Electrical transport properties of polyacetylene tetrachloroferrate - [CH(FeCl4)y]x]

Solid State Communications, Vol. 48, No. 10, pp. 893-896, 1983. Printed in Great Britain.

0038-1098/83 $3.00 + .00 Pergamon Press Ltd.

ELECTRICAL TRANSPORT PROPERTIES OF POLYACETYLENE TETRACHLOROFERRATE - [CH(FeCln)~]x M. Przybylski and B.R. Bulka Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznarl, Poland and I. Kulszewicz and A. Prod Department of Chemistry, Technical University of Warsaw, Noakowskiego 3, 00-664 Warsaw, Poland (Received 1 July 1983 by M. Cardona)

Conductivity and thermopower measurements of polyacetylene doped with FeC14 are reported. The conductivity changes over 10 orders of magnitude and reaches the maximum value of 2 0 0 ~ -~ cm -1 aty = 0.07. The thermopower reveals the semiconductor to metal transition aty ~ 0.002, with high and temperature independent Seebeck coefficient in the dilute limit and a metallic like dependence in the heavily doped samples. I. INTRODUCTION CHEMICAL OR ELECTROCHEMICAL oxidation of polyacetylene (sometimes called p-type doping) is a well established synthetic procedure leading to the formation of highly conducting derivatives of (CH)x. In the above reactions neutral chains of polyacetylene are converted into polycarbonium cations and the equivalent number of anions is introduced between the polymer chains to neutralize the positive charge imposed upon oxidation. Recently a few paramagnetic halide anions of iron and titanium were introduced to (CH)x by chemical [1, 2] or electrochemical [3] oxidation. To date polyacetylene/ferric chloride system has been most extensively studied. Both chemical oxidation carried out in FeCla/nitromethane solution and electrochemical oxidation performed in LiC1/FeC13/propylene carbonate or nitromethane electrolytes led to the insertion of FeCl~ anion into (CH)x as evidenced by elemental analysis [3], EPR and M6ssbauer spectroscopy [4, 5] and XAFS studies [6]. [CH(FeC14)y] x is reasonably stable in air but its prolonged exposure to air results in the chlorination of the double bond with concomitant transformation of FeC14 into hydrated FeC12 [7]. A great interest in polyacetylene was stimulated by the discovery of the semiconductor to metal transition manifested by the increase of the conductivity over twelve orders of magnitude during doping [8]. One can distinguish three concentration regions: (1) A dilute region (semiconducting phase,y ~< 0.001), where the conductivity is strongly temperature dependent, i.e. o ~ T n (n "" 13), and the thermopower is constant (about 850#VK -l [8, 9]). Experimental facts are in good agreement with the Kivelson's theory [ 10] of 893

intersoliton electron hopping. However, Roth et al. [11 ] pointed out that more recent Summerfield's theory [ 12] of general hopping between random sites accounts for all the previous data and in addition fits the imaginary part of the conductivity, which is less consistent with intersoliton hopping. (2) In an intermediate region (0.001 < y < 0.07) the conductivity is higher than 0.1 (~2 cm)-1 (at room temperature) and a ~ exp [-(To/T)~], (v = 1/4 or 1/2) [13, 14]. The thermopower is very small with a metallic like temperature dependence [15]. It suggests a variable range hopping (in 3-D or 1-D system) but To > 106K and the conductivity is many orders of magnitude below the experimental value [ 13, 9]. (3) For heavily doped samples (y ~> 0.07) the conductivity is weakly temperature dependent, a ~ T ~ with a ~< 1 [8, 14] and with the large value of the Pauli susceptibility [8] suggesting the metallic type of electronic structure with the density of states 0.1 states/eVC-atom at the Fermi level. The aim of this work is to investigate the transport properties of FeCI4 doped polyacetylene. 2. EXPERIMENTAL Polyacetylene films used in all experiments were prepared as cis-rich (85% cis) using a modification of the method o f l t o et al. [16]. It was isomerized to all trans by heating in vacuum sealed glass tube for 15 min at 180°C as recommended in [17]. Trans (CH)x was oxidized (doped) chemically in 0.05 molar nitromethane solution of FeCla for 0.001 to 0.019 doping levels in 0.2 molar solution for 0.02 to 0.10 doping levels. The excess of the dopant was removed by repeated washing with dry nitromethane. For thermoelectric power and conductivity measurements silver or platinum wires

