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Evidence of implantation doping in polyacetylene
Nuclear
166
EVIDENCE
OF IMPLANTATION
J. DAVENAS,
Instruments
and Methods
in Physics Research B32 (1988) 166-169 North-Holland, Amsterdam
DOPING IN POLYACETYLENE
X.L. XU, M. MAITROT,
C. MATHIS
* and B. FRANCOIS
*
ISIDT, Universitt!Claude Bernard Lyon I, 43 Boulevard du II Novembre 1918, 69622 Vdleurbanne CPdex, France
Ion implantation has been used in order to dope polyacetylene. DC measurements have been performed between two electrodes evaporated on the irradiated surface of the sample. Rutherford backscattering spectroscopy shows a diffusion of the electrodes in the film which ensures the electrical contact with the implanted layer. I(V) characteristics show a modification of the electrical behaviour which becomes ohmic after irradiation. Two successive elements of the periodic table, Cl+ and Ar+ ions, have been implanted at the same energy, 50 keV, in order to produce a comparable rate of damage in the polyacetylene film. The conductivity resulting from implantation with electrically active ions (Cl+ ) is one order of magnitude higher than the conductivity associated to inert ions (Ar+ ). A larger conductivity enhancement (4 orders of magnitude) is observed with iodine (I+ ions implanted at 100 keV) which is known to be a more efficient dopant of polyacetylene than chlorine. These results are consistent with the existence of a doping effect limited by the irradiation induced damage. The conductivities which may be calculated by assuming the thickness of the
conducting layer to be equal to the width of the implanted distribution in the film remain, however, lower than the conductivities obtained by chemical doping.
1. Introduction
2. Experimental procedure
Ion beams have become widely used for the modification of material properties. In spite of the early work of Charlesby (11 and Chapiro [2] on the effects of ionizing radiations induced in polymers, which gave several important technical applications, it is only recently that the interest for the modification of organic materials by ion beams increased rapidly, in particular through the impetus given by Venkatesan [3] from Bell Labs. The new optical and electrical properties, induced by ion beams, are generally attributed to the formation of peculiar carbon phases, which may be at the origin of high levels of conductivity [4]. Less attention was paid to the doping effects associated to ion implantation than to the modifications of the bombarded materials. However Weber et al. [5] demonstrated the formation of conducting polyacetylene by implantation with doping ions, exhibiting an unusual resistance to air oxidation. We also showed [6] that an increase by more than 4 orders of magnitude of the conductivity could be reached by implantation of polyacetylene with 5 X 1016 I +/cm’. The purpose of this paper is to study the modification of the electrical properties of polyacetylene films implanted with an inert ion (Ar+) and an electronically active ion (Cl’) in order to establish the role of the implanted ions.
The polyacetylene films have been grown according to the usual Shirakawa synthesis [7] using a Ziegler-Natta catalyst at the Institut Charles Sadron of Strasbourg (CNRS). After several washings in appropriate solvents the film does not show any detectable trace of the catalyst by Rutherford backscattering spectroscopy. Polyacetylene has been completely isomerized into tram-polyacetylene by a thermal treatment at 150°C during 15 min. The polyacetylene films where implanted at liquid nitrogen temperature in order to limit the rate of defect production. The sheet resistance was measured between two electrodes evaporated after implantation on the irradiated surface of the film. AC measurements were performed with a General Radio 1621 Bridge, which provides the equivalent impedance (Gr , C,) of the sample.
* Centre de Recherches sur les Macromolecules, singeault, 67083 Strasbourg Cedex, France. 0168-583X/88/$03.50 (North-Holland
0 Elsevier Physics
Publishing
Science
6 Rue Bous-
Publishers
Division)
B.V.
3. Results 3. I. Characterization
of the metal-polymer
We used a beam of determine the distribution film and to characterize trode and the polymer. backscattered a-particles evaporated electrode in hanced for an electrode tion of the polyacetylene
contact
2 MeV a-particles in order to of the implanted ions in the the interface between the elecThe energy spectrum of the shows a diffusion of the the film. This diffusion is enevaporated after ionic irradiafilm. The simplest explanation
J.
