Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile
It has been shown experimentally that the ferroelectric nano-particles, in common with other molecular additives, shift the clearing temperature, Tc, extending or reducing the two-phase coexistent region and changing the average order parameter of the single-component nematics. An increase of up to...
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| Цитувати: | Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile / O. Kurochkin, H. Atkuri, O. Buchnev, A. Glushchenko, O. Grabar, R. Karapinar, V. Reshetnyak, J. West, Yu. Reznikov // Condensed Matter Physics. — 2010. — Т. 13, № 3. — С. 33701:1-9. — Бібліогр.: 37 назв. — англ. |
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irk-123456789-321102012-04-09T12:24:04Z Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile Kurochkin, O. Atkuri, H. Buchnev, O. Glushchenko, A. Grabar, O. Karapinar, R. Reshetnyak, V. West, J. Reznikov, Yu. It has been shown experimentally that the ferroelectric nano-particles, in common with other molecular additives, shift the clearing temperature, Tc, extending or reducing the two-phase coexistent region and changing the average order parameter of the single-component nematics. An increase of up to 11°C or a smaller decrease of as much as 3°C of Tc, and corresponding changes of the order parameter were observed. In all cases the order parameter of the colloid followed the universal temperature behaviour, which is a characteristic of LCs with molecular additives. 2010 Article Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile / O. Kurochkin, H. Atkuri, O. Buchnev, A. Glushchenko, O. Grabar, R. Karapinar, V. Reshetnyak, J. West, Yu. Reznikov // Condensed Matter Physics. — 2010. — Т. 13, № 3. — С. 33701:1-9. — Бібліогр.: 37 назв. — англ. 1607-324X PACS: 77.84.Nh, 42.70.Df, 78.15.+e, 47.57.J-, 77.84.Lf, 78.67.Sc http://dspace.nbuv.gov.ua/handle/123456789/32110 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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It has been shown experimentally that the ferroelectric nano-particles, in common with other molecular additives, shift the clearing temperature, Tc, extending or reducing the two-phase coexistent region and changing the average order parameter of the single-component nematics. An increase of up to 11°C or a smaller decrease of as much as 3°C of Tc, and corresponding changes of the order parameter were observed. In all cases the order parameter of the colloid followed the universal temperature behaviour, which is a characteristic of LCs with molecular additives. |
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Kurochkin, O. Atkuri, H. Buchnev, O. Glushchenko, A. Grabar, O. Karapinar, R. Reshetnyak, V. West, J. Reznikov, Yu. |
| spellingShingle |
Kurochkin, O. Atkuri, H. Buchnev, O. Glushchenko, A. Grabar, O. Karapinar, R. Reshetnyak, V. West, J. Reznikov, Yu. Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile Condensed Matter Physics |
| author_facet |
Kurochkin, O. Atkuri, H. Buchnev, O. Glushchenko, A. Grabar, O. Karapinar, R. Reshetnyak, V. West, J. Reznikov, Yu. |
| author_sort |
Kurochkin, O. |
| title |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile |
| title_short |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile |
| title_full |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile |
| title_fullStr |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile |
| title_full_unstemmed |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile |
| title_sort |
nano-colloids of sn2p2s6 in nematic liquid crystal pentyl-cianobiphenile |
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Інститут фізики конденсованих систем НАН України |
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2010 |
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http://dspace.nbuv.gov.ua/handle/123456789/32110 |
| citation_txt |
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile / O. Kurochkin, H. Atkuri, O. Buchnev, A. Glushchenko, O. Grabar, R. Karapinar, V. Reshetnyak, J. West, Yu. Reznikov // Condensed Matter Physics. — 2010. — Т. 13, № 3. — С. 33701:1-9. — Бібліогр.: 37 назв. — англ. |
