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This article was downloaded by: [University of Lucknow ] On: 07 March 2013, At: 23:29 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20 Abnormal switching behavior of nanoparticle composite systems Pankaj Kr. Tripathi a, Abhishek Kr. Misra a, Kamal Kr. Pandey a, Satya P. Yadav a & Rajiv Manohar a a Liquid Crystal Research Laboratory, Physics Department, University of Lucknow, Lucknow, India Version of record first published: 01 Feb 2013. To cite this article: Pankaj Kr. Tripathi, Abhishek Kr. Misra, Kamal Kr. Pandey, Satya P. Yadav & Rajiv Manohar (2013): Abnormal switching behavior of nanoparticle composite systems, Phase Transitions: A Multinational Journal, DOI:10.1080/01411594.2012.761698 To link to this article: http://dx.doi.org/10.1080/01411594.2012.761698 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-andconditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Phase Transitions, 2013 http://dx.doi.org/10.1080/01411594.2012.761698 Abnormal switching behavior of nanoparticle composite systems Pankaj Kr. Tripathi, Abhishek Kr. Misra, Kamal Kr. Pandey, Satya P. Yadav and Rajiv Manohar* Liquid Crystal Research Laboratory, Physics Department, University of Lucknow, Lucknow, India (Received 24 September 2012; final version received 19 December 2012) We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 khz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells. Keywords: nanoparticle; liquid crystal; dielectric anisotropy; switching; splay elastic constant 1. Introduction Liquid crystals (LCs) are a mesostate between solids and liquids. These share the anisotropic properties of optical (uniaxial and biaxial) crystals and the fluid properties of isotropic liquids [1,2]. These materials are extremely sensitive to the small external factors (electric and magnetic fields, surface effects, temperature, etc.) and possess order and mobility at microscopic and macroscopic levels [3]. The thermotropic LCs are technologically most important among all the LC mesophases, and the nematic mesophase is one of them which have broad applications in many engineering devices. Another important application of nematic LCs is their utilization in holographically formed polymer-dispersed LCs. These switchable diffraction gratings have broad engineering applications: video displays, switchable focus lenses, and photonic time-delay generators for optically assisted phased-array radars [3 6]. The remarkable advances in LC technology have led to the appearance of LC-based spatial light modulators (SLMs) and display applications [7]. The correct performance of all the above-mentioned devices requires LC materials that are stable over a long period of time, have high thermal stability and thermal range, high dielectric and optical anisotropy, and low switching voltage and switching times. These various requirements are achieved by using chemical synthesis and carefully designed mixtures of different LC materials such as dye, polymer, and nanoparticle [8,9]. It is important to note that in optimizing one property usually results in changes in other properties, mostly in an undesirable direction. Therefore, designing right materials for commercial applications is a challenging task. *Corresponding author. Email: rajiv.manohar@gmail.com Ó 2013 Taylor & Francis