Davenas et
al. / Implantation
167
doping in polyacetylene I
\
I Ar.CI lo-’
1o-8
1o-g m-
20
40 DEPTH
60
80
100
XIOOA
-)
Fig. 1. Distribution of Ar+ and Cl+ ions implanted at 50 keV, I+ ions implanted at 100 keV, in polyactylene. Diffusion profile of Al from the electrode in polyacetylene. (1) Nonirradiated film, (2) implanted with 3 X lOI Ar+ ions cm-*.
of this diffusion comes from the fibrillar structure of polyacetylene films grown by the Shirakawa method and the modification of the porosity induced by the irradiation. However, electron microscopy does not show clear evidence of an increase of the porosity. Fig. 1 shows the implantation profile of 50 keV Ar+ and Cl+ ions, and of 100 keV iodine ions, together with the distribution of aluminium from the electrode in the film, which have been deduced from the RBS spectrum. The characteristics of the implantation profile have been summarized in table 1. The figure shows the overlapping of the diffusion profile of the electrode with the implantation distribuion, which should ensure a good electrical contact with the implanted layer. A modification of the electrical contact is evidenced in fig. 2 which presents the I(V) characteristics of nonimplanted polyacetylene (curve 1) and of a film implanted with 3 X 1015 Ar’/cm’. The current-voltage curve, which is supralinear before irradiation (j proportional to E’) becomes linear
Table 1 Range (R,)
and straggling (AR,,)
of the ions implanted in
(CH),
50 keV Cl+ 50 keV Ar+ 100 keV I+
R, IAl
AR,
2240 2120 2040
800 800 1100
IAl
10-10 1
10
1oov
Fig. 2. Current-voltage curve for (1) the pristine film, (2) implanted with 3 x 10” Ar+ ions, (3) implanted with 3 X lOI Cl+ ions cm-*.
after implantation. The ohmic behaviour of the contact may be attributed to the modifications induced by the irradiation at the surface of the film before the deposition of the contact. This change may be related to the improvement of the electrical contact, which has been demonstrated in fig. 1 by RBS. The formation of an ohmic contact seems to be a general property of polyacetylene irradiated with ion beams since the same behaviour is exhibited in fig. 5 for (CH), bombarded with iodine ions. 3.2. Influence conductivity
of the implanted
species on the electrical
Fig. 2 shows the same modification of the contact for a polyacetylene film implanted with 3 X 1015 Cl+ (linear variation of Z versus V) However, an increase of the conductivity greater than one order of magnitude may be observed when going from the I(V) curve (2) associated with Ar+ to Cl+ (curve 3) ions. (The conductance between the two electrodes is given directly by the ordinate corresponding to 1 V since the Z(V) curve has been plotted according to log-log coordinates.) The frequency dependences of the equivalent conductance GP and capacitance Cr, have been studied according to frequency. In fact the interpretation of the capacitance variation cannot be easily done for this geometry, since the usual equivalent circuit is valid for two opposite electrodes. The conductance, which is the only meaningIII. ORGANICS
168
J. Davenas et al. / Implantation
doping
in poiyacetylene
r GP -lo-7 pristine
fCHlx -lo-8
-10-g
-10-‘0
-10-‘l
10-a.
---__
10-g.
____________
‘\
GP_
_c’_ -
-
-.,o-7
- 10-8
\
- lOJ0
-10-f’
Ar
Cp
7 10-3
10-Z
10-l
1
10
lo2
103
104
f
Hz
Fig. 3. Dielectric spectrum of polyacetylene; upper curves: pristine film, lower curves: films implanted with 3 x 1015 Ar+ (full line) and 3X10” Cl+ cm-’ (dotted line). Gr,, conductance and C,, capacitance of the equivalent circuit.
ful parameter, is hightly modified by the implantation. The conductance becomes slightly frequency dependent with the disappearance of its abrupt decrease at very low frequencies, which may be related to the ohmic behaviour of the contact. However, the level of the plateau is slighly lower for the film implanted with Ar+ than for the pristine film, showing a degradation of the long range conduction. For the film implanted with chlorine ions the conductance becomes nearly constant for the whole frequency range, which may be an indication of the appearance of a metallic behaviour. The conductance increase reaches two orders of magnitude over a large range of frequencies. A second criterion of metallic conductivity is the thermal behaviour of the conductivity. A large decrease of the activation energy of the conduction may be determined, varying from 1 eV for the pristine film to 0.2 eV for the film implanted with chlorine ions. The residual value of the activation energy shows that the enhanced conductivity is not purely metallic.
