Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles
Electrical properties of mesogenic cadmium octanoate composites containing CdS nanoparticles (NPs) have been studied for the first time. Semiconductor CdS spherical NPs (sizes of 2.5 nm) were chemically synthesized in the thermotropic ionic liquid crystalline phase (smectic A) of cadmium octanoat...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2014
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irk-123456789-1183622017-05-31T03:04:59Z Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles Zhulay, D.S. Fedorenko, D.V. Koval’chuk, A.V. Bugaychuk, S.A. Klimusheva, G.V. Mirnaya, T.A. Electrical properties of mesogenic cadmium octanoate composites containing CdS nanoparticles (NPs) have been studied for the first time. Semiconductor CdS spherical NPs (sizes of 2.5 nm) were chemically synthesized in the thermotropic ionic liquid crystalline phase (smectic A) of cadmium octanoate that was used as nanoreactor. We compared the electrical properties of both clean matrix and nanocomposite to clarify the role of semiconductor CdS NPs with different concentrations. We have investigated electrical characteristics at different temperatures, which correspond to the different phases of the composites. The conductivity of nanocomposites has an activation nature both in anisotropic glassy and smectic A phase. The conductivity of the nanocomposite along the cation-anion layers is by two orders of magnitude higher than that across the cation-anion layers, which confirms anisotropy of the nanocomposite regardless of the phase of material. In the glassy phase, the electronic type conductivity is observed. Increasing the nanoparticles concentration brings additional free charge carriers or increases their mobility. For the smectic A phase, increasing the CdS NPs concentration brings additional traps for the carriers that travel in plane of the cation-anion layers. On the other hand, the nanoparticles deform the cation-anion layers and increase the mobility of carriers across the layers. 2014 Article Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles / D.S. Zhulay, D.V. Fedorenko, A.V. Koval'chuk, S.A. Bugaychuk, G.V. Klimusheva, T.A. Mirnaya // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 1. — С. 56-60. — Бібліогр.: 7 назв. — англ. 1560-8034 PACS 81.07.-b, 81.16.-c http://dspace.nbuv.gov.ua/handle/123456789/118362 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| description |
Electrical properties of mesogenic cadmium octanoate composites containing
CdS nanoparticles (NPs) have been studied for the first time. Semiconductor CdS
spherical NPs (sizes of 2.5 nm) were chemically synthesized in the thermotropic ionic
liquid crystalline phase (smectic A) of cadmium octanoate that was used as nanoreactor.
We compared the electrical properties of both clean matrix and nanocomposite to clarify
the role of semiconductor CdS NPs with different concentrations. We have investigated
electrical characteristics at different temperatures, which correspond to the different
phases of the composites. The conductivity of nanocomposites has an activation nature
both in anisotropic glassy and smectic A phase. The conductivity of the nanocomposite
along the cation-anion layers is by two orders of magnitude higher than that across the
cation-anion layers, which confirms anisotropy of the nanocomposite regardless of the
phase of material. In the glassy phase, the electronic type conductivity is observed.