2 P.Kr. Tripathi et al. During the past four decades, the doping of nanomaterials in LCs has been extensively studied [9] and this area of research is now completely mature and can be used for device fabrication. The interpretation of measured properties of LCs and proper understanding of LC devices depends on the knowledge of the geometry of LC systems and anchoring energy. The doping of different nanoparticles such as BaTiO 3 nanoparticles, silica nanoparticles, and cadmium telluride quantum dots has been used in recent times for highquality performance of the devices [10 13]. Most of the groups have presented results of doping in ferroelectric LCs (FLCs) especially for display applications [14,15]. They observed enhancement of the E O properties is attributed to the variation of the order parameter and dielectric studies of BaTiO 3 nanoparticles dispersed in a FLC matrix. [14] Gold nanoparticles change the elastic parameter and rotational viscosity of the FLCs. Gold nanoparticle doping creates a strong intrinsic field inside the sample generating high tilt and a reproducible observation of memory effect [16]. Each type of these nanoparticles has its own effect on alternation of the LC material properties. It has been established that the impurity ions in LC systems strongly impact the device performance, reducing the LC display quality particularly at higher temperature. Therefore, in the present paper, we have observed the change in the nature of dielectric anisotropy with changing the electro-optical parameters. We have used ZnO nanoparticles, which provided best alignment of LC molecules in our cells. The switching behavior of LC molecules has been affected due to the presence of nanoparticles in pure LCs. The dielectric and electro-optical parameters have been evaluated for pure and nanoparticle-doped LCs. The temperature dependence of the different dielectric and electro-optical parameters, such as dielectric anisotropy, threshold voltage, splay elastic constant and rotational viscosity, has also been discussed with the help of different theories of LCs. 2. Experimental details 2.1. Materials The investigated LC used in the present study is p-butoxybenzylidene, p-heptylaniline (BBHA). The phase sequence of BBHA is smectic 20 C! nematic 55 C! isotropic 83 C. The chemical structure of BBHA is shown in Figure 1. The use of nematic LCs for photonic applications is still of great interest, and so it can be electrically modulated within this wide working temperature range. The Cu 2þ -doped ZnO nanoparticles have been used in the present study; ZnO (wide band gap) is one of the most important materials for blue and ultraviolet (UV) optical device applications. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O 2 and Zn 2þ ions, heaped alternatively along the c-axis. The tetrahedral coordination in ZnO leads to a non-central symmetric structure, which is one of the most important structural characteristics of wurtzite nanostructured materials. ZnO shows strong electromechanical coupling due to its unique structure, which results in strong piezoelectric and pyroelectric properties. The other important structural characteristic of ZnO is its polar surfaces. ZnO nanoparticles have a normal dipole moment and spontaneous polarization along the c-axis [17]. The synthesis of these copper-doped ZnO nanoparticles has been given by our synthesis group Sharma et al. [17]. Figure 2 shows the X-ray diffraction pattern for the 8W nanoparticle used for doping in LCs. The spectra show broad peaks at the positions of 31.63, 34.50, 36.25, 47.50, 56.60, 62.80, 66.36, 67.92 and 68.91, as shown in Figure 2. The values are in good

Phase Transitions 3 Figure 1. Molecular structure of LC BBHA. agreement with the standard JCPDS file for ZnO (JCPDS number 36-1451, a ¼ b ¼ 3.249 Å, c ¼ 5.206 Å) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P6 3mc [17]. The average particle size of a ZnO:Cu 2þ nanoparticle is 13 nm, estimated by the Debye Scherer equation [17,18]. The uniform nano rod-shaped particles having diameter 1215 nm and length 4080 nm were observed from Scanning Electron Microscopic (SEM) images, shown in Figure 3 [17]. 2.2. Preparation for sample cells The planar as well as homeotropic sample cells have been used in the present study. The sandwiched-type (capacitor) cells were made by using two optically plane glass substrates Figure 2. X-ray diffraction spectra of ZnO for 8% doping of Cu 2þ.

4 P.Kr. Tripathi et al. Figure 3. SEM images of ZnO:Cu 2þ (8% Cu 2þ doped ZnO) nanoparticles. coated with conducting indium tin oxide (ITO) layers. Planar alignment has been achieved by conducting layers with an adhesion promoter and coating with polymer nylon (6/6). After drying the polymer layers, both the substrates were rubbed unidirectionally by velvet cloth [18 22]. The substrates were then placed one over another to form a capacitor. The cell thickness was fixed by placing a Mylar spacer (2.5 mm in our case) in between the two substrates and then it was sealed by a UV sealant. Similarly, for homeotropic alignment, the glass substrates were coated with a dilute solution of lecithin (cetyl trimethyl ammonium bromide). The substrates were dried at 220 C for 10 hours before assembling the cell. Sample cells were calibrated using analytical reagent grade carbon tetrachloride (CCl 4 ) and benzene (C 6 H 6 ) as standard references for dielectric studies. We prepared two mixtures of different concentrations of 8W, i.e., 0.5% and 1.0% wt/wt, in LCs. The nanoparticles were dispersed in pure LCs physically. Each mixture was heated at room temperature from the smectic to isotropic phase and cooled back to room temperature. Then the heating cooling cycle was repeated. The mixture was viewed under a polarizing microscope to assure homogeneous distribution of the nanoparticles. The assembled cells were filled with the suspension and the pure nematic liquid crystal (NLC) at a temperature higher than the isotropic temperature of NLCs by the capillary method [20 22]. Nanoparticle composite systems are prepared by dispersion of 8% copper-doped zinc oxide nanoparticles (8W) in pure BBHA LCs. These samples are hereafter denoted as mixture 1 and mixture 2. 2.3. Dielectric measurement The dielectric measurements have been carried out by using a computer-controlled impedance/gain phase analyzer (HP4194A) in the frequency range 100 Hz to 10 MHz. The measurements in the high frequency range have been limited to 10 MHz because of the dominating effect of the finite sheet resistance of ITO coating on the glass plates and the lead inductance [21 23]. The temperature has been maintained by using a computercontrolled hot plate (Instec Corporation, USA). Experiments were performed by ramping

