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Effect of an azo dye (DR1) on the dielectric parameters of a nematic liquid crystal system
ARTICLE IN PRESS
Physica B 390 (2007) 101–105 www.elsevier.com/locate/physb
Effect of an azo dye (DR1) on the dielectric parameters of a nematic liquid crystal system S. O¨zdera, M. Okutanb,, O. Ko¨ysalb, H. Go¨ktas-c, S.E. Sanb Departement of Physics, C - anakkale Onsekiz Mart University, 17100 C - anakkale, Turkey Department of Physics, Gebze Institute of Technology, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey c Nuclear Research and Training Center, 06983 Saray, Ankara, Turkey a
b
Received 17 August 2005; received in revised form 22 May 2006; accepted 28 July 2006
Abstract The dielectric parameters and relaxation properties of azo dye (DR1) doped E7 and pure E7 liquid crystal (LC) have been investigated in a wide frequency range of 10 k–10 MHz through the dielectric spectroscopy method at room temperature. Dielectric anisotropy (De) property of the LC changes from the positive type to negative type and dielectric anisotropy values decrease with doping of DR1. The relaxation frequency fr of E7 and E7/DR1 LC was calculated by means of Cole–Cole plots. Influence of bias voltage on the dielectric parameters has also been investigated. r 2006 Elsevier B.V. All rights reserved. Keywords: Liquid crystals; Azo dye; Dielectric spectroscopy; Relaxation frequency
1. Introduction Organic molecules and liquid crystals (LC) are known to be favorable to photonic and nonlinear optical device applications because of their large optical nonlinearity and rapid optical response [1,2]. In a mixture of LC and dichroic dye, the collective orientation of the LC molecules under the action of an electric field influences that of the dye molecules. This phenomenon is called guest–host interaction. Azo dyes and their derivatives have been widely used as guest additives in condensed optical materials to develop novel optoelectronic devices. It had previously been found that the presence of a dichroic dye in a LC affects some properties of the pure host. Furthermore, influence of a dye on the dielectric properties of a liquid has been observed [3–5]. For example, a dye can change refractive indices and the order parameter of LCs. Molecular orientation of LC molecules determines the electro-optical behavior of the system and external effects may cause molecules to reorient by molecular interactions. Electro-optical measurements are useful such that researchCorresponding author. Fax: +90 262 6538490.
E-mail address: [email protected] (M. Okutan). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.073
ers could demonstrate molecular-reorientation-based changes in capacitance, impedance, dielectric coefficients, and refractive index dispersion. Such experiments have been previously performed on dye-doped and polymerdoped LC [6–8]. There are various works concentrating on the electrooptical characterization of LC [9–12]. The dielectricspectroscopy technique (DST) has been used by various workers for the study of systems in different phases. This method has been found to be one of the best for the measurement of permittivity and dielectric loss with high accuracy and sensivity [13,14]. The dielectric anisotropy is expressed as D ¼ jj ? , where jj and e? are the parallel and perpendicular components of the electric permittivity, respectively. This is because the critical-frequency (fc) rise alters the structure type from positive to negative. Regarding the dielectric constant, there are two structure types. One is named as positive dielectric anisotropy (p-type) and its dielectric constant along the director axis is larger than that along the axes perpendicular to the director; De is greater than zero in this case. The other type is named as negative dielectric anisotropy (n-type), where De is less than zero [15,16]. Variation of De with respect to the spot frequencies
ARTICLE IN PRESS S. O¨zder et al. / Physica B 390 (2007) 101–105
102