a distinct behaviour for chlorine ions which induce an enhancement of the conductivity reaching two orders of magnitude, from argon ions which induced a slight degradation of the conduction. We have done a calculation of the energy dissipated in the polymer by the bombarding ions, before their implantation when they have lost all their kinetic energy, according to the electronic processes and the elastic collisions involved in the stopping power. The results of this calculation, done with the Biersack algorithm [S], are reproduced in fig. 4. We may observe a very slight modification of the rates of energy deposition for argon and chlorine ions implanted at the same energy. It is then possible to consider that similar rates of damage (within 10%) have been created by these two implantations. The two orders of magnitude between the implantations with argon and chlorine are then to be attributed to the chemical nature of the ions, since chlorine is known to be electronicallly active in chemically doped polyacetylene, whereas argon is inert. Furthermore, the degradation of the polymer by the ion bombardment tends to lower the level of conductivity, as has been observed in the dielectric spectrum of the Ar+ implanted film. These experiments demonstrate the existence of a doping effect in polyacetylene doped by implantation with active ions. In fig. 5, we recall the result of the implantation of iodine ions in polyacetylene [9] which shows a larger improvement of the conductivity (4 orders of magnitude) for this element which has a better efficiency for polyacetylene doping. It is then possible to assume that the conducting layer corresponds to the implanted thickness
I
4
4
8
12
16
20
24
w
E
I
0 4. Discussion The implantation of polyacetylene films with two successive elements of the periodical table clearly shows
ARGON
CHLORINE
4
a
PENETRATION
12
16 DEPTH
20
24 -
xlooA
Fig. 4. Energy loss profiles of 50 keV Ar+ and Cl+ ions in (CH),.
(1) Electronic stopping power; (2) nuclear stopping power; (3) total stopping power.
J. Davenas et al. / Implantation doping in polyacetylene
I
1 /
#’
10
-Y 16’16
10
,I
_7
/
2
!
10
10
29
!
!
-9
,/
10
1
10
TO2
V
Fig. 5. I(V) characteristics of (CH), films: (1) pristine, (2) implanted with 3~10’~ Cl+ cm-‘, (3) 1015 I+ cmm2, (4) 5 X lOi I+ cm*.
Table 2 Conductivity induced by implantation in (CH), films Ion
FIuence [cm-*]
Conductance [Q-‘]
Conductivity [Q-’ cm-‘]
Ar+ cl+ I+ I+
3x10’5 3x10’5 1 x lOI 5 x 10’6
0.3 x1o-9 1o-8 0.25~10-~ 0.5 x10-s
1.2x10-5 3.7x10-4 6.8x10-3 1.4x10-r
(2AR,). We have done an estimation of the wnductivity deduced from the sheet conductance with this assumption. The results have been summarized in table 2 for the different implanted ions. These conductivity levels remain however, lower than those obtained by chemical doping.
5. Conclusion Implanted polyacetylene shows a considerable modification of its electrical properties, which depends on the nature of the doping ion. The transformation of the contact of the film with a deposited electrode, which becomes ohmic, appears to be a general result of the modifications induced by the irradiation in the film structure. A slight degradation of the electrical conduc-
169
tivity is obtained with inert ions, whereas an enhancement of the conductivity is observed for doping ions such as halogens (this property has also been obtained in the case of alkali ions). The doping effect appears, however, to be severely limited by the degradation of the polyacetylene. The behaviour of polyacetylene has in fact to be distinguished from the properties of the saturated polymers, since the role of irradiation is to induce unsaturations in these latter ones through the modification of the chemical bonds, whereas the formation of saturated bonds in (CH), is the only possible pathway to introduce interruptions of the conducting chains. Polyacetylene thus appears not to be well suited for obtaining high levels of conductivity by implantation. However, the small gap of polyacetylene offers interesting possibility of applications, as illustrated by the formation of a p-n junction by implantation of sodium in a p-doped polyacetylene film: (CHI,), [lO,ll].
