Journal of Molecular Liquids 147 (2009) 145–148

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Journal of Molecular Liquids
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q

Enhancement of Freedericksz's threshold voltage of nematic liquid crystals inthin cells
Surajit Dhara ⁎School of Physics, University of Hyderabad, Hyderabad 500040, Indiaa r t i c l ei n f o

Article history:
Received 9 May 2008
Received in revised form 2 January 2009
Accepted 26 January 2009Available online 1 February 2009
Keywords:Nematic liquid crystal
Thin cell
Freedericksz's threshold voltage

a b s t r a c t
We report the measurement of Freedericksz's threshold voltage (Vth) in two pure calamatic nematic as well
as a mixture of a calamatic nematic and a bent core (banana) liquid crystal at different thicknesses, in theranges 1–3 μm and 7 to 15 μm cells. Rubbed polyimide-coated glass plates are used to get planar alignment of
the nematic director. We show that the transmitted intensity varies gradually across the transition region
with the applied voltage in the thin cells compared to the thick cells. We ﬁnd a signiﬁcant enhancement ofVth in thin cells in both the compounds and in the mixture and the enhancement is larger at as thetemperature is lowered in the nematic phase. Part of it is contributed by the enhancement of theorientational order parameter (V2th ~ S) in thin cells. Finally we discuss the role of ionic contribution toincrease the space charge polarization in thin cells in further enhancing the threshold voltage.
© 2009 Elsevier B.V. All rights reserved.

1. IntroductionLiquid crystals are orientationally ordered liquids, made of anisotropic shape [1] organic molecules. There are a variety of liquid crystall;ine phases — among them nematic is the widely-used one. The averageorientation direction of the molecules is speciﬁed by a unit vector n̂
called the director which is apolar in nature in the nematic (N) phase.
The material combines anisotropies in dielectric, optical, and otherproperties with a facility to ﬂow and hence exhibits remarkable electrooptic effects, which are exploited in ﬂat panel liquid crystal display(LCD) devices [2]. When the liquid crystal is conﬁned between thesubstrates in the presence of electric or magnetic ﬁelds the competition
between the effects of wall and the effects of external ﬁeld results in a
director distribution beyond a particular ﬁeld. The reorienting torquearises through a coupling between an external ﬁeld and the corresponding susceptibility anisotropy. Twisted nematic display devices make useof electric ﬁeld (E) effects in materials with positive dielectric anisotropy(Δε =ε|| −ε⊥ N 0), which favors an alignment of the director parallel tothe electric ﬁeld. The response time for nematic is τ ~ηd2 /π2k [1] whereη is the effective viscosity, K is an effective curvature elastic constant andd is the sample thickness. A typical LCD cell has a thickness ~5 μm hence
a low response time. The response time of the device can be reduced
considerably by reducing the sample thickness. In addition, in thebistable twisted nematic displays [3,4] and surface-stabilized ferroelectric devices [5], the thickness has to be quite small, 2 μm or lower.
When the thickness of the sample is reduced, the physical as well as theelectro-optical properties of the cell becomes signiﬁcantly different. The⁎ Fax: +91 40 23010227.
E-mail address: sdsp@uohyd.ernet.in.
0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2009.01.010

effect of cell thickness on the various physical properties of nematic
liquid crystal was reported by several authors [6–9]. Our recent
experiments show that the orientational order parameter of nematic
liquid crystals in thin cells is enhanced considerably with respect to thatin thick cells in all the compounds with aromatic cores [6]. We have also
reported the dielectric properties of nematic liquid crystal in the low
frequency range in thin cells [7]. The time dependence of switching timein a thin boundary layer bistable nematic display has been discussed byThruston et al [8]. Mora et al have reported the Freedericksz's transition
measurements on the anisotropic polymer-stabilized liquid crystal
network and the dependence of threshold voltage on the thickness wasexplained due to the coupling of the polymer network and nematic
elasticity [9]. However, there are no direct measurements of Freedericksz's threshold voltage in thin cells (1–3 μm) in the planar-aligned
nematic liquid crystals.
In the present paper we report the measurement of Freedericksz's
threshold voltage as a function of temperature in two calamatic
nematic liquid crystals as well as a mixture of a calamatic nematic andbanana liquid crystal in different cell thicknesses and show the effectof space charge polarization on the threshold voltage in thin cells.
2. Experimental
Our main aim was to investigate the effect of cell thickness on theFreedericksz's threshold voltage in nematic liquid crystals. We havechosen two calamatic compounds and a bent core compound. The
chemical structures and phase transition temperatures of thecompounds are shown in Fig. 1. The compound-1, (PCH-5) obtained
from Merck, is highly polar. It exhibits a nematic phase at room
temperature and is widely used in display applications. The directionof the permanent dipole moment in this molecule is along the long146

