Influence of temperature and frequency on the electrical conductivity and the dielectric properties of nickel phthalocyanine

Organic Electronics 7 (2006) 261–270 www.elsevier.com/locate/orgel

Influence of temperature and frequency on the electrical conductivity and the dielectric properties of nickel phthalocyanine M.M. EL-Nahass *, A.F. EL-Deeb, F. Abd-El-Salam Physics Department, Faculty of Education, Ain Shams University, Heliopolies, Cairo 11757, Egypt Received 29 December 2005; received in revised form 2 March 2006; accepted 16 March 2006 Available online 18 April 2006

Abstract The temperature and frequency dependence of the AC conductivity rac(x), the dielectric constant e 0 (x) and the dielectric loss e00 (x) were studied on pellet samples of nickel phthalocyanine (NiPc) with evaporated ohmic Au electrodes in the frequency range from 20 kHz to 10 MHz and within the temperature range from 303 to 600 K. The DC conductivity rdc has also been measured in the considered range of temperature. Two temperatures – induced changes in the thermal activation energy DE have been observed. For T 6 435 K, DE1 = 0.322 eV; for 435 K 6 T 6 525 K, DE2 = 0.497 eV; for T P 525 K, DE3 = 0.703 eV. These variations in the activation energy were attributed to a partial phase transformation from a- to b-NiPc phase and as a change from extrinsic to intrinsic conduction mechanism. The AC conductivity rac(x) showed temperature independence and it has been found to vary with angular frequency as xs with the index s 6 1 suggesting a hopping conduction mechanism at low temperatures and high frequency. At higher temperatures and lower frequencies a free-band conduction mechanism was observed. Both the dielectric constant e 0 (x) and the dielectric loss e00 (x) increased with temperature and decreased with frequency in the investigated ranges. Such characteristics, reveal that the tested organic NiPc exists in the form of molecular dipoles which remain frozen at low temperature, whereas at higher temperatures, when the dipoles attaining rotational freedom, the dielectric constant was found to decrease with increasing frequency and increase with increasing temperature. The increase in the dielectric loss e00 (x) with increasing temperature at low frequencies can be understood in terms of an increase in DC conductivity.  2006 Elsevier B.V. All rights reserved. PACS: 72.20.i; 77.22.d Keywords: Organic semiconductor; Nickel phthalocyanine; Electrical conductivity; Dielectric properties

1. Introduction

*

Corresponding author. Tel.: +20 012 4168621. E-mail address: [email protected] (M.M. Nahass).

EL-

Phthalocyanines are aromatic compounds having semiconducting properties. Besides they are chemically stable and have dense colors; arising from an intense absorption in the UV–VIS region of spectra,

1566-1199/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.03.007

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that make them suitable to be employed as dyes and pigments in textile and paint industries. There are more than 70 different phthalocyanine complexes that can be obtained by replacing the two hydrogen atoms at the center of the molecular structure of metal-free phthalocyanine, H2Pc, molecule [1]. Phthalocyanine compounds may have the potential to be used as gas sensors [2–6], optical data storage systems [7], solar cells [8,9], light emitting diodes [10,11] and to generate various types of switching devices [12]. In addition, zinc phthalocyanine, ZnPc, was employed in some medical application due to its selective binding to tumor-selective antibodies, and it has been used in the synthesis of a novel compound appropriate in photodynamic therapy of cancer [13,14]. The DC electrical properties of phthalocyanines have received the greatest attention in both single crystal [15–17] and thin film forms [18–20]. The AC electrical properties of metal phthalocyanine have attracted several researchers in the last decades, such as cobalt phthalocyanine, CoPc [21–24], copper phthalocyanine, CuPc [25–27], zinc phthalocyanine, ZnPc [28,29], molybdenum phthalocyanine, MoPc [30] and nickel phthalocyanine, NiPc [26,28]. AC measurements in various phthalocyanines have generally shown an r a xs dependence for low temperatures and high frequencies, corresponding to hopping conduction. At higher temperatures and lower frequencies free – band conductivity with activation energy of a few tenths of an electron volt is fairly common [31]. In the present work, we report on AC electrical conductivity and the dielectric measurements of NiPc in pellets form with evaporated Au electrodes. Studies covered the frequency range from 20 kHz to 10 MHz and in the temperature range from 303 to 600 K. The DC conductivity has been measured in the same temperature range to determine the predominant electrical conduction mechanism in nickel phthalocyanine. 2. Experimental techniques The nickel phthalocyanine (NiPc) powder used in this study was obtained from Kodak, UK. The pellets of NiPc were prepared (diameter 1 cm; thickness 0.13 cm) by compressing the powder in a die under a pressure 1.96 · 108 N m2. The pellets were sandwiched between two evaporated gold electrodes which provide ohmic contacts to the phthalocyanines. The AC and DC conductivity measurements were made in air in the temperature range from