Increasing the nanoparticles concentration brings additional free charge carriers or
increases their mobility. For the smectic A phase, increasing the CdS NPs concentration
brings additional traps for the carriers that travel in plane of the cation-anion layers. On
the other hand, the nanoparticles deform the cation-anion layers and increase the mobility
of carriers across the layers. |
| format |
Article |
| author |
Zhulay, D.S. Fedorenko, D.V. Koval’chuk, A.V. Bugaychuk, S.A. Klimusheva, G.V. Mirnaya, T.A. |
| spellingShingle |
Zhulay, D.S. Fedorenko, D.V. Koval’chuk, A.V. Bugaychuk, S.A. Klimusheva, G.V. Mirnaya, T.A. Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Zhulay, D.S. Fedorenko, D.V. Koval’chuk, A.V. Bugaychuk, S.A. Klimusheva, G.V. Mirnaya, T.A. |
| author_sort |
Zhulay, D.S. |
| title |
Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles |
| title_short |
Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles |
| title_full |
Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles |
| title_fullStr |
Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles |
| title_full_unstemmed |
Electro-conductive properties of cadmium octanoate composites with CdS nanoparticles |
| title_sort |
electro-conductive properties of cadmium octanoate composites with cds nanoparticles |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118362 |
| citation_txt |
Electro-conductive properties of cadmium octanoate composites
with CdS nanoparticles / D.S. Zhulay, D.V. Fedorenko, A.V. Koval'chuk, S.A. Bugaychuk, G.V. Klimusheva, T.A. Mirnaya // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 1. — С. 56-60. — Бібліогр.: 7 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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2025-07-08T13:47:24Z |
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2025-07-08T13:47:24Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
56
PACS 81.07.-b, 81.16.-c
Electro-conductive properties of cadmium octanoate composites
with CdS nanoparticles
D.S. Zhulay1, D.V. Fedorenko1, A.V. Koval’chuk2, S.A. Bugaychuk1, G.V. Klimusheva1, T.A. Mirnaya3
1Institute of Physics, NAS of Ukraine,
46, prospect Nauky, 03028 Kyiv, Ukraine, e-mail: klimush@iop.kiev.ua
2Kyiv National University of Technologies and Design,
2, Nemirovich-Danchenko str., 01011 Kyiv, Ukraine
3V. Vernadskii Institute of General and Inorganic Chemistry, NAS of Ukraine,
32/34, prospect Palladina, 03142 Kyiv, Ukraine
Abstract. Electrical properties of mesogenic cadmium octanoate composites containing
CdS nanoparticles (NPs) have been studied for the first time. Semiconductor CdS
spherical NPs (sizes of 2.5 nm) were chemically synthesized in the thermotropic ionic
liquid crystalline phase (smectic A) of cadmium octanoate that was used as nanoreactor.
We compared the electrical properties of both clean matrix and nanocomposite to clarify
the role of semiconductor CdS NPs with different concentrations. We have investigated
electrical characteristics at different temperatures, which correspond to the different
phases of the composites. The conductivity of nanocomposites has an activation nature
both in anisotropic glassy and smectic A phase. The conductivity of the nanocomposite
along the cation-anion layers is by two orders of magnitude higher than that across the
cation-anion layers, which confirms anisotropy of the nanocomposite regardless of the
phase of material. In the glassy phase, the electronic type conductivity is observed.
Increasing the nanoparticles concentration brings additional free charge carriers or
increases their mobility. For the smectic A phase, increasing the CdS NPs concentration
brings additional traps for the carriers that travel in plane of the cation-anion layers. On
the other hand, the nanoparticles deform the cation-anion layers and increase the mobility
of carriers across the layers.
Keywords: octanoate, CdS nanoparticles, ionic liquid crystal, nanocomposite, electrical
properties.
Manuscript received 12.11.13; revised version received 30.01.14; accepted for
publication 20.03.14; published online 31.03.14.
1. Introduction
The new class of ionic liquid crystals based on metal
alkanoates possesses a number of unique properties,
such as intrinsic ionic conductivity, high solvating
power and ability to form time-stable mesomorphic
glasses [1]. The mesophase of metal alkanoates can be
used as a nanoreactor for chemical synthesis and
stabilization of semiconductor nanoparticles (NPs).
Earlier, the electrical conductivity of lyotropic and
thermotropic ionic liquid crystals of different metal
alkanoates was studied [2]. High electrical conductivity
was observed in the potassium caproate lyotropic ionic
liquid crystals (LILC). It appeared to be higher than
that in isotropic water electrolytes [3], which arises
from ordering the structure of lyotropic smectic A
liquid crystals. On applying an electric field, the
potassium cations can easily migrate along the cation-
anion layers with water of LILC. The electrical
conductivity of the cobalt decanoate mesophase along
the cation-anion layers is by four orders of magnitude
larger than that in the perpendicular direction due to the
homeotropic layer alignment of the thermotropic
smectic A phase [2].