reveals that LC orientation has p-type property at low frequencies, and as the frequency increases the dielectric anisotropy character shifts to n-type. The measurement of the dielectric relaxation at different frequencies gives information about the dynamics of polar groups and about molecular motion. Materials containing azobenzene units have been intensively studied because they are promising systems for electro-optical applications such as holographic and digital storage, LC command surfaces, and nonlinear optical devices. Dielectric spectroscopy has been used to investigate the molecular orientation and motions in LC doping with azo dye. Azo dyes are stable and quite polar, having a large dipole moment. Thus, the dielectric permittivities can be sensitively influenced by the orientation motions of the dipoles. Due to the fact that these materials are very interesting and promising for practical applications, it can be suitable for fast electro-optical displays like switches, guest host display and many others. In this paper, we present experimental study of the dielectric anisotropy analysis in DR1 doped nematic LC E7. It was observed that doping LC with DR1 altered dielectric behavior and relaxation frequency fr of nematic LCs. We have also extracted dielectric anisotropy De and critical frequency fc value as well as some other key electrical parameter for pure E7 and E7/DR1 hybrid LC. 2. Experimental Before the construction of the cells, indium tin oxide (ITO) covered glass windows were treated with polyvinyl alcohol for planar alignment. Measurement cells were made up of two glass slides separated by mylar sheets having 6.2 mm thickness. These cells were filled in capillary action with the samples at room temperature. The material used was composed of nematic LC from Merck and DR1 powder (95%). Molecular structures of the samples’ components are depicted in Fig. 1. As seen, the DR1 molecule consists of [N-ethyl-N-2-ethylhydroxy]amino group as the electron donor group (D), and the nitro-electron acceptor group (A), located at the opposite ends of the conjugated chain azobenzene bridge (B). E7 is the mixture of four nematogens (51% K15, 25% K21, 16% M24, and 8% T15). Two samples were prepared; one of them contains pure E7, the other one was filled with E7/DR1 0.5% (w/w). The measurements were performed at room temperature by using HP 4192ALS impedance analyzer. 3. Results and discussion The complex dielectric constant of the LC- and DR1doped E7 is described by ðoÞ ¼ 0 ðoÞ i00 ðoÞ, 0
(1) 00
where e is the real and e the imaginary part of the dielectric constant. The spectrums of real and imaginary
CH3CH2
N
N
HOCH2CH2
(a)
D
N
B
NO2
A
51%
C5H11
CN
25%
C7H15
CN
16%
OC8H17
CN
8% (b)
C5H11
CN
Fig. 1. Molecular structures of the samples; (a) azo dye DR1 and (b) nematic host E7.
parts are respectively called dispersion and absorption curves. Figs. 2(a) and (b) show the real part of the dielectric constant of the E7 and DR1-doped E7 LCs as a function of frequency at different voltages, respectively. As seen in the figures, at a certain frequency value the real part of the dielectric constant e0 increases exponentially with increasing applied voltage. Afterwards, e0 almost does not change with applied voltage. The dielectric constant dependence of the voltage applied has valuable information about reorientation. It is seen in Figs. 2(a) and (b) that when there is no voltage applied, dielectric constant has a minimum value where molecules are in their original orientation. When the applied voltage is increased, orientation starts and as a result of this, the dielectric constant increases with applied voltage. Figs. 3(a) and (b) show the frequency dependence of the imaginary part of the dielectric constant at different voltages for the samples. We see that the loss is minimum without any bias voltage (0 V) but at 20 V dielectric loss is the highest. The imaginary dielectric constant attains a maximum at a critical frequency fc, which is influenced by an applied voltage. This maximum value corresponds to a dielectric relaxation peak, where a transition takes places from positive to negative dielectric anisotropy. The orientation configurations (pure E7 and E7/DR1) influence the position and intensity of the relaxation peak. The critical frequency fc values, for the pure E7 and E7/DR1 cells were determined from the position of the peaks, as shown in Figs. 3(a) and (b), and are given in Table 1. The position and the intensity of the peak shift to lower frequencies with increasing applied voltages. The relaxation phenomenon is characterized by Cole– Cole plot. These curves give useful information about the
ARTICLE IN PRESS S. O¨zder et al. / Physica B 390 (2007) 101–105
Fig. 2. Dielectric permittivity e0 as a function of frequency at different voltages for (a) E7 and (b) E7/DR1.
