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Electro-optic and dielectric studies ofsilica nanoparticle doped ferroelectricliquid crystal in SmC* phaseA. Chaudhary a , P. Malik a , R. Mehra a & K.K. Raina ba Department of Physics, Dr B R Ambedkar National Institute ofTechnology, Jalandhar 144 011, Punjab, Indiab Materials Research Laboratory, School of Physics and MaterialsScience, Thapar University, Patiala 147 004, Punjab, India

Available online: 10 Nov 2011

To cite this article: A. Chaudhary, P. Malik, R. Mehra & K.K. Raina (2011): Electro-optic anddielectric studies of silica nanoparticle doped ferroelectric liquid crystal in SmC* phase, PhaseTransitions, DOI:10.1080/01411594.2011.624274

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Phase Transitions2011, 1–11, iFirst

Electro-optic and dielectric studies of silica nanoparticle doped

ferroelectric liquid crystal in SmC* phase

A. Chaudharya, P. Malika*, R. Mehraa and K.K. Rainab

aDepartment of Physics, Dr B R Ambedkar National Institute of Technology, Jalandhar 144 011,Punjab, India; bMaterials Research Laboratory, School of Physics and Materials Science,

Thapar University, Patiala 147 004, Punjab, India

(Received 26 May 2011; final version received 14 September 2011)

We present the results based on the electro-optic and dielectric properties of silicananoparticle (SNP) doped ferroelectric liquid crystal (FLC) in SmC* phase.Switching time, spontaneous polarization and rotational viscosity decreases withincrease in the silica concentration. An improvement in switching time afterdoping the silica nanoparticle is due to enhancement in anchoring energy existbetween silica nanoparticle and ferroelectric liquid crystal. We noticed that thedielectric permittivity and dielectric strength decreases with increasing theconcentration of silica nanoparticle in SmC* phase. Relaxation frequencyincreases with increasing the silica concentration and temperature in SmC* anddecreases as we approaches towards transition temperature.

Keywords: ferroelectric liquid crystal; silica nanoparticle; response timespontaneous polarization; dielectric permittivity

1. Introduction

Dispersed liquid crystal composite materials have received significant attention by researchcommunity due to their potential application in electro-optic displays [1, 2]. Immense workwas done on dispersed liquid crystal materials doped with polymers and dyes tounderstand the liquid crystal behavior and improve the physical, electro-optical andelectrical properties of existing liquid crystals (LCs) [3, 4]. Among dispersed LC systems,nanomaterials such as CdSe, CdTe quantum dots, BaTiO3, Sn2P2S6, Pd, Au, MgO, ZnO,TiO2, SiO2, etc [5–20] doped liquid crystals systems have shown tremendous interest due tothe unique thermal, mechanical, and electrical properties of nanoparticles.

Nanoparticles (NPs) doped liquid crystal systems have been studied due to theirinteresting properties and prospective applications in electronic industry for viewingvarious important phenomena such as electro-optic [13,14,17,21], dielectric [22,23], andmemory effects [20]. Due to the increasing demand of these novel materials for displayapplications, the improvement in existing properties of LC materials is highly desirable.The improvement in physical and electro-optical properties of liquid crystals depends onvarious factors of NPs. These factors include selection of materials, size, type,

*Corresponding author. Email: pmalik100@yahoo.com

ISSN 0141–1594 print/ISSN 1029–0338 online

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concentration, intrinsic characteristic and mutual interaction between NPs and LCs. Low

concentration of NPs are usually preferred to yield a more stable and even distribution on

the LCs, which lowers the interaction forces between the LC and NPs [24]. The size of NPs

should be of comparable with the LCs, which would not cause serious disruption in the

orderly arrangement of LCs. Initially dispersion of NPs into nematic liquid crystals

(NLCs) was focused on the dielectric properties and only few results were reported under

the influence of aerosol particles on the phase transitions of LCs [25,26]. The addition of

NPs into NLCs result a significant changes in electro-optical, morphological, periodic and

dielectric properties [27–30]. Dynamical behavior of liquid crystal near phase transitions

and at interface properties in silica dispersed FLC systems has been observed by Jakli et al.

[16,26]. Silica nanoparticles have been employed to improve the electro-optical and

dielectric properties in the form of low threshold voltage, better optical characteristics and

memory effect [16,31].In this article, we study the effects of silica nanoparticle concentration on the electro-

optic and dielectric properties of FLC in SmC* phase and results have been compared with

pure ferroelectric liquid crystal material.

