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Electrical transport in electron beam irradiated polyacetylene
Solid State Communications, Printed in Great Britain.
ELECTRICAL
Vol. 46, No. 5, pp. 405-408,
TRANSPORT
1983.
0038-1098/83/170405-04$03.00/0 Pergamon Press Ltd.
IN ELECTRON BEAM IRRADIATED
POLYACETYLENE
K. Yoshino, S. Hayashi, G. Ishii and Y. Inuishi Faculty of Engineering,
Osaka University,
Yamada-Oka,
Suita, Osaka, Japan
(Received 24 January 1983 by H. Kawamura) Electrical conductivity of non-doped (CH), decreases a little and its activation energy increases a little by the electron beam irradiation. The increase of the electrical conductivity of the irradiated (CH), by 12 doping is not so remarkable compared with that of non-irradiated (CH),. The conductivity of heavily irradiated (CH), is very low even after halogen dopings. On the other hand, the effect of the electron beam irradiation on the conductivity of previously I2 doped (CH), is much less compared with that doped after the electron beam irradiation. These results are explained tentativelv bv the role of I, molecules in the initial stage of radiolysis and the suppressibn of the cro&nking.
RECENTLY, POLYACETYLENE (CH), has attracted much attention, because its electrical conductivity increases by many orders of magnitude with the doplngs of various acceptors or donors, changing from an insulator to a metal [ 1, 21. However, the understanding of the electrical transport in (CH), is still poor in spite of many intensive efforts. A soliton model has been proposed to explain the properties of lightly doped (CH), [3,4]. However, it is not clear whether the soliton is necessary to explain the observed results. Someone infers that the inhomogeneous doping is more important to understand experimental results and the idea of so&on is not always necessary [ 51. Though the conductivity of the heavily doped (CH), lies in metallic range, its temperature dependence is much different from the typical characteristics of an ideal metal. Namely, the conductivity of doped (CH), does not increase remarkably with decreasing temperature [6]. We have reported that various defects on conjugated double bonds such as cross-linking, chain breaks, oxidation etc. have severe influence on the conductivity [7], because in the case of one-dimensional metal, the carrier transport along one conduction path is heavily hindered by the introduction of defects, contrary to the case of the three-dimensional metal. We have already indicated that the conductivity of (CH), irradiated with the electron beam does not increase remarkably even by 12 doping compared with that of pristine non-irradiated (CH), [71. In this paper, we will indicate that the effect of the electron beam irradiation depends on whether doping of Iz is done before the irradiation or after the irradiation.
Cis-(CH), samples are synthesized by the method of Shirakawa et al. [8]. Tram-(CM), was obtained by the heat treatment of cis-(CH), at 160°C for about 1 hr under vacuum. The electron beam of 1.8 MeV was irradiated on the (CH), sample sealed in Pyrex tubes. Figure 1 shows temperature dependences of the electrical conductivity of electron beam irradiated (CH), films and those of IZ doped trans-(CH), films after the electron beam irradiation. In this case, I? was doped after the electron beam irradiation under the same condition (same vapour pressure, same time interval and same temperature). As evident from this figure, the electrical conductivity of (CH), decreases with increasing irradiation dose and its activation energy also increases a little. The electrical conductivity of nondoped (CH), decreases only one order of magnitude even by the irradiation of the electron beam as high as 160 Mrad. On the other hand, the conductivity of IZ doped (CH), after the electron beam irradiation is strongly dependent on the irradiation dose. The effect of the irradiation is quite remarkable in this case as evident from this figure. The conductivity of non-irradiated (CH), increases remarkably with IZ doping. However, the increase of the conductivity of the electron beam irradiated sample with IZ doping is much less. This should be explained by the cutting of a conduction channel (conjugated double bonds in (CH),) by the formation of cross-linkings, chain breaks etc. by the irradiation. Figure 2 shows the increase of the conductivity during IZ doping for various samples. It should be noted that the conductivity increases a little slowly in the
405
406
ELECTRON BEAM IRRADIATED
Vol. 46, No. 5
POLYACETYLENE
r ”
d non-doped Pristine (CH& o Irrdia~fxKl~ A 18Mrad 0 42 Mrad 0 180Mrad e
Pristine
Ipoped .
