Thin-film heterostructures comprising ferroelectric (FE) copolymer layers of vinylidene fluoride with
trifluoroethylene (PVDF/TrFE) and linear dielectric (LD) layers of azodye prepared by Langmuir-Blodgett
technique are obtained and studied. A simple physical model of a local electric field amplification in layers
of FE and LD are presented. Experimental studies of the ferroelectric switching confirm that, for heterostructures,
the external effective coercitive voltage decreases and the switched polarization increases.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3514

Alpatova, A.V.

Lazarev, V.V.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 1, 01NTF15(4pp) (2013)  2304-1862/2013/2(1)01NTF15(4) 01NTF15-1  2013 Sumy State University Ferroelectric Behavior of Nano-Layered Heterostructures Composed of Ferroelectric -Linear Dielectric Organic Layers  A.V. Alpatova*, V.V. Lazarev†, S. P. Palto, S.G. Yudin  Shubnikov Institute of Crystallography, Russ. Acad. Sci, Leninsky prosp. 59, 119333, Moscow, Russia  (Received 20 May 2013; published online 31 August 2013)  Thin-film heterostructures comprising ferroelectric (FE) copolymer layers of vinylidene fluoride with trifluoroethylene (PVDF/TrFE) and linear dielectric (LD) layers of azodye prepared by Langmuir-Blodgett technique are obtained and studied. A simple physical model of a local electric field amplification in layers of FE and LD are presented. Experimental studies of the ferroelectric switching confirm that, for hetero-structures, the external effective coercitive voltage decreases and  the  switched polarization increases.  Keywords: Nanostructured Langmuir-Blodgett films, Ferroelectric switching.   PACS numbers: 73.50.Pz, 77.80.Fm, 77.55.tf                                                           * annalpatova@bk.ru   † lcl@ns.crys.ras.ru  1. INTRODUCTION  In the last years we see a reviving interest in study-ing ferroelectric materials, especially ultrathin ferroe-lectric films. There is also an evident trend to hybridi-zation of ferroelectric and semiconductor materials in new electronic devices [1, 2]. Most of such devices are based on traditional crystalline ferroelectrics but some-times one has serious difficulties with crystalline mate-rials in preparing ultrathin films compatible with mod-ern electronics. In many applications polymer ferroe-lectrics are more desirable [3, 4]. Recently, a possibility for studying ferroelectricity in ultra-thin (even below 10 nm) P(VDF-TrFE) copolymer films prepared by Langmuir-Blodgett (LB) technique has been demon-strated [5, 6]. Langmuir-Blodgett technique allows one to design nano-structured films of different materials. In such complex sandwiches (heterostructures), the ferroelectric layers can alternate with other functional-ized layers of magnetic materials, dyes, luminophores, biological molecules, etc., which would suffer a very strong built-in electric field of the ferroelectric layers as high as 108-109 V/m. In addition, the polymer structure allows incorporation of almost any solid nanoparticles. Hence, one may look forward to new discoveries and technical applications in such areas as organic transis-tors [2], conductivity switchers [7], multiferroic hetero-structures [8], and high all-organic capacity data stor-age devices prepared by modern nanoimprinting tech-niques [9]. Here we report on preparing and studying all-organic heterostructures prepared by Langmuir-Blodgett (LB) technique and consisting of two subsys-tems of molecular layers. The first subsystem is pre-sented by molecules of ferroelectic copolymer polyvinyl-idene fluoride - trifluoroethylene (PVDF/TrFE). For the second subsystem molecules of an azodye MEL-26 were chosen, the films of which were well investigated early [10]. Two problems were considered and studied in this work. Firstly, the problem of retaining ferroelectricity in such a heterostructure is arising, since for the na-nometer-thick films the presence of nonferroelectric azodye molecular layers might, in principle, influence the ferroelectric phase state of the copolymer. Secondly, from the simple physical model presented here, it fol-lows that, if the ferroelectric phase of the copolymer is retained, the switching of spontaneous polarization should have an effect on the instantaneous electric field in both the subsystem of azodye molecules and the sub-system of ferroelectric copolymer. Thus, the question