In the present work, we investigate electrical
properties of both pure cadmium octanoate matrix and
cadmium octanoate composites with different
concentration of synthesized CdS NPs.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
57
2. Sample preparation and experimental techniques
Cadmium octanoate ( 2
157
2 COOHCCd , abbreviation
– CdC8) exists in the form of the polycrystalline powder
at room temperature. Within the temperature range
98…180 C, cadmium octanoate forms the smectic A
mesophase (Fig. 1) that can be used for chemical
synthesis of semiconductor nanoparticles (NPs) [4]. The
smectic A phase of CdC8 can be frozen by quick cooling
to room temperature.
We study cadmium octanoate composites
containing CdS NPs, which were synthesized directly in
the CdC8 matrix by the template-controlled method [1]:
a cadmium octanoate polycrystalline powder
impregnated with a saturated aqueous-alcoholic solution
of thiourea was put in a furnace at 150 C (the
temperature of the mesophase of cadmium alkanoates) in
argon atmosphere for one hour. Under these conditions,
CdS NPs grow in the matrix. After cooling the
mesophase of composite to room temperature, the
nanocomposite contained CdS NPs (Fig. 2). The starting
concentrations of thiourea for preparation of composites
were 2, 4, 6 mol.%. It was shown that increasing the
concentration of sulphide ions in the matrix does not
affect the size of CdS NPs but increases their
concentration in the matrix [4].
Fig. 1. Model of the layered structure (smectic A) of cadmium
octanoate CdC8.
On the basis of X-ray measurements, the size of
synthesized CdS NPs was estimated ≈2.5 nm. Our
results show that NPs have a small dispersion of their
sizes, and their shape is spherical. The nanocomposites
are stable over a long period of time (years) and ordered
in a layered matrix (Fig. 2) [5].
The cadmium octanoate composites with CdS NPs
placed in flat cell form smectic A domains with
homeotropic orientation. The cells were prepared using
nickel electrodes and glass supports. Two geometric
configurations of samples were used to investigate
anisotropy of electrical properties of matrix and
composites (Fig. 3). We placed the electrodes in such a
way to direct the current perpendicular (Fig. 3a) and
parallel (Fig. 3b) to the ionic layers of the
nanocomposite. The spacers in the case 3a adjust the
thicknesses of the samples. In the case 3b, the thickness
is given by the thickness of the electrodes. The
thicknesses of samples are chosen to be 30 μm in the
case 3a and 1 mm for the case 3b.
The electric conductivity was measured using the
oscilloscopic technique (Fig. 4) [6, 7]. The voltage
signal had a triangular shape with the pick amplitude
0.25 V. The signal frequency was changed within the
range from 50 up to 106 Hz. It was found that the
resistance of the samples had no dispersion at
frequencies above 5103 Hz. Therefore, the electric
conductivity was measured at the frequency 104 Hz.
The bulk resistance was directly measured in our
experiments. Thus, the geometric parameter, k, must be
known for each sample to make the calculation of the
possible conductivity, Rk . The geometric
parameter, )( zdlk , depends on the sample
thickness d, the length of the metal electrode z , and the
distance between electrodes l .
To make the temperature measurements available,
the samples were placed into a thermostat. The
temperature was changed from 20 C up to 200 C.
Fig. 2. Model of the nanocomposite with incorporated
nanoparticles.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
58
(Ni) electrodes
Glass substrate
b)
S
am
pl
e
S
am
ple
b) Charge transfer along the cation-anion layera) Charge transfer across cation-anion layer
Z
Z
а)
Fig. 3. Configuration of the cells used in experiments.
Generator Resistance
Oscilloscope
X Y
Sample
Thermostat
ILCs
Fig. 4. Setup for measuring the electric conductivity.
2.4 2.6 2.8 3.0 3.2 3.4
150 125 100 75 50 25
T, oC
, O
hm
m
1
1
2
3
4
Fig. 5. Electric conductivity of matrices CdC8 (1), CdC8 +
2% CdS (2), CdC8 + 4% CdS (3) and CdC8 + 6% CdS (4)
versus inverse temperature.