References 111 A. Charlesby, J. Polymer Sci. 10 (1953) 201.11 (1953) 513, 11 (1953) 521. [2] A. Chapiro, J. Chem. Phys. 51 (1954) 165, 52 (1955) 246, 53 (1956) 295; Proc. Roy. Sot. 215A (1952) 187. [3] T. Venkatesan, L. Calcagno, B.S. Ehnan and G. Foti, Beam Modification of Materials, vol. 2, eds. P. Mazzoldi and G.W. Arnold (Elsevier, Amsterdam. 1987) p. 301. [4] J. Davenas, G. Boiteux, X.L. Xu and E. Adem, these Proceedings (REI-4) Nucl. Instr. and Meth. B32 (1988) 136. [5] D.C. Weber, P. Brandt and C.A. Carosella, Proc. Materials Research Society Symp. on Metastable Materials Formation by Ion Implantation, eds. S.T. Picraux and W.J. Choyke (North-Holland, New York, 1982) vol. 7, p. 167. [6] J. Davenas, C. Dupuy, X.L. Xu, Maitrot, J.J. Andre and B. Francois, Radiat. Eff. 74 (1983) 209. [7] H. Shirakawa and S. Ikeda, Polymer J. 2 (1971) 3; T. Ito, H. Shirakawa and S. Ikeda, J. Polymer Sci. Lett., Polymer. Chem. Ed. 12 (1974) 11. [8] J.P. Biersack, Nucl. Instr. and Meth. 182/183 (1981) 199. [9] J. Davenas, C. Dupuy, S.L. XII, M. Ma&rot, J.J. Andre and B. Francois, Radiat. Eff. 74 (1983) 209. [lo] N. Koshida and Y. Wachi, Appl. Phys. Lett. 45 (1984) 436. [ll] T. Wada, A. Takeno, M. Iwaki, H. Sasabe and Y. Kobayashi, J. Chem. Sot. Chem. Commun. 17 (1985) 1194.
III. ORGANICS
166
EVIDENCE
OF IMPLANTATION
J. DAVENAS,
Instruments
and Methods
in Physics Research B32 (1988) 166-169 North-Holland, Amsterdam
DOPING IN POLYACETYLENE
X.L. XU, M. MAITROT,
C. MATHIS
* and B. FRANCOIS
*
ISIDT, Universitt!Claude Bernard Lyon I, 43 Boulevard du II Novembre 1918, 69622 Vdleurbanne CPdex, France
Ion implantation has been used in order to dope polyacetylene. DC measurements have been performed between two electrodes evaporated on the irradiated surface of the sample. Rutherford backscattering spectroscopy shows a diffusion of the electrodes in the film which ensures the electrical contact with the implanted layer. I(V) characteristics show a modification of the electrical behaviour which becomes ohmic after irradiation. Two successive elements of the periodic table, Cl+ and Ar+ ions, have been implanted at the same energy, 50 keV, in order to produce a comparable rate of damage in the polyacetylene film. The conductivity resulting from implantation with electrically active ions (Cl+ ) is one order of magnitude higher than the conductivity associated to inert ions (Ar+ ). A larger conductivity enhancement (4 orders of magnitude) is observed with iodine (I+ ions implanted at 100 keV) which is known to be a more efficient dopant of polyacetylene than chlorine. These results are consistent with the existence of a doping effect limited by the irradiation induced damage. The conductivities which may be calculated by assuming the thickness of the
conducting layer to be equal to the width of the implanted distribution in the film remain, however, lower than the conductivities obtained by chemical doping.