of dynamics of ferroelectric switching of such a hetero-structure is arising. So, these two questions are ad-dressed to this work.  2. HETEROSTRUCTURE  FERROELECTRIC -LINEAR DIELECTRIC   2.1 Samples preparing  We used copolymer vinylidene fluoride with trifluo-roethylene P(VDF/TrFE) in composition 70:30. Such a composition guarantees the existence of the ferroelec-tric phase under heating up to the temperature of about ~120oC. Langmuir-Blodgett films were obtained by the following way. Weak-concentration (0.01% by weight) solution of copolymer in cyclohexanone and azodye (p-octadecyloxi-p’-sulphopiperidid azobenzene, MEL-26) in chloroform were prepared. Then, using home-done set-up [11] two subsystems of molecular layers were deposited onto a glass substrate by a hori-zontal lift technique [12] (at first copolymer, then azo-dye layers). Al electrodes (both the bottom and the top at the final step) were deposited by vacuum evapora-tion technique. Monolayers were transferred from the air-water interface onto the solid substrates at room temperature and a surface pressure of 3 mN/m (for copolymer) and 15 mN/m (for azodye layers). A special design of the device was used as schematically shown in Fig. 2.1. The design consists of three capacitor ele-ments on a common substrate prepared in one techno-logical procedure with a common copolymer layer for all three capacitors. One of these three capacitors com-prises only common ferroelectric layer consisting of sixty transferred copolymer monolayers (60 TL). The  A.V. ALPATOVA , V.V. LAZAREV , S. P. PALTO, S.G. YUDIN PROC. NAP 2, 01NTF15 (2013)   01NTF15-2 two other capacitors comprise additional azodye trans-ferred layers (3 TL and 7 TL respectively for the 2d and 3d capacitor elements).   z1 z2 Al, 1 Al, 2 S u b s t r a t e 1 2 Al LD FE   Fig. 1 – The scheme of a design with three capacitors fabri-cated on a common glass substrate. The right part is a hetero-structure capacitor comprising the region 1 (molecular layers of copolymer, PVDF/TrFE - ferroelectric, FE) and the region 2 (3 or 7 molecular layers of MEL-26 azodye - linear dielectric, LD) confined between common bottom and top Al-electrodes. The left part is a capacitor comprising only individual layers of the copolymer confined between common bottom and inter-mediate top Al-electrodes.  2.2 Model of a local electric field amplification in a ferroelectric heterostructure  Considering the heterostructure (Fig. 1) to be ho-mogeneous in the substrate plane (x, y-directions) and supposing electric conductance of materials to be equal to 0, one can write the following relationships between the electric field strength E  0, 0, Ez) in different parts of the heterostructure and the potential difference UU12   1 -  2 between the electrodes:   1 1 2 2, ,z zE z E z U    , (2.1)  11 1 0 2 2, , , ,z z s z zE P E    , (2.2)  where indices “1” and “2” refer to the space regions oc-cupied by the ferroelectric copolymer (the thickness z1) and the dye (the thickness z2), respectively; Ps is the spontaneous polarization normal to the substrate surface, εo  8.85x10-12 F/m, εz,1,2 are dimensionless (relative) components of dielectric permittivity tensor along normal direction (z-direction). Equation (2.2) con-cerns one of the Maxwell’s Equations (div D  0), re-quiring a continuity of the normal component of the electric displacement vector D, and in the absence of free space charge results in Dn,1  Dn,2. Real LB films have nonzero conductance and Eqns (2.1) and (2.2) would satisfactorily hold just in alternating electric fields of frequency much larger than reverse Maxwell relaxation time (2f >> (z,1,2 o z,1,2)-1, where z,1,2 is the specific resistance of the materials along the nor-mal) under the condition of spontaneous polarization switching. Thus, we proceed from an assumption that the external voltage U is always higher than some min-imal coercive voltage Uc that ensures switching the ferroelectric subsystem. It is the fast polarization switching that excludes polarization screening by free charges. From Eqs. (2.1) and (2.2) one can easily find the electric field in ferroelectric and dye regions:     112 1 1 0 2 1 1 2, , , ,( ) ( ) ( )z z s z zE t U t z P t z z           (2.3)      12,11,21022,,1 )()()(   zzszz zztPztUtE   (2.4)  In Eqs. (2.3) and (2.4) the alternating character of the electric field is expressed by introducing the time-dependent values (E, U, Ps). As