111

CHAPTER IV

GRAPHENE OXIDE INDUCED HOMEOTROPIC

ALIGNMENT IN FERROELECTRIC LIQUID

CRYSTALS

In this chapter, the studies based on uniform and defect free homeotropic

(HMT) alignment of negative dielectric anisotropic ferroelectric liquid crystal (FLC)

has been described. One can achieve HMT alignment by the treatment of silane over

the surfaces of substrates of FLC samples. In contrast, doping of graphene oxide

(GO) into FLC material favours for a good quality HMT alignment without any

surface treatment of the substrates. The observed HMT alignment has been confirmed

by dielectric and optical studies.

4.1 INTRODUCTION

Ferroelectric liquid crystals (FLCs) are the most promising materials which can be

used in various display devices due to their high speed, better optical contrast and fast

response to external stimuli such as electric field, magnetic field, temperature and

pressure, etc. [1, 2]. Noticeable investigations have been carried out extensively in

order to understand the complex phase transition from ferroelectric (Sm C*) to

paraelectric (Sm A*) phase of FLCs [3-6]. Among all FLCs, a special class of FLC

with considerably wide range of Sm A* phase, is very interesting due to its ultra fast

response and non-layer shrinkage [7-13]. These FLCs are known as electroclinic

liquid crystals (ELCs) and described on the basis of symmetry arguments by Garoff

and Meyer [7, 8].

Thereafter, Andersson et al. put forward the possible application of such type of

liquid crystals (LCs) [13]. ELCs are remained the topic of academic interest for about

a decade. Detailed discussion on ELCs has been given in the first chapter of present

thesis (Chapter I).

112

The liquid crystalline state is anisotropic owing to the ordering of the constituent

molecules; therefore their alignment with respect to the applied electric field affects

their dielectric and electro-optical properties. The alignment of the FLCs is extremely

important both for elucidation of the molecular processes as well as for practical

applications because a good alignment layer provides high optical contrast. In an

aligned sample, the director points, on an average, in the same direction over the

entire sample. There are mainly three configurations of alignments, namely,

homogeneous (HMG), homeotropic (HMT) and hybrid configurations. The detail

description of all these alignments is given in Chapter II of present thesis. The

orientation of the molecules on the surface is characterized by the pretilt angle (θ,

angle between molecules to the plane of the surface) and the anchoring energy.

Depending on the molecular alignment angles in case of HMG, θ will be 0°, in case of

HMT, θ will be π/2 while in case of tilted or hybrid alignment, θ will lie in between

0° to π /2 [14].

However, the nature of alignment required differs in different geometries depending

on the display mode used. In twisted nematic (TN) or surface stabilized ferroelectric

liquid crystal (SSFLC) displays; HMG alignment is required whereas HMT alignment

is useful in applications as liquid crystal displays (LCDs) for high information display

devices, large area LCD TVs, and digital medical imaging [15, 16]. Negative

dielectric anisotropy LCs have recently seen extensive use in flat panel display

applications, especially in the vertically aligned mode since such a structure has good

contrast combined with fast switching times [17].

4.1.1 HMT alignment in LCs by surface treatment

In HMT alignment, all of the long molecular axes are perpendicular to the substrate.

The surfaces of the substrates such as glass, oxides, and metals exhibit the ability to

align LC molecules vertically [18]. It is reported that the HMT alignment of LC is

favored when the surface is hydrophobic and has a lower polar surface energy [19,

20]. The pretilt angle increases as the polar surface energy of the alignment film

decreases, thus the weak polar surface energy plays an important role in the HMT

alignment of LC [20]. To achieve HMT alignment, the glass substrates are treated

113

with the surfactant materials such as hexadecyltrimethyl-ammonium-bromide

(HTAB), N,N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilyl chloride

(DMOAP), silane, lecithin, etc. [21, 22]. The molecules of such surfactants tend to

align themselves perpendicular to the substrate and thus impart the HMT alignment to

LC molecules. Kahn has demonstrated the importance of physicochemical forces,

e.g,. vander waals, hydrogen bonding and dipolar forces that play an important role in

the alignment of molecules [23]. The deposition of the silane layer and large alkyl

side chain alcohols onto the indium tin oxide (ITO) layer makes the surface

hydrophobic and also decreases the surface energy value which is attributed to the

long alkyl chain in the silane molecule [24]. Apart from these methods, Hiroshima et

al. proposed rotational oblique evaporation of silicon monoxide (SiO) in order to

obtain HMT alignment [25-26]. Matsumoto et al. proposed that tetrachloro-µ-

hydroxo-µ-carboxylatodichromium (III) complexes could be used to induce HMT

alignment [27]. The HMT alignment of LCs using fluorinated diamond like carbon

thin films is an example of such techniques [28]. The alignment achieved in all above

cases propagated from the substrate surface to the bulk of the LC and hence is clearly

dependent on the surface treatments. Cheng et al. have succeeded to achieve vertical

alignment by adding an appropriate dopant in to LC material in contrast to the surface

contact treatment techniques. They explored the possibility of favorable HMT

alignment of LCs having large negative dielectric anisotropy (∆ε) by the addition of

dopant material possessing a longitudinal dipole and therefore a positive ∆ε in LCs

[29].

