Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal
Solutions of fullerene molecules С60 with chemically attached molecules of diamine (С60D) in planar oriented nematic liquid crystal (NLC) were obtained by only heating and ultrasonic processing. The С60D concentration changes from 0 up to 3.0 wt.%. Within the ranges of frequencies 10⁻¹ – 10⁶ Hz and...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2011
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| Cite this: | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal / V.E. Vovk, O.V. Kovalchuk, M.P. Gorishnyj, T.M. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 2. — С. 256-260. — Бібліогр.: 10 назв. — англ. |
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| author | Vovk, V.E. Kovalchuk, O.V. Gorishnyj, M.P. Kovalchuk, T.M. |
| author_facet | Vovk, V.E. Kovalchuk, O.V. Gorishnyj, M.P. Kovalchuk, T.M. |
| citation_txt | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal / V.E. Vovk, O.V. Kovalchuk, M.P. Gorishnyj, T.M. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 2. — С. 256-260. — Бібліогр.: 10 назв. — англ. |
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| description | Solutions of fullerene molecules С60 with chemically attached molecules of diamine (С60D) in planar oriented nematic liquid crystal (NLC) were obtained by only heating and ultrasonic processing. The С60D concentration changes from 0 up to 3.0 wt.%. Within the ranges of frequencies 10⁻¹ – 10⁶ Hz and temperatures 298-343 K, dielectric properties of solutions were investigated. It was shown that, at frequencies higher than 100 Hz, the frequency dispersion of the components of complex dielectric permittivity is absent. A value of conductivity of the solution was determined. It was also shown that the activation energy for the temperature dependence of the conductivity in nematic and isotropic phases does not depend on the concentration of molecules С60D. Obtained and explained were the reasons of the nonmonotonic conductivity dependence of solutions on the concentration of С60D molecules. For frequencies lower than 100 Hz, the dispersion of the components of complex dielectric permittivity is observed. It was shown that the dispersion can be described by the Debye equation. The temperature dependence of a value inverse to the relaxation time correlates with the temperature dependence of conductivity. Presence of С60D molecules in NLC tends to increasing the voltage for the Frederiksz transition. Made was the assumption that this effect may be explained by increase in viscosity of NLC as a consequence of aggregation of fullerene molecules.
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 2. P. 256-260.
PACS 73.61.Wp, 78.66.Tr
Dielectric and electro-optical properties of solutions
of chemically modified fullerene С60 in nematic liquid crystal
V.E. Vovk1, O.V. Kovalchuk1, M.P. Gorishnyj1, T.M. Kovalchuk2
1Institute of Physics, NAS of Ukraine,
46, prospect Nauky, 03028 Kyiv, Ukraine
2V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: vovk@iop.kiev.ua
Abstract. Solutions of fullerene molecules С60 with chemically attached molecules of
diamine (С60D) in planar oriented nematic liquid crystal (NLC) were obtained by only
heating and ultrasonic processing. The С60D concentration changes from 0 up to
3.0 wt.%. Within the ranges of frequencies 10–1 – 106 Hz and temperatures 298-343 K,
dielectric properties of solutions were investigated. It was shown that, at frequencies
higher than 100 Hz, the frequency dispersion of the components of complex dielectric
permittivity is absent. A value of conductivity of the solution was determined. It was also
shown that the activation energy for the temperature dependence of the conductivity in
nematic and isotropic phases does not depend on the concentration of molecules С60D.
Obtained and explained were the reasons of the nonmonotonic conductivity dependence
of solutions on the concentration of С60D molecules. For frequencies lower than 100 Hz,
the dispersion of the components of complex dielectric permittivity is observed. It was
shown that the dispersion can be described by the Debye equation. The temperature
dependence of a value inverse to the relaxation time correlates with the temperature
dependence of conductivity. Presence of С60D molecules in NLC tends to increasing the
voltage for the Frederiksz transition. Made was the assumption that this effect may be
explained by increase in viscosity of NLC as a consequence of aggregation of fullerene
molecules.
Keywords: fullerene molecule, nematic liquid crystal, dielectric and electro-optical
properties.
Manuscript received 10.02.11; accepted for publication 16.03.11; published online 30.06.11.
1. Introduction
The recent decade may be characterized by wide
application of liquid crystals (LC) in manufacturing the
displays of varied types. At the same time, intense
researches in the field of nanotechnology have begun.
Therefore, it is reasonable to use nanoparticles to
improve certain parameters of LC and to develop
materials with new properties that would be inherent
neither to nanoparticles, nor to LC [1, 2].
