J in other important systems, notably carbon nanotube networks.