relaxation mechanism of the cells. Figs. 4(a) and (b) show the complex dielectric constant plots of e00 vs. e0 at different applied voltages. The plots indicate semi-circles and pass through the origin. The plots obey Cole–Cole type dispersion with a distribution of relaxation times [17]. The Cole–Cole plots show a semi-circle. The presence of the semicircle in the complex dielectric constant shows the monodispersive nature of the dielectric properties of the pure E7 and E7/DR1 LC samples. The complex dielectric dispersion curves are described by the Cole–Cole relation [18–21] ðoÞ ¼ 1 þ
o 1 , 1 þ ðiotÞ1a
(2)
where e*(o) is the complex dielectric constant, e0 the limiting low-frequency dielectric constant, eN the limiting high-frequency dielectric constant, t the average relaxation time, o the average angular frequency, and a the distribution parameter. If the centers of the semicircles lie on e0 axis, a is zero (Debye type) otherwise the center is
103
Fig. 3. Frequency dependence of dielectric loss e00 for (a) E7 and (b) E7/DR1.
Table 1 The critical frequency and low-frequency dielectric constant e0 values of E7 and E7/DR1 44 fc (Hz)
0V
5V
10 V
15 V
20 V
E7 E7/DR1
218547 131783
198747 98617
146440 98600
146440 65986
146440 65983
e0 E7 E7/DR1
10.6 11.23
15.83 14.90
23.19 23.24
25.11 25.86
25.9 26.67
00max E7 E7/DR1
4.43 4.52
6.81 5.78
11.02 11.00
11.69 12.44
12.13 12.88
below the e0 axis and a6¼0 (nonDebye type) [18]. The values of a were determined from the Cole–Cole plots. The plots and calculated values suggest that dielectric relaxation process is a Debye type relaxation behavior. This behavior is probably due to the dipolar rotation around the long
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Fig. 5. Plots of log10(v/u) vs. log10 (f) for (a) E7 and (b) E7/DR1. Fig. 4. Cole–Cole plots of the LCs at different voltages for (a) E7 and (b) E7/DR1.
molecular axis. The e0 and 00max values were calculated from the Cole–Cole curves and are given in Table 1. e0 values increase with applied voltage. This suggests that e0 values of the relaxation process are associated with the reorientation of molecules. The relaxation frequency fr can be evaluated by using the expression [17] v ¼ ðotÞ1a , (3) u where h i1=2 2 2 v ¼ ðo 0 ðoÞÞ þ ð00 ðoÞÞ , (4) h i1=2 2 2 , u ¼ ð0 ðoÞ 1 Þ þ ð00 ðoÞÞ
(5)
and e0 (o) is the dielectric permittivity at a particular frequency. A plot of log10 ðv=uÞ vs. log10 ð f Þ shown in Figs. 5(a) and (b) gives a straight line for E7 and E7/DR1 LCs samples at different voltages. The intercept on the abscissa
Table 2 Dielectric relaxation parameters of E7 and E7/DR1 LCs fr (Hz)
0V
5V
10 V
15 V
20 V
E7 E7/DR1
275642.6 133472.5
159320.8 95092.8
109377.0 60677.5
98019.5 53385.3
92367.9 51208.6
gives the magnitude of relaxation frequency and is given in Table 2. It is observed that the relaxation frequency decreases with increasing voltages for both samples. Furthermore, the relaxation frequency is affected by the addition of azo dye DR1. Doping of DR1 decreases the relaxation frequency depending on the bias voltage. Typical behavior of the relaxation frequency as a function of bias voltage for pure E7 and doping with azo dye E7 LC at room temperature is shown in Fig. 6. The relaxation frequency shows a monotonic decrease up to 10 V but remains stable beyond this bias voltage for E7/DR1.
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quency depending on the bias voltage. Thus, switching values decrease with application bias voltage. Dielectric anisotropy parameters of E7 and E7/DR1 LCs are given in Table 3. In the Cole–Cole plots of the samples, a semicircle was observed, indicating a monodispersive nature of the dielectric properties of the pure E7 and E7/DR1 samples. The goal of this work comes from the fact that determinations of dielectric properties provide an optimization basis for selecting proper LC materials and azo dyes for electro-optical devices. Also such measurements are valuable according to the reorientation mechanisms and thresholds. References
Fig. 6. Effect of bias voltage on the relaxation frequency of the LC samples; (a) E7 and (b) E7/DR1.