Phase Transitions 5 Figure 4. (a) Applied arbitrary wave and (b) detected shapes of the waveform in the oscilloscope. the temperature at a very slow heating rate of 0.5 Cmin 1 and temperature stability was better than 0.1 C [23]. 2.4. Electro-optical measurement for the rise and fall time An arbitrary AC signal voltage (20V pp and 1 Hz) was applied to the cells using a function generator. He Ne laser beam with a wavelength 632 nm as the input signal is detected by a new focus photodetector (Instec-PD02LI) connected directly to a digital storage oscilloscope (Tektronix TDS-2024C). From the detected shapes of the waveform in Figure 4, we calculated the rise time (t on ) and fall time (t off ) for pure LCs and nanoparticle-doped LCs. Here, t on is the time required for the transmittance to rise from 10% to 90% and t off is the time required for the transmittance to fall from 90% to 10% [7,23]. The cell is set at angle 45 crossed polarizer and analyzer for ensuring maximum optical transmittance. Thus, the cell works as a phase retarder, thereby altering the polarization of light. When voltage is applied, the LC molecules are aligned in the applied field direction. Again when the voltage is switched off, the LC molecules relax and return to their initial state. 3. Results and discussion Dielectric anisotropy has been calculated by using dielectric data of planar as well as homeotropic alignments. The value of dielectric permittivity for both the alignments has been corrected due to sheet inductances and lead inductances of the cell [20 22]. By least-squares fitting, we have evaluated the corrected dielectric permittivity for both alignments and then the dielectric anisotropy was calculated by [7] using De ¼ e e?, where e and e? represent the dielectric permittivity for the applied electric field E parallel and perpendicular to the macroscopic molecular orientation n, respectively. The extracted dielectric anisotropy for pure LCs and nanoparticle composite systems is plotted as a function of temperature in Figure 5. The value of dielectric anisotropy is enhanced for nanoparticle composite systems. The dielectric anisotropy is generally constant for one phase (smectic) and suddenly decreases for other phase (nematic) for pure LCs [24,25]. Figure 5 shows the little change in magnitude of dielectric anisotropy after doping of nanoparticles in the pure LC. It can be seen that the value of dielectric anisotropy increases with the increase in the temperature for pure LCs in negative order, while its value is in positive order for nanoparticle composite systems. We consider that the pure LC molecule exhibits a continuous symmetry breaking phase transition on varying a

6 P.Kr. Tripathi et al. Figure 5. Dielectric anisotropy of pure LC and ZnO nanoparticle doped composite systems at different temperatures. control parameter. The dielectric anisotropy was found to increase with increasing concentration of nanoparticles in pure LCs. The dielectric anisotropy (De) increases significantly for pure LCs in negative order, but ultimately changes its sign for nanoparticle composite systems. The dielectric anisotropy (De) of LCs mainly depends upon the change of effective dipole moment of LC molecules [26]. The change in the molecular polarizability of nanoparticle composite systems will also change the dielectric anisotropy according to the equation De ¼ e e? ¼ NhF e 0 Da þ m2 F 2K B T ð3cos2 b 1Þ S: ð1þ Here N is the number density, h and F are the internal field factors, e 0 is the permittivity of vacuum, De isthedielectricanisotropyandda is the anisotropy of the polarizability, S is the orientational order parameter, K B T is the thermal energy, and b is the angle between the molecular net permanent dipole moment and the long molecular axis of the molecule. The resultant dipole moment of the LC molecule attained a new orientation with respect to the long molecular axis. Therefore, it can be concluded that the 8W nanoparticle significantly influenced the geometrical orientation of the used BBHA molecule in suspension. The contribution of the electronic polarizability to dielectric permittivity is greater in the direction along the molecular long axis than perpendicular to it. Consequently, De is positive, as there are additional contributions from dipolar relaxation because the ZnO nanoparticle with diameter 12 nm possesses dipole moment > 100 D which is much larger than that of an LC molecule. This large value of dipole moment on ZnO nanoparticles