894

PROPERTIES OF POLYACETYLENE TETRACHLOROFERRATE

were attached to the samples before doping using Electrodag contacts. Compositions of the films were determined by the mass uptake assuming FeC12~as the dopant species. It should be stressed that for the dopant concentrations exceedingy = 0.07 the side reaction of the chlorination of the double bond occurs [2] leading to the partial degradation of the system. This phenomenon will result in the conductivity decrease due to the lowering of the carriers concentration and their mobility. The conductivity measurements were performed by a conventional four-probe technique [18]. The values of conductivities o vs temperature were obtained directly from the measured resistance and the measured sample dimensions (usually of 6 x 2 × 0.2 mm3). Contacts were checked to be ohmic for currents applied from 10-1° to 10-SA for undoped (CH)= and from 1 0 -6 t o 10-4A for metallic samples. The samples of pristine and doped (CH)x have been mounted on d.c. and thermal EMF holders in air with exposure time kept to a minimum (typically of 2 - 5 minutes). Both conductivity and thermoelectric power measurements were carried out in dry nitrogen atmosphere. For the thermal EMF measurements a rectangular samples (6 x 1 x 0.2 mm 3) were cut from the polymer film and mounted lengthwise between two silver electrodes [18]. The alternating temperature gradient was produced by two silver rods wound with wire heaters. The maximum temperature drop AT across the samples was less than 2 K. At given temperature the Seebeck coefficient was determined as the slope of thermopower against the temperature gradient traced by an X - Y recorder. After measurements of the polymersamples the thermopower of platinum wire with Electrodag has been measured as well. The experimental error of Seebeck coefficient determined by this method was less than 10%.

Vol. 48, No. 10

103 Xxxx xx xx x xxxxx

102

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10

3

4

5

6

7

8

9 10 11 m-',xl0 3[K "~]

12

Fig. I. Conductivity o vs 1/T for [CH(FeCI4)~] x for different values of dopent concentration y. 10 3

RT •

I

i800

700

101

600 500

o 10-1 E

u)

400 ~

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c=]

300

LID

200

10 .3

loo

3. EXPERIMENTAL RESULTS AND DISCUSSION The temperature dependences of conductivity of [CH(FeC14)y ] x for a variety of dopant concentrations y are shown in Fig. 1, whereas room temperature conductivities as a function of concentration are shown in Fig. 2. The measured values o f o cover more than 10 orders of magnitude. The room temperature conductivty of undoped t r a n s ~ C H ) x is about 1.5 x 10-s~2 -1 cm -1. The temperature dependence is very strong and experimental results can be fitted by a power law dependence o ~ T n [10] with n --~ 14. The values oRr and n are in good agreement with those obtained earlier by other authors [8, 9]. When the dopant concentrationy increases the conductivity o increases as well while the temperature dependence of o is getting weaker. For y = 0.001 n is equal to 4.1 and ORT = 6 X 10-3~2 -1 cm -1.

X

10-5

Xl

i

i

X I

i

I

a61'003 a65 a07 a09

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o.ll

CONCENTRATION (Y)

Fig. 2. Room temperature conductivity (e) and thermopower (x) as a function of dopant concentration for [CH(FeCI4)y ] xThe sign of thermopower is positive throughout the entire concentration range indicatingp-type behaviour consistent with FeC14 formulation as the dopant species. The thermopower of undoped t r a n s ( C H ) x was found to be temperature independent and S = 770 -+ 100/aVK -1. It is in agreement with the Kivelson's model giving the following formula [10]: S = k [(n + 2)/2 + In ( Y n / Y e h ) ] . e

(1)

Vol. 48, No. 10

PROPERTIES OF POLYACETYLENE TETRACHLOROFERRATE

895

Table 1. Parameters derived from the conductivity for variable range hopping [equation (2)] Y

0.003 0.008 0.0185 0.03

OrtT

To

A

ot-1

N(EF)

( ~ - ' cm -1)

(K)

(~2 -1 cm -1K 1/2)

(h)

(states/eV-C-atom)

0.38 0.50 20 70

2.0 4.9 3.4 1.2

5.7 5.2 9.1 1.4

0.3 6 14 15

83.4 0.04 0.05 0.11

X 10 6

x l0 s x 104 x 104

x x x x

104 10 a 103 104

y - dopant concentration, trRT -- conductivity at the room temperature, To = 19Aaa/kN(EF), A = 0.44e 2 cwph/x/To, a -1 - localization length, N ( E F ) - density of states at the Fermi level.

