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Investigation of refractive index dispersion and electrical properties in carbon nano-balls’ doped nematic liquid crystals
ARTICLE IN PRESS
Physica B 362 (2005) 180–186 www.elsevier.com/locate/physb
Investigation of refractive index dispersion and electrical properties in carbon nano-balls’ doped nematic liquid crystals Mustafa Okutana,, S. Eren Sana, Og˘uz Ko¨ysala, Fahrettin Yakuphanoglub a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Turkey b Department of Physics, Firat University, 23169 Elazig, Turkey
Received 20 October 2004; received in revised form 1 February 2005; accepted 7 February 2005
Abstract Electrical and refractive index dispersion properties of a carbon nano-balls’ (Fullerene C60) doped Liquid crystal (LC) composite, are investigated by concerning the frequency-dependent dielectrical properties, laser-induced effects on current–voltage (I2V ) characteristics and optical characteristics suggest that the samples exhibit voltage-controlled differential negative resistance behavior. Dielectric spectra reflect the presence of different relaxation modes at higher frequencies. The refractive index dispersion parameters (oscillator energy and strength, optical moments) were also determined by optical characterization. As a result, C60 doping changes photoconductivity, dielectric and refractive index properties of LC in a favorable manner. r 2005 Elsevier B.V. All rights reserved. PACS: 61.30.Vx; 77.84.Nh; 64.70.Md Keywords: Liquid crystal; Fullerene (C60); Current–voltage; Impedance spectroscopy
1. Introduction Liquid crystals (LCs) are highly nonlinear optical materials due to their susceptible property activating under even relatively low optical fields. Several nonlinear mechanisms investigated so far have revealed the promising characters of these Corresponding author. Tel.: +902626538497;
fax: +902626538490. E-mail address: [email protected] (M. Okutan).
materials. The difference in refractive indices, for fields polarized along, and perpendicular to, the director axis brings about a large birefringence property from visible to infrared spectral regime. This property is an opportunity for various applications [1]. Director axis reorientation based effects causing the change of refractive index and observations of several interesting dynamic and storage wave-mixing effects have been extensively studied so far [1–5]. Compared with others, LCbased systems require lower characteristic voltages
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.02.009
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
to be applied for the realization of molecular gratings and relatively lower light power for efficient modulation of refractive index. Fullerene doped LC systems were extensively studied due to their positive effects in possible technological applications [6,7]. Photorefractive like reorientation is a famous molecular mechanism in which photo excited dopants bring about the formation of space-charge complexes and their induced fields force the molecules to reorient by enhancing the photoconductivity. Effect of fullerene on reorientation process is explained by this approach indeed [6–8]. Molecular orientation of LC molecules determine the electro-optical behavior of the system and because laser molecule interaction causes molecules to reorient in these systems, our aim was to examine electro optical measurements so that we could demonstrate the molecular reorientation-based changes in capacitance, impedance and dielectric coefficients and refractive index dispersion. Such works have been performed on dye doped and polymer doped LC previously [9–11].
2. Experimental Measurement cells were made up of two glass slides separated by Mylar sheets having 8.5 mm thicknesses. Before the construction of the cells, Indium tin oxide (ITO)-coated glass substrates were spin coated with polyvinyl alcohol (PVA) at 2000 rpm and they were cured at 50 1C for 2 h. The thickness of the coating is 100 nm and these coating layers were exposed to surface treatment of unidirectional rubbing with velvet in order to obtain preliminary molecular orientation. The ultimate form of the constructed cell is planar with 21 rubbing tilt. Fullerene (C60) was dissolved within toluene and toluene fraction was evaporated to eliminate fullerene powders, which later mixed to LC under the reinforcement of ultrasonic effect. Chemical formulas of fullerene and nematic host are depicted in Fig. 1. Experimental setup is shown in Fig. 2a. Agilent 4396 B 100 kHz–1.8 GHz Network/Spectrum/Impedance Analyzer was used in the measurements
51%
C5H11
181
CN
25%
C7H15
CN
16%
OC8H17
CN
8%
C5H11
CN
(a)
(b) Fig. 1. Chemical formulas of: (a) nematic host, E7; (b) fullerene, C60.
