Journal of Molecular Liquids 188 (2013) 230–236

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Journal of Molecular Liquids

j ourna l homepage: www.e lsev ie r .com/ locate /mol l iq

Influence of ZnO nanoparticle concentration on electro-optic anddielectric properties of ferroelectric liquid crystal mixture

Ashok Chaudhary a, Praveen Malik a,⁎, Rohit Mehra a, K.K. Raina b

a Department of Physics, Dr B R Ambedkar National Institute of Technology, Jalandhar 144 011, Punjab, Indiab Materials Research Laboratory, School of Physics and Materials Science, Thapar University, Patiala 147 004, Punjab, India

⁎ Corresponding author. Tel.: +91 181 2690301x3209181 2690320.

E-mail address: pmalik100@yahoo.com (P. Malik).

0167-7322/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.molliq.2013.09.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 November 2012Received in revised form 20 September 2013Accepted 23 September 2013Available online 10 October 2013

Keywords:Ferroelectric liquid crystalZnO nanoparticlesResponse timeSpontaneous polarizationDielectric permittivity

We present here the electro-optic and dielectric properties of ZnO nanoparticle doped ferroelectric liquid crystal(FLC) mixtures. A small quantity of ZnO nanoparticles (0.0, 0.1, 0.3, 0.5 and 1.0 wt.%) was dispersed into FLCwith a thickness of 5.0 ± 0.2 μm. The enhancement in physical parameters is found to be dependent on ZnOnanoparticle concentration. The lower concentration of ZnO nanoparticles (0.1–0.3wt.%) enhances the value ofspontaneous polarization and decreases for higher concentration (0.5–1.0wt.%). The fastening of response timehas also been noticed in the doped sample cells up to 0.3 wt.% concentration. Dielectric measurements revealeda concentration dependence effect in all sample cells. A low threshold voltage and improved contrast ratio hasbeen noticed for 0.3wt.% concentration over other doped samples.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ferroelectric liquid crystals (FLCs) have attracted a significant deal ofattraction due to their various promising applications like opticalantennas, spatial light modulator, flat panel display, etc. and have vitalrole in faster switching time, lower threshold voltage and better opticalresponse [1–7]. Various kinds of display devices, particularly liquid crys-tal displays have attracted significantly due to their unique electro-opticcharacteristics. Recently, the nanomaterial dispersion in liquid crystalsplays an important role for producing the enhancement in physicalparameters of liquid crystals (LCs) [8–17]. It has been reported earlierthat with the dispersion of silica nanoparticles in ferroelectric liquidcrystal a significant improvement in response time and memory effecthas been observed [12,16]. The addition of gold nanoparticles in FLCmaterials enhanced the electro-optic properties of the systems [1,8].With the addition of nanotube in liquid crystals a significant modifica-tion in the threshold voltage, response time, contrast ratio and sponta-neous polarization has already reported by various groups [11,17].There are number of factors on which the interaction of nanomaterialsand liquid crystals depends like size, shape, preparation methods andconcentration etc. [18,19]. Among the different kinds of nanomaterials,semiconductor nanoparticles have attracted a great deal of attentiondue to their optical importance and good agreement with physicalparameters [20–23]. Here ZnO is widely used due to their variety ofapplications in devices such as gas sensors, solar cells, luminescent

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materials, optical properties and electrical properties [24–26]. Joshiet al. observed the lower power operation and better optical contrastwith ZnO doped FLC [21]. The impact of ZnO on dielectric and electro-optic responses was studied by Manohar et al. [27]. Huang et al. ex-plored the importance of addition of ZnO nanocrystals into the FLCsby probing the molecular binding of FLC molecules which surroundedthe ZnO [28]. Li et al. proposed amodel to show the interaction betweenZnO and FLC molecules surrounding them [29].

In the present paper, we demonstrated the effect of ZnO nanoparti-cles concentration on the electro-optic and dielectric responses of FLCmaterial. The effect on physical parameters like spontaneous polariza-tion, response time, rotational viscosity, threshold voltage and dielectricparameters (dielectric permittivity, relaxation frequency) have alsobeen investigated and discussed.

2. Experimental

In this study, ferroelectric liquid crystal material (W206E) has beentaken. The phase sequence of material is as follows

Crystal↔SmC �↔86:6 �CSmA↔

92:5 �CN �↔97:6 �C

Iso:

Zinc oxide (ZnO) nanoparticles have been used in our present studyas a dopant. The XRD and TEM image patterns of ZnO are shown inFigs. 1 and 2 respectively. The typical size of ZnO nanoparticles wasfound ~10 nm. Empty LC sample cells of thickness 5.0 ± 0.2 μm wereprepared using indium tin oxide (ITO) coated transparent glasssubstrates of sheet resistance 100 Ω. Five cells assigned as pure anddoped FLC samples were prepared using doping of ZnO in 0.0, 0.1, 0.3,

Fig. 1. XRD pattern of ZnO nanoparticles.

