Effect of ion trapping behavior of TiO2 nanoparticles on different parameters of weakly polar nematic liquid crystal
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In the present investigation, TiO2-doped weakly polar nematic liquid crystal (NLC) has been studied. This study mainly focuses on dielectric, electro-optical and optical properties of pure NLC and doped systems. Variation in different parameters like dielectric permittivity, dielectric loss, dielectric anisotropy and conductivity has been noticed. Permittivity of the doped system is almost the same as undoped system. With doping concentration, the ionic contribution to the dielectric loss as observed in low-frequency region has been suppressed and the shift in relaxation frequency toward higher frequency side has also been observed. The electrical conductivity and threshold voltage have been decreased with increasing concentration of nanoparticles (NPs). The continuous increase in dielectric anisotropy with increasing the concentration of NPs has also been observed. These results have been attributed to the trapping capability of free ions by TiO2 NPs. Further, we focused on the study of photoluminescence (PL), UV absorbance and Fourier transformed infrared spectroscopy (FTIR) of pure and doped systems. The continuous increase in PL intensity without any shift in emission peak has been observed for doped systems. Enhancement in UV absorbance with increasing concentration of NPs has also been observed. The effect of NPs doping on molecular dynamics of NLC can be clearly seen by FTIR study. The results suggest that the TiO2 NP-doped weakly polar NLC can have significant improved dielectric, electro-optical and optical properties. This makes the weakly polar NLC to be a potential candidate for many applications.
KeywordsNematic liquid crystal TiO2 nanoparticles Dielectric anisotropy Photoluminescence UV absorbance
NLCs have been widely studied over last few years, and it has variety of applications in our daily life. These applications include liquid crystal displays (LCD) , tunable lenses , retarders , filters , wave plates , diffractive optical elements , optical shutters  and smart windows . Extreme work has been done by many researchers to improve the dielectric, electro-optical and optical properties of NLCs [7, 8]. With the advancement of technology, the enhancement in the basic properties of available NLCs is highly required. Two basic approaches have been adopted in order to modify these properties. One is to synthesize the new LC material according to required parameters and the other one is to alter the basic properties of LC materials by doping it with different nanomaterials like dye [9, 10], metallic NP [11, 12], semiconducting NP [13, 14], carbon nanotube  and some others [16, 17, 18]. Doping nanomaterials in the LCs to optimize the properties is potentially cost-effective, non-complicated and do not require great deal of efforts in comparison with synthesizing new one. Effect of doping nanomaterials on the properties of LCs depends on the type of nanomaterials used. Each nanomaterial has its own effect on the modification of properties of LCs . Almost, all the liquid crystal-based devices have one thing in common that they are driven by electric field. Thus, the ions normally present in the liquid crystal in small quantity can alter the performance of LC devices . Due to having an optical property of uniaxial crystal, NLCs have become dominating materials in the field of liquid crystal displays. Nowadays, the LCDs are expected to achieve high electro-optical properties with low power consumption as well. It has been observed that even a small fraction of impurity ions originating from LC material, sealing glue and alignment layer are strongly affecting the device performance. These impurity ions are deteriorating the quality of LCDs by reducing the voltage holding ratio, increasing threshold voltage, image sticking, gray-level shift and slowing down of response. There are also electro-optical devices like optical shutters and smart windows which rely on ions in liquid crystals [5, 6]. Despite, the negative effect of ionic contamination in display techniques, LCs with high ionic conductivity may be used in non-display applications [20, 21]. In recent years, doping nanoparticles in LCs has played an effective role in affecting the ionic impurities of LCs. Therefore, it becomes very important to understand how nanodopants can affect the ions in liquid crystals. Garbovskiy  discussed about the physical factors determining the type of the nanoparticle behavior and their effects on the concentration of ions in liquid crystals. For detailed quantitative analysis of the temperature effect in liquid crystals doped with NPs, reference can be made of .
The ionic species in the thermotropic liquid crystals are fully ionized, so the concentration of mobile ions does not depend on the temperature. But the LCs doped with nanoparticles exhibit different behavior. The concentration of ions in the NP-doped NLC systems becomes temperature dependence [24, 25]. The 100% pure NPs can only decrease the concentration of ions in the LC by means of adsorption or absorption process. The incorporation of contaminated NPs in the host LC can affect the concentration of ions in three different regimes: the purification, contamination and no change in the concentration of ions . Generally for NP-doped LCs, the increase in the concentration of mobile ions with temperature is observed but under certain conditions, the concentration of ions in LCs decreases with increasing temperature. This effect has been modeled for pure and contaminated NPs systems .
TiO2 NP is one of the most prominent NPs, which is highly employed to suppress the ionic effect. TiO2 is an insulating NP that occurs in three different crystalline phases: anatase, brookite and rutile . Among these three phases, anatase is chemically and optically active and therefore it is suitable to use as a doping material. The anatase structure is also preferred because of its high electron mobility, low dielectric constant and low density as compare to metallic or semiconductor NPs. The insulating NPs interact weakly or do not actually interact with NLC molecules . Shcherbinin et al. studied the impact of TiO2 NPs on nematic LC with different initial ionic contaminations. They explained that these types of NPs can be used to prevent uncontrolled ionic contamination that occurs during LC device production and utilization . TiO2 nanoparticles dispersion can actually change the relaxation parameters and dielectric anisotropy. Concentration and size of NPs play a crucial role in the alteration of LCs properties. Tang et al.  studied the electrical properties of NLC and observed that both the ionic concentration and diffusion constant have been reduced in TiO2-doped NLC. Low-frequency dielectric spectroscopy can be used to explain the ionic transport behavior. Chen et al.  reported that insulating TiO2 NPs, used as dopant in nematic LC, can reduce the ionic impurity contamination, thereby lowering the threshold voltage. Enhancement in electro-optical performance of NLC has been reported by Lee et al. . Yadav et al.  explained the effect of TiO2 NPs toward the suppression of screening effect in NLC. The photoluminescence study of nanoparticle-doped LC system promotes the better understanding of the interaction between nanoparticles and LC molecules. Photoluminescence behavior of NLC doped with TiO2 NPs has been studied by Roy et al. , and the enhanced PL intensity for the doped system has been reported. Pathak et al.  observed the induced photoluminescence behavior in TiO2 NP-doped NLC system.