which are performed at room temperature with a high accuracy (1% typ.) and Source Meter (Keithley Model 2400) was used alternatively during the measurements. During the I–V measurements an incoming laser beam, 20 mW He–Cd l ¼ 441:6 nm; was employed and it was arranged to cover the whole surface of the sample cell by a diverging lens and design of an opaque masking permitted to precisely illuminate only the filled portion, which is roughly at the order of 16 16 mm2. Polarization of laser is parallel to what it is called as director axis n, along which the overall orientation of molecules take place with minor fluctuations and this polarization is convenient for laser molecule interaction. The transmittance spectra of the LC cells were measured with the experimental configuration depicted in Fig. 2b. The light from a 100 W halogen lamp was used as input to the monochromator (JY Triax 550, focal length ¼ 0.5 m, wavelength positioning accuracy 10–3 nm). The output slit width of the monochromator was set to
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
182
9
M
8
He-Cd LASER
E7 dark E7 20mW
7
I (µ Amp)
6
SM
IA DL
4 3 2
PC M
5
1
S
0
(a)
–1 0
DS
Computer
2
4
6
8
(a)
10
12
14
16
18
20
12
14
16
18
20
V (Volt) 800 700
L
Monochromator
500
LC
P
P
I (n Amp)
PD
(b)
E7/C60 Dark E7/C60 20 mW
600
100 0 –100 –200 0
(b)
3. Results and discussion 3.1. The current (I)– voltage (V) characteristics I–V plots of samples, under dark and illuminated circumstances, are given in Fig. 3(a) and (b).
300 200
Fig. 2. (a) Experimental set up for electro optical measurements. M: Mirror, DL: Diverging lens, S: Sample, IA: Impedance analyzer, SM: Source meter, (b) Optical arrangement of the experimental setup. L; lamp, P; polarizer, DS; analog to digital converter, PD; photodedector, LC; LC cell.
0.2 mm to reduce the spectral bandwidth of incident light that impinges onto the LC. As the spectral dispersion of the monochromator with 1200 lines/mm grating is 1 nm/mm, the temporal coherence length of incident light is E1.5 mm for this slit width. Monochromatic light is focused onto the LC by a lens and transmitted light passing through this LC was converted to electrical signal in silicon photodetector (PD). The analog electric signal was then converted to digital signal by analog to digital converter (DS) whereby transmittance data were collected.
400
2
4
6
8
10
V (Volt)
Fig. 3. Current–voltage (I2V ) plot of samples under dark and illuminated circumstances of: (a) nematic host, E7; (b) fullerene, C60 doped LC nematic host.
In the plots, the current increases sharply at low voltage until it reaches a maximum and afterwards, the current decreases to a minimum. After this minimum, the current continues to rise linearly with increasing voltage. The value of voltage at turn over points is in the range of 0–2.5 V. A current peak in figures is observed. The maximum peak positions of E7 and C60 doped samples take place at different current values. The current values of the peak positions for E7 and C60 doped samples are 1.05 mA (for dark) 1.05 mA (for 20 mW) and 257.92 nA (for dark), 286.29 nA (for 20 mW), respectively. The below and above the current peak, I–V plots show two different regions, when a positive voltage is applied to the samples.
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
3.2. Dielectric spectroscopy study Fig. 4 shows the impedance of the samples as a function of frequency. The complex impedance for the LC is described as Z ¼ R þ iX ,
(1)
where R is the real part, i.e. resistance and X is the imaginary part, i.e., reactance. The impedance spectrum of the samples shows different relaxation peaks. The position of the peak changes with C60 doping. The complex dielectric constant of the LC
400
E7 E7/C60
300
IZI (Ω)
The current peak suggests that the samples exhibit voltage-controlled differential negative resistance (VCNR) behavior [12]. At the higher voltages region, 46 V, the current for the samples indicates an ohmic behavior. As seen in figures, the illuminated current of C60 doped LC composite is higher than that of its dark current. This suggests that the doping of C60 into LC has an important effect on the photoconductivity, i.e. laser illumination increases the photoconductivity of C60 doped LC. This difference between photoconductivity and dark conductivity might arise from the molecular alignment of the C60 into LC. On the other hand, the illuminated current LC is smaller than that of its dark current. Dark current or conductivity in LC cells is ionic [13]. It is evaluated that the photocurrent is limited by residual ionic space charge, which shields the charge carriers photoexcited by illumination and prevent them from contributing in current and therefore, photocurrent decreases. The reason for reduction of current in C60 doped sample can be explained as; the current in the C60 doped sample is probably affected by dopant concentration. Because, the tunneling transport of carrier charge depends on the polarization energy and the distance between C60 molecules. The negative resistance effect of C60 doped sample is clear more than that of E7 sample and the negative resistance appears to be caused by tunneling transport of carrier charges. Therefore, it is evident that C60 dopant does not provide the species producing the current charge carriers. This is a reason for the reduction of current in the C60 doped sample.
