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Electro-optical and ferroelectric switching of Langmuir–Blodgett films made of a chiral smectic-C⁎ liquid crystalline compound
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Thin Solid Films 516 (2008) 8905 – 8908 www.elsevier.com/locate/tsf
Electro-optical and ferroelectric switching of Langmuir–Blodgett films made of a chiral smectic-C⁎ liquid crystalline compound V.V. Lazarev ⁎, L.M. Blinov, S.P. Palto, S.G. Yudin Institute of Crystallography, Russian Academy of Sciences., Leninsky prosp. 59, Moscow, 119333, Russia Available online 22 November 2007
Abstract Electric and electro-optical properties of a smectic liquid crystal (LC) compound formed by rod-like molecules have been investigated in the thin film sandwich geometry typically used in solid-state technology. Thin (~ 10–30 nm) Langmuir–Blodgett (LB) films of the compound (tetradecyl homologue (n = 14) of the series p-oxybenzylidene-p′-amino-2-methylbutyl(α-cyano)cinnamate) were transferred onto glasses covered with a transparent electrode and supplied with a top electrode evaporated in vacuum. In contrast to the LC bulk, thin films of the compound manifest only a solid crystalline phase throughout the whole temperature range (20–110 °C) investigated. Ferroelectric properties of the films (covering linear electro-optic effect, pyroeffect and polarization switching) have been observed in the whole temperature range studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; Ferroelectricity; Liquid crystal; Polarization; Pyroeffect
Liquid crystals are well known as materials for displays and other electro-optic applications [1]. Particularly interesting are phenomena of ferro- and antiferroelectricity, which are observed in many smectic (lamellar) phases. Among them are chiral SmC⁎, SmCA⁎ and other phases formed by rod-like molecules and achiral B1–B7 and other ones formed by bent shape molecules [2]. All remarkable properties and electro-optic applications of liquid crystals are based on easy field-induced molecular reorientation that is possible due to a fluid nature of mesophases. Correspondingly, a conventional electro-optic cell consists of two electrode-coated glass substrates, which form a flat capillary with a 2–10 μm gap filled with a liquid crystal. In the so-called in-plane geometry, the electrodes, e.g. interdigitated, can be made on one of the substrates forming the capillary. Very viscous, e.g. polymer liquid crystals can be studied in this geometry even without a top glass [3,4]. Moreover, smectic liquid crystals can form free-standing films on holes made in solid substrates and ferroelectric properties can be studied using in-plane electrodes made on the substrates [5]. On the other hand, thin and ultrathin films of molecular solids, e.g. of organic semiconductors are typically studied in a ⁎ Corresponding author. E-mail address: [email protected] (V.V. Lazarev). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.065
planar sandwich geometry with bottom electrodes deposited onto substrates and top electrodes evaporated onto the organic films. The overwhelming majority of practical results on solid organic films have been obtained using just the planar technology that, up to now, is considered not suitable for liquid crystal films. In some cases ferroelectric properties were also reported for thin film sandwiches [6]. However, in the course of our study of different liquid crystal substances deposited onto electroded substrates by Langmuir– Blodgett (LB) technique (from 10 to 50 monolayers) and equipped with evaporated top electrodes in a low temperature crystalline phase we have discovered that, upon heating, the sandwiches keep their integrity even in high-temperature liquid crystal phases. This allowed us to study ferroelectric and even electro-optic properties of thin and ultrathin liquid crystal films formed by bent (banana) shape molecules [7]. The aim of the present work is to use the same sandwich geometry for investigations of the ferroelectric and electro-optical properties of another mesogenic compound (with rod-like molecules), which, in the bulk, manifests a chiral smectic liquid crystal phase. The substance investigated is a little-studied tetradecyl homologue (n = 14) of the series p-oxybenzylidene-p′-amino2-methylbutyl(α-cyano)cinnamate (TDOBAMBCC, for its structure see Inset in Fig. 1). Its molecule is chiral, possesses
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Fig. 1. π–A isotherms of TDOBAMBCC on the water surface upon compression (filled squares) and expansion (open squares) process. T is the operating point for the film transfer. Inset: chemical formula of TDOBAMBCC.