303 to 600 K, using a temperature controller (model TC-15 A) which can maintain constant temperature within ±0.5 K. The AC measurements were measured in the frequency range from 20 kHz to 10 MHz using a Hewlett–Packard (model 4275A) LCR bridge equipped with a three-terminal test fixture to avoid any stray capacitance and minimize the experimental error. The AC conductivity rac(x) was calculated by using the relation; rtot(x) = rac(x) + rdc, where rdc (DC conductivity) was measured using a Keithley electrometer (model 6512 programmable electrometer) in series with a standard regulated power supply. The dielectric constant e 0 (real part of the dielectric constant) was calculated using the relation: e 0 = Ct/e0a, where t is the disk thickness, C is the capacitance of the sample, a is the cross-sectional area and e0 is the permittivity of free space. The dielectric loss e00 (imaginary part of the dielectric constant) was calculated from the relation: e00 = e 0 tan d, where (d = 90  u), u is the phase angle. The experimental error during the measurements is ±2%.

3. Results and discussion 3.1. The frequency and temperature dependence of conductivity Fig. 1 demonstrates the measured total conductivity rtot(x) as a function of reciprocal temperature at various frequencies, along with the DC conductivity rdc of NiPc sample. This figure reveals that, frequency has a pronounced effect on rtot(x), while there is no effect of temperature on it except at high temperatures region. At low frequencies and high temperatures the values of rtot(x) and rdc approach each other. The DC curve shows an increase in the conductivity starting from 303 K up to 345 K, followed by a decrease in it with increasing temperature from 345 K to 370 K. Above 370 K the DC conductivity increases again with increasing the temperatures. This variation in the conductivity may be caused by the oxygen exhaustion from the sample. Similar behaviour has been observed in both thin films and pressed pellets of H2Pc [32,33], in a-CuPc thin films [34], in NiPc thin films [35] and in ZnPc thin films [36]. These workers have ascribed this behaviour to oxygen exhaustion too. Above 370 K, the change of the DC conductivity with temperature shows three regions associated with three activation energies separated by two transition temperatures,

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263

-4 -6 -8 -10

ln σtot

-12 -14 40 kHz 100 kHz 200 kHz 400 kHz 1 MHz 2 MHz 4 MHz 10 MHz

-16 -18

ln σdc

-20 -22 -24 1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

-1

1000/T (K ) Fig. 1. Temperature dependence of the measured total electrical conductivity rtot(x) at different frequencies and DC electrical conductivity for NiPc sample.

T1 = 435 K and T2 = 525 K. The activation energies obtained from the slopes of the three straight line segments using the well-known relationship [37]: rdc ¼ r0 expðDE=kT Þ;

ð1Þ

where rdc is the DC conductivity at temperature T, r0 is the pre-exponential factor, k is Boltzmann’s constant and DE is the thermal activation energy. For T 6 435 K, DE1 = 0.322 eV; for 435 K 6 T 6 525 K, DE2 = 0.497 eV; for T P 525 K, DE3 = 0.703 eV. Phthalocyanines exist in several crystalline polymorphs, including the a-, b- and c-structures [38]. Sharp [39] showed that in phthalocyanine compounds, the a-form is converted to the b-form by annealing in the temperature range of 473–573 K. Hence in Fig. 1, region 1 for T 6 435 K may represents the a-form having thermal activation energy DE1 = 0.322 eV; region 2 extending over the range 435 K 6 T 6 525 K may be related to the existing of preferential orientation in the a-form prior to the phase change associated with activation energy DE2 = 0.497 eV. On passing through the second transition temperature T P 525 K, we get region 3 which represents a smooth progression from the a-phase to the b-phase having thermal activation energy DE3 = 0.703 eV, while the transformation to b-NiPc phase is completed at 623 K [40]. The change in the activation energy could be interpreted as a change from extrinsic to intrinsic conduction too. The presence of extrinsic behaviour in phthalocya-