[1] [2] [3] [4] [5]
Table 3 Dielectric anisotropy parameters of E7 and E7/DR1 LCs
[6] [7]
De
10 kHz
100 kHz
1 MHZ
E7 E7/DR1
15.05 14.44
12.36 7.92
2.10 1.18
[8] [9] [10] [11] [12]
4. Conclusion Dielectric anisotropy and relaxation properties of azo dye DR1 in E7 nematic LC have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property and dielectric relaxation parameters change with DR1 dopant. The dielectric anisotropy of the samples changes from the positive type to the negative type, and the samples exhibit dielectrically controlled positive dielectric anisotropy (p-type De) with, at a critical frequency, a transition to negative dielectric anisotropy (n-type De) behavior. Doping the LC materials with azo dye changes the dielectric anisotropy and decreases the critical fre-
[13] [14] [15] [16] [17] [18] [19] [20] [21]
K.Y. Wong, et al., J. Chem. Phys. 100 (1994) 6818. B.W. Nie, Adv. Mater. 5 (1993) 520. D. Bauman, W. Haase, Mol. Cryst. Liq. Cryst. 168 (1989) 155. D. Bauman, Mol. Cryst. Liq. Cryst. 174 (1989) 1. D. Bauman, G. Czechowski, J. Jadz˙ yn, Acta Phys. Pol. A 75 (1989) 697. M. Okutan, O. Ko¨ysal, S.E. San, Displays 24 (2003) 81. S.E. San, M. Okutan, O. Ko¨ysal, H. Ono, N. Kawatsuki, Opt. Commun. 238 (2004) 79. M. Okutan, S.E. San, O. Ko¨ysal, Dyes Pigm. 65 (2005) 169. H. Allinson, H.F. Gleesson, Liq. Cryst. 19 (4) (1995) 421. G. Klimusheva, A. Koval’chuk, N. Volynets, A. Vakhnin, Issue Series Title: Opto-electron. Rev. 10 (1) (2002) 39. J. Parka, Opto-electron. Rev. 10 (1) (2002) 83. T. Grudniewski, J. Parka, R. Dabrowski, A. Januszko, A. Miniewicz, Opto-electron. Rev. 10 (1) (2002) 11. A.M. Biradar, D. Kilian, S. Wrobel, W. Haase, Liq. Cryst. 27 (2000) 225. G.K. Johri, K. Srivastava, M. Johri, K. Yoshino, Mol. Cryst. Liq. Cryst. 366 (2001) 673. H. Kawamoto, Proc. IEEE 90 (2002) 4. M. Okutan, S.E. San, O. Koysal, Dyes Pigm. 65 (2005) 169. R. Singh, V.K. Agarwal, K.K. Raina, B. Bahadur, Curr. Appl. Phys. 5 (6) (2005) 588. A.K. Johscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. K.S. Cole, R.H. Cole, J. Chem. Phys. 9 (1969) 341. I. Bunget, M. Popescu, Physics of Solid Dielectrics, Elsevier, Amsterdam, Oxford, New York, Tokyo, 1984. A. Chelkowski, Dielectric Physics, Elsevier, Amsterdam, Oxford, New York, Warszawa, 1980.