The perspective direction of these researches is
modulation of the LC structure by using nanoparticles
that, under certain conditions, can create elements of
structures with the certain ordering inside it. This
structural ordering can be easily controlled because the
structure basic elements are in a liquid anisotropic
phase [3].
We showed earlier [4] that presence of 3 wt.%
С60D in ferroelectric LC results in disappearance of
ferroelectric properties. In this case, all the phases and
temperatures of transitions between them were not
changed. Made was the assumption that the reason of
disappearance of the ferroelectric properties is the
substantial growth of rotary viscosity in the solution
С60D in LC as a consequence of formation of bonds
between the molecules С60D. These bonds can rather
easily arise between the С60D molecules due to
presence of symmetric diamine molecules.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
256
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 2. P. 256-260.
The purpose of this work was to research the
possibilities for manifestation of the structural ordering
of С60D molecules in NLC and to define what methods
serving as a basis of the changes in NLC properties can
be investigated most efficiently [5, 6].
2. Materials and methods
As NLC, we used nematic mixture E25M, while as an
impurity we used fullerene C60 with attached molecules
of 1,8-octanediamine (NH2-(CH2)8-NH2). They were
created in Universidad Nacional Autónoma de México
[6]. The С60D concentration was changed within the
range 0-3.0 wt.%. Observation by using the polarized
microscope has shown that, in the NLC-С60D solution,
the temperature of phase transitions within the limits of
experimental error 0.5 K was the same as in the pure
NLC one.
The studies were carried out with the use of
sandwich cells. Transparent layers of In2O3 deposited on
a glass plate were used as electrodes. Each electrode was
separated by measuring and protecting sections via
etching. The protecting electrode was grounded during
the measurements. To create a planar orientation of the
molecules, we used polyamide.
The thickness of a composite layer d varying within
the range of 20-23 μm was obtained by introducing a
Teflon film between the glass plates over the protecting
electrode. Due to a low viscosity of the NLC-С60D
composite, filling the cells was achieved via pressurizing
this composite between the electrodes. The distance
between electrodes was set by two Teflon film strips.
The cell was not completely filled with this composite.
In that place of the cell where the composite was absent,
the distance between the electrodes d was measured by
using the interferometric method. The cell assembled
was sealed with glue along its perimeter.
Temperature stabilization with an error less than
0.2 K was carried out in a custom-designed thermostat
with a low level of electromagnetic noise. The
measurements were performed within the temperature
range 298-343 K. The sample capacity C and resistance
R were measured within the frequency range
by means of the oscilloscopic method [7].
The measured signal had the triangular shape. The peak
voltage value was U
Hz1010 61 −−
0 = 0.25 V. Based on the data
obtained, the frequency dependence was analyzed for ε′
and ε″ components of the complex dielectric
permittivity.
Electro-optical researches were carried out using
the standard technique [8]. The angle between the axes
of the polarizer and analyzer was 90°.
3. Experimental results and discussion
3.1. Dielectric properties
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Shown in Fig. 1 are the frequency dependences of real ε′
(curves 1, 3) and imaginary ε″ (2, 4) components of the
complex dielectric permittivity for the planar oriented
NLC (1, 2) and Е25М+3 wt.% С60D (3, 4). All the
measurements have been carried out at the temperature
298 K. The obtained spectra can be separated by two
parts A and B. In the low-frequency area A (f < 100 Hz),
observed is an essential (by orders) increase in ε′ and ε″
when the frequency decreases. As it was shown in [9],
the presence of this part of the dielectric spectrum is
caused by near-electrode phenomena. Properties of this
layer will be analyzed after the analysis of bulk
properties of the samples under study (part B of the
dielectric spectrum).
3.1.1. Bulk region
As it follows from Fig. 1, in the part of the dielectric
spectrum where f > 100 Hz, the value ε′ did not depend
on frequency, and the value ε″ linearly decreased with
frequency. The latter is caused by that, for this area of
frequencies, the resistance did not depend on frequency.
It enabled us to find the conductivity of samples by
using alternating current σАС .
In the part of the linear dependence ε″ on frequency
(it is marked by lines in Fig. 1), the value σАС was
determined as
ωε ′′ε=σ 0AC , (1)
where ε0 is the dielectric permittivity in vacuum, and
ω = 2πƒ is the cyclic frequency.