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0

x ~ ~ 'xxxx

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5'0

100

150 200 T[K]

250

300

Fig. 3. Temperature dependence of the thermopower for [CH(FeC14)y] x for different values of dopant concentration ( y ~> 0.01). For n = 14, the number of neutral solitons

Yn = 3 x 10-4 [9] and the obtained thermopower value 770 pV K -1 the number o f charged solitonsyeh can be estimated as 1.3 × 1 0 -4. A t y = 0.001 the thermopower is still very high S = 200 -+ 15 pV K -1 and temperature independent. This result is also consistent with the above formula, w h e n n = 4.1,yn = 3 x 10-4 and Yeh = 7 X 10-4. However, the power n, defined as the ratio of the half width of the optical phonon spectrum to an average optical frequency, is weakly dependent on dopant concentration. This concentration is near the semiconductor-metal transition and one can expect a soliton band formation. In the Kivelson's approach the soliton band is degenerate (much narrower than K T ) and a soliton-impurity interaction, which may drastically change the electronic structure [19], is neglected. At concentrations higher t h a n y ~ "" 0.002 the thermopower (shown in Fig. 3) is metallic like. Its value is small, 5 0 p V K -1 a t y -- 0.01, and decreases with increasing concentration up to the value of 1 6 # V K -1 at y = 0.06. The temperature dependence of the conductivity gradually becomes weaker and reaches the maximum value of 200~2 -1 cm-: at y = 0.07. The slight decrease o f the conductivity of the sample doped t o y = 0.098 as compared with the sample doped to y -- 0.07 is consistent with the postulate of the partial chlorination of the system at doping levels exceeding y = 0.07 [21.

There exists no quantitative theory of transport for these concentrations. Mele and Rice [20] calculated the electronic structure of such a system and showed that for a few percent of dopants the soliton and valence band tails overlap. Thus, there is a pseudogap which disappears for higher concentrations. In such a situation, the thermopower is metallic like, and its sign and value depend on d lnN(E)/dE at the Fermi level (N(E) is the density of states). It is in a qualitative agreement with a decreasing Seebeck coefficient as the concentration increases (see Fig. 2) and a proper sign can be obtained if the Fermi level lies in the valence band shoulder (not in soliton band!). Conductivity and thermopower measurements of polyacene quinone radical polymers performed by Colson and Nageles [21 ], which reveal similarities to our data, were interpreted in the framework of the variable range hopping. The conductivity for the variable range hopping can be expressed by [22] :

o(T) = 0.44e2auph(To T) -1/2 exp [-- (To~T)1/4],

(2)

where To = 19.4aa/kN(EF), a -1 is the localization length and Vph is the phonon optical frequency. The fit of obtained experimental data to the formula (2) is very good and for uph = 3.6 X 1013 S-1 [23] the localization length a -1 andN(EF) can be determined (see Table 1). The values of N(EF) and a -1 are in good agreement with values estimated from the magnetic susceptibility measurements [8, 9]. A t y = 0.003 the localization length is much too small and it implies very large N(EF). Just above the critical value Y c r o n e may expect a peculiar form of the density o f states and a deviation from equation (2). In this case, a ~ exp [-- (To~T) V] with 1/4 ~< v ~< 1/2 is expected [24]. Note also that the formula (2) has been derived for one sinlge hop, whereas in reality one has to include the contributions of all hops from one end of a sample to the other one [21]. Fibrilal structure of polyacetylene caused many speculations on tunnelling between metallic islands or fibrils. The electric field dependent transport experiments [13] exclude this hypothesis. Moreover, a rapid increase of the thermopower with decreasing temperature

896

PROPERTIES OF POLYACETYLENE TETRACHLOROFERRATE

at low temperatures was not observed as one could expect if the contribution of the interfibril hopping were significant.