Phase Transitions 7 interacts strongly with dipolar species present in the LC mixtures. This dipolar interaction enhances the anchoring and hence the ordering of LC molecules which surround the ZnO nanoparticles. For composite systems, the dipole moment of ZnO nanoparticles, which contributes slightly more in the parallel case than in the perpendicular component? For nanoparticle composite systems, a reversal in the sign of the dielectric anisotropy has been seen both in the nematic and in the smectic A (SmA) phases [24]. The change in sign is caused by a decrease in the value of e? with increase in the concentration of nanoparticles in LCs on entering the smectic and nematic phases, whereas e increases anomalously. The decrease of e? in nanoparticle systems results from the smaller distance between the nanoparticle and LC molecules, leading to an increased antiparallel correlation between the components of the dipole moments along the molecular long axis. Consequently, the effective dipole moment in this direction is reduced, causing a decrease in e?. The absolute value of the static dielectric permittivity for LCs with a weak dipole moment is much smaller than for ZnO nanoparticles with a strong dipole moment. The electro-optical properties have been studied on planar alignment for pure LCs and nanoparticle composite systems. The alignment layer greatly affects the electro-optical properties of the LCs [27]. The more confined and uniform alignment quality leads to a better LC device performance. It is known that at an applied external field, the electrooptical response of LCs is related to the surface anchoring energy which depends on the boundary surface energy [7,27]. The alignment of the LC molecule director is determined by the competition among the surface interactions, bulk interactions, visco-elastic properties and the externally applied electric field. As the surface anchoring strength decreases, the LC response time also decreases, and the electro-optical switching becomes faster [7]. The threshold voltage V th of the LC is given by the following equation [7,28]: rffiffiffiffiffiffiffiffiffiffi K 11 V th ¼ p : ð2þ e 0 De Here K 11 is the splay elastic constant. The response time from the OFF state to ON state, t on, and that from the ON state to OFF state, t off, are, respectively, given by the following equations [7,28]: gd 2 t on ¼ e 0 DeðV 2 Vth 2 Þ ð3þ and t off ¼ gd2 p 2 K 11 : ð4þ Here d is the cell thickness and g is the rotational viscosity. All these parameter are proportional to the order parameter of LCs. From this equation, the rise time depends on both applied voltage and threshold voltage. However, the threshold voltage depends on the LC dielectric anisotropy, which is dependent on the applied frequency. Thus, the rise time depends on the applied frequency as well. Therefore, for a cell of fixed thickness and a specific LC, the rise time (or the switch on time) depends mainly on the driving frequency and applied field strengths. Figure 6 shows the variation of the rise time with temperature in the nematic phase for the pure LC and nanoparticle composite systems. The figure indicates that from 60 Cto 71 C the rise time is constant for pure LCs, however, the rise time for mixture 1 and

8 P.Kr. Tripathi et al. Figure 6. systems. Variation of the rise time with temperature for pure LC and nanoparticle composite mixture 2 decreases in this temperature range. The rise time decreases from 72 Cto83 C temperature for all systems. It is also clear that the rise time basically depends upon the viscosity and order parameter. The viscosity and order parameter always decreases with increase in temperature. The rise time is greater for nanoparticle composite systems compared to pure LCs. The rise time of mixture 2 is less than mixture 1, but its value is higher as compared to the pure LC. The reason is that the nanoparticle produces the maximum viscosity and interaction on the surface boundary. So the rise time increases with increase in the concentration of nanoparticles in LCs. The other reason is that the interaction between nanoparticles and LC molecules is weak, but the interaction between LC molecules and the surface electrode is strong. So the anchoring strength is strong for mixture 1, while.the interaction between nanoparticles and LC molecules is strong as compared to the interaction between LC molecules and the surface electrode for mixture 2 Therefore, the anchoring strength of mixture 2 is weak as compared to mixture 1. The response time of LC molecules depends upon the anchoring strength of the sample cell, which in turn means the response of LC molecules depends upon the surface anchoring of the alignment layer. However, nanoparticles also increase the response time. Therefore, the doping of such nanoparticles in this LC is not a perfect requirement in the application of display technology. If anyhow we can develop a process to decrease the rise time of the LC sample, then this process should be useful for display devices. Figure 7 shows the temperature variation of fall time in the nematic phase for the pure LC and nanoparticle composite systems. Generally, the fall time does not depend upon the applied field, but it depends upon the elastic constant and rotational viscosity. If the rise time decreases, then the fall time increases with variation of temperature [7]. The fall time of mixture 1 and mixture 2 is longer as compared to the pure LC, because the nanoparticle opposes the LC molecules to reach their initial position. Therefore, the LC