231A. Chaudhary et al. / Journal of Molecular Liquids 188 (2013) 230–236

0.5 and1.0wt.% ratio into FLC. The homogenizedmixture (ZnOand FLC),so prepared was then filled between empty LC sample cells by capillaryaction at the isotropic temperature of FLC and finally sealed using Nor-land Optical Adhesive (NOA-65). The electrodes were connected atthe ITO surface of the cells using indium material. The sample cellswere kept in a hot stage coupled with temperature controller(LINKAM-TP 95, THMSE 600) and can be stabilized to an accuracy of0.5 °C/min. The change in phases was analyzed with the help of opticalpolarizing microscope (Nikon Eclipse LV100POL). The electro-opticalresponses were recorded on a digital storage oscilloscope (TektronixTDS2024B). The dielectric measurements were performed in thefrequency range of 50Hz to 1 MHz using LCR meter (Fluke-PM6306).The experimental setup used for the measurement of electro-opticparameters is shown in Fig. 3.

3. Results and discussion

3.1. Interaction between ZnO and FLC

Fig. 4 shows the micro-texture observed in pure and doped samplesunder cross polarizer at 30 °C. The micro-texture shows that up to0.3wt.% doping of ZnO in FLC material slightly enhances the alignmentof FLC molecules (Fig. 4(c)) and above 0.3%, a small perturbation in FLCgeometry can be seen in Fig. 4(d,e). The mutual interaction mechanismbetween ZnO and FLC molecules may have an influence on the good

Fig. 2. TEM image of ZnO nanoparticles.

coupling between ZnO-FLC molecules up to a specific concentration[0.3wt.%] than the higher concentration.

3.2. Electro-optic studies

The concentration dependence on spontaneous polarization (Ps) forpure and doped sample cells at 30V is shown in Fig. 5. The value of Psincreases remarkably by increasing the ZnO nanoparticle concentrationup to 0.3 wt.% in FLC and further then, it starts to decrease to 0.5 and1.0wt.% concentrated samples. The modification in the polarization ofFLC with doping ZnO nanoparticles could be explained by the accountof dipole moment of ZnO nanoparticles and interaction between ZnOnanoparticles and FLC. The doping of ZnO nanoparticles adds up the di-pole moment into the FLC material and hence a corresponding increasein Ps up to 0.3wt.% ZnO nanoparticles doped sample. The higher value ofPs in 0.3wt.% of ZnO nanoparticles is due to existence of large dipole–dipole interaction between ZnO and FLCmolecules than higher concen-tration of ZnO (1.0wt.%). Here higher concentration of ZnO nanoparti-cles may produce a hindrance in rotation of FLC molecules and resultsa decrease in Ps. This behavior does not mean that the dipole momentof ZnO nanoparticles does not contribute to the overall dipole momentof FLC-ZnO nanoparticles at higher concentration, but there are someother factors such as intrinsic properties of ZnO nanoparticles (wideband gap, electric conductivity) and threshold effectmay play an impor-tant role in doped compositions. The variation in tilt angle with concen-tration of ZnO nanoparticles is given in Fig. 6. A slight variation in tiltangle with doping of ZnO nanoparticles has been noticed. Fig. 7 showsthe behavior of rotational viscosity (η) in all sample cells at 30 V. Wefound that η decreases with increasing the ZnO nanoparticle concentra-tion up to 0.3wt.% and then increases beyond 0.3wt.% ZnO concentra-tion. It may be due to the remarkable change in strength of dipolarinteraction between the ZnO nanoparticles and FLC molecules at theinterface [21]. The electro-optic response time (τs) (i.e. the time takenby the molecules to switch from ON to OFF state) was calculated andexplained by using the relation [17]

τs ¼η

Ps � Eð1Þ

where E is the applied electric field. Concentration and temperature de-pendence on τs for pure and doped samples at 30V (50Hz) are plotted inFig. 8. τs decreases by increasing the concentration and temperature indoped samples of up to 0.3wt.% concentration and after that it starts in-creasing. The value of Ps increases and ηdecreases up to 0.3wt.% concen-tration and beyond 0.3 wt.% both parameters have an vice versabehavior. The combined effect of Ps and η show a faster response timein doped samples at low concentration which is also supported byEq. (1). The decrease in τs with temperature for all samples is due tothe decrease in order parameter of FLC molecules. The value of τs de-creases by increasing the concentration of ZnO nanoparticles andshows faster response time in 0.3 wt.% doped cell over other samplecells. The recorded output waveform pattern of response time in theform of hump is shown in Fig. 9.