seen from (2.3) the switched polarization can increase the instantaneous field strength in the region of the dye subsystem and such an increase takes place in the dye region after switching the polarization, when polarization is di-rected along the external electric field applied. The same field strengthening in the dye part of the hetero-structure was earlier demonstrated by electrooptical measurements in [13, 14]. Of importance, from our point of view, is the rela-tionship (2.4). It determines electric field acting in fer-roelectric subsystem. Let us consider the situation when the negative external voltage is switched up to the positive one. In this case, up to the next-in-turn switching, the spontaneous polarization is directed op-posite to the external electric field (if U is a positive value, then Ps is the negative one). However, since the term with Ps in the numerator of Eq. (2.4) is negative, the both terms in the numerator turn out to be of the same sign, and the effect of field amplification is possi-ble up to the very instant of switching. The latter should promote an acceleration of the switching process and reducing the external coercive voltage across the heterostructure as a whole. Therefore, the effect of field strengthening in the dye subsystem is possible after switching of the spon-taneous polarization, whereas the field in the ferroelec-tric subsystem can be amplified before the instant of polarization switching. The effect of field strengthening in the ferroelectric layer can be expressed by the ratio of the field (Eq.2.4) to the corresponding electric field in the absence of the dye layer (z2  0). From Eq.2.4 follows:  111 1 0 21 2 2 2 11,, , ,z sz z zE z P zF zU U z                  (2.5)  The strengthening effect is realized if the  factor F1 >1, i.e., when   1 0 1 1, ,z csUz zP      (2.6)  In Fig. 2 there are given dependencies of the electric field amplification factor, F1, versus the thickness of azodye layer, z2, for some fixed thicknesses of ferroe-lectric layer, z1 (from 2 nm to 120 nm), at an external voltage U  20 V and for the material parameters close to the experimental ones: Ps  0.04 C/m2; ε1  7; ε2  2.5.  FERROELECTRIC BEHAVIOR OF NANO-LAYERED HETEROSTRUCTURES… PROC. NAP 2, 01NTF15 (2013)   01NTF15-3   0 20 40 60 80 1000123z1  =  20 nmz1  = 5 nmz1  =  60 nmz1  =  80 nmz1  = 100 nmF1 z2  (nm)z1  = 120 nmz1  =  30 nm  Fig. 2 – Dependence of the electric field amplification factor, F1, versus the thickness of azodye layer, z2, at different fixed thicknesses of ferroelectric layer, z1, (U  20 V; Ps  0.04 C/m2; ε1  7; ε2  2.5).  Thus, the presence of the additional dielectric lay-ers (dye subsystem) can strengthen the local electric field in the region of ferroelectric if thickness of the ferroelectric layers is sufficiently large. The latter can positively affect the dynamics of ferroelectric switching and the value of switched part of polarization.  3. EXPERIMENTAL RESULTS  Figure 3 shows the temperature behavior of the ca-pacitance of the heterostructure and the individual copolymer film during the thermal cycling. The both structures reveal pronounced temperature hysteresis characteristic of first-order ferroelectric phase transi-tion.   20 40 60 80 100 120123Copolymer-FilmHeterostructure-FilmCapacity,  nFTemperature,   oC   Fig. 3 – Temperature dependencies of the capacity of hetero-structure (18 layers of copolymer and 3 layers of azodye) and the capacity of individual copolymer film (18 layers of copoly-mer) taken at and sine-form voltage of 0.35 V frequency of 68 Hz (overlapped active area S   ~1x1 mm2 for both capacitors). Arrows along the curves indicate the direction of the tempera-ture run.   Merz switching technique [15] was used for charac-terizing repolarization properties of capacitors. Ampli-fied triangle waveform signal from functional generator was applied to the capacitor tested and in series con-nected load resistor. Oscillograms taken from the load resistor were registered and saved by a memory oscillo-scope joint with a personal computer. The totally switched polarization was obtained after integrating the area under current curves i(t) in Fig.  4 and sub-tracting the linear contributions of the resistive and capacitive components of the corresponding current response. Figure 4 shows some switching curves of ferroelec-tric repolarization for 3 capacitor elements obtained for different amplitudes (from 10 V to 30 V) of the triangle waveform voltage applied. Beginning with amplitudes of about ~12 V there is a distinctive repolarization bump that, with increasing voltage amplitude, is al-ways larger in value for the heterostructure capacitor compared to the copolymer homo-film capacitor. Here we once more give emphasis to the fact that each of the three capacitors (heterostructures comprised and indi-vidual copolymer layer comprised) are placed on the same common substrate and comprised common ferroe-lectric layer. That minimizes inaccuracy in repolariza-tion measurements associated with a  possible nonuni-formity in thickness of the ferroelectric layer, the com-pared capacitors might have if prepared as separate units on three different substrates.  10 1502040608002040608010 150204060800.00.20.40.60.810 15008004080 6543i ,   mkAt ,   msa12FE (60 layers) FE + LD(3 layers)654321bt ,  msUtr  FE+LD(7 layers)654321ct ,  ms  Utr  Utr Fig. 4 – Oscillograms i(t) of repolarization current response for individual copolymer PVDF/TrFE film (60 layers) (a), and two heterostructures (60 layers of copolymer + 3 layers of azodye) (b) and (60 layers of copolymer + 7 layers of azodye) (c). Oscillograms are obtained by Merz technique under apply-ing cycling voltage of the triangle form (f  80 Hz )  of different amplitudes:10 V (1), 12.5 V (2), 15 V (3), 20 V (4), 25 V (5), 30 V (6)- the same  for (a, b, c) curve series. Only parts of the curves for the positive half-period of applied triangle form voltage Utr are given.   The measurements given in Fig. 4 were carried out at the same amplitudes of triangle voltage for all three series. As seen from the figures the switched polariza-tion is occurred larger for the heterostructure (Psw  5.7 mkC/cm2 for Utr  30 V,curve 6, Fig. 4c) than that for the copolymer capacitor(Psw 5.0 mkC/cm2 for the same Utr  30 V, curve 6, Fig. 4a ). This is a clear indication of the presence of the local field amplifying effect in the case of the heterostructure. Moreover, the maximum of the current response in the heterostruc-ture corresponds to a lower instant external voltage (~11.7 V) as compared to the instant voltage (~13.7 V) for the current maximum of the individual copolymer layer (compare curves 6 in Figs. 4a and 4c). The latter means that the effective external coercive voltage Uc is lower for the heterostructure.  4. CONCLUSIONS  We have demonstrated that polarization switch-ing in a heterostructure can be more effective than  A.V. ALPATOVA , V.V. LAZAREV , S. P. PALTO, S.G. YUDIN PROC. NAP 2, 01NTF15 (2013)   01NTF15-4 in an individual ferroelectric film. This circumstance is due to the effect of amplification of local electric field acting in the ferroelectric part of the hetero-structure.   AKNOWLEDGEMENTS  The work was supported by Russian Foundation for Basic Research (project no. 12-02-00214a). The authors are grateful to Dr. L.M. Blinov (Institute of Crystallog-raphy, RAS) for fruitful discussions.  REFERENCES  1. R.C.G. Naber, C. Tanase, P.W.M. Blom, G.N. Gelink, A.W. Marsman, F.J. Touwslager, S. Setayesh and D.M. De Leeuw, Nature Materials 4, 243 (2005). 2.  I. Stolichnov, S.W.E. Riester, H.J. Trodahl, N. Setter, A.W. Rushforth, K.W. Edmonds, R.P. Campion, C.T. Fox-on, B.L. Gallagher and T. Jungwirth, Nature Mater. 7, 464 (2008). 3. R. G. Kepler, in Ferroelectric Polymers (chemistry, physics and applications), (ed. N.S. Nalwa, Marcel Dekker) (New Yourk: 1995). 4. T. Furukawa, Phase Transitions 18, 143 (1989). 5. A.V. Bune, V.M. Fridkin, S. Ducharme, L.M. Blinov, S.P. Palto, A.V. Sorokin, S.G. Yudin and A. Zlatkin, Nature 391, 874 (1998). 6. V.V. Lazarev, L.M. Blinov, S.P. Palto, S.G. Yudin, Phys. Rev. B 82, 134122 (1-6) (2010). 7. K. Asadi, D. M. De Leeuw, B. De Boer, and P. W. M. Blom, Nature Mater. 7, 547 (2008). 8. R. Ramesh and N. A. Spaldin, Nature Mater. 6, 21 (2007). 9. Z. Hu, M. Tian, B. Nysten, and A. M. Jonas, Nature Mater. 8, 62 (2009). 10. L.M. Blinov, S.P. Palto, S.G. Yudin, Opt. Spectr.60, 756 (1986). 11. S.G.Yudin, S.P.Palto, V.A. Khavrichev, S.V.Mironenko and M.I.Barnik, Thin Solid Films 210/211, 46 (1992). 12. I. Langmuir, V.J. Schaeffer, J. Am. Chem. Soc. 60, 557 (1938). 13. S.G. Yudin, L.M. Blinov, V.V. Lazarev, S.P. Palto, Physics of the Solid State 54, 940 (2012).  14. L.M. Blinov, V.V. Lazarev, S.G. Yudin, and S.P. Palto, JETP Letters 95, 160 (2012). 15. W.J. Merz. J. Appl. Phys. 27, 938 (1956).  