a lateral dipole moment, and has a rod-like shape. The chirality originates from an asymmetric carbon atom on the end part of the rod-like molecule. In the bulk, the compound is crystalline at room temperature and is in the isotropic phase above 105 °C. Upon cooling down the TDOBAMBCC-bulk shows a monotropic chiral SmC⁎ phase in the 70–54 °C temperature range with small value of spontaneous polarization (of about ~ 10– 20 μC/m2) typical of all compounds of the said series [8,12]. TDOBAMBCC molecules have slightly hydrophilic central rigid cores and strongly hydrophobic long alkyl ending groups, therefore, like other substances of the same type, TDOBAMBCC forms dense layers on the water surface. We prepared such layers from a 0.01% solution of the substance in chloroform by slow spreading 1 ml of the solution over 0.1 m2 area of the water surface in our home-made Langmuir trough. After evaporation of the solvent we have studied “pressure (π)–molecular area (A)” isotherms at a temperature T = 25 °C. An example of the π–A isotherm is shown in Fig. 1. Then, using the Langmuir–Schaefer technique [9], at pressure π = 5–6 mN/m (operating point T, see Fig. 1), we made a transfer of a certain number (from 15 to 50) of layers from the water surface onto Al (or ITO)-coated glass substrates. Note that, at point T, the area per molecule (~0.28 nm2) is very close to the expected value for a dense-packed array of benzene rings arranged perpendicularly to the water surface (~0.23 nm2). Therefore we believe that the TDOBAMBCC molecules form dense monolayers on the water surface with molecules standing up. Finally, top Al electrodes were evaporated onto the deposited films in vacuum and solid sandwich structures Al(or ITO)– TDOBAMBCC–Al formed. The thickness of the TDOBAMBCC films was obtained from capacitance measurements assuming low frequency value of dielectric constant ε ≈ 5–6 rational for polar Langmuir–Blodgett films (for example, typical ε-values of ferroelectric Langmuir–Blodgett films made of polyvinylidene fluoride and its copolymers are varied in the range of ε ≈ 4–16 [10]). We have found nearly linear dependence of the film thickness on the number of transfers, d ≈ αN with coefficient α ≈ 0.6 nm/transfer. This layer thickness is close to the transverse size (~ 0.52 nm) of an alkyl chain calculated from the molecular model. Since our transfer ratio is close to 1, we conclude that the TDOBAMBCC molecules are
lying almost in the plane of our substrates, i.e. they tumbled through 90° either upon transfer or due to subsequent film crystallisation. The cells were placed in a thermostat and studied under a polarizing microscope equipped with a photodetector. For the measurements of repolarization currents and electro-optic response, the triangular or rectangular voltage forms were used with the amplitude and frequency varied. The temperature dependence of the cell capacitance was measured with the sineform voltage and a lock-in technique described in detail in [11]. In this work we present the results obtained on films consisting of 17 and 51 transferred layers of TDOBAMBCC (film thickness d ≈ 10 nm and 30 nm, correspondingly, the area of overlapping electrodes A ≈ 1 mm2). For comparison, we have also studied the dielectric properties and the field-induced optical transmission of bulky TDOBAMBCC-layers in a conventional capillary cell (cell thickness d ≈ 7 μm, electrode overlapping area A ≈ 9 mm2) with two ITO electrodes covered by polyimide orienting layers rubbed unidirectionally. Fig. 2 shows the temperature dependence of capacity (curve 1) of such a cell with distinct sequence of phase transitions the bulky TDOBAMBCC undergoes during heating-cooling cycles in good agreement with data in [8,12]: upon heating cycle 25– 80 °C (solid phase)–80–105 °C (smectic A mesophase)–105 °C and higher (isotropic phase); upon cooling cycle 103–70 °C (smectic A mesophase)–70–54 °C (chiral smectic C mesophase) and below 54 °C again solid phase. Note that solely in the chiral SmC⁎ phase, the bulky TDOBAMBCC layer exhibits the field-induced optical response (curve 2 in Fig. 2 shows the cell optical transmittance under polarized microscope at the sine-shape voltage driving, Uampl = 10 V, f = 28 Hz) and correspondingly in the same phase a small switched polarization (~ 10–20 μC/m2) measured by Merz repolarization current technique was only possible to observe. On the contrary, the temperature dependence of capacity of a TDOBAMBCC film comprising 51 transferred layers (Fig. 3) shows no distinct phase transition points even in a larger
Fig. 2. Temperature dependencies of capacity (curve 1) and field-induced optical transmittance (curve 2) of the bulk TDOBAMBCC layer taken at a frequency of 80 Hz and sine-form voltage of 1 V in geometry of a conventional capillary cell (overlapped active area S = ~ 9 mm2, gap thickness d = ~7 μm). Arrows along the curves indicate the direction of temperature runs.
