CHAPTER VI EFFECT OF ZINC OXIDE NANOPARTICLES ON THE ...shodhganga.inflibnet.ac.in/bitstream/10603/28318/13/13_chapter 6.pdf · THE TRANSITION TEMPERATURE AND DIELECTRIC PROPERTIES

154

CHAPTER VI

EFFECT OF ZINC OXIDE NANOPARTICLES ON

THE TRANSITION TEMPERATURE AND

DIELECTRIC PROPERTIES OF

ELECTROCLINIC LIQUID CRYSTALS

In this chapter, the effect of zinc oxide nanoparticles (ZnO NPs) on the dielectric

relaxation behavior of electroclinic liquid crystal (ELC) materials has been described.

The addition of ZnO NPs into ELCs, caused a remarkable shift in ferro to paraelectric

(Sm C* to Sm A*) phase transition temperature which was investigated by the dielectric

and electro-optical measurements. The different behavior of ZnO NPs doped ELC from

pure ELC has been explained by determining the dielectric strength, distribution

parameter and the corresponding relaxation frequency, etc. and these results have been

compared with the data calculated by using theoretical model. Effect of ZnO NPs

addition on physical parameters such as spontaneous polarization (Ps) and rotational

viscosity (η) has also been discussed in this chapter.

6.1 INTRODUCTION

Nanoscience and nanotechnology is one of the vital frontiers in scientific research. A

broad area of research topics from fundamental physical, biological and chemical

phenomenon to material science has been addressing by the scientific society at the

nanoscale [1-3]. Nanoparticles (NPs) doping technology provides a more convenient

and flexible approach for the modification of liquid crystal (LC) materials and

designing new and improved devices based upon LCs. As discussed in previous

chapter, by adding copper oxide (CuO) decorated multi-walled carbon nanotubes

(MWCNTs) into ferroelectric liquid crystals (FLCs), one can improve the response of

the device (Chapter V). LC is itself a flexible material whose properties can be

customized by mixing different chemicals mutually. A small addition of NPs into LC

155

materials has improved many special characteristics in the form of frequency

modulation response, non-volatile memory effect, fast electro-optic response and low

driving voltage [4-7]. Among all the fascinating materials emerging from the field of

nanotechnology, the metal NPs such as silver(Ag), gold(Au), palladium(Pd),

platinum(Pt) or their alloys doped liquid crystal displays (LCDs) exhibited the faster

response time than that of the LCDs with conventional driving methods [8-12] and

continue to attract immense research interests. However, still a lot of efforts needs to

put in the improvement of nanomaterials research, particularly, the agglomeration or

the aggregation tendency of NPs over time due to which the alignment and

performance of the LCD device get affected [13].

Comparing with other metallic NPs, ZnO NPs have attracted increasing attention

owing to wide application in nano-generators, gas sensors, highly efficient solar cells,

field-emission transistors, ultraviolet photo detectors and in biomedical systems such

as cancer detecting biosensors and ultra sensitive DNA sequence detectors [14-18].

Due to its large dipole moment (> 100 D), ZnO NPs are very much appropriate for LC

material as ZnO NPs generates a powerful field inducing dipolar interaction resulting

into the enhancement of anchoring of LC molecules which may give rise to the well

ordered molecular structure [19]. Numerous research groups have explored that the

addition of ZnO NPs into FLCs reduced the threshold voltage and improved the

optical contrast of the devices based on LCs [20, 21]. In the starting of 2008, Huang et

al. explored that the doping of ZnO NPs into surface stabilized ferroelectric liquid

crystals (SSFLCs), can improved the alignment and field induced reorientation

processes of FLCs [22]. In 2009, Li et al. proposed a physical model, which shows an

interaction of ZnO NPs with surrounding FLC molecules [23]. Till now, researchers

mainly emphasis on the addition of ZnO NPs into FLCs to improve the material

parameters. The effect of ZnO NPs on the phase transition temperature and dielectric

relaxation processes of FLC materials has not been reported.

The frequency domain dielectric spectroscopy (FDDS) is a promising tool to detect

several collective/non-collective dielectric relaxations in FLCs in chiral smectic C

(Sm C*) to chiral smectic A (Sm A*) phase transitions [24-27]. The dielectric

relaxation behaviour of FLCs provides important information regarding its dipolar

response to external stimulus. Various dielectric modes have been found to exist in

156

Sm C* phase of the FLC materials owing to either collective dielectric processes or

the molecular reorientation processes connected with the polarization of the

molecules. To investigate the dielectric relaxation behaviors of FLCs in Sm C* phase,

one can give details of two modes, Goldstone and soft modes, which have been

discussed in previous chapter of the thesis (Chapter II). Moreover, there may be

some additional dynamic modes produced by the dynamics of the molecules or by the

molecular interaction between the molecules of the FLCs and some dopant materials

[28-32].

In the present study, noticeable investigations have been carried out extensively in

order to understand the shift in phase transition from ferro (Sm C*) to paraelectric (Sm

A*) phase of electroclinic liquid crystal (ELC), which is a special type of FLC. In

ELCs, the Sm A* mesophases are capable to show the induced polarization (PS)

similar to Sm C* mesophases. ELC materials have been discussed in detail in the first

chapter of the thesis (Chapter I). The effect of ZnO NPs on the transition temperature

of ELCs has been demonstrated by using FDDS. After analyzing the dielectric

relaxation behavior of ZnO NPs doped ELC, a low frequency peak along with the

Goldstone mode in the Sm C* phase has also been observed near the transition

temperature. Further, it has been observed that this low frequency peak depends

strongly on temperature and electric field. The results have also been compared with

the data calculated by using theoretical model (Cole-Cole model). The effect of

doping of ZnO NPs, on the physical characteristics of materials such as spontaneous

polarization (Ps) and rotational viscosity (η) has also been discussed.