183
200
100
0 200.0M
400.0M
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 4. The variation of the impedance with frequency.
is described as [14] ¼ 0 i00 ,
(2)
where 0 is the real and 00 is the imaginary parts of the dielectric constant. The real part is expressed as 0 ¼ C
d , o A
(3)
where d is the thickness, A the effective area, o the permittivity of free space and the imaginary part is expressed as 00 ¼ 0 tan d,
(4)
where the loss tangent of the LC as dielectric material is denoted as tan d. The frequency dependence of the real and imaginary parts of the dielectric constant is shown in Fig. 5(a) and (b). The relaxation peaks are observed at different frequencies. C60 dopant affects the maximum peak positions and intensities. It is seen that the real part of the dielectric constant shows a slight decrease by doping of C60. Fig. 6 shows the frequency dependence of the dielectric loss. The dielectric loss attains a maxima (f1 ¼ 360 MHz). After this maximum, at higher frequencies, other peaks are observed at about 372 MHz and 720 MHz and these peaks reflect the presence of different relaxation modes at higher frequencies. These are due to the collective motions of the molecules in the liquid crystalline phase. It is
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
184 0.025
ε'
0.020
0.015
E7 E7/C60
0.010
0.005 200.0M
400.0M
(a)
600.0M
800.0M
1.0G
Frequency (Hz) 0.020 E7 E7/C60
ε"
0.015
0.010
0.005
3.3. Refractive index dispersion analysis
0.000 200.0M
400.0M
(b)
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 5. The variation of real and imaginary parts of the complex dielectric constant.
2.0
1.5
tan (δ)
are, respectively, due to the rotation of molecules and the vibrational motion of the molecules. The relaxation strength for the f 2 and f 3 peaks decreases with increasing frequency. The decrease in relaxation strength can be explained as; the oriented LC molecules tend to form interacting pairs, thereby tending to cancel each moment. The decrease of the relaxation strength for the peaks f 2 and f 3 is partly caused by this effect [15]. The intensity of the first relaxation peak in the nematic phase increases with C60 dopant, while it decreases for the other peaks. The position of the first relaxation peak shifts also towards lower frequency. But, the peak position of the other peaks does almost not change with frequency and C60 dopant. There are two main polar groups –CN or OCnHn+1. These groups can contribute the permanent dipole moment. The direction of this moment may not correspond to the molecular long axis. Therefore, in figures different dispersion regions should appear.
T¼
E7 E7/C60 f1
ð1 RÞ2 ead , 1 R2 e2ad
(5)
where a is the absorption coefficient, d is the thickness of sample and R the reflectance, which is expressed as [15]
1.0 f3
f2
R¼
0.5
0.0 200.0M
The transmittance T and reflectance R of the samples are shown in Fig. 7(a) and (b). The transmittance and reflectance can be expressed by the following relationships [16]:
400.0M
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 6. Tan d vs. frequency plots of the samples.
