On the metallic temperature dependence of the conductivity of doped polyacetylene

Solid State Communications, Printed in Great Britain.

Vol.47,No.lO,

ON THE METALLIC TEMPERATURE

pp.759-761,

1983.

OO38-1098/83 $3.00 + .OO Pergamon Press Ltd.

DEPENDENCE OF THE CONDUCTIVITY

OF DOPED POLYACETYLENE

Wan Meixiang, Wang Ping, Cao Yong and Qian Renyuan Instutute of Chemistry, Academia Sinica, Beijing, China and Wang Fosong, Zhao Xiaojian and Gong Zhi Changchun Institute of Applied Chemistry, Academia Sinica, Changchun,

China

(Received 21 April 1983 by A.A. Maradudin) The true metallic temperature dependence of the conductivity of various doped polyacetylene films of both fibrillar and granular morphology has been experimentally demonstrated by voltage shorted compaction measurements.

The metallic conduction in doped polyacetylene films has so far been revealed from their optical propertieslr thermoelectric power 2 and magnetic susceptibility measurements 3. Because of the fibrillar morphology of the polyacetylene film obtained on polymerization the metallic temperature dependence of its conductivity is not observed but a thermally activated conduction characteristic of a semiconductor. This is attributed to the contact resistance in the interfibrillar, intergranular or intercrystallitic regions of the film. Similar problem has been encountered in the measurement of the temperature dependence of conductivity of polycrystalline powder compactions. However, Coleman devised a voltage shorted compaction (VSC) method 4 to effectively short circuit the intercrystallitic contact resistance so as to get the true temperature dependence of conductivity on four probe conductivity measurements. Based on a careful manipulation and detailed analysis of the VSC configuration 5 we have succeeded to observe the true metallic temperature dependence of conductivity of various doped polyacetylene films of both fibrillar and granular morphology.

] (b) (a)

Fig. la. VSC measuring device. 1-insulating frame, 2-fixing adhesive, 3-Au wire, 4,6-conducting paste, 5-sample specimen. lb. Schematic showing the cross-section of the region between terminals 2,3 of the VSC device. A-conducting paste layer, B-wetted layer, C-organic conductor sample layer.

Experimental Polyacetylene films of fibrillar and granular morphology were obtained by polymerization of acetylene with Shirakawa catalyst in toluene 6 and in tetrahydrofuran 7 as solvent respectively. Doping with HCIO 4 and CIS03H took place in gas phase while doping with NOCIO 4 took place in nitromethane solution. Standard four probe conductivity measurement was carried out on rectangular shaped specimen with 20 ]/m diameter Au wire as leads (Fig. la), and the region between two voltage terminals was painted with Ag paste for VSC measurement. A constant current of 1-5 mA was passed through the specimen while the potential difference between the voltage terminals was measured with a Keithley model 148 Nanovoltmeter. The measurements covered a temperature range of 77-300K.

Table i. RT Conductivities of Doped Polyacetylene Films

(CH.~)x Dopant A CIS03H NOCIO 4 HCIO 4 12

Film Morphology* F G F G F G F G

* F-fibrillar morphology,

Results and Discussion

Y 0.05 00115 00124 0.108 0.i0 0.13 0.225 0.172

~RT, ~-icm-i 218 3 418 89 194 48 187 15

G-granular mophology.

It is apparent that the fibrillar films showed higher conductivity at room temperature than granular films, presumably due to the fact that there are much more interfibrillar contacts than

Ordinary four probe conductivity measurements. Results of conductivity measurements of polyacetylene films of fibrillar and granular morphology are shown in Table i. 759

~ETALLIC TEMPERATURE DEPENDENCE

760

spaces are filled with the conducting paste (Fig. ib), we arrive at the following equations 5

intergranular contacts. The apparent temperature dependence of the conductivity of these films gave nice straight line plots for ig(OT I/2) vs T -I/4 as shown in Fig. 2. This behavior has been

10 4

OVSC = CI~c + ( I - C I ) ~ I ~ s

,

C I = dl/(dl+d2)

4F

dOvsc do c dO s dT = Cld--T- + (l-Cl)~sld--T-- .