4.1.2 Nanomaterials induced HMT alignment in LCs

The effect of addition of various nanomaterials into LCs has been studied by various

groups around the world for observing improvements in performance of LCs based

display and non-display devices. A little amount of nanomaterials into LC materials

has improved many features of the latter in the form of frequency modulation

response, better optical contrast, non-volatile memory effect, faster electro-optic

response, low driving voltage and enhanced photoluminescence [30-34]. Addition of

different type of nanoparticles (NPs) provides a way to align LC molecules by a non-

114

contact technique or without any surface treatment of the substrates. The NPs of

polyhedral oligomeric silsesquioxane (POSS), induced HMT alignment in nematic

liquid crystals (NLCs) with positive dielectric anisotropy [35]. The electro-optical

properties of POSS NPs induced vertically aligned LC cells were found similar to the

conventional HMT aligned cells. It has also been discussed that the aligning

capability induced by POSS is due to adsorption of POSS on the inner surfaces of the

substrates and also influenced by doping concentration of POSS [35, 36]. The

physical properties of FLCs are quite different from those of the NLCs due to the

differences in the degree of order, the molecular arrangement and the presence of a

permanent dipole moment. It is for this very fact, that nematic phases have been

studied very extensively in both the HMT and HMG state [37-40] compared to few

studies for the HMT alignment in FLCs [41, 42].

Now a days, graphene and graphene oxide (GO) are the interesting nanomaterial for

dynamic research studies. The ease of synthesizing GO and its solution processing

compatibility make it attractive for large area applications including transparent

conductors [43-46], electrical energy storage devices and polymer composites, etc.

[47]. In 2008, Blake et al. used graphene to fabricate electrodes for the LC device and

observed excellent performance with a high contrast ratio [48]. In the beginning of

2010, Safavi and Tohidi designed the LC-Graphite composite electrodes and were

found these electrodes perfect for the application of electric field in different

electrochemical and biosensing applications [49]. GO is an intriguing nanomaterial

and shows very high anisotropic morphology. Becerril et al. evaluated the potential of

highly reduced GO thin films as transparent conductors and proposed a possible route

for addressing the LC devices [43]. Till now graphene and GO have mainly been used

in the fabrication of electrodes and applied in the form of thin film but doping of GO

in LCs is rarely reported in literature. It is for the first time, GO nanomaterial was

doped in ELCs [50].

In this chapter, we described in detail the techniques to achieve perfect HMT

alignment. The silane layer over the surface of ITO coated glass substrate induced

HMT alignment in the studied ELC material [51] while by adding GO into ELC

material we can achieve a perfect HMT alignment without any surface treatment of

the substrate. After applying a bias voltage HMT alignment can be converted in to

115

HMG alignment. Optical and dielectric studies allowed us to confirm the HMT

alignment (geometry). These results will provide a fascinating tool to align FLC

materials in HMT manner with and without surface treatment and such alignment

have important implications for modern LCD technology.

The current chapter also describes the effect of doping of GO on the physical

parameters of the ELC material.

4.2 EXPERIMENTAL DETAILS

GO used in the present study was synthesized by Hummer’s method [52]. Hummer et

al. prepared GO by using a mixture of sulphuric acid (H2SO4), sodium nitrate

(NaNO3) and potassium permanganate (KMnO4). Only the purification step has been

replaced with washing of products step with known concentration of nitric acid

(HNO3). One can remove the impurities of other metal ions and enhance the oxidation

of graphite by this replacement. The product was thoroughly washed with distilled

water and at the end, a golden yellow powder of GO nanomaterial was obtained. The

structural characterization of GO was carried out in detail by powder x-ray diffraction

(XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) [53].

AFM images of GO nanomaterial showed layering morphology having thickness of

~25 nm consisting of each stack thickness of ~5 nm.

Sample cells for the dielectric relaxation processes and electro-optical studies were

prepared by using highly conducting (10-18 Ω/ ) indium tin oxide (ITO) glass

substrates. Two types of sample cells were prepared. In first type cells, we treated two

electrodes by dipping into the silane (phenyl trichlorosilane : toluene in the ratio of 1 :

100) solution for 6-7 minutes, washed in isoproponaol to remove any adsorbed silane

molecules, baking was performed at 90 °C for 20 minutes and finally gently rubbed (~

10-12 strokes) with velvet cloth. In second type cells, both the electrodes were left

untreated (i.e., neither the polymer nor silane was coated or rubbed) and they were

called untreated cells. The morphology of the alignment layer is of great consequence

for the achievement of the desired orientation of the LC molecules. The glass

substrates were assembled facing the electrodes to each other, in the form of cells

maintaining a uniform thickness by using Mylar spacers [54]. The filling of cells has

been done at isotropic temperature of the material.