evaluated that the first relaxation peak results from the interfacial polarization. The last two relaxation peaks denoted by f 2 and f 3 frequencies
ðn 1Þ2 þ k2 , ðn þ 1Þ2 þ k2
(6)
where n is the refractive index and k is extinction coefficient (k ¼ ad=4p). The variation of the refractive index of the samples with wavelength is shown in Fig. 8. The behavior of these curves is characteristic for the electronic transitions. A maximum in n curve of the E7 sample is observed and it corresponds to optical band-to-band transition between extrema of valence and conduction band. The refractive index dispersion of the
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
Transmittance (%)
where n is the refractive index, and Eo is the average excitation energy for electronic transitions and Ed is the dispersion energy which is a measure of the strength of interband optical transitions. This model describes the dielectric response for transitions below the optical gap. In order to determine the oscillator energy and strength, we used Eq. (7). Plotting 1=ðn2 1Þ as a function of ðhnÞ2 results in a straight line having the slope ðE o E d Þ1 ; and intercept with the vertical axis, E o =E d (Fig. 9). The obtained Eo and Ed values are given Table 1. These values are in good agreement with those obtained by Wemple [18]. It is seen that the values obtained for E7 is smaller than that of C60 doped E7 sample. The different values may be attributed to the C60 doping. It is reasonable to surmise that the oscillator parameters are strongly influenced by the doping. The aspect of refractive index for the samples corresponds to normal dispersion and can be described by the following equation; the singleterm Sellmeir oscillator model was used as [18] 2 ðn21 1Þ lo ¼1 , (8) 2 ðn 1Þ l
E7 E7/C60
65
64
63 400
450
500
(a)
550
600
650
700
Wavelength (nm) 37
Reflectance (%)
E7 E7/C60
36
35
400
450
500
(b)
550
600
650
700
Wavelength (nm)
Fig. 7. The transmittance and reflectance spectra of the samples. 2.55
E7 E7/C60
n
2.52
where 1; is the high-frequency dielectric constant, lo is the average oscillator wavelength. 1; and lo values were obtained from the linear parts of 1=ðn2 1Þ vs. l2 and are given in Table 1. Another form of the relation (8) is given by the following: n2 1 ¼
So l2o , 1 l2o =l2
(9)
2.49
0.21
2.46
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 8. The variation of the refractive index of the samples with wavelength.
samples can be expressed as [17] EdEo , n ¼1þ 2 Eo E2
0.19
0.18 3.0
(7)
E7 E7/C60
0.20
1/(n2–1)
2.43 350
2
185
3.5
4.0
(hv)2 (eV)2
Fig. 9. The 1=n2 1 vs. hn2 plots of the samples.
4.5
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
186 Table 1 The optical parameters of the samples Sample
E o (eV)
E d (eV)
n1;
lo (nm)
E o =So (eV m2)
So (m2)
M1
M3 (eV)2
E7 E7/C60
4.43 4.79
18.62 20.49
5.20 5.27
279.9 258.8
1.86 1014 1.56 1014
5.63 1013 6.39 1013
4.20 4.27
0.21 0.18
where So is the average oscillator parameter which is the strength of the individual dipole oscillator. The S o values for the samples were calculated using Eq. (9) and are given in Table 1. It is seen that So values increase with dopant concentration. The M 1 and M 3 moments of the optical spectra can be obtained from the relationship [19] E 2o ¼
M 1 M3 ; E 2d ¼ 1 , M 3 M 3
(10)
The obtained values are given in Table 1. It is seen that the values of M 1 increase by C60 doping, while M 3 values decrease. The moments are measure of the average bond strength. Eq. (10) indicates a single oscillator approximation to the dielectric response of these materials. The optical moments are related to the macroscopic quantities like effective dielectric constant, effective number of valence electrons in materials investigated [20].
4. Conclusions Electrical and refractive index dispersion properties of a carbon nano-balls’ (Fullerene C60) doped LC composite, are investigated by concerning the frequency-dependent dielectrical properties, laser-induced effects on current–voltage characteristics and optical characterization. The C60 doping changes photoconductivity, dielectric and refractive index properties of LC.
References [1] I.C. Khoo, M. Shih, M.V. Wood, B.D. Guenther, P.H. Chen, F. Simoni, S.S. Slussarenko, O. Francescangeli, L. Lucchetti, Proc. IEEE 87 (11) (1999) 1897. [2] S. Bartkiewicz, A. Januszko, A. Miniewicz, J. Parka, Pure Appl. Opt. 5 (1996) 799. [3] S.E. San, O. Ko¨ysal, F.N. Ecevit, Opt. Commun. 212 (4–6) (2002) 405. [4] Y.J. Wang, G.O. Carlisle, J. Mater. Sci.: Mater. Elect. 13 (2002) 73. [5] A. Miniewicz, S. Bartkiewicz, J. Parka, Opt. Commun. 149 (1998) 89. [6] W. Lee, H. Chen, S. Yeh, Opt. Exp. 10 (11) (2002) 482. [7] W. Lee, C. Chiu, Opt. Lett. 26 (8) (2001) 521. [8] I.C. Khoo, M.Y. Shih, A. Shishido, P.H. Chen, M.V. Wood, Opt. Mater. 18 (2001) 85. [9] M. Okutan, O. Ko¨ysal, S.E. San, Displays 24 (2003) 81. [10] S. Eren San, M. Okutan, O. Ko¨ysal, H. Ono, N. Kawatsuki, Opti. Commun. 238 (2004) 79–84. [11] M. Okutan, S.E. San, O. Ko¨ysal, Dyes Pigments 65 (2005) 169. [12] A.Y. Sergei, S. Orlov, G.A. Rakulijic, J. Opt. Soc. Am. B 13 (1996) 513. [13] L.M. Blinov, V.G. Chigrinov, Electro-optical Effects in Liquids Crystals, Springer, New York, 1996. [14] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. [15] H. Kresse, et al., Phys. Stat. Sol. 106 (1988) 89. [16] T.S. Moss, Semiconductor Optoelectronics, Butterworths, London, 1973. [17] M. DiDomenico, S.H. Wemple, J. Appl. Phys. 40 (1969) 720. [18] N.A. Subrahamanyam, A textbook of Optics, ninth ed, Brj Laboratory, Delhi, India, 1977. [19] S.H. Wimple, M. DiDomenico, Phys. Rev. Lett. 23 (1969) 1156. [20] L. Ley, in: J.D. Joannopolos, G. Lucovsky (Eds.), The Physics of Hydrogenated Amorphous Silicon II, Springer, Berlin, 1984.
