Impact on Bulk and Interfacial Dielectric properties of Nematic Liquid Crystals due to Silver Nanoparticles

Shivaraja S J 1,a, R K Gupta 1,b, and Manjuladevi.V 1,c

1 Department of Physics, Birla Institute of Technology and Science, Pilani (BITS Pilani),Pilani Campus, Rajasthan – 333031, India;

a)p20170007@pilani.bits-pilani.ac.in b)raj@pilani.bits-pilani.ac.in

c)Corresponding author: manjula@pilani.bits-pilani.ac.in

Abstract: Doping 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.

INTRODUCTION

Impurity ions present in the liquid crystal (LC) cell play an important role in deciding the quality of the picture and durability of the devices [1]. So it is essential to understand the complex behavior of ions inside the cell and at the interface. Impedance spectroscopy is a versatile and easy method to study the dielectric properties, electrical conductivity and polarization properties of ions in the liquid crystal cells. This method will give information about the mobility, diffusion properties of ions at the interface. There are several reports devoted to the impedance spectroscopy analysis of LCs at the electrode-LC interface [2,3,4]. It is already well known fact that, addition of nanomaterials into the host LC has improved the display parameters such as switching time, threshold voltage and contrast ratio due to the improved physical properties. In this work, we are presenting the results of impedance spectroscopy investigations on pure and f-AgNPs doped nanocomposites of 5PCH in the homeotropic configuration. The doping of f-AgNPs in 5PCH enhanced bulk capacitance and resistance of the sample for 0.05 wt% compared to pure 5PCH. From the studies of electrical properties it is clear that, low concentrations of f-AgNPs can be used to suppress the unwanted ion effects in the LC.

EXPERIMENTAL

The hexane thiol functionalised silver nanoparticles (f-AgNPs) of average size 3-5nm [5] were used as a dopant in the host nematic liquid crystal, 4-trans-pentyal-cyclohexyl cyanobenzene (5PCH) procured from Merck. Two different concentrations (0.05 and 1 wt %) of nanocomposites f-AgNPs in 5PCH were prepared.

To obtain the homeotropic alignment, the indium tin oxide (ITO) coated glass plates were coated with Octadecyl trimethoxy silane (ODS) then cured at 150oC for 1hr. The ODS coated ITO plates were glued together using epoxy glue mixed with glass spacers to obtain the desired thickness. The average thickness of the cells used in this work are 5 ± 0.1 휇푚 . The LC nanocomposites were filled into the cells at temperature above isotropic to nematic transition temperature (TIN) by capillary action and cooled slowly to room temperature. The optical texture of LC cells at 25oC was captured through camera mounted on polarizing optical microscope (POM) (OLYMPUS BX53M). Impedance spectroscopy measurements in nematic phase were carried out in the frequency range from 20Hz to 12MHz in the cooling cycle starting from isotropic phase using the impedance analyzer (KEYSIGHT E4990 A). The temperature of

the LC cell placed inside the hot stage was varied using a temperature controller (MTDC600, MICRO OPTIC) with an accuracy of ±0.1℃.

RESULTS AND DISCUSSIONS

The POM images of homeotropic cells filled with pure and f-AgNPs doped nanocomposites of 5PCH observed

under crossed polarizer at 25oC are shown in figure 1. The texture shows dark image even after rotating LC sample stage between crossed polarizers indicating perfect homeotropic alignment of LC molecules. Also POM image shows uniform dispersion of f-AgNPs for 0.05wt% and aggregation in case of 1wt% f-AgNPs.

The simple equivalent electrical circuit (EEC) model for LC cell consists of a resistor in series with a parallel set of capacitor and resistor. In order to account for accumulation of space charge near the interface and diffusion process of ions, we used additional components of capacitance and Warburg element as shown in the inset of figure 2 (a). The semicircle of the nyquist plot corresponds to the parallel combination of RLC and C LC which represent the bulk resistance and bulk capacitance of the samples respectively. The edge of the semicircle towards the left will reveal electrode resistance RCR. The tail towards the right side of the graph represents parallel combination of CDL and W which correspond to double layer capacitance and Warburg diffusion element respectively. The data of parameters obtained by fitting with EEC model [6] is shown in table1. The value of double layer capacitance CDL decreased for 0.05wt % and then increased for 1 wt% of f-AgNPs compared to pure 5PCH. Two types of Warburg coefficients were obtained which explain two different mechanisms of diffusion of ions. Wsr indicates the diffusion of ions from bulk to the interface and Wsc deals with the diffusion of ions into the double layer. The increase of sample resistance RLC in the 0.05 wt% doped samples compared to pure 5PCH indicates trapping of ions by the f-AgNPs. The decrease of RLC for 1 wt% is due to the aggregation f-AgNPs.

TABLE 1. Fitting Parameters of the equivalent electrical circuit for the LC cell filled with pure and f-AgNPs nanocomposites of

5PCH. Composition

(Wt% of f-AgNPs

in 5PCH)

CLC(nF) CDL(nF) RCR(훀) RLC(M훀) Wsr(M훀풔 ퟏ) Wsc(훀풔 ퟏ)

0 2.138 25.11 68.9 0.1778 1.175 9.63

0.05 2.212 18.68 76.6 0.1962 1.278 9.99

1 1.872 25.72 177.1 0.1374 0.630 9.99

FIGURE 1. POM images of LC cells (kept under crossed polarizer) filled with pure and f-AgNPs doped nanocomposites of 5PCH in homeotropic configuration. The scale bar denotes 100흁풎.