3.3. Dielectric studies

Dielectric spectroscopy is an important tool to understand themolecular structure and mechanism of the molecular process. The rela-tionship between the dielectric permittivity can be written as [30–34].

ε� ¼ ε0−jε} ð2Þ

ε0 ¼ ε∞ þ ε0−ε∞ð Þ1þω2τ2� � ð3Þ

Digital Storage Oscilloscope

TDS 2024

CHI CHII Gnd

Interface with Computer

PR C

Sample holder RS 232cum hot stage

Function Generator Scientech ST 4060

Temperature controller THMS 600

Fig. 3. Experimental setup to study the polarizing switching response using the field reversal technique.

232 A. Chaudhary et al. / Journal of Molecular Liquids 188 (2013) 230–236

ε} ¼ ε0−ε∞ð Þωτ1þω2τ2� � : ð4Þ

The symbols have usual meaning and defined earlier [19]. Thedielectric relaxation spectrum can be described with the help of thefollowing generalized Cole–Cole Eq. as [30,32,34]

ε� ¼ ε∞ þX Δεð Þ

1þ jωτð Þ1−α þ σj∈oω

kð5Þ

where, Δε, α, σ and ∈o are dielectric strength, distribution parameter,electric conductivity and electric permittivity of free space respectively.

Fig. 4.Micro textures of pure and doped sample cells at 30°C at a magnification of 10× for(a) 0.0, (b) 0.1, (c) 0.3, (d) 0.5 and (c) 1.0% ZnO nanoparticles.

The behavior of dielectric permittivity with temperature for pure anddoped sample cells at 500 Hz is shown in Fig. 10(a–b). Fig. 10(a)shows that in each sample real part of permittivity increases in SmC*up to SmC*–SmA transition temperature (Tc) and after that it start de-creasing for particular value of concentration. A similar type of behaviorhas also been noticed in Fig. 10(b) for ε″with respect to temperature aswell as the concentration. Themaximum value of dielectric permittivity(ε″) in SmC* near Tc indicatesmaximum strength of helix in SmC* phaseand on further increase in the temperature, a sharp decrease in thedielectric permittivity occurred due to the unwinding of the helix[35–39]. The increment in permittivity up to 0.3% concentration mayalso be due to the contribution of ZnO nanoparticle dipole moment.However beyond 0.3wt.%, it creates a hindrance in the rotation of FLCmolecules which results to a decrease in permittivity. Fig. 11 representsthe appearance of single relaxation mode in Cole–Cole form whichclearly suggests a goldstone mode (GM) in the low frequency region(~200Hz). The calculated values of distribution parameter (α) as a func-tion of ZnO concentration are also presented in Fig. 11. The α has mini-mum value of 0.172 at 0.3wt.% concentration while it starts increasingwith the ZnO concentration. The observed lowest value of α for0.3 wt.% concentration suggests the dominating nature of GM at thisconcentration in the SmC* phase than the other concentrations. Thelowest value of α with the dispersion of ZnO nanoparticles in the pure

0.0 0.2 0.4 0.6 0.8 1.010

11

12

13

14

15

16

Spon

tane

ous

pola

riza

tion

( nC

/cm

2 )

Concentration (wt./wt%)

30oC

40oC

50oC

70oC

Fig. 5. Concentrations versus spontaneous polarization for pure and ZnO nanoparticledoped samples at 30 V (f=50Hz).

0.0 0.2 0.4 0.6 0.8 1.0

22.0

22.5

23.0

23.5

24.0

24.5

25.0

25.5

30oC

40oC

50oC

70oC

Concentration (wt./wt%)

Tilt

ang

le (

deg)

Fig. 6. Variation in tilt angle with concentration of ZnO nanoparticles.

1 >

2 >

Fig. 9. Output waveform pattern for response time in SmC* phase under applied voltage.