Ferroelectric Behavior of Nano-Layered Heterostructures Composed of Ferroelectric - Linear Dielectric Organic Layers

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 1, 01NTF15(4pp) (2013) 
 
2304-1862/2013/2(1)01NTF15(4) 01NTF15-1  2013 Sumy State University 
Ferroelectric Behavior of Nano-Layered Heterostructures Composed of Ferroelectric -
Linear Dielectric Organic Layers 
 
A.V. Alpatova*, V.V. Lazarev†, S. P. Palto, S.G. Yudin 
 
Shubnikov Institute of Crystallography, Russ. Acad. Sci, Leninsky prosp. 59, 119333, Moscow, Russia 
 
(Received 20 May 2013; published online 31 August 2013) 
 
Thin-film heterostructures comprising ferroelectric (FE) copolymer layers of vinylidene fluoride with 
trifluoroethylene (PVDF/TrFE) and linear dielectric (LD) layers of azodye prepared by Langmuir-Blodgett 
technique are obtained and studied. A simple physical model of a local electric field amplification in layers 
of FE and LD are presented. Experimental studies of the ferroelectric switching confirm that, for hetero-
structures, the external effective coercitive voltage decreases and  the  switched polarization increases. 
 
Keywords: Nanostructured Langmuir-Blodgett films, Ferroelectric switching. 
 
 PACS numbers: 73.50.Pz, 77.80.Fm, 77.55.tf 
 
 
                                                        
* annalpatova@bk.ru   
† lcl@ns.crys.ras.ru  
1. INTRODUCTION 
 
In the last years we see a reviving interest in study-
ing ferroelectric materials, especially ultrathin ferroe-
lectric films. There is also an evident trend to hybridi-
zation of ferroelectric and semiconductor materials in 
new electronic devices [1, 2]. Most of such devices are 
based on traditional crystalline ferroelectrics but some-
times one has serious difficulties with crystalline mate-
rials in preparing ultrathin films compatible with mod-
ern electronics. In many applications polymer ferroe-
lectrics are more desirable [3, 4]. Recently, a possibility 
for studying ferroelectricity in ultra-thin (even below 
10 nm) P(VDF-TrFE) copolymer films prepared by 
Langmuir-Blodgett (LB) technique has been demon-
strated [5, 6]. Langmuir-Blodgett technique allows one 
to design nano-structured films of different materials. 
In such complex sandwiches (heterostructures), the 
ferroelectric layers can alternate with other functional-
ized layers of magnetic materials, dyes, luminophores, 
biological molecules, etc., which would suffer a very 
strong built-in electric field of the ferroelectric layers as 
high as 108-109 V/m. In addition, the polymer structure 
allows incorporation of almost any solid nanoparticles. 
Hence, one may look forward to new discoveries and 
technical applications in such areas as organic transis-
tors [2], conductivity switchers [7], multiferroic hetero-
structures [8], and high all-organic capacity data stor-
age devices prepared by modern nanoimprinting tech-
niques [9]. 
Here we report on preparing and studying all-
organic heterostructures prepared by Langmuir-
Blodgett (LB) technique and consisting of two subsys-
tems of molecular layers. The first subsystem is pre-
sented by molecules of ferroelectic copolymer polyvinyl-
idene fluoride - trifluoroethylene (PVDF/TrFE). For the 
second subsystem molecules of an azodye MEL-26 were 
chosen, the films of which were well investigated early 
[10]. Two problems were considered and studied in this 
work. Firstly, the problem of retaining ferroelectricity 
in such a heterostructure is arising, since for the na-
nometer-thick films the presence of nonferroelectric 
azodye molecular layers might, in principle, influence 
the ferroelectric phase state of the copolymer. Secondly, 
from the simple physical model presented here, it fol-
lows that, if the ferroelectric phase of the copolymer is 
retained, the switching of spontaneous polarization 
should have an effect on the instantaneous electric field 
in both the subsystem of azodye molecules and the sub-
system of ferroelectric copolymer. Thus, the question of 
dynamics of ferroelectric switching of such a hetero-
structure is arising. So, these two questions are ad-
dressed to this work. 
 