6.2 EXPERIMENTAL DETAILS

The LC sample cells for the thermal and dielectric studies of ELC materials were

prepared by using by photolithographic technique and the rubbed polyimide technique

was used to obtain the homogeneous (HMG) alignment. The detail procedure of

sample cell fabrication has given in the previous chapters. The commercially available

ELC materials used in our present study are BDH 764E and Felix-20 and the phase

sequences of the materials are as follows:

157

Cryst. →← °− C7Sm C* →← Cº28

Sm A* →← Cº73N →← C92º-89

Iso. (BDH 764E)

Cryst. →← °− C8 Sm C* →← − Cº1815

Sm A* →← Cº75N →← C92º

Iso. (Felix-20)

A small amount (1 wt%) of ZnO NPs was doped into ELC materials and then ZnO

doped ELC materials were introduced into the cells by means of capillary action at

elevated temperature (~95°C) to ensure that filling takes place in isotropic phase of

the ELC materials.

The molecular and collective dielectric studies were carried out by dielectric

spectroscopy using Wayne Kerr 6540A impedance analyzer in the frequency range of

20 Hz to 1 MHz. The dielectric set up was fully computer controlled and automated.

The sample temperature was controlled within the accuracy of ± 0.01oC using

temperature controller JULABO F-25 HE. Optical tilt angle measurements were taken

with the sample cell mounted on a rotatable stage of the polarizing optical microscope

(Axioskop-40) interfaced with a canon digital camera. Automatic liquid crystal tester

(ALCT-P), which works on the principle of current measurement with time on the

application of triangular pulse [33], was used for measuring spontaneous polarization

(Ps) and rotational viscosity (η). The sample holder was kept thermally isolated from

the external sources.

6.3 RESULTS AND DISCUSSION

The ZnO NPs used in the present study, were synthesized in alcoholic medium at

room temperature by using zinc acetate and lithium hydroxide. The characteristic size

of synthesized ZnO NPs estimated by XRD pattern was found to be around ~ 7 nm

which is calculated using Debye–Scherrer formula [34]. The XRD pattern of ZnO-

NPs is shown in Fig. 6.1.

158

Figure 6.1: XRD pattern of zinc oxide nanoparticles.

For the comparative studies, we prepared and used the sample cells containing ZnO

NPs doped ELC materials and pure ELC materials. Addition of ZnO NPs into pure

ELC materials results in the redistribution of intermolecular interaction energies

which can affect almost all the physical parameters of pure materials.

Dielectric relaxation spectroscopy is one of the important tool to study the molecular

relaxation and dielectric properties of the materials. Temperature dependence of

dielectric relaxation can be described by Debye theory but if collective dielectric

processes exhibits a continuous distribution of relaxation time for LCs then it can be

described by Cole-Cole equation as given by:

11,2...

( )*( )

1 ( ) i

i i

o

i iiα

ε εε ω ε

ωτ∞

∞ −=

−= +

+∑ (6.1)

where, ε0, ε∞ and τ stand for the static dielectric permittivity, frequency independent

permittivity at high frequency and relaxation time, respectively. ‘i’ represent the

number of relaxation processes and ‘α’ stands for the distribution parameter. α is a

measure of the width of the relaxation distribution and if the value of α is very small

or equal to zero then the above Cole-Cole equation will obey the Debye process.

The real and imaginary part of permittivity can be separated out easily from the

complex function by the relation:

159

ε*(ω) = ε' (ω) - iε "(ω) (6.2)

where, ε' denotes the real part of the complex dielectric permittivity, ε" is the

imaginary part of the permittivity and ω is the angular frequency of applied electric

field.

Figure 6.2 shows the real part of dielectric permittivity (ε') as a function of frequency

for a large temperature range of ELC material (BDH 764E) doped with 1 wt % ZnO

NPs, while the dielectric permittivity (ε') of pure ELC material is shown in the inset of

Fig. 6.2 and the solid lines represents the best theoretically fit data by using Cole–

Cole model. As seen from the inset, the dielectric permittivity (ε') of pure ELC

material continuously decreases in Sm C* phase near the transition temperature of Sm

C* to Sm A* (~28°C). The value of dielectric permittivity (ε') in ELC is very high at

lower frequencies and almost constant at higher frequencies in both Sm C* and Sm

A* phases. The ELC material (BDH 764E) doped with ZnO NPs shows a drastic

change in the dielectric relaxation processes as shown in Fig. 6.2.

Figure 6.2: Dispersion curves of dielectric permittivity (εεεε′′′′ vs frequency) at

different temperatures for ZnO NPs doped BDH 764E and in inset, for pure

BDH 764E sample cells, under no bias. Here, solid lines represent the

theoretically calculated data.

102

103

104

105

106

0

50

100

150

200

102

103

104

105

106

0

20

40

60

80

100

120

140

160

180

Pure BDH 764E

εε εε'

Frequency (Hz)

20oC

20oC

22oC

22oC

24oC

24oC

25oC

25oC

26oC

26oC

28oC

28oC

29oC

29oC

31oC

31oC

32oC

32oC

εε εε'

Frequency (Hz)

20 o

C

20 o

C

24 o

C

24 o

C

26 o

C

26 o

C

28 o

C

28 o

C

30 o

C

30 o

C

32 o

C

32 o

C

34 o

C

34 o

C

35 o

C

35 o

C

36 o

C

36 o

C

37 o

C

37 o

C

38 o

C

38 o

C

40 o

C

40 o

C

160

The high dielectric permittivity (ε׳) which is due to phason mode in Sm C* phase

appears up to 36°C. This shows the shifting in transition temperature (around 8°C) of

Sm C* to Sm A* phase of BDH 764E material by doping ZnO NPs in it.

Figure 6.3 shows the behavior of dielectric loss factor (tan δ) versus frequency of

ELC material (BDH 764E) doped with 1 wt % ZnO NPs, at different temperatures

while the inset of this figure shows the behavior of loss factor (tan δ) versus frequency

of pure BDH 764E material at same parameters.