V.V. Lazarev et al. / Thin Solid Films 516 (2008) 8905–8908
Fig. 3. Temperature dependence of low-voltage capacitance of the 51-layer TDOBAMBCC film taken at the voltage amplitude Uo = 1 V and frequency f = 80 Hz. Inset: oscillograms of repolarization current i(t) obtained at T = 23 °C with the triangular voltage form of amplitudes Utr = ±0.5; 1.0; 1.5; 3.0; 4.0 V and frequency f = 80 Hz.
temperature range (over 80 °C) where the bulk of the material would exist in the isotropic liquid state. Nevertheless, oscillograms of repolarization current of the film, obtained under the triangular-shape voltage driving, show distinct bumps corresponding to the field-induced reversal of polarization. The polarization reversal is observed in the whole temperature range of TDOBAMBCC films studied (~ 25–110 °C). The value of the switched part of polarization Psw, (calculated from the bump area with the capacitive and ohmic current contribution subtracted) depends on voltage amplitudes and temperature (for current oscillograms see Insets to Figs. 3, 4). The temperature dependence of Psw is shown in the main plot of Fig. 4. With an increasing temperature, Psw increases from ~ 0.8 mC/m2 at 23 °C to ~ 3 mC/m2 at 105 °C tending to saturation at higher temperatures. The pyroelectric coefficient of the film in this temperature range (calculated from Psw(T)dependence) is varied within the range of γ = 40–20 μC/m2 K, comparable with that measured on the LB films of the same
Fig. 4. Temperature dependence of switched part of polarization of the 51-layer TDOBAMBCC film during the heating cycle (solid squares) and cooling cycle (open squares). Dotted line is for the eye guide. Inset (left-up plot): oscillograms of repolarization current i(t) obtained at T = 32 °C with the triangular-shape voltage of different amplitudes Utr = ±1.5; 2.5; 3.5 V and frequency f = 80 Hz. Inset (right-down plot): oscillograms of repolarization current i(t) obtained at T = 106 °C and 101 °C with the triangular voltage amplitude of Utr = ±3.0 V and frequency f = 80 Hz.
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Fig. 5. Typical example of the polarization hysteresis loop for the 51-layer TDOBAMBCC film. The curves are obtained by integrating repolarization current oscillograms recorded under the following conditions: triangular waveform of driving, Utr = ±4 V, f = 80 Hz, and T = 23 °C (open squares) and 40 °C (open circles). Arrows along the curves indicate the direction of the voltage change.
thickness prepared from ferroelectric polymer polyvinylidene fluoride and its copolymers [13]. Fig. 5 shows typical polarization hysteresis loops (at 23 °C and 40 °C) for 51-layer thick LB film of TDOBAMBCC obtained from Mertz technique measurements using triangular driving waveform of an amplitude Utr = ± 4 V and frequency f = 80 Hz (with the capacitive and ohmic current contribution subtracted). Under this driving voltage coercive field of 51layer thick film is of Ec ~ 34 V/μm. The coercive field is clearly growing with decreasing film thickness as seen from Fig. 6 where a dependence of the switched part of polarization Psw on applied electric field E is given for two TDOBAMBCC films, namely for 51-layer film (d ~ 30 nm), Ec ~ 34 V/μm and 17-layer film (d ~ 10 nm), Ec ~ 200 V/μm. Fig. 7 shows the temperature dependence of the field-induced optical transmission for the 47-layer sandwich cell with semitransparent SnO2–Al electrodes placed between crossed polarizers. The curve was measured on the fresh sample using the lock-in technique at the first harmonic (f = 48 Hz) of the rectangular form voltage applied (Ur = ±8 V). The corresponding
Fig. 6. Dependence of the switched part of polarization on applied electric field for two TDOBAMBCC films (51-layer film (d ~ 30 nm), Ec ~ 40 V/μm and 17-layer film (d ~ 10 nm), Ec ~ 200 V/μm). Dotted lines are for the eye guide.
