Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal
First estimated were the parameters of dense part of double electrical layer
 (DEL) at the interface electrode – solution of the dye in liquid crystal, based on analysis
 of capacitance-voltage characteristics obtained at low frequencies, taking the series
 resistance into ac...
Saved in:
| Published in: | Semiconductor Physics Quantum Electronics & Optoelectronics |
|---|---|
| Date: | 2011 |
| Main Author: | |
| Format: | Article |
| Language: | English |
| Published: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2011
|
| Online Access: | https://nasplib.isofts.kiev.ua/handle/123456789/117748 |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Journal Title: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Cite this: | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal / O.V. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 321-324. — Бібліогр.: 11 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860258633710829568 |
|---|---|
| author | Kovalchuk, O.V. |
| author_facet | Kovalchuk, O.V. |
| citation_txt | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal / O.V. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 321-324. — Бібліогр.: 11 назв. — англ. |
| collection | DSpace DC |
| container_title | Semiconductor Physics Quantum Electronics & Optoelectronics |
| description | First estimated were the parameters of dense part of double electrical layer
(DEL) at the interface electrode – solution of the dye in liquid crystal, based on analysis
of capacitance-voltage characteristics obtained at low frequencies, taking the series
resistance into account. DELs with various parameters near each electrode were obtained
owing to their different chemical compositions. It was shown that the obtained difference
in the DEL parameters is caused not only by different chemical composition of the
electrodes, but also different orientations of molecules near each of them (planar
orientation near one electrode and homeotropic one near another). It was estimated the
barrier height, thickness of near-electrode layer, and concentration of ionized states in the
dense parts of DEL near each electrode. It was shown that these parameters differ by 2 to
3 times, and an assumption was made about the reasons that can cause this difference.
|
| first_indexed | 2025-12-07T18:52:39Z |
| format | Article |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 321-324.
PACS 61.30.Gd, 68.08.-p, 77.22.Ch
Method and estimation of parameters of dense part
of double electrical layer at the interface electrode-solution
of the dye in liquid crystal
O.V. Kovalchuk
1Institute of Physics, NAS of Ukraine, 03680 Kyiv, Ukraine
E-mail: akoval@iop.kiev.ua
Abstract. First estimated were the parameters of dense part of double electrical layer
(DEL) at the interface electrode – solution of the dye in liquid crystal, based on analysis
of capacitance-voltage characteristics obtained at low frequencies, taking the series
resistance into account. DELs with various parameters near each electrode were obtained
owing to their different chemical compositions. It was shown that the obtained difference
in the DEL parameters is caused not only by different chemical composition of the
electrodes, but also different orientations of molecules near each of them (planar
orientation near one electrode and homeotropic one near another). It was estimated the
barrier height, thickness of near-electrode layer, and concentration of ionized states in the
dense parts of DEL near each electrode. It was shown that these parameters differ by 2 to
3 times, and an assumption was made about the reasons that can cause this difference.
Keywords: liquid crystal, double electric layer, planar and homeotropic orientation.
Manuscript received 02.06.11; accepted for publication 14.09.11; published online 21.09.11.
1. Introduction
Analyzing electro-optical characteristics of liquid crystal
(LC), it should be considered near-electrode processes in
most cases. In LC, as in electrolytes, these processes are
caused by double electric layers (DEL) [1-4] that are
mainly formed due to different mechanisms of
conductivity in liquid (ion conductivity) and electrode
(electron one). As known, the DEL structure at the
interface electrode-liquid is complex and includes a
diffuse (Gouy layer) and dense (Helmholtz layer) parts
[5]. Parameters of the diffuse part of DEL in LC were
estimated in a large number of articles (for example, see
[3, 4]). The parameters of the dense part are much less
studied.
From literature [5, 6], it is known that even for
strong electrolytes determination of parameters for the
dense part of DEL is a complex task. As it was
demonstrated by our researches, the methods that were
developed to estimate the Helmholtz layer parameters
for strong electrolytes are unusable for analysis of DEL
in LC.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
In the study of dielectric spectra in LC, we also
showed that, at low frequencies (tens of Hz or less) and
amplitudes of the measuring signal U0<2 V, the voltage
applied to thin near-electrode layers at each electrode is
almost equal to the voltage applied to the sample [7].