Figure 6.3: Behavior of dielectric loss factor (tan δ) with frequency at different

temperatures for ZnO NPs doped BDH 764E and in inset, for pure BDH 764E

sample cells, under no bias.

From these graphs, one can confirm the change in phase transition temperature. As

seen from the inset of Fig. 6.3, there is an abrupt increment in the value of relaxation

frequency near 29oC while for ZnO NPs doped BDH 764E, the increment in

relaxation frequency is about 36ºC. In Sm C* phase, there are two or more than two

relaxation peaks present in ZnO NPs doped BDH 764E, which was totally absent in

pure BDH 764E material.

The phase transition (Sm C* to Sm A*) can be varied depending upon the

concentration (C) of ZnO NPs in BDH 764E material. In our study, we have found

102

103

104

105

106

0.0

0.5

1.0

1.5

2.0

2.5

3.0

102

103

104

105

106

0.0

0.5

1.0

1.5

2.0

2.5

3.0

26 C

27 C

28 C

29 C

30 C

31 C 32 C

(Pure BDH 764E)

tan δδ δδ

Frequency (Hz)tan

δδ δδ

Frequency (Hz)

26o

C

27o

C

28o

C

29o

C

30o

C

32o

C

34o

C

35o

C

36o

C

37o

C

38o

C

161

that when the value of C of ZnO NPs is around 1 wt%, then the shift in transition

temperature is maximum. The above graphs shows the behavior of dielectric

permittivity (ε') and loss factor (tan δ) with frequency, for BDH 764E material doped

with 1 wt % ZnO NPs [Figs. 6.2 & 6.3]. Figure 6.4 shows the behavior of dielectric

permittivity (ε') and loss factor (tan δ) with frequency for different values of C (< 1

wt %, i.e., 0.5 wt % and >1 wt %, i.e., 2 wt %).

Figure 6.4: Frequency dependences of (a) dielectric permittivity (εεεε') and (b)

dielectric loss factor (tan δ) for 0.5 wt % ZnO NPs doped BDH 764E, while (c)

dielectric permittivity (εεεε') and (d) dielectric loss factor (tan δ) for 2 wt % ZnO

NPs doped BDH 764E sample cells, at different temperatures and under no bias.

102

103

104

105

106

0.0

0.5

1.0

1.5

2.0

2.5

3.00.5 wt % ZnO NPs doped BDH 764E

tan

δδ δδ

Frequency (Hz)

24oC

26oC

28oC

29oC

30oC

32oC

34oC

35oC

36oC

38oC

40oC

(b)

102

103

104

105

1060

40

80

120

160

0.5 wt % ZnO NPs doped BDH 764E

εε εε'

Frequency (Hz)

24oC

26oC

28oC

29oC

30oC

33oC

35oC

36oC

38oC

40oC

(a)

102

103

104

105

106

0

40

80

120

160

200

ε ε ε ε '

2 wt % ZnO NPs doped BDH 764E

Frequency (Hz)

24oC

25oC

26oC

27oC

28oC

30oC

32oC

34oC

35oC

(C)

102

103

104

105

106

0.0

0.5

1.0

1.5

2.0

2.5

3.0

(d) 2 wt % ZnO NPs doped BDH 764E

24oC

26oC

27oC

28oC

29oC

30oC

31oC

32oC

34oC

35oC

tan

δδ δδ

Frequency (Hz)

162

From the figures one can see that as the value of C increases, the shift of the transition

temperature decreases. At C ~ 2 wt %, there is no shift in transition temperature with

the pure BDH 764E sample as shown in Figs 6.4 (c) and (d). However, at C ~ 0.5

wt %, there is a notable shift in transition temperature (from 28oC to 35oC) as shown

in Figs 6.4 (a) and (b).

The behavior of dielectric loss factor (tan δ) of 0.5 wt % ZnO NPS doped BDH 764E

with frequency, is almost similar as in case of 1 wt %, however, the shift in transition

temperature is little less (around 1 to 2oC). The comparable enhancement in transition

temperature with ZnO NPs addition has also been checked in another ELC material,

Felix-20. Figure 6.5 (a) shows the behaviour of dielectric permittivity (ε') as a

function of frequency for a large temperature range of Felix-20 material doped with

ZnO NPs having

Figure 6.5: Frequency dependences of (a) dielectric permittivity (εεεε') and (b)

dielectric loss factor (tan δ) of 1 Wt % ZnO NPs doped ELC (Felix-20) material

and in inset, for pure Felix-20 sample, at different temperatures and under no

bias.

C ~ 1 wt%, while the dielectric permittivity (ε') of pure Felix-20 has shown in the

inset of Fig. 6.5 (a). As seen from the inset, the dielectric permittivity (ε') of pure

Felix-20 continuously decreases in Sm C* phase near the transition temperature of Sm

102

103

104

105

106

0

40

80

120

160

200

240

100

101

102

103

104

0

20

40

60

80

11o C

13o C

15o C

17o C

19o C

21o C

23o C

ε ε ε ε '

Frequency (Hz)

Pure Felix-20

εε εε '

Frequency (Hz)

15o C

18o

C

22o C

24o

C

25o

C

26o

C

27o

C

28o

C

30o

C

ZnO NPs doped Felix-20 (a)

102

103

104

105

106

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Frequency (Hz)

ZnO NPs doped Felix-20

15o C

18o C

20 oC

22o C

24o C

25o C

26o C

28o C

30o C

tan

δδ δδ

(b)

163

C* to Sm A* (18°C). The value of dielectric permittivity (ε') in Felix-20 is very high

at lower frequencies in Sm C* phase. The ELC material (Felix-20) doped with ZnO

NPs shows a drastic change in the dielectric relaxation processes as shown in Fig.

