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The electrical conductivity of doped oligo[aromatic diimidoselenide]
European Polymer Journal 39 (2003) 1541–1552 www.elsevier.com/locate/europolj
The electrical conductivity of doped oligo[aromatic diimidoselenide] Ali El-Shekeil *, Khalid Y. Abid, Omar M. Al-ShujaÕa Chemistry Department, Faculty of Science, Sana’a University, P.O. Box 12463, Sana’a, Yemen Received 14 February 2003; received in revised form 14 February 2003; accepted 18 February 2003
Abstract DC electrical conductivity of oligo[aromatic diimidoselenide] is studied in the temperature range 300–500 K after doping. The dopants used are I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . Doping is done by mixing with 10% of the dopant, and by chemical doping. The DC electrical conductivity of the two types of doped materials is measured, compared and results interpreted. A trend of high DC electrical conductivity in the case of chemical doping especially with I2 has been noticed. A conduction of 107 S cm1 is obtained at ambient or higher temperatures. This is related to a charge transfer complex formation between the oligomers and I2 . The complexation is confirmed from the electronic spectra of the chemically doped materials which showed a decrease in the p–p energy absorption bands and an increase in the n–p energy absorption bands. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Oligo[aromatic diimidoselenide]; DC electrical conductivity; Doping; UV–visible spectrum; Variation of conductivity with temperature; Charge transfer
1. Introduction Adding a dopant to a polymer was found to enhance the electrical conductivity [1]. Doping of polyacetylene with iodine as an acceptor began more than 25 years ago [2]. Different authors tried the same idea with other polymers and different dopants [3]. The dopant plays a fundamental role in both intra-chain and inter-chain carrier transport [4]. In the polymers, electrical conductivity is attributed to disorder, defect, and/or relaxation of the polymer chain. These are the bases of the idea of the quasi-particles; soliton, polaron and bipolaron [5]. A recently published communication [6] have shown a superconductivity value under a pressure of
* Corresponding author. Tel.: +967-732-15234; fax: +967123-4233. E-mail addresses: [email protected], profi[email protected] (A. El-Shekeil).
16.5 kbar of a TTF derivative as an unprecedented pelectron donor with AsF6 . In the first part of this work, we dealt with four oligo[aromatic diimidoselenide]s [7]. The aim of the study is to understand the structure–property relation [8]. The DC electrical conductivity at room temperature is poor (in the order of 1010 S cm1 ). The effect of the temperature follows Arrhenius equation (r ¼ r0 expðDE=KT Þ) in a simple physical process. Even with the rise of temperature the conductivity is still low. This publication, the second part of the work, deals with the DC electrical conductivity of three oligo[aromatic diimidoselenide]s prepared. Iodine, ferric chloride, zinc chloride, sodium perchlorate and copper sulfate are the five dopants (as acceptors) used in this study. Doping is done by mixing with 10% of the dopant, and by chemical doping. The DC electrical conductivity of the two types of doped materials are measured, compared and results interpreted. A comprehensive study is carried out and discussed in detail to understand the nature of the electrical conduction.
0014-3057/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00040-5
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2. Experimental
10 - 6
T ºC
-7
PADIS1Ann 0%I2 PADIS1 2 .5%I2 PADIS1 5 %I2 PADIS1 1 0%I2 PADIS1 2 0%I2
-8
10 15
-9
10
10
10
-10
-11
2
2.5
3
3.5
1000 /T(º K -1 )
Fig. 1. The DC electrical conductivity vs. 1=T for PADIS1 at different I2 concentrations.
3. Results and discussion Oligo[aromatic diimidoselenide]s (PADIS1-3) are prepared by the reaction of the para-substituted diaminoaromatic compounds, specifically; 1,4-phenylenediamine, 4,40 -thiodianiline, and 1,10 -biphenylene-4, 40 -diamine, with selenium dioxide in refluxing absolute ethanol (Scheme 1). The characterization of these materials has been published elsewhere [7]. 3.1. The effect of different dopants upon electrical conductivity The proper concentration required to give the best electrical conductivity is investigated first. Fig. 1 shows
H2N Ar
10
100
Conductivity
Electrical conductivity measurements are performed as described in previous publications [9–11]. Briefly, the materials in the form of a compact disc (1 mm in thickness and 1 cm in diameter) are produced by subjecting a specific amount of powder (200 mg) to a pressure of 104 kg cm2 . A V –I homemade circuit is used for the electrical measurements. The voltage is kept constant at 10 V in all studies. The measurements are taken under vacuum (103 mm Hg). The system is allowed to stabilize for 1 h prior to taking the readings to eliminate any accumulated electrostatic surface charge. The rate of temperature change with time is kept to a minimum (2 °C min1 ) to assure that the specimen environment is at thermal equilibrium. Doping is done in two ways. The first method is performed for the three materials under study by mixing 10% (wt/wt) I2 with the annealed material. The second way, chemical doping, is carried out by dissolving the dopant (I2 ) in the minimum amount of ethanol followed by addition of an equivalent weight of PADIS1. The mixture is refluxed for 20 min, filtered, washed, dried and compressed into pellets for electrical conductivity measurement. Annealing is achieved by heating the dry material in a vacuum oven for 24 h at 100 °C. The room temperature UV–visible absorption spectra are measured in DMSO with a Cecil CE 599 Automatic Scanning Spectrophotometer.
200
NH2
Ar:
the DC electrical conductivity of PADIS1 vs. 1=T at different I2 concentrations. The concentration of I2 is doubled from 2.5%, to 5%, to 10% to 20% (wt/wt). The highest DC electrical conductivity at ambient temperature is noticed for the highest dopant concentration of 20% I2 . This means that the DC electrical conductivity increased at low temperatures with increasing dopant concentration. The lowest DC electrical conductivity at ambient temperature is seen at zero I2 concentration, i.e. the annealed PADIS1. At higher temperature, the highest DC electrical conductivity is noticed for 5% I2 . The differences between the highest (20%) and lowest concentration (0%) is very small at high temperatures.
+ SeO2
N
S
, Scheme 1.
Ar
N
,
Se
n
+ H2O
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
The 10% (wt/wt) concentration is selected for the study to give an optimum dopant concentration at ambient and higher temperatures.