3. Experimental results and discussion
The temperature dependence of the conductivity
along the cation-anion layers (see Fig. 3b) is shown in
Fig. 5. Different symbols and lines correspond to the
conductivity of the nanocomposites with a different
concentration of NPs: matrix CdC8 (1), CdC8 + 2% CdS
(2), CdC8 + 4% CdS (3) and CdC8 + 6% CdS (4). Their
analysis shows that the temperature dependences )(T ,
plotted in the Arrhenius coordinates )/1(ln T , can be
approximated by straight lines for both investigated
phases (glass and ionic liquid crystals). Thus, like for the
majority of liquid crystals, the temperature dependence
of nanocomposite conductivity can be represented as:
Tk
Ea
B
0 exp (1)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
59
where 0 is the parameter depending on the phase of
liquid crystal, aE – activation energy of conduction, Bk
– Boltzmann constant.
Similar dependences were measured for )/1( T ,
see Fig. 6.
Comparison of the values of T/1 in Fig. 5 and
)/1( T in Fig. 6 confirms anisotropy of our materials.
The difference by two orders of magnitude arises from
the large difference in the mobility of charge carriers
along and across the cation-anion layers. Each
dependence T/1 and )/1( T can be
approximated by two straight lines. Thus, the activation
energy, aE , can be established for both phases and for
each concentration of the nanoparticles by using
formula (1).
Let us describe the glassy phase first. The
dependence of the activation energy, aE , on the
concentration of NPs is presented as the following plot
(Fig. 7).
The values of the activation energy are indicative
of the electronic nature of conductivity. As stated above,
the mobility of electrons in the cation-anion layers is
higher than that perpendicular to them. With increasing
the nanoparticle concentration, the activation energy
decreases. The increase of the number of free charge
carriers or their mobility may cause this decrease.
Second, the conductivity significantly grows at
temperatures above 100 C when the glassy
nanocomposite turns to the smectic A phase. The
anisotropy of conductivity is still as high as in the glassy
phase. But the values of the activation energy get much
higher (approximately by two orders of magnitude),
which states a different type of conductivity. Therefore,
we suggest that Cd2+ cations are main charge carriers in
the smectic A phase of the nanocomposite. The
dependences of the activation energy in the smectic A
phase of nanocomposites on the concentration of CdS
NPs are presented in Fig. 8.
2.4 2.6 2.8 3.0 3.2 3.4
150 125 100 75 50 25
, O
hm
m
103/T, K1
1
2
3
T, oC
Fig. 6. Electric conductivity of matrices CdC8 (1), CdC8 +
2% CdS (2), CdC8 + 4% CdS (3) versus inverse temperature.
0 2 4 6
0.00
0.01
0.02
0.03
0.04
a
a
E
a ,
eV
c, mol.%
Fig. 7. Conductivity activation energy on the concentration of
CdS NPs in glassy nanocomposites across and along the
cation-anion layers.
0 2 4 6
0.75
1.00
1.25
1.50 a
a
E
a
, e
V
c, mol.%
Fig. 8. Conductivity activation energy on the concentration of
CdS NPs for the smectic A phase of nanocomposites across
and along the cation-anion layers.
Here we can see that the activation energies behave
principally different when charge carriers occur either
across or along the cation-anion layers. When the carriers
move inside the cation-anion layers, aE grows with the
concentration of NPs. This non-evident result can be
explained by appearance of new traps due to increasing
the number of nanoparticles on the way of carriers. Thus,
we have to state that the nanoparticles not only increase
the number of charge carriers or mobility, but also create
“potential wells” for moving ions, thereby increasing the
height of energetic barriers on the way of ions. On the
other hand, the nanoparticles deform the alkanoate chains
and increase the mobility of carriers across the layers.