6.5(a). The high dielectric permittivity (ε׳) in Sm C* phase appears up to 26°C. This

shows the shifting in transition temperature of Sm C* to Sm A* phase (from 18°C to

about 26°C) of Felix-20 material by doping ZnO NPs in it. Figure 6.5 (b) shows the

behavior of dielectric loss factor (tan δ) versus frequency of Felix-20 material doped

with ZnO NPs, at different temperatures. From this graph, one can confirm the change

in phase transition temperature. There is an abrupt increment in the value of relaxation

frequency of ZnO NPs doped Felix-20 material up to the temperature about 26ºC, In

Sm C* phase, there are two or more than two relaxation peaks present in ZnO NPs

doped Felix-20 material as in case of another ELC material (BDH 764E) doped with

ZnO NPs. The edge of the high frequency dielectric loss above 100 KHz in Figs. 6.3

and 6.5 (b), occurs due to the finite resistance of the ITO coating on the glass

substrate of the sample cell. This ITO effect on dielectric measurements has also been

reported elsewhere [35].

Figure 6.6 shows the Cole-Cole plots at four different temperatures, three in Sm C*

Figure 6.6: The Cole-Cole plots for ZnO NPs doped BDH 764E sample in Sm C*

(25ºC, 30ºC and 34ºC) and Sm A*(38ºC) phases, under no bias field.

0 50 100 150 200 2500

50

100

150

200

34 o

C

30 o

C

25o

C

0 10 20 30 40 50 60 700

10

20

30

40

50

60

αααα2=0.139

αααα=0.3

αααα1=0.025

εε εε''

εεεε'

38oC

34oC

αααα1=0.011

αααα=0.028

αααα2=0.139

αααα2=0.056

αααα=0.3

αααα1=0.025

εε εε''

εεεε'

38 o

C

ZnO NPs doped BDH 764E

164

phase (25oC, 30

oC and 34

oC) and one in Sm A* phase (2

o above transition

temperature) for ZnO NPs added BDH 764E material while inset of the figure shows

the magnifying Cole–Cole plots for ZnO NPs doped ELC at 2o below (34oC) and 2o

above (38oC) the transition temperature (38oC), respectively. From Fig. 6.6, one can

see that in deep Sm C* phase (i.e., 25oC) by fitting the expression of equation (1),

only one relaxation behavior is present which is attributed to Goldstone mode while

further increase in temperature or near transition temperature (i.e., 30oC and 34

oC),

one can observe that the Cole-Cole diagrams are the superposition of two semicircles,

i.e., at this temperature range, two modes are contributing to the dielectric response.

At higher temperature or in Sm A* phase (38ºC), only one relaxation behavior is

present which is attributed to soft mode in case of ZnO doped ELC material.

Figure 6.7 shows the Cole-Cole plots at three different temperatures, two plots in Sm

C* phase (20ºC and 26ºC) and one in Sm A* phase (31ºC) for pure ELC (BDH 764E)

Figure 6.7: The Cole-Cole plots for pure BDH 764E sample in Sm C* (20ºC and

26ºC) and Sm A* (31ºC) phases, under no bias field.

material. From all above Cole-Cole plots for pure ELC material, one can observe that

there is only single relaxation behavior in complete temperature range.

0 30 60 90 120 150 1800

30

60

90

120

150

26o

C

31o

C

20o

C

αααα=0.011

αααα=0.11

αααα=0.028εε εε''

εεεε'

Pure BDH 764E

165

The behavior of relaxation frequency (νR,ε'') in Sm C* and Sm A* phases has been

shown in Fig. 6.8, calculated by the formula , " ,tanR R oε δν ν ε ε∞= [36]. Figure 6.8(a)

shows the relaxation frequency versus temperature for ZnO NPs doped ELC and inset

shows the same for pure ELC material, without any bias application. The solid line

represents the best theoretically calculated data by using Cole-Cole model through the

experimental points. As seen from the inset of Fig. 6.8(a), the relaxation frequency is

found to be independent of temperature in deep Sm C* phase and at transition from

Sm C* to Sm A* phase there is a continuous increment in the value of relaxation

frequency for pure ELC material. In case of ZnO NPs doped ELC material, two

relaxation peaks present which we have been discussed earlier in Fig. 6.3. In Fig.

6.8(a) the behavior of both relaxation peaks have been shown with respect to

temperature.

Figure 6.8: (a) Behavior of relaxation frequency with respect to temperature for

ZnO NPs doped ELC (BDH 764E) under no bias and in inset, the same behavior

for pure ELC sample. The symbols show the experimental points where the filled

square shows first dielectric process and filled circles shows the second dielectric

process. Solid lines represents the theoretically calculated data for the first and

second dielectric processes by using Cole-Cole equation, while (b) shows the

experimental behavior of relaxation frequency with different bias (0 V, 5 V and

10 V) for ZnO doped BDH 764E.

20 25 30 35 40

0

10

20

30

40

50

60

(a)

20 22 24 26 28 30 32

0

2

4

6

8

10

12

14

νν ννR

, εε εε''(

KH

z)

Temperature(oC)

Temperature (oC)

νν ννR, εε εε

'' (

KH

z)

20 25 30 35 40

0

15

30

45

60

75(b)

νν ννR

, εε εε'' (

KH

z)

Temperature (oC)

0 V

5 V

10 V

166

The solid lines of theoretically calculated data and the experimental data are almost

matching with each-other. As seen in the Fig. 6.8(a), one is able to separate both the

dielectric realaxation peaks in ZnO NPs doped ELC material. The frequency

separation of both relaxation peaks increases with temperature and the peak along

with the low frequency side almost vanishes after transition temperature of ZnO NPs

doped ELC material. After transition temperature only the regular soft mode process

remains which shows the usual behavior of ELC materials. Such type of low

frequency relaxation peak has also been observed in water added and graphene oxide

(GO) added ELC samples [32, 37].