10
1543
-6
200
100
T ºC
3.2. DC electrical conductivity of doped PADIS1 -7
1020
PADIS1Ann PADIS1Chm.I2 PADIS1 Chm.FeCl3 PADIS1 Chm.ZnCl2
10
-8
PADIS1 Chm.NaClO4 PADIS1 Chm.CuSO4
Conduct ivit y
Fig. 2 displays the DC electrical conductivity vs. 1=T of PADIS1 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . The highest DC electrical conductivity at ambient and higher temperature is noticed for FeCl3 , while the lowest DC electrical conductivity is for the annealed PADIS1 at ambient temperature and the second highest at high temperature. ZnCl2 gave the lowest DC electrical conductivity at higher temperature. Fig. 3 shows the effect of chemical doping of PADIS1 on the DC electrical conductivity vs. 1=T with the same five dopants mentioned. The chemical doping of PADIS1 showed a great enhancement of DC electrical conductivity especially for I2 . This is attributed to the formation of PADIS1–I2 complex as will be verified in
10
-9
-10
-6
10
200
100
10
T ºC
PADIS1Ann PADIS1 10%I 2
20 -7
10
-11
10
2
PADIS1 10%FeCl 3 PADIS1 10%ZnCl 2
2.5
3
3.5
1000 /T(º K -1)
PADIS1 10%NaClO4 PADIS1 10%CuSO 4
Fig. 3. The DC electrical conductivity vs. 1=T of chemically doped PADIS1 with I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
-8
Conductivity
1015
-9
10
-10
10
-11
10
2
3
2.5
3.5
1000 /T(º K -1)
Fig. 2. The DC electrical conductivity vs. 1=T for PADIS1 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
the electronic spectra. Ferric chloride, which is well known to easily make charge transfer complexes [12], gave the second highest DC electrical conductivity at high temperature. The other three dopants ZnCl2 , NaClO4 and CuSO4 are similar in action and the lowest. The action of chemical doping by I2 on DC electrical conductivity enhancement of PADIS1 is noticed at low temperature and remained highest at all temperatures. When doped by mixing with 10% I2 , PADIS1 showed a DC electrical conductivity vs. 1=T that reached its highest value at 500 K. PADIS1, when doped with FeCl3 , chemically or by mixing, showed a similar action on DC electrical conductivity which increased by heat to reach its maxima at 500 K. ZnCl2 doped PADIS1, chemically or by mixing, gave similar DC electrical conductivity enhancement that did not respond to heat at ambient and low temperatures then started to increase by heat to reach the highest DC electrical conductivity at 500 K. NaClO4 and CuSO4 ,
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either chemically or by mixing, showed a similar effect to that of ZnCl2 . 3.3. DC electrical conductivity of doped PADIS2 PADIS2 when doped, by mixing or chemically, showed a peculiar effect. DC electrical conductivity did not increase by heat at ambient and low temperatures until above 126 °C. This increase in conductivity has been attributed mainly to the variation of the carrier concentration with temperature as in the case of semiconductors [13]. I2 and FeCl3 showed the highest DC electrical conductivity followed by ZnCl2 then NaClO4 and CuSO4 , respectively. Fig. 4 shows the DC electrical conductivity vs. 1=T of PADIS2 after doping by mixing with I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . When chemically doped, PADIS2 showed the same trends noticed in dry doping. I2 showed the highest DC electrical conductivity at ambient, low and high tem-
peratures. This is explained by the formation of PADIS2–I2 complex. FeCl3 gave the second highest DC electrical conductivity after I2 . ZnCl2 and NaClO4 showed the lowest conductivity at low and high temperatures. Fig. 5 shows the DC electrical conductivity vs. 1=T of chemically doped PADIS2. With regard to PADIS2 that has been doped with I2 , by mixing or chemically, the same trend is noticed as in case of PADIS1. The chemically doped PADIS2 with I2 responded nicely to heat and conductivity started to increase until it reached its highest DC electrical conductivity at 130 °C. When doped by mixing with 10% I2 , PADIS2 did not respond to heat until at 110 °C where DC electrical conductivity increased until it reached its highest value. PADIS2 that has been doped with FeCl3 , either chemically or by mixing, gave almost the same enhancement to DC electrical conductivity. In both cases,
10
-6
200
100
T ºC
-6
10
200
T ºC
100
10
-7
PADIS2 Ann -7
PADIS2Chm.I2
10
PADIS2 Chm.FeCl3 PADIS2 Ann PADIS2 10%I 2 PADIS2 10%FeCl 3 PADIS2 10%ZnCl 2 PADIS2 10%NaClO4 PADIS2 10%CuSO 4
-8
PADIS2 Chm.ZnCl2 PADIS2 Chm.NaClO4
10
-8
10
-9
)
(S /cm
Conductivity
Conductivity
1015
-9
10
10
-10
-10
10
10
-11
-11
10
2
2.5
3
3.5
1000/ T(º K -1)
Fig. 4. The DC electrical conductivity vs. 1=T for PADIS2 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
2
2.5
3
3.5
1000 /T(º K -1)
Fig. 5. The DC electrical conductivity vs. 1=T for PADIS2 with the four different dopants (chemical doping) namely; I2 , FeCl3 , ZnCl2 and NaClO4 .
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
the conductivity enhancement started above 140 °C to reach its highest value at 500 K.
1545
-6
10
200
100
T ºC PADIS3Ann PADIS3Chm.I 2
3.4. DC electrical conductivity of doped PADIS3
PADIS3Chm.FeCl3 PADIS3Chm.ZnCl 2
10
200
10
PADIS3Chm.NaClO4
-8
-9
10-10
10-11
T ºC
100
10
Conductivity
The action of doping of PADIS3 on DC electrical conductivity is generally similar to PADIS1 and PADIS2. Doping by mixing, however, gave a higher enhancement of DC electrical conductivity at higher temperatures for NaClO4 then FeCl3 followed by ZnCl2 and I2 . Except for NaClO4 none of the other dopants responded to enhancement of DC electrical conductivity by heat at low temperature. Fig. 6 shows the DC electrical conductivity vs. 1=T of PADIS3 that has been doped by mixing with I2 , FeCl3 , ZnCl2 and NaClO4 . Chemically I2 doped PADIS3 showed DC electrical conductivity enhancement at low and high temperatures and gave the highest electrical conductivity noticed. The other three dopants gave similar action and none of them responded to heat at low temperature. Fig. 7 shows the DC electrical conductivity vs. 1=T of the chemically doped PADIS3 by I2 , FeCl3 , ZnCl2 and NaClO4 .
10
-7
2
2.5
3
3.5
1000 /T(º K -1)
Fig. 7. The DC electrical conductivity vs. 1=T of chemically doped PADIS3 with I2 , FeCl3 , ZnCl2 and NaClO4 . 10 PADIS3 Ann PADIS3 1 0%I2 PADIS3 1 0%FeCl3 PADIS3 1 0%ZnCl2 PADIS3 1 0%NaClO4
The chemically doped PADIS3 showed high DC electrical conductivity at ambient temperature that increased by heat until it reached its highest value at 190 °C, after that no increase in electrical conductivity is noticed. In case of PADIS3 that has been doped by mixing, conductivity did not respond to heat below 80 °C where DC electrical conductivity started to increase until it reached its highest value.
Conductivity
10
10
3.5. The electronic absorptions of the chemically doped oligomers
10
10 2
2.5
3
3.5
1000 /T(º K -1)
Fig. 6. The DC electrical conductivity vs. 1=T for PADIS3 with the four different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 and NaClO4 .
The main electronic absorptions of PADIS1 and its five chemically doped states are summarized in Table 1. To investigate the action of I2 chemical doping on PADIS1 the UV spectrum was measured. The electronic absorptions of PADIS1 that has been chemically doped with I2 are shown in Fig. 8. PADIS1–I2 doped material revealed two p–p transitions at 47,620 and 35,090 cm1 . These bands are shifted to lower energies by 1160 and 4910 cm1 , respectively, relative to the undoped material. A similar shift to a higher energy of 6350 cm1 is also observed for the n–p transition. Two of the bands [at 285 nm (35,090 cm1 ) and 350 nm (28,570 cm1 )] of
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transfer complexes at 291 and 355 nm [14]. The change in UV absorption of the chemically doped PADIS1 confirms the addition of the I2 to the selenium atom [15– 17]. Addition of auxochromes like I2 are known to raise the energy of n–p transitions and lower the energy of p–p transitions [18]. In case of the other dopants used this trend has not been noticed clearly. For PADIS1+FeCl3 , the n–p did not change at all and the change in the p–p absorption band was very small; only from 40,000 to 38,460 cm1 although the 48,780 cm1 band disappeared. The PADIS1+ZnCl2 showed a similar behavior to the p–p absorption band and the second n–p absorption band of PADIS1+FeCl3 , however, it showed a new band at 24,390 cm1 . The PADIS1+NaClO4 showed a similar behavior to the p–p absorption band and the second n–p absorption band of PADIS1+FeCl3 however it showed a new band at 25,000 cm1 . PADIS1+CuSO4 showed two new bands at the p–p region at 50,000 and 36,360 cm1 but gave almost the same absorption band of the n–p at 22,730 cm1 .
Table 1 The electronic absorptions of PADIS1 and its five chemically doped states kmax /nm (cm1 ) p–p
n–p 450 (22,220)
PADIS1+FeCl3 PADIS1+ZnCl2
205 250 210 285 260 260
PADIS1+NaClO4
260 (38,460)
PADIS1+CuSO4
200 (50,000), 250 (40,000), 275 (36,360)
PADIS1 PADIS1+I2
(48,780), (40,000) (47,620), (35,090) (38,460) (38,460)
350 (28,570) 450 410 455 400 450 440
(22,220) (24,390), (21,980) (25,000), (22,220) (22,730)
the chemically doped PADIS1 are similar to two charge transfer bands observed in the spectra of some I2 -charge
0.8 0.7 PADIS1 PADIS1+I2
0.6
0.4 .