0
trans-KHh,
1
. . + . Electron beam irrodiated trw-(G-iX,
3 4 2 Doping time (hour)
1
Fig. 1. Temperature dependence of the electrical conductivity of electron beam irradiated non-doped (CH), (broken line) and those of Ia doped (CH), after the electron beam irradiation (solid line), irradiated sample. The decrease of the conductivity after the evacuation of the IZ doped sample is also different for the samples of different irradiation dose as indicated by the arrows in Fig. 2. These facts seem to indicate that the velocity or mechanism of I2 doping should be influenced by the creation of cross-linking etc. Details of the irradiation effect on doping process are also now under study. Fig. 3 shows the dependence of the electrical conductivity of Ia pre-doped (CH), on the electron beam irradiation. In this case, I, was doped in cis-(CH), at first and then the electron beam was irradiated. It should be noted that the decrease of the conductivity by the electron beam irradiation is much less compared with that of (CH), doped after the irradiation (Fig. 1). As already known, the cis-(CH), is transformed into nuns-(CH), by halogen dopings and the conductivity of IZ doped trans-(CH), by this procedure from cis-(CH), is much higher than that of Ia doped trans(CH), after the isomerization by the heat treatment. The difference of the electrical conductivitv of I, doued
5
Fig. 2. Increase of the electrical conductivity irradiated at various dose by the Ia doping.
‘0 L
20I
(CHl,,z
1~
40
60
I
80 Dose
I00
120
140
of (CH)x
160
180
(M rad)
Fig. 3. Dependence of the electrical conductivity of IZ doped (U-I), on the electron beam irradiation. In this case. 1, was doned before irradiation.
ELECTRON BEAM IRRADIATED
Vol. 46, No. 5
(CH1o.d~ o Pristine 0 18 Mrad 0 42Mrad l 90 M rod v 180Mrad
E.(oV) 0.010 0.010 0.012 0.015 0 018
I
I
10.
Inverse
5 of
15 Temperature
‘pge
(I/K) Fig. 4. Temperature dependence of the electrical conductivity of the electron beam irradiated (CH), after 1s doping. non-irradiated (CH), in Fig. 1 and Fig. 3 should be due to this difference of the isomerization process into truns0%. Figure 4 shows the temperature dependences of the electrical conductivity of the electron beam irradiated (CH), films after Is doping. Though the activation energy of the conductivity increases also with increasing irradiation dose, its change is much less compared with (CH), doped after the irradiation indicated in Fig. 1. Although the mechanism of the difference of the irradiation effect on pre-doped and after-doped samples is not clear at this stage of experiments, it can be explained qualitatively as follows. Generally, the irradiation of the high energy electron will ionize the polymer, creating a secondary high energy electron. Then the highly excited state of the polymer is produced by the recombination of the ionized polymer with this secondary electron. During the relaxation of this excited state, various reactions like radical formation, cross-linking and chain breaks etc. are considered to occur [9]. This initial processes of the radiolysis generally observed in various polymers will also occur in (CH), . In such processes, the acceptor dopants like I2 will either capture ionized electron before recombination to form excited state, or react with polymer cation radical forming polymer-iodine complex. Then the formation
POLYACETYLENE
407
of the cross-linking will be much suppressed, which is consistent with the mechanical properties of the irradiated (CI-I), sample after 1s doping. Namely, Is doped (CH), after the electron beam irradiation is very brittle but the sample irradiated after doping does not indicate any remarkable change of the mechanical strength, indicating the less formation of the cross-linking in the latter case. Namely, the existence of the electron acceptor seems to prevent the formation of the cross-linking, which explain the difference of the properties of both the samples. There should also remain another possibility that the existence of free carriers in conducting polymers may influence strongly on the initial process of the radiolysis, resulting in the much different characteristics from insulating polymers. We can expect very interesting effect from the above experimental results. Namely, the effect of the doping generally depends on the dopant molecules and only limited number of dopants like strong Lewis acid are known to increase electrical conductivity of (CH), remarkably. However, other dopants are not so effective even if it has ability to capture electrons as a scavenger. Our results suggest that the irradiation of the electron beam on the (CH), doped with dopants which is not so effective will increase the electrical conductivity remarkably by the electron beam induced complex formation. This consideration will be applied for most of the conducting polymers. We are now continuing the experiment with various polymers-dopants systems. Acknowledgements - The authors would like to express their sincere thanks to Mr. S. Murakami, Mr. G. Okube, Mr. M. Shirai, M. S. Ohta, Mr. A. Ichiba, Mr. Y. Kawasaki and Mr. T. Moriya of Sumitomo Electric Industries Ltd. for providing electron irradiation facilities. They also thank Prof. K. Hayashi and Prof. M. Irie of the Institute of Science and Industrial Research of Osaka University for the stimulated discussion.
