Study on Dielectric and Optical Properties of ZnO Doped Nematic Liquid Crystal in Low Frequency Region

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1 2 Study on Dielectric and Optical Properties of ZnO Doped Nematic Liquid Crystal in Low Frequency Region Pankaj Kumar Tripathi, M.Sc, Abhishek Kr Misra, Ph.D, Kamal Kr Pandey, Ph.D and Rajiv Manohar, 1 Liquid Crystal Research Lab, Department of Physics, University of Lucknow, Lucknow- 2267, India Abstract In the present paper, we have examined the effect of nano particle and ionic contribution on the dielectric and optical properties for pure nematic liquid crystal (NLC) and nano particle doped NLC system. Detailed studies of the dielectric parameters (dielectric permittivity and tangent loss) as a function of frequency (1Hz to 1 KHz) with temperature range 25 C - 4 C) were carried out. It was found that as ZnO concentration increases in NLC, the value of dielectric permittivity decreases. The optical transmittance and activation energy have also measured in the nematic phase. Measurement of optical transmittance as a function of temperature and bias voltage suggests that the compound have an optically anisotropic nature. The optical textures have also been observed from cooling phase to room temperature as seen by polarizing microscope. Keywords Dielectric permittivity, Liquid crystals, Nanoparticle, Optical transmittance. N I. INTRODUCTION EMATIC liquid crystals (NLCs) are known for more than a century, and their applications are omnipresent in our daily life. With technological advances, the liquid crystal display (LCD) is now a day s expects to achieve high electro-optical performance as well as to be of low power consumption [1]. Instead of synthesizing a number of high quality LC compounds as constituents for a suitable 1Author to correspond: Dr. Rajiv Manohar Associate Professor Liquid Crystal Research Lab Physics Department, University of Lucknow Lucknow, India rajiv.manohar@gmail.com Mob mixture used in a display, doping nano materials into LCs for post synthesis optimization is a potentially cost effective way to attain the goal. The doping of nano particle into NLC has emerged as a fascinating area of research to the application point of view. LC dispersions containing various types of nano particles have been developed in the recent years. These nano dopant materials investigated include metallic nano particles [1, 2], semiconducting nano particles [3 5], ferroelectric nano particles [6, 7], carbon-related nano particles [8-1] and the other like [11, 12]. Each type of these nano particles has its own effect on alteration of the LC material properties. It has been established that the impurity ions in LC systems strongly impact the device performance, reducing the LCD quality particularly at higher temperatures. In order to overcome the ion-induced problems such as the reduction in dielectric permittivity, increase in threshold voltage, image sticking, grey-level shift, image flicker and the slowdown of response, this study concentrates on several essential ionic properties and intends to provide a promising solution of the ion-effect issue. In recent years, ZnOnano particles have attracted considerable attention owing to their potential in materials and device applications [5]. Our groups have observed the increasing threshold voltage, splay elastic constant and rotational viscosity of NLC display cell for nano doped NLC system in recent time [5]. The other group has also reported that the dispersion characteristics of CNT doped ferroelectric liquid crystal system for low and high concentrations of SWCNTs at different temperatures. It suggested that TiO2 nano particles can effectively reduce the threshold voltage [13]. Both these previous studies claim that the of nano particle and ion effect play an important role in the behavior of NLC as functional electronic materials. Low-frequency dielectric spectroscopy can be used to explicate the ion transport behaviour. In the present paper, on effort has been made to study the effect of ZnOnano particle on the dielectric 2

2 21 studies of nematic liquid crystal in the low frequency range 1Hz-1KHz. The effect of bias voltage on dielectric permittivity and optical transmittance is also a subject of present research. II. EXPERIMENTAL METHODS The NLC material used in the present study is p-methoxybenzylidene p-ethylaniline (MBEA), having phase sequence is Nematic (28 C) Isotropic (57 C). The MBEA has the length ~2Å and width ~5Å. This feature makes MBEA very interesting for fundamental investigation on NLC. The chemical structure of the MBEA is shown in the figure 1. Optical textures have been taken for pure NLC and nano particle doped NLC by polarizing microscope as shown in figure 2. The Zinc oxide nano particle doped sample of MBEA was prepared by dispersion of Zinc oxide nano particles (NPs) in.5% wt/wt and 1.