116

In the present study, we used a commercially available ELC material (BDH 764 E)

having phase sequence as follows:

Cryst. →←− Co7 Sm C* →← Cº28

Sm A* →← Cº73N →← C92º-89

Iso.

where, Cryst. is the crystalline phase, Sm C* is chiral smectic C phase, Sm A* is the

chiral smectic A phase and Iso. is the isotropic phase of studied ELC material,

respectively.

A small amount (0.1- 0.2 wt %) of GO was dispersed in the ELC material at room

temperature and then the composition was introduced into the LC sample cells in

isotropic phase of the ELC material by means of capillary action.

The molecular and collective dynamic dielectric processes were investigated by

broadband dielectric spectroscopy in an electrically shielded parallel plate capacitor

using impedance analyzer 6540A (Wayne Kerr, U.K.) in the frequency range of 20 Hz

to 1 MHz and the probe amplitude for the dielectric measurement was fixed at 0.5 V.

The optical micrographs (i.e. textures) observations of sample cells with different bias

voltages have been recorded using high-resolution crossed polarizing microscope

(AX-40, Carl Zeiss, Germany) [Chapter II]. The material constants such as

spontaneous polarization (Ps) and rotational viscosity (η) have been determined by

using an automatic liquid crystal tester (ALCT, Instec, U.S.A.). The temperature

controller (JULABO F-25 HE) equipment with temperature stability of ± 0.01ºC was

used for controlling the temperature of the sample cell. The sample holder was kept

insulated from the external sources (electrical and thermal).

4.3 RESULTS AND DISCUSSION

In the present study, the HMT configuration in sample cell filled with ELC material

has been achieved either by silane treatment over the surface of substrate or by adding

GO into ELC material without any surface treatment. In HMT configuration,

molecules are aligned perpendicular to the ITO coated glass substrates. The first

section of this chapter is dealing with HMT alignment achieved by silane coating over

the surface of substrate while second section of the chapter is focussed on HMT

alignment induced by GO doping into ELC material.

117

4.3.1 Analysis of alignment in ELCs by surface treatment

In this section, we are describing the HMT configuration achieved by silane treatment

over the surface of ELC samples. The optical and dielectric observations confirmed

the HMT configuration and after applying a bias voltage such HMT alignment can be

converted in to HMG alignment. The experiments have been carried out on both,

HMT aligned and planar or HMG converted (by applying dc bias), sample cells.

4.3.1.1 Optical observations

The optical micrographs of surface treated ELC samples recorded by using polarizing

microscope to confirm the alignment configuration have shown in Figs. 4.1(a-d). The

texture in Fig. 4.1(a) confirms a perfect HMT alignment because no light transmission

occurs through the sample cell and hence a dark field of view appears under the

crossed polarizers.

Figure 4.1: Optical micrographs of HMT aligned ELC material in Sm C* phase

under (a) no bias (b) 10 V (c) 20 V, and (d) 25 V (which changes the orientation

from HMT to HMG configuration).

(c)

(a) (b)

(d)

118

Surprisingly, on applying a bias of 25 V in Sm C* phase, one can obtained a HMG

aligned state [Fig. 4.1(d)] which was confirmed by the changes in the transmission as

seen by the rotation of the cell under the crossed polarizers. Figs. 4.1(a)-(c) show the

textural observation of the HMT aligned sample. After applying 10 V bias, there is no

change in texture [Fig. 4.1 (b)] while by applying 20 V bias, there is a little

transmission of light as shown in Fig. 4.1 (c). It is worth to notice here that the planar

state was preserved even after removing the field up to very high temperature (about

65°C). Hence the alignment observed in silane treated surfaces is not only in HMT

geometry but also it changes into HMG configuration on the application of a

significant field in the Sm C* phase. In order to explore such effect in the Sm A*

phase, the cell was further heated up to the isotropic phase (90°C) and then cooled

slowly to retrieve the HMT state. At a temperature of 38°C, i.e., in the HMT aligned

Sm A* phase [Fig. 4.2(a)], we observed a transition to the planar state on application

of 55 V [Fig. 4.2(b)]. Even after removal of the field, the cell remained in planar

orientation. Chen et al. have described a device geometry using inter digital electrodes

that enables the electrically driven reorientation of Sm A* layers from HMG state to

HMT state and back again [41].

Figure 4.2: Optical micrographs of HMT aligned ELC material in Sm A* phase

under (a) no bias, and (b) 55 V (which changes the orientation from HMT to

HMG configuration).

(a) (b)

119

The observance of dark state is probably due to the expansion of strains and a

balanced torque from electric field, in the cells which align the LC molecules

perpendicular to the substrate instead of aligning along the rubbing direction and

forcing the smectic layers normal to be tilted by some angle. The surface anchoring

energy of the orthogonal boundary alignment layers imply that the surface effects do

play a role in obtaining a perfect HMT alignment.