Phase Transitions 9 Figure 7. Variation of fall time with temperature for pure LC and nanoparticle composite systems. molecule could not easily attain its original position. The rise time, fall time, rotational viscosity, splay elastic coefficient, saturated voltage, threshold voltage and dielectric anisotropy have been evaluated experimentally, as given in Table 1, at 72 C temperature. Threshold voltage increases for mixture 1 and it decreases for mixture 2. Theoretically, the threshold voltage depends upon the magnitude of dielectric anisotropy as shown in Equation (1). The rise time and fall time also have been measured as a function of the applied voltage. Figure 8(a) and (b) show the applied field strength effects for the pure LC cells and nanoparticle-doped cells at a frequency of 1 Hz. The field strength increased from 5 volts to 10 volts. The rise time strongly depends on the applied voltage according to Equation (3). When the applied voltage is just above the threshold voltage for mixture 1, mixture 2 and pure LCs, the rise time is comparatively higher. However, the rise time is reduced effectively just by increasing the applied voltage. The decrement in the rise time found in this study is 33.76 ms, 14.00 ms, and 10.6 ms for mixture 1, mixture 2, and pure LC, respectively, as shown by the dotted line in the figure. At below 7 volts, the rise time is very large for mixture 2 in comparison to the pure LC and mixture 1. The threshold voltage is minimum for mixture 2 as compared to mixture 1 and the pure LC. In this system, the maximum rise time achieved due to presence of nanoparticles, and some LC molecules respond at this voltage. But a higher concentration of nanoparticles disrupts the alignment of LC molecules. Other reason is the surface anchoring of the alignment layer produced due to the interaction of nanoparticles between the substrates. The threshold voltage behavior exists when the pretilt angle is zero. However, in most LC devices a non-zero pretilt angle is required in order to avoid domain formation during molecular reorientation. In nanoparticle composite systems, some LC molecules also tilt

10 P.Kr. Tripathi et al. Table 1. Experimental value of the rise time, fall time, rotational viscosity, splay elastic coefficient, saturated voltage, threshold voltage, and dielectric anisotropy for pure LC and nanoparticle composite systems at 72 C temperature. Sample at 72 C temperature Dielectric anisotropy (De) Thickness (d) Threshold voltage (Vth) (in volts) Saturation voltage (Vsat) (in volts) Rise time (t on ) (in milliseconds) Fall time (t off ) (in milliseconds) Splay elastic coefficient (K 11 ) in order of magnitude (10 11 ) Rotational viscosity (g) (in poise) Pure 2.9069 6 mm 2.13 5.83 23.4 44.4 1.1838 1.428 Mixture 1 þ0.6537 6 mm 4.10 5.10 45.4 144.5 0.9863 3.903 Mixture 2 þ0.5871 6 mm 1.36 3.90 22.4 122.4 0.0974 0.032

Phase Transitions 11 Figure 8. (a) Variation of rise time with applied voltage for pure LC and nanoparticle composite systems. (b) Variation of fall time with applied voltage for pure LC and nanoparticle composite systems. due to the presence of nanoparticles. In addition to this, the nanoparticles also generate a multidomain in the bulk of LC cells. From 7 volts to 10 volts the rise time has small dependence for mixture 1, mixture 2 and the pure LC. Above 7 volts, the nanoparticle aligns in the direction of the electric field and does not easily reach its own position. Therefore, the rise time is reduced for nanoparticle-doped systems. Figure 8(b) shows the fall time with variation of the applied electric field (voltage) for pure LCs and nanoparticle-doped systems. The fall time increases with the applied field for mixture 1 increases suddenly from 6 volts to 8 volts while it changes little for pure LCs. This suggests that the rise time remains minimum at 8 volts and 9 volts, which means the LC molecules show very fast response at that condition and molecules do not immediately attain their initial position. The fall time decreases with the rise in voltage from 5 volts to 7 volts for mixture 2 and after that the fall time saturates with increasing voltage from 7 volts to 10 volts. At this applied field, the nanoparticle opposes the response of LC molecules. They do not attain their initial position. Above 8 volts, the fall time of mixture 2 saturates with the applied field. In this condition, the nanoparticle is fully aligned to the direction of the applied electric field and therefore only LC molecules are responses to it. The response of LC molecules is related to rotational viscosity. The connectivity of the LC cavity network has some effects on the switching times (rise and fall time). In conventional nanoparticle composite systems, fall times typically exceed rise times, since relaxation is driven only by elastic energy, with no electric field. The fall time is proportional to the characteristic size of nanoparticles and LC molecules. This model predicts that isolated, spherical LC droplets should have anomalously long fall times [29 31]. Figure 8(a) and (b) provide a comparison of rise and fall times for various applied electric fields for both the pure LC and the nanoparticle composite systems. For the nanoparticle composite system, the shorter rise versus fall times agree with expectations. In a continuum of this study, we have also determined the rotational viscosity and splay elastic constant for pure LC and nanoparticle composite systems. The splay elastic constant is obtained from Frederick s threshold voltage with the help of Equation (1) [32]. Figures 9 and 10 show the temperature behavior of rotational viscosity and splay elastic constant for the pure LC and nanoparticle composite systems [33,34]. One can conclude from the figures that they would have both changed in the similar way with respect to the nanoparticle concentration and this similar change is proportional to the order

