Department of Physics
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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 Formation of H-type liquid crystal dimer at air-water interface(AIP, 2014) Manjuladevi, V.; Gupta, Raj KumarWe have formed the Langmuir monolayer of H-shaped Azo linked liquid crystal dimer molecule at the air-water interface. Isocycles of the molecule showed hysteresis suggesting the ir-reversible nature of the monolayer formed. The thin film deposited on the silicon wafer was characterized using Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM). The images showed uniform domains of the dimer molecule. We propose that these molecules tend to take book shelf configuration in the liquid phase.Item META Ion trapping in nematic liquid crystal by incorporating functionalised silver nanoparticles. P. 4(INIS, 2019-07) Manjuladevi, V.; Gupta, Raj KumarIn this work, the effect of functionalised silver nanoparticles (f-AgNPs) on the dielectric constant, electrical conductivity and birefringence of a rod shaped NLC 4-trans-pentyl cyclohexylcyanobenzene (5PCH) has been investigated in the planar cell as a function of temperature. The magnitude of dielectric anisotropy, elastic constants and birefringence in nanocomposites of 5PCH were enhanced with increasing concentration of f-AgNPs indicating enhancement of order parameter in the nematic medium. From the study it is observed that, the conductivity has reduced in f-AgNps nanocomposites of 5PCH compared to pure 5PCH due to the absorption of ions by f-AgNPs which is further confirmed by measuring transient current and ion transport number. So our results show that f-AgNPs can be used as ion trapping agent with other improved physical properties suitable for display devicesItem Impact on bulk and interfacial dielectric properties of nematic liquid crystals due to silver nanoparticles(AIP, 2021-09) Manjuladevi, V.; Gupta, Raj KumarDoping of nanomaterials into the liquid crystals (LC) can greatly enhance the physical properties of host LC. Since, it is not possible to obtain all the desired parameters of a display device in a single LC, several researchers are trying to optimize the display parameters by using different nanomaterials of different functionalization. In this work, we report the effect of hexane thiol functionalised silver nanoparticles (f-AgNPs) on the dielectric relaxation and electrical properties of nematic liquid crystal 4-trans-pentyal-cyclohexyl cyanobenzene (5PCH) in the homeotropic alignment. The impedance spectroscopy response of pure and f-AgNPs doped nanocomposites of 5PCH are studied in the frequency range of 20Hz - 12MHz. The electrical properties such as bulk capacitance, electrode resistance, double layer capacitance, bulk resistance and Warburg diffusion coefficients were determined by fitting a suitable equivalent electrical circuit model to the experimental data. Temperature dependence of the dielectric relaxation frequency for pure and f-AgNPs doped nanocomposites of 5PCH were also investigated.Item Langmuir monolayer assisted formation of cadmium sulfide nanoparticles at the air–water interface and their role in the alignment of bulk liquid crystals(RSC, 2016-04) Manjuladevi, V.; Gupta, Raj KumarMetal-sulfide nanoparticles can be fabricated by a one-step synthetic route by forming a Langmuir monolayer of long chain fatty acids over a subphase possessing metal ions and H2S gas in an air medium. Due to an interfacial reaction, the metal-sulfide nanoparticles are fabricated. The one-step synthesis can be further simplified by the appropriate choice of the amphiphilic molecules. The Langmuir monolayer of octadecanethiol (ODT) was reported to be stable over an ultrapure ion-free water subphase and can be perturbed significantly due to the presence of metal ions in the subphase. In this paper, we report the Langmuir monolayer assisted formation of CdS nanoparticles due to the interfacial interaction between the organosulfur compound, ODT and cadmium ions in the aqueous subphase. The films of nanoparticles were deposited on a hydrophilic quartz substrate by the Langmuir–Blodgett (LB) technique. UV-VIS spectra of the LB films of CdS nanoparticles at different target surface pressures and different Cd2+ ion concentrations depicted the signature of CdS nanoparticles. X-ray diffraction (XRD) measurements confirmed the crystalline structure of the CdS nanoparticles to be cubic. The average size of the CdS nanoparticles was estimated using field emission scanning electron microscopy, atomic force microscopy and XRD results and it was found to be in the range of 22–25 nm. We also incorporated CdS nanoparticles in the monolayer matrix of rod shaped liquid crystal (4-n-octyl-cyanobiphenyl, 8CB) molecules at an A/W interface, and transferred the film to solid substrates by the LB technique. Such LB films were employed as the alignment layer during the fabrication of liquid crystal cells and the alignment of bulk liquid crystal molecules in a nematic phase was studied using polarizing optical microscopy. The presence of CdS nanoparticles in the 8CB monolayer matrix reduces the surface anchoring of the liquid crystal molecule and thereby enhances the planar alignment of the bulk liquid crystal molecules.Item Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals(Taylor & Francis, 2019-02) Manjuladevi, V.; Gupta, Raj KumarThe properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir‒Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.Item Effect of octadecylamine-functionalised SWCNTs on the elastic constants and electro-optic response of a liquid crystal(Taylor & Francis, 2015-03) Manjuladevi, V.; Gupta, Raj KumarFor efficient and high-performance liquid crystal (LC) devices, the physical properties of the LC materials can be suitably altered by incorporating nanomaterials like carbon nanotubes (CNTs), graphene and quantum dots. In the present work, the effect of incorporation of octadecylamine-functionalised single-walled carbon nanotubes (ODA-SWCNT) in a LC exhibiting nematic and smectic phases on its physical properties is investigated. The electro-optic threshold voltage in the nematic phase of ODA-SWCNT nanocomposite of LC is enhanced as compared to that of pure LC. The dielectric anisotropy as a function of reduced temperature decreases with an increase in the concentration of ODA-SWCNT. The ratio of bend to splay elastic constants of ODA-SWCNT nanocomposite of LC is enhanced considerably as compared to that of pure LC at low temperature. The electro-optic response rise time decreased and decay time increased at higher concentrations of ODA-SWCNT. The temperature range of smectic phase is increased with an increase in the concentration of ODA-SWCNT. At the lower concentration of ODA-SWCNTs (0.05 wt%), the anchoring of the nanotubes at the LC–electrode interface is favoured. At higher concentration of the ODA-SWCNTs, in addition to the anchoring at the interface, the nanotubes that are dispersed in the LC medium increases the orientational ordering.Item Effect of functionalized carbon nanotube on electro-optic and dielectric properties of a liquid crystal(Elsevier, 2012-07) Gupta, Raj Kumar; Manjuladevi, V.The effect of octadecylamine functionalized carbon nanotube (ODACNT) on the electro-optic response and dielectric measurements of a liquid crystal (LC) were investigated. The threshold electric field of ODACNT doped LC 'less as compared to that of pure LC whereas the dielectric anisotropy is enhanced. The dielectric measurements reveal that the ODACNT doped LC exhibits a relaxation frequency of 30 Hz as compared to 3 Hz for pure LC. The tunneling current images obtained using atomic force microscope in spreading resistance mode indicate presence of ODACNT in close proximity of alignment layer. These results clearly indicate that ODACNT is influencing the nematic anchoring as well as the ordering in the bulk due to π − π interaction between LC, ODACNT and the alignment layer.