S. Dhara / Journal of Molecular Liquids 147 (2009) 145–148

Fig. 1. Chemical structures, phase sequences and the transition temperatures of the compounds used in the experiments. The transition temperatures are given in degree Celsius.

axis and hence the dielectric anisotropy is positive (Δε N 0). The
dielectric anisotropy of the nematic is chosen positive so that nematicshows the realignment of director above the Freedericksz's thresholdvoltage. The compound-2 and compound-3 were synthesized in ourlaboratory. The compound-2 is also highly polar with a long alkyl
chain. It exhibits the nematic phase at relatively high temperatures.
The direction of the permanent dipole moment in this molecule is also
along the long axis and hence the dielectric anisotropy is positive. The
compound-3 is made of bent shaped molecules with a transverse
dipole moment and is highly biaxial. The mixture with 50:50 wt.% of
the compounds-2 and compound-3, gives rise to a uniaxial nematic of
15-degree temperature range and positive dielectric anisotropy. This
mixture was studied to explore the effect of molecular structure on
the threshold voltage.
The cells are constructed using ITO coated glass plates. The plates
are treated with polyimide and cured at a temperature of 280 °C for

90 min. After curing, the plates are rubbed for homogeneousalignment of the molecules in the nematic phase. The plates wereassembled such that the rubbing directions are antiparallel. Glassbeads are mixed with epoxy glue, which is spread outside theelectrode region to get the required cell thickness (N2 μm). However
to make cells of thickness of b2 μm we do not use any spacers. In thiscase several cells were prepared at a time and a few cells withthickness of b2μm were only selected. The glue used does not affectthe liquid crystals and is usually used for making liquid crystal
displays. The thickness of the cell is measured at several positions
inside the overlapped electrode area using an interferometrictechnique. The cells with uniform thickness are selected for theexperiment. The cell thickness d is found to be constant within 1%. The
cell is ﬁlled with the sample in the isotropic phase and cooled slowly
to the nematic phase, resulting to a good homogenious alignment ofthe director. The alignment was checked using a polarizing microscope (Leitz ORTHOPLAN). The experimental setup is shown in Fig. 2.
The temperature of the sample is controlled by using a heating stage(INSTEC HS1) within an accuracy of 0.1 °C. A helium-neon laser (ORIEL
3 mW, λ = 632.8 nm) beam is used to illuminate the sample. A beam
splitter and a reference photodiode (PD2) are arranged to monitor theFig. 2. Schematic diagram of the experimental setup. Photodiodes (PD1, PD2). Polariser(POL), Analyzer (ANL), Multimeter (MUL), Computer (COM). Signal generator (SIG).

Fig. 3. Variation of transmitted intensity as function of applied voltage of compounds-2at various temperatures. Frequency: 3037 Hz. Cell thickness: 1.5 μm.

S. Dhara / Journal of Molecular Liquids 147 (2009) 145–148

Fig. 4. Variation of transmitted intensity as a function of applied voltage of 50 wt.
%:50 wt.% mixture. Open squares and open circles correspond to the data in 1.4 µm cellthickness at temperatures 105 °C and 115 °C respectively. Open triangles and open stars
correspond to the data in 16 µm cell at temperatures 105 °C and 115 °C respectively.

stability of the reference beam intensity. The laser beam is adjusted to
be incident on the sample through a polariser and the transmitted
intensity is passed through an analyser, which is crossed with respect
to the polariser. The optic axis of the sample is set 45° with respect tothe polarizer to optimize the measurement. The transmitted intensity
is collected by the photodiode PD1. All the photodiodes are connected
to a digital multimeter (Keithley-2000). A signal generator (WaveTek)
is used to apply an electric ﬁeld to the cell. The transmitted intensity ismeasured as a function of applied sinusoidal voltage at varioustemperatures. The frequency used in the experiment is 3.037 kHz
which is typically in the range of LCD application. Finally all theinstruments are controlled by a computer (COM). A suitable programis used to run the experiment.