The dependence of diffusion coefficient (D) on Wsr and Wsc is as follows [7] 푊 =

√ (1)

푊 =√

(2) Where R is the gas constant, T is absolute temperature, NAis Avogardro’s number, F is Faraday constant, S is

electrode area of LC cell, ns is concentration of ions on the surface and 훿 is the thickness of Nernst diffusion layer. From equation (1) and (2), the diffusion coefficient is inversely proportional to square of Wsr and Wsc. The value of Wsr increased for 0.05 wt% doping indicating decrease in diffusion coefficient and ns, which suggests decrease of ion concentration at the electrode due to the trapping of ions at the bulk. The value of Wsr decreased for 1 wt% doping of f-AgNPs due to the presence of f-AgNPs at the interface as well as at the bulk because of aggregation. There is not much change in the Wsc values of pure and f-AgNPs doped nanocomposites of 5PCH.

The observed dielectric relaxation frequency from the imaginary part of dielectric permittivity versus frequency graph (Figure 2 (b)) of pure sample at temperature (T-TIN)= -9oC is 1.27MHz, while for 0.05 and 1 wt% f-AgNPs doped nanocomposites, the relaxation frequencies are 1.14 and 0.78 MHz, respectively. This relaxation frequency corresponds to the flip-flop motion of the molecules around their short axis. The decrease of relaxation frequency in the f-AgNPs doped samples may be due to the decrease of rotational viscosity.

The behavior of Cole-Cole plot (Figure 3 (a)) of f-AgNPs doped nanocomposites of 5PCH is similar to that of the pure 5PCH. The tail at low frequency region of Cole-Cole plot decreased for 0.05 wt% of f-AgNPs indicating decrease of ion concentration at the interface.

The temperature dependence of the dielectric relaxation frequencies (fR) of pure and f-AgNPs doped nanocomposites of 5PCH increases with increasing temperature, as shown in Figure 3(b). This exhibits a standard Arrhenius like behavior.

푓 = 퐴푒 (3) Where 퐸 is the activation energy, T is the temperature, 푘 is the Boltzmann constant and A is a pre-exponential

factor. The values of activation energy 퐸 are determined by using the slopes of the linear plots of log fR versus 1000/T. The value of activation energy for pure, 0.05 and 1 wt% f-AgNPs doped nanocomposites of 5PCH are 7.24, 6.14 and 6.78 kJ/mol respectively.

FIGURE 2. (a) Nyquist plot of pure and f-AgNPs doped nanocomposites of 5PCH at temperature (T-TIN) =-9oC. The solid red color line represents EEC model fitted data. Inset shows the equivalent electrical circuit (EEC) model used to fit the data. (b) Imaginary part of dielectric

permittivity as a function of frequency for pure and f-AgNPs doped nanocomposites of 5PCH at temperature (T-TIN) =-9oC

(a) (b)

The ac conductivity of pure and f-AgNPs doped nanocomposites of 5PCH as a function of frequency is plotted in

figure 4. From the graph it is clear that, the conductivity of pure 5PCH reduced with the addition of 0.05 wt% f-AgNPs. The decrease of conductivity in the 0.05 wt% doped sample may be due to the trapping of ions on the surface of hexane thiol functionalised f-AgNPs which is also confirmed from the ion transport measurement in our previous work [8,9]. The increase of conductivity in the 1wt% nanocomposite may be due to the aggregation of f-AgNPs as indicated in POM image.

FIGURE 3. (a) Cole–Cole plot (휺 versus 휺 ) of pure and f-AgNPs nanocomposites of 5PCH at temperature (T-TIN) =-9oC. (b) Temperature dependence of the dielectric relaxation frequencies as a function of inverse absolute temperature for pure

and f-AgNPs nanocomposites of 5PCH.

(a) (b)

FIGURE 4. Variation of conductivity as a function of frequency for pure and f-AgNPs doped nanocomposites of 5PCH at temperature (T-TIN) =-9oC. Inset

shows the enlarged graph at low frequency region.

CONCLUSION

The doping of f-AgNPs into the 5PCH nematic liquid crystal shows uniform dispersion for 0.05 wt% of f-AgNPs and aggregation for 1wt%. The EEC model fitted with experimental data shows decrease of double layer capacitance in the lower doping concentration. The dielectric relaxation frequency corresponding to the flip-flop motion of the molecules around their short axis for 5PCH shifts towards lower frequency with the addition of f-AgNPs. The temperature dependence of dielectric relaxation frequency versus inverse temperature follows Arrhenius like behavior. From the Warburg diffusion coefficients and conductivity results we conclude that, lower concentrations of nanocomposites of f-AgNPs with NLC are good candidate for the suppression of impurity ions.

ACKNOWLEDGEMENTS

The author SJS would like to thank UGC India for the support of fellowship. The author V.M gratefully acknowledges DST, SERB for the support provided under the project number (EMR/2016/005782).

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