nine is attributed to the presence of energy states above the valence band, and act as donors’ levels. The presence of the energy states is confirmed by the SCLC measurements [16,40]. The intrinsic conduction probably results from the removal of states. Similar behaviour has also been observed in DC conductivity measurements of both NiPc single crystals [16] and thin films [35], which were considered to be due to oxygen exhaustion and phase transformation from a- to b-phase. Abdel-Malik et al. [16] attributed the variation of the activation energy in b-NiPc single crystals to a transition from an extrinsic to a non-extrinsic conduction mechanism in a partially compensated specimen. To further check the decrease in the DC conductivity in the temperature range from 345 to 370 K, we measured the conductivity of another NiPc sample and recycled the temperature of the sample between room temperature and 385 K. Fig. 2 shows the DC conductivity for temperature increasing up to 385 K (curve A), followed by the characteristics obtained with temperature decreasing down to room temperature (curve B). The disappearance of the conductivity peak during the decreasing temperature measurements can be attributed to the oxygen exhaustion from the sample during the heating cycle. To ensure that the peak is not due to a hysteretic effect or a phase transition, the sample was heated again to 385 K. No peak was observed in the second heating cycle, confirming the oxygen drain from the

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M.M. EL-Nahass et al. / Organic Electronics 7 (2006) 261–270 -19.8

Heating Cooling

-20.0 -20.2 -20.4

ln σdc

(A) -20.6 -20.8 -21.0

(B)

-21.2 -21.4 -21.6 2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

1000/T (K-1) Fig. 2. The DC conductivity dependence on inverse temperature for increasing temperature (curve A), and decreasing temperature (curve B) for NiPc sample.

-2 40 kHz 100 kHz 200 kHz 400 kHz 1 MHz 2 MHz 4 MHz 10 MHz

-4

ln σac

-6

-8

-10

-12

-14

-16 1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

-1

1000/T (K ) Fig. 3. Temperature dependence of AC conductivity rac(x) for NiPc at different frequencies.

sample. Therefore it is apparent that annealing or heating the sample to high temperature will stabilize the electrical properties due to oxygen exhaustion and structural changes [1,16,29,33–36,40]. Fig. 3 shows the AC conductivity rac(x), obtained by subtracting the DC conductivity from the measured total conductivity, as a function of reciprocal temperature at various frequencies. This

figure depicts the temperature independence and the frequency dependence of rac(x) in the investigated temperature and frequency ranges; this means that rac(x) is not thermally activated in this range of temperature. The frequency dependence of the AC conductivity for different temperatures is shown in Fig. 4. The conductivity obeyed the power law [37]:

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265

-7 -8 -9

ln σac

-10

320 K 360 K 400 K 440 K 480 K 520 K 560 K 580 K

-11 -12 -13 -14 -15 11

12

13

14

15

16

17

18

19

ln ω Fig. 4. Frequency dependence of AC conductivity rac(x) for NiPc at various temperatures.

1.40 1.35 1.30 1.25 1.20

s

1.15 1.10 1.05 1.00 0.95 0.90 300

350

400

450

500

550

600

T (K) Fig. 5. Temperature dependence of the frequency exponent s for NiPc sample.

rac ðxÞ ¼ Axs ;

ð2Þ

where A is a constant dependent on temperature, x is the angular frequency and the exponent s denotes the frequency dependence of rac(x). As can be seen from Fig. 4, the conductivity shows strong frequency dependence at low temperatures, while at higher temperatures and low frequencies the conductivity becomes less frequency dependent. The index s in the present work was found to vary with temperature, as can be seen from Fig. 5, where it has values very close to unity at low temperatures,

while at high temperatures (T > 460 K) the index s increases with increasing temperature reaching a maximum value of 1.37 at T = 580 K. It is apparent from the derived values of the index s that the hopping conduction process as proposed by Elliott [41] is applicable for the present low temperatures case. In this model, hopping of carriers is most likely to occur between adjacent localized sites. Additionally, the AC conductivity is given by the following equation [42]: rac ðxÞ ¼ ðnp3 =24ÞN 2 xee0 R6x ;