REFERENCES 1.
2. 3. 4. 5. 6.
C.K. Chiang, C.R. Fincher Jr., Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau & A.G. MacDiarmid. Phvs. Rev. Lett. 39.1098 (1977). ’ ’ C.K. Chiang, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis & A.G. MacDiarmid, J. Chem. Phys:69,5098 (1979). W.P. Su, J.R. Schrieffer & A.J. Heeger, Phys. Rev. Lett. 42,1698 (1979). W.P. Su, J.R. Schrieffer & A.J. Heeger. Phys. Rev. B22,2099 (1980). Y. Tomkiewicz, T.D. Schultz, H.B. Brom, A.R. Taranko, T.C. Clark & G.B. Street, Phys. Rev. Lett. 43, 1532 (1979). J.F. Kwak, T.C. Clark, R.L. Green & G.B. Street, Solid State Commun. 31,3 55 (1979).
408 7. 8.
ELECTRON BEAM IRRADIATED K. Yoshino, S. Hayashi & Y. Inuishi, Japan J. AppZ. Phys. 21, L569 (1982). H. Shirakawa, T. Ito & S. Ikeda, J. Poly. 4,460 (1973).
9.
POLYACETYLENE
Vol. 46, No. 5
A. Chapiro, Radiation Chemistry of Polymeric System, Interscience Publishers (1962).
ELECTRICAL
Vol. 46, No. 5, pp. 405-408,
TRANSPORT
1983.
0038-1098/83/170405-04$03.00/0 Pergamon Press Ltd.
IN ELECTRON BEAM IRRADIATED
POLYACETYLENE
K. Yoshino, S. Hayashi, G. Ishii and Y. Inuishi Faculty of Engineering,
Osaka University,
Yamada-Oka,
Suita, Osaka, Japan
(Received 24 January 1983 by H. Kawamura) Electrical conductivity of non-doped (CH), decreases a little and its activation energy increases a little by the electron beam irradiation. The increase of the electrical conductivity of the irradiated (CH), by 12 doping is not so remarkable compared with that of non-irradiated (CH),. The conductivity of heavily irradiated (CH), is very low even after halogen dopings. On the other hand, the effect of the electron beam irradiation on the conductivity of previously I2 doped (CH), is much less compared with that doped after the electron beam irradiation. These results are explained tentativelv bv the role of I, molecules in the initial stage of radiolysis and the suppressibn of the cro&nking.
RECENTLY, POLYACETYLENE (CH), has attracted much attention, because its electrical conductivity increases by many orders of magnitude with the doplngs of various acceptors or donors, changing from an insulator to a metal [ 1, 21. However, the understanding of the electrical transport in (CH), is still poor in spite of many intensive efforts. A soliton model has been proposed to explain the properties of lightly doped (CH), [3,4]. However, it is not clear whether the soliton is necessary to explain the observed results. Someone infers that the inhomogeneous doping is more important to understand experimental results and the idea of so&on is not always necessary [ 51. Though the conductivity of the heavily doped (CH), lies in metallic range, its temperature dependence is much different from the typical characteristics of an ideal metal. Namely, the conductivity of doped (CH), does not increase remarkably with decreasing temperature [6]. We have reported that various defects on conjugated double bonds such as cross-linking, chain breaks, oxidation etc. have severe influence on the conductivity [7], because in the case of one-dimensional metal, the carrier transport along one conduction path is heavily hindered by the introduction of defects, contrary to the case of the three-dimensional metal. We have already indicated that the conductivity of (CH), irradiated with the electron beam does not increase remarkably even by 12 doping compared with that of pristine non-irradiated (CH), [71. In this paper, we will indicate that the effect of the electron beam irradiation depends on whether doping of Iz is done before the irradiation or after the irradiation.