147

Fig. 6. V2th at a few temperatures of the compound-2. The open circles and open squares
correspond to 1.4 μm and 7 μm thick cells respectively. Dotted lines are drawn as guides
to the eye. Frequency: 3037 Hz.

as It =Io sin2(πΔnd/λ), where Δn, the birefringence, d, the sample
thickness and λ, the wave length of light. The very different nature of theintensity in thick cells arises due to comparatively larger value of theoptical path difference (Δnd). In the mixture the threshold voltage in
1.4 µm is also higher with respect to 16-µm cell at corresponding

3. Results and discussion
A typical variation of transmitted intensity in 1.5 µm cell as a function of applied voltage at different temperatures for the compound-1 isshown in Fig. 3. The threshold voltage decreases with increasing
temperature and the transmitted intensity varies gradually across thetransition region. In Fig. 4 we show the variation of intensity in twodifferent cell thicknesses namely 1.4 µm and 16 µm at two different
temperatures for the 50 wt.%:50 wt.% of compound-2 and compound-3.
The nature of variation of ﬁeld dependent transmitted intensity in themixtures at two temperatures is very different. The transmitted
intensity in nematic between the two crossed polarisers can be written

Fig. 5. Variation of birefringence as a function of applied voltage at two different cellthicknesses: 1.5 µm, and 14 µm. Temperature 137 °C. Frequency: 3037 Hz.

Fig. 7. Variation of V2th at a few temperatures in three different cells thicknesses of thecompound-3. Open circles (1.5μm), open triangles (2.2μm), open squares (14μm),.
Dotted lines are drawn as guides to the eye. Frequency: 3037Hz.

Fig. 8. V2th at a few temperatures in three different cells of the 50 wt.%:50 wt.% mixturesof compound-2 and compound-3. Open circles (1.4 μm), open triangles (2.3 μm), opensquares (16 μm). Dotted lines are drawn as guides to the eye. Frequency: 3037 Hz.

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S. Dhara / Journal of Molecular Liquids 147 (2009) 145–148

Table 1
Relative enhancement of orientational order parameter computed from birefringenceand Freedericksz's threshold voltage.
Thicknesses Compoundsof thin andthick cells

TNI(TNI −T)0 ΔnthinΔn−thickΔnthick × 100
in °C

1.4 and 7 μm Compound-1
55.5 12
1.5 and 14 μm Compound-2
154.2 12118.5 12
1.4 and 16 μm Mixture ofcompound-2
and compound-3(50:50 wt.%)