Physica B 362 (2005) 180–186 www.elsevier.com/locate/physb
Investigation of refractive index dispersion and electrical properties in carbon nano-balls’ doped nematic liquid crystals Mustafa Okutana,, S. Eren Sana, Og˘uz Ko¨ysala, Fahrettin Yakuphanoglub a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Turkey b Department of Physics, Firat University, 23169 Elazig, Turkey
Received 20 October 2004; received in revised form 1 February 2005; accepted 7 February 2005
Abstract Electrical and refractive index dispersion properties of a carbon nano-balls’ (Fullerene C60) doped Liquid crystal (LC) composite, are investigated by concerning the frequency-dependent dielectrical properties, laser-induced effects on current–voltage (I2V ) characteristics and optical characteristics suggest that the samples exhibit voltage-controlled differential negative resistance behavior. Dielectric spectra reflect the presence of different relaxation modes at higher frequencies. The refractive index dispersion parameters (oscillator energy and strength, optical moments) were also determined by optical characterization. As a result, C60 doping changes photoconductivity, dielectric and refractive index properties of LC in a favorable manner. r 2005 Elsevier B.V. All rights reserved. PACS: 61.30.Vx; 77.84.Nh; 64.70.Md Keywords: Liquid crystal; Fullerene (C60); Current–voltage; Impedance spectroscopy
1. Introduction Liquid crystals (LCs) are highly nonlinear optical materials due to their susceptible property activating under even relatively low optical fields. Several nonlinear mechanisms investigated so far have revealed the promising characters of these Corresponding author. Tel.: +902626538497;
fax: +902626538490. E-mail address: [email protected] (M. Okutan).
materials. The difference in refractive indices, for fields polarized along, and perpendicular to, the director axis brings about a large birefringence property from visible to infrared spectral regime. This property is an opportunity for various applications [1]. Director axis reorientation based effects causing the change of refractive index and observations of several interesting dynamic and storage wave-mixing effects have been extensively studied so far [1–5]. Compared with others, LCbased systems require lower characteristic voltages
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.02.009
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
to be applied for the realization of molecular gratings and relatively lower light power for efficient modulation of refractive index. Fullerene doped LC systems were extensively studied due to their positive effects in possible technological applications [6,7]. Photorefractive like reorientation is a famous molecular mechanism in which photo excited dopants bring about the formation of space-charge complexes and their induced fields force the molecules to reorient by enhancing the photoconductivity. Effect of fullerene on reorientation process is explained by this approach indeed [6–8]. Molecular orientation of LC molecules determine the electro-optical behavior of the system and because laser molecule interaction causes molecules to reorient in these systems, our aim was to examine electro optical measurements so that we could demonstrate the molecular reorientation-based changes in capacitance, impedance and dielectric coefficients and refractive index dispersion. Such works have been performed on dye doped and polymer doped LC previously [9–11].