Abs
0.5
0.3 0.2 0.1 0 -0.1
0
100
200
300
400
500
600
700
800
900
λ nm
Fig. 8. The UV–visible absorptions of PADIS1 that has been chemically doped with I2 .
1
0 .8
PADIS2 PADIS2+I2
Abs
0 .6
0 .4
0 .2
0 0
100
200
300
400
500
600
700
800
900
-0 .2 λ nm
Fig. 9. The UV–visible absorptions of PADIS2 that has been doped chemically with I2 .
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552 Table 2 The electronic absorptions of PADIS2 and its four chemically doped states
Table 3 The electronic absorptions of PADIS3 and its four chemically doped states
kmax /nm (cm1 )
PADIS2 PADIS2+I2 PADIS2+FeCl3 PADIS2+ZnCl2 PADIS2+NaClO4
280 240 290 270 270 265
kmax /nm (cm1 ) n–p
p–p
(35,700) (41,670), (34,480) (37,040) (37,040) (37,740)
1547
520 (19,200) 360 (27,780)
PADIS3 PADIS3+I2
500 (20,000) 500 (20,000) 500 (20,000)
PADIS3+FeCl3 PADIS3+ZnCl2 PADIS3+NaClO4
The electronic absorptions of PADIS2 and PADIS2 that has been doped chemically with I2 are shown in Fig. 9. The main electronic bands of PADIS2 and its four chemically doped states are summarized in Table 2. The I2 doped oligomer showed the typical absorptions usually noticed in case of charge transfer complexes; the p–p absorption bands decreased from 35,700 to 34,480 cm1 by 1220 cm1 and a new band showed up at 41,670 cm1 . On the other side, the n–p band increased by 8580 cm1 from 19,200 to 27,780 cm1 . As in the case of PADIS1, the p–p decreased and the n–p increased indicating a charge transfer complex. In case of the other dopants used with PADIS2 this trend has not been noticed clearly. The action of FeCl3 , ZnCl2 and NaClO4 on PADIS2 showed a similar behavior. In the three cases, the n–p did not change much, only from 19,200 to 20,000 cm1 , and the change in the p–p absorption band increased from 35,700 to 37,040 cm1 . The electronic absorptions of PADIS3 and PADIS3 that has been doped chemically with I2 are shown in Fig. 10. The main electronic bands of PADIS3 and its four chemically doped states are summarized in Table 3. The I2 doped oligomer showed the typical absorptions usually noticed in case of charge transfer complexes; the
p–p
n–p
310 295 370 220 305 245 300 230 245 275 305
460 (21,700) 425 (23,530)
(32,260) (33,900), (27,030) (45,450), (32,790) (40,820), (33,330) (43,480), (40,820), (36,360), (32,790)
0.8
PADIS3 PADIS3+I2
0.6
.
Abs
0.4 0.2
0
100
200
300
450 (22,220) 450 (22,220)
p–p absorption bands showed a decrease by 5230 cm1 from 32,260 to 27,030 cm1 and a new band appeared at 33,900 cm1 . On the other side, the n–p band increased from 21,700 to 23,530 cm1 by 1850 cm1 . For PADIS3+FeCl3 , the change in the p–p absorption band was very small; only from 32,260 to 32,790 cm1 , however, a new band appeared at 45,450 cm1 . The n–p absorption band of PADIS3+FeCl3 changed only from 21,700 to 22,990 cm1 . The PADIS3+ZnCl2 showed a similar behavior to PADIS3+FeCl3 . The change in the p–p absorption band was very small from 32,260 to 33,330 cm1 and a new band showed up at 40,820 cm1 . The second n–p absorption band of PADIS3+ZnCl2 , however, appeared at 22,220 with very little increase from 21,700 cm1 . The PADIS3+NaClO4 showed a similar behavior to PADIS3+ZnCl2 in case of the n–p absorption band with very little increase from 21,700 to 22,220 cm1 , however, it showed two new bands at 43,480 and 36,360 cm1 in case of p–p absorptions beside the two seen in PADIS3+ZnCl2 .
1
0
435 (22,990)
400
500
600
700
800
900
-0.2 -0.4
λ nm
Fig. 10. The UV–visible absorptions of PADIS3 that has been doped chemically with I2 .
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3.6. The activation energies and electronic gaps of doped polymers The activation energies of the doped samples showed some interesting trends. The activation energies (Ea ) and temperature ranges of PADIS1 and its five doped states, chemically and by mixing, are summarized in Table 4. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. All the highest region for I2 and FeCl3 together with PADIS1 itself showed an Ea of 0.27–0.28 eV. The other three dopants used did not show up in this region. At high temperature, Ea of all samples ranged from 0.68 to 1.04 eV. I2 was the smallest (0.68) and ZnCl2 was the highest (1.04 eV). At the medium temperature region, all samples gave an Ea of 0.26–0.30 eV except I2 which gave 0.38 eV.
The annealed PADIS1 gave similar values and within the range mentioned at the different segments, i.e. 0.27 at the higher segment, 0.83 at the middle segment and 0.33 at the lower segment. The observed behavior of different segments has been linked to phase transition [19]. Chemically doped PADIS1 (five dopants) showed in the upper segments an Ea of 0.40–1.10 eV. The medium segmentÕs Ea ranged from 0.48 to 0.68 eV. The lowest temperatureÕs segment showed an Ea of 0.00 eV except I2 which showed an Ea of 0.13 eV. The activation energies and temperature ranges of PADIS2 and its five doped states, chemically and by mixing are summarized in Table 5. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. The Ea noticed in the upper segments were 0.84–1.53 eV. In the lower temperature segments an Ea of 0.25–0.68 eV were observed.
