OPTO-ELECTRONICS REVIEW 16(3), 271–276
DOI: 10.2478/s11772-008-0008-1
Technical aspects of dielectric spectroscopy measurements
of liquid crystals
P. PERKOWSKI*, D. £ADA, K. OGRODNIK, J. RUTKOWSKA, W. PIECEK,
and Z. RASZEWSKI
Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland
The dielectric spectroscopy measurements were performed for antiferroelectric liquid crystalline mixture. For this purpose,
the cells with ITO electrodes were prepared. It was found that it is not possible to detect some important relaxation modes in
Sm A*, Sm C*, and Sm CA* phases. The own cell mode (related to cell properties, i.e., capacity and resistivity) covers the dielectric response of liquid crystalline medium. Dielectric measurements in cells with gold electrodes were done to show all
possible relaxations in antiferroelectric liquid crystals (LCs).
Keywords: smectic liquid crystals, dielectric spectroscopy, relaxation modes.
1. Introduction
Dielectric spectroscopy is a very important and useful experimental technique for characterization of liquid crystals
phases. The electrooptical response is strongly related to
dielectric modes observed in LC phases. Particularly, collective dielectric modes are very interesting from this point
of view. If dielectric relaxation mode is fast, electrooptical
switching related to this mode can be fast as well.
Typically, the cells with ITO electrodes are used for
electrooptical investigations. Their main advantage is that
they are transparent for visible light. But unfortunately, we
found that there is a big obstacle. For fast collective modes
in smectic phases, the own cell relaxation mode covers the
dielectric response of liquid crystalline medium, put into
the measuring cell [1,2].
ferroelectric Sm CA* phases. We chose such “broad phases”
mixture to be sure about dielectric modes we can observe and
to avoid effects observed close to the phase transitions.
For ITO cells, the ITO layer with resistivity of 50 W/o
was used as well as the cells with gold electrodes. We approximated the own resistivity of gold layers as less than 1 W/o.
All cells were prepared in our laboratory. Connection between
wires and cell was made with ultrasonic welding unit ULTRASONIC-400 (Industrial Technologies AB). LC sample,
with 5-µm gap and planar alignment, was slowly cooled from
isotropic phase with 0.1°C/min rate. For this purpose, Linkam
TMHS 600 hot stage with Linkam TMS93 temperature controller was used. Dielectric measurements were prepared by
HP 4192A (Hewlett Packard impedance analyzer) for the frequencies from 100 Hz up to 10 MHz with weak AC measuring field (0.1 V) to avoid nonlinear dielectric response.
2. Experimental procedures
3. Experimental results
We have investigated the liquid crystalline mixture with broad
paraelectric Sm A*, ferroelectric Sm C* as well as anti-
For ITO cell, the following results were obtained (Figs.
1–8).
Fig. 1. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (105°C) measured in ITO cell.
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Technical aspects of dielectric spectroscopy measurements of liquid crystals
Fig. 2. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (100°C) measured in ITO cell.
Soft mode (SM) is weakly detected.
Fig. 3. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (95°C) measured in ITO cell.
Soft mode (SM) is easily detected.
Fig. 4. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm C* phase (90°C) measured in ITO cell.
Soft mode (SM) is well detected.
Fig. 5. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm C* phase (85°C), measured in ITO cell.
Goldstone mode (GM) is well detected.
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Fig. 6. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (80°C) measured in ITO cell.
In-phase PL mode (PLM) is detected.
Fig. 7. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (70°C) measured in ITO cell.
In-phase PL mode (PLM) is detected.
Fig. 8. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (60°C) measured in ITO cell.
In-phase PL mode (PLM) is detected.
One can notice that for all phases the relaxation around
1 MHz is presented. This relaxation is not related to any
modes in liquid crystals. It is the own cell relaxation, connected with relatively high resistivity of ITO layer. The relaxation frequency of CR circuit is connected with capacity
and resistance by equation
f ITO =
1 1
2p RC
To make the observation for higher frequencies, we decided to use gold electrodes. The cell gap was the same (we
used the same distances as well as the same polyimide layers). When resistivity is lower, the relaxation frequency of
the cell will be higher, and it will be possible to observe the
higher LC modes. Results of measurements for gold cell
are presented below.
Comparing the results for the same phase and the same
temperature one can see that for Sm A* (Figs. 3 and 11) in
Opto-Electron. Rev., 16, no. 3, 2008
ITO cell, the soft mode is covered by cell relaxation while
in a gold cell well defined soft mode is measured. The
same can be observed in antiferroelectric Sm CA* phase
(Figs. 8 and 16). PL mode can be noticed in ITO cell while
we cannot observe PH mode in ITO cell. Both PL and PH
modes are well visible in gold cell. Goldstone (GM) mode
in Sm C* (Figs. 5 and 13) is easily detected in ITO as well
as gold cells because of Goldstone mode relaxation frequency is far from cell relaxation frequency (fGM = 15 kHz,
fITO = 1 MHz).
4. Discussion and conclusions
Commercially available are the cells with ITO resistivity
around 100 W/o. For such cells, the relaxation frequency
fITO is around 200 kHz. For the cells with better ITO layer
(50 W/o), one can notice the higher frequency fITO = 1
MHz. The dielectric spectroscopy of smectic phases is sen-
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Technical aspects of dielectric spectroscopy measurements of liquid crystals
Fig. 9. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (105°C) measured in gold cell.
Soft mode (SM) is easily detected.
Fig. 10. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (100°C) measured in gold cell.
Soft mode (SM) is easily detected.
Fig. 11. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm A* phase (95°C) measured in gold cell.
Soft mode (SM) is easily detected.
Fig. 12. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm C* phase (90°C) measured in gold cell.
Soft mode (SM) is easily detected.
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Fig. 13. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm C* phase (85°C) measured in gold cell.
Goldstone mode (GM) is detected.
Fig. 14. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (80°C) measured in gold
cell. Anti-phase mode (PHM) is easily detected while in-phase mode (PLM) is weakly detected.
Fig. 15. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (70°C) measured in gold
cell. Anti-phase mode (PHM) is easily detected while in-phase mode (PLM) is weakly detected.
Fig. 16. Real (eps’) and imaginary (eps”) parts of dielectric permittivity and Cole-Cole plot for Sm CA* phase (60°C) measured in gold
cell. Anti-phase mode (PHM) is easily detected and in-phase mode (PLM) is well detected.
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Technical aspects of dielectric spectroscopy measurements of liquid crystals
sible in ITO cell with low resistivity (10 W/o) or in cells
with gold electrodes.
The dielectric studies of fast collective modes in
smectics (soft and PH modes) cannot be done in a cell with
high resistivity ITO layers. For the same reason, electrooptical measurements connected with fast switching
(electroclinic effect, etc.) can be performed in a cell with
good (low resistivity) ITO layer. Otherwise the dielectric
properties of the cell can influence the driving of the LC
medium in the cell.
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Acknowledgements
This work was supported by the project of the Military
University of Technology WAT PBW 980.
References
2. S.T. Lagerwall, Ferroelectric and Antiferroelectric Liquid
Crystals, Wiley-VCH, 1999.
3. W. Haase and S. Wróbel, Relaxation Phenomena, SpringerVerlag, 2003.
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