4.3.1.2 Dielectric observations

Dielectric spectroscopy is a powerful method to study dipolar ordering and molecular

dynamics of collective and non-collective behavior of molecules. This technique also

provides an insight into molecular dynamics of various cell configurations

(HMT/HMG). Therefore, a study of temperature and frequency dependent dielectric

response of ELCs is worthwhile to examine the molecular dynamics of different

phases. The dielectric spectra of FLCs in the Sm C* phase for HMG alignment, in

general, shows two collective relaxation modes, i.e. Goldstone and soft mode [55-57].

The Goldstone mode appears in the Sm C* phase and the soft mode dominates over

the Goldstone mode near transition (Sm C* to Sm A*) temperature. The principal

molecular reorientations, i.e. the rotation around the long molecular axis give separate

relaxation regions falling within the microwave frequency range (3 GHz to several

GHz) [58, 59].

The frequency-temperature dependence of the complex dielectric permittivity

qualitatively can be explained by the well-known Debye’s equation as given by:

∑=

−

∞∞

+

−+=

SGi i

ii

o

ii,1)(1

)()(*

αωτ

εεεωε (4.1)

where, α stands for the distribution parameter. G & S stands for Goldstone and soft

mode respectively. The real and imaginary part can be separated out easily from the

complex function by the relation:

ε*(ω) = ε' (ω) – i ε "(ω) (4.2)

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 (Ref. Chapter II for more details).

120

Generally, in HMG aligned samples, where we applied an electric field perpendicular

to the molecular director, the dielectric permittivity (ε'⊥) is very high [3, 60]. In HMT

configuration, however, the dielectric spectrum exhibits absorption peaks related to

the rotation around short axis and in HMG alignment the re-orientation of the long

molecular axis around the short axis falling in the radio (20 kHz-30 MHz) and

microwave frequency range, respectively [59]. These relaxation processes are non-

collective modes. In HMT alignment, with electric field applied parallel to the

molecular director, contribution of the dielectric permittivity is denoted by ε'|| and the

values of ε'|| are relatively lower than their ε'

⊥ counterpart [60]. Thus, the values of the

ε* allow us to differentiate between the two alignments configurations provided other

circumstances remain the same.

The variation of ε'|| as a function of frequency (20 Hz – 1 MHz) at different

temperatures (15°C – 45°C) in the HMT aligned sample, is shown in Fig. 4.3(a). It is

observed that the values of ε'|| vary between ~5.5 and 6, even though a wide

temperature range has been studied, pointing to the fact that cell is indeed in the HMT

configuration.

Figure 4.3: Dielectric permittivity (εεεε'|||| |||| ) in the HMT-aligned cell of ELC material

as a function of frequency (a) under no bias for the temperature range 15 – 45°°°°C,

and (b) at room temperature with different bias voltages (0-20V).

102

103

104

105

106

3.5

4.0

4.5

5.0

5.5

6.0

6.5

(a)

εε εε '

ll

Frequency (Hz)

15oC

20oC

22oC

24oC

27oC

28oC

30oC

40oC

45oC

102

103

104

105

106

3.5

4.0

4.5

5.0

5.5

6.0

ε ε ε ε ' ll

Frequency (Hz)

0 V

1 V

2 V

5 V

10 V

15 V

20 V

(b)

121

Also, it can be seen from this figure that the ε'|| component exhibits a low-frequency

dispersion region around few kHz, which is associated with the well-known molecular

rotation around the short axis and such reorientation appears only when the measuring

electric field excites the longitudinal part of the permanent dipole moment of

molecules [61]. Figure 4.3(b) shows the variation of dielectric permittivity (ε'||) with

frequency at room temperature for different bias voltages. It is clear from Fig. 4.3(b)

that the value of ε'|| does not suppress even at higher values of applied voltages which

suggesting the alignment to be in HMT geometry and further confirms that the

contribution to the permittivity is a result of the excitation of the longitudinal

component of dipolar moment, i.e., molecular orientation around short axis. This is so

because, neither the permittivity (ε'||) shows high value in the low-frequency region (in

view of the Goldstone mode strength in planar alignment) nor it falls to a low value

on applying a bias value up to 10 V (in view of the suppression of Goldstone mode).

Figure 4.4 shows the absorption curve showing the frequency dependence of

dielectric loss factor (tan δ) in the temperature range 15 - 45°C for HMT aligned

Figure 4.4: Frequency dependence of dielectric loss factor (tan δδδδ) measured in

the temperature range 15 – 45°°°°C in the HMT aligned sample cell of ELC

material.