Note added after the submission of the manuscript Park et al. [25] reported about measurements of the conductivity and thermopower in polyacetylene with y = 0.061 and 0.148 of FeC17~as the dopant. In the present communication a greater variety of FeC14 concentrations within the limit of the chemical stability (0 < y < 0.07) of the system has been studied. For y = 0.06 their data overlap with our results; it is not trivial in view of the seemingly ill-defined material.

10. 11.

12.

13. 14.

REFERENCES 1. 2.

3. 4. 5. 6. 7.

8. 9.

A. Profi, I. Kulszewicz, D. BiUaud & J. PrzyCuski, J.C.S. Chem. Comm. 783 (1981). D. Billaud, I. Kulszewicz, A. Profi, P. Bernier & S. Lefrant, Proc. Int. Conf. on the Physics and Chemistry of Conducting Polymers, Les Arcs, December 11-15, 1982 (in press); I. Kulszewicz & A. Profi, (to be pubfished). J. Przytuski, M. Zag6rska, K. Conder & A. Profi, Polymer 23, 1872 (1982). A. Profi, P. Bernier, D. Billaud & S. Lefrant, Solid State Commun. 46, 587 (1983). A. Profi, M. Zag6rska, J. Kucharski, M. Lukasiak & J. Suwalski, Mat. Res. Bull 17, 1505 (1982). H. Shirakawa, Proc. Int. Conf. on the Physics and Chemistry on Conducting Polymers, Les Arcs, December 11-15, 1982 (in press). Z. Kucharski, M. Lukasiak, J. Suwalski & A. Profi, Int. Conf. on the Physics and Chemistry of Conducting Polymers, Les Arcs, Dece,ner 11-15, 1982 (in press). D. Moses, A. Denenstein, J. Chen, P. McAndrew, T. Woerner, A.J. Heeger, A.G. MacDiarmid & Y.W. Park, Phys. Rev. B25, 7652 (1982). D. Moses, J. Chen, A. Denenstein, M. Kaveh,

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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T.-C. Chung, A.J. Heeger, A.G. MacDiarmid & Y.-W. Park, Solid State Commun. 40, 1007 (1981). S. Kivelson,Phys. Rev. B25, 3798 (1982). S. Roth, K. Ehinger, K. Menko, M. Peo & R.J. Schweizer, Proc. Int. Conf. on the Physics and Chemistry of Conducting Polymers, Les Arcs, December 11-15, 1982 (in press). J.A. Chroboczek & S. Summerfield, Proc. Int. Conf. on the Physics and Chemistry of Conducting Polymers, Les Arcs, December 11-15, 1982 (in press). A.J. Epstein, H.W. Gibson, P.M. Chaikin, W.G. Clark & G. Griiner, Phys. Rev. Lett. 45, 1730 (1980). K. Ehinger, W. Bauhofer, K. Menke & S. Roth, Proc. Int. Conf. of the Physics and Chemistry of Conducting Polymers, Les Arcs, December 11-15, 1982 (in press). Y.W. Park, A. Denenstein, C.K. Chiang, A.J. Heeger & A.G. MacDiarmid, Solid State Commun. 29, 747 (1979). T. I to, H. Shirakawa & S. Ikeda, J. Polym. Sei. Polym. Chem. Ed. 12, 11 (1974). B. Francois, M. Bernard & J.J. Andre,J. Chem. Phys. 75, No. 8, 4142 (1981). M. Przybylski & A. Graja, Physica 104B, 278 (1981); M. Przybylski, Physica 114B, 307 (1982). B.R. Bulka, Phys. Status Solidi (b) 118, K57 (1983). M.J. Rice & E.J. Mele, Chemica Seripta 17, 121 (1981); E.J. Mele & M.J. Rice, Phy~ Rev. B23, 5397(1981). R. Colson & P. Nageles, Phil. Mag. 38,503 (1978). P. Nageles, Amorphous Semiconductors, (Edited by M.H. Brodsky) p. 146. Moscow, Mir (1982) (in Russian). C.R. Fincher, M. Ozaki, A.J. Heeger & A.G. MacDiarmid, Phys. Rev. B19, 4140 (1979). N.F. Mott & J.H. Davies, Phil. Mag. B42,845 (1980). Y.W. Park, J.C. Woo, K.H. Yoo, W.K. Han& C.H. Choi, Solid State Commun. 46, 731 (1983).