| series |
Condensed Matter Physics |
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2025-07-03T12:38:36Z |
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2025-07-03T12:38:36Z |
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| fulltext |
Condensed Matter Physics 2010, Vol. 13, No 3, 33701: 1–9
http://www.icmp.lviv.ua/journal
Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal
Pentyl-cianobiphenile
O. Kurochkin1∗, H. Atkuri2, O. Buchnev1,3, A. Glushchenko4, O. Grabar5, R. Karapinar6,
V. Reshetnyak7, J. West2, Yu. Reznikov1
1 Institute of Physics, National Academy of Sciences of Ukraine, 46 Nauky avenue, Kyiv 03028, Ukraine
2 Liquid Crystal Institute, Kent State University, Kent, Ohio, 44242, USA
3 University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom
4 University of Colorado at Colorado Springs, Department of Physics, Colorado Springs, CO 80918, USA
5 Institute of Solid State Physics and Chemistry, Uzhgorod State University, 46 Pidhirna Str., Uzhgorod
294000, Ukraine
6 Faculty of Arts and Sciences Department of Physics, Yuzuncu Yil University, Van, 65080, Turkey
7 National Taras Shevchenko University of Kyiv, Physics Faculty, Volodymyrs’ka Str. 64, Kyiv, 01601, Ukraine
Received December 18, 2009, in final form June 9, 2010
It has been shown experimentally that the ferroelectric nano-particles, in common with other molecular addi-
tives, shift the clearing temperature, Tc, extending or reducing the two-phase coexistent region and changing
the average order parameter of the single-component nematics. An increase of up to 11◦C or a smaller de-
crease of as much as 3◦C of Tc, and corresponding changes of the order parameter were observed. In all
cases the order parameter of the colloid followed the universal temperature behaviour, which is a characteristic
of LCs with molecular additives.
Key words: liquid crystals, ferroelectric nanoparticles
PACS: 77.84.Nh, 42.70.Df, 78.15.+e, 47.57.J-, 77.84.Lf, 78.67.Sc
1. Introduction
Different types of nano-particles (dielectric [1, 2], ferroelectric [3], ferromagnetic [4] and metal [5])
have been dispersed in liquid crystals (LCs) to form unique anisotropic colloids. Nano-particles can
significantly change the basic physical properties of LC matrix by “sharing” their intrinsic proper-
ties with the LC host. Previous research has shown that doping a nematic host with ferromagnetic
nano-particles introduces ferromagnetic properties into the diamagnetic matrix. This doping also
makes the resulting colloid super-sensitive to a magnetic field [4]. Colloids of ferroelectric nano-
particles dispersed in a nematic host introduce ferroelectricity; they become sensitive to the sign
of an electric field [3].
Recently the unique properties of dilute colloids of ferroelectric nano-particles in LCs were
reported [3, 6–9, 37]. These colloids are stable because the particles do not agglomerate at low
concentrations (less than 0.1–1% by weight). The important point is that the single nano-particles
are so small that the anchoring parameter ξ = WR/K � 1 (W is the anchoring energy, K
is the elastic constant, R is the characteristic size of the particles). For this condition the nano-
particles do not substantially disturb the orientation of the LC director producing a macroscopically
uniform alignment. As a result, the ferroelectric colloid appears similar to a pure LC with no
readily apparent evidence of dissolved particles. At the same time the nano-particles may produce
substantial changes in the physical properties of the liquid crystal host – they enhance the order
parameter, birefringence and dielectric anisotropy of nematics [3, 6], improving the alignment order
of a surface-stabilized ferroelectric LCs [9].
∗E-mail: kurochkin@iop.kiev.ua
c© O. Kurochkin, H. Atkuri, O. Buchnev, A. Glushchenko, O. Grabar, R. Karapinar,
V. Reshetnyak, J. West, Yu. Reznikov
33701-1
http://www.icmp.lviv.ua/journal
Kurochkin O. et al.