This gave grounds to use the analysis of capacitance-
voltage characteristics (CVC) to estimate the parameters
of the dense part of DEL. For LC, like to all liquids but
unlike semiconductors, it is impossible to create an
ohmic contact at least with one of the electrodes.
Therefore, analyzing the CVC, one always should take
account that he deals with a two-barier structure.
Theoretical analysis of this structure can be essentially
simplified provided that the parameters of barriers are
different.
Formation of barriers with different parameters
near each electrode was first realized by us in glycerol
[8] through a long-term (several hours) action of direct
electric field at the temperature 340 K. Obtained in these
samples were the CVC asymmetric as to the direction of
electric field, and evaluated were the barrier parameters
(height, thickness and concentration of ionized states).
Unfortunately, this method to form different parameters
of DEL could not be used for LC. Therefore, it was
decided to use a method traditional for contact
phenomena metal-semiconductor to form DEL with
different parameters through using the electrodes with
different chemical compositions.
321
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 321-324.
The aim of this work was to develop technology of
creating the LC structures with asymmetric DELs based
on electrodes of different chemical composition and to
estimate parameters of the dense part of DEL in these
structures.
2. Materials and methods
For researches, we used a mixture of LCD ZLI4803.
From the electrodes that would be the most neutral as to
its interaction with molecules of liquid, the platinum Pt
electrode was chosen. We chose another electrode out of
Al, Ni, Cu, metal oxides In2O3, SnO2, and mixtures of
these oxides ITO. The best results regarding asymmetry
of CVC were obtained for the Cu electrode. However,
for “pure” LC mixture ZLI4803 (without implemented
impurities), to obtain CVC, by using which one could
clearly separate the contribution of each of the barriers,
was impossible. This effect was obtained for 2 wt.%
solution of dye D in LC.
The structural formula of the dye D is shown in
Fig. 1. It is seen that its high solubility (compared to that
of other dyes) in liquid crystal is caused by the fact that
the structure of molecules D is close to that of LC
molecules.
The main part of our researches was performed
using the structures Pt/ZLI4803 + 2 wt.%D/Cu. In
addition, we investigated the structures Pt/ZLI4803 +
2 wt.% D/Pt and Cu/ZLI4803 + 2 wt.% D/Cu.
At the initial stage, we created a flat capacitor of a
Pt plate that was purified in organic solvents and
polished as well as a Cu plate that was buffed and
cleaned with organic solvents, too. The thickness of this
capacitor was specified by using teflon spacers with the
thickness 25 μm. Long-term fixation of the electrodes
was provided by gluing the electrodes along the edges.
With this aim, used were the electrodes possessing the
same dimensions.
In order that the teflon spacers did not effect the
capacity of the cells, there used Cu films deposited on
glass fiber plastic, and by etching in the Cu electrode,
created was a central part (measuring electrode) and a
peripheral part (guard electrode). Since the teflon spacer
was on the guard electrode, which was connected to
ground when measurements were carried out, it did not
effect the value of capacitance. The factual thickness of
the cell was estimated in the value of capacitance of an
empty cell. It was equal to 30 μm.
After measuring its capacitance, the cell was filled
using the capillary effect with liquid crystal. To speed up
this process fillup took place at the temperature 340 K.
When measuring the frequency dependences for the
components of the complex permittivity and CVC, we
used the oscilloscopic method [9] within the frequency
range . The measuring signal amplitude
was equal to 0.5 V. At the frequencies f < 0.01 Hz,
oscillograms were recorded by using the self-recording
device N307. Then, the obtained oscillograms were
digitized and analyzed by computer. According to the
results of this analysis, CVC were plotted.
Hz1010 64 −−
Based on the analysis of these oscillograms at the
frequencies determined was the value of
the permittivity and conductivity of the bulk part of
samples. All the measurements were performed at 293 K.