2. Experimental Measurement cells were made up of two glass slides separated by Mylar sheets having 8.5 mm thicknesses. Before the construction of the cells, Indium tin oxide (ITO)-coated glass substrates were spin coated with polyvinyl alcohol (PVA) at 2000 rpm and they were cured at 50 1C for 2 h. The thickness of the coating is 100 nm and these coating layers were exposed to surface treatment of unidirectional rubbing with velvet in order to obtain preliminary molecular orientation. The ultimate form of the constructed cell is planar with 21 rubbing tilt. Fullerene (C60) was dissolved within toluene and toluene fraction was evaporated to eliminate fullerene powders, which later mixed to LC under the reinforcement of ultrasonic effect. Chemical formulas of fullerene and nematic host are depicted in Fig. 1. Experimental setup is shown in Fig. 2a. Agilent 4396 B 100 kHz–1.8 GHz Network/Spectrum/Impedance Analyzer was used in the measurements
51%
C5H11
181
CN
25%
C7H15
CN
16%
OC8H17
CN
8%
C5H11
CN
(a)
(b) Fig. 1. Chemical formulas of: (a) nematic host, E7; (b) fullerene, C60.
which are performed at room temperature with a high accuracy (1% typ.) and Source Meter (Keithley Model 2400) was used alternatively during the measurements. During the I–V measurements an incoming laser beam, 20 mW He–Cd l ¼ 441:6 nm; was employed and it was arranged to cover the whole surface of the sample cell by a diverging lens and design of an opaque masking permitted to precisely illuminate only the filled portion, which is roughly at the order of 16 16 mm2. Polarization of laser is parallel to what it is called as director axis n, along which the overall orientation of molecules take place with minor fluctuations and this polarization is convenient for laser molecule interaction. The transmittance spectra of the LC cells were measured with the experimental configuration depicted in Fig. 2b. The light from a 100 W halogen lamp was used as input to the monochromator (JY Triax 550, focal length ¼ 0.5 m, wavelength positioning accuracy 10–3 nm). The output slit width of the monochromator was set to
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
182
9
M
8
He-Cd LASER
E7 dark E7 20mW
7
I (µ Amp)
6
SM
IA DL
4 3 2
PC M
5
1
S
0
(a)
–1 0
DS
Computer
2
4
6
8
(a)
10
12
14
16
18
20
12
14
16
18
20
V (Volt) 800 700
L
Monochromator
500
LC
P
P
I (n Amp)
PD
(b)
E7/C60 Dark E7/C60 20 mW
600
100 0 –100 –200 0
(b)
3. Results and discussion 3.1. The current (I)– voltage (V) characteristics I–V plots of samples, under dark and illuminated circumstances, are given in Fig. 3(a) and (b).
300 200
Fig. 2. (a) Experimental set up for electro optical measurements. M: Mirror, DL: Diverging lens, S: Sample, IA: Impedance analyzer, SM: Source meter, (b) Optical arrangement of the experimental setup. L; lamp, P; polarizer, DS; analog to digital converter, PD; photodedector, LC; LC cell.
0.2 mm to reduce the spectral bandwidth of incident light that impinges onto the LC. As the spectral dispersion of the monochromator with 1200 lines/mm grating is 1 nm/mm, the temporal coherence length of incident light is E1.5 mm for this slit width. Monochromatic light is focused onto the LC by a lens and transmitted light passing through this LC was converted to electrical signal in silicon photodetector (PD). The analog electric signal was then converted to digital signal by analog to digital converter (DS) whereby transmittance data were collected.
400
2
4
6
8
10
V (Volt)
Fig. 3. Current–voltage (I2V ) plot of samples under dark and illuminated circumstances of: (a) nematic host, E7; (b) fullerene, C60 doped LC nematic host.
In the plots, the current increases sharply at low voltage until it reaches a maximum and afterwards, the current decreases to a minimum. After this minimum, the current continues to rise linearly with increasing voltage. The value of voltage at turn over points is in the range of 0–2.5 V. A current peak in figures is observed. The maximum peak positions of E7 and C60 doped samples take place at different current values. The current values of the peak positions for E7 and C60 doped samples are 1.05 mA (for dark) 1.05 mA (for 20 mW) and 257.92 nA (for dark), 286.29 nA (for 20 mW), respectively. The below and above the current peak, I–V plots show two different regions, when a positive voltage is applied to the samples.