ð3Þ

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where e is the dielectric constant of the material, e0 that of free space, N the concentration of localized sites, n = 1 for single-electron hopping, n = 2 for hopping of two electrons and Rx is the hopping distance at a given frequency; given by

to move the electron from one site to the infinite. The frequency exponent s for this model is evaluated as: s¼1

2

Rx ¼

pee0 ½W M

ne ; þ kT lnðxs0 Þ

ð4Þ

where s0 is the effective relaxation time of approximately 1013 s [42], e the electronic charge, k the Boltzmann, s constant and WM is the energy required

6kT ; W M þ ½kT lnðxs0 Þ

ð5Þ

which, to a first approximation reduces to the simple expression: s¼1b¼1

6kT ; WM

ð6Þ

20 18 16

ε′ (ω)

14

20 KHz 40 kHz 100 kHz 200 kHz 400 kHz 1 MHz

12 10 8 6 4 300

350

400

450

500

550

600

T (K) Fig. 6. Variation of the dielectric constant e 0 (x) with temperature at different frequencies for NiPc.

18 320 K 340 K 350 K 360 K 380 K 400 K 420 K 440 K 460 K 480 K 500 K 520 K 540 K 560 K 580 K

16

ε′ (ω)

14

12

10

8

6 11

12

13

14

15

16

17

ln ω Fig. 7. Frequency dependence of the dielectric constant e 0 (x) at various temperatures for NiPc.

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where the parameter b is small in comparison to unity. It is worth mentioning here that the derived values of the index s, particularly at low temperatures approaches one, and is in good qualitative agreement with the predicted value as proposed by Elliott model. At temperatures exceeding 460 K, the index s behaviour reveals a free band conduction mechanism as suggested by Vidadi et al. [43]. Such frequency dependence is in reasonable agreement with those observed for other phthalocyanines compounds like CoPc [22,25], CuPc [25–27,43,44], H2Pc [26,33], ZnPc [29,36], PbPc [25], NiPc [26,35] and MgPc [43]. These researchers pointed out that for low temperatures and high frequencies hopping was believed to occur, and was mainly by free-band conductivity with an activation energy of a few tenths of an electron volt at higher temperatures and lower frequencies.

267

to the electric field which is accompanied by the applied frequencies. Such field will cause some ordering inside the sample as well as the formation of an electric moment in the entire volume of the dielectric and in each separate polarizing molecule. The molecular dipoles in polar material cannot orient themselves at low temperature. When the temperature rises the dipoles orientation is facilitated, and this increases the dielectric constant e 0 (x). In slowly varying fields at low frequency, the dipoles align themselves along the field direction and fully contribute to the total polarization. As the frequency is increased, the variation in the field become too rapid for the molecular dipoles to follow, so that their contribution to the polarization becomes less with a measurable lag because of internal frictional forces. The phase transformation from a- to b-phase in phthalocyanine compounds is accompanied by a gradual change of the electronic configuration inside the sample [15]. So, the space charge polarization is expected to give a drastic increase in e 0 (x) around 525 K and is responsible for the behaviour of the dielectric constant at different frequencies. The variation in the dielectric loss e00 (x) as a function of temperature and frequency is similar to that for the dielectric constant e 0 (x), this is clear from Figs. 8 and 9. These figures illustrate that e00 (x) exhibit strong temperature dependence at higher temperatures and lower frequencies. There are two types of polarization [45], deformational polarization

3.2. Frequency and temperature dependence of the dielectric properties Figs. 6 and 7 show the temperature and the frequency dependences of the dielectric constant e 0 (x). These figures indicate that the increase in the dielectric constant e 0 (x) with temperature is more clear at lower frequencies, and a strong dielectric dispersion occurs at temperatures greater than 525 K. The increase in the dielectric of the sample is due

10 9 8 7

ε′′ (ω)

6

20 KHz 40 KHz 100 KHz 200 KHz 400 KHz 1 MHz

5 4 3 2 1 0 -1 300

350

400

450

500

550

600

T (K) Fig. 8. Variation of the dielectric loss e00 (x) with temperature at different frequencies for NiPc.