Table 4 The activation energies and temperature ranges of PADIS1 and its five doped states, chemically and by mixing, with 10% (wt/wt) of the dopant PADIS1 Ea /eV (Temp. °C) Annealed 0.33 (25–67) 0.83 (67–145) 0.27 (145–205) 0.00 (205–225) Dopants I2
FeCl3
ZnCl2
NaClO4
Eg p–p
Eg n–p
6.0 5.0
2.8
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
Eg p–p
Eg n–p
0.00 0.38 0.68 0.28
(22–34) (34–108) (108–181) (181–225)
0.13 (22–44) 0.00 (44–225)
6.0 4.4
3.5
0.00 0.30 0.99 0.28 0.17
(22–42) (42–92) (92–139) (139–160) (160–225)
0.00 (22–43) 0.68 (43–98) 0.40 (98–225)
4.8
2.8
4.8
2.7 3.0
4.8
3.1 2.8
0.00 (22–73) 0.26 (73–143) 1.04 (143–225)
0.00 (22–62) 0.29 (62–124) 0.64 (124–225)
0.00 (22–55) 0.31 (55–112) 0.74 (112–225)
0.00 0.17 0.48 1.10
(22–47) (47–109) (109–196) (196–225)
0.00 0.22 0.59 0.25 0.71
(22–56) (56–104) (104–152) (152–178) (178–225)
CuSO4 0.00 (22–62) 0.30 (62–111) 0.74 (111–225)
6.2 5.0 4.5
2.8
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
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Table 5 The activation energies and temperature ranges of PADIS2 and its five doped states, chemically and by mixing with 10% (wt/wt) of the dopant PADIS2 Ea /eV (Temp. °C) Annealed 0.00 (25–144) 0.47 (144–199) 2.58 (199–225) Dopants
ZnCl2
NaClO4
CuSO4
Eg n–p
4.4
2.4
Eg p–p
Eg n–p
5.2 4.3
3.4
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
0.00 (22–107) 0.41 (107–153) 1.42 (153–225)
0.71 0.27 0.76 0.28 0.00
0.05 (25–131) 0.58 (131–169) 1.53 (169–225)
0.06 (22–153) 1.42 (153–196) 0.50 (196–225)
5.0
2.3
0.00 (22–67) 0.25 (67–207) 0.84 (207–225)
0.00 (22–139) 0.37 (139–197) 1.53 (197–225)
5.0
2.4
0.000 (22–162) 0.86 (162–225)
0.00 (22–152) 0.94 (152–202) 2.83 (202–225)
5.0
2.4
–
–
–
I2
FeCl3
Eg p–p
0.00 (22–161) 0.25 (161–225)
(22–31) (31–53) (53–109) (109–139) (139–225)
For PADIS2 the peculiar effect of the charge transfer complex between PADIS2 and I2 is clear. The complex responded to heat starting from ambient temperature until about 140 °C where it reached its highest DC electrical conductivity. The Ea in the PADIS2–I2 complex alternated between 0.71 and 0.27 eV. For the other three dopants, Ea of the upper segments were 1.42–1.53. The medium segmentÕs Ea was 0.48–0.68 eV. The lowest temperatureÕs segment was 0.00 eV. The activation energies and temperature ranges of PADIS3 and its four doped states, chemically and by mixing, are summarized in Table 6. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. The upper segments of PADIS3–I2 and FeCl3 gave an Ea of 1.16 and 0.86 eV. The medium segments gave an Ea of 0.44 and 0.42 eV. The lower segment gave an Ea of 0.00 eV. The Ea of NaClO4 is around 0.51 eV from ambient to high temperature. The Ea of ZnCl2 is 0.51, 0.19 and 0.00 eV in the upper, medium and higher segments, respectively. In case of PADIS3–I2 , the complex formation showed an increase of two orders of magnitude in DC electrical conductivity from 1010 to 108 S cm1 . Ea of the four
segments from high to low temperature increased from 0.12 to 0.31 then decreased to 0.10 then to 0.00 eV. The three other dopantsÕ Ea ranged from 0.00 to 0.10 to a maximum of 0.25 eV in the lower segments. The medium segments showed Ea of 0.70–0.83 eV. The higher segmentsÕ Ea were 0.10–0.27 eV. A schematic proposed energy band diagram between bulk valence band (BCB) and bulk conduction band (BVB) of PADIS1 and PADIS3 and their I2 doped materials either by mixing and chemically is shown in Fig. 11 for comparison. The same applies to PADIS2 but its figure has not been shown because of its big size. It can be noted that the energy band gaps are much smaller in case of the I2 -chemically doped oligomers compared to the annealed or the doped samples, as a result of the charge transfer complex formation. 3.7. The effect of chemical structure on the DC electrical conductivity The effect of chemical structure upon the DC electrical conductivity is illustrated in Fig. 12. PADIS1 displayed the best electrical conductivity at room temperature.
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Table 6 The activation energies and temperature ranges of PADIS3 and its four doped states, chemically and by mixing with 10% (wt/wt) of the dopant PADIS3
Dopants
Ea /eV (Temp. °C)
Eg p–p
Eg n–p
Annealed 0.42 (25–63) 1.24 (63–73) 0.56 (73–107) 0.32 (107–162) 0.86 (162–190) 0.00 (190–225)
4.0
2.7
Eg p–p
Eg n–p
4.2
3.4 2.9
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
0.00 (25–79) 0.44 (79–169) 1.16 (169–225)
0.12 0.31 0.10 0.00
0.00 0.42 0.86 0.52
0.10 (22–96) 0.70 (96–209) 0.27 (209–225)
I2
FeCl3
ZnCl2
NaClO4
(22–67) (67–112) (112–189) (189–225)
0.00 (22–69) 0.19 (69–99) 0.51 (99–225) 0.00 (22–40) 0.51 (40–217) 0.22 (217–225)
0.00 0.25 0.75 0.16
(22–46) (46–107) (107–189) (189–225)
(22–48) (48–92) (92–178) (178–225)
0.00 (22–82) 0.83 (82–225)
There is one selenium atom to every aromatic ring; in other words there is one aromatic ring to every Se–I2 charge transfer complex. PADIS3 showed the second highest DC electrical conductivity at room temperature. There are two aromatic rings to every charge transfer complex. PADIS2 showed the lowest electrical conductivity at room temperature. The reason is that there is only one charge transfer complex to every two aromatic rings and a sulfur atom. It can be concluded that the higher the ratio of the charge transfer complexes in the system the higher will be the DC electrical conductivity at ambient temperature. Although showed a lower electrical conductivity at ambient temperature, PADIS2 and 3 displayed the same value of 107 S cm1 like PADIS1 by heat.
4. Conclusion Three new selenium based oligomers were synthesized and studied. The study is based mainly on UV–visible
5.2 4.1
2.8
5.0 3.9
2.8
5.3 5.1 4.5 4.1
2.8
spectra and DC electrical conductivity at 300 and 500 K. The oligomers were doped with five different dopants: I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . The doping is performed in two ways; by mixing as 10% (wt/wt) and chemically by refluxing a weight equivalent of the dopant in ethyl alcohol for 20 min. The I2 chemically doped oligomers showed higher electrical conductivity at ambient temperature than the other dopants. Especially PADIS1, when chemically doped with I2 , gave a DC electrical conductivity of 107 S cm1 [20]. This oligomer contains one selenium atom to each benzene ring. The case of two benzene rings to every selenium atom (PADIS3) gave a lower conductivity. PADIS2 that has two benzene rings to every selenium atom showed an even lower DC electrical conductivity because of the presence of a sulfur atom between the two benzene rings that disturbs the chain linearity. It reacted with I2 in the ratio of 1:1. The high electrical conductivity noticed in case of I2 by chemical doping is attributed to the formation of charge transfer complexes with the oligomers that is
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
1551
Fig. 11. Schematic proposed energy band diagram of PADIS1 and PADIS3 and their I2 doped materials by mixing and chemically.
10
-6
Conductivity
10
10
-7
PADIS1
-8
PADIS3 10
-9
PADIS2 10
10
-10
-11
2
2.5
1000 /T(º K -1 )
3
3.5
Fig. 12. The effect of chemical structure on the DC electrical conductivity of the polymers.
confirmed by the electronic spectra of the doped materials which showed a decrease in the p–p absorption bands and an increase in the n–p absorption bands. To sum it up, the more selenium atoms in the system that can be chemically complexed with I2 the higher conductivity will be noticed.
References [1] Baeriswyl D, Harbeke G, Kiess H, Meyer W. Conducting polymers: polyacetylene. In: Mort J, Pfister G, editors. Electronic properties of polymers, vol. 267. New York: John Wiley; 1982.
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[2] Ching CK, Fisher CR, Park YW, Heeger AJ, Shirakawa H, Louis EJ, Gau SC, MacDiarmid AG. Phys Rev Lett 1977;39:1098. [3] Lacaze PC, Aeiyach S, Lacroix JC. Poly(p-phenylenes): preparation techniques and properties. In: Nalwa HS, editor. Handbook of organic conductive molecules and polymers, vol. II/205. West Sussex: John Wiley; 1997. [4] Yang C, Jenekhe SA. Macromolecules 1995;28:1180. [5] Rehwald W, Kiess HG. Charge transport in polymers. In: Kiess HG, editor. Conjugated conducting polymers, vol. 135. Berlin: Springer-Verlag; 1992. [6] Nishikawa H et al. J Am Chem Soc 2002;124:730. [7] El-Shekeil A, Abid K, Al-Shujaa O. J Inorg Organomet Polym 2002;11:217. [8] Katon JE. In: Organometallic semiconducting polymers, vol. 162. New York: Marcel Dekker; 1968. [9] El-Shekeil A, Khalid MA, Al-Maydama H, Al-Karbooly A. Eur Polym J 2001;37:575. [10] Khalid MA, El-Shekeil A, Al-Yusufy F. Eur Polym J 2001;37:1423.