We have previously considered the change of the average order parameter of the LC, S̄, due to
the strong electric field around the ferroelectric nano-particles to be the primary factor affecting the
macroscopic properties of nematics [3, 6]. In this sense, the behaviour of the nano-particles in a LC
is reminiscent of the behaviour of low-molecular weight molecular dopants in nematics. Chen and
Luchkhrust as far back in 1969 concluded that the temperature dependence of S̄(τ), τ = T/(Tc +
273), for a nematic LC with additives is the universal function of the reduced temperature [12]. It
was also shown that the function S̄(τ) did not depend on the chemical structure of the additives and
coincided with the dependence S̄0(τ) for the pure LC matrix. Later, Pinkevich et al. [13] showed
that this universality follows from the standard Grandjean-Maier-Saupe (GMS) molecular field
theory. The sign of the shift of τ and Tc is determined by the relationship between the amplitude of
inter-molecular interaction of LC molecules, Glc−lc, and the amplitude of interaction “LC molecule
– additive molecule”, Glc−imp. In the case of non-mesogenic additives, Glc−lc > Glc−imp, the local
order parameter around the additive is smaller than in a pure LC, and the shift of the clearing
temperature, ∆Tc < 0. Mesogenic dopants can interact with liquid crystal molecules more strongly
than liquid crystal molecules with themselves Glc−lc < Glc−imp. Therefore, mesogenic additives can
increase the local ordering and increase the clearing temperature, demonstrating that ∆Tc > 0.
The goal of the present work is to follow the analogy between the effects of the ferroelectric
nano-particles and low-molecular weight dopants on the properties of nematic LCs. It was found
experimentally that the nano-particles, by analogy with molecular additives, show the universal
temperature behaviour of the average order parameter. The experimental results are confirmed
within the framework of GMS molecular field formalism in which the colloid is considered as a
LC host with undisturbed order parameter, SLC, with clusters of LC molecules, having order
parameter, Scl, which differs from SLC due to the nano-particles.
2. Materials
To our knowledge, three types of ferroelectric materials have been studied as dopants of nano-
particles in LC matrixes: Sn2P2S6 [3, 7, 8, 14–16], BaTiO3 [6, 7, 17] and ZnO [9]. Since the majority
of the experimental data were collected for the material Sn2P2S6 (tin thiohypodiphosphate) and
the nano-colloids based on this are extremely promising for photorefractive applications [14, 15,
18], this ferroelectric material was chosen for our experiments. At room temperature macroscopic
Sn2P2S6 crystals have a spontaneous polarization of 14µC cm−2 parallel to the [100 ] direction in the
monoclinic unit cell chosen according to [19]. The Curie temperature of Sn2P2S6 in a macroscopic
crystal is TCurie ≈ 64◦C [20, 21]. The value of the dielectric constants of Sn2P2S6 strongly depends
on the quality of the samples [22]. Typically, the low-frequency dielectric constant measured along
the polar axis at room temperature is ε‖ ≈ 300 in a single domain crystal, and is about two times
larger in a polydomain state, due to a domain wall contribution to the dielectric permittivity. On
approaching Tc the dielectric constant follows the Curie-Weiss law, and near TCurie reaches the
peak value of the order of 104–105 [23].
Crystals of Sn2P2S6 were ground together with oleic acid as a surfactant in a weight ratio 4:1 in
a Fritsch 00-502 vibration micro-mill. The particles had been milling for 8–10 hours per day, 130 h
in total. The resulting dispersion was characterized by an NT-MDT atomic force microscope. This
was accomplished by dispersing the ferroelectric particles in methanol in a weight ratio 1:3000. A
drop of the dispersion was deposited on a microscope glass slide and an AFM scan was obtained
after evaporation of the methanol. It was found that the particles had quasi-spherical shape with
the average size, d ≈ 20 nm and the size dispersion δd ≈ 10 nm. At this size the ferroelectric
particles have a single domain structure [24, 25]. Some of the particles were combined in the
agglomerates containing 3–5 particles. The milling did not destroy the ferroelectric phase of the
Sn2P2S6. We observed a peak of the ferroelectric-paraelectric transition at TCurie ≈ 64◦C using
differential scanning calorimetry technique (Perkin-Elmer). This corresponds well with the phase
transition point of the bulk Sn2P2S6 crystals.