Fig. 2 shows a temperature dependence of the
conductivity for NLC and NLC+C60D with various
concentrations of C60D. As it follows from the data
obtained, within mesophase limits, a linear dependence
of the logarithm of conductivity on inverse temperature
(Arrhenius coordinates) is observed
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ Δ
−σ=σ σ
Tk
E
AC exp0 , (2)
where σ0 is the direct current conductivity, ΔEσ is the
activation energy of conductivity, k is the Boltzmann
constant.
10-1 100 101 102 103 104 105 106
10-1
100
101
102
103
104
4
3
2
1
ε
f, Hz
Fig. 1. Frequency dependences of real ε′ (1, 3) and imaginary
ε″ (2, 4) components of the complex dielectric permittivity for
the planar oriented NLC (1, 2) and Е25М+3 wt.% С60D (3, 4).
257
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 2. P. 256-260.
2,9 3,0 3,1 3,2 3,3 3,4
10-8
10-7
10-6
10-5
5
4
3
2
1
NI
σ A
C
, O
hm
-1
m
-1
103/T, K-1
Fig. 2. Temperature dependence of the conductivity for NLC
and NLC+C60D with E25M (1), 0.03 wt.% C60D (2),
0.3 wt.% C60D (3), 1 wt.% C60D (4), 3 wt.% C60D (5). I –
isotropic phase and N – nematic phase.
As it follows from Fig. 2, the slope of temperature
dependences for conductivity in each mesophase of LC
(within the limits of experimental errors of
measurements) does not depend on presence of С60D
and its concentration. It follows from our estimations
that in the nematic phase ΔЕσ = 0.55±0.06 eV, and in the
isotropic phase ΔЕσ = 0.22±0.06 eV.
Fig. 3 shows the concentration dependence of
conductivity for the solution NLC+С60D at the
temperature 298 K for the fullerene concentrations
с ≥ 0.03 wt.%. Being based on the data obtained, it may
be inferred that this dependence corresponds to the
equation
4/1cbAC =σ , (3)
where b is the factor of proportionality. In accord with
[8], for the solution NLC+С60D it is impossible to
explain the process of charge carrier formation as based
on the simple model of generation-recombination of
charge carriers, but it is necessary to take into account
participation of complexes with charge carriers in
formation of charge carriers.
10-1 100 101
2x10-7
4x10-7
6x10-7
8x10-7
10-6
σ A
C
, O
hm
-1
m
-1
c, wt. %
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Fig. 3. Concentration dependence of conductivity for the
solution NLC+С60D at the temperature 298 K for the fullerene
concentrations с ≥ 0.03 wt.%.
3.1.2. Near-electrode region
As remarked above, the part of a dielectric spectrum A is
caused by near-electrode processes. It follows from the
work [9] that for planar oriented LC with positive
anisotropy of dielectric permittivity (NLC corresponds
to these characteristics), characteristic is the relaxation
process caused by dipole polarization of molecules in a
near-electrode layer. At low frequencies, almost all the
voltage is applied to this layer of the sample, and an
alternating current can be caused by oscillations of
molecular dipoles within the angles that correspond to
fluctuations of the order parameter. Inside the sample
bulk, the electric field is much less and the electric
current is mainly provided by ion carriers. In this case,
the sample can be separated by three parts (two near-
electrode layers and bulk) with different mechanisms of
charge transfer. When the layers with various electric
parameters are in contact, the Маxwell-Wagner
polarization may arise [10]. It is this polarization that is
responsible for appearance of the relaxation process
inside the sample bulk.
Our researches have shown [9] that, for nematic
and isotropic phases of LC, the relaxation process
caused by dipole polarization in a near-electrode layer is
described by the Debye equation:
τω+
ε−ε
+ε=ε ∞
∞
∗
i
s
1
, (4)
where ε* is the complex dielectric permittivity, εs and ε∞
are the dielectric permittivities for frequencies f = 0 and
ƒ = ∞, accordingly, τ is the time of dielectric relaxation.
The analysis of the frequency dependences ε′ and
ε″ obtained experimentally in the part of a dielectric
spectrum A has shown that, for all the samples, the
dependence ε″(ε′) (Cole-Cole diagram) is approximated
by a semicircle. These dependences between ε′ and ε″,
according to theoretical representations, correspond to
the Debye relaxation and are described by the
equation (4).
2,9 3,0 3,1 3,2 3,3 3,4
100
101
102
3
2
1
NI
τ-1
, s
-1
103/T, K-1
Fig. 4. Temperature dependence τ–1 for NLC and NLC+C60D
with E25M (1), 1 wt.% C60D (2), 3 wt.% C60D (3). I –
isotropic phase and N – nematic phase.
