Department of Physics
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Item Impact of ZnO nanosphere on ionic conductivity and dielectric relaxation of nematic liquid crystal(Springer, 2025-05) Gupta, Raj Kumar; Manjuladevi, V.Liquid crystal (LC) devices, including displays, switchable optical components, and sensors, are widely used today. The fundamental physical properties of LCs can be controlled through external physical fields, allowing for dynamic configuration, and can also be modified by incorporating nanoparticles. Ions present in a minuscule amount can affect the reorientation of liquid crystals through the screening effect at the interfaces. Dielectric relaxation, which impacts switching time through the dipole dispersion phenomenon, gets altered by the incorporation of nanoparticles in the bulk regime. Therefore, understanding the effects of nanoparticle dispersion in liquid crystals is crucial for both existing and emerging technologies that rely on LC materials. Here, we present the dielectric spectroscopy results of ZnO nanosphere-incorporated nematic liquid crystal (E7), using an electric field as an external stimulus. In this study, we have synthesized the ZnO nanospheres and dispersed these ZnO nanospheres in E7 at various concentrations. We found that 0.02 wt.% of ZnO nanosphere dispersed in the E7 mixture exhibits a reduction of ionic conductivity as well as relaxation frequency.Item Conventional and nonconventional materials for Langmuir monolayer and LB film studies(Elsevier, 2025) Gupta, Raj Kumar; Manjuladevi, V.Langmuir monolayer can be considered as a classical two-dimensional system for studying thermodynamics and molecular interactions at the air-water (a/w) interface. In general, the amphiphilic organic molecules exhibiting a proper balance between hydrophobicity and hydrophilicity can yield a stable Langmuir monolayer at the a/w interface. Such monolayers exhibit variety of surface phases which are important not only from fundamental understanding but also for device applications. These monolayers in these phases can be transferred onto solid substrates layer-by-layer in a highly controlled manner using the Langmuir–Blodgett (LB) technique. Several amphiphilic molecules such as fatty acids, cholesterol and derivatives, lipids, liquid crystals, and polymers were widely studied by forming Langmuir monolayer and Langmuir–Blodgett (LB) films. The field of Langmuir monolayer and LB films is not limited by the requirement of amphiphilicity of the molecules, however there are variety of technologically important hydrophobic materials viz. nanomaterials, liquid crystals, and polymers can form stable Langmuir monolayer at the a/w interface and thereby can be deposited through the LB technique for device applications. In this chapter, we will present an extensive review on such conventional materials and non-conventional materials forming Langmuir monolayer and LB films and some of their applications.Item Tuning liquid crystal properties with 0D carbon dots: exploring the impact of functionalization of carbon dots(RSC, 2025) Manjuladevi, V.; Gupta, Raj KumarFunctionalization of nanomaterials is an efficient way to prevent the aggregation of particles and improve the dispersibility in solvents. However, we propose that if unfunctionalized nanoparticles are capable of forming stable dispersions in solvents and miscible in the LC matrix without aggregation or accumulation at interfaces, they could be a better alternative than their functionalized counterparts for improving the physical properties of NLC. In this study, the effect of functionalization of nanomaterials on various physical properties, such as dielectric, electro-optic and conductivity properties, of nematic liquid crystals is investigated. To explore the validity of our hypothesis, we investigated the properties of NLC, 7CB incorporated with carbon dots and octadecylamine-functionalized carbon dots. Dielectric permittivity and elastic constant measurements suggested that quantum dots were rearranged in the nematic matrix in such a way to minimize the free energy of the composite, and functionalization did not significantly affect the global ordering of NLC molecules. We also observed that the conductivity of C-dot composites decreased when compared to pure NLC but increased with the dispersion of ODA C-dot in NLC compared to pure NLC. It was observed that the ligand molecules of the functionalized quantum dots did not add to the conductivity of the dispersions but act as a trap for ionic impurities, and the partial release of these impurities upon interactions of the ligand shell with the uniaxial nematic host could be the source for the increased conductivity. This study is expected to impart substantial insights into designing high-performance nanocomposites of LCs for device applications.Item Unusual polar ordering and room-temperature blue phase stabilization in tetrafluorinated bent-shaped mesogens(RSC, 2025) Manjuladevi, V.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 (Ncyb 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.Item Facile hydrothermal synthesis of α-MnO2 nanorods for low-cost, scalable and stable photoresponsive devices(RSC, 2025-09) Gupta, Raj Kumar; Manjuladevi, V.Morphologically well-defined α-MnO2 nanorods were synthesized through a facile one-pot hydrothermal method. The structural, morphological and electronic characteristics were systematically investigated using XRD, XPS, FESEM, FTIR, UV-vis and ultraviolet photoelectron spectroscopy. Analysis confirmed the formation of pure α-phase MnO2, exhibiting n-type semiconducting behavior with an indirect bandgap of ∼1.24 eV (from Tauc analysis) and a work function of 4.53 eV, as determined from UPS. XPS confirms the coexistence of Mn3+, along with Mn4+, in the synthesized MnO2. To evaluate the optoelectronic properties, a simple photoresponsive device was fabricated with an in-plane geometry, where the ITO substrate was patterned via a straightforward etching process to define lateral electrodes, followed by drop-casting α-MnO2 nanorods across the active region. The device exhibited a distinct photoresponse under varying illumination conditions (dark, red, and green lasers at different intensities). Under 532 nm green laser excitation, the photocurrent increased by ∼34%, attributed to enhanced charge carrier separation and electron–hole recombination. The fabricated device demonstrated robust stability over repeated