In the majority of previous studies the multi-component nematic LC-mixtures were utilized as
a host medium for the colloid preparation. In this case the physical interpretation of the results is
33701-2
Nano-colloids of Sn2P2S6 in NLC 5CB
seriously complicated by the many factors affecting the properties of the multi-component LC nano-
particle dispersions. For instance, different components of the LC-mixture may be preferentially
adsorbed on the nano-particle surface. This will result in the enrichment of one component at the
particle surface and its depletion in the “bulk” of the host, and will clearly affect the measured
macroscopic properties of the dispersion. Also, some molecular components of LC mixtures have
permanent electric dipoles different for each component. This will result in the spatial separation of
the molecular components on a nanometer scale due to the strong electric field gradient produced
near the ferroelectric nano-particles. Another serious problem that appears during preparation of
the colloids in a multi-component mixture is a possible change of molecular composition during
the preparation procedure. As a consequence, the properties of the final matrix may differ from
the initial one [26]. The difference in the properties due to the change of the composition may be
comparable with the changes associated with the presence of the particles and may mask the real
action of the nanoparticles.
To avoid the complications associated with a multi-component host, we used a single-component
nematic, pentyl-cianobephenyle (5CB) from EMI (clearing temperature, Tc = 34.5 ◦C) as the
host medium for the colloid. We can therefore assume that any observed changes are caused by
interactions of the particles with the surrounding host rather than by any change in the composition
and spatial separation of the components of the matrix.
The mixture of the nano-particles with the surfactant was dispersed in heptane at the weight
concentration 7%. After precipitation of the big agglomerates at the bottom of the vessel, the top
2/3 of the mixture volume was decanted, stirred ultrasonically for 30 s and added to 5CB in a
weight ratio 1:2. This mixture was stirred ultrasonically again and put onto a hot-stage at T = 45◦C
for solvent evaporation. The full evaporation of the heptane was confirmed by IR spectroscopy
measurements. The resulting nematic colloid had a volume fraction of particles Cv ≈ 0.3 vol.%.
We prepared several colloids with the described procedure using Sn2P2S6 crystals from the same
batch, as well as from different batches.
The colloids were studied in cells comprised of two ITO coated glass substrates. The substrates
were covered with rubbed polyamide polymer layers to produce uniform homogenous alignment.
The substrates were separated with rod-like polymer spacers to produce a cell gap in the range
of d ≈ 20 µm. The cells were capillary filled at elevated temperature, T > Tc. Observations of all
samples in a polarizing microscope showed the same uniform planar structures in the cells filled
with the pure nematic and with the colloid.
3. Results
The ferroelectric nano-particles act as molecular dopants if (a) they do not disturb the director;
and (b) the particles change the clearing temperature, extend the two-phase region of “nematic-
isotropic phase” transition and change the order parameter in a way that the dependence S̄col(τ)
is a universal function of the reduced temperature τ that coincides with the dependence S̄LC(τ)
for pure LC [12, 27, 28, 37].
The validity of the conditions listed above was checked by the following experiments.
(a). The detailed observation of the samples in a polarization microscope did not show any
additional distortions of the director in the colloid compared with the pure LC. This corresponds
to our measurements of the size of the particles, d ≈ 20 nm at which the anchoring parameter
ξ � 1 and no director orientational defects should be formed. The colloids in the cells were stable
for at least several months and we did not find, with time, any visible aggregations of the particles
or formation of defects by optical microscope observations. Therefore, condition (a) is valid.
We suggest that besides the surfactant coating, the high stability of the colloid is caused by the
orientational elasticity of mesophase. If the particles agglomerate, their effective size increases and
at some point requires the creation of a high energy defect. This will prevent agglomeration and
maintain the dispersion as single particles, or agglomerations with dimensions below the threshold
requiring the formation of a defect.
(b). In order to check the condition (b), the clearing temperature, Tc, of the pure 5CB and
33701-3
Kurochkin O. et al.
Table 1. Samples of different ferroelectric colloids on the basis of 5CB.
Sample
Number
Shift ∆Tc,
◦C Width of
two-phase
region, ∆TNI,
◦C
Comment
1 +3.0 0.5 Sn2P2S6 from batch #1
2 +11.1 1.6 Sn2P2S6 from batch #2
3 +3.3 0.9 Sn2P2S6 from batch #3
4 +1.5 n/a Sn2P2S6 from batch #3.