2. HETEROSTRUCTURE  FERROELECTRIC -
LINEAR DIELECTRIC  
 
2.1 Samples preparing 
 
We used copolymer vinylidene fluoride with trifluo-
roethylene P(VDF/TrFE) in composition 70:30. Such a 
composition guarantees the existence of the ferroelec-
tric phase under heating up to the temperature of 
about ~120oC. Langmuir-Blodgett films were obtained 
by the following way. Weak-concentration (0.01% by 
weight) solution of copolymer in cyclohexanone and 
azodye (p-octadecyloxi-p’-sulphopiperidid azobenzene, 
MEL-26) in chloroform were prepared. Then, using 
home-done set-up [11] two subsystems of molecular 
layers were deposited onto a glass substrate by a hori-
zontal lift technique [12] (at first copolymer, then azo-
dye layers). Al electrodes (both the bottom and the top 
at the final step) were deposited by vacuum evapora-
tion technique. Monolayers were transferred from the 
air-water interface onto the solid substrates at room 
temperature and a surface pressure of 3 mN/m (for 
copolymer) and 15 mN/m (for azodye layers). A special 
design of the device was used as schematically shown 
in Fig. 2.1. The design consists of three capacitor ele-
ments on a common substrate prepared in one techno-
logical procedure with a common copolymer layer for 
all three capacitors. One of these three capacitors com-
prises only common ferroelectric layer consisting of 
sixty transferred copolymer monolayers (60 TL). The 
 A.V. ALPATOVA , V.V. LAZAREV , S. P. PALTO, S.G. YUDIN PROC. NAP 2, 01NTF15 (2013) 
 
 
01NTF15-2 
two other capacitors comprise additional azodye trans-
ferred layers (3 TL and 7 TL respectively for the 2d and 
3d capacitor elements). 
 
 
z1 
z2 
Al, 1 
Al, 2 
S u b s t r a t e 
1 
2 
Al 
LD 
FE 
 
 
Fig. 1 – The scheme of a design with three capacitors fabri-
cated on a common glass substrate. The right part is a hetero-
structure capacitor comprising the region 1 (molecular layers 
of copolymer, PVDF/TrFE - ferroelectric, FE) and the region 2 
(3 or 7 molecular layers of MEL-26 azodye - linear dielectric, 
LD) confined between common bottom and top Al-electrodes. 
The left part is a capacitor comprising only individual layers 
of the copolymer confined between common bottom and inter-
mediate top Al-electrodes. 
 
2.2 Model of a local electric field amplification 
in a ferroelectric heterostructure 
 
Considering the heterostructure (Fig. 1) to be ho-
mogeneous in the substrate plane (x, y-directions) and 
supposing electric conductance of materials to be equal 
to 0, one can write the following relationships between 
the electric field strength E  0, 0, Ez) in different parts 
of the heterostructure and the potential difference 
UU12   1 -  2 between the electrodes: 
 
 1 1 2 2, ,z zE z E z U    , (2.1) 
 11 1 0 2 2, , , ,z z s z zE P E  
  , (2.2) 
 
where indices “1” and “2” refer to the space regions oc-
cupied by the ferroelectric copolymer (the thickness 
z1) and the dye (the thickness z2), respectively; Ps is 
the spontaneous polarization normal to the substrate 
surface, εo  8.85x10-12 F/m, εz,1,2 are dimensionless 
(relative) components of dielectric permittivity tensor 
along normal direction (z-direction). Equation (2.2) con-
cerns one of the Maxwell’s Equations (div D  0), re-
quiring a continuity of the normal component of the 
electric displacement vector D, and in the absence of 
free space charge results in Dn,1  Dn,2. Real LB films 
have nonzero conductance and Eqns (2.1) and (2.2) 
would satisfactorily hold just in alternating electric 
fields of frequency much larger than reverse Maxwell 
relaxation time (2f >> (z,1,2 o z,1,2)-1, where z,1,2 is 
the specific resistance of the materials along the nor-
mal) under the condition of spontaneous polarization 
switching. Thus, we proceed from an assumption that 
the external voltage U is always higher than some min-
imal coercive voltage Uc that ensures switching the 
ferroelectric subsystem. It is the fast polarization 
switching that excludes polarization screening by free 
charges. 
From Eqs. (2.1) and (2.2) one can easily find the 
electric field in ferroelectric and dye regions: 
 