233A. Chaudhary et al. / Journal of Molecular Liquids 188 (2013) 230–236

FLC suggests that the relaxation phenomenon gets closer to the Debyetype of relaxation phenomenon. The calculated value of distribution pa-rameter is clearly matched with theoretically values [27]. The behaviorof relaxation frequency (fr) for pure and doped cells in SmC* phase isshown in Fig. 12. It is found that fr increases up to SmC* phase and startsdecreasing near the transition temperature (Tc). This decrease in fr withtemperature near Tc may be due to the decrement in the order parame-ter of these molecules or distortion in molecular arrangement. Thefr also increases with increasing the ZnO concentration up to 0.3 wt.%

0.0 0.2 0.4 0.6 0.8 1.0

80

100

120

140

160

180

Rot

atio

nal v

isco

sity

(kg

/ms)

Concentration (wt./wt%)

30oC

40oC

50oC

70oC

Fig. 7. Variation of rotational viscosity with concentration for pure and ZnO nanoparticledoped samples.

0.0 0.2 0.4 0.6 0.8 1.00.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Res

pons

e tim

e (m

s)

Concentration (wt./wt%)

30oC

40oC

50oC

70oC

Fig. 8. Behavior of response time with concentration for pure and ZnO doped samples.

and above it show a decreasing behavior. This behavior may beexplained using Landau free energy expansion [19]

f r ¼K33q

2

2πηð6Þ

where K33 is the elastic constant and q is thewave vector of helical pitchof material. From Eq. (6), it is clear that fr depends only upon η and q,which play an important role [35]. It can be seen from Fig. 12 that inSmC* phase fr and η follow the similar behavior as presented inEq. (6). The effect of ZnO nanoparticles on dc conductivity at 30 °Cis also shown in Fig. 13, which increases by increasing the ZnO

5

10

15

20

25

30

SmASmC*

0.0 % 0.1 % 0.3 % 0.5 % 1.0 %

Temperature (oC)

30 40 50 60 70 80 90 100

Temperature (oC)

30 40 50 60 70 80 90 1000

5

10

15

20

25

SmC* SmA

0.0 % 0.1 % 0.3 % 0.5 % 1.0 %

ε'ε''

(a)

(b)

Fig. 10. Temperature dependence on dielectric permittivity in pure and ZnO nanoparticledoped samples at 500Hz for (a) ε′ and (b) ε″.

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

35

40

Samples 0.0 0.180 0.1 0.178 0.3 0.172 0.5 0.173 1.0 0.174

0.0 % 0.1 % 0.3 % 0.5 % 1.0 %

30oCε"

ZnO nanoparticles in %

ε'

α

Fig. 11. Cole–Cole plots for pure and ZnO nanoparticle doped samples at 30 °C.

30 40 50 60 70 80200

300

400

500

600

700

0.0 % 0.1 % 0.3 % 0.5 % 1.0 %

Temperature (oC)

Rel

axat

ion

freq

uenc

y (H

z)

Fig. 12. Temperature dependence on relaxation frequency dependence in SmC* phase atdifferent concentrations.

0.0 0.2 0.4 0.6 0.8 1.0

0.228

0.229

0.230

0.231

0.232

0.233

0.234

Concentration (wt./wt%)

Act

ivat

ion

ener

gy (

eV)

30oC

Fig. 14. Concentration dependence on activation energy at 30 °C.

234 A. Chaudhary et al. / Journal of Molecular Liquids 188 (2013) 230–236

nanoparticle concentration. The activation energy (Eτ) was obtainedusing the following Arrhenius relationship [19]

τ ¼ τo exp − EτkB:T

� �ð7Þ

where τ is the relaxation time, τo the relaxation time at high tempera-ture, T the absolute temperature and kB the Boltzmann constant.The behavior of activation energy with ZnO nanoparticle concentration

0.0 0.2 0.4 0.6 0.8 1.01.4x10-5

1.5x10-5

1.5x10-5

1.6x10-5

1.6x10-5

ZnO nanoparticles in %

Samples A n

0.0 5.53E-17 0.670 0.1 5.83E-17 0.681 0.3 6.27E-17 0.668 0.5 4.85E-17 0.709 1.0 5.05E-17 0.677

Concentration (wt./wt.%)

dc c

ondu

ctiv

ity (

ohm

-1m

-1)

Fig. 13. Variation of dc conductivity with ZnO nanoparticle concentrations at 30 °C.

is shown in Fig. 14. We predict that for higher concentration(0.5–1.0 wt.%) sample activation energy is higher than those in thecase of pure and 0.1–0.3 wt.% concentrations, which show that thehigher concentration samples required more energy for the rotation ofFLC molecule orientation.