103

104

105

106

0.00

0.05

0.10

0.15

0.20

0.25

15o

C

20o

C

24o

C

26o

C

27o

C

28o

C

30o

C

35o

C

40o

C

45o

C

tan

δδ δδ

Frequency (Hz)

122

sample cell. It shows the existence of a single relaxation process that depends on

temperature and presents relaxation frequencies of about ten to hundreds of kHz. This

implies that the dielectric relaxation process connected with rotation around the short

axis is a thermally activated process.

To ascertain the activation behavior of molecular processes of various phases, the

temperature dependence of relaxation frequency (νR) is plotted in Fig. 4.5. It

represents the well-known Arrhenius–type behavior governed by the equation [61]:

(4.3)

where, νR represents the relaxation frequency, Ea is the activation energy of LC phase,

T is the absolute temperature and kB is the Boltzmann’s constant.

Figure 4.5: Arrhenius plot of the relaxation frequency (ννννR) in the HMT aligned

cell of ELC material. Dashed line is the linear fit obtained for Sm C* and Sm A*

phases.

In the studied sample, we observed that the Arrhenius plot is linear in the Sm C* as

well as in the Sm A* phase implying that the activation process remains same

throughout. The low activation energy of about 39 kJ mol-1 K-1 is due to the easier

movements of the molecules around short axis.

3.1 3.2 3.3 3.4 3.50.4

0.8

1.2

1.6

2.0

2.4

Sm A* Sm C*

ln νν νν

R (

kH

z)

1000 / T (K -1)

T)(-E

oR

a

e νν Bk=

123

By applying of a dc field of 25 V (using an external dc power supply), we observed a

transition from the HMT to the HMG state in the Sm C* phase as shown in Fig.

4.1(d). The dielectric studies of on field induced state confirmed that this state is

indeed the HMG state. Figure 4.6(a) shows the dielectric permittivity (ε′⊥) as a

function of frequency for HMG configuration at different temperatures under no bias

application in which ε′⊥ is very high for lower frequencies and falls near Sm C* to Sm

A* transition temperature. The high value of ε′⊥ for lower frequencies in Sm C* phase

is attributed to the well-known phase fluctuations, i.e., the Goldstone mode.

Figure 4.6: Dielectric permittivity (εεεε′′′′⊥⊥⊥⊥) in the ‘‘field-induced state’’ of ELC

material as a function of frequency (a) under no bias for the temperature range

15–45°°°°C, and (b) at room temperature with different bias voltages (0-20V).

On application of successive bias field (0-20 V, using an impedance analyzer) in this

field induced state, one can observe a significant change and suppression of ε′⊥ values

as shown in Fig. 4.6(b). The jump in the dielectric permittivity (ε') value from ~6

(HMT alignment) to ~120 (‘‘field-induced state’’), shows the fact that the transition

in the alignment configuration is in fact from the perpendicular (HMT) to nearly

parallel (HMG) orientation of the molecules with respect to the electrodes.

Figure 4.7 shows the variation of νR as a function of inverse of absolute temperature

under the application of 10 V bias in the field induced HMG aligned sample cell.

102

103

104

105

1060

20

40

60

80

100

120

140

εε εε '

(b)

0 V

1 V

2 V

5 V

10 V

15 V

20 V

Frequency (Hz)

102

103

104

105

1060

20

40

60

80

100

120

140

(a)

εε εε '

Frequency (Hz)

15oC

20oC

24oC

26oC

27oC

28oC

30oC

35oC

40oC

45oC

124

Figure 4.7: Arrhenius plot of ννννR in the field-induced HMG aligned cell of ELC

material. Dashed line is the linear fit obtained for Sm C* and Sm A* phases.

A linear best fit, using Eq. 4.3, in the region of Sm C* and Sm A* phases, results into

activation energies (Ea) as 17.34 KJ/mole-K and 74.58 KJ/mole-K, respectively. The

value of νR is constant near transition temperature in the Sm C* phase. This is

attributed to the phason fluctuation and random modes near transition temperature

[62]. Each phase of LCs is uniquely represented by one activation energy. However,

in our studies no significant change in the relaxation frequency (νR) has been

observed at the phase transition. In general, the bias application suppresses the phason

mode (Goldstone mode) and the value of νR falls near transition temperature and one

can observe the soft mode. However, in present studied material, it is observed that

the value of νR maintains a constant value near transition under 10 V bias and

Goldstone mode is suppressed only partially in this case causing it to be difficult to

observe the soft mode in the Sm C* phase near transition temperature even by

applying a strong bias [63].

3.24 3.28 3.32 3.36

3.4

3.6

3.8

4.0

4.2

4.4

Sm A* Sm C*

(Bias = 10 V)

ln νν νν

R (

kH

z)

1000 / T (K -1)

125

4.3.1.3 Dielectric anisotropy

From dielectric observations it has been confirmed that the value of ε'⊥ remains

clearly higher than ε'|| in the entire frequency range, which indicates a negative

dielectric anisotropy in the studied sample (BDH 764E).