Sample prepared in a week after the
sample #3
5 –0.4 0.2 Sn2P2S6 from batch #3.
Sample prepared in two weeks after the
sample #3
6 +3.2 n/a Sn2P2S6 from batch #4
7 –1.9 n/a Sn2P2S6 from batch #5
8 –2.8 0.3 Sn2P2S6 from batch #6
colloids was measured by observing the LC textures in a polarizing microscope at slow temperature
change (∼ 0.1◦C min−1) in a hot-stage. Most previous experiments showed that the addition of the
Sn2P2S6 nano-particles leads to an increase in the clearing temperature of the multi-component
LC [3, 7, 13–15], but a decrease of Tc in some experiments was also observed [8, 9]. In order to
identify whether the sign of Tc, and accordingly, S̄col is caused by changes in the composition of
the multi-component mixtures [3, 7–9, 13–15], or exists also in a single component LC, we have
measured the values of Tc in seven samples of Sn2P2S6 particles dispersed in 5CB and compared
them with Tc of the pure 5CB. Despite identical fabrication procedure, in some cells we observed
an increase of the average order parameter and clearing temperature of the LC while in others we
observed a decrease (table 1). The difference in the results obtained for the different batches of
the Sn2P2S6 may be explained by uncontrolled stehiometry of the initial crystals which can arise
during their growth. Concerning the samples 3, 4 and 5, which were produced from the same batch
of Sn2P2S6, the only difference is the time between the preparation of the milled nano-particles and
fabrication of the sample (dispersion of the particles in heptanes, adding of 5CB and evaporation
of heptanes).
Expansion of the two phase region, ∆TNI, (the co-existence of the nematic and isotropic phases
region), in comparison with the pure LC (∆TNI 6 0.1◦C) was also observed (table 1). The increase
of the width of the two-phase region, ∆TNI corresponds to the increase of the clearing point, ∆Tc
This corresponds to the effect [29] on the phase transition in a LC with non-mesogenic molecular
additives.
The measured values ∆TNI and Tc remained constant for at least 10 months. Moreover, a
repeated heating of the cells well into the isotropic phase, in the presence of a high electric field
did not affect the values of ∆TNI and Tc. For example, sample #3 was heated above Tc seven times
in the presence of an electric field of 3 V µm−1 without any change in the value of ∆TNI and Tc.
The average order parameter of the colloid, S̄col, and the pure LC, S̄5CB, as a function of
temperature was measured using the guest-dye technique [30]. Methyl Red (MR) was used as the
guest dye. In the guest-dye technique the order parameter is determined from the optical density of
the probe dye molecules absorption measured for light polarization parallel (A‖) and perpendicular
33701-4
Nano-colloids of Sn2P2S6 in NLC 5CB
(A⊥) to the LC director orientation:
S̄ =
A‖ − A⊥
A‖ + 2A⊥
. (1)
Strictly speaking, this method does not yield the value of the average order parameter of a LC,
S̄5CB, but the average order parameter of dye molecules orientation, S̄dye. However, the comparison
of the S̄dye values, obtained in our experiments with the S̄5CB values obtained by NMR technique
in [31] shows the difference of only 3%. Therefore, it is reasonable to consider that S̄5CB ≈ S̄dye. The
dye is even more reliable at monitoring changes in the order parameter as compared to measuring
the absolute value of the order parameter.
(a) (b)
Figure 1. Dependences of the order parameters of the pure 5CB and the colloids (samples #1;7)
on the temperature (a) and the reduced temperature (b).
In order to obtain the temperature dependencies of the order parameter of pure 5CB and
the colloid, we chose the samples #1;7 that showed similar absolute values but opposite signs of
∆Tc (table 1). The results of the measurements S̄5CB(T ) and S̄col(T ) are presented in figure 1a.
In sample #1 (∆Tc = 3.0◦C) an increase of the average order parameter was observed, however
a decrease of order parameter was observed in sample #7 (∆Tc = −1.9◦C) compared to pure
5CB. The important point is that despite the value and even the sign of the changes of S and
Tc varying from sample to sample, in all cases the curves in the temperature dependences of
S̄5CB(τ) and S̄col(τ) plotted against the reduced temperature τ = (T + 273)/(Tc + 273) perfectly
coincided (figure 1a) as is seen for LC doped with molecular additives.