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Fig. 7. Temperature dependence of the field-induced optical signal (transmission) of the fresh 47-layer sandwich with semitransparent SnO2–Al electrodes taken at crossed polarizers, Utr = ±8 V, frequency f = 48 Hz. Inset: an oscillogram of the modulated intensity of the transmitted light at T = 25 °C (f = 48 Hz). The maximum light modulation depth is of ~ 1.5%.
oscillogram (at room temperature having maximum light modulation depth of ~1.5%) is presented in the Inset. This linear electro-optic effect also confirms the ferroelectric nature of TDOBAMBCC films. The high value of the switched polarization (up to 2.8 mC/ m2, Fig. 4) and strong linear electro-optical response (about 2% for a 25–30 nm thick film, see Inset to Fig. 7) points to the ferroelectric nature of our Langmuir–Blodgett films. However, the mechanism of the ferroelectricity seems to be irrelevant to the chiral structure of the compound studied. Indeed, in the bulk sample, the polarization is much smaller (Psw ≈ 0.02 mC/m2) and observed only in the monotropic SmC⁎ phase. It is difficult to imagine that the same mechanism of improper ferroelectricity would result in the 100 times increase of the Psw in a solid phase of the same compound. Thus one should look for another explanation. It is interesting that after several heating-cooling cycles exceeding 100 °C, ferroelectric properties of TDOBAMBCC Langmuir–Blodgett films markedly degraded or even disappeared at all. This might be attributed to the existence of water molecules left, for instance, in the interlayer gaps during the transfer of monolayers. Hydroxyl–OH groups and bound water molecules are well known for their role in ferroelectricity of many solid inorganic ferroelectrics, e.g. from the potassium hydro phosphate or the alum classes [14] because transfer of protons from one to another molecular site results in large permanent (but field switchable) dipole moments of the materials. Probably in our case, the dissociation or evaporation of water molecules at a high temperature are responsible for disappearance of permanent polarization and degradation of
ferroelectric properties of the films. Evidently, further investigations are needed to explain a mechanism of the observed phenomena. In conclusion, many specific features of the ferroelectric behaviour are observed for thin Langmuir–Blodgett films made of TDOBAMBCC chiral smectic LC. At elevated temperatures, at which the bulk of TDOBAMBCC would be in the liquid crystal phase, the thin films are still in a solid crystalline phase that can be attributed to very strong confinement of TDOBAMBCC molecules in sandwich structures. The switched polarization measured for a film of 30 nm thick is of Psw ~ 1–3 mC/m2 depending on temperature. That leads to pyroelectric coefficient γ = 40–20 μC/m2 K in the temperature range of 20–110 °C. Usage of LC compounds expands a variety of materials for preparing thin ferroelectric films by Langmuir–Blodgett technology. Acknowledgement We acknowledge the financial support from the Russian Fund for Basic Research (grants no. 0402-16466). References [1] L.M. Blinov, V.G. Chigrinov, Electro-Optic Effects in Liquid Crystal Materials, Springer Verlag, New York, 1994. [2] S.T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals, WileyVCH, Weinheim, 1999. [3] S. Pfeiffer, R. Shashidhar, T.L. Fare, J. Nacri, J. Adams, R.S. Duran, Appl. Phys. Lett. 63 (1993) 1285. [4] S. Okazaki, S. Uto, K. Yoshino, K. Skarp, B. Helgee, Appl. Phys. Lett. 71 (1997) 3373. [5] C.Y. Young, R. Pindak, N.A. Clark, R.B. Meyer, Phys. Rev. Lett. 40 (1978) 773. [6] S.G. Yudin, L.M. Blinov, N.N. Petukhova, S.P. Palto, JETP Lett. 70 (1999) 633. [7] L.M. Blinov, A.R. Geivandov, V.V. Lazarev, S.P. Palto, S.G. Yudin, G. Pelzl, W. Weissflog, Appl. Phys. Lett. 87 (2005) 241913. [8] G. Durand, Ph. Martinot-Lagarde, Ferroelectrics 24 (1980) 89. [9] Langmuir, V.J. Shaefer, J. Am. Chem. Soc. 60 (1938) 1351. [10] L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin, Thin Solid Films 284–285 (1996) 469. [11] N.M. Shtykov, M.I. Barnik, S.P. Palto, L.M. Blinov, G. Pelzl, W. Weissflog, JETP 121 (2002) 739. [12] L.A. Beresnev, L.M. Blinov, Zh.Vses. Khim. Obshch. 28 (1983) 149 (in Russ.). [13] S. Ducharme, S.P. Palto, V.M. Fridkin, in: H.S. Nalwa (Ed.), Handbook of Thin Films Materials, Ferroelectric and Dielectric Thin Films, chapter 11, vol. 3, Academic Press, 2002, p. 1. [14] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectric and related Materials, Clarendon Press, Oxford, 1977.