% wt/wt concentration in the pure NLC sample and termed as nano particle doped NLC system. ZnOnano particle is one of the most important wurtzite crystals, exhibiting one of the simplest uniaxial structures. The ideal wurtzite structure of ZnOnano particle is shown in figure 3(a). Fig. 1.Molecular structure of used nematic liquid crystals (MBEA). Fig. 2.Texture of planar aligned pure and nano particle doped nematic liquid crystal figure (a, b, c) shows nematic phase (4 C). Fig. 3.(a)Wurtzite structure of used ZnOnano particle. (b) X-ray diffraction spectra of ZnOnano crystals and Inset show the corresponding TEM (Transmission Electron Microscopy) image. The nano particles used in present study were synthesized as follows- An appropriate amount of zinc acetate di-hydrate (analytical reagent, AR) was dissolved in methanol (1 ml) while stirring for two hours at room temperature (Solution A). At the same time, potassium hydroxide (AR) (14 mmol) solution was prepared in methanol (1 ml) while refluxing through water condenser for two hours at 5 C (Solution B). Both the solutions were then mixed and refluxed through water condenser with constant stirring for two hours. Thus obtained solution was maintained at ph 1 by adding essential amount of ammonium hydroxide solution, and aged overnight at room temperature. The product obtained by centrifuging and repeated washing was placed in a vacuum oven for 24 hours at 5 C yielding white ZnOnano particles. X-ray diffraction (XRD) was performed on Rigaku D/max-22 PC diffractometer using CuK 1 (=1.54Å) radiation. The Tecnai G 2 3 S-Twin electron microscope (3kV) was used to record the transmission electron microscopy (TEM) images. Figure 3(b) shows the X-ray diffraction (XRD) spectra of the synthesized ZnOnano crystals. Broad peaks at the positions of 31.63, 34.5, 36.25º, 47.5, 56.6, 62.8, 66.36, and were observed. These peaks are in good agreement with the standard JCPDS file for ZnO (JCPDS number , a = b = Å, c = Å) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P6 3mc. The Scherer s formula shows average crystallite size of ~3 nm with indication of amorphous character which is obvious due to such small size (~3 nm) of these nano crystals. The results relating to size and crystallinity are well supported by TEM measurements.as is evident from the inset of figure 3(b), spherical particles with diameters from 2 nm ~5 nm were observed. The dielectric study of nano particle doped NLC were conducted on planar geometry. The sandwiched type (capacitor) cells were made using two optically flat glass substrates coated with Indium tin oxide (ITO) layers. To obtain planar alignment the conducting layer were treated 21

3 22 with the adhesion promoter and coated with polymer nylon (6/6). After drying the polymer layer, substrates were rubbed unidirectionally. The substrates were then placed one over another to form a capacitor. The cell thickness was fixed by placing a Mylar spacer (6m in our case) in between the plates and then sealed with UV sealant. The empty sample cells were calibrated using analytical reagent (AR) grade CCl 4 (Carbon tetrachloride) and C 6 H 6 (Benzene) as standard references for dielectric study. The assembled cells were filled with the pure and doped NLC at temperature slightly higher than the isotropic temperature by means of capillary method and then cooled gradually up to room temperature. This ensures the uniform distribution of NPs in the NLC. The alignment of the sample was checked by the polarizing microscope under the crossed polarizer-analyzer arrangement. Fig. 4. Experimental setup for the optical transmittance measurement of planar aligned nematic liquid crystal. The dielectric measurements have been performed by a computer controlled Impedance/Gain Analyzer (Solartron SI 126) attached with a temperature controller in the frequency range 1Hz to 1Hz. The dielectric measurements have been carried out as a function of temperature by placing the sample on a computer controlled hot plate INSTEC (HCS-32). The temperature stability was better than ±.1 C. The experimental set up for the optical transmittance measurement of planar aligned NLC as shown in figure 4. For the optical transmittance measurement sample holder is placed between two-crossed polarizer of polarizing microscope model CENSICO (7626) fitted with a hot stage and light intensity coming through eyepiece has been measured by light dependent resistance (LDR). The resistance value of LDR corresponding to varying light intensity due to temperature variation of the sample is proportional to the inverse of optical transmittance and has been directly measured by attached digital multimeter. The % and 1% optical transmittance have also been measured for empty and black ink filled sample holder to calculate the percentage optical transmittance. III. RESULTS AND DISCUSSION The dielectric studies can be described in terms of the complex dielectric permittivity as given by [5], ε', j", ε* Where denotes the real part of the complex dielectric permittivity, is the imaginary part and is the angular frequency of applied