12 P.Kr. Tripathi et al. Figure 9. systems. Temperature behavior of rotational viscosity for the pure LC and nanoparticle composite Figure 10. Temperature behavior of splay elastic constant for the pure LC and nanoparticle composite systems.

Phase Transitions 13 parameter. The splay elastic constant remains constant for all samples. The value of the splay elastic constant of nanoparticle composite systems has been found to be less as compared to the pure LC. The splay elastic constant basically depends upon threshold voltage and dielectric anisotropy. The magnitude of dielectric anisotropy of pure LCs is high as compared to the nanoparticle composite system. In addition to this, the value of splay elastic constant of mixture 2 is less in comparison to mixture 1. The rotational viscosity of the pure LC and nanoparticle composite systems has been determined with the help of Equation (3). The rotational viscosity of aligned LCs represents an internal friction among LC directors during the rotation process. The magnitude of rotational viscosity depends on the detailed molecular constituents, structures, intermolecular associations and temperature. As temperature increases, rotational viscosity decreases rapidly. Rotational viscosity is an important parameter for many electro-optical applications employing LCs, because the response time of the LC is linearly proportional to the rotational viscosity. According to the molecular theory developed by Osipov and Terentjiv [7,34], the rotational viscosity of mixture 1 is larger than the pure LC. It is also clear that the molecular association between nanoparticles and LC molecules is strong as compared to an LC LC molecule. But for mixture 2 the intermolecular association of nanoparticles is very strong as compared to a nanoparticle LC molecule. At a higher concentration, the nanoparticle is more effective than an LC LC molecule, so the interaction of nanoparticle nanoparticle is very strong for mixture 2. Therefore, LCs with weak intermolecular association reduce the rotational viscosity significantly. Another reason is that the relaxation time is proportional to the viscosity size of nanoparticles and the splay elastic constant in LC cells. The splay elastic constant decreases with the addition of nanoparticles in pure LCs. The reason is that the number of nanoparticles increases in the LC system, decreasing the splay elastic constant for nanoparticle composite systems. According to the mean field theory [7,28], the splay elastic constant depends upon the number and size of the molecules. K 11 ¼ C 11S 2 ; ð5þ C 11 ¼ 3A 2 Vn 7=3 1 L 3; ð6þ where C 11 is called the reduced splay elastic constant, V n is the mole volume, L is the length of a molecule, m is the number of molecules, A is constant and S is the order parameter. mg 2 1 4. Conclusion Nanoparticle-induced electro-optical parameters and dielectric anisotropy have been demonstrated in LCs. The dielectric anisotropy has been found to be of positive order for nanoparticle composite systems and of negative order for pure LCs. In addition to this, the rise time and fall time have also been measured for both pure and nanoparticle composite systems with variation of voltage and temperature. The rise time has increased for nanoparticle LCs compared to pure LCs. The electro-optical performance of these doped systems is not good from the application point of view. The threshold voltage and saturation voltage have decreased for nanoparticle composite systems. The dielectric anisotropy