Hz1010 62 −−
3. Results and analysis
When studying LC or solutions of certain substances in
LC, it is important to know orientation of the molecules.
Essentially, the orientation is fitted by using an
observation in polarization microscope or an analysis of
images of this microscope. In this case, at least one of
this plates that creates this flat capacitor must be
transparent to light. In our case, the electrodes were not
transparent. Therefore, a conclusion about orientation of
liquid crystal near the Pt and Cu electrodes was made
using measurements of permittivity for the structures
Pt/ZLI4803 + 2 wt.%D/Pt and Cu/ZLI4803 +
2 wt.%D/Cu.
Our analysis of the obtained results showed that the
permittivity for the structures Pt/ZLI4803 + 2 wt.%D/Pt is
close to that of ZLI4803 at planar orientation of
molecules, and the permittivity for Cu/ZLI4803 +
2 wt.%D/Cu is close to that of ZLI4803 at homeotropic
orientation of molecules. From these results, it was
concluded that near the Pt electrode the LC molecules
have planar orientation and near the Cu electrode –
homeotropic one, for the samples Pt/ZLI4803 +
2 wt.% D/Cu hybrid orientation of LC molecules is
realized.
Shown in Fig. 2 are CVC for the structure
Pt/ZLI4803 + 2 wt.%D/Cu at various frequencies of the
measuring signal. It is clearly seen that the obtained
CVC are asymmetrical concerning the polarity of the
electric field. It can be mainly caused not only by
different materials of electrodes, but by different
orientation of molecules near each of the electrodes.
Based on the analysis of Fig. 2, one may make the
following conclusions. In the dependence ( )UC 2− , there
are straight-line portions, with increasing the frequency
CH3O C
O
HC CH N
CH3
CH3
Fig. 1. Structural formula of D dye.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
322
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 321-324.
these portions are shifted towards larger values of C–2
(lower values of capacity), the slope of straight-line
portions does not change at different frequencies. The
presence of the straight-line portion in ( )UC 2− -
characteristics makes it possible to assert that, in the
space charge region (SCR), electrically active centers (in
our case, these are ionized molecules of near-electrode
layer) are distributed uniformly.
Based on the results presented in [10, 11], one can
conclude that the main cause of displacement of straight-
line portion in -characteristics when changing the
frequency is an effect of series resistance R
( )UC 2−
S. In this case,
the value of displacement of -characteristics
relative to its position at f = 0 is proportional to the
frequency squared [11]
( )UC 2−
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
222 2 SRC ω≈Δ − . (1)
Our analysis of Fig. 2 showed that the displacement of
CVC relative to the axis in the left and right sides
may be described by Eq. (1) with a small error. As a
result of this analysis, the value R
2−C
S was estimated. Like
to the case of glycerol [8], it reaches tens of MOhm,
which by more than 5 orders of magnitude exceeds the
resistance of the sample bulk part. Because these
structures is two-barier, one can foresee that the series
resistance for each barrier is the resistance of another
barrier providing a current flows in the forward
direction.
Analyzing the CVC in Fig. 2, by using Eq. (1)
estimated was a position of -characteristics at
f = 0 (curves 1a and 1b). This enabled us to determine
the height of barriers near each electrode. The obtained
data are listed in the table. It is seen that the barrier
height near the Pt electrode is almost three times lower
than that near the Cu electrode.
( )UC 2−
-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6
0
2
4
6
8
10
1b
1a
ϕPt
ϕCu
4
3
2
C-2x10-8, F-2
Fig. 2. Capacitance-voltage characteristics of the Pt/ZLI4803 +
2 wt.% D/Cu structure for various frequencies f, Hz:
(2), (3), (4). The sign of voltage
is indicated relatively to the Cu electrode. Curves 1a and 1b are
obtained using extrapolation of experimental data to the
frequency f = 0 via Eq. (1).