electric field. In order to characterize the temperature dependence of the observed dielectric ε', ε", relaxations, and by the Debye formula as follows [5] ', ", can be described where is the static dielectric permittivity, is the high frequency limit of dielectric permittivity, and is the relaxation time. The generalization of Debye formulation describes a single dielectric relaxation process. If the dielectric relaxation is exhibiting more than one process, then the relaxation process can be described by the Cole Cole equation as [5], 1 1 j j Here is the distribution parameter for a particular relaxation process, is the electric permittivity of free space and σ is the electric conductivity. In the present investigation, three types of cells were prepared, one of pure NLC material and other two with ZnOnano particle doped NLC material, for comparative study. Figure 5 shows the dielectric dispersion curves, of the pure NLC and ZnOnano particle doped NLC system. The dielectric permittivity of pure NLC and.5% doped ZnOnano particle NLC systems have decreased with rise frequency. But 1% ZnOnano particle NLC system, the permittivity is constant for this frequency range (1Hz-1Hz). The dielectric permittivity of pure NLC system have obtained to be very high for low frequency region, while the dielectric permittivity obtained for ZnO doped NLC system is low. In higher concentration (1% ZnO) the dielectric permittivity is half times as compared to low concentration (.5% ZnO) as very low frequency region (1Hz) as shown in figure. The dielectric permittivity of the liquid crystals medium itself, which depends on the induced dipole moment and orientation polarization contribution of the molecules can be considered to be essentially frequency independent in range & 22

4 23 (<1KHz) in which the measurement have been made. This medium also has a uniform distribution of ions, which produces a frequency independent conductivity, which in turn to produces a bulk resistance of the medium. The application of electric field generates a non-uniform distribution of ions, which depends on the frequency and in turn contributes both to the real and imaginary part of the dielectric permittivity. It is also noticed that the space charge density is higher at lower frequency in the bulk as well as near the surfaces. With increasing the frequency the space charge density is reduced in the bulk as well as near the surfaces (see in figure). Therefore, at lower frequency the space charge effect is dominant. When nano particles were doped into the NLC system, the orientation of nano particle was such that the dipole moment of nano particle is opposing the dipole moment of NLC molecule and that s why the dielectric permittivity decreases for nano doped systems. In low frequency region, when the electric field is applied on the NLC and nano particle doped NLC system, the ionic polarizations are induced. Therefore, the net dipole moment of nano particle doped NLC system is decreased as compared to pure NLC system. The figure 6 shows the behavior of tan (loss factor) for the planar aligned pure NLC system and ZnOnano particle doped NLC system with variation of frequency which shows a sharp relaxation peak at 5Hz frequency for pure NLC. But dielectric relaxation has not been found for doped NLC system in this frequency range. This low frequency peak is related to the low frequency relaxations of the ions. This low frequency relaxation mode cannot be assigned any relaxation mode due to space charge accumulation near the substrate surface. The occurrence of this relaxation is related to ionic impurities present in the materials. The addition of nano particles (NPs) has improved many characteristic properties of NLC. Jim and Kim studied the low frequency dielectric relaxation of a non-chiral (8CB) NLC material [14]. They performed a numerical simulation and concluded that the ionic impurities contributes to low frequency dielectric relaxation in two separate ways-from fast ions in the single particle diffusion and slow ions in the ionization recombination assisted diffusion. The low frequency mode in pure NLC appeared due to the ionization-recombination assisted diffusion of slow ions in planar aligned configuration. In the nano particle doped NLC system, the LC ions and nano particle ions to get cancel to each other and no ions accumulate to that system. Therefore, relaxation mode does not come in nano particle doped NLC system. Fig. 5. Behavior of dielectric permittivity as a function of frequency for pure and nano particle doped NLC at 35 C. Fig. 7. (a) Behavior of dielectric permittivity ( ) as a function of temperature for pure and nano particle doped NLC system at 2Hz frequency. (b) Behavior of dielectric permittivity ( ) as a function of applied voltage for pure and nano particle doped NLC at 2Hz frequency. Fig. 6. Behavior of dielectric loss factor (tan δ) as a function of frequency for pure and nano particle doped NLC at 35 C. The figure 7(a) shows the relationship between the permittivity and temperature for pure NLC and ZnOnano particle doped NLC system in a 2Hz frequency. The permittivity has increased drastically with increases temperature. We can see from figure that there is very large difference in the permittivity for pure NLC and ZnOnano particle doped NLC systems, whereas the permittivity of.5% ZnO and 1% ZnO doped system have obtained a very little difference, the maximum permittivity for 1% ZnO 23