14 P.Kr. Tripathi et al. increases with increase in the concentration of the nanoparticles doped in LCs, indicating that the nanoparticle changes the order parameter of the rod-like LC molecule. The splay elastic constant remains constant for mixture 2 with variation of temperature, whereas it increases significantly for mixture 1 and pure LCs with increasing temperature, suggesting the coupling of the LCs due to distortion of nanoparticles. Acknowledgements The authors are sincerely thankful to ISRO for providing assistance in the RESPOND program in the form of the project entitled Designing of SLMs based on nematic LCs doped with nonmesogenic molecules. One of the authors (AKM) is thankful to UGC, New Delhi, for the thirdyear grant of Dr D.S. Kothari Post Doctoral Fellowship No. F.4.2/2006 (BSR)-13-234/2008(BSR). References [1] Kato T, Mizoshita N, Kishimoto K. Functional liquid-crystalline assemblies: self-organized soft materials. Angew. Chem. Int. Ed. Chem. 2006;45:38 68. [2] Goodby JW, Saez IM, Cowling SJ, Gortz V, Draper M, Hall AW, Sia S, Cosquer G, Lee SE, Raynes EP. Transmission and amplification of information and properties in nanostructured liquid crystals. Angew. Chem. Int. Ed. Chem. 2008;47:2754 2787. [3] Tschierske C. Liquid crystal engineering-new complex mesophase structures and their relations to polymer morphologies, nanoscale patterning and crystal engineering. Chem. Soc. Rev. 2007;36:1930 1970. [4] Liao LY, Shieh PY, Huang YP. Marginal electrodes with over-drive method for fast response liquid crystal lens applications. SID Symp. Dig. Tech. Pap. 2010;41:1766 1769. [5] Park MS, Yi J, Kwon JH, Gwag JS. Field-controlled vertical aligned nematic mode for high performance LCD application. SID Symp. Dig. Tech. Pap. 2010;41:1751 1754. [6] Tang CY, Huang SM, Lee W. Electrical properties of nematic liquid crystals doped with anatase TiO 2 nanoparticles. J. Phys. D: Appl. Phys. 2011;44:355102-1-3551025. [7] Blinov LM, Chigrinov VG. Electro-optical effects in liquid crystal materials. Vol. 3. New York: Springer-Verlag;1994. [8] Manohar R, Srivastava AK, Misra AK. Electro-optical behavior for dye-doped FLC. Soft Mater. 2010;8(1):1 13. [9] Manohar R, Srivastava AK, Tripathi PK, Singh DP. Dielectric and electro-optical study of ZnO nano rods doped ferroelectric liquid crystals. J. Mat. Sci. 2011;46:5969 5976. [10] Paul SN, Dhar R, Verma R, Sharma S, Dabrowski R. Change in dielectric and electro-optical properties of a nematic material (6CHBT) due to the dispersion of BaTiO 3 nanoparticles. Mol. Cryst. Liq. Cryst. 2011;545:105 111. [11] Dhar R, Pandey AS, Pandey MB, Kumar S, Dabrowski R. Optimization of the display parameters of a room temperature twisted nematic display material by doping single-wall carbon nanotubes. Appl. Phys. Exp. 2008;1:121501-1 121501-3. [12] Pandey AS, Dhar R, Kumar S, Dabrowski R. Enhancement of the display parameters of 4-pentyl-4-cyanobiphenyl due to the dispersion of functionalised gold nano particles. Liq. Cryst. 2011;38:115 120. [13] Neeraj, Raina KK. Influence of silica nanoparticles on dielectric spectroscopy and polarization switching responses of novel ferroelectric liquid crystals. Phase Trans. 2010;83:615 626. [14] Mikulko A, Arora P, Glushchenko A, Lapanik A, Haase W. Complementary studies of BaTiO3 nanoparticles suspended in a ferroelectric liquid-crystalline mixture. Euro Phys. Lett. 2009;87:27009-p1 27009-p5. [15] Kumar A, Prakash J, Mehta DS, Biradar AM, Haase W. Enhanced photoluminescence in gold nanoparticles doped ferroelectric liquid crystals. Appl. Phys. Lett. 2009;95:023117-1 023117-3. [16] Kaur S, Singh SP, Biradar AM, Choudhary A, Sreenivas K. Enhanced electro-optical properties in gold nanoparticles doped ferroelectric liquid crystals. Appl. Phys. Lett. 2007;91:023120-1 023120-4. [17] Sharma PK, Dutta RK, Pandey AC. Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu 2 þ nanorods. J. Magn. Mater. 2009;321:4001 4003.

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