4102.1 −× 4100.2 −× 4100.3 −×
Table. Parameters of the dense part of DEL near Pt and Cu
electrodes as based on our analysis of CVC for the
structures Pt/ZLI4803 + 2 wt.% D/Cu. For comparison, the
data for glycerol [8] are listed.
Substance Elect-
rode
ϕ,
V
W,
nm
n,
m-3
Glycerol [8] SnO2 0.40±0.02 2.0 (3.0±0.5)
×1026
ZLI4803 +
2 wt.% D
Cu 0.50±0.02 1.6 (3.2±0.5)
×1027
ZLI4803 +
2 wt.% D
Pt 0.14±0.02 0.50 (6.0±0.5)
×1027
By using the intersection of the curves 1a and 1b
with the axis drawn through the point U = 0,
estimated were the capacities at zero displacement for
each of the barriers. Based on the formula for
capacitance of flat capacitor, we estimated the thickness
of space charge region W for each of the barriers. These
data are also shown in Table.
2−C
It is seen that this thickness, as for glycerol, is a
few monolayers, and the W value of near-electrode area
at the Pt electrode is approximately three times lower
than that near the Cu electrode. For each of the barriers,
values of the concentration inherent to ionized states n
were estimated via the slopes of curves 1a and 1b by
using the ratio
( )
2
0
2 2
enSdU
Cd
εε
=
−
, (2)
where 0ε is the electric constant, is the permittivity of
the solution, and S – area of the sample. It follows from
the table that the difference in the n value for each
barrier is smaller than that observed for the values of the
barrier height and thickness of the near-electrode layer
W. In contrast to glycerol [8], it was difficult to estimate
the concentration of molecules in the near-electrode
area, because a mixture of liquid crystals was used, and
it was not known what part of the total amount of the
molecules in solution comprise the dye molecules in
near-electrode area. However, based on comparison of
the n values for the structure studied in this work with
the results of [8], we can assume that almost all the
molecules of the near-electrode layer are ionized. The
main reason that leads to this effect is rather high value
of electric field in the dense part of DEL. To estimate its
magnitude, we assume that the voltage 0.1 V is applied
to the dense part of DEL. The thickness of the dense part
of DEL according to Table is approximately equal to
1 nm. Then, we get that the electric field value in the
dense part of DEL is 10
ε
8 V/m. Long-term action of this
field is in principle sufficient to ionize the molecules.
4. Conclusion
We first show that being based on analysis of CVC at
low frequencies one can estimate the parameters of the
323
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 321-324.
dense part of DEL at the interface electrode – solution of
a dye in liquid crystal. To implement this method,
created were the samples with different parameters of
DEL near each electrode by using the electrodes of
different chemical compositions. From the analysis of
properties of these samples, the difference in parameters
of DEL near each electrode is caused by not only
different chemical composition of the electrodes, but by
different orientations of LC molecules near them (planar
orientation near the Pt electrode and homeotropic one
near the Cu electrode).
It is shown that, like to glycerol, the obtained
barriers that are asymmetric relative to polarity of the
applied voltage can be evaluated taking the series
resistance into account. For each of the barriers, the
series resistance is the resistance of another barrier, if the
current flows in the forward direction.
Through the analysis of CVC for the structures
Pt/ZLI4803 + 2 wt.% D/Cu, estimated were the
parameters of the dense part of DEL separately near Pt
electrode and Cu one. It was shown that the barrier
height and thickness of near-electrode layer at the Pt
electrode differs triply from those at the Cu electrode. It
is obvious that this difference in the parameters is
mainly caused by different orientation of LC molecules
near each electrode.
Also it was shown that the concentration of ionized
centers near the Pt electrode is almost two times higher
than that near the Cu one. The very value of the
concentration of ionized centers is close to the
concentration of LC molecules. So, we can assume that
the majority of molecules in the dense part of DEL is
ionized. This may be caused by rather high value (about
108 V/m) of external electric field in the dense part
of DEL.
References
1. G.J. Sprokel, Resistivity, permittivity and the
electrode space charge of nematic liquid crystals //
Mol. Cryst. Liq. Cryst. 22, No.3/4, p. 249-260
(1973).