5 24 doped NLC system is 3.5, occurred at 34 C temperature after this temperature the permittivity decreases for 1% ZnO doped NLC system. It seems to indicate that nano particle are broken the symmetry of NLC molecule, the dipole moment of nano particle are anti-parallel direction of the dipole moment of NLC molecule. Therefore, one can see that the doping of ZnO Nano particle effectively reduced the permittivity of the NLC materials with its large electric dipole moment. At 3 C, the dielectric permittivity for pure NLC and nano particle doped NLC shows a pronounced dependence on the external bias field as shown in figure 7(b). When a bias voltage is applied to the NLC, the director tends to orient along the electric field direction and therefore the permittivity remain constant with the bias field at about 3.75 volt. Above 3.75 volt, the dielectric permittivity suddenly increases for pure NLC and nano particle doped NLC system. Thus the alignment of NLC changes from planar to homeotropic as seen by polarizing microscope. But the used NLC is a negative dielectric anisotropy nature, so the direction of dipole moment of NLC molecule is aligned to the direction of electric field, in which the nematic molecules do slightly change their orientation. In other words we can say that the dielectric permittivity depends upon the frequency, a positive dielectric anisotropy (> ) will be aligned homotropically at low frequency and planar at high frequency [15]. But here only obtained to low frequency region. Therefore, above the 3.75 volt the dielectric permittivity is larger. Thus the electrically induced alignment changes at certain frequency for NLC cell. In order to study the bias field dependence of the dielectric permittivity of the nano particle doped NLC system, significantly higher bias fields are necessary. The relaxation time for pure NLC and nano particle doped NLC have been calculated by using this equation as follows [15] 1 tan.2f The time scale is discussed in terms of the Arrhenius plot, the relaxation time τ is plotted against the inverse temperature [15] as shown in figure 8(a). For liquid crystal devices the relaxation time is extreme importance, which further depends on the cell gap, the degree of molecular alignment and many other parameters. Figure 8(a) shows the temperature dependence of the relaxation time, namely Arrhenius plot, for pure NLC and nano particle doped NLC system. When a high voltage is applied to the sample cell it effects relaxation time [15] and all these properties depends on the activation energy, which play a crucial role in many applications. The temperature dependence of the relaxation time in the nematic phase follows Arrhenius law that is given by [15] E A exp KBT where E A is the activation energy, is the pre-exponential factor, k B is Boltzmann s constant and T is the absolute temperature. The activation energy is determined by fitting the Arrhenius plot in line with the Arrhenius law, the value of E A in the isotropic and nematic phase are consistent with those reported in other literatures [16-18]. The activation energy for pure NLC is.69 mev, which is quite to remarkable meaning that activation energy found for this NLC is much higher as compared nano particle doped NLC system, while in 5CB NLC activation energy has been as less than 1meV [19]. Fig. 8.(a) Variation of relaxation time with temperature for pure and nano particle doped NLC system at 2Hz frequency (b) Variation of activation energy with concentration of nano particle doped in NLC system. Fig. 9.(a) Temperature dependence of optical transmittance for pure and nano particle doped NLC system, (b) Variation of optical transmittance with applied voltage for pure and nano particle doped NLC system. Figure 8(b) shows the variation of activation energy with concentration of nano particle in NLC. The activation energy is connected with the reorientation of the molecule round an axis perpendicular to the director. In this case pure nematic system, the potential barrier shape is symmetrical. On the other hand, in the nano particle doped NLC system, the potential barrier is asymmetrical according to preferential direction. This can be interpreted by the decreases in activation energy in nano particle doped NLC system. These results have been suggested that addition interaction appear for the nano particle doped NLC system. 24