In Fig. 6.8 (b), the relaxation frequency has been plotted with respect to temperature

for ZnO NPs doped ELC material at different bias values (0 V, 5 V and 10 V). The

behavior of νR,ε'' at 0 V bias for pure and ZnO NPs doped ELC is same, as below

transition temperature (Sm C* to Sm A*) it is independent of the temperature and

above transition there is a continuous increment in the value of νR,ε''. Now by applying

different bias values (5 V and 10 V) in deep Sm C* phase, the relaxation frequency is

independent of the temperature which starts decreasing near the transition

temperature. After transition temperature there is a abrupt increase in the value of

νR,ε''. The behavior of νR,ε'' with bias in ZnO NPs doped ELC material is similar to pure

ELC and it does not obey the Curie- Weiss law in Sm C* phase due to high

electroclinic coefficient of such ELC materials. Thakur et al. have reported earlier that

pure ELC materials do not obey the Curie-Weiss law near the transition temperature

[38]. In Sm A* phase the relaxation frequency is temperature dependent like in other

FLC materials.

The parameters like dielectric strength (∆ε), distribution parameter (α) and the

corresponding relaxation frequency (νR,ε'') obtained experimentally in a wide

temperature range have been given in Table 6.1 and 6.2, for both pure and ZnO NPs

doped ELC material and these parameters compared with the data calculated from

Cole-Cole model. One can observe from both the tables that there is an increment in α

with respect to temperature. The small values of α for both pure and ZnO NPs doped

ELC suggest that dielectric process is very close to Debye type of relaxation. The

increase of α with temperature indicates that the both samples become less dispersive

167

at higher temperatures and must show more than one relaxation process at some

higher temperatures in Sm C* phase.

Table 6.1: Variation of dielectric strength (∆ε), distribution parameter (α) and

relaxation frequency (νR,ε'') with temperature for pure ELC material and

comparison with theoretically calculated data.

Temperature

(oC)

∆ε α

(Experimental)

α

(Theoretical)

υR,ε'' (KHz)

(Experimental)

υR,ε'' (KHz)

(Theoretical)

20 166.25 0.011 0.010 0.222 0.232

22 161.56 0.017 0.030 0.225 0.256

24 150.33 0.022 0.032 0.232 0.294

25 138.70 0.022 0.035 0.241 0.312

26 128.47 0.028 0.045 0.268 0.333

27 109.45 0.028 0.050 0.322 0.335

28 83.65 0.031 0.074 0.453 0.498

29 40.75 0.033 0.080 2.205 1.923

30 28.86 0.100 0.090 3.674 3.122

31 18.16 0.110 0.096 7.420 6.670

32 12.66 0.144 0.100 11.110 9.523

168

Table 6.2: Variation of dielectric strength (∆ε), distribution parameter (α) and

relaxation frequency (νR,ε'') with temperature for ZnO NPS doped ELC material

and comparison with theoretically calculated data.

Temperature

(oC)

∆ε α

(Experimental)

α

(Theoretical)

υR,ε'' (KHz)

(Experimental)

υR,ε'' (KHz)

(Theoretical)

20 209.01 0.011 0.010 0.120 0.140

22 203.85 0.017 0.017 0.120 0.150

24 190.70 0.028 0.020 0.124 0.172

25 190.03 0.028 0.026 0.124 0.183

26 187.57 0.033 0.032 0.125 0.200

27 176.59 0.033 0.034 0.128 0.200

28 150.61 0.056 0.065 0.183 0.208

30 109.11 0.011 0.056 0.020 0.080 0.103 1.680 0.127 1.600

32 83.70 0.022 0.133 0.030 0.140 0.122 3.890 0.161 4.454

34 59.83 0.025 0.139 0.035 0.140 0.170 5.010 0.192 5.879

35 40.66 0.028 0.139 0.058 0.190 0.250 6.500 0.220 6.676

36 29.30 0.031 0.144 0.068 0.200 0.354 8.630 0.300 9.069

37 19.88 0.033 0.278 0.080 0.330 0.520 9.990 0.580 9.990

38 10.74 0.300 0.260 9.401 6.900

40 3.62 0.310 0.220 48.230 45.450

169

The bias dependent relaxation processes have been studied in Sm C* phase of ZnO

NPs doped and pure ELC material (inset) as shown in Fig. 6.9 (a). Figure 6.9 (a)

shows the behavior of dielectric loss factor (tan δ) at different applied biases for both

doped and pure ELC material. At lower bias voltages (in the range of 0.1 V to 1 V) in

Sm C* phase, we see two or more than two low frequency relaxation peaks in case of

ZnO NPs added ELC samples which can also be confirmed by Fig. 6.9 (b).

Figure 6.9: (a) Behavior of dielectric loss factor (tan δ) for ZnO NPs doped ELC

and in inset, for pure ELC material with frequency at different values of applied

voltages at 29°°°°C (Sm C* phase), while (b) shows the Cole-Cole plot for the ZnO

NPs doped ELC at 29°°°°C for 0.3 V bias and in inset the solid line shows the

theoretical calculated data for the same experimental data.

Figure 6.9 (b) shows the Cole-Cole plot at 0.3 V in Sm C* phase (29oC) for ZnO NPs

added ELC material. The Cole-Cole semicircles in Fig 6.9 (b), show the presence of

more than two relaxation behaviors. Inset of Fig. 6.9 (b) shows the theoretical fitting

for the same experimental data and exhibit three relaxation processes (three different

values of α), while the pure ELC sample exhibit only one relaxation process due to a

characteristic Goldstone mode as shown in inset of Fig. 6.9 (a). The appearance of

two or more than two relaxation peaks with the application of bias in Sm C* phase

suggest that there is a strong interaction between ZnO NPs and the molecules of ELC

material and the low frequency process is associated with the Goldstone mode of ELC

102

103

104

105

106

0.0

0.4

0.8

1.2

1.6

2.0(a)

102

103

104

105

106

0.0

0.2

0.4

0.6

0.8

1.0

tan

δδ δδ

Frequency(Hz)

Pure BDH 764 E 0 V

0.2 V

0.4 V

0.8 V

1 V

2 V 5 V

10 V

tan

δδ δδ

Frequency (Hz)

0 V

0.1 V

0.2 V

0.3 V

0.4 V

0.5 V

0.6 V

0.7 V

1 V

2 V

5 V

10 V

0 20 40 60 80 100 1200

20

40

60

80

100(b)

102

103

104

105

106

0

20

40

60

80

100

αααα3

αααα2

αααα1

Theoretical data

εε εε'

Frequency(Hz)

αααα3= 0.056

αααα2= 0.072

αααα1= 0.194

(Bias = 0.3V)

ε ε ε ε ''

εεεε'

170

material. Such additional low frequency relaxation peaks have been observed in the

GO added ELC sample also, which can be seen by Fig. 6.10.