[11] El-Shekeil A, Al-Maydama H, Al-Karbooly A. J Inorg Organomet Polym 1997;7:121. [12] Kuivalainen P, Stubb H, Isotalo P, Yli-Lahti P, Holmstrom C. Phys Rev B 1985;31:7900. [13] Amin M, El-Shekeil A, Mounir M, Abu El-Ez M. Ang Makromol Chem 1991;193:13. [14] Ataiw SH, El-Hefnawey GB, Abdel Ghany NT. Ann Chem 1982;72:95. [15] Maddox H, McCullough JD. J Inorg Chem 1966;5:522. [16] Chao GY, McCullough JD. Acta Cryst 1961;14:40. [17] Dance NS, McWhinnie WR, Mallaki J, Mirzai ZM. J Organomet Chem 1980;198:131. [18] Pecsoc R, Shields L, Cairns T, McWilliam I. Modern methods of chemical analysis, vol. 228. 2nd ed. New York: John Wiley; 1976. [19] Bube RH. Electronic properties of crystalline solids. London: Academic Press; 1974. [20] Nalwa HS, editor. Handbook of organic conductive molecules and polymers. West Sussex: John Wiley; 1997.
The electrical conductivity of doped oligo[aromatic diimidoselenide] Ali El-Shekeil *, Khalid Y. Abid, Omar M. Al-ShujaÕa Chemistry Department, Faculty of Science, Sana’a University, P.O. Box 12463, Sana’a, Yemen Received 14 February 2003; received in revised form 14 February 2003; accepted 18 February 2003
Abstract DC electrical conductivity of oligo[aromatic diimidoselenide] is studied in the temperature range 300–500 K after doping. The dopants used are I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . Doping is done by mixing with 10% of the dopant, and by chemical doping. The DC electrical conductivity of the two types of doped materials is measured, compared and results interpreted. A trend of high DC electrical conductivity in the case of chemical doping especially with I2 has been noticed. A conduction of 107 S cm1 is obtained at ambient or higher temperatures. This is related to a charge transfer complex formation between the oligomers and I2 . The complexation is confirmed from the electronic spectra of the chemically doped materials which showed a decrease in the p–p energy absorption bands and an increase in the n–p energy absorption bands. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Oligo[aromatic diimidoselenide]; DC electrical conductivity; Doping; UV–visible spectrum; Variation of conductivity with temperature; Charge transfer
1. Introduction Adding a dopant to a polymer was found to enhance the electrical conductivity [1]. Doping of polyacetylene with iodine as an acceptor began more than 25 years ago [2]. Different authors tried the same idea with other polymers and different dopants [3]. The dopant plays a fundamental role in both intra-chain and inter-chain carrier transport [4]. In the polymers, electrical conductivity is attributed to disorder, defect, and/or relaxation of the polymer chain. These are the bases of the idea of the quasi-particles; soliton, polaron and bipolaron [5]. A recently published communication [6] have shown a superconductivity value under a pressure of
* Corresponding author. Tel.: +967-732-15234; fax: +967123-4233. E-mail addresses: [email protected], profi[email protected] (A. El-Shekeil).
16.5 kbar of a TTF derivative as an unprecedented pelectron donor with AsF6 . In the first part of this work, we dealt with four oligo[aromatic diimidoselenide]s [7]. The aim of the study is to understand the structure–property relation [8]. The DC electrical conductivity at room temperature is poor (in the order of 1010 S cm1 ). The effect of the temperature follows Arrhenius equation (r ¼ r0 expðDE=KT Þ) in a simple physical process. Even with the rise of temperature the conductivity is still low. This publication, the second part of the work, deals with the DC electrical conductivity of three oligo[aromatic diimidoselenide]s prepared. Iodine, ferric chloride, zinc chloride, sodium perchlorate and copper sulfate are the five dopants (as acceptors) used in this study. Doping is done by mixing with 10% of the dopant, and by chemical doping. The DC electrical conductivity of the two types of doped materials are measured, compared and results interpreted. A comprehensive study is carried out and discussed in detail to understand the nature of the electrical conduction.
0014-3057/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00040-5
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2. Experimental
10 - 6
T ºC
-7
PADIS1Ann 0%I2 PADIS1 2 .5%I2 PADIS1 5 %I2 PADIS1 1 0%I2 PADIS1 2 0%I2
-8
10 15
-9
10
10
10
-10
-11
2
2.5
3
3.5
1000 /T(º K -1 )
Fig. 1. The DC electrical conductivity vs. 1=T for PADIS1 at different I2 concentrations.
3. Results and discussion Oligo[aromatic diimidoselenide]s (PADIS1-3) are prepared by the reaction of the para-substituted diaminoaromatic compounds, specifically; 1,4-phenylenediamine, 4,40 -thiodianiline, and 1,10 -biphenylene-4, 40 -diamine, with selenium dioxide in refluxing absolute ethanol (Scheme 1). The characterization of these materials has been published elsewhere [7]. 3.1. The effect of different dopants upon electrical conductivity The proper concentration required to give the best electrical conductivity is investigated first. Fig. 1 shows
H2N Ar
10
100
Conductivity
Electrical conductivity measurements are performed as described in previous publications [9–11]. Briefly, the materials in the form of a compact disc (1 mm in thickness and 1 cm in diameter) are produced by subjecting a specific amount of powder (200 mg) to a pressure of 104 kg cm2 . A V –I homemade circuit is used for the electrical measurements. The voltage is kept constant at 10 V in all studies. The measurements are taken under vacuum (103 mm Hg). The system is allowed to stabilize for 1 h prior to taking the readings to eliminate any accumulated electrostatic surface charge. The rate of temperature change with time is kept to a minimum (2 °C min1 ) to assure that the specimen environment is at thermal equilibrium. Doping is done in two ways. The first method is performed for the three materials under study by mixing 10% (wt/wt) I2 with the annealed material. The second way, chemical doping, is carried out by dissolving the dopant (I2 ) in the minimum amount of ethanol followed by addition of an equivalent weight of PADIS1. The mixture is refluxed for 20 min, filtered, washed, dried and compressed into pellets for electrical conductivity measurement. Annealing is achieved by heating the dry material in a vacuum oven for 24 h at 100 °C. The room temperature UV–visible absorption spectra are measured in DMSO with a Cecil CE 599 Automatic Scanning Spectrophotometer.
200
NH2
Ar:
the DC electrical conductivity of PADIS1 vs. 1=T at different I2 concentrations. The concentration of I2 is doubled from 2.5%, to 5%, to 10% to 20% (wt/wt). The highest DC electrical conductivity at ambient temperature is noticed for the highest dopant concentration of 20% I2 . This means that the DC electrical conductivity increased at low temperatures with increasing dopant concentration. The lowest DC electrical conductivity at ambient temperature is seen at zero I2 concentration, i.e. the annealed PADIS1. At higher temperature, the highest DC electrical conductivity is noticed for 5% I2 . The differences between the highest (20%) and lowest concentration (0%) is very small at high temperatures.
+ SeO2
N
S
, Scheme 1.
Ar
N
,
Se
n
+ H2O
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
The 10% (wt/wt) concentration is selected for the study to give an optimum dopant concentration at ambient and higher temperatures.