6 25 The interaction of nano particle created the reorientation motion of NLC molecules. Therefore the activation energy decreases for nano particle doped NLC system. Figure 9(a) shows the temperature dependence of the optical transmittance for pure NLC and nano particle doped NLC system. The optical transmittance remains almost constant with increasing the temperature for pure and nano particle doped NLC system. At a particular temperature the value of optical transmittance has decreased, it is show that the nematic phase changes to isotropic phase. The optical transmittance has also increased with addition of nano particle in LC. Admittedly, the temperature dependence of the optical transmittance through nano particle doped NLC is a complex problem. Laser induced change in the temperature and density of the liquid crystals (in nematic phase) may produce its refractive index. Liquid crystals are anisotropic and optically non linear materials; their physical properties are easily changed for pure NLC and nano particle doped NLC system. Often large change in density and temperature of liquid crystal may give rise to flows and liquid crystal molecule director axis reorientation. All of these effects may contribute to change in the refractive index of the material. Figure 9(b) shows that the optical transmittance varying with applied voltage (d.c.volt). The optical transmittance intensity increases with rise voltage, but less than threshold voltage the optical transmittance intensity is remains constant, when above the threshold voltage is apply the optical intensity increases at increasing the bias voltage, but after a particular voltage the optical intensity is constant. This particular voltage is knows as transition voltage. At transition voltage the one can see that the achievable alignment change from planar to homeotropic for pure and nano particle doped NLC cell. The doping of nano particles gave rise to long-range inter particle interactions of NLC molecules, surrounded the ZnOnano particles. This interaction is found to be dependent on the orientation and the local ordering of the NLC molecules with respect to the nano particles. It has been observed that a ZnOnanocrystal can interact with surrounding NLC dipolar molecules and tie them together to respond to an external driving field in more unison [2]. The origin of permanent dipole moment is based on its structure to some extent. The ideal wurtzite structure never exists in which each tetrahedron has T d symmetry, but, in a real wurtzite compound AB, a slight displacement of the A and B sublattices along the hexagonal c-axis occurs. The c/a ratio [which is defined as the ratio of magnitude of the third axis (c) to the axis lying in the basal plane (a), where a and c are the lattice parameters] should be whereas in case of ZnO it is [21]. Thus, the presence of a permanent dipole moment in real wurtzite, e.g., ZnO, can be attributed to C 3v distortion of the elementary AB 4 tetrahedron. Shim and Guyot-Sionnest proposed that a major contribution for the possible origins of the large dipole moments includes internal bonding geometry; shape asymmetry, surface strain, and the surface localized charges [22].The ZnOnano particle with diameter 2~5 nm possess dipole moment >1 D which is much larger than that of a NLC molecule (~1.5 D). This large value of dipole moment on ZnOnano particles interacts strongly with dipolar species present in the NLC mixture. This dipolar interaction enhances the anchoring and hence the ordering of NLC molecules which surround the ZnOnano particles. This enhanced ordering of NLC molecules has been resulted in the form of improved optical transmittance of ZnOnano particles doped MBEA material. The magnitude of the electrical torque experienced by ZnOnano particles is larger due to their higher dipole moments and hence the NLC molecules coupled with these ZnOnano particles could be switched by the application of lower value of applied electric field. IV. CONCLUSIONS In summary, doping a NLC with a small amount of ZnOnano particles strongly affects the dielectric properties of the system. In particular, the ZnOnano particles result in a decrease the dielectric permittivity of NLC molecules in low frequencies region, and also decrease the amplitude of loss factor for nano particle doped NLC system and not found to any relaxation mode in doped system. This suggested that the dipole moment of nano particles do not support the dipole moment of NLC molecule and interrupt the nematic ordering. The nano particles also affect the activation energy, optical transmittance and threshold voltage for doped system. The activation energy have decreased due to potential barrier is asymmetrical about to reorientation motion of NLC molecules. ACKNOWLEDGMENTS V. ACKNOWLEDGMENTS The authors are sincerely thankful to ISRO, for providing assistance in RESPOND program in the form of project entitled Designing of SLMs based on nematic liquid crystals doped with non-mesogenic molecules. One of the authors (AKM) is thankful to UGC, New Delhi for the grant of Dr. D.S. Kothari Post-DoctoralFellowship No.F.4.2/26 (BSR) /28(BSR). REFERENCES [1] ShiraishiY, ToshimaN, MaedaK, YoshikawaH, XuJand KobayashiS.Frequency modulation response of a liquid-crystal electro-optic device doped with nanoparticles.appl. Phys. Lett.22; 81:2845. [2] KobayashiS, MiyamaT, NishidaN, SakaiY, ShirakiH, ShiraishiY and Toshima N. 