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
3.2. Dielectric spectroscopy study Fig. 4 shows the impedance of the samples as a function of frequency. The complex impedance for the LC is described as Z ¼ R þ iX ,
(1)
where R is the real part, i.e. resistance and X is the imaginary part, i.e., reactance. The impedance spectrum of the samples shows different relaxation peaks. The position of the peak changes with C60 doping. The complex dielectric constant of the LC
400
E7 E7/C60
300
IZI (Ω)
The current peak suggests that the samples exhibit voltage-controlled differential negative resistance (VCNR) behavior [12]. At the higher voltages region, 46 V, the current for the samples indicates an ohmic behavior. As seen in figures, the illuminated current of C60 doped LC composite is higher than that of its dark current. This suggests that the doping of C60 into LC has an important effect on the photoconductivity, i.e. laser illumination increases the photoconductivity of C60 doped LC. This difference between photoconductivity and dark conductivity might arise from the molecular alignment of the C60 into LC. On the other hand, the illuminated current LC is smaller than that of its dark current. Dark current or conductivity in LC cells is ionic [13]. It is evaluated that the photocurrent is limited by residual ionic space charge, which shields the charge carriers photoexcited by illumination and prevent them from contributing in current and therefore, photocurrent decreases. The reason for reduction of current in C60 doped sample can be explained as; the current in the C60 doped sample is probably affected by dopant concentration. Because, the tunneling transport of carrier charge depends on the polarization energy and the distance between C60 molecules. The negative resistance effect of C60 doped sample is clear more than that of E7 sample and the negative resistance appears to be caused by tunneling transport of carrier charges. Therefore, it is evident that C60 dopant does not provide the species producing the current charge carriers. This is a reason for the reduction of current in the C60 doped sample.
183
200
100
0 200.0M
400.0M
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 4. The variation of the impedance with frequency.
is described as [14] ¼ 0 i00 ,
(2)
where 0 is the real and 00 is the imaginary parts of the dielectric constant. The real part is expressed as 0 ¼ C
d , o A
(3)
where d is the thickness, A the effective area, o the permittivity of free space and the imaginary part is expressed as 00 ¼ 0 tan d,
(4)
where the loss tangent of the LC as dielectric material is denoted as tan d. The frequency dependence of the real and imaginary parts of the dielectric constant is shown in Fig. 5(a) and (b). The relaxation peaks are observed at different frequencies. C60 dopant affects the maximum peak positions and intensities. It is seen that the real part of the dielectric constant shows a slight decrease by doping of C60. Fig. 6 shows the frequency dependence of the dielectric loss. The dielectric loss attains a maxima (f1 ¼ 360 MHz). After this maximum, at higher frequencies, other peaks are observed at about 372 MHz and 720 MHz and these peaks reflect the presence of different relaxation modes at higher frequencies. These are due to the collective motions of the molecules in the liquid crystalline phase. It is
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
184 0.025
ε'
0.020
0.015
E7 E7/C60
0.010
0.005 200.0M
400.0M
(a)
600.0M
800.0M
1.0G
Frequency (Hz) 0.020 E7 E7/C60
ε"
0.015
0.010
0.005
3.3. Refractive index dispersion analysis
0.000 200.0M
400.0M
(b)
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 5. The variation of real and imaginary parts of the complex dielectric constant.
2.0
1.5
tan (δ)
are, respectively, due to the rotation of molecules and the vibrational motion of the molecules. The relaxation strength for the f 2 and f 3 peaks decreases with increasing frequency. The decrease in relaxation strength can be explained as; the oriented LC molecules tend to form interacting pairs, thereby tending to cancel each moment. The decrease of the relaxation strength for the peaks f 2 and f 3 is partly caused by this effect [15]. The intensity of the first relaxation peak in the nematic phase increases with C60 dopant, while it decreases for the other peaks. The position of the first relaxation peak shifts also towards lower frequency. But, the peak position of the other peaks does almost not change with frequency and C60 dopant. There are two main polar groups –CN or OCnHn+1. These groups can contribute the permanent dipole moment. The direction of this moment may not correspond to the molecular long axis. Therefore, in figures different dispersion regions should appear.