258
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 2. P. 256-260.
As shown in [9], the time for dipole polarization in
the LC near-electrode layer is in inverse proportion to
the conductivity of liquid. Therefore, for comparison of
the data obtained when analyzing the near-electrode
processes with the results summarized in Fig. 2, we have
analyzed the temperature dependence of the value
inverse to the relaxation time (τ–1).
The temperature dependence τ–1 is shown in Fig. 4.
It follows from comparison of Figs 2 and 4 that
generally confirmed is the conclusion that was made in
the work [9] about the relation between the values of
relaxation time and conductivity of samples, because
well-defined correlation between the temperature
dependences τ–1 and σАС is observed in this case.
However, distinction between the data shown in
Figs 2 and 4 is also observed. First, for the temperature
dependence τ–1, the activation energy is equal to
0.70±0.06 eV for the nematic phase, and 0.37±0.06 eV for
isotropic one, which slightly exceeds the corresponding
values for the activation energy of conductivity. It can be
explained by the fact that the relaxation time depends not
only on conductivity, but also on other parameters
possessing the same temperature dependence.
3.2. Electro-optic properties
Shown in Fig. 5 is the dependence of the sample
transmission normalized by the maximal value on the
applied voltage for NLC (1) and NLC+3 wt.% С60D (2).
It is clearly seen that introduction of 3 wt.% С60D into
LC essentially changes the dependence of conductivity
on the voltage. The reason is the non-homogeneous
orientation of NLC in the solution NLC+3 wt.% С60D
owing to aggregation of fullerene molecules. In this
case, the observed ordering of molecules caused by
orienting surfaces is imposed by the new ordering
caused by aggregation nanoparticles. To determine at
what concentration this effect manifests itself most
clearly, it is necessary to choose a parameter that
characterizes electro-optical properties. This parameter
is the voltage of transition from one orientation of
molecules to another (in our case, from planar to
homeotropic one). This is the voltage of the Frederiksz
transition UF that can be found when analyzing the
dependence of transmission on the voltage [8].
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
The dependence of the value UF on the
concentration is depicted in Fig. 6. It follows from the
data obtained that, both for the dependence on the
fullerene concentration and for the dependence U
1−τ
F(с),
the greatest changes are observed when the content of
fullerene in a solution varies from 0.1 up to 0.3 wt.%.
Just within the limits of these concentrations, observed is
intensive formation of bonds between the fullerene
molecules. As a consequence, the viscosity of a solution
of fullerene in LC sharply increases, which is the main
cause for changes in the value UF. In other words, like to
the parameters of near-electrode processes, the
parameters of the electro-optical processes are sensitive
to ordering of fullerene molecules in LC.
0 2 4 6 8 10
0,0
0,2
0,4
0,6
0,8
1,0
2
1
I /
I m
ax
U, V
Fig. 5. The dependence of the sample transmission normalized
by the maximal value on the applied voltage for NLC (1) and
NLC+3 wt.% С60D (2).
0,0 0,2 0,4 0,6 0,8 1,0
1,0
1,2
1,4
1,6
1,8
2,0
U
F, V
c, wt %
Fig. 6. The dependence of the value UF on the concentration of
C60D.
4. Conclusions
1. The dielectric spectra of the solution Е25М+С60D
within the ranges of fullerene concentrations (0-
3.0 wt.%) and frequencies (10–1 – 106 Hz) can be
separated by two parts. For frequencies f < 100 Hz,
observed is the relaxation process caused by dipole
polarization of molecules (the electric field is applied to
a near-electrode layer). In the case f > 100 Hz, the
electric field in a sample is homogeneous, its parameters
characterize a bulk part of the structures under study.
2. The conductivity of samples using the alternating
current σАС in each phase linearly changes in the
coordinates ( )1ln −TACσ . The activation energy in each
phase of LC does not depend on the fullerene
concentration and is equal to 0.55±0.06 eV for the
nematic phase, and 0.22±0.06 eV for isotropic one. For
fullerene concentrations с ≥ 0.03 wt.%, σАС ∼ с1/4. This
dependence can be caused by generation of charge
carriers through an intermediate state with charge
transfer. Using the temperature and concentration
dependences σАС, it is difficult to determine at what
concentrations aggregation of molecules can take place.
259
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 2. P. 256-260.