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7 26 Nematic Liquid Crystal: Dielectric and Electro-Optical Study.Jpn. J. Appl. Phys.29; 48: [6] Reznikov Y, BuchnevO, TereshchenkoO, ReshetnyakV, GlushchenkoA and WestJ. Ferroelectric nematic suspension.appl. Phys. Lett.23; 82: [7] LiF, BuchnevO, CheonCI, GlushchenkoA, ReshetnyakV, ReznikovY, SluckinTJ and WestJL. Orientational Coupling Amplification in Ferroelectric Nematic Colloids. Phys. Rev. Lett.26; 97: , [8] LeeW, WangCY and ShihYC.Effects of carbon nanosolids on the electro-optical properties of a twisted nematic liquid-crystal host.appl. Phys. Lett. 24; 85: [9] ChenPS, HuangCC, Liu YW and ChaoCY. Effect of insulating-nanoparticles addition on ion current and voltage-holding ratio in nematic liquid crystal cells.appl. Phys. Lett. 27; 9: , [1] Lee CW and ShihWP.Quantification of ion trapping effect of carbon nanomaterials in liquid crystal.mater. Lett.21; 64: [11] HirstLS, KirchhoffJ, Inman R and GhoshS.Quantum dot self-assembly in liquid crystal media.proc. SPIE21; 7618:7618F. [12] Kinkead B and HegmannT. Effects of size, capping agent, and concentration of CdSe and CdTe quantum dots doped into a nematic liquid crystal on the optical and electro-optic properties of the final colloidal liquid crystal mixture. J. Mater. Chem. 21; 2: [13] TangCY, Huang SM and LeeW.Electrical properties of nematic liquid crystals doped with anatase TiO 2 nanoparticles.j. Phys. D: Appl. Phys. 211; 44: , [14] Jin MY and KimJJ. Low-frequency dielectric relaxations of a nonchiral liquid crystal, 8CB. J. Phys. Condens. Matt. 21; 13: [15] L. M. Blinov and V. G. Chigrinov, Electrooptical Effects in Liquid Crystal Materials, Springer-Verlag, New York, [16] AliewFM, BengoecheaMR, GaoCY, CochramHDand DaiS.Dielectric relaxation in liquid crystals confined in a quasi-one-dimensional system. J. Non Cryst. Solids. 25; 351: [17] LeysJ.Broadband dielectric spectroooocopy of confined liquid crystals and Hydrogen bonded liquids, PhD dissertation, Katholicke Universiteit Leuven, 27. [18] RozanskiSA, SinhaGP, and ThoenJ. Influence of hydrophilic and hydrophobic aerosol particles on the molecular modes in the liquid crystal 4 n pentyl 4 cyanobiphenyl. Liq. Cryst. 26; 33: [19] KreulHG, UrbanS and WurflingerA.Dielectric studies of liquid crystals under high pressure: Static permittivity and dielectric relaxation in the nematic phase of pentylcyanobiphenyl (5CB).Phys.Rev. A 1992; 45: [2] Li LS and HuangJY.Tailoring switching properties of dipolar species in ferroelectric liquid crystal with ZnO nanoparticles.j. Phys. D: Appl. Phys29; 42: , [21] Nann Tand SchneiderJ.Origin of permanent electric dipole moments in wurtzite nanocrystals.chem. Phys. Lett.24; 384: [22] Shim Mand SionnestP G.Permanent dipole moment and charges in colloidal semiconductor quantum dots.j. Chem. Phys.1999; 111: Liquid Crystal-Nanomaterial composite systems. He has also published many research papers in repute journals describing the properties of liquid crystals and some nanoparticle disperses in LC materials. Abhishek Kumar Misra is a Post-Doctoral Fellow (Dr. D.S.Kothari Post-Doctoral Fellow University Grant Commission, New Delhi India) in Physics Department University of Lucknow, Lucknow India. He obtained his Ph.D. at the University of Lucknow, Lucknow India. His research is on dielectric, electro-optical properties of dye doped liquid crystal. He has published more than 25 articles in international reputed journal. The research paper published in Soft Materials Vol. 5(4), , 27 has got 5th rank in most cited article of this journal in year Dr. Rajiv Manohar obtained his Ph.D. degree in 1999 from University of Lucknow, Lucknow (India). Currently, he is an associate professor in the department of Physics, Lucknow University. His field of interest in research is the study and characterization of the pure and doped liquid crystals. He has published more than 7 international research papers in repute journal. He has also been awarded Young scientist by Indian Science congress and Indian liquid crystal society. He is a life time member of International liquid crystal society, Indian liquid crystal society and Indian science congress. He is also a member of editorial board of some repute international journals. Pankaj Kumar Tripathi is a Senior Research Fellow (ISRO) in the department of Physics, University of Lucknow, Lucknow (India). He completed his B.Sc. and M.Sc. degree from the Dr. R.M.L. (Avadh) university Faizabad, Faizabad, India. Presently, he is a Ph.D student in Liquid Crystal Research Lab, Department of physics, University of Lucknow, Lucknow India since 29 under the supervision of Dr. Rajiv Manohar. His research interest involves the study of 26