measurement cycles, with response and recovery times of 76.5 s and 77.5 s, respectively, at room temperature. A maximum responsivity of 8.66 mA W−1 at 4 V bias was achieved under 17.2 mW cm−2 green laser illumination, along with an external quantum efficiency (ηEQE) of 2.018% at room temperature. The device shows superior performance at elevated temperatures, demonstrating a response time of ∼12 seconds. At 160 °C temperature, the device shows a responsivity of 240.29 mA W−1 at 4 V bias and an ηEQE of 56.2%, highlighting its applications in next-generation optoelectronic devices. Temperature-dependent measurements confirmed the role of thermally activated carrier transport, revealing enhanced photocurrent at elevated temperatures due to increased carrier mobility. This study establishes α-MnO2 nanorods as a promising platform for cost-effective and stable photoresponsive devices, highlighting the role of band alignment and temperature-activated dynamics in advancing next-generation MnO2-based photodetectors and energy-harvesting systems.Item Ultrathin langmuir-blodgett film of functionalized single-walled carbon nanotubes for enhanced acetone sensing(Elsevier, 2024-05) Manjuladevi, V.; Gupta, Raj KumarAcetone is one of the important volatile organic compounds which needs to be detected from the industrial effluent and as a bio-marker from breath of diabetic patients. Here, ultrathin Langmuir-Blodgett (LB) film of octadecylamine- functionalized single-walled carbon nanotubes (ODA-CNT) was formed on interdigitated gold electrodes and was employed for acetone vapor sensing using the impedance spectroscopy (IS) at room temperature. The LB film deposited in liquid-like phase of the Langmuir monolayer of ODA-CNT revealed aligned nanotubes. Being a reducing agent, acetone interacts readily with the CNTs through the transfer of electrons. Using IS, multiparameter were measured with respect to both frequency and acetone vapor concentration. Principal component analysis (PCA) of the IS data revealed capacitance as the most suitable parameter for acetone vapor sensing using the ODA-CNT film. A very low limit of detection (0.5 ppm) and a wide detectable concentration range (1–300 ppm) for acetone sensing using the LB film of ODA-CNT was obtained. The 2D calibration map revealed that the sensing performance of acetone using LB film was much better as compared to that of drop-cast film of ODA-CNT and LB film of pristine CNTs. The superior sensing performance of the LB film of ODA-CNT is attributed to the uniform and aligned nature of the nanotubes, leading to a coherent behavior due to interaction with the acetone molecules. This study shows the potential of LB films of ODA-CNT for sensitive and low level detection of acetone vapor at room temperature.Item Optical anisotropy and surface phases of cholesterol derivative monolayer at air–water interface(Elsevier, 2024-11) Manjuladevi, V.; Gupta, Raj KumarCholesterol and its derivatives play crucial roles in regulating processes of biological membranes. Langmuir monolayer can mimic a bio-membrane which can be used to investigate the molecular interaction governing the important physical phenomena. The cholesterol derivative exhibiting mesophases (CHLC molecules) was synthesized and spread at the air–water (A/W) interface to investigate the surface behavior. The monolayer exhibited a variety of surface phases such as gas, liquid expanded (LE), low density liquid like (L1) and liquid condensed (L2) phases. Treating the CHLC molecules to be rod-like, the average tilt of the molecules with respect to the surface normal in these phases are found to be different. The tilt angle decreases systematically from LE to L2 phase. The optical anisotropy of the ultrathin Langmuir-Blodgett (LB) films of CHLC molecules in these phases was measured using the surface plasmon resonance (SPR) spectroscopy. The high tilted molecules in the ultrathin LB film displayed a high value of optical anisotropy. The ultrathin film of CHLC molecules at different interfaces was investigated using Brewster angle microscopy, X-ray reflectivity (XRR), SPR spectroscopy and atomic force microscopy. This study is useful for the systems where the physical phenomena are governed by tilt of the molecules.Item Alignment Of Liquid Crystalson Lb Films Of Stearic Acid(CIB Tech, 2012) Gupta, Raj Kumar; Manjuladevi, V.We formed the Langmuir monolayer of stearic acid at the air-water (A-W) interface. The Langmuir-Blodgett (LB) films were deposited on solid substrates at different surface pressures. The liquid crystal cells were prepared over such LB films. We found that these cells exhibit a tendency to align in planar orientation for the liquid crystal. We found that the liquid crystal alignment improves in cells prepared with substrates deposited at higher surface pressure like 25mN/m. We present the results on the liquid crystal alignment as a function of number of layers of LB films deposited on the substrates at various surface pressures. We propose that LB films of stearic acid can be used as an aligning agent for liquid crystals.Item Surface Plasmon Resonance Study Of Langmuir-blodgett Films Of Stearic Acid(CIB Tech, 2012) Manjuladevi, V.; Gupta, Raj KumarStearic acid is one of the commonly studied fatty acid. It exhibits interesting behavior on surfaces. The Langmuir Blodgett films of stearic acid were fabricated on solid substrates. We have developed a surface plasmon resonance (SPR) instrument in the Kretschmann configuration. The change in SPR angle due to molecular specific interaction is potentially employed for sensing application. In this paper we report our studies on SPR of stearic acid deposited on solid substrates. We find a significant change in the resonance angle.Item 14 Ultrathin films of nanomaterials: a lyotropic liquid crystalline system and its sensing application(De Gruyter, 2021) Gupta, Raj Kumar; Manjuladevi, V.The ultrathin films of nanomaterials provide remarkable features that can be potentially employed for efficient device fabrication. In this chapter, an overview of ultrathin films of nanomaterials at different interfaces, along with their sensing capabilities, is discussed. From the symmetry point of view, the ultrathin films and sensing applications of the nanoparticles with spherical geometry (eg amphiphilic gold and TiO2 nanoparticle), one-dimensional system (single-walled carbon nanotube) and two-dimensional systems (functionalized graphene) are highlighted.