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M.M. EL-Nahass et al. / Organic Electronics 7 (2006) 261–270 9 8

320 K 340 K

ε′′ (ω)

7 6

350 K 360 K

5

380 K 400 K

4

420 K 440 K 460 K

3

480 K

2

500 K 520 K

1

540 K 560 K

0

580 K

-1 11

12

13

14

ln ω

15

16

17

Fig. 9. Frequency dependence of the dielectric loss e00 (x) at various temperatures for NiPc.

(electronic and ionic polarization) and relaxation polarization (orientation and interfacial polarization). Electronic polarization is the displacement of electrons with respect to the atomic nucleus, it can be observed in all dielectrics and occur during a very brief interval of time (1015 s). Ionic polarization is the mutual displacement of ions forming a heteropolar (ionic) molecule. It occurs during a short time (1013–1012 s), which is longer than that of electronic polarization. The process of deformational polarization is practically unaffected by the temperature of the dielectric and is not connected with an irreversible dissipation of energy. So, the samples polarization here is not of the deformational type. Orientational polarization is not only a direct rotation of polar molecules under the action of an electric field, but it is connected with the thermal motion of molecules. So temperature must exert an appreciable effect on the phenomenon of dipole polarization. In fact, the rise in temperature and the resulting drop in viscosity exert a double effect on the amount of losses due to the friction of the rotating dipoles: on one hand, the degree of dipole orientation increases, and on the other hand, there is a reduction in the energy require to overcome the resistance of the viscous medium (internal friction of matter) when the dipole rotates through a unit angle. Interfacial or space charge polarization arises from the migration of electrons or ions over

distances of macroscopic magnitude. Some of these charge carries may be trapped and accumulated at the interfaces of different dielectrics; lattice defects, impurity centers, voids, strains, or at electrode surfaces. So it distort the field and produce an apparent increase in the dielectric loss e00 (x) [46]. As distinct from deformational polarization, relaxation polarization requires a relatively long time and dissipates electric energy which transforms into heat in a dielectric, i.e., this energy causes dielectric losses. Owing to Stevels [47], the origins of the dielectric loss are conduction losses, dipole losses and vibrational losses. As the temperature increases, the electrical conduction losses increase which increases the dielectric loss e00 (x). A comparative study to the following equation [48]: ~eðxÞ ¼ e0 ðxÞ  i½e00 ðxÞ þ rdc =x ¼ e0 f1 þ v0 ðxÞ  i½v00 ðxÞ þ rdc =e0 xg;

ð7Þ

where ~eðxÞ denotes the effective permittivity as measured by the instrument, v 0 (x) and v00 (x) are the real and imaginary parts of the susceptibility of the material medium itself, and rdc is the DC conductivity. The significance of the last term in the above equation is that the DC conductivity makes a contribution to the apparent dielectric loss measured by a bridge or other instrument. This is not a true dielectric response, since it is not accompanied by any contribution to the real part of the permittivity

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269

18 16

ε′′

14

ε′′dc

12

ε′′

10 8 6 4 2 0 300

350

400

450

500

550

600

T (K) Fig. 10. Temperature dependence of the observed dielectric loss e00 (x) and the DC conduction loss e00dc at frequency 20 kHz for NiPc sample.

and it arises because no instrument can distinguish between true dielectric response which does not contain rdc and the effective which does. To ascertain the contribution of the DC conduction loss e00dc to the dielectric dispersion; e00dc was calculated from the above relation [48]: e00dc ¼ rdc =e0 x.