6%
11%
13%

ðVth2 Þthin −ðVth2 Þthick× 100ðVth2 Þthick

16%
43%30%

temperatures. More importantly the intensity varies gradually acrossthe transition region in thin cell and the variation is comparatively
steeper in the 16-µm cell. In order to compare the relative sharpness ofthe transition we estimate the slope by ΔI / ΔV in the transition region.
The magnitude of the slope of the transition in 1.4-µm cell is ~5 whereas
it is ~30 in case of the 16-µm thick cell. We also measured the thresholdvoltage from the birefringence measurement in two different cell
thicknesses namely 1.4 µm and 14 µm in compound-2. The birefringence
was measured using a tilting compensator as a function of applied
voltage as shown in Fig. 5. It is noticed that the birefringence in thinnercell (1.4 µm) is higher (6%) compared to the thick cell (14 µm). Similar
enhancement of birefringence in the absence of a ﬁeld was also reported
by us on several other nematic liquid crystals and the origin of suchenhancement is not completely understood [6].
In order to bring out the strong inﬂuence of cell thickness we show
the variation of threshold voltage in two compounds and the mixturein Figs. 6–8. We plot V2th as a function of temperature instead of Vthand the reason will be discussed later. As we can see from Fig. 6, V2th ishigher in 1.4-μm cell than that in 7-μm cell of the compound-1. V2th,
measured in three different cells of the compound-2 are shown at afew temperatures in Fig. 7. Similarly V2th for the mixture is shown at afew temperatures in Fig. 8. It is observed that with decreasing cell
thickness V2th is increased at a ﬁxed temperature in all the systems. Itis also noticed that due to the addition of banana molecules in themixture, the threshold voltage measured in all the cells is increased
signiﬁcantly compared to that of the compound-2. Thepthreshold
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
voltage of the Freedericksz's transition is given by Vth = π K1 = e0 Δe,where K1 is the splay elastic constant and Δɛ is the dielectricanisotropy. In the mean ﬁeld model the elastic constant, K1∝S2 where
S is the orientational order parameter and the dielectric anisotropyΔε∝S. Therefore V2th∝S. Thus Vth, can get enhanced in thinner cellsdue to the enhancement of orientational order parameter.
In order to estimate the enhancement in Vth in thin cells which canarise due to the enhancement of order parameter alone, we collect theΔn data from Ref. [6] at a ﬁxed temperature below the transition of theabove compounds and summarize in Table 1. In compound-1, V2th is ~16%
larger in 1.4-μm cell than that in 7-μm cell at ~12° below TNI whereas Δnis ~6% larger at the same temperature. In the compound-2, V2th is ~43%
larger in 1.5 μm than that in 14-μm cell whereas Δn is ~11% larger at thesame temperature. Similarly in the mixture V2th is 30% larger in 1.4-μm
cell than that in 16-μm cell whereas Δn is ~13% larger at the sametemperature. These discrepancies can be explained as follows. Though
the liquid crystals are dielectric but they exhibit some pronounced ioniceffect which are generally not observed in ordinary dielectric liquids.
One of such effect is the space charge polarization which depends on
concentration of ions, applied voltage and cell thickness. The source ofions in liquid crystal are not only residual impurities but probably also
decomposition of the basic liquid crystal compounds. The transit time
(t =d2 / μV, where μ, is the mobility, V is the applied voltage and d is thecell thickness) of these ions in the thin cells is small and hence the ionscan easily reach the appropriate electrodes when the ﬁeld is reversed,

producing a space charge. This effect is more in the low frequency
region. The space charges partially screen the external electric ﬁeld.
Therefore the effective ﬁeld inside the thin cells is decreased. The large
value of V2th in thin cell arises due to the screening effect of the spacecharge apart from the enhancement of S. It is also noticed from Table 1
that in the compound-1 and in the mixture the enhancement of V2th in
thin cells is ~2.5 times larger than that of Δn whereas this is ~4 times
larger in compound-2. In this compound the nematic phase occurs at
high temperatures (105.5–154.2 °C). The density of ions increases with
the increase of temperatures and as a result the screening effect isincreased. The larger threshold voltage in case of the mixture withrespect to the pure compound may indicate the stiffening of splay elastic
constant in the mixtures due to the addition of the banana-shaped
molecules. The transition is ~6 times sharper in thick cell (16 µm)
compared to the thinner (1.4 µm) one. This effect may arise due to thestronger surface anchoring energy in thinner cell compared to thethicker one.
Finally we want to comment on the effect of addition of bent coremolecules in the mixtures. V2th(~S) in the mixture is larger comparedto the pure compound in all the cells indicating that the orientationalorder parameter is increased. However, the understanding of mutualorientations of two differently shaped molecules is important toexplain the results further. At present we are pursuing someinvestigations to understand the mutual orientations.
4. Conclusion
Our experiments show that the Freedericksz's transition in thin(~ 1–3 μm) cell is much less steeper compared to the thicker cells(~ 7–16 μm). The threshold voltage of nematic liquid crystals in thincells is enhanced considerably with respect to that in thick cells formaterials with positive dielectric anisotropy. Part of the enhancement arises due to the enhancement of orientational orderparameter in thin cells. The space charge polarization exhibits strong
screening effect in thin cells at higher temperatures as a result thenonuniform electric ﬁeld arising from the space charge effects
enhances the Freedericksz's threshold voltage. Liquid crystals which
are used in very thin display devices have to be highly puriﬁed tominimize the effect of space charge polarization to the electro-optic
characteristics.
Acknowledgement
The author gratefully acknowledges Prof. B.K. Sadashiva and Prof.
N.V. Madhusudana of Raman Research Institute, Bangalore forproviding samples and various discussions. The author also gratefully
acknowledges the support from the DST, Govt. of India for the Project
Number SR/FTP/PS-48/2006 under the SERC Fast Track Scheme.
Reviewers of this paper are also greatly acknowledged for providing
valuable suggestions.
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