Thus, the doping of 5CB with Sn2P2S6 particles changes the clearing temperature and the order
parameter in a way that the temperature dependence S̄col(τ) coincides with the dependence S̄LC(τ)
for pure LC, and the presence of the particles expands the two-phase region of “nematic-isotropic
phase” transition, i. e. the condition (b) is valid.
The results described above show that the particles change the average order parameter of the
LC matrix. Since the order parameter strongly affects the key parameters of LCs, such as dielectric
anisotropy, birefringence, Frank constants, etc., it makes possible the control of LC properties by a
non-chemical method. From this point of view it is very important to know if the properties of the
particles, such as the optical polarizabilities and the low frequency dielectric constants, effect the
values na and εa of the colloid. To do this, we measured the temperature dependences of εa(T ) and
na(T ) in the pure 5CB and in the colloid. The temperature dependences na(T ) were determined
using a retardation technique that allows the measurement of the phase shift between e- and o-
waves ϕ = πdna/λ in planar cells (d = 50 µm). An impedance analyzer was used to measure the
dielectric constants of the colloid and the pure LC in the cells with homeotropic and planar LC
orientation. The cells were 20 µm thick and contained guard electrodes. The dielectric constants
parallel to the director,ε‖, and perpendicular to the director, ε⊥, were determined measuring the
capacitance of the cells with homeotropic and planar LC orientation respectively.
33701-5
Kurochkin O. et al.
The birefringence and dielectric constants of the sample #2 was measured. Changes in the
birefringence in the colloid explained by the change of the order parameter of the matrix, and
the direct contribution of the material particles to the value of ncol
a is negligible. This is not
surprising taking into account that the birefringence of the Sn2P2S6 single crystals, npart
a ≈ 0.15
is close to the birefringence of the 5CB (nNLC
a ≈ 0.17) and the concentration of particles is small
(Cv ≈ 0.3 vol.%). Alternatively, the spontaneous polarization of the Sn2P2S6 single crystals is very
high (P ≈ 14 µC cm−2) and the dielectric constant of Sn2P2S6 , ε ∼ 102 − 103 is much higher
than the dielectric constant of 5CB. Therefore, one may expect that the dielectric constant of
the particles can notably contribute to the effective dielectric constant of the colloid [32]. Indeed,
a strong enhancement of the dielectric anisotropy that increases near the phase transition was
observed. At the same time we found a small discrepancy between the dependence of εLC(τ)
and εcol(τ), but it was difficult to conclude whether it was an objective result or a systematic
experimental error (the measurements of ε‖ and a decrease of ε⊥ were carried out in different
cells). Therefore, we assume that the nano-particles have a small, if any, contribution to the effective
dielectric constants of the colloid.
We also compared the Freedericksz transition voltage in a symmetric planar cell filled with
the pure 5CB to one filled with the colloid (sample #2). The transition voltage for pure a LC is
determined by the formula [30]
U = π
√
4πK11
εa
(2)
and for the colloid of ferroelectric nano-particles the same formula applies, except that εa is now
the anisotropy of the effective dielectric constant, εa,eff , that depends on the concentration and
polarization of the ferroelectric particles [33].
The dielectric anisotropy of a LC is proportional to the order parameter, εa ∼ S, while the
elastic constant is proportional to the square of the of the order parameter, K11 ∼ S2 [34]. Assuming
the same dependencies on S in the colloid and applying the experimental data for εa,LC/εa,part =
SLC/Spart = 0.86 and K11,LC/K11,part = S2
LC/S2
part = 0.79 at room temperature, a very weak
increase in Freedericksz transition voltage for the colloid, Ucol ≈ 1.04ULC, is expected. Experimental
results confirmed this estimation; we found that ULC = 1.84± 0.02 V and Ucol = 1.87± 0.02 V.