3. The temperature dependence of the parameter,
inverse to the time of dielectric relaxation (τ–1),
correlates with temperature dependence σАС. This fact
confirms that the relaxation process in near-electrode
layer is caused by molecular dipoles oscillations within
the angles that do not exceed fluctuations of the order
parameter.
4. The dependence of transmission on the applied
voltage essentially varies when introducing fullerene.
The voltage of transition from planar to homeotropic
orientations of molecules UF increases with growth of
the fullerene concentration. One of the main reasons for
presence of this effect is the increase of the solution
Е25М+С60D viscosity with increasing the fullerene
concentration. Therefore, analysis of the UF value
dependence on c enables to determine at what
concentration of nanoparticles one can observe fullerene
aggregation.
Acknowledgments
This work was supported in part by Projects 1.4.1.
В/134. Authors also thank Dr. E.V. Basiuk (Golovataya-
Dzhymbeeva), Universidad Nacional Autónoma de
México for providing the sample of modified fullerene
C60 and Dr. A.B. Nych for his help with obtaining the
microphotos.
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(Golovataya-Dzhymbeeva), Dielectric properties of
(C60+C70) ferroelectric liquid crystal composite //
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in Liquid Crystal Materials. Springer, New York,
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260
2. Materials and methods
Acknowledgments
|
| id | nasplib_isofts_kiev_ua-123456789-117724 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2025-11-29T13:13:02Z |
| publishDate | 2011 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Vovk, V.E. Kovalchuk, O.V. Gorishnyj, M.P. Kovalchuk, T.M. 2017-05-26T13:11:46Z 2017-05-26T13:11:46Z 2011 Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal / V.E. Vovk, O.V. Kovalchuk, M.P. Gorishnyj, T.M. Kovalchuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 2. — С. 256-260. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 73.61.Wp, 78.66.Tr https://nasplib.isofts.kiev.ua/handle/123456789/117724 Solutions of fullerene molecules С60 with chemically attached molecules of diamine (С60D) in planar oriented nematic liquid crystal (NLC) were obtained by only heating and ultrasonic processing. The С60D concentration changes from 0 up to 3.0 wt.%. Within the ranges of frequencies 10⁻¹ – 10⁶ Hz and temperatures 298-343 K, dielectric properties of solutions were investigated. It was shown that, at frequencies higher than 100 Hz, the frequency dispersion of the components of complex dielectric permittivity is absent. A value of conductivity of the solution was determined. It was also shown that the activation energy for the temperature dependence of the conductivity in nematic and isotropic phases does not depend on the concentration of molecules С60D. Obtained and explained were the reasons of the nonmonotonic conductivity dependence of solutions on the concentration of С60D molecules. For frequencies lower than 100 Hz, the dispersion of the components of complex dielectric permittivity is observed. It was shown that the dispersion can be described by the Debye equation. The temperature dependence of a value inverse to the relaxation time correlates with the temperature dependence of conductivity. Presence of С60D molecules in NLC tends to increasing the voltage for the Frederiksz transition. Made was the assumption that this effect may be explained by increase in viscosity of NLC as a consequence of aggregation of fullerene molecules. This work was supported in part by Projects 1.4.1. В/134. Authors also thank Dr. E.V. Basiuk (GolovatayaDzhymbeeva), Universidad Nacional Autónoma de México for providing the sample of modified fullerene C₆₀ and Dr. A.B. Nych for his help with obtaining the microphotos. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal Article published earlier |
| spellingShingle | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal Vovk, V.E. Kovalchuk, O.V. Gorishnyj, M.P. Kovalchuk, T.M. |
| title | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal |
| title_full | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal |
| title_fullStr | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal |
| title_full_unstemmed | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal |
| title_short | Dielectric and electro-optical properties of solutions of chemically modified fullerene С60 in nematic liquid crystal |
| title_sort | dielectric and electro-optical properties of solutions of chemically modified fullerene с60 in nematic liquid crystal |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/117724 |
| work_keys_str_mv | AT vovkve dielectricandelectroopticalpropertiesofsolutionsofchemicallymodifiedfullerenes60innematicliquidcrystal AT kovalchukov dielectricandelectroopticalpropertiesofsolutionsofchemicallymodifiedfullerenes60innematicliquidcrystal AT gorishnyjmp dielectricandelectroopticalpropertiesofsolutionsofchemicallymodifiedfullerenes60innematicliquidcrystal AT kovalchuktm dielectricandelectroopticalpropertiesofsolutionsofchemicallymodifiedfullerenes60innematicliquidcrystal |