Cis-(CH), samples are synthesized by the method of Shirakawa et al. [8]. Tram-(CM), was obtained by the heat treatment of cis-(CH), at 160°C for about 1 hr under vacuum. The electron beam of 1.8 MeV was irradiated on the (CH), sample sealed in Pyrex tubes. Figure 1 shows temperature dependences of the electrical conductivity of electron beam irradiated (CH), films and those of IZ doped trans-(CH), films after the electron beam irradiation. In this case, I? was doped after the electron beam irradiation under the same condition (same vapour pressure, same time interval and same temperature). As evident from this figure, the electrical conductivity of (CH), decreases with increasing irradiation dose and its activation energy also increases a little. The electrical conductivity of nondoped (CH), decreases only one order of magnitude even by the irradiation of the electron beam as high as 160 Mrad. On the other hand, the conductivity of IZ doped (CH), after the electron beam irradiation is strongly dependent on the irradiation dose. The effect of the irradiation is quite remarkable in this case as evident from this figure. The conductivity of non-irradiated (CH), increases remarkably with IZ doping. However, the increase of the conductivity of the electron beam irradiated sample with IZ doping is much less. This should be explained by the cutting of a conduction channel (conjugated double bonds in (CH),) by the formation of cross-linkings, chain breaks etc. by the irradiation. Figure 2 shows the increase of the conductivity during IZ doping for various samples. It should be noted that the conductivity increases a little slowly in the
405
406
ELECTRON BEAM IRRADIATED
Vol. 46, No. 5
POLYACETYLENE
r ”
d non-doped Pristine (CH& o Irrdia~fxKl~ A 18Mrad 0 42 Mrad 0 180Mrad e
Pristine
Ipoped .
0
trans-KHh,
1
. . + . Electron beam irrodiated trw-(G-iX,
3 4 2 Doping time (hour)
1
Fig. 1. Temperature dependence of the electrical conductivity of electron beam irradiated non-doped (CH), (broken line) and those of Ia doped (CH), after the electron beam irradiation (solid line), irradiated sample. The decrease of the conductivity after the evacuation of the IZ doped sample is also different for the samples of different irradiation dose as indicated by the arrows in Fig. 2. These facts seem to indicate that the velocity or mechanism of I2 doping should be influenced by the creation of cross-linking etc. Details of the irradiation effect on doping process are also now under study. Fig. 3 shows the dependence of the electrical conductivity of Ia pre-doped (CH), on the electron beam irradiation. In this case, I, was doped in cis-(CH), at first and then the electron beam was irradiated. It should be noted that the decrease of the conductivity by the electron beam irradiation is much less compared with that of (CH), doped after the irradiation (Fig. 1). As already known, the cis-(CH), is transformed into nuns-(CH), by halogen dopings and the conductivity of IZ doped trans-(CH), by this procedure from cis-(CH), is much higher than that of Ia doped trans(CH), after the isomerization by the heat treatment. The difference of the electrical conductivitv of I, doued
5
Fig. 2. Increase of the electrical conductivity irradiated at various dose by the Ia doping.
‘0 L
20I
(CHl,,z
1~
40
60
I
80 Dose
I00
120
140
of (CH)x
160
180
(M rad)
Fig. 3. Dependence of the electrical conductivity of IZ doped (U-I), on the electron beam irradiation. In this case. 1, was doned before irradiation.
ELECTRON BEAM IRRADIATED
Vol. 46, No. 5
(CH1o.d~ o Pristine 0 18 Mrad 0 42Mrad l 90 M rod v 180Mrad
E.(oV) 0.010 0.010 0.012 0.015 0 018
I
I
10.