Figure 6.10: Behavior of dielectric loss factor (tan δ) with frequency for GO

doped ELC (BDH 764E) sample at (a) different temperatures and under no bias,

and (b) different values of applied voltages at 26°°°°C (Sm C* phase).

Figure 6.10(a) shows the behavior of dielectric loss factor (tan δ) with frequency of

GO doped ELC (BDH 764E) sample with temperature. The low frequency relaxation

processes peaks can be clearly seen in dielectric loss factor (tan δ) vs frequency

curves which was not observable in pure ELC sample. The frequency separation of

both relaxation peaks increases with temperature and the low frequency peak almost

vanishes near transition temperature. For further investigation, the bias dependent

relaxation process for GO doped ELC sample has been studied with bias in Sm C*

phase. Figure 6.10(b) shows the behavior of dielectric loss factor (tan δ) vs frequency

curves with different applied bias values in Sm C* phase. The slow process frequency

peak shows reduction in characteristic process whereas the general Goldstone mode

process peak shows the increment with bias. The low frequency peak disappears

completely up to 0.8 V biases, which shows the complete suppression with bias and

only Goldstone mode exists there. Pure ELC sample exhibits no slow relaxation

process either with temperature or with bias but a regular Goldstone mode is

observable as shown in the insets of Figs. 6.3 and 6.9 (a). Effect of GO on other

102

103

104

105

106

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

GO doped BDH 764E

tan

δδ δδ

Frequency (Hz)

10o C

15o C

18o C

20o C

24o C

26o C

27o C

28o C

29o C

30o C

35o C

(a)

102

103

104

105

106

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(b)

tan

δδ δδFrequency(Hz)

0 V

0.1 V

0.2 V

0.3 V

0.4 V

0.5 V

0.6 v

0.7 V

0.8 V

1 V

2 V

3 V

4 V

5 V

10 V

GO doped BDH 764E

171

physical parameters of ELC material has been discussed in our earlier chapter of

thesis (Chapter IV).

In order to confirm the origin of the low relaxation peak, a non dipolar organic

material (heptane) has been added in the same ELC material and in same aspect ratio.

Figure 6.11(a) shows the behavior of dielectric loss factor (tan δ) with frequency for

heptane doped ELC (BDH 764E) sample at different temperatures while Fig. 6.11(b)

shows the behavior of dielectric loss factor (tan δ) with different values of applied

biases. From both the figures one can observed that there is only single relaxation

process present in complete temperature range and the relaxation process is almost

similar to pure ELC material.

Figure 6.11: Behavior of dielectric loss factor (tan δ) with frequency for heptane

doped ELC (BDH 764E) sample at (a) different temperatures and under no bias,

and (b) different values of applied voltages at 26°°°°C (Sm C* phase).

These observations show that the low frequency process in GO doped ELC could be

due to dipolar contribution of dopant material along with the main dipolar component

of used ELC material.

Figures 6.12(a) and (b) show the behavior of spontaneous polarization (PS) and the tilt

angle (θ) with respect to temperature for both ZnO NPs doped and pure BDH 764E

102

103

104

105

106

0

1

2

3

4

5 Heptane doped BDH 764E

tan

δδ δδ

Frequency (Hz)

15OC

20OC

24OC

26OC

27OC

28OC

29OC

30OC

32OC

35OC

(a)

102

103

104

105

106

0

1

2

3

4

Heptane doped BDH 764Eta

n δδ δδ

Frequency (Hz)

0 V

0.1 V

0.2 V

0.3 V

0.4 V

0.5 V

0.6 V

0.7 V

0.8 V

1 V

2 V

5 V

(b)

172

samples. From Fig. 6.12(a), it can be observed that there is an improvement in the

polarization (Ps) with respect to temperature by doping ZnO NPs into ELC as

compared to the pure ELC material. The improved polarization (Ps) is due to the

arrangement of dipole moments of all ZnO NPs in the direction of the applied field

which will contribute to the dipole moment of pure ELC material.

Figure 6.12: Behavior of (a) spontaneous polarization (PS) and (b) tilt angle (θ)

with respect to temperature, for both ZnO NPs doped and pure BDH 764E

samples.

Figure 6.12(b) shows the plot of the measured tilt angle (θ) for both pure and ZnO

NPs doped ELC materials with respect to temperature. There is no remarkable

difference in tilt angle (θ) of both pure and ZnO NPs doped ELC with temperature.

This is because the ZnO NPs have suppressed the randomized scattering of molecules

around diffuse cone but the tilt of individual molecule remains the same around the

diffuse cone due to which the switching remains in the limit of cone. The phase of

ELC material has been extended by ZnO NPs only in the limit of electroclinic effect

in Sm A* phase, therefore the larger the electroclinic effect the greater the phase

extension (increase of transition temperature from Sm C* to Sm A* phases). This

phase extension has been observed only in ELCs and not in conventional FLCs.