Physica B 390 (2007) 101–105 www.elsevier.com/locate/physb
Effect of an azo dye (DR1) on the dielectric parameters of a nematic liquid crystal system S. O¨zdera, M. Okutanb,, O. Ko¨ysalb, H. Go¨ktas-c, S.E. Sanb Departement of Physics, C - anakkale Onsekiz Mart University, 17100 C - anakkale, Turkey Department of Physics, Gebze Institute of Technology, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey c Nuclear Research and Training Center, 06983 Saray, Ankara, Turkey a
b
Received 17 August 2005; received in revised form 22 May 2006; accepted 28 July 2006
Abstract The dielectric parameters and relaxation properties of azo dye (DR1) doped E7 and pure E7 liquid crystal (LC) have been investigated in a wide frequency range of 10 k–10 MHz through the dielectric spectroscopy method at room temperature. Dielectric anisotropy (De) property of the LC changes from the positive type to negative type and dielectric anisotropy values decrease with doping of DR1. The relaxation frequency fr of E7 and E7/DR1 LC was calculated by means of Cole–Cole plots. Influence of bias voltage on the dielectric parameters has also been investigated. r 2006 Elsevier B.V. All rights reserved. Keywords: Liquid crystals; Azo dye; Dielectric spectroscopy; Relaxation frequency
1. Introduction Organic molecules and liquid crystals (LC) are known to be favorable to photonic and nonlinear optical device applications because of their large optical nonlinearity and rapid optical response [1,2]. In a mixture of LC and dichroic dye, the collective orientation of the LC molecules under the action of an electric field influences that of the dye molecules. This phenomenon is called guest–host interaction. Azo dyes and their derivatives have been widely used as guest additives in condensed optical materials to develop novel optoelectronic devices. It had previously been found that the presence of a dichroic dye in a LC affects some properties of the pure host. Furthermore, influence of a dye on the dielectric properties of a liquid has been observed [3–5]. For example, a dye can change refractive indices and the order parameter of LCs. Molecular orientation of LC molecules determines the electro-optical behavior of the system and external effects may cause molecules to reorient by molecular interactions. Electro-optical measurements are useful such that researchCorresponding author. Fax: +90 262 6538490.
E-mail address: [email protected] (M. Okutan). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.07.073
ers could demonstrate molecular-reorientation-based changes in capacitance, impedance, dielectric coefficients, and refractive index dispersion. Such experiments have been previously performed on dye-doped and polymerdoped LC [6–8]. There are various works concentrating on the electrooptical characterization of LC [9–12]. The dielectricspectroscopy technique (DST) has been used by various workers for the study of systems in different phases. This method has been found to be one of the best for the measurement of permittivity and dielectric loss with high accuracy and sensivity [13,14]. The dielectric anisotropy is expressed as D ¼ jj ? , where jj and e? are the parallel and perpendicular components of the electric permittivity, respectively. This is because the critical-frequency (fc) rise alters the structure type from positive to negative. Regarding the dielectric constant, there are two structure types. One is named as positive dielectric anisotropy (p-type) and its dielectric constant along the director axis is larger than that along the axes perpendicular to the director; De is greater than zero in this case. The other type is named as negative dielectric anisotropy (n-type), where De is less than zero [15,16]. Variation of De with respect to the spot frequencies
ARTICLE IN PRESS S. O¨zder et al. / Physica B 390 (2007) 101–105
102
reveals that LC orientation has p-type property at low frequencies, and as the frequency increases the dielectric anisotropy character shifts to n-type. The measurement of the dielectric relaxation at different frequencies gives information about the dynamics of polar groups and about molecular motion. Materials containing azobenzene units have been intensively studied because they are promising systems for electro-optical applications such as holographic and digital storage, LC command surfaces, and nonlinear optical devices. Dielectric spectroscopy has been used to investigate the molecular orientation and motions in LC doping with azo dye. Azo dyes are stable and quite polar, having a large dipole moment. Thus, the dielectric permittivities can be sensitively influenced by the orientation motions of the dipoles. Due to the fact that these materials are very interesting and promising for practical applications, it can be suitable for fast electro-optical displays like switches, guest host display and many others. In this paper, we present experimental study of the dielectric anisotropy analysis in DR1 doped nematic LC E7. It was observed that doping LC with DR1 altered dielectric behavior and relaxation frequency fr of nematic LCs. We have also extracted dielectric anisotropy De and critical frequency fc value as well as some other key electrical parameter for pure E7 and E7/DR1 hybrid LC. 2. Experimental Before the construction of the cells, indium tin oxide (ITO) covered glass windows were treated with polyvinyl alcohol for planar alignment. Measurement cells were made up of two glass slides separated by mylar sheets having 6.2 mm thickness. These cells were filled in capillary action with the samples at room temperature. The material used was composed of nematic LC from Merck and DR1 powder (95%). Molecular structures of the samples’ components are depicted in Fig. 1. As seen, the DR1 molecule consists of [N-ethyl-N-2-ethylhydroxy]amino group as the electron donor group (D), and the nitro-electron acceptor group (A), located at the opposite ends of the conjugated chain azobenzene bridge (B). E7 is the mixture of four nematogens (51% K15, 25% K21, 16% M24, and 8% T15). Two samples were prepared; one of them contains pure E7, the other one was filled with E7/DR1 0.5% (w/w). The measurements were performed at room temperature by using HP 4192ALS impedance analyzer. 3. Results and discussion The complex dielectric constant of the LC- and DR1doped E7 is described by ðoÞ ¼ 0 ðoÞ i00 ðoÞ, 0
(1) 00
where e is the real and e the imaginary part of the dielectric constant. The spectrums of real and imaginary
CH3CH2
N
N
HOCH2CH2
(a)
D
N
B
NO2
A
51%
C5H11
CN
25%
C7H15
CN
16%
OC8H17
CN
8% (b)
C5H11
CN
Fig. 1. Molecular structures of the samples; (a) azo dye DR1 and (b) nematic host E7.