4. Conclusions
The conductivity of nanocomposites has an activation
dependence both in glassy and smectic A phases. The
conductivity of the nanocomposite along the cation-
anion layers is by two orders of magnitude higher than
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
60
that across the cation-anion layers, which confirms
anisotropy of the nanocomposite regardless of the phase
of materials. In the glassy phase, electrons carry the
charge. The increase of the nanoparticle concentration
brings additional free charge carriers or increases their
mobility. For the smectic phase, increasing the
nanoparticle concentration brings additional traps for the
carriers that travel in plane of the cation-anion layers. On
the other hand, the nanoparticles deform the alkanoate
chains and increase the mobility of carriers across the
layers. In general, we can conclude that the new
nanocomposites based on metal alkanoates with
synthesized CdS nanoparticles have very interesting
electric properties and are promising for applications in
electro-optical devices.
References
1. T.A. Mirnaya, S.V. Volkov, Ionic liquid crystals as
universal matrices (solvents): Main criteria for
ionic mesogenicity, in: Green Industrial
Applications of Ionic Liquids. Eds. R.D. Rogers,
K.R. Seddon, S.V. Volkov. Kluwer Academic
Publishers, London, 2002, pp.439-456.
2. Yu. Garbovskiy, A.V. Kovalchuk, A. Grydyakina,
S.A. Bugaychuk, T. Mirnaya, G.V. Klimusheva,
Electrical conductivity of lyotropic and
thermotropic ionic liquid crystals consisting of
metal alkanoates // Liquid Crystals, 34(5), p. 599-
603 (2007).
3. D.R. Lide (Eds), Handbook of Chemistry and
Physics. 84th Ed. CRC Press, 2003-2004.
4. T.A. Mirnaya, V.N. Asaula, S.V. Volkov,
A.S. Tolochko, D.A. Melnik, G.V. Klimusheva,
Synthesis and optical properties of liquid
crystalline nanocomposites of cadmium octanoate
with CdS quantum dots // Physics and Chemistry of
Solid State, 13(1), p. 131-135 (2012).
5. G.V. Klimusheva, I. Dmitruk, T. Mirnaya,
A. Tolochko, S.A. Bugaychuk, A. Naumenko, D.
Melnik, V. Asaula, Monodispersity and ordering of
semiconductor quantum dots synthezed in ionic
liquid crystalline phase of cadmium alkanoates //
Liquid Crystals, 40(7), p. 980-988 (2013).
6. 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).
7. A.V. Koval’chuk, Low- and infra-low dielectric
spectroscopy liquid crystal – solid state interface.
Sliding layers // Ukr. J. Phys. 41, No.10, p. 991-
998 (1996).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 1. P. 56-60.
PACS 81.07.-b, 81.16.-c
Electro-conductive properties of cadmium octanoate composites
with CdS nanoparticles
D.S. Zhulay1, D.V. Fedorenko1, A.V. Koval’chuk2, S.A. Bugaychuk1, G.V. Klimusheva1, T.A. Mirnaya3
1Institute of Physics, NAS of Ukraine,
46, prospect Nauky, 03028 Kyiv, Ukraine, e-mail: klimush@iop.kiev.ua
2Kyiv National University of Technologies and Design,
2, Nemirovich-Danchenko str., 01011 Kyiv, Ukraine
3V. Vernadskii Institute of General and Inorganic Chemistry, NAS of Ukraine,
32/34, prospect Palladina, 03142 Kyiv, Ukraine
Abstract. Electrical properties of mesogenic cadmium octanoate composites containing CdS nanoparticles (NPs) have been studied for the first time. Semiconductor CdS spherical NPs (sizes of 2.5 nm) were chemically synthesized in the thermotropic ionic liquid crystalline phase (smectic A) of cadmium octanoate that was used as nanoreactor. We compared the electrical properties of both clean matrix and nanocomposite to clarify the role of semiconductor CdS NPs with different concentrations. We have investigated electrical characteristics at different temperatures, which correspond to the different phases of the composites. The conductivity of nanocomposites has an activation nature both in anisotropic glassy and smectic A phase. The conductivity of the nanocomposite along the cation-anion layers is by two orders of magnitude higher than that across the cation-anion layers, which confirms anisotropy of the nanocomposite regardless of the phase of material. In the glassy phase, the electronic type conductivity is observed. Increasing the nanoparticles concentration brings additional free charge carriers or increases their mobility. For the smectic A phase, increasing the CdS NPs concentration brings additional traps for the carriers that travel in plane of the cation-anion layers. On the other hand, the nanoparticles deform the cation-anion layers and increase the mobility of carriers across the layers.