20 24 28 32 36 400

4

8

12

16 Pure BDH 764E

ZnO NPs doped BDH 764E

Til

t a

ng

le (

θθ θθ)

Temperature (oC)

(b)

20 24 28 32 36 40

0

10

20

30

40

50

60

ZnO NPs doped BDH 764E

PS (

nC

/cm

2)

Temperature (oC)

Pure BDH 764E (a)

173

Figure 6.13 shows the variation in the ratio of polarization (Ps) and optical tilt angle

(θ), i.e., (Ps/θ) with respect to temperature for both pure and ZnO NPs doped ELC

materials. There is a rapid fall in the value of Ps/θ for pure material as compared to

ZnO NPs added material. ZnO NPs doped ELC shows a gradual fall in the ratio which

suggests a rise in the order of ELC molecules. As the value of spontaneous

polarization (PS) has increased and there is almost no change in the value of tilt angle

(θ), hence, the ratio Ps/θ will be more in case of ZnO NPs doped ELC material. One

can also validate the shift in Sm C* to Sm A* phase transition temperature in the ZnO

NPs doped ELC material from above Fig. 6.13.

Figure 6.13: Behavior of the ratio of coupling constant (Ps/θ) with respect to

temperature, for both ZnO NPS doped and pure BDH 764E samples.

Figure 6.14 shows the variation in rotational viscosity (η) with respect to temperature

for both pure and ZnO NPs doped ELC samples without any bias application. There is

an increment in the values of rotational viscosity (η) with respect to temperature, in

ZnO NPs doped ELC as compared to pure ELC.

20 24 28 32 36 40

0

1

2

3

4

5 Pure BDH 764E

ZnO NPs doped BDH 764E

Ps /

θθ θθ

Temperature (oC)

174

Figure 6.14: Behavior of rotational viscosity (η) with respect to temperature, for

both ZnO NPs doped and pure BDH 764E samples.

The increment in rotational viscosity (η) decreases with respect to temperature which

means as we increase the temperature the gap between the values of rotational

viscosity (η) in case of pure and ZnO NPs added ELCs decreases. This increment of

rotational viscosity (η) is a result of the strong interaction between ZnO NPs with

ELC molecules and the possible formation of clusters.

Addition of ZnO NPs into ELCs causes redistribution of interaction energies of ELC

molecules and long-range molecular interactions of the system. This interaction is

found to be dependent on the orientation and the local ordering of the ELC molecules

with respect to the ZnO NPs. It has been observed that ZnO NPs can interact with

surrounding ELC dipolar molecules and tie them together to respond to an external

driving field in more unison [23]. The origin of permanent dipole moment is based on

ZnO structure to some extent. The ideal wurtzite structure never exists in which each

tetrahedron has Td symmetry, but in a real wurtzite compound AB, a slight

displacement of the A and B sublattices along the hexagonal c-axis occurs. The c/a

ratio [which is defined as the ratio of magnitude of the third axis (c) to the axis lying

in the basal plane (a); where a and c are the lattice parameters] should be 1.633

whereas in case of ZnO it is 1.6018 [39]. Thus, the presence of a permanent dipole

moment in real wurtzite, e.g., ZnO, can be attributed to C3v-distortion of the

20 24 28 32 36 40

0

100

200

300

400

500

600

η

η

η

η (( ((m

Pa S

)) ))

Temperature (oC)

Pure BDH 764E

ZnO NPs doped BDH 764E

175

elementary AB4 tetrahedron. Shim and Guyot-Sionnest proposed that a major

contribution for the possible origins of the large dipole moments includes internal

bonding geometry, shape asymmetry, surface strain, and the surface localized charges

[40]. The value of dipole moment of ELC molecules (>1.5 D) is very small as

compared to ZnO NPs (>100 D) having diameter ~7 nm and such huge dipole

moment of the ZnO NPs generate a powerful field inducing dipolar interaction that

compete with spontaneous molecular interaction and this dipolar interaction enhances

the anchoring of ELC molecules around the ZnO NPs. Such strong anchoring of ELC

molecules around ZnO NPs bring about long range orientational distortions which

may give rise to well ordered molecular structure of ELC materials and enhance the

Sm C* to Sm A* phase transition temperature. The dielectric relaxation behavior of

ZnO NPs doped ELC confirmed the existence of a low frequency peak along with the

Goldstone mode in the Sm C* phase. We found the same low frequency behavior in

ELC material by doping of different NPs or fluids [33, 37]. All the NPs or fluids that

have been used in earlier studies (i.e., GO, water, glycerol and Au NPs), have some

dipolar moment [41-43] and when we doped the ELC material by a non dipolar

organic material (heptane) then there was no additional dielectric peak. Hence, the

effective dipolar contribution of NPs or fluids into ELC dipole moment could be the

probable reason for the occurrence of the additional peak. The large value of dipole

moment of ZnO NPs also affects the dielectric relaxation processes in ELCs due to

the strong interaction between ZnO NPs and ELC molecules.

6.4 CONCLUSIONS

The results presented in this chapter confirmed that the transition temperature of ferro

to para electric ( Sm C* to Sm A*) phase of ELC materials can be increased by the

doping of ZnO NPs. Due to the large dipole moment of ZnO NPs, a strong molecular

interaction takes place between ZnO NPs and ELC molecules which can reduce the

randomization of ELC molecules. Our experimental outcome illustrates the existence

of one or more than one additional low frequency relaxation peaks along with

Goldstone mode in Sm C* phase and these relaxation peaks vanishes at the transition

temperature. The doping of 1 wt% of ZnO NPs makes a significant increase in the

176

values of spontaneous polarization (PS) and rotational viscosity (η) of ELC material.

This work is also helpful for various dynamic studies of ELCs and open up innovative

ways to implement ELCs for potential applications at higher temperature.

177

References

[1] W. H. De Jong and P. J. A. Borm , Int. J. Nanomedicine 3, 133 (2008).

[2] G. Reiss and A. Hutten, Nature Mater. 4, 725 (2005).