Thin Solid Films 516 (2008) 8905 – 8908 www.elsevier.com/locate/tsf
Electro-optical and ferroelectric switching of Langmuir–Blodgett films made of a chiral smectic-C⁎ liquid crystalline compound V.V. Lazarev ⁎, L.M. Blinov, S.P. Palto, S.G. Yudin Institute of Crystallography, Russian Academy of Sciences., Leninsky prosp. 59, Moscow, 119333, Russia Available online 22 November 2007
Abstract Electric and electro-optical properties of a smectic liquid crystal (LC) compound formed by rod-like molecules have been investigated in the thin film sandwich geometry typically used in solid-state technology. Thin (~ 10–30 nm) Langmuir–Blodgett (LB) films of the compound (tetradecyl homologue (n = 14) of the series p-oxybenzylidene-p′-amino-2-methylbutyl(α-cyano)cinnamate) were transferred onto glasses covered with a transparent electrode and supplied with a top electrode evaporated in vacuum. In contrast to the LC bulk, thin films of the compound manifest only a solid crystalline phase throughout the whole temperature range (20–110 °C) investigated. Ferroelectric properties of the films (covering linear electro-optic effect, pyroeffect and polarization switching) have been observed in the whole temperature range studied. © 2007 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; Ferroelectricity; Liquid crystal; Polarization; Pyroeffect
Liquid crystals are well known as materials for displays and other electro-optic applications [1]. Particularly interesting are phenomena of ferro- and antiferroelectricity, which are observed in many smectic (lamellar) phases. Among them are chiral SmC⁎, SmCA⁎ and other phases formed by rod-like molecules and achiral B1–B7 and other ones formed by bent shape molecules [2]. All remarkable properties and electro-optic applications of liquid crystals are based on easy field-induced molecular reorientation that is possible due to a fluid nature of mesophases. Correspondingly, a conventional electro-optic cell consists of two electrode-coated glass substrates, which form a flat capillary with a 2–10 μm gap filled with a liquid crystal. In the so-called in-plane geometry, the electrodes, e.g. interdigitated, can be made on one of the substrates forming the capillary. Very viscous, e.g. polymer liquid crystals can be studied in this geometry even without a top glass [3,4]. Moreover, smectic liquid crystals can form free-standing films on holes made in solid substrates and ferroelectric properties can be studied using in-plane electrodes made on the substrates [5]. On the other hand, thin and ultrathin films of molecular solids, e.g. of organic semiconductors are typically studied in a ⁎ Corresponding author. E-mail address: [email protected] (V.V. Lazarev). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.065
planar sandwich geometry with bottom electrodes deposited onto substrates and top electrodes evaporated onto the organic films. The overwhelming majority of practical results on solid organic films have been obtained using just the planar technology that, up to now, is considered not suitable for liquid crystal films. In some cases ferroelectric properties were also reported for thin film sandwiches [6]. However, in the course of our study of different liquid crystal substances deposited onto electroded substrates by Langmuir– Blodgett (LB) technique (from 10 to 50 monolayers) and equipped with evaporated top electrodes in a low temperature crystalline phase we have discovered that, upon heating, the sandwiches keep their integrity even in high-temperature liquid crystal phases. This allowed us to study ferroelectric and even electro-optic properties of thin and ultrathin liquid crystal films formed by bent (banana) shape molecules [7]. The aim of the present work is to use the same sandwich geometry for investigations of the ferroelectric and electro-optical properties of another mesogenic compound (with rod-like molecules), which, in the bulk, manifests a chiral smectic liquid crystal phase. The substance investigated is a little-studied tetradecyl homologue (n = 14) of the series p-oxybenzylidene-p′-amino2-methylbutyl(α-cyano)cinnamate (TDOBAMBCC, for its structure see Inset in Fig. 1). Its molecule is chiral, possesses
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Fig. 1. π–A isotherms of TDOBAMBCC on the water surface upon compression (filled squares) and expansion (open squares) process. T is the operating point for the film transfer. Inset: chemical formula of TDOBAMBCC.