2. G. Barbero, G. Durand, Selective ion adsorption
and nonlocal anchoring energy in nematic liquid
crystal // J. Appl. Phys. 67(5), p. 2678-2680 (1990).
3. H. Mada, H. Suzuki, Reverse hysteresis loop of
nematic liquid crystal in characteristic due
to static electric field // Jpn. J. Appl. Phys. 26(7),
p. L1092-L1094 (1987).
VC −
4. A. Sawada, K. Tarumi, S. Naemura, Effects of
electric double layer and space charge polarization
by plural kinds of ions on complex dielectric
constant on liquid crystal materials // Jpn. J. Appl.
Phys. 38(3A), p. 1418-1422 (1999).
5. Hiroyuki Ohshima, Theory of Colloid and
Interfacial Electric Phenomena. Academic Press,
2006.
6. A.M. Gabovich, Yu.A. Reznikov, A.I. Voitenko,
Excess nonspecific Coulomb ion adsorption at the
metal electrode/electrolyte solution interface: Role
of the surface layers // Phys. Rev. E, 73, 021606
(2006).
7. A.V. Koval’chuk, Low-frequency spectroscopy as
an investigation method of the electrode-liquid
interface // Functional Materials, 5, No.3, p. 426-
430 (1998).
8. A.V. Koval’chuk, Generation of charge carriers
and formation of antisymmetric double electric
layers in glycerine // J. Chem. Phys. 108(19),
p. 8190-8194 (1998).
9. A.J. Twarowski, A.C. Albrecht, Depletion layer in
organic films: low frequency measurements in
polycrystalline tetracene // J. Chem. Phys. 20 (5),
p. 2255-2261 (1979).
10. O.V. Konstantinov, O.A. Mezrin, Influence of
series resistance of Schottky diode on its effective
capacitance // Fizika tekhnika poluprovodnikov,
17(2), p. 305-311 (1983), in Russian.
11. Yu.A. Goldberg, O.V. Ivanova, T.V. Lvova,
B.V. Tsarenkov, Influence of series resistance on
the capacitance-voltage characteristics of surface
barier structure // Fizika tekhnika poluprovodnikov,
17(6), p. 1068-1072 (1983), in Russian.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
324
|
| id | nasplib_isofts_kiev_ua-123456789-117748 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2025-12-07T18:52:39Z |
| publishDate | 2011 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Kovalchuk, O.V. 2017-05-26T15:51:24Z 2017-05-26T15:51:24Z 2011 Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal / O.V. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 321-324. — Бібліогр.: 11 назв. — англ. 1560-8034 PACS 61.30.Gd, 68.08.-p, 77.22.Ch https://nasplib.isofts.kiev.ua/handle/123456789/117748 First estimated were the parameters of dense part of double electrical layer
 (DEL) at the interface electrode – solution of the dye in liquid crystal, based on analysis
 of capacitance-voltage characteristics obtained at low frequencies, taking the series
 resistance into account. DELs with various parameters near each electrode were obtained
 owing to their different chemical compositions. It was shown that the obtained difference
 in the DEL parameters is caused not only by different chemical composition of the
 electrodes, but also different orientations of molecules near each of them (planar
 orientation near one electrode and homeotropic one near another). It was estimated the
 barrier height, thickness of near-electrode layer, and concentration of ionized states in the
 dense parts of DEL near each electrode. It was shown that these parameters differ by 2 to
 3 times, and an assumption was made about the reasons that can cause this difference. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal Article published earlier |
| spellingShingle | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal Kovalchuk, O.V. |
| title | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| title_full | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| title_fullStr | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| title_full_unstemmed | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| title_short | Method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| title_sort | method and estimation of parameters of dense part of double electrical layer at the interface electrode-solution of the dye in liquid crystal |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/117748 |
| work_keys_str_mv | AT kovalchukov methodandestimationofparametersofdensepartofdoubleelectricallayerattheinterfaceelectrodesolutionofthedyeinliquidcrystal |