   
11
2 1 1 0 2 1 1 2, , , ,( ) ( ) ( )z z s z zE t U t z P t z z   
        (2.3) 
 
    12,11,21022,,1 )()()(   zzszz zztPztUtE   (2.4) 
 
In Eqs. (2.3) and (2.4) the alternating character of 
the electric field is expressed by introducing the time-
dependent values (E, U, Ps). As seen from (2.3) the 
switched polarization can increase the instantaneous 
field strength in the region of the dye subsystem and 
such an increase takes place in the dye region after 
switching the polarization, when polarization is di-
rected along the external electric field applied. The 
same field strengthening in the dye part of the hetero-
structure was earlier demonstrated by electrooptical 
measurements in [13, 14]. 
Of importance, from our point of view, is the rela-
tionship (2.4). It determines electric field acting in fer-
roelectric subsystem. Let us consider the situation 
when the negative external voltage is switched up to 
the positive one. In this case, up to the next-in-turn 
switching, the spontaneous polarization is directed op-
posite to the external electric field (if U is a positive 
value, then Ps is the negative one). However, since the 
term with Ps in the numerator of Eq. (2.4) is negative, 
the both terms in the numerator turn out to be of the 
same sign, and the effect of field amplification is possi-
ble up to the very instant of switching. The latter 
should promote an acceleration of the switching process 
and reducing the external coercive voltage across the 
heterostructure as a whole. 
Therefore, the effect of field strengthening in the 
dye subsystem is possible after switching of the spon-
taneous polarization, whereas the field in the ferroelec-
tric subsystem can be amplified before the instant of 
polarization switching. 
The effect of field strengthening in the ferroelectric 
layer can be expressed by the ratio of the field (Eq.2.4) 
to the corresponding electric field in the absence of the 
dye layer (z2  0). From Eq.2.4 follows: 
 
11
1 1 0 2
1 2 2 2 1
1
,
, , ,
z s
z z z
E z P z
F z
U U z

  
   
            
 (2.5) 
 
The strengthening effect is realized if the  factor F1 
>1, i.e., when 
 
 1 0 1 1, ,z c
s
U
z z
P
      (2.6) 
 
In Fig. 2 there are given dependencies of the electric 
field amplification factor, F1, versus the thickness of 
azodye layer, z2, for some fixed thicknesses of ferroe-
lectric layer, z1 (from 2 nm to 120 nm), at an external 
voltage U  20 V and for the material parameters close 
to the experimental ones: Ps  0.04 C/m2; ε1  7; ε2  2.5. 
 FERROELECTRIC BEHAVIOR OF NANO-LAYERED HETEROSTRUCTURES… PROC. NAP 2, 01NTF15 (2013) 
 
 
01NTF15-3 
  
0 20 40 60 80 100
0
1
2
3
z
1
  =  20 nm
z
1
  = 5 nm
z
1
  =  60 nm
z
1
  =  80 nm
z
1
  = 100 nm
F
1
 z
2
  (nm)
z
1
  = 120 nm
z
1
  =  30 nm
 
 
Fig. 2 – Dependence of the electric field amplification factor, 
F1, versus the thickness of azodye layer, z2, at different fixed 
thicknesses of ferroelectric layer, z1, (U  20 V; 
Ps  0.04 C/m2; ε1  7; ε2  2.5). 
 
Thus, the presence of the additional dielectric lay-
ers (dye subsystem) can strengthen the local electric 
field in the region of ferroelectric if thickness of the 
ferroelectric layers is sufficiently large. The latter can 
positively affect the dynamics of ferroelectric switching 
and the value of switched part of polarization. 
 
3. EXPERIMENTAL RESULTS 
 
Figure 3 shows the temperature behavior of the ca-
pacitance of the heterostructure and the individual 
copolymer film during the thermal cycling. The both 
structures reveal pronounced temperature hysteresis 
characteristic of first-order ferroelectric phase transi-
tion. 
  
20 40 60 80 100 120
1
2
3
Copolymer-Film
Heterostructure-Film
C
a
p
a
c
it
y
, 
 n
F
Temperature,   
o
C 
 
 
Fig. 3 – Temperature dependencies of the capacity of hetero-
structure (18 layers of copolymer and 3 layers of azodye) and 
the capacity of individual copolymer film (18 layers of copoly-
mer) taken at and sine-form voltage of 0.35 V frequency of 68 
Hz (overlapped active area S   ~1x1 mm2 for both capacitors). 
Arrows along the curves indicate the direction of the tempera-
ture run.  
 