10 2

IG

i01

I

(2)

As dOc/dT is usually small and nearly constant, the temperature dependence of VSC conductivity reflects essentially the temperature dependence of the conductivity of the material under test. It is obvious from eq. (2) that small d I and large d 2 in the manipulation of conducting paste application are necessary to ensure good result. Fig. 3 shows the results obtained from VSC measurements on various doped polyacetylene

-iN

24

(i)

provided that ~c<<~s and Os<
i0 3

I-

Vol. 47, No. i0

I

I

26

I

I

I

28

I

30

I

f

i

32

I

34

l I00/T 4

vsc

Fig. 2. Apparent temperature dependence of the conductivity of doped polyacetylene films. Dopant: 1,2,3,4-CIS03H, NOCI04, HCI04, 12 • F-fibrillar morphology, G-granular morphology.

observed by many authors previously and has been interpreted after Mott model of variable range hopping 8 among localized states near the Fermi surface. As will be clear in the following that this is not the true temperature dependence of conductivity intrinsic to polyacetylene film, the T I/4 dependence of igO is nothing but the characteristics of the interfibrillar or intergranular contacts. In fact many compressed pellets of polycrystalline molecular solids 9 and ~-copper phthalocyanine evaporated film I0 also show such temperature dependence of their conductivity. Analogously it seems dubious to consider the frequency dependent conductivity obtained from the measurements below 1 MHz on fibrillar polyacetylene films II as the intrinsic electronic property of polyacetylene. VSC measurements. Using the model of two layers connected in parallel, in which one is the top conducting paste layer of thickness d I and the other is the underlayer of thickness d 2 of the material under test being fully wetted by the conducting paste so that the interfibrillar or intergranular

I

i

i

I00

i

200

Fig. 3. VSC measurements on doped polyacetylene films. Dopant: 1,2,3-CIS03H , NOCI04, HC!O 4 F-fibrillar morphology, G-granular morphology. films of both fibrillar and granular morphology. All these curves show an increase of conductivity with decreasing temperature from 300 to 77K, typical of metallic conduction. This demonstrates the successful application of VSC measurement to observe the true metallic conduction in doped polyacetylene films so far masked by interfibrillar or intergranular contact resistance. It remains to see how these films behave down to lower temperatures.

References i. C.R. Fincher, D.L. Peebles, A.J. Heeger, M.A. Druy, Y.Matsumura, A.G0 MacDiarmid,

5(30

T(K)

H. Shirakawa and S. Ikeda, Solid State C o ~ u n i c a t i o n s , 27, 489 (1978).

Vol. 47, No. IO

METALLIC TEMPERATURE

2, Y.W, Park, A, Denenstein r C.K. Chiang, A.J. Heeger and A.G. MacDiarmid, Solid State Communications, 29, 747 (1979). 3. B.R. Weinberger, J. Kaufer, A.J. Heeger, A. Pron and A.G. MacDiarmid, Physical Review, B20, 233 (1979). 4. L.B. Coleman, Review of Scientific Instrumentation, 49, 58 (1978). 5. M. Wan, D. Zhou, M. Li and R. Qian, Kexue Tongbao (Chinese Ed.), 1982, 1495. 6. T. Ito, H. Shirakawa and S. Ikeda, Journal of Polymer Science, Polymer Chemistry Edition, 12, ii (1974). 7. F. Wang, X. Zhao, Z. Gong, Y. Cao, Q. Yang and R. Qian, Makromolekulare Chemie, Rapid

DEPENDENCE

761

Co~unications, ~, 929 (1982). 8. N.F. Mott and E.A. Davis, "Electronic Processes in Non-Crystalline Materials", Clarendon Press, Oxford, 2nd ed., 1979,p.34. 9. C°S. Anithkumar and N. Umakantha, Philosophical Magazine, B44, 615 (1981). i0. z.Shi, J0 Zhang, Z. Cai, W. Huang and R. Qian, Abstracts, China-Japan Joint Symposium on the Conduction and PhotoConduction in Organic Solids and Related Phenomena, Beijing, 1983. ii. A.J. Epstein, H. Rommelmann, M. Abkowitz and H.W. Gibson, Physical Review Letters, 47, 1549 (1981).