10
1543
-6
200
100
T ºC
3.2. DC electrical conductivity of doped PADIS1 -7
1020
PADIS1Ann PADIS1Chm.I2 PADIS1 Chm.FeCl3 PADIS1 Chm.ZnCl2
10
-8
PADIS1 Chm.NaClO4 PADIS1 Chm.CuSO4
Conduct ivit y
Fig. 2 displays the DC electrical conductivity vs. 1=T of PADIS1 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . The highest DC electrical conductivity at ambient and higher temperature is noticed for FeCl3 , while the lowest DC electrical conductivity is for the annealed PADIS1 at ambient temperature and the second highest at high temperature. ZnCl2 gave the lowest DC electrical conductivity at higher temperature. Fig. 3 shows the effect of chemical doping of PADIS1 on the DC electrical conductivity vs. 1=T with the same five dopants mentioned. The chemical doping of PADIS1 showed a great enhancement of DC electrical conductivity especially for I2 . This is attributed to the formation of PADIS1–I2 complex as will be verified in
10
-9
-10
-6
10
200
100
10
T ºC
PADIS1Ann PADIS1 10%I 2
20 -7
10
-11
10
2
PADIS1 10%FeCl 3 PADIS1 10%ZnCl 2
2.5
3
3.5
1000 /T(º K -1)
PADIS1 10%NaClO4 PADIS1 10%CuSO 4
Fig. 3. The DC electrical conductivity vs. 1=T of chemically doped PADIS1 with I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
-8
Conductivity
1015
-9
10
-10
10
-11
10
2
3
2.5
3.5
1000 /T(º K -1)
Fig. 2. The DC electrical conductivity vs. 1=T for PADIS1 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
the electronic spectra. Ferric chloride, which is well known to easily make charge transfer complexes [12], gave the second highest DC electrical conductivity at high temperature. The other three dopants ZnCl2 , NaClO4 and CuSO4 are similar in action and the lowest. The action of chemical doping by I2 on DC electrical conductivity enhancement of PADIS1 is noticed at low temperature and remained highest at all temperatures. When doped by mixing with 10% I2 , PADIS1 showed a DC electrical conductivity vs. 1=T that reached its highest value at 500 K. PADIS1, when doped with FeCl3 , chemically or by mixing, showed a similar action on DC electrical conductivity which increased by heat to reach its maxima at 500 K. ZnCl2 doped PADIS1, chemically or by mixing, gave similar DC electrical conductivity enhancement that did not respond to heat at ambient and low temperatures then started to increase by heat to reach the highest DC electrical conductivity at 500 K. NaClO4 and CuSO4 ,
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either chemically or by mixing, showed a similar effect to that of ZnCl2 . 3.3. DC electrical conductivity of doped PADIS2 PADIS2 when doped, by mixing or chemically, showed a peculiar effect. DC electrical conductivity did not increase by heat at ambient and low temperatures until above 126 °C. This increase in conductivity has been attributed mainly to the variation of the carrier concentration with temperature as in the case of semiconductors [13]. I2 and FeCl3 showed the highest DC electrical conductivity followed by ZnCl2 then NaClO4 and CuSO4 , respectively. Fig. 4 shows the DC electrical conductivity vs. 1=T of PADIS2 after doping by mixing with I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . When chemically doped, PADIS2 showed the same trends noticed in dry doping. I2 showed the highest DC electrical conductivity at ambient, low and high tem-
peratures. This is explained by the formation of PADIS2–I2 complex. FeCl3 gave the second highest DC electrical conductivity after I2 . ZnCl2 and NaClO4 showed the lowest conductivity at low and high temperatures. Fig. 5 shows the DC electrical conductivity vs. 1=T of chemically doped PADIS2. With regard to PADIS2 that has been doped with I2 , by mixing or chemically, the same trend is noticed as in case of PADIS1. The chemically doped PADIS2 with I2 responded nicely to heat and conductivity started to increase until it reached its highest DC electrical conductivity at 130 °C. When doped by mixing with 10% I2 , PADIS2 did not respond to heat until at 110 °C where DC electrical conductivity increased until it reached its highest value. PADIS2 that has been doped with FeCl3 , either chemically or by mixing, gave almost the same enhancement to DC electrical conductivity. In both cases,
10
-6
200
100
T ºC
-6
10
200
T ºC
100
10
-7
PADIS2 Ann -7
PADIS2Chm.I2
10
PADIS2 Chm.FeCl3 PADIS2 Ann PADIS2 10%I 2 PADIS2 10%FeCl 3 PADIS2 10%ZnCl 2 PADIS2 10%NaClO4 PADIS2 10%CuSO 4
-8
PADIS2 Chm.ZnCl2 PADIS2 Chm.NaClO4
10
-8
10
-9
)
(S /cm
Conductivity
Conductivity
1015
-9
10
10
-10
-10
10
10
-11
-11
10
2
2.5
3
3.5
1000/ T(º K -1)
Fig. 4. The DC electrical conductivity vs. 1=T for PADIS2 with the five different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 .
2
2.5
3
3.5
1000 /T(º K -1)
Fig. 5. The DC electrical conductivity vs. 1=T for PADIS2 with the four different dopants (chemical doping) namely; I2 , FeCl3 , ZnCl2 and NaClO4 .
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
the conductivity enhancement started above 140 °C to reach its highest value at 500 K.
1545
-6
10
200
100
T ºC PADIS3Ann PADIS3Chm.I 2
3.4. DC electrical conductivity of doped PADIS3
PADIS3Chm.FeCl3 PADIS3Chm.ZnCl 2
10
200
10
PADIS3Chm.NaClO4
-8
-9
10-10
10-11
T ºC
100
10
Conductivity
The action of doping of PADIS3 on DC electrical conductivity is generally similar to PADIS1 and PADIS2. Doping by mixing, however, gave a higher enhancement of DC electrical conductivity at higher temperatures for NaClO4 then FeCl3 followed by ZnCl2 and I2 . Except for NaClO4 none of the other dopants responded to enhancement of DC electrical conductivity by heat at low temperature. Fig. 6 shows the DC electrical conductivity vs. 1=T of PADIS3 that has been doped by mixing with I2 , FeCl3 , ZnCl2 and NaClO4 . Chemically I2 doped PADIS3 showed DC electrical conductivity enhancement at low and high temperatures and gave the highest electrical conductivity noticed. The other three dopants gave similar action and none of them responded to heat at low temperature. Fig. 7 shows the DC electrical conductivity vs. 1=T of the chemically doped PADIS3 by I2 , FeCl3 , ZnCl2 and NaClO4 .
10
-7
2
2.5
3
3.5
1000 /T(º K -1)
Fig. 7. The DC electrical conductivity vs. 1=T of chemically doped PADIS3 with I2 , FeCl3 , ZnCl2 and NaClO4 . 10 PADIS3 Ann PADIS3 1 0%I2 PADIS3 1 0%FeCl3 PADIS3 1 0%ZnCl2 PADIS3 1 0%NaClO4
The chemically doped PADIS3 showed high DC electrical conductivity at ambient temperature that increased by heat until it reached its highest value at 190 °C, after that no increase in electrical conductivity is noticed. In case of PADIS3 that has been doped by mixing, conductivity did not respond to heat below 80 °C where DC electrical conductivity started to increase until it reached its highest value.
Conductivity
10
10
3.5. The electronic absorptions of the chemically doped oligomers
10
10 2
2.5
3
3.5
1000 /T(º K -1)
Fig. 6. The DC electrical conductivity vs. 1=T for PADIS3 with the four different dopants 10% (wt/wt) namely; I2 , FeCl3 , ZnCl2 and NaClO4 .
The main electronic absorptions of PADIS1 and its five chemically doped states are summarized in Table 1. To investigate the action of I2 chemical doping on PADIS1 the UV spectrum was measured. The electronic absorptions of PADIS1 that has been chemically doped with I2 are shown in Fig. 8. PADIS1–I2 doped material revealed two p–p transitions at 47,620 and 35,090 cm1 . These bands are shifted to lower energies by 1160 and 4910 cm1 , respectively, relative to the undoped material. A similar shift to a higher energy of 6350 cm1 is also observed for the n–p transition. Two of the bands [at 285 nm (35,090 cm1 ) and 350 nm (28,570 cm1 )] of
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A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
transfer complexes at 291 and 355 nm [14]. The change in UV absorption of the chemically doped PADIS1 confirms the addition of the I2 to the selenium atom [15– 17]. Addition of auxochromes like I2 are known to raise the energy of n–p transitions and lower the energy of p–p transitions [18]. In case of the other dopants used this trend has not been noticed clearly. For PADIS1+FeCl3 , the n–p did not change at all and the change in the p–p absorption band was very small; only from 40,000 to 38,460 cm1 although the 48,780 cm1 band disappeared. The PADIS1+ZnCl2 showed a similar behavior to the p–p absorption band and the second n–p absorption band of PADIS1+FeCl3 , however, it showed a new band at 24,390 cm1 . The PADIS1+NaClO4 showed a similar behavior to the p–p absorption band and the second n–p absorption band of PADIS1+FeCl3 however it showed a new band at 25,000 cm1 . PADIS1+CuSO4 showed two new bands at the p–p region at 50,000 and 36,360 cm1 but gave almost the same absorption band of the n–p at 22,730 cm1 .