a lateral dipole moment, and has a rod-like shape. The chirality originates from an asymmetric carbon atom on the end part of the rod-like molecule. In the bulk, the compound is crystalline at room temperature and is in the isotropic phase above 105 °C. Upon cooling down the TDOBAMBCC-bulk shows a monotropic chiral SmC⁎ phase in the 70–54 °C temperature range with small value of spontaneous polarization (of about ~ 10– 20 μC/m2) typical of all compounds of the said series [8,12]. TDOBAMBCC molecules have slightly hydrophilic central rigid cores and strongly hydrophobic long alkyl ending groups, therefore, like other substances of the same type, TDOBAMBCC forms dense layers on the water surface. We prepared such layers from a 0.01% solution of the substance in chloroform by slow spreading 1 ml of the solution over 0.1 m2 area of the water surface in our home-made Langmuir trough. After evaporation of the solvent we have studied “pressure (π)–molecular area (A)” isotherms at a temperature T = 25 °C. An example of the π–A isotherm is shown in Fig. 1. Then, using the Langmuir–Schaefer technique [9], at pressure π = 5–6 mN/m (operating point T, see Fig. 1), we made a transfer of a certain number (from 15 to 50) of layers from the water surface onto Al (or ITO)-coated glass substrates. Note that, at point T, the area per molecule (~0.28 nm2) is very close to the expected value for a dense-packed array of benzene rings arranged perpendicularly to the water surface (~0.23 nm2). Therefore we believe that the TDOBAMBCC molecules form dense monolayers on the water surface with molecules standing up. Finally, top Al electrodes were evaporated onto the deposited films in vacuum and solid sandwich structures Al(or ITO)– TDOBAMBCC–Al formed. The thickness of the TDOBAMBCC films was obtained from capacitance measurements assuming low frequency value of dielectric constant ε ≈ 5–6 rational for polar Langmuir–Blodgett films (for example, typical ε-values of ferroelectric Langmuir–Blodgett films made of polyvinylidene fluoride and its copolymers are varied in the range of ε ≈ 4–16 [10]). We have found nearly linear dependence of the film thickness on the number of transfers, d ≈ αN with coefficient α ≈ 0.6 nm/transfer. This layer thickness is close to the transverse size (~ 0.52 nm) of an alkyl chain calculated from the molecular model. Since our transfer ratio is close to 1, we conclude that the TDOBAMBCC molecules are
lying almost in the plane of our substrates, i.e. they tumbled through 90° either upon transfer or due to subsequent film crystallisation. The cells were placed in a thermostat and studied under a polarizing microscope equipped with a photodetector. For the measurements of repolarization currents and electro-optic response, the triangular or rectangular voltage forms were used with the amplitude and frequency varied. The temperature dependence of the cell capacitance was measured with the sineform voltage and a lock-in technique described in detail in [11]. In this work we present the results obtained on films consisting of 17 and 51 transferred layers of TDOBAMBCC (film thickness d ≈ 10 nm and 30 nm, correspondingly, the area of overlapping electrodes A ≈ 1 mm2). For comparison, we have also studied the dielectric properties and the field-induced optical transmission of bulky TDOBAMBCC-layers in a conventional capillary cell (cell thickness d ≈ 7 μm, electrode overlapping area A ≈ 9 mm2) with two ITO electrodes covered by polyimide orienting layers rubbed unidirectionally. Fig. 2 shows the temperature dependence of capacity (curve 1) of such a cell with distinct sequence of phase transitions the bulky TDOBAMBCC undergoes during heating-cooling cycles in good agreement with data in [8,12]: upon heating cycle 25– 80 °C (solid phase)–80–105 °C (smectic A mesophase)–105 °C and higher (isotropic phase); upon cooling cycle 103–70 °C (smectic A mesophase)–70–54 °C (chiral smectic C mesophase) and below 54 °C again solid phase. Note that solely in the chiral SmC⁎ phase, the bulky TDOBAMBCC layer exhibits the field-induced optical response (curve 2 in Fig. 2 shows the cell optical transmittance under polarized microscope at the sine-shape voltage driving, Uampl = 10 V, f = 28 Hz) and correspondingly in the same phase a small switched polarization (~ 10–20 μC/m2) measured by Merz repolarization current technique was only possible to observe. On the contrary, the temperature dependence of capacity of a TDOBAMBCC film comprising 51 transferred layers (Fig. 3) shows no distinct phase transition points even in a larger
Fig. 2. Temperature dependencies of capacity (curve 1) and field-induced optical transmittance (curve 2) of the bulk TDOBAMBCC layer taken at a frequency of 80 Hz and sine-form voltage of 1 V in geometry of a conventional capillary cell (overlapped active area S = ~ 9 mm2, gap thickness d = ~7 μm). Arrows along the curves indicate the direction of temperature runs.