parts are respectively called dispersion and absorption curves. Figs. 2(a) and (b) show the real part of the dielectric constant of the E7 and DR1-doped E7 LCs as a function of frequency at different voltages, respectively. As seen in the figures, at a certain frequency value the real part of the dielectric constant e0 increases exponentially with increasing applied voltage. Afterwards, e0 almost does not change with applied voltage. The dielectric constant dependence of the voltage applied has valuable information about reorientation. It is seen in Figs. 2(a) and (b) that when there is no voltage applied, dielectric constant has a minimum value where molecules are in their original orientation. When the applied voltage is increased, orientation starts and as a result of this, the dielectric constant increases with applied voltage. Figs. 3(a) and (b) show the frequency dependence of the imaginary part of the dielectric constant at different voltages for the samples. We see that the loss is minimum without any bias voltage (0 V) but at 20 V dielectric loss is the highest. The imaginary dielectric constant attains a maximum at a critical frequency fc, which is influenced by an applied voltage. This maximum value corresponds to a dielectric relaxation peak, where a transition takes places from positive to negative dielectric anisotropy. The orientation configurations (pure E7 and E7/DR1) influence the position and intensity of the relaxation peak. The critical frequency fc values, for the pure E7 and E7/DR1 cells were determined from the position of the peaks, as shown in Figs. 3(a) and (b), and are given in Table 1. The position and the intensity of the peak shift to lower frequencies with increasing applied voltages. The relaxation phenomenon is characterized by Cole– Cole plot. These curves give useful information about the
ARTICLE IN PRESS S. O¨zder et al. / Physica B 390 (2007) 101–105
Fig. 2. Dielectric permittivity e0 as a function of frequency at different voltages for (a) E7 and (b) E7/DR1.
relaxation mechanism of the cells. Figs. 4(a) and (b) show the complex dielectric constant plots of e00 vs. e0 at different applied voltages. The plots indicate semi-circles and pass through the origin. The plots obey Cole–Cole type dispersion with a distribution of relaxation times [17]. The Cole–Cole plots show a semi-circle. The presence of the semicircle in the complex dielectric constant shows the monodispersive nature of the dielectric properties of the pure E7 and E7/DR1 LC samples. The complex dielectric dispersion curves are described by the Cole–Cole relation [18–21] ðoÞ ¼ 1 þ
o 1 , 1 þ ðiotÞ1a
(2)
where e*(o) is the complex dielectric constant, e0 the limiting low-frequency dielectric constant, eN the limiting high-frequency dielectric constant, t the average relaxation time, o the average angular frequency, and a the distribution parameter. If the centers of the semicircles lie on e0 axis, a is zero (Debye type) otherwise the center is
103
Fig. 3. Frequency dependence of dielectric loss e00 for (a) E7 and (b) E7/DR1.