The dielectric anisotropy (∆ε) can be calculated by the following relation between ε'||

and ε'⊥ :

It is difficult to determine the dielectric anisotropy (∆ε) of FLC materials, especially

in the low frequency region where Goldstone mode is dominant in the planar state. In

view of this, ε'|| and ε'

⊥ acquired at 10 kHz and 100 kHz under no bias and after

applying 10 V bias is plotted as a function of temperature, which is shown in Figs.

4.8(a) and (b), respectively. As can be observed, ε'⊥ is comparatively higher than ε'

||

component in the entire temperature range. This difference is due to the very high

negative dielectric anisotropy of the studied sample.

Figure 4.8: A comparison of the dielectric permittivity (εεεε'⊥⊥⊥⊥ : HMG state; εεεε'

|||| |||| :

HMT state) as a function of temperature for different frequency values (10 and

100 kHz) under (a) no bias, and (b) 10 V bias for silane treated sample cell.

20 25 30 35 40 45

4

6

8

10

12

'

εεεε'II

εεεε'I

εεεε'II

εεεε'I

(Bias = 10 V)

εε εε'

Temperature (oC)

10 KHz

10 KHz

100 KHz

100 KHz

(b)

10 20 30 40 50 60 70 80

4

6

8

10

12

14

εεεε'II

εεεε'II

εεεε'I

εεεε'I

10 KHz

10 KHz

100 KHz

100KHz

(a)

εε εε'

Temperature (oC)

''l l εε∆ε ⊥−= (4.4)

126

In Fig. 4.8(a), a peak in the ε'⊥ component (10 kHz) is observed near the Sm C* to Sm

A* transition temperature, which is due to the contribution from soft mode in the field

induced planar state. In Fig. 4.8(b) it is observed that, on application of a bias voltage

of 10 V, the peak in εεεε'⊥⊥⊥⊥ [Fig. 4.8(a)] is suppressed in the same temperature regime. It

is clear from the figure that the material under study has a high negative dielectric

anisotropy (−4) as compared to the very low value of dielectric anisotropy in

conventional FLCs (~ 0.2-1) [64].

It is well-known that LC molecules, can possess permanent or induced dielectric

moments both along (parallel component, pll) and across (transverse component, p⊥⊥⊥⊥)

the long axis of the molecules [61]. Thus, on application of electric field, the molecule

tends to orient in a manner such that the stronger of either the components lies along

the electric field [65]. Knowing how the molecules respond to the applied electric

field, it is thus understandable that since in the present case ε'⊥ is greater than ε'

||, the

molecules tend to orient perpendicular to the field or in other words, p⊥⊥⊥⊥ tends to orient

along the field. It is for this reason that we could observe a transition from the HMT

state to the HMG state on application of a bias of 25 V in the Sm C* phase and

similarly in the Sm A* phase. Moreover, since the transverse component, p⊥⊥⊥⊥ is

stronger than pll, the strong coupling of electric field with p⊥⊥⊥⊥ in the ELC does not

allow switching back from HMG to HMT state in either of the phases (Sm C* and Sm

A*).

Hence, these investigation provides a simple way of acquiring a perfect HMT

alignment using silane which never been obtained earlier in the Sm C* phase of ELCs

and the transition from HMT to HMG state has been understood in view of the high

negative dielectric anisotropy. The attainment of such perfect HMT alignment for an

ELC has substantial potential and advantage over the existing use of nematic based

devices because of their faster response time.

4.3.2 Analysis of alignment in ELCs by doping GO nanomaterial

In this section, we are describing the HMT alignment observed by doping GO

nanomaterial into ELC material even without any surface treatment (neither polymer

nor silane) of the substrates. The optical and dielectric observations confirmed the

127

HMT configuration. The HMT alignment can be converted in to HMG alignment by

applying a significant bias voltage. The experiments have been carried out on virgin

sample cell in HMT configuration and same cell converted into HMG configuration

by applying a bias voltage.

4.3.2.1 Optical observations

In order to confirm the HMT configuration in GO doped ELC sample, it was initially

investigated using polarizing microscope. The optical micrographs of GO added

untreated sample cell of ELC at different bias voltages in Sm C* phase, are shown in

Fig. 4.9.

Figure 4.9: Optical micrographs of HMT aligned sample cell of ELC material

doped with GO nanomaterial in Sm C* phase under (a) no bias (b) 10 V (c) 20 V,

and (d) 30 V (which changes the orientation from HMT to HMG configuration).

(a)

(d) (c)

(b)

128

From Fig. 4.9(a), it is clear that the cell having HMT configuration, shows perfect

dark state under crossed polarizers at 0 V bias. The dark state remains unaffected even

if we rotate the sample cell under crossed polarizers. Now after applying a bias of 30

V in the Sm C* phase, we obtained a HMG configuration [Fig. 4.9(d)] which has been

confirmed by the changes in the transmission as seen by the rotation of the cell under

the crossed polarizers. However, the cell does not retain its mono domain orientation

in the HMG state, which could be hindrance for its use in good contrast display

applications. It is worth to be noted here that once the sample goes in the HMG

configuration then it can not go back to the HMT configuration which means that the

transition from HMT to HMG configuration is not reversible. HMT alignment on the

untreated substrates could be possible, when the surface energy (due to the

intermolecular interaction among LC molecules) is more than the substrate surface

energy. However, after the bias field application the non-uniformity in the bright state

might be due to the reduction of LC surface energy as compared to substrate surface

energy.