Inverse
5 of
15 Temperature
‘pge
(I/K) Fig. 4. Temperature dependence of the electrical conductivity of the electron beam irradiated (CH), after 1s doping. non-irradiated (CH), in Fig. 1 and Fig. 3 should be due to this difference of the isomerization process into truns0%. Figure 4 shows the temperature dependences of the electrical conductivity of the electron beam irradiated (CH), films after Is doping. Though the activation energy of the conductivity increases also with increasing irradiation dose, its change is much less compared with (CH), doped after the irradiation indicated in Fig. 1. Although the mechanism of the difference of the irradiation effect on pre-doped and after-doped samples is not clear at this stage of experiments, it can be explained qualitatively as follows. Generally, the irradiation of the high energy electron will ionize the polymer, creating a secondary high energy electron. Then the highly excited state of the polymer is produced by the recombination of the ionized polymer with this secondary electron. During the relaxation of this excited state, various reactions like radical formation, cross-linking and chain breaks etc. are considered to occur [9]. This initial processes of the radiolysis generally observed in various polymers will also occur in (CH), . In such processes, the acceptor dopants like I2 will either capture ionized electron before recombination to form excited state, or react with polymer cation radical forming polymer-iodine complex. Then the formation
POLYACETYLENE
407
of the cross-linking will be much suppressed, which is consistent with the mechanical properties of the irradiated (CI-I), sample after 1s doping. Namely, Is doped (CH), after the electron beam irradiation is very brittle but the sample irradiated after doping does not indicate any remarkable change of the mechanical strength, indicating the less formation of the cross-linking in the latter case. Namely, the existence of the electron acceptor seems to prevent the formation of the cross-linking, which explain the difference of the properties of both the samples. There should also remain another possibility that the existence of free carriers in conducting polymers may influence strongly on the initial process of the radiolysis, resulting in the much different characteristics from insulating polymers. We can expect very interesting effect from the above experimental results. Namely, the effect of the doping generally depends on the dopant molecules and only limited number of dopants like strong Lewis acid are known to increase electrical conductivity of (CH), remarkably. However, other dopants are not so effective even if it has ability to capture electrons as a scavenger. Our results suggest that the irradiation of the electron beam on the (CH), doped with dopants which is not so effective will increase the electrical conductivity remarkably by the electron beam induced complex formation. This consideration will be applied for most of the conducting polymers. We are now continuing the experiment with various polymers-dopants systems. Acknowledgements - The authors would like to express their sincere thanks to Mr. S. Murakami, Mr. G. Okube, Mr. M. Shirai, M. S. Ohta, Mr. A. Ichiba, Mr. Y. Kawasaki and Mr. T. Moriya of Sumitomo Electric Industries Ltd. for providing electron irradiation facilities. They also thank Prof. K. Hayashi and Prof. M. Irie of the Institute of Science and Industrial Research of Osaka University for the stimulated discussion.
REFERENCES 1.
2. 3. 4. 5. 6.
C.K. Chiang, C.R. Fincher Jr., Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau & A.G. MacDiarmid. Phvs. Rev. Lett. 39.1098 (1977). ’ ’ C.K. Chiang, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis & A.G. MacDiarmid, J. Chem. Phys:69,5098 (1979). W.P. Su, J.R. Schrieffer & A.J. Heeger, Phys. Rev. Lett. 42,1698 (1979). W.P. Su, J.R. Schrieffer & A.J. Heeger. Phys. Rev. B22,2099 (1980). Y. Tomkiewicz, T.D. Schultz, H.B. Brom, A.R. Taranko, T.C. Clark & G.B. Street, Phys. Rev. Lett. 43, 1532 (1979). J.F. Kwak, T.C. Clark, R.L. Green & G.B. Street, Solid State Commun. 31,3 55 (1979).
408 7. 8.
ELECTRON BEAM IRRADIATED K. Yoshino, S. Hayashi & Y. Inuishi, Japan J. AppZ. Phys. 21, L569 (1982). H. Shirakawa, T. Ito & S. Ikeda, J. Poly. 4,460 (1973).
9.
POLYACETYLENE
Vol. 46, No. 5
A. Chapiro, Radiation Chemistry of Polymeric System, Interscience Publishers (1962).