Table 1 The critical frequency and low-frequency dielectric constant e0 values of E7 and E7/DR1 44 fc (Hz)
0V
5V
10 V
15 V
20 V
E7 E7/DR1
218547 131783
198747 98617
146440 98600
146440 65986
146440 65983
e0 E7 E7/DR1
10.6 11.23
15.83 14.90
23.19 23.24
25.11 25.86
25.9 26.67
00max E7 E7/DR1
4.43 4.52
6.81 5.78
11.02 11.00
11.69 12.44
12.13 12.88
below the e0 axis and a6¼0 (nonDebye type) [18]. The values of a were determined from the Cole–Cole plots. The plots and calculated values suggest that dielectric relaxation process is a Debye type relaxation behavior. This behavior is probably due to the dipolar rotation around the long
ARTICLE IN PRESS 104
S. O¨zder et al. / Physica B 390 (2007) 101–105
Fig. 5. Plots of log10(v/u) vs. log10 (f) for (a) E7 and (b) E7/DR1. Fig. 4. Cole–Cole plots of the LCs at different voltages for (a) E7 and (b) E7/DR1.
molecular axis. The e0 and 00max values were calculated from the Cole–Cole curves and are given in Table 1. e0 values increase with applied voltage. This suggests that e0 values of the relaxation process are associated with the reorientation of molecules. The relaxation frequency fr can be evaluated by using the expression [17] v ¼ ðotÞ1a , (3) u where h i1=2 2 2 v ¼ ðo 0 ðoÞÞ þ ð00 ðoÞÞ , (4) h i1=2 2 2 , u ¼ ð0 ðoÞ 1 Þ þ ð00 ðoÞÞ
(5)
and e0 (o) is the dielectric permittivity at a particular frequency. A plot of log10 ðv=uÞ vs. log10 ð f Þ shown in Figs. 5(a) and (b) gives a straight line for E7 and E7/DR1 LCs samples at different voltages. The intercept on the abscissa
Table 2 Dielectric relaxation parameters of E7 and E7/DR1 LCs fr (Hz)
0V
5V
10 V
15 V
20 V
E7 E7/DR1
275642.6 133472.5
159320.8 95092.8
109377.0 60677.5
98019.5 53385.3
92367.9 51208.6
gives the magnitude of relaxation frequency and is given in Table 2. It is observed that the relaxation frequency decreases with increasing voltages for both samples. Furthermore, the relaxation frequency is affected by the addition of azo dye DR1. Doping of DR1 decreases the relaxation frequency depending on the bias voltage. Typical behavior of the relaxation frequency as a function of bias voltage for pure E7 and doping with azo dye E7 LC at room temperature is shown in Fig. 6. The relaxation frequency shows a monotonic decrease up to 10 V but remains stable beyond this bias voltage for E7/DR1.
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105
quency depending on the bias voltage. Thus, switching values decrease with application bias voltage. Dielectric anisotropy parameters of E7 and E7/DR1 LCs are given in Table 3. In the Cole–Cole plots of the samples, a semicircle was observed, indicating a monodispersive nature of the dielectric properties of the pure E7 and E7/DR1 samples. The goal of this work comes from the fact that determinations of dielectric properties provide an optimization basis for selecting proper LC materials and azo dyes for electro-optical devices. Also such measurements are valuable according to the reorientation mechanisms and thresholds. References
Fig. 6. Effect of bias voltage on the relaxation frequency of the LC samples; (a) E7 and (b) E7/DR1.
[1] [2] [3] [4] [5]
Table 3 Dielectric anisotropy parameters of E7 and E7/DR1 LCs
[6] [7]
De
10 kHz
100 kHz
1 MHZ
E7 E7/DR1
15.05 14.44
12.36 7.92
2.10 1.18
[8] [9] [10] [11] [12]
4. Conclusion Dielectric anisotropy and relaxation properties of azo dye DR1 in E7 nematic LC have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property and dielectric relaxation parameters change with DR1 dopant. The dielectric anisotropy of the samples changes from the positive type to the negative type, and the samples exhibit dielectrically controlled positive dielectric anisotropy (p-type De) with, at a critical frequency, a transition to negative dielectric anisotropy (n-type De) behavior. Doping the LC materials with azo dye changes the dielectric anisotropy and decreases the critical fre-
[13] [14] [15] [16] [17] [18] [19] [20] [21]
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