Keywords: octanoate, CdS nanoparticles, ionic liquid crystal, nanocomposite, electrical properties.
Manuscript received 12.11.13; revised version received 30.01.14; accepted for publication 20.03.14; published online 31.03.14.
1. Introduction
The new class of ionic liquid crystals based on metal alkanoates possesses a number of unique properties, such as intrinsic ionic conductivity, high solvating power and ability to form time-stable mesomorphic glasses [1]. The mesophase of metal alkanoates can be used as a nanoreactor for chemical synthesis and stabilization of semiconductor nanoparticles (NPs). Earlier, the electrical conductivity of lyotropic and thermotropic ionic liquid crystals of different metal alkanoates was studied [2]. High electrical conductivity was observed in the potassium caproate lyotropic ionic liquid crystals (LILC). It appeared to be higher than that in isotropic water electrolytes [3], which arises from ordering the structure of lyotropic smectic A liquid crystals. On applying an electric field, the potassium cations can easily migrate along the cation-anion layers with water of LILC. The electrical conductivity of the cobalt decanoate mesophase along the cation-anion layers is by four orders of magnitude larger than that in the perpendicular direction due to the homeotropic layer alignment of the thermotropic smectic A phase [2].
In the present work, we investigate electrical properties of both pure cadmium octanoate matrix and cadmium octanoate composites with different concentration of synthesized CdS NPs.
2. Sample preparation and experimental techniques
Cadmium octanoate (
(
)
2
15
7
2
COO
H
C
Cd
-
+
, abbreviation – CdC8) exists in the form of the polycrystalline powder at room temperature. Within the temperature range 98…180 (C, cadmium octanoate forms the smectic A mesophase (Fig. 1) that can be used for chemical synthesis of semiconductor nanoparticles (NPs) [4]. The smectic A phase of CdC8 can be frozen by quick cooling to room temperature.
We study cadmium octanoate composites containing CdS NPs, which were synthesized directly in the CdC8 matrix by the template-controlled method [1]: a cadmium octanoate polycrystalline powder impregnated with a saturated aqueous-alcoholic solution of thiourea was put in a furnace at 150 (C (the temperature of the mesophase of cadmium alkanoates) in argon atmosphere for one hour. Under these conditions, CdS NPs grow in the matrix. After cooling the mesophase of composite to room temperature, the nanocomposite contained CdS NPs (Fig. 2). The starting concentrations of thiourea for preparation of composites were 2, 4, 6 mol.%. It was shown that increasing the concentration of sulphide ions in the matrix does not affect the size of CdS NPs but increases their concentration in the matrix [4].
Fig. 1. Model of the layered structure (smectic A) of cadmium octanoate CdC8.
On the basis of X-ray measurements, the size of synthesized CdS NPs was estimated ≈2.5 nm. Our results show that NPs have a small dispersion of their sizes, and their shape is spherical. The nanocomposites are stable over a long period of time (years) and ordered in a layered matrix (Fig. 2) [5].
The cadmium octanoate composites with CdS NPs placed in flat cell form smectic A domains with homeotropic orientation. The cells were prepared using nickel electrodes and glass supports. Two geometric configurations of samples were used to investigate anisotropy of electrical properties of matrix and composites (Fig. 3). We placed the electrodes in such a way to direct the current perpendicular (Fig. 3a) and parallel (Fig. 3b) to the ionic layers of the nanocomposite. The spacers in the case 3a adjust the thicknesses of the samples. In the case 3b, the thickness is given by the thickness of the electrodes. The thicknesses of samples are chosen to be 30 μm in the case 3a and 1 mm for the case 3b.
The electric conductivity was measured using the oscilloscopic technique (Fig. 4) [6, 7]. The voltage signal had a triangular shape with the pick amplitude 0.25 V. The signal frequency was changed within the range from 50 up to 106 Hz. It was found that the resistance of the samples had no dispersion at frequencies above 5(103 Hz. Therefore, the electric conductivity was measured at the frequency 104 Hz.