4. Discussion
All the experimental data described unambiguously indicate that the ferroelectric nano-particles
in single-component LC behave as molecular dopants, and the change of the order parameter of
the LC is the dominant mechanism that determines the difference between the properties of the
colloid and the pure single component LC.
Considering the nature of the order parameter changes in the colloid, there are two basic
mechanisms by which the ferroelectric nano-particles can change the order parameter. The first
mechanism is a change of the order parameter near a boundary surface due to a difference between
the “LC-anchoring surface” interaction and a molecular interaction in the bulk LC [35]. As an
example, one can refer to the recent paper of Cordoyiannis et al., which observed a decrease of up to
4.9◦C in the I-N transition when the liquid crystal octylcyanobiphenyl was doped by ferromagnetic
nano-particles with an organic coating [36]. In our opinion, this mechanism cannot be responsible
for the observed increase of the average order parameter and the clearing temperature. Indeed,
interaction of the ferroelectric nano-particles coating (oleic acid) with 5CB molecules is smaller
than the intermolecular interaction in 5CB (doping of 5CB with oleic acid decreases Tc). Therefore,
this coating can only decrease the order parameter near the particles. The second basic mechanism
deals with the electric field produced by the ferroelectric nano-particles. This mechanism can result
in an increase of Tc and S due to enhancement of the orientation coupling in a LC matrix, either
due to dipole-dipole interaction between the nanoparticles and surrounding molecules, or due to
an increase of the intermolecular interaction in a dipole field from the nano-particles [6]. In the last
case the increase of the clearing point, ∆Tc is related to a permanent polarization of the particles
33701-6
Nano-colloids of Sn2P2S6 in NLC 5CB
by the formula [6]:
P =
√
27ε0
zNpartγ2
a
(
L
R
)3
4.54kB∆Tc , (3)
where z is a number of nearest neighbor molecules, R is the radius of the particles, L is the
intermolecular distance, Npart is the concentration of the particles, γa is the anisotropy of the
polarizability of LC molecules. For our experimental conditions z ≈ 6, R ≈ 35 nm, L ≈ 0.5 nm,
Npart ≈ 2.8 · 1019 m−3, βa ≈ 1 and P ≈ 0.04C m−2, the largest observed value of ∆Tc was 11◦C 1.
The calculated value of P is 3.5 times less than the reported value P = 0.14 C m−2 for macroscopic
Sn2P2S6 crystals. The reduced value P as compared with the polarization of macroscopic crystals is
reasonable because the permanent intrinsic polarization of the particles may be partially screened
by charged ions that are always present in a LC. Thus, the mechanism dealing with the electric field
produced by the ferroelectric particles explains the observed increase of S and Tc. The question
of why in some experiments S and Tc decreases, requires additional studies. For the time being
we suggest that the permanent polarization of ferroelectric particles, which is definitely preserved
after the milling of the Sn2P2S6 crystals (see the part “Materials” of the paper) is screened by
external charges in the experiments where the decrease of S and Tc was observed. This suggestion
is supported by the fact that the initial shift ∆Tc ≈ 3◦C decreased to ∆Tc ≈ 1.5◦C for the cell
assembled one week after the milling of the particles and changed the sign, ∆Tc ≈ −0.4◦C for
the cell assembled after two weeks (table 1). We speculate that in the case of the negative Tc
shift the main factor affecting the values of S and Tc is either the oleic acid coating or free oleic
acid molecules that are always present in the LC bulk [10]. Our preliminary measurements of IR
spectra of the colloids confirmed the presence of free oleic acid molecules in the bulk LC and the
oleic acid molecules bounded to the surfaces of the particles. We also found that 0.5 weight percent
of oleic acid dissolved in 5CB, decreases the clearing temperature by ∆Tc ≈ −1.6◦C. This value is
comparable with the characteristic value of the negative shift of Tc in the colloid. It suggests that
this factor alone may explain the negative component of the clearing temperature shift.