4.3.2.2 Dielectric observations

Dielectric spectroscopy is very effective tool to detect stochastic reorientation of

collective molecular dipole moment processes particularly in 1 Hz to 1 MHz

frequency domain. Figures 4.10(a) and (b), reveal the real part of ε′ in HMT aligned

(in virgin sample) and HMG aligned (aligned by dc electric field) samples. HMT

alignment is the result of filling the GO doped ELC material in the sample cell having

untreated ITO coated glass substrates. The molecular alignment has been found

vertical in virgin sample cell, therefore, the value of ε′ in low frequency domain is

less as compared to the HMG aligned sample, which has shown in Fig. 4.10(a). The

decrease in ε′ value is due to the low dielectric strength along the long molecular axis.

The HMG alignment was achieved by applying dc electric field of 30 V, as can be

seen in Fig. 4.10 (b). Its corresponding ε′ curve of HMG aligned sample as a function

of frequency shows very high value of ε′ as compared to HMT aligned sample.

129

Figure 4.10: Dielectric permittivity (εεεε') as a function of frequency with respect to

temperature for ELC material doped with GO in (a) HMT aligned and (b) field

induced HMG aligned sample cells.

Figure 4.10(b) shows the permittivity (ε′⊥) as a function of frequency at different

temperatures without any external bias application in which ε′⊥ is very high for lower

frequencies and falls near Sm C* to Sm A* transition temperature. The high value of

ε′⊥ for lower frequencies in Sm C* phase is attributed to the well-known mode due to

phase fluctuations, i.e., Goldstone mode.

Figure 4.11 shows the behavior of ε' with respect to frequency at room temperature

for GO doped ELC material in both HMT aligned sample [Fig. 4.11(a)] and field

induced HMG aligned [Fig. 4.11(b)] samples, at different bias voltages. It is clear

from the Fig 4.11(a) that the value of ε'|| does not get suppressed even at higher values

of applied voltages suggesting the alignment to be in HMT configuration and the low

value of ε'|| is a result of the excitation of the longitudinal component of dipolar

moment, i.e., molecular orientation around short axis. In field induced state, one can

observe a significant change and suppression in ε′⊥ values by applying the successive

bias fields as shown in Fig. 4.11(b), and this can be attributed to quenching of the

Goldstone mode.

102

103

104

105

0

50

100

150

200

250

300

εε εε' I

Frequency (Hz)

(b) 15 oC

20 oC

22 oC

25 oC

26 oC

27 oC

28 oC

30 oC

35 oC

40 oC

102

103

104

105

106

3.5

4.0

4.5

5.0

5.5

6.0

εε εε'II

Frequency (Hz)

15 oC

20 oC

25 oC

28 oC

30 oC

35 oC

40 oC

50 oC

60 oC

(a)

130

Figure 4.11: Dielectric permittivity (εεεε') as a function of frequency with respect to

different applied voltages for ELC material doped with GO in (a) HMT aligned

and (b) field induced HMG aligned sample cells, at room temperature.

Figure 4.12 shows the frequency dependence of dielectric loss factor (tan δ) in the

temperature range 15 – 60oC for HMT aligned ELC sample doped with GO. From the

graph, we observed a single relaxation process in HMT aligned cell that depends

strongly on temperature and this relaxation process connected with rotation around

the short axis while in HMG converted sample cell, we observed more than one

relaxation processes.

102

103

104

105

1063.0

3.5

4.0

4.5

5.0

5.5

εε εε' II

Frequency (Hz)

0 V

2 V

5V

10V

(a)

102

103

104

105

1060

40

80

120

160

200

εε εε' I

Frequency (Hz)

0 V

1 V

2 V

5 V

10 V

15 V

20 V

(b)

131

Figure 4.12: Frequency dependence of dielectric loss factor (tan δδδδ) measured in

the temperature range 15 – 60°°°°C in the HMT aligned sample cell of ELC

material doped with GO.

The detailed study of dielectric relaxation behaviour in field induced HMG aligned

sample of GO doped ELC material is presented in next chapter of thesis (Chapter

VI).

4.3.2.3 Dielectric anisotropy

Figure 4.13 shows the dielectric permittivity versus temperature for both dielectric

components, HMG (ε'⊥) and HMT (ε'

||), for frequencies 10 KHz and 100 KHz. The

value of ε'⊥ remains higher than ε'

|| in the entire frequency range in GO doped ELC

material as in silane treated surface of substrate.