3.4. Optical properties

Transmission (T), contrast ratio (CR) and threshold voltage (Vth) areimportant characteristic parameters for display applications. Transmis-sion is defined as

T ¼ IoutIin

ð8Þ

where Iin and Iout are the intensities of light before and after passingthrough the cell respectively [17,40]. Transmission–voltage (T–V)curve under crossed polarizer at 30 °C for pure and doped samples isshown in Fig. 15. At lower voltage a proportional rise in transmissionwas observed and it saturates above ~7–8 V in all samples. It is foundthat at low concentration (0.1–0.3 wt.%) of ZnO nanoparticles, dopedsample shows nearly ~66% transmission value than high concentrated(1.0 wt.%) doped sample. The concentration dependence on contrastratio (the ratio of transmittance in bright state of cell to transmittancein real dark state of cell) is shown in Fig. 16. It can be seen that contrastratio (CR) increases with increasing the nanoparticle concentration andhigher for 0.3wt.% doped cell than pure and other doped sample cells.Above 0.3wt.%, CR decreases due to some perturbation which leads to

0 3 6 9 12 15 18 21

9

12

15

18

21

24

27

30

33

0.0% 0.1% 0.3% 0.5% 1.0%

Applied voltage (Volt)

Tra

nsm

issi

on (

a.u.

)

Fig. 15. Transmission–voltage curve for pure and ZnO nanoparticle doped samples at 30 °C.

Table 1Physical parameters of pure and ZnO nanoparticle doped FLC sample cells at 30 °C.

Samples(ZnO nanoparticles in %)

Ps(nC/cm2)

τs(ms)

η(kg/ms)

θ(deg)

fr(Hz)

Eτ(eV)

Vth

(V)

0.0 14.98 1.53 171.89 25.2 240 0.228 5.70.1 15.40 1.46 169.12 25.4 250 0.230 4.10.3 16.04 1.38 166.63 25.6 272 0.233 3.60.5 15.75 1.57 186.36 25.5 267 0.234 8.31.0 15.36 1.62 187.51 25.1 238 0.234 8.6

0.0 0.2 0.4 0.6 0.8 1.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Concentration (wt./wt%)

Con

tras

t rat

io

Fig. 16. Concentration versus contrast ratio behavior for pure and ZnO nanoparticle dopedsamples at 30 °C.

235A. Chaudhary et al. / Journal of Molecular Liquids 188 (2013) 230–236

low CR. The behavior of threshold voltage with concentration depen-dence is also presented in Fig. 17 and measured by using T–V curve(Fig. 15). The decrease in Vth is due to the decrease in the order param-eter of liquid crystal with temperature. We observed a minimumthreshold voltage for 0.3 wt.% ZnO doped cell and beyond 0.3 wt.% itstart increasing. The various physical parameters obtained from theabove studies are summarized in Table 1.

4. Conclusions

This work deals mainly with the effect of ZnO nanoparticle concen-tration on electro-optic and dielectric response of FLC mixture. Thepresent results can be summarized as follows:

1. The enhancement in spontaneous polarization was observed inlower concentration doped cells (0.1–0.3 wt.%) as compared topure FLC and high concentration cells. It may be due to the large di-pole moment interaction of FLC molecules with ZnO nanoparticlesat lower concentration than that of higher concentration. The fasterresponse time has also been noticed for lower concentration cells.

2. Addition of ZnO nanoparticles into pure FLC mixture results thedeviation in the value of dielectric permittivity. The behavior of di-electric permittivity depends upon the interactionmechanismand as-sociated parameters between ZnO nanoparticles and FLC molecules.

3. The relaxation frequency increases up to a critical concentration andafter that it starts decreasing. This behavior is due to the combined

0.0 0.2 0.4 0.6 0.8 1.0

3

4

5

6

7

8

9 30oC

40oC

50oC

70oC

Thr

esho

ld v

olta

ge (

Vol

t)

Concentration (wt./wt%)

Fig. 17. Effect of concentration on threshold voltage for pure and ZnO nanoparticle dopedsamples.

effect of rotational viscosity, elastic constant and wave vector of FLCnanoparticle molecules in doped cells.

4. A low threshold voltage and improved contrast ratio have beenobserved at lower concentration (0.3 wt.%) of ZnO nanoparticledoped cell.

Acknowledgments

Authors thank Director, NIT Jalandhar and Thapar University Patialafor providing the experimental facility. Authors are also thankful to DrAkashdeep, Scientist CSIO, Chandigarh, India and Prof. R. Dabrowski,Poland for providing the nanoparticle and FLC material. The work wasalso supported by DST, New Delhi, India under project no. SR/FT/CS-50/2011.

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