[3] J. F. Scott, Ferroelectric memories, Berlin: Springer, pp. 191-206, (2000).

[4] Y. Shiraishi, N. Toshima, K. Maeda, H. Yoshikawa, J. Xu, and S. Kobayashi,

Appl. Phys. Lett. 81, 2845 (2002).

[5] J. Prakash, A. Choudhary, A. Kumar, D. S. Mehta, and A. M. Biradar, Appl.

Phys. Lett. 93, 112904 (2008).

[6] J. Prakash, A. Choudhary, D. S. Mehta, and A. M. Biradar, Phys. Rev. E 80,

012701 (2009).

[7] W. Lee, C. Y. Wang, and Y. C. Shih, Appl. Phys. Lett. 85, 513 (2004).

[8] N. Mizoshita, K. Hanabusa, and T. Kato, Adv. Funct. Mater. 13, 313 (2003).

[9] J. Thisayukta, H. Shiraki, Y. Sakai, T. Masumi, S. Kundu, Y. Shiraishi,

N. Toshima, and S. Kobayashi, Jpn. J. Appl. Phys. 43, 5430 (2004).

[10] H. Yoshikawa, K. Maeda, Y. Shiraishi, J. Xu, H. Shiraki, N. Toshima, and

S. Kobayashi, Jpn. J. Appl. Phys. 41, L1315 (2002).

[11] Y. Sakai, N. Nishida, H. Shiraki, Y. Shiraishi, T. Miyama, N. Toshima, and

S. Kobayashi, Mol. Cryst. Liq. Cryst. 441, 143 (2005).

[12] H. Shiraki, S. Kundu, Y. Sakai, T. Masumi, Y. Shiraishi, N. Toshima, and

S. Kobayashi, Jpn. J. Appl. Phys. 43, 5425 (2004).

[13] A. Kumar and A. M. Biradar, Phys. Rev. E 83, 041708 (2011).

[14] N. Hongsith, C. Viriyaworasakul, P. Mangkorntong, N. Mangkorntong, and

S. Choopun, Ceram. Inter. 34, 823 (2008).

[15] X. Wang, J. Song, and Z. L. Wang, J. Mater. Chem. 17, 711 (2007).

[16] M. S. Arnold, P. Avouris, Z. W. Pan, and Z. L. Wnag, J. Phys. Chem. B 107,

659 (2003).

[17] N. Kumar, A. Dorfman, and J. I. Hahm, Nanotechnology 17, 2875 (2006).

[18] J. H. Jun, H. Seong, K. Cho, B. M. Moon, and S. Kim, Ceram. Inter. 35, 2797

(2009).

[19] A. Malik, A. Choudhary, P. Silotia, and A. M. Biradar, J. Appl. Phys. 110,

064111 (2011).

178

[20] T. Joshi, A. Kumar, J. Prakash, and A. M. Biradar, Appl. Phys. Lett. 96, 253109

(2010).

[21] H. Jiang and N. Toshima, Chem. Letts. 38, 566 (2009).

[22] J. Y. Huang, L. S. Li, and M. C. Chen, J. Phys. Chem. C 112, 5410 (2008).

[23] L. S. Li and J. Y. Huang, J. Phys. D: Appl. Phys. 42, 125413 (2009).

[24] T. Carlsson, B. Zeks, C. Filipic, and A. Levstik, Phys. Rev. A 42, 877 (1990).

[25] J. Pavel, M. Glogarova, and S. S. Bawa, Ferroelectrics 76, 221 (1987).

[26] F. Gouda, G. Andersson, M. Matuszczyk, K. Skarp, and S. T. Lagerwall, J.

Appl. Phys. 67, 180 (1990).

[27] A. M. Biradar, S. Wrobel, and W. Haase, Phys. Rev. A 39, 2693 (1989).

[28] A. Levstik, T. Carlsson, C. Filipic, I. Levstik, and B. Zeks, Phys. Rev. A 35,

3527 (1987).

[29] S. Hiller, L. A. Beresnev, S. A. Pikin, and W. Haase, Ferroelectrics 180, 153

(1996).

[30] S. A. Rozanski and J. Thoen, Liq. Cryst. 32, 331 (2005).

[31] A. M. Biradar, D. Kilian, S. Wrobel, and W. Haase, Liq. Cryst. 27, 225 (2000).

[32] A. Malik, A. Choudhary, P. Silotia, A. M. Biradar, V. K. Singh, and N. Kumar,

J. Appl. Phys. 108, 124110 (2010).

[33] K. Miyasato, S. Abe, H. Takezoe, A. Fukuda, and E. Kuze, Jpn. J. Appl. Phys.

22, L661 (1983).

[34] P. Scherrer, Gött Nachr 2, 98 (1918).

[35] F. Gouda, K. Skarp, and S. T. Lagerwall, Ferroelectrics 113, 165 (1991).

[36] A. Chelkowski, Dielectric Physics, Elsevier Scientific Publishing Co., New

York, (1980).

[37] G. Singh, A. Choudhary, G. V. Prakash, and A. M. Biradar, Phys. Rev. E 81,

051707 (2010).

[38] A. K. Thakur, G. K. Chadha, S. Kaur, S. S. Bawa, and A. M. Biradar, J. Appl.

Phys. 97, 113514 (2005).

[39] T. Nann and J. Schneider, Chem. Phys. Lett. 384, 150 (2004).

[40] M. Shim and P. G. Sionnest, J. Chem. Phys. 111, 6955(1999).

[41] A. Kumar, J. Prakash, A. Choudhary, and A. M. Biradar, J. Appl. Phys. 105,

124101 (2009).

179

[42] T. Joshi, A. Kumar, J. Prakash, and A. M. Biradar, Liq. Cryst. 37, 1433 (2010).

[43] A. Malik, G. Singh, J. Prakash, P. Ganguly, P. Silotia, and A. M. Biradar,

Ferroelectrics 431, 6 (2012).