1. Introduction
2. Experimental procedure
Ion beams have become widely used for the modification of material properties. In spite of the early work of Charlesby (11 and Chapiro [2] on the effects of ionizing radiations induced in polymers, which gave several important technical applications, it is only recently that the interest for the modification of organic materials by ion beams increased rapidly, in particular through the impetus given by Venkatesan [3] from Bell Labs. The new optical and electrical properties, induced by ion beams, are generally attributed to the formation of peculiar carbon phases, which may be at the origin of high levels of conductivity [4]. Less attention was paid to the doping effects associated to ion implantation than to the modifications of the bombarded materials. However Weber et al. [5] demonstrated the formation of conducting polyacetylene by implantation with doping ions, exhibiting an unusual resistance to air oxidation. We also showed [6] that an increase by more than 4 orders of magnitude of the conductivity could be reached by implantation of polyacetylene with 5 X 1016 I +/cm’. The purpose of this paper is to study the modification of the electrical properties of polyacetylene films implanted with an inert ion (Ar+) and an electronically active ion (Cl’) in order to establish the role of the implanted ions.
The polyacetylene films have been grown according to the usual Shirakawa synthesis [7] using a Ziegler-Natta catalyst at the Institut Charles Sadron of Strasbourg (CNRS). After several washings in appropriate solvents the film does not show any detectable trace of the catalyst by Rutherford backscattering spectroscopy. Polyacetylene has been completely isomerized into tram-polyacetylene by a thermal treatment at 150°C during 15 min. The polyacetylene films where implanted at liquid nitrogen temperature in order to limit the rate of defect production. The sheet resistance was measured between two electrodes evaporated after implantation on the irradiated surface of the film. AC measurements were performed with a General Radio 1621 Bridge, which provides the equivalent impedance (Gr , C,) of the sample.
* Centre de Recherches sur les Macromolecules, singeault, 67083 Strasbourg Cedex, France. 0168-583X/88/$03.50 (North-Holland
0 Elsevier Physics
Publishing
Science
6 Rue Bous-
Publishers
Division)
B.V.
3. Results 3. I. Characterization
of the metal-polymer
We used a beam of determine the distribution film and to characterize trode and the polymer. backscattered a-particles evaporated electrode in hanced for an electrode tion of the polyacetylene
contact
2 MeV a-particles in order to of the implanted ions in the the interface between the elecThe energy spectrum of the shows a diffusion of the the film. This diffusion is enevaporated after ionic irradiafilm. The simplest explanation
J.
Davenas et
al. / Implantation
167
doping in polyacetylene I
\
I Ar.CI lo-’
1o-8
1o-g m-
20
40 DEPTH
60
80
100
XIOOA
-)
Fig. 1. Distribution of Ar+ and Cl+ ions implanted at 50 keV, I+ ions implanted at 100 keV, in polyactylene. Diffusion profile of Al from the electrode in polyacetylene. (1) Nonirradiated film, (2) implanted with 3 X lOI Ar+ ions cm-*.
of this diffusion comes from the fibrillar structure of polyacetylene films grown by the Shirakawa method and the modification of the porosity induced by the irradiation. However, electron microscopy does not show clear evidence of an increase of the porosity. Fig. 1 shows the implantation profile of 50 keV Ar+ and Cl+ ions, and of 100 keV iodine ions, together with the distribution of aluminium from the electrode in the film, which have been deduced from the RBS spectrum. The characteristics of the implantation profile have been summarized in table 1. The figure shows the overlapping of the diffusion profile of the electrode with the implantation distribuion, which should ensure a good electrical contact with the implanted layer. A modification of the electrical contact is evidenced in fig. 2 which presents the I(V) characteristics of nonimplanted polyacetylene (curve 1) and of a film implanted with 3 X 1015 Ar’/cm’. The current-voltage curve, which is supralinear before irradiation (j proportional to E’) becomes linear
Table 1 Range (R,)
and straggling (AR,,)
of the ions implanted in
(CH),
50 keV Cl+ 50 keV Ar+ 100 keV I+
R, IAl
AR,
2240 2120 2040
800 800 1100
IAl
10-10 1
10
1oov
Fig. 2. Current-voltage curve for (1) the pristine film, (2) implanted with 3 x 10” Ar+ ions, (3) implanted with 3 X lOI Cl+ ions cm-*.