The bulk resistance was directly measured in our experiments. Thus, the geometric parameter, k, must be known for each sample to make the calculation of the possible conductivity,
R
k
=
s
. The geometric parameter,
)
(
z
d
l
k
×
=
, depends on the sample thickness d, the length of the metal electrode
z
, and the distance between electrodes
l
.
To make the temperature measurements available, the samples were placed into a thermostat. The temperature was changed from 20 (C up to 200 (C.
Fig. 2. Model of the nanocomposite with incorporated nanoparticles.
2.4
2.6
2.8
3.0
3.2
3.4
10
-5
10
-4
150
125
100
75
50
25
T,
o
C
s
||
, Ohm
-1
m
-
1
10
3
/T, K
-1
1
2
3
4
Fig. 5. Electric conductivity
||
s
of matrices CdC8 (1), CdC8 + 2% CdS (2), CdC8 + 4% CdS (3) and CdC8 + 6% CdS (4) versus inverse temperature.
(Ni) electrodes
Glass substrate
b)
Sample
Sample
b) Charge transfer along the cation-anion layera) Charge transfer across cation-anion layer
ZZ
а)
3. Experimental results and discussion
The temperature dependence of the conductivity
||
s
along the cation-anion layers (see Fig. 3b) is shown in Fig. 5. Different symbols and lines correspond to the conductivity of the nanocomposites with a different concentration of NPs: matrix CdC8 (1), CdC8 + 2% CdS (2), CdC8 + 4% CdS (3) and CdC8 + 6% CdS (4). Their analysis shows that the temperature dependences
)
(
T
s
, plotted in the Arrhenius coordinates
)
/
1
(
ln
T
s
, can be approximated by straight lines for both investigated phases (glass and ionic liquid crystals). Thus, like for the majority of liquid crystals, the temperature dependence of nanocomposite conductivity can be represented as:
÷
÷
ø
ö
ç
ç
è
æ
-
s
=
s
T
k
E
a
B
0
exp
(1)
where
0
s
is the parameter depending on the phase of liquid crystal,
a
E
– activation energy of conduction,
B
k
– Boltzmann constant.
Similar dependences were measured for
)
/
1
(
T
^
s
, see Fig. 6.
Comparison of the values of
(
)
T
/
1
||
s
in Fig. 5 and
)
/
1
(
T
^
s
in Fig. 6 confirms anisotropy of our materials. The difference by two orders of magnitude arises from the large difference in the mobility of charge carriers along and across the cation-anion layers. Each dependence
(
)
T
/
1
||
s
and
)
/
1
(
T
^
s
can be approximated by two straight lines. Thus, the activation energy,
a
E
, can be established for both phases and for each concentration of the nanoparticles by using formula (1).
Let us describe the glassy phase first. The dependence of the activation energy,
a
E
, on the concentration of NPs is presented as the following plot (Fig. 7).
The values of the activation energy are indicative of the electronic nature of conductivity. As stated above, the mobility of electrons in the cation-anion layers is higher than that perpendicular to them. With increasing the nanoparticle concentration, the activation energy decreases. The increase of the number of free charge carriers or their mobility may cause this decrease.
Second, the conductivity significantly grows at temperatures above 100 (C when the glassy nanocomposite turns to the smectic A phase. The anisotropy of conductivity is still as high as in the glassy phase. But the values of the activation energy get much higher (approximately by two orders of magnitude), which states a different type of conductivity. Therefore, we suggest that Cd2+ cations are main charge carriers in the smectic A phase of the nanocomposite. The dependences of the activation energy in the smectic A phase of nanocomposites on the concentration of CdS NPs are presented in Fig. 8.
2.4
2.6
2.8
3.0
3.2
3.4
10
-7
10
-6
150
125
100
75
50
25
s
^
, Ohm
-1
m
-1
10
3
/T, K
-
1
1
2
3
T,
o
C
Fig. 6. Electric conductivity
^
s
of matrices CdC8 (1), CdC8 + 2% CdS (2), CdC8 + 4% CdS (3) versus inverse temperature.