2. Experimental

FLC material trade name KCFLC10R (obtained from Kingston Chemicals, UK) has been

used in present investigations. FLC material has spontaneous polarization 23 nC cm�2, tilt

angle 22�, optical birefringence 0.18 at 25�C and phase sequence as

K !5�C

SmC� !64:5�C

SmA !99:5�C

N� !112�C

I:

SNP was obtained from M/S Sigma Aldrich, India. These spherical nanoparticles have

particle size 5–15 nm and molecular weight 60.08. SNP in different concentrations (0.01,

0.02, 0.03wt./wt.% of FLC) were doped into FLC. To get a completely homogenize

mixture of SNP -FLC, both materials were properly mixed at room temperature and than

ultrasonication was done at a frequency of 36 kHz for 1 h. Planar aligned cells made of

indium tin oxide (ITO) coated glass substrates of thickness 12 mm were used for detailed

studies. These glass substrates were initially spin coated with a solution of nylon 6/6 and

m-cresol and then rubbed uni-directionally for planar alignment. The prepared homog-

enize mixture was filled in empty sample cell by capillary action at the isotropic

temperature of FLC. Prepared pure FLC and doped sample cells were placed in hot stage

(model THMS 600) attached with temperature controller (model Linkam TP94) for

electro-optic and dielectric investigations. Electro-optic studies have been carried out by

triangular wave method and output response were seen on a digital storage oscilloscope

(model- Tektronix TDS2024) attached with computer. Phase transition in the samples was

observed under cross polarizer at 10x magnification with the help of polarizing microscope

(model Olympus BX-51P) interfaced with charge couple device camera. Optical tilt angle

of cells was measured through rotating the stage assembled with microscope. The electric

field was applied through function generator (model Philips FG-8002). The dielectric

studies have been carried out in SmC* phase in the frequency range of 50Hz to 1MHz by

LCR meter (model Fluke- PM6306). To avoid the low and high frequency corrections data

were calibrated with air and benzene [32].

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3. Results and discussion

Figure 1 shows the temperature dependence on spontaneous polarization (Ps) for pure and

doped sample cells at 10V. Temperature dependence on Ps in SmC* phase of liquid crystal

follows the relation [33,34]

PsðTÞ ¼ PoðTc � TÞ� ð1Þ

where � ¼ 0.5 from the mean field model, Po is the constant, T is the temperature at

which measurement is taken and Tc is the SmC*-SmA transition temperature.

The calculated value of � is found in the range of 0.52 to 0.60 and decreasing with

increasing the silica concentrations. For a certain external applied voltage, the switching

response through sample cell in the form of current mainly consists of three contributions

e.g. ionic, capacitance and polarization. It can be seen from Figure 1 that temperature

dependence on polarization follows the Equation (1) for pure and silica doped samples.

The polarization value is decreased on increasing the silica concentrations. Nearly 5–6�C

decrease in Tc was observed in doped sample cells (Figure 1).This decrease in Tc

may be due to change in ordering of FLC molecules and existence of impurity

in NPs. The switching time (�s) for pure and doped cells has been calculated using the

relationship

�s ¼�

Ps:Eð2Þ

where �s is switching time, � is rotational viscosity and E is the applied field respectively.

The behavior of �s with temperature for silica doped samples is shown in Figure 2.

The value of �s is decreased on increasing the concentrations of silica and found minimum

value for 0.01wt% silica doped sample. The corresponding rotational viscosity behavior

with temperature is also shown in Figure 3. The decrease in electro-optic parameters such

as �s, �, and Ps in doped sample cells may be explained on anchoring phenomena.

It has been reported that addition of nanoparticle increase the surface anchoring and

hence corresponding effect on LC switching time. The LC switching time relation with the

Figure 1. Temperature dependence on spontaneous polarization (Ps) for pure and silica doped cellat 10V.

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anchoring energy for weak and strong anchoring respectively can be expressed as

follows [35]

�s ��

k�2d2 þ

4dk

W

� �For strong anchoring ð3:1Þ

�s �4�d

W�2For weak anchoring ð3:2Þ

where d cell thickness, k bend elastic constant and W is anchoring energy coefficient.

In both cases (strong and weak anchoring) the switching time is inversely propositional to

anchoring energies. It means that if the anchoring energy would have been increased by

doping silica nanoparticles, the switching time should be lowered. In our results we found

similar trends for switching time as predicated from Equations (3.1) and (3.2).

Figure 2. Temperature dependence on switching time (�s) at 10V for pure and silica dopedsample cells.

Figure 3. Temperature dependence on rotational viscosity for pure FLC and silica doped samples.

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Figure 4. Variation of applied voltage on tilt angle for pure and silica doped samples at 30�C.