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Fig. 3. Temperature dependence of low-voltage capacitance of the 51-layer TDOBAMBCC film taken at the voltage amplitude Uo = 1 V and frequency f = 80 Hz. Inset: oscillograms of repolarization current i(t) obtained at T = 23 °C with the triangular voltage form of amplitudes Utr = ±0.5; 1.0; 1.5; 3.0; 4.0 V and frequency f = 80 Hz.
temperature range (over 80 °C) where the bulk of the material would exist in the isotropic liquid state. Nevertheless, oscillograms of repolarization current of the film, obtained under the triangular-shape voltage driving, show distinct bumps corresponding to the field-induced reversal of polarization. The polarization reversal is observed in the whole temperature range of TDOBAMBCC films studied (~ 25–110 °C). The value of the switched part of polarization Psw, (calculated from the bump area with the capacitive and ohmic current contribution subtracted) depends on voltage amplitudes and temperature (for current oscillograms see Insets to Figs. 3, 4). The temperature dependence of Psw is shown in the main plot of Fig. 4. With an increasing temperature, Psw increases from ~ 0.8 mC/m2 at 23 °C to ~ 3 mC/m2 at 105 °C tending to saturation at higher temperatures. The pyroelectric coefficient of the film in this temperature range (calculated from Psw(T)dependence) is varied within the range of γ = 40–20 μC/m2 K, comparable with that measured on the LB films of the same
Fig. 4. Temperature dependence of switched part of polarization of the 51-layer TDOBAMBCC film during the heating cycle (solid squares) and cooling cycle (open squares). Dotted line is for the eye guide. Inset (left-up plot): oscillograms of repolarization current i(t) obtained at T = 32 °C with the triangular-shape voltage of different amplitudes Utr = ±1.5; 2.5; 3.5 V and frequency f = 80 Hz. Inset (right-down plot): oscillograms of repolarization current i(t) obtained at T = 106 °C and 101 °C with the triangular voltage amplitude of Utr = ±3.0 V and frequency f = 80 Hz.
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Fig. 5. Typical example of the polarization hysteresis loop for the 51-layer TDOBAMBCC film. The curves are obtained by integrating repolarization current oscillograms recorded under the following conditions: triangular waveform of driving, Utr = ±4 V, f = 80 Hz, and T = 23 °C (open squares) and 40 °C (open circles). Arrows along the curves indicate the direction of the voltage change.
thickness prepared from ferroelectric polymer polyvinylidene fluoride and its copolymers [13]. Fig. 5 shows typical polarization hysteresis loops (at 23 °C and 40 °C) for 51-layer thick LB film of TDOBAMBCC obtained from Mertz technique measurements using triangular driving waveform of an amplitude Utr = ± 4 V and frequency f = 80 Hz (with the capacitive and ohmic current contribution subtracted). Under this driving voltage coercive field of 51layer thick film is of Ec ~ 34 V/μm. The coercive field is clearly growing with decreasing film thickness as seen from Fig. 6 where a dependence of the switched part of polarization Psw on applied electric field E is given for two TDOBAMBCC films, namely for 51-layer film (d ~ 30 nm), Ec ~ 34 V/μm and 17-layer film (d ~ 10 nm), Ec ~ 200 V/μm. Fig. 7 shows the temperature dependence of the field-induced optical transmission for the 47-layer sandwich cell with semitransparent SnO2–Al electrodes placed between crossed polarizers. The curve was measured on the fresh sample using the lock-in technique at the first harmonic (f = 48 Hz) of the rectangular form voltage applied (Ur = ±8 V). The corresponding
Fig. 6. Dependence of the switched part of polarization on applied electric field for two TDOBAMBCC films (51-layer film (d ~ 30 nm), Ec ~ 40 V/μm and 17-layer film (d ~ 10 nm), Ec ~ 200 V/μm). Dotted lines are for the eye guide.