Most of the studies on TiO2 NP-doped LCs have been carried out on strongly polar NLCs by different research groups. Here is an attempt to explain the effect of Titania NPs on the dielectric, electro-optical and optical properties of weakly polar nematic LC. Weakly polar LCs is those which have weak dipolar strength; however, the dipole moment has a very little influence on the liquid crystalline properties . In the present work, the dielectric permittivity and dielectric loss have been carried out to understand the charge storage and charge transportation phenomenon in doped NLC systems. To verify the ion trapping capability of TiO2 NPs, the conductivity and dielectric anisotropy have also been studied. Threshold voltage of doped systems is decreasing with the increase in doping concentration which is also the consequence of ion trapping phenomenon. Photoluminescence behavior of TiO2 NP-doped weakly polar NLC has also been obtained, and it is being absorbed that intensity of PL is continuously increasing with the concentration while its peak value is constant. UV–visible absorbance of pure and doped systems has been analyzed, and it is observed that the intensity of absorbance has been decreased in doped systems. Through the study of FTIR spectra, it is demonstrated that some physical and chemical changes are occurring in the doped systems.
Anatase TiO2 NPs used as a doped material in this study have spherical shape and particle size 20–24 nm [38, 39]. These NPs have been purchased from Aldrich UK. The resistivity of anatase TiO2 is 1015 Ω cm . NPs have been used without any further purification. The planar aligned sample cells with ITO-coated glass plate are used to prepare sandwiched type cells. Firstly, the conducting layers are treated with adhesive promoter and then coated with nylon (6/6) to obtain a planar alignment. Thickness of the cell was fixed by placing a Mylar spacer (6 μm in our case) between the glass plates and then sealed with a UV sealant. The obtained empty cell is calibrated by using analytical reagent (AR) grade benzene and CCl4 as standard reference for dielectric study. The composite has been prepared by mixing the Titania NPs with NLC D5AOB. The concentration of the dopant dispersed in the NLC was 0.05 wt%/wt (mix1), 0.1 wt%/wt (mix2) and 0.2 wt%/wt (mix3). For preparing the composites, the proper amount of TiO2 NPs was first dispersed in propanol-2 and then the solution was ultrasonicated for 24 h to assure the proper dissolving of NPs. After that the appropriate volume concentration is mixed with fixed amount of NLC. This mixture is then repeatedly passed through the heating and cooling process until it is properly mixed. The empty planar cell is then filled with pure and doped LC above their isotropic temperature by capillary action.
The dielectric measurements of pure and Titania-doped NLCs were carried out by using an impedance gain/analyzer HP4194A. Instec hotplate (HCS-302) with an accuracy of ± .001 °C has been used to control the temperature of sample cell. Dielectric response is studied in the frequency range 100 Hz–40 MHz. The POM (Progress CT-3 Radical) has been used to record the optical texture of pure and composite systems. Carry eclipse fluorescence spectrophotometer (Agilent technology) is used to record the photoluminescence spectra of all the samples. A xenon lamp is used as an excitation source within the fluorescence spectrophotometer. All measurements are taken at room temperature, and the slit width of source is kept 5 nm. UV–visible spectrophotometer (ELICO, SL 210) has been used for UV–visible absorption study of pure and composite systems for wavelength range 190–500 nm. FTIR study in the wavelength range 400–4000 cm−1 was performed by Fourier transform of infrared spectrophotometer (IR affinity-1 Shimsdzu).
Results and discussion
The obtained results demonstrate the effect of doping TiO2 NPs on the dielectric, electro-optical and optical properties of weakly polar nematic liquid crystal. Consistency in the variation of dielectric permittivity with frequency for both pure and doped systems reveals the negligible effect of TiO2 NPs on the net dipole moment of NLC molecules. The value of dielectric loss is decreasing with concentration at lower part of frequency region which may be attributed to the trapping of free ions by TiO2 NPs at its surface. The study also shows that the conductivity of doped NLC system has been decreased due to the reduction in the effective transported ion concentration. The decreased value of threshold voltage further confirms the suppression of screening effect in TiO2 NP-doped systems. The dielectric anisotropy has been increased for all doped system. Increased value of \(\Delta \varepsilon\) can be beneficial in the enhancement of electro-optical properties. For optical study, the photoluminescence, UV–visible absorbance and FTIR study has also been performed. The presence of TiO2 NPs enhances the intensity of photoluminescence. This is due to constructive combination of emissions from NPs and NLC molecules. The UV absorbance is increasing continuously with the increase in concentration of NPs. This enhancement in the UV absorbance is attributed to coupling between electromagnetic wave and phonons. FTIR study confirms the effect of TiO2 NPs on the molecular dynamics of NLC system. On the basis of these results, we conclude that the TiO2 NPs are promising materials for enhancing the characteristic properties of D5AOB NLC. The most engrossing result of this study is the reduction in ionic contaminations present in NLC material, which can be highly beneficial for display applications.
The author Rajiv Manohar is thankful to UGC for the Grant of MID CAREER AWARD.
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