Department of Physics
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Item Scanning tunneling microscopy of liquid crystals, perylene‐tetracarboxylic‐dianhydride, and phthalocyanine(AIP, 1994-05) Dey, SrijataScanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene‐tetracarboxylic‐dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the first time.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 Liquid crystals under compression and tension(SPIE, 2004-09) Manjuladevi, V.We report measurements of the temperature dependences of the birefringence of a number of nematogens under high pressures. In a compound whose molecules have large longitudinal dipole moments, we detect a nematic to nematic transition when the order parameter is sufficiently high. We also describe a technique of subjecting aligned liquid crystal droplets to large negative pressures and measuring the temperature dependence of birefringence under isochoric conditions. The temperature range of the partial bilayer smectic Ad phase gets shrunk at elevated pressures but gets enlarged under tension.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 Experimental study for the conditions of analog switching in ferroelectric liquid crystal cells(AIP, 2007-08) Manjuladevi, V.The authors study the effect of the spontaneous polarization of a ferroelectric liquid crystal mixture with compensated helix and of the thickness of the alignment layer on the analog switching in a cell. The quality of analog switching is established in terms of its contrast ratio, texture in the dark state, and the electrostatic energy. The latter approximates to for ⪡ (thickness of the ferroelectric liquid crystal cell layer).Item High pressure studies on a nematogen with highly polar molecules: Evidence for a nematic–nematic transition(Current Science Association, 2003-10) Manjuladevi, V.We have modified a high pressure optical set-up to enable measurements on the optical path difference of aligned liquid crystals. We present the phase diagram of a polar nematogen, viz. p-cyanophenyl p-n heptylbenzoate which is shown to exhibit a nematic (Nd) to nematic (N1) transition in the bulk at elevated pressures. We have also measured the temperature variations of the orientational order parameter (S) at various pressures to find that S has constant values at both the nematic to isotropic and Nd to N1 transitions at different pressuresItem Phase Transitions in Liquid Crystals Under Negative Pressures(APS, 2002-01) Manjuladevi, V.We report the first measurements of orientational order parameters and phase transition temperatures in nematic and smectic A liquid crystals under negative pressures generated by an isochoric cooling of small droplets embedded in a glass former. Comparison of isobaric and isochoric measurements allows us to estimate the coefficients coupling the order parameter and density of an extended Landau–de Gennes model of the nematic phase.