Table 1 The electronic absorptions of PADIS1 and its five chemically doped states kmax /nm (cm1 ) p–p
n–p 450 (22,220)
PADIS1+FeCl3 PADIS1+ZnCl2
205 250 210 285 260 260
PADIS1+NaClO4
260 (38,460)
PADIS1+CuSO4
200 (50,000), 250 (40,000), 275 (36,360)
PADIS1 PADIS1+I2
(48,780), (40,000) (47,620), (35,090) (38,460) (38,460)
350 (28,570) 450 410 455 400 450 440
(22,220) (24,390), (21,980) (25,000), (22,220) (22,730)
the chemically doped PADIS1 are similar to two charge transfer bands observed in the spectra of some I2 -charge
0.8 0.7 PADIS1 PADIS1+I2
0.6
0.4 .
Abs
0.5
0.3 0.2 0.1 0 -0.1
0
100
200
300
400
500
600
700
800
900
λ nm
Fig. 8. The UV–visible absorptions of PADIS1 that has been chemically doped with I2 .
1
0 .8
PADIS2 PADIS2+I2
Abs
0 .6
0 .4
0 .2
0 0
100
200
300
400
500
600
700
800
900
-0 .2 λ nm
Fig. 9. The UV–visible absorptions of PADIS2 that has been doped chemically with I2 .
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552 Table 2 The electronic absorptions of PADIS2 and its four chemically doped states
Table 3 The electronic absorptions of PADIS3 and its four chemically doped states
kmax /nm (cm1 )
PADIS2 PADIS2+I2 PADIS2+FeCl3 PADIS2+ZnCl2 PADIS2+NaClO4
280 240 290 270 270 265
kmax /nm (cm1 ) n–p
p–p
(35,700) (41,670), (34,480) (37,040) (37,040) (37,740)
1547
520 (19,200) 360 (27,780)
PADIS3 PADIS3+I2
500 (20,000) 500 (20,000) 500 (20,000)
PADIS3+FeCl3 PADIS3+ZnCl2 PADIS3+NaClO4
The electronic absorptions of PADIS2 and PADIS2 that has been doped chemically with I2 are shown in Fig. 9. The main electronic bands of PADIS2 and its four chemically doped states are summarized in Table 2. The I2 doped oligomer showed the typical absorptions usually noticed in case of charge transfer complexes; the p–p absorption bands decreased from 35,700 to 34,480 cm1 by 1220 cm1 and a new band showed up at 41,670 cm1 . On the other side, the n–p band increased by 8580 cm1 from 19,200 to 27,780 cm1 . As in the case of PADIS1, the p–p decreased and the n–p increased indicating a charge transfer complex. In case of the other dopants used with PADIS2 this trend has not been noticed clearly. The action of FeCl3 , ZnCl2 and NaClO4 on PADIS2 showed a similar behavior. In the three cases, the n–p did not change much, only from 19,200 to 20,000 cm1 , and the change in the p–p absorption band increased from 35,700 to 37,040 cm1 . The electronic absorptions of PADIS3 and PADIS3 that has been doped chemically with I2 are shown in Fig. 10. The main electronic bands of PADIS3 and its four chemically doped states are summarized in Table 3. The I2 doped oligomer showed the typical absorptions usually noticed in case of charge transfer complexes; the
p–p
n–p
310 295 370 220 305 245 300 230 245 275 305
460 (21,700) 425 (23,530)
(32,260) (33,900), (27,030) (45,450), (32,790) (40,820), (33,330) (43,480), (40,820), (36,360), (32,790)
0.8
PADIS3 PADIS3+I2
0.6
.
Abs
0.4 0.2
0
100
200
300
450 (22,220) 450 (22,220)
p–p absorption bands showed a decrease by 5230 cm1 from 32,260 to 27,030 cm1 and a new band appeared at 33,900 cm1 . On the other side, the n–p band increased from 21,700 to 23,530 cm1 by 1850 cm1 . For PADIS3+FeCl3 , the change in the p–p absorption band was very small; only from 32,260 to 32,790 cm1 , however, a new band appeared at 45,450 cm1 . The n–p absorption band of PADIS3+FeCl3 changed only from 21,700 to 22,990 cm1 . The PADIS3+ZnCl2 showed a similar behavior to PADIS3+FeCl3 . The change in the p–p absorption band was very small from 32,260 to 33,330 cm1 and a new band showed up at 40,820 cm1 . The second n–p absorption band of PADIS3+ZnCl2 , however, appeared at 22,220 with very little increase from 21,700 cm1 . The PADIS3+NaClO4 showed a similar behavior to PADIS3+ZnCl2 in case of the n–p absorption band with very little increase from 21,700 to 22,220 cm1 , however, it showed two new bands at 43,480 and 36,360 cm1 in case of p–p absorptions beside the two seen in PADIS3+ZnCl2 .
1
0
435 (22,990)
400
500
600
700
800
900
-0.2 -0.4
λ nm
Fig. 10. The UV–visible absorptions of PADIS3 that has been doped chemically with I2 .
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3.6. The activation energies and electronic gaps of doped polymers The activation energies of the doped samples showed some interesting trends. The activation energies (Ea ) and temperature ranges of PADIS1 and its five doped states, chemically and by mixing, are summarized in Table 4. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. All the highest region for I2 and FeCl3 together with PADIS1 itself showed an Ea of 0.27–0.28 eV. The other three dopants used did not show up in this region. At high temperature, Ea of all samples ranged from 0.68 to 1.04 eV. I2 was the smallest (0.68) and ZnCl2 was the highest (1.04 eV). At the medium temperature region, all samples gave an Ea of 0.26–0.30 eV except I2 which gave 0.38 eV.
The annealed PADIS1 gave similar values and within the range mentioned at the different segments, i.e. 0.27 at the higher segment, 0.83 at the middle segment and 0.33 at the lower segment. The observed behavior of different segments has been linked to phase transition [19]. Chemically doped PADIS1 (five dopants) showed in the upper segments an Ea of 0.40–1.10 eV. The medium segmentÕs Ea ranged from 0.48 to 0.68 eV. The lowest temperatureÕs segment showed an Ea of 0.00 eV except I2 which showed an Ea of 0.13 eV. The activation energies and temperature ranges of PADIS2 and its five doped states, chemically and by mixing are summarized in Table 5. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. The Ea noticed in the upper segments were 0.84–1.53 eV. In the lower temperature segments an Ea of 0.25–0.68 eV were observed.