after implantation. The ohmic behaviour of the contact may be attributed to the modifications induced by the irradiation at the surface of the film before the deposition of the contact. This change may be related to the improvement of the electrical contact, which has been demonstrated in fig. 1 by RBS. The formation of an ohmic contact seems to be a general property of polyacetylene irradiated with ion beams since the same behaviour is exhibited in fig. 5 for (CH), bombarded with iodine ions. 3.2. Influence conductivity
of the implanted
species on the electrical
Fig. 2 shows the same modification of the contact for a polyacetylene film implanted with 3 X 1015 Cl+ (linear variation of Z versus V) However, an increase of the conductivity greater than one order of magnitude may be observed when going from the I(V) curve (2) associated with Ar+ to Cl+ (curve 3) ions. (The conductance between the two electrodes is given directly by the ordinate corresponding to 1 V since the Z(V) curve has been plotted according to log-log coordinates.) The frequency dependences of the equivalent conductance GP and capacitance Cr, have been studied according to frequency. In fact the interpretation of the capacitance variation cannot be easily done for this geometry, since the usual equivalent circuit is valid for two opposite electrodes. The conductance, which is the only meaningIII. ORGANICS
168
J. Davenas et al. / Implantation
doping
in poiyacetylene
r GP -lo-7 pristine
fCHlx -lo-8
-10-g
-10-‘0
-10-‘l
10-a.
---__
10-g.
____________
‘\
GP_
_c’_ -
-
-.,o-7
- 10-8
\
- lOJ0
-10-f’
Ar
Cp
7 10-3
10-Z
10-l
1
10
lo2
103
104
f
Hz
Fig. 3. Dielectric spectrum of polyacetylene; upper curves: pristine film, lower curves: films implanted with 3 x 1015 Ar+ (full line) and 3X10” Cl+ cm-’ (dotted line). Gr,, conductance and C,, capacitance of the equivalent circuit.
ful parameter, is hightly modified by the implantation. The conductance becomes slightly frequency dependent with the disappearance of its abrupt decrease at very low frequencies, which may be related to the ohmic behaviour of the contact. However, the level of the plateau is slighly lower for the film implanted with Ar+ than for the pristine film, showing a degradation of the long range conduction. For the film implanted with chlorine ions the conductance becomes nearly constant for the whole frequency range, which may be an indication of the appearance of a metallic behaviour. The conductance increase reaches two orders of magnitude over a large range of frequencies. A second criterion of metallic conductivity is the thermal behaviour of the conductivity. A large decrease of the activation energy of the conduction may be determined, varying from 1 eV for the pristine film to 0.2 eV for the film implanted with chlorine ions. The residual value of the activation energy shows that the enhanced conductivity is not purely metallic.
a distinct behaviour for chlorine ions which induce an enhancement of the conductivity reaching two orders of magnitude, from argon ions which induced a slight degradation of the conduction. We have done a calculation of the energy dissipated in the polymer by the bombarding ions, before their implantation when they have lost all their kinetic energy, according to the electronic processes and the elastic collisions involved in the stopping power. The results of this calculation, done with the Biersack algorithm [S], are reproduced in fig. 4. We may observe a very slight modification of the rates of energy deposition for argon and chlorine ions implanted at the same energy. It is then possible to consider that similar rates of damage (within 10%) have been created by these two implantations. The two orders of magnitude between the implantations with argon and chlorine are then to be attributed to the chemical nature of the ions, since chlorine is known to be electronicallly active in chemically doped polyacetylene, whereas argon is inert. Furthermore, the degradation of the polymer by the ion bombardment tends to lower the level of conductivity, as has been observed in the dielectric spectrum of the Ar+ implanted film. These experiments demonstrate the existence of a doping effect in polyacetylene doped by implantation with active ions. In fig. 5, we recall the result of the implantation of iodine ions in polyacetylene [9] which shows a larger improvement of the conductivity (4 orders of magnitude) for this element which has a better efficiency for polyacetylene doping. It is then possible to assume that the conducting layer corresponds to the implanted thickness
I
4
4
8
12
16
20
24
w
E
I
0 4. Discussion The implantation of polyacetylene films with two successive elements of the periodical table clearly shows
ARGON
CHLORINE
4
a
PENETRATION
12
16 DEPTH
20
24 -
xlooA
Fig. 4. Energy loss profiles of 50 keV Ar+ and Cl+ ions in (CH),.
(1) Electronic stopping power; (2) nuclear stopping power; (3) total stopping power.