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Fig. 7. Temperature dependence of the field-induced optical signal (transmission) of the fresh 47-layer sandwich with semitransparent SnO2–Al electrodes taken at crossed polarizers, Utr = ±8 V, frequency f = 48 Hz. Inset: an oscillogram of the modulated intensity of the transmitted light at T = 25 °C (f = 48 Hz). The maximum light modulation depth is of ~ 1.5%.
oscillogram (at room temperature having maximum light modulation depth of ~1.5%) is presented in the Inset. This linear electro-optic effect also confirms the ferroelectric nature of TDOBAMBCC films. The high value of the switched polarization (up to 2.8 mC/ m2, Fig. 4) and strong linear electro-optical response (about 2% for a 25–30 nm thick film, see Inset to Fig. 7) points to the ferroelectric nature of our Langmuir–Blodgett films. However, the mechanism of the ferroelectricity seems to be irrelevant to the chiral structure of the compound studied. Indeed, in the bulk sample, the polarization is much smaller (Psw ≈ 0.02 mC/m2) and observed only in the monotropic SmC⁎ phase. It is difficult to imagine that the same mechanism of improper ferroelectricity would result in the 100 times increase of the Psw in a solid phase of the same compound. Thus one should look for another explanation. It is interesting that after several heating-cooling cycles exceeding 100 °C, ferroelectric properties of TDOBAMBCC Langmuir–Blodgett films markedly degraded or even disappeared at all. This might be attributed to the existence of water molecules left, for instance, in the interlayer gaps during the transfer of monolayers. Hydroxyl–OH groups and bound water molecules are well known for their role in ferroelectricity of many solid inorganic ferroelectrics, e.g. from the potassium hydro phosphate or the alum classes [14] because transfer of protons from one to another molecular site results in large permanent (but field switchable) dipole moments of the materials. Probably in our case, the dissociation or evaporation of water molecules at a high temperature are responsible for disappearance of permanent polarization and degradation of
ferroelectric properties of the films. Evidently, further investigations are needed to explain a mechanism of the observed phenomena. In conclusion, many specific features of the ferroelectric behaviour are observed for thin Langmuir–Blodgett films made of TDOBAMBCC chiral smectic LC. At elevated temperatures, at which the bulk of TDOBAMBCC would be in the liquid crystal phase, the thin films are still in a solid crystalline phase that can be attributed to very strong confinement of TDOBAMBCC molecules in sandwich structures. The switched polarization measured for a film of 30 nm thick is of Psw ~ 1–3 mC/m2 depending on temperature. That leads to pyroelectric coefficient γ = 40–20 μC/m2 K in the temperature range of 20–110 °C. Usage of LC compounds expands a variety of materials for preparing thin ferroelectric films by Langmuir–Blodgett technology. Acknowledgement We acknowledge the financial support from the Russian Fund for Basic Research (grants no. 0402-16466). References [1] L.M. Blinov, V.G. Chigrinov, Electro-Optic Effects in Liquid Crystal Materials, Springer Verlag, New York, 1994. [2] S.T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals, WileyVCH, Weinheim, 1999. [3] S. Pfeiffer, R. Shashidhar, T.L. Fare, J. Nacri, J. Adams, R.S. Duran, Appl. Phys. Lett. 63 (1993) 1285. [4] S. Okazaki, S. Uto, K. Yoshino, K. Skarp, B. Helgee, Appl. Phys. Lett. 71 (1997) 3373. [5] C.Y. Young, R. Pindak, N.A. Clark, R.B. Meyer, Phys. Rev. Lett. 40 (1978) 773. [6] S.G. Yudin, L.M. Blinov, N.N. Petukhova, S.P. Palto, JETP Lett. 70 (1999) 633. [7] L.M. Blinov, A.R. Geivandov, V.V. Lazarev, S.P. Palto, S.G. Yudin, G. Pelzl, W. Weissflog, Appl. Phys. Lett. 87 (2005) 241913. [8] G. Durand, Ph. Martinot-Lagarde, Ferroelectrics 24 (1980) 89. [9] Langmuir, V.J. Shaefer, J. Am. Chem. Soc. 60 (1938) 1351. [10] L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin, Thin Solid Films 284–285 (1996) 469. [11] N.M. Shtykov, M.I. Barnik, S.P. Palto, L.M. Blinov, G. Pelzl, W. Weissflog, JETP 121 (2002) 739. [12] L.A. Beresnev, L.M. Blinov, Zh.Vses. Khim. Obshch. 28 (1983) 149 (in Russ.). [13] S. Ducharme, S.P. Palto, V.M. Fridkin, in: H.S. Nalwa (Ed.), Handbook of Thin Films Materials, Ferroelectric and Dielectric Thin Films, chapter 11, vol. 3, Academic Press, 2002, p. 1. [14] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectric and related Materials, Clarendon Press, Oxford, 1977.