T¼
E7 E7/C60 f1
ð1 RÞ2 ead , 1 R2 e2ad
(5)
where a is the absorption coefficient, d is the thickness of sample and R the reflectance, which is expressed as [15]
1.0 f3
f2
R¼
0.5
0.0 200.0M
The transmittance T and reflectance R of the samples are shown in Fig. 7(a) and (b). The transmittance and reflectance can be expressed by the following relationships [16]:
400.0M
600.0M
800.0M
1.0G
Frequency (Hz)
Fig. 6. Tan d vs. frequency plots of the samples.
evaluated that the first relaxation peak results from the interfacial polarization. The last two relaxation peaks denoted by f 2 and f 3 frequencies
ðn 1Þ2 þ k2 , ðn þ 1Þ2 þ k2
(6)
where n is the refractive index and k is extinction coefficient (k ¼ ad=4p). The variation of the refractive index of the samples with wavelength is shown in Fig. 8. The behavior of these curves is characteristic for the electronic transitions. A maximum in n curve of the E7 sample is observed and it corresponds to optical band-to-band transition between extrema of valence and conduction band. The refractive index dispersion of the
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
Transmittance (%)
where n is the refractive index, and Eo is the average excitation energy for electronic transitions and Ed is the dispersion energy which is a measure of the strength of interband optical transitions. This model describes the dielectric response for transitions below the optical gap. In order to determine the oscillator energy and strength, we used Eq. (7). Plotting 1=ðn2 1Þ as a function of ðhnÞ2 results in a straight line having the slope ðE o E d Þ1 ; and intercept with the vertical axis, E o =E d (Fig. 9). The obtained Eo and Ed values are given Table 1. These values are in good agreement with those obtained by Wemple [18]. It is seen that the values obtained for E7 is smaller than that of C60 doped E7 sample. The different values may be attributed to the C60 doping. It is reasonable to surmise that the oscillator parameters are strongly influenced by the doping. The aspect of refractive index for the samples corresponds to normal dispersion and can be described by the following equation; the singleterm Sellmeir oscillator model was used as [18] 2 ðn21 1Þ lo ¼1 , (8) 2 ðn 1Þ l
E7 E7/C60
65
64
63 400
450
500
(a)
550
600
650
700
Wavelength (nm) 37
Reflectance (%)
E7 E7/C60
36
35
400
450
500
(b)
550
600
650
700
Wavelength (nm)
Fig. 7. The transmittance and reflectance spectra of the samples. 2.55
E7 E7/C60
n
2.52
where 1; is the high-frequency dielectric constant, lo is the average oscillator wavelength. 1; and lo values were obtained from the linear parts of 1=ðn2 1Þ vs. l2 and are given in Table 1. Another form of the relation (8) is given by the following: n2 1 ¼
So l2o , 1 l2o =l2
(9)
2.49
0.21
2.46
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 8. The variation of the refractive index of the samples with wavelength.
samples can be expressed as [17] EdEo , n ¼1þ 2 Eo E2
0.19
0.18 3.0
(7)
E7 E7/C60
0.20
1/(n2–1)
2.43 350
2
185
3.5
4.0
(hv)2 (eV)2
Fig. 9. The 1=n2 1 vs. hn2 plots of the samples.
4.5
ARTICLE IN PRESS M. Okutan et al. / Physica B 362 (2005) 180–186
186 Table 1 The optical parameters of the samples Sample
E o (eV)
E d (eV)
n1;
lo (nm)
E o =So (eV m2)
So (m2)
M1
M3 (eV)2
E7 E7/C60
4.43 4.79
18.62 20.49
5.20 5.27
279.9 258.8
1.86 1014 1.56 1014
5.63 1013 6.39 1013
4.20 4.27
0.21 0.18
where So is the average oscillator parameter which is the strength of the individual dipole oscillator. The S o values for the samples were calculated using Eq. (9) and are given in Table 1. It is seen that So values increase with dopant concentration. The M 1 and M 3 moments of the optical spectra can be obtained from the relationship [19] E 2o ¼
M 1 M3 ; E 2d ¼ 1 , M 3 M 3
(10)
The obtained values are given in Table 1. It is seen that the values of M 1 increase by C60 doping, while M 3 values decrease. The moments are measure of the average bond strength. Eq. (10) indicates a single oscillator approximation to the dielectric response of these materials. The optical moments are related to the macroscopic quantities like effective dielectric constant, effective number of valence electrons in materials investigated [20].
4. Conclusions Electrical and refractive index dispersion properties of a carbon nano-balls’ (Fullerene C60) doped LC composite, are investigated by concerning the frequency-dependent dielectrical properties, laser-induced effects on current–voltage characteristics and optical characterization. The C60 doping changes photoconductivity, dielectric and refractive index properties of LC.
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