J. Davenas et al. / Implantation doping in polyacetylene
I
1 /
#’
10
-Y 16’16
10
,I
_7
/
2
!
10
10
29
!
!
-9
,/
10
1
10
TO2
V
Fig. 5. I(V) characteristics of (CH), films: (1) pristine, (2) implanted with 3~10’~ Cl+ cm-‘, (3) 1015 I+ cmm2, (4) 5 X lOi I+ cm*.
Table 2 Conductivity induced by implantation in (CH), films Ion
FIuence [cm-*]
Conductance [Q-‘]
Conductivity [Q-’ cm-‘]
Ar+ cl+ I+ I+
3x10’5 3x10’5 1 x lOI 5 x 10’6
0.3 x1o-9 1o-8 0.25~10-~ 0.5 x10-s
1.2x10-5 3.7x10-4 6.8x10-3 1.4x10-r
(2AR,). We have done an estimation of the wnductivity deduced from the sheet conductance with this assumption. The results have been summarized in table 2 for the different implanted ions. These conductivity levels remain however, lower than those obtained by chemical doping.
5. Conclusion Implanted polyacetylene shows a considerable modification of its electrical properties, which depends on the nature of the doping ion. The transformation of the contact of the film with a deposited electrode, which becomes ohmic, appears to be a general result of the modifications induced by the irradiation in the film structure. A slight degradation of the electrical conduc-
169
tivity is obtained with inert ions, whereas an enhancement of the conductivity is observed for doping ions such as halogens (this property has also been obtained in the case of alkali ions). The doping effect appears, however, to be severely limited by the degradation of the polyacetylene. The behaviour of polyacetylene has in fact to be distinguished from the properties of the saturated polymers, since the role of irradiation is to induce unsaturations in these latter ones through the modification of the chemical bonds, whereas the formation of saturated bonds in (CH), is the only possible pathway to introduce interruptions of the conducting chains. Polyacetylene thus appears not to be well suited for obtaining high levels of conductivity by implantation. However, the small gap of polyacetylene offers interesting possibility of applications, as illustrated by the formation of a p-n junction by implantation of sodium in a p-doped polyacetylene film: (CHI,), [lO,ll].
References 111 A. Charlesby, J. Polymer Sci. 10 (1953) 201.11 (1953) 513, 11 (1953) 521. [2] A. Chapiro, J. Chem. Phys. 51 (1954) 165, 52 (1955) 246, 53 (1956) 295; Proc. Roy. Sot. 215A (1952) 187. [3] T. Venkatesan, L. Calcagno, B.S. Ehnan and G. Foti, Beam Modification of Materials, vol. 2, eds. P. Mazzoldi and G.W. Arnold (Elsevier, Amsterdam. 1987) p. 301. [4] J. Davenas, G. Boiteux, X.L. Xu and E. Adem, these Proceedings (REI-4) Nucl. Instr. and Meth. B32 (1988) 136. [5] D.C. Weber, P. Brandt and C.A. Carosella, Proc. Materials Research Society Symp. on Metastable Materials Formation by Ion Implantation, eds. S.T. Picraux and W.J. Choyke (North-Holland, New York, 1982) vol. 7, p. 167. [6] J. Davenas, C. Dupuy, X.L. Xu, Maitrot, J.J. Andre and B. Francois, Radiat. Eff. 74 (1983) 209. [7] H. Shirakawa and S. Ikeda, Polymer J. 2 (1971) 3; T. Ito, H. Shirakawa and S. Ikeda, J. Polymer Sci. Lett., Polymer. Chem. Ed. 12 (1974) 11. [8] J.P. Biersack, Nucl. Instr. and Meth. 182/183 (1981) 199. [9] J. Davenas, C. Dupuy, S.L. XII, M. Ma&rot, J.J. Andre and B. Francois, Radiat. Eff. 74 (1983) 209. [lo] N. Koshida and Y. Wachi, Appl. Phys. Lett. 45 (1984) 436. [ll] T. Wada, A. Takeno, M. Iwaki, H. Sasabe and Y. Kobayashi, J. Chem. Sot. Chem. Commun. 17 (1985) 1194.
III. ORGANICS