5. Summary
We have experimentally found that ferroelectric nano-particles, in common with other molec-
ular additives, shift the clearing temperature, Tc, extending the two-phase coexistent region and
changing the average order parameter of the single-component nematics. An increase of up to 11◦C
or a smaller decrease of as much as 3◦C of Tc, and corresponding changes of the order parameter
were observed. For both of these cases the order parameter of the colloid showed the universal
temperature behavior, which is a characteristic of the LCs with molecular additives. We suggest
that the increase of the order parameter and clearing temperature are caused by a strong electric
field produced by ferroelectric particles that can be screened by outside charges in the case of the
decrease of Tc. The measurements of the dielectric anisotropy and the birefringence showed that
their enhancement in the colloid is determined by the increase of the order parameter of the LC
matrix, and the direct contribution of the particles to the values of the dielectric anisotropy and
the birefringence is small.
Acknowledgement
The authors are very grateful to Christophe Blank, Lena Lopatina, Maurizo Nobili, Tim Sluckin
and Jonathan Selinger for useful discussions and advice. The research described in this paper was
partially supported by the grants “Composite liquid crystal and polymer materials for informa-
tion technologies” and “Self-organization and structuring, physical properties of anisotropic system
based on liquid crystals matrix” of the National Academy of Sciences of Ukraine, by the NATO
Collaborative Linkage Grant “Colloids of ferroelectric nano-particles in nematic and cholesteric
1The estimations according to model of ferroelectric nanoparticles in nematic LCs described in the recent paper
of Lopatina and Selinger [11] give a close value of P .
33701-7
Kurochkin O. et al.
liquid crystals”, a grant ECO-NET 2008 no. 18843TM, the Royal Society joint project “Nanoengi-
neering of liquid crystals for strong photorefraction response”, a joint grant of the Scientific, and
Technical Research Council of Turkey and the National Academy of Sciences of Ukraine “Investiga-
tion of Electro-optical Properties of Nano-particles in Liquid Crystals”, and Research Corporation
Cottrell College Science Award no. CC6945.
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33701-8
Nano-colloids of Sn2P2S6 in NLC 5CB
Наноколоїди Sn2P2S6 в рiдкому кристалi
пентил-цiанобiфенил
О. Курочкiн1, Х. Аткурi2, О. Бучнєв1,3, А. Глущенко4, О. Грабар5, Р. Карапiнар6,
В. Решетняк7, Д. Вест2, Ю. Рєзнiков1
1 Iнститут фiзики, Нацiональна Академiя наук України, просп. Науки 46, Київ, 03028, Україна
2 Iнститут Рiдких Кристалiв, Державний Унiверситет Кенту, Кент, Огайо, 44242, США
3 Унiверситет Саутхемптона, Хемпшир, Саутхемптон, SO17 1BJ, Велика Британiя
4 Унiверситет Колорадо в Колорадо Спрiнгс, Департамент фiзики, Колорадо Спрiнгс, CO 80918, США
5 Iнститут фiзики та хiмiї твердого стану, Ужгородський державний Унiверситет, Пiдгiрна 46, Ужгород,
294000, Україна
6 Факультет науки та мистецтв, Департамент фiзики, Юзунсу Йiл Унiверситет, Ван, 65080, Туреччина
7 Київський Нацiональний Унiверситет iм. Тараса Шевченка, Фiзичний факультет,
вул. Володимирська 64, Київ, 01601, Україна
В роботi експериментально показано, що сегнетоелектричнi наночастинки, як i iншi молекулярнi
домiшки, змiщують температуру просвiтлення, Tc, при цьому збiльшуючи або зменшуючи область
iснування двофазного стану, та змiнюючи середнiй параметр порядку однокомпонентних немати-
чних рiдких кристалiв (РК). В деяких сегнетоелектричних колоїдах спостерiгалося пiдвищення Tc
на величину до 11 ◦C, в iнших – невеликi зниження на величину до 3 ◦C, а також вiдповiднi змi-
ни параметра порядку. У всiх випадках температурнi кривi параметру порядку колоїду збiгаються з
унiверсальними кривими для РК з молекулярними домiшками.
Ключовi слова: рiдкi кристали, сегнетоелектричнi наночастинки
33701-9
Introduction
Materials
Results
Discussion
Summary
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