Table 4 The activation energies and temperature ranges of PADIS1 and its five doped states, chemically and by mixing, with 10% (wt/wt) of the dopant PADIS1 Ea /eV (Temp. °C) Annealed 0.33 (25–67) 0.83 (67–145) 0.27 (145–205) 0.00 (205–225) Dopants I2
FeCl3
ZnCl2
NaClO4
Eg p–p
Eg n–p
6.0 5.0
2.8
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
Eg p–p
Eg n–p
0.00 0.38 0.68 0.28
(22–34) (34–108) (108–181) (181–225)
0.13 (22–44) 0.00 (44–225)
6.0 4.4
3.5
0.00 0.30 0.99 0.28 0.17
(22–42) (42–92) (92–139) (139–160) (160–225)
0.00 (22–43) 0.68 (43–98) 0.40 (98–225)
4.8
2.8
4.8
2.7 3.0
4.8
3.1 2.8
0.00 (22–73) 0.26 (73–143) 1.04 (143–225)
0.00 (22–62) 0.29 (62–124) 0.64 (124–225)
0.00 (22–55) 0.31 (55–112) 0.74 (112–225)
0.00 0.17 0.48 1.10
(22–47) (47–109) (109–196) (196–225)
0.00 0.22 0.59 0.25 0.71
(22–56) (56–104) (104–152) (152–178) (178–225)
CuSO4 0.00 (22–62) 0.30 (62–111) 0.74 (111–225)
6.2 5.0 4.5
2.8
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Table 5 The activation energies and temperature ranges of PADIS2 and its five doped states, chemically and by mixing with 10% (wt/wt) of the dopant PADIS2 Ea /eV (Temp. °C) Annealed 0.00 (25–144) 0.47 (144–199) 2.58 (199–225) Dopants
ZnCl2
NaClO4
CuSO4
Eg n–p
4.4
2.4
Eg p–p
Eg n–p
5.2 4.3
3.4
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
0.00 (22–107) 0.41 (107–153) 1.42 (153–225)
0.71 0.27 0.76 0.28 0.00
0.05 (25–131) 0.58 (131–169) 1.53 (169–225)
0.06 (22–153) 1.42 (153–196) 0.50 (196–225)
5.0
2.3
0.00 (22–67) 0.25 (67–207) 0.84 (207–225)
0.00 (22–139) 0.37 (139–197) 1.53 (197–225)
5.0
2.4
0.000 (22–162) 0.86 (162–225)
0.00 (22–152) 0.94 (152–202) 2.83 (202–225)
5.0
2.4
–
–
–
I2
FeCl3
Eg p–p
0.00 (22–161) 0.25 (161–225)
(22–31) (31–53) (53–109) (109–139) (139–225)
For PADIS2 the peculiar effect of the charge transfer complex between PADIS2 and I2 is clear. The complex responded to heat starting from ambient temperature until about 140 °C where it reached its highest DC electrical conductivity. The Ea in the PADIS2–I2 complex alternated between 0.71 and 0.27 eV. For the other three dopants, Ea of the upper segments were 1.42–1.53. The medium segmentÕs Ea was 0.48–0.68 eV. The lowest temperatureÕs segment was 0.00 eV. The activation energies and temperature ranges of PADIS3 and its four doped states, chemically and by mixing, are summarized in Table 6. The electronic gaps (Eg ) of the chemically doped materials are included for comparison. The upper segments of PADIS3–I2 and FeCl3 gave an Ea of 1.16 and 0.86 eV. The medium segments gave an Ea of 0.44 and 0.42 eV. The lower segment gave an Ea of 0.00 eV. The Ea of NaClO4 is around 0.51 eV from ambient to high temperature. The Ea of ZnCl2 is 0.51, 0.19 and 0.00 eV in the upper, medium and higher segments, respectively. In case of PADIS3–I2 , the complex formation showed an increase of two orders of magnitude in DC electrical conductivity from 1010 to 108 S cm1 . Ea of the four
segments from high to low temperature increased from 0.12 to 0.31 then decreased to 0.10 then to 0.00 eV. The three other dopantsÕ Ea ranged from 0.00 to 0.10 to a maximum of 0.25 eV in the lower segments. The medium segments showed Ea of 0.70–0.83 eV. The higher segmentsÕ Ea were 0.10–0.27 eV. A schematic proposed energy band diagram between bulk valence band (BCB) and bulk conduction band (BVB) of PADIS1 and PADIS3 and their I2 doped materials either by mixing and chemically is shown in Fig. 11 for comparison. The same applies to PADIS2 but its figure has not been shown because of its big size. It can be noted that the energy band gaps are much smaller in case of the I2 -chemically doped oligomers compared to the annealed or the doped samples, as a result of the charge transfer complex formation. 3.7. The effect of chemical structure on the DC electrical conductivity The effect of chemical structure upon the DC electrical conductivity is illustrated in Fig. 12. PADIS1 displayed the best electrical conductivity at room temperature.
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Table 6 The activation energies and temperature ranges of PADIS3 and its four doped states, chemically and by mixing with 10% (wt/wt) of the dopant PADIS3
Dopants
Ea /eV (Temp. °C)
Eg p–p
Eg n–p
Annealed 0.42 (25–63) 1.24 (63–73) 0.56 (73–107) 0.32 (107–162) 0.86 (162–190) 0.00 (190–225)
4.0
2.7
Eg p–p
Eg n–p
4.2
3.4 2.9
10% (wt/wt)
Chem. (w/w)
Ea /eV (Temp. °C)
Ea /eV (Temp. °C)
0.00 (25–79) 0.44 (79–169) 1.16 (169–225)
0.12 0.31 0.10 0.00
0.00 0.42 0.86 0.52
0.10 (22–96) 0.70 (96–209) 0.27 (209–225)
I2
FeCl3
ZnCl2
NaClO4
(22–67) (67–112) (112–189) (189–225)
0.00 (22–69) 0.19 (69–99) 0.51 (99–225) 0.00 (22–40) 0.51 (40–217) 0.22 (217–225)
0.00 0.25 0.75 0.16
(22–46) (46–107) (107–189) (189–225)
(22–48) (48–92) (92–178) (178–225)
0.00 (22–82) 0.83 (82–225)
There is one selenium atom to every aromatic ring; in other words there is one aromatic ring to every Se–I2 charge transfer complex. PADIS3 showed the second highest DC electrical conductivity at room temperature. There are two aromatic rings to every charge transfer complex. PADIS2 showed the lowest electrical conductivity at room temperature. The reason is that there is only one charge transfer complex to every two aromatic rings and a sulfur atom. It can be concluded that the higher the ratio of the charge transfer complexes in the system the higher will be the DC electrical conductivity at ambient temperature. Although showed a lower electrical conductivity at ambient temperature, PADIS2 and 3 displayed the same value of 107 S cm1 like PADIS1 by heat.
4. Conclusion Three new selenium based oligomers were synthesized and studied. The study is based mainly on UV–visible
5.2 4.1
2.8
5.0 3.9
2.8
5.3 5.1 4.5 4.1
2.8
spectra and DC electrical conductivity at 300 and 500 K. The oligomers were doped with five different dopants: I2 , FeCl3 , ZnCl2 , NaClO4 and CuSO4 . The doping is performed in two ways; by mixing as 10% (wt/wt) and chemically by refluxing a weight equivalent of the dopant in ethyl alcohol for 20 min. The I2 chemically doped oligomers showed higher electrical conductivity at ambient temperature than the other dopants. Especially PADIS1, when chemically doped with I2 , gave a DC electrical conductivity of 107 S cm1 [20]. This oligomer contains one selenium atom to each benzene ring. The case of two benzene rings to every selenium atom (PADIS3) gave a lower conductivity. PADIS2 that has two benzene rings to every selenium atom showed an even lower DC electrical conductivity because of the presence of a sulfur atom between the two benzene rings that disturbs the chain linearity. It reacted with I2 in the ratio of 1:1. The high electrical conductivity noticed in case of I2 by chemical doping is attributed to the formation of charge transfer complexes with the oligomers that is
A. El-Shekeil et al. / European Polymer Journal 39 (2003) 1541–1552
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Fig. 11. Schematic proposed energy band diagram of PADIS1 and PADIS3 and their I2 doped materials by mixing and chemically.
10
-6
Conductivity
10
10
-7
PADIS1
-8
PADIS3 10
-9
PADIS2 10
10
-10
-11
2
2.5
1000 /T(º K -1 )
3
3.5
Fig. 12. The effect of chemical structure on the DC electrical conductivity of the polymers.
confirmed by the electronic spectra of the doped materials which showed a decrease in the p–p absorption bands and an increase in the n–p absorption bands. To sum it up, the more selenium atoms in the system that can be chemically complexed with I2 the higher conductivity will be noticed.
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