Figure 5. Dielectric permittivity (a) real part (b) imaginary part as a function of temperature forpure and silica doped samples at 500Hz.

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The variation of applied voltage on tilt angle (�) at 30�C is presented in Figure 4. It can be

seen that tilt angle decreases with increasing the applied voltage and silica concentration.Dielectric spectroscopy is an important tool to understand the molecular relaxation

process in liquid crystals. The complex dielectric spectrum can be described with the help

of the following generalised Cole–Cole Equation [36–40]

"� ¼ "0 � j"00 ð4Þ

"� ¼ "1 þXi

ðD"Þi1þ ð j!�iÞ

1��iþ

A

!nþ

�

j 2 o!k� jBf m ð5Þ

Where "1 is the high frequency dielectric permittivity, ðD"Þi, �i and �i are dielectric

strength, relaxation time and distribution parameter of the ith mode, respectively. The

third and fourth terms in Equation (5), represent the contribution of the electrode

polarization capacitance and ionic conductance at low frequencies where A and n are

constants [41]. The fifth imaginary term Bf m [42] is included in Equation (5) partially to

account for the high-frequency ITO effect, where B and m are constants since correction

Figure 6. Frequency dependence on dielectric permittivity (a) real part (b) imaginary part for pureand silica doped samples at 30�C.

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terms are small. 2 o is electric permittivity of free space. Temperature dependenceon dielectric permittivities for pure and doped sample cells at 500Hz is shown inFigure 5(a)–(b). Figure 5(a) shows that in each sample cells real part of permittivity moreor less increases in SmC* up to Tc. A decrease in permittivity (�36%) is found in 0.03wt%silica doped sample than pure FLC sample. It can be seen from Figure 5(b) that "00 alsodecreases with increasing the silica concentration from 0.01% to 0.03%. Frequencydependence behavior on "0 and "00 at 30�C are presented in Figure 6. Figure 6(a) infers thatin higher doped sample cells, "0 decreases with increasing the silica concentration. While atfrequencies (�1 kHz) no significant variation in "0 has been observed. Figure 6(b) follows atypical absorption behavior observed for pure and doped samples. It can be seen fromFigure 6(b) that permittivity continuously decreases with increasing the silica concentra-tion. A relaxation mode which appears at �100Hz in pure FLC cell slightly shift to higherfrequency �200Hz for doped sample cells. The observed Cole-Cole behavior for pure anddoped samples at 30�C is shown in Figure 7. The temperature dependence on dielectricstrength for GM (D"GM) for pure and doped cells is given in Figure 8. It is found thatdielectric strength slightly increases with increasing the temperature in SmC* and then

Figure 7. Cole – Cole plots for pure and silica doped samples at 30�C.

Figure 8. Temperature dependence of dielectric strength for GM in pure and doped samples.

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drops considerably to a very small value near Tc. Here dielectric strength is decreased by�52% in 0.03wt% silica doped samples as compared to pure sample cell. This decreasecan be understood on the basis of molecular interaction between FLC and silicananoparticles. It is assumed that doping of silica particles enhances the disorder in FLC. Inthis situation it is difficult for FLC molecules to reorient easily in a particular direction.The reduction in the order of the FLC molecule lowers the dielectric strength of thesamples. The effect of silica concentration on relaxation frequency (fr) at differenttemperature in SmC* phase is shown in Figure 9. It is clearly seen that initially fr increaseswith increasing silica concentrations and then decreases towards approaching the Tc. Thisincrease in fr with silica concentration may be due to the decrease in rotational viscosity ata particular temperature, however near Tc disorder is increased due to both factors e.g.temperature and silica concentrations [43].

4. Conclusions

In the present study, effects of silica nanoparticle on the electro-optic and dielectricproperties in SmC* phase of FLC have been investigated. Switching time, spontaneouspolarization and rotational viscosity decreases with increasing the silica concentrationwhich is due to increase in anchoring energies between silica nanoparticle and FLCmolecules. A decrease in dielectric permittivity (�36%) and transition temperature(5–6�C) was noticed in the doped sample cell, which also lowers the dielectric strength.Relaxation frequency increases with increasing the silica concentration and temperature inSmC* and decreases towards approaching the Tc. Our results show that silicananoparticles is not only responsible for fastening the switching response of FLC butalso provide a better understanding to develop the novel materials of improved physicalproperties for display applications.

Acknowledgments

The authors sincerely thank the Director, NIT Jalandhar for continuous encouragement andsupport. They also sincerely thank the referee for his valuable suggestions. Ashok Chaudhary isthankful to MHRD, New Delhi, India for financial assistance.

Figure 9. Relaxation frequency of GM with silica concentration in SmC* phase.

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