0
2
4
6
0.00
0.01
0.02
0.03
0.04
E
a
^
E
a
||
E
a
, eV
c
, mol.%
Fig. 7. Conductivity activation energy on the concentration of CdS NPs in glassy nanocomposites across and along the cation-anion layers.
0
2
4
6
0.75
1.00
1.25
1.50
E
a
^
E
a
||
E
a
, eV
c
, mol.%
Fig. 8. Conductivity activation energy on the concentration of CdS NPs for the smectic A phase of nanocomposites across and along the cation-anion layers.
Here we can see that the activation energies behave principally different when charge carriers occur either across or along the cation-anion layers. When the carriers move inside the cation-anion layers,
||
a
E
grows with the concentration of NPs. This non-evident result can be explained by appearance of new traps due to increasing the number of nanoparticles on the way of carriers. Thus, we have to state that the nanoparticles not only increase the number of charge carriers or mobility, but also create “potential wells” for moving ions, thereby increasing the height of energetic barriers on the way of ions. On the other hand, the nanoparticles deform the alkanoate chains and increase the mobility of carriers across the layers.
4. Conclusions
The conductivity of nanocomposites has an activation dependence both in glassy and smectic A phases. The conductivity of the nanocomposite along the cation-anion layers is by two orders of magnitude higher than that across the cation-anion layers, which confirms anisotropy of the nanocomposite regardless of the phase of materials. In the glassy phase, electrons carry the charge. The increase of the nanoparticle concentration brings additional free charge carriers or increases their mobility. For the smectic phase, increasing the nanoparticle concentration brings additional traps for the carriers that travel in plane of the cation-anion layers. On the other hand, the nanoparticles deform the alkanoate chains and increase the mobility of carriers across the layers. In general, we can conclude that the new nanocomposites based on metal alkanoates with synthesized CdS nanoparticles have very interesting electric properties and are promising for applications in electro-optical devices.
References
1.
T.A. Mirnaya, S.V. Volkov, Ionic liquid crystals as universal matrices (solvents): Main criteria for ionic mesogenicity, in: Green Industrial Applications of Ionic Liquids. Eds. R.D. Rogers, K.R. Seddon, S.V. Volkov. Kluwer Academic Publishers, London, 2002, pp.439-456.
2.
Yu. Garbovskiy, A.V. Kovalchuk, A. Grydyakina, S.A. Bugaychuk, T. Mirnaya, G.V. Klimusheva,
Electrical conductivity of lyotropic and thermotropic ionic liquid crystals consisting of metal alkanoates // Liquid Crystals, 34(5), p. 599-603 (2007).
3.
D.R. Lide (Eds), Handbook of Chemistry and Physics. 84th Ed. CRC Press, 2003-2004.
4.
T.A. Mirnaya, V.N. Asaula, S.V. Volkov, A.S. Tolochko, D.A. Melnik, G.V. Klimusheva, Synthesis and optical properties of liquid crystalline nanocomposites of cadmium octanoate with CdS quantum dots // Physics and Chemistry of Solid State, 13(1), p. 131-135 (2012).
5.
G.V. Klimusheva, I. Dmitruk, T. Mirnaya, A. Tolochko, S.A. Bugaychuk, A. Naumenko, D. Melnik, V. Asaula, Monodispersity and ordering of semiconductor quantum dots synthezed in ionic liquid crystalline phase of cadmium alkanoates // Liquid Crystals, 40(7), p. 980-988 (2013).
6.
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).
7.
A.V. Koval’chuk, Low- and infra-low dielectric spectroscopy liquid crystal – solid state interface. Sliding layers // Ukr. J. Phys. 41, No.10, p. 991-998 (1996).
�
Fig. 3. Configuration of the cells used in experiments.
�
Fig. 4. Setup for measuring the electric conductivity.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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GeneratorResistance
Oscilloscope
X
Y
Sample
Thermostat
ILCs
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