Merz switching technique [15] was used for charac-
terizing repolarization properties of capacitors. Ampli-
fied triangle waveform signal from functional generator 
was applied to the capacitor tested and in series con-
nected load resistor. Oscillograms taken from the load 
resistor were registered and saved by a memory oscillo-
scope joint with a personal computer. The totally 
switched polarization was obtained after integrating 
the area under current curves i(t) in Fig.  4 and sub-
tracting the linear contributions of the resistive and 
capacitive components of the corresponding current 
response. 
Figure 4 shows some switching curves of ferroelec-
tric repolarization for 3 capacitor elements obtained for 
different amplitudes (from 10 V to 30 V) of the triangle 
waveform voltage applied. Beginning with amplitudes 
of about ~12 V there is a distinctive repolarization 
bump that, with increasing voltage amplitude, is al-
ways larger in value for the heterostructure capacitor 
compared to the copolymer homo-film capacitor. Here 
we once more give emphasis to the fact that each of the 
three capacitors (heterostructures comprised and indi-
vidual copolymer layer comprised) are placed on the 
same common substrate and comprised common ferroe-
lectric layer. That minimizes inaccuracy in repolariza-
tion measurements associated with a  possible nonuni-
formity in thickness of the ferroelectric layer, the com-
pared capacitors might have if prepared as separate 
units on three different substrates. 
 
10 15
0
20
40
60
80
0
20
40
60
80
10 15
0
20
40
60
80
0.0
0.2
0.4
0.6
0.8
10 15
0
0
80
0
40
80
 
6
5
4
3i 
, 
  
m
k
A
t ,   ms
a
1
2
FE (60 layers)
 
FE + LD(3 layers)
6
5
4
3
2
1
b
t ,  ms
Utr
  
FE+LD(7 layers)
6
5
4
3
2
1
c
t ,  ms
 
 
Utr
  
Utr
 
Fig. 4 – Oscillograms i(t) of repolarization current response 
for individual copolymer PVDF/TrFE film (60 layers) (a), and 
two heterostructures (60 layers of copolymer + 3 layers of 
azodye) (b) and (60 layers of copolymer + 7 layers of azodye) 
(c). Oscillograms are obtained by Merz technique under apply-
ing cycling voltage of the triangle form (f  80 Hz )  of different 
amplitudes:10 V (1), 12.5 V (2), 15 V (3), 20 V (4), 25 V (5), 
30 V (6)- the same  for (a, b, c) curve series. Only parts of the 
curves for the positive half-period of applied triangle form 
voltage Utr are given.  
 
The measurements given in Fig. 4 were carried out 
at the same amplitudes of triangle voltage for all three 
series. As seen from the figures the switched polariza-
tion is occurred larger for the heterostructure 
(Psw  5.7 mkC/cm2 for Utr  30 V,curve 6, Fig. 4c) than 
that for the copolymer capacitor(Psw 5.0 mkC/cm2 for 
the same Utr  30 V, curve 6, Fig. 4a ). This is a clear 
indication of the presence of the local field amplifying 
effect in the case of the heterostructure. Moreover, the 
maximum of the current response in the heterostruc-
ture corresponds to a lower instant external voltage 
(~11.7 V) as compared to the instant voltage (~13.7 V) 
for the current maximum of the individual copolymer 
layer (compare curves 6 in Figs. 4a and 4c). The latter 
means that the effective external coercive voltage Uc is 
lower for the heterostructure. 
 
4. CONCLUSIONS 
 
We have demonstrated that polarization switch-
ing in a heterostructure can be more effective than 
 A.V. ALPATOVA , V.V. LAZAREV , S. P. PALTO, S.G. YUDIN PROC. NAP 2, 01NTF15 (2013) 
 
 
01NTF15-4 
in an individual ferroelectric film. This circumstance 
is due to the effect of amplification of local electric 
field acting in the ferroelectric part of the hetero-
structure. 
 
 
AKNOWLEDGEMENTS 
 
The work was supported by Russian Foundation for 
Basic Research (project no. 12-02-00214a). The authors 
are grateful to Dr. L.M. Blinov (Institute of Crystallog-
raphy, RAS) for fruitful discussions. 
 
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Ferroelectric Behavior of Nano-Layered Heterostructures Composed of Ferroelectric - Linear Dielectric Organic Layers

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