The difference between the εεεε' of HMG aligned (ε'⊥) and HMT aligned (ε'

||) samples is

due to the very high negative dielectric anisotropy of the studied material. In Figure

4.13, due to the presence of soft mode a peak in the ε'⊥ component (10 kHz) is

observed near Sm C* to Sm A* transition temperature in the field induced HMG

state. It is clear from the figure, that the material under study has a high negative

dielectric anisotropy (~ − 4).

103

104

105

106

0.0

0.1

0.2

0.3

15 0C

20 0C

25 0C

27 0C

28 0C

30 0C

35 0C

40 0C

50 0C

60 0C

tan

δδ δδ

Frequency (Hz)

132

Figure 4.13: A comparison of the dielectric permittivity (εεεε'⊥⊥⊥⊥ : HMG state; εεεε'

|||| |||| :

HMT state) as a function of temperature for different frequency values (10 and

100 kHz) for GO doped ELC sample.

4.3.3 Effect of doping of GO nanomaterial on the physical

parameters of ELCs

In this section, the effect of GO nanomaterial on the physical parameters of the ELC

material (BDH 764E) has been discussed. The effect of GO nanomaterial on the

material constants has shown in Fig. 4.14. Figure 4.14 (a) shows a little increment in

the values of spontaneous polarization (Ps) of GO doped ELC sample in Sm C* phase

and minimization of spontaneous polarization (Ps) near 30ºC is the indication of

transition before 30ºC. Figure 4.14(b) shows increment in the values of rotational

viscosity (η) of GO doped sample and drops to minimum at 30ºC while in pure it

drops to minimum at 27ºC which is a general behavior of ELC materials and this

could happen because of the highly electroclinic nature of ELC material near

transition temperature.

20 30 40 50 602

4

6

8

10

12

εεεε'II

εεεε'II

εεεε'I

εεεε'I

10 KHz

10 KHz

100 KHz

100KHz

εε εε'

Temperature (oC)

133

Figure 4.14: Behavior of (a) spontaneous polarization (PS) and (b) rotational

viscosity (η) with temperature for both GO doped and pure ELC sample cells.

This can also reflect the effect of anchoring on the surface substrate without any

alignment layer. The conversion from HMT to HMG configuration is not perfect due

to the vertical anchoring of ELC molecules with ITO substrate through GO

molecules. The possible interaction of oxide-oxide (graphene and ITO) and the

affected molecular ordering may result vertical anchoring.

Furthermore, another measurement for comparison of substrate attachment with ELC

has been carried out on the ITO substrate coated with nylon 6/6 and rubbed with

velvet cloth for proper HMG alignment of ELC molecules. It is to examine whether

the vertical alignment is due to ITO surface anchoring of molecules or any bulk

induced effect. Figure 4.15 shows the relevant textures of planar alignment of GO

doped ELC material filled in the treated sample cell, i.e., the attachment of ELC

material is mediated by nylon 6/6 layer. The dark and bright state has been obtained in

virgin sample cell, as shown in Figs. 4.15(a) and (b).

This exhibits the effect of GO in bulk and not on the surface. The major differences

have been observed in an untreated sample where there is a direct exposure of ITO to

GO doped ELC sample.

22 24 26 28 30 32

0

50

100

150

200

250

300

(b) Pure ELC

GO Doped ELC

Temperature (oC)

ηη ηη (

m P

a s

)

22 24 26 28 30 32

0

5

10

15

20

25

30

35

40

PS

(n

C/C

m2)

Temperature (oC)

Pure ELC

GO Doped ELC(a)

134

Figure 4.15: Optical micrographs of a HMG aligned (nylon 6/6 treated) sample

cell of GO doped ELC material in (a) dark state and (b) bright state.

This means that the vertical alignment in untreated sample substrates is only due to

the interaction of GO and ITO components. GO works as an order-affecting agent,

which affects the viscosity of ELC and makes it easier to be aligned along the GO

molecules attached to the ITO surface in vertical direction. Hence, GO interacts with

ITO layer and supports the ELC molecules to align in the direction of GO molecules.

4.4 CONCLUSIONS

A perfect HMT alignment has been observed in ELC material by using two methods.

In first method, ITO coated glass substrates are treated with silane and followed by

rubbing. In second method by adding the GO nanomaterial in ELC material, we

achieved a perfect HMT alignment without any surface treatment. We proposed that

the reason behind such good vertical alignment without any surface treatment is the

consequence of coupling between ELC molecules and ITO film mediated by GO

nanomaterial. On application of high bias voltage, we can convert HMT configuration

into HMG configuration in both the cases and this transition from HMT to HMG state

has been understood in terms of the high negative dielectric anisotropy. Furthermore,

the achievement of such perfect HMT alignment of an ELC material has provided

substantial prospective and the technique can be adopted for various dynamic studies

of ELC materials.

(a) (b)

135

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