CHAPTER IV GRAPHENE OXIDE INDUCED HOMEOTROPIC...
Transcript of CHAPTER IV GRAPHENE OXIDE INDUCED HOMEOTROPIC...
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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).
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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
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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-
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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
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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.
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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.
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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)
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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)
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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).
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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)
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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)
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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=
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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)
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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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