ð8Þ

From Fig. 10 it has been observed that the DC conduction loss is comparable to the observed loss at a certain frequency (20 kHz). The DC conduction loss e00dc is smaller than the observed loss e00 (x) up to a certain temperature 500 K, and after that e00dc dominates over the observed dielectric loss e00 (x). It can be assumed that the increase in the dielectric loss e00 (x) may be due to an increase in the DC conductivity. Similar trend has been reported by other workers [49,50]. 4. Summary and conclusion AC conductivity measurements have been carried out on pellet samples of NiPc sandwiched between two evaporated Au electrodes. The measurements covered the temperature range of 303–600 K and frequency range of 20 kHz to 10 MHz. The AC conductivity shows a strong frequency dependence and temperature independence in the investigated

temperature and frequency ranges. It has been observed that the AC conductivity vary with the angular frequency as xs with index s 6 1 at low temperatures, while at high temperatures the index s increased with increasing temperature having a maximum value of 1.37 at 580 K. Such behaviour indicating a hopping conduction mechanism at lower temperatures and a band conduction mechanism dominates at higher temperatures. The DC conductivity has also been measured in the considered range of temperature. There is a decrease in the DC conductivity in the temperature range from 345 K to 370 K, then after that started to increase once again. This behaviour was attributed to drain of oxygen molecules out of the sample during heating. This phenomenon was not a hysteretic effect since the peak disappeared in a second heating cycle. A partial phase transformation from a- to b-phase in NiPc was identified with the change in the thermal activation energy DE from 0.497 eV to 0.703 eV around 525 K. The dielectric constant e 0 (x) and the dielectric loss e00 (x) were found to decrease with increasing frequency and increase with increasing temperature. Such behaviour reveals that the tested organic NiPc exists in the form of molecular dipoles. The phase transformation from a- to b-phase in NiPc is accompanied by a gradual change of the electronic configuration which gives a drastic increase in e 0 (x) around

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525 K. The increase in the dielectric loss e00 (x) with increasing the temperature at low frequencies can be understood in terms of an increase in DC conductivity. In conclusion, the results of the present study are qualitatively in good agreement with the results of other phthalocyanine compounds. Despite of this agreement, more research effort is seriously required to fully understand the electrical behaviour of these compounds due to their importance in science, technology and industry. The proposed study should include more refine investigation of the compounds that include a wider temperature and frequency ranges. References [1] E. Orti, J.L. Brades, J. Chem. Phys. 89 (1988) 1009. [2] J.W. Gardner, M.Z. Iskandari, B. Bott, Sens. Actuators B 9 (1992) 133. [3] A. Mrwa, M. Friedrich, A. Hofman, Sens. Actuators B 24 (1995) 596. [4] L. Hou, L. Cao, X. Li, H. Cui, D. Jiang, G. Zeng, S. Xi, Thin Solid Films 365 (2000) 129. [5] K. Chuan Ho, Y. Ham Tsou, Sens. Actuators B 77 (2001) 253. [6] R. Rellaa, A. Rzzob, A. Licciullic, P. Sicilianoa, L. Troisid, L. Vallic, Mater. Sci. Eng. C 22 (2002) 439. [7] L. Kilmmert, D. Haarer, Adv. Mater. 7 (1995) 495. [8] H.R. Kerb, E.E. Van Faassen, Chem. Phys. Lett. 5 (2000) 332. [9] M. Pfeiffer, A. Beyer, B. Plonnigs, A. Nollau, T. Fritz, K. Leo, D. Schlettwein, S. Hiller, D. Wohrle, Sol. Energy Mater. Sol. Cells 63 (2000) 83. [10] S.T. Lee, Y.M. Wang, X.Y. Hou, C.W. Tang, Appl. Phys. Lett. 74 (1999) 670. [11] Z. Zhilin, J. Xueyin, Z. Wenquing, Z. Buxin, X. Shaohong, J. Phys. D Appl. Phys. 34 (2001) 188. [12] F.Z. Henari, J. Opt. A Pure Appl. Opt. 3 (2001) 188. [13] U. Drechsles, M. Ptaff, M. Hanack, Eur. J. Org. Chem. 1999 (1999) 3411. [14] L. Gao, X. Qian, L. Zhang, Y. Zhang, J. Photochem. Photobiol. 65 (2001) 35. [15] G.H. Helimeier, S.E. Harrison, Phys. Rev. 132 (1963) 2010. [16] T.G. Abdel-Malik, G.A. Cox, J. Phys. C: Solid State Phys. 10 (1977) 63. [17] A.S. Riad, A.E. El-Samahy, S.M. Khalil, Physica B 215 (1995) 217. [18] R.D. Gould, Thin Solid Films 125 (1985) 63.

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