EPR studies of binary alkali metal doped polyacetylene

~

Solid State Communications, Vol.64,No.1, pp.69-72, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00. Pergamon Journals Ltd.

EPR Studies of Binary Alkali Metal Doped Polyacetylene J. Ghanbaja*, C. Goulon**, J.F. Mar~ch~* and D. Billaud* * Laboratoire de Chimie Min~rale Appliqu~e, UA CNRS N ° 158 ** Laboratoire de Chimie Th~orique, UA CNRS N ° 22 Universit~ de Nancy I, B.P. 239, 54506 Vandoeuvre les Nancy C~dex, France (Received

2 May 1987 by E.F. BERTAUT)

The evolution of the EPR characteristics (AHpp, A/B and g) of vapour phase alkali metal doped polyacetylene has been examined as a function of the doping duration and the nature of the dopant (No, K, Rb, Cs). Different kinds of spins are responsible for the two observed signals. At intermediate doping levels, concomittant lightly and heavily doped parts of the material lead to the superposition of two signals or to an anomalous signal the form of which is discussed. For high doping levels, the linewidth AHDD of homogeneous samples follows a Z~ law with = 2.9 ~ 0.5. Ou~ results are compared with other data related to polyacetylene doped with alkali-metals in solvent media.

Int roduct ion

method of Ito et al. 9. Vapour phase alkali metel doping was carried out in the £PR tube in order to avoid any further sample transfer ~. EP~ soectra were recorded at room temperature with an Xband (9.8 ~Hz) 8ruker ER 200 D spectrometer.

Considerable attention is currently being paid to the compounds obtained by doping polyacetylene with alkali metals. Reduction of the polymer is carried out either chemically I using a solution of the alkali metal - naphtalene complex in, for example I tetrahydrofuran (THF) or electrochemicallyZ,3 using electrolytes obtained by the dissolution of alkali metal salts in a solvent such as THF. Under these conditions the intercalated alkali metals are more or less solvated according to the nature of the alkali and the solvent. We have shown that alkali metals in the vapour phase react directly with polyacetylene to form compounds of high electrical conductivity4,5. More recently, Moses et al. 6 have also described the vapour phase dopin~ of (CH)x with Na, Rb and Cs. The room temperature conductivity of the materi~ls obtained by these authors was, however, smaller by an order of magnitude than t~at of the compounds obtained by Ghanbaja et al.'. Annealing of samples obtained by using a solvent results in an improved crystallographic arrangement of the doped material and consequently leads to an increased electrical conductivity 8. The inhomogeneous distribution of the dopant within the polymer would thus likely be the source of the dispersion in experimental results. In this work, we have examined the evolution of the EPR signal during doping of polyacetylene by the alkali metals : M=Na, K, Rb, Cs. This is the first EPR study carried out on the compounds (MyCH) x synthesized by the direct action of the metal vapour on (CH)x.

Results and Discussion Figures 1 and 2 show the evolution of the room temperature EPR signal of cis-rich samples doped with rubidium and potassium for in creasing lengths of time. Considerable evolution of the behaviour is noted. The spectrum of the (CH) x utilized (70 % cis) for this study is represented on figure la : a single, symmetrical signal is observed with AHo- o = 7.5 ~ and g = 2.0026 ~ 0.0002. After a few hours of doping (figure Ib), a second type of behaviour is noted : the signal has narrowed, its intensity has increased and the signal is no longer symmetric. The disappearance of the wide siqnal of the initial cis (CH) x can b e interpreted in terms of the cis + trans isomerisation which is induced both by the heatin~ and by the diffusion of the dopant between the chains of the material 10 : the unpaired spins (neutral solitons) which are the source of the EPR signal become delocalized over increasingly longer trans sequences thus provoking a decrease in the width of the signal. This is ~n agreement with the relationship AHp = AD-I/3 Proposed by Holczer et al. 11 in w~ich A is a parameter accounting for the intensity of the interaction between the unpaired electrons and the nuclear spins of the hydrogen atoms and also between the electrons themselves ; D is the diffusion constant of the electron spins. The spin-spin relaxation time T2 of the paramagnetic centers will thus increase because of the inverse relationship relating it to ~Hp_p. It should be noted that the doping induced isomerisation process

Experimental Films of polyacetylene (CH)x of typical thickness 150-200 pm were synthesized using the 69

70

EPR STUDIES OF BINARY ALKALI METAL DOPED POLYACETYLENE

//~

t=0h

Vol. 64, No.

g=5103

(a) '213'

/~

(b)'

2G

t=Sh g=104

I

t=48h g=1.6 104

t=12h

(c)'

26

g=1.25 104

(b)'

J

5013

A/B =2~ AHpp=20.5 G

A,/B+EF=4.2 AHpp=I.2 G

' t=48h g =6.3 104 t =20h

(d)'213,

~

g=3.2 104

(c)'

2G,

A/B+B, =5.1 AHpp=l~ 5 G

~AHpp=l.2 G t=36h

20013 (e)~.._~,, A/B=~

AHpp=IgOG

.• ~,~

t=66h g=3.2 104

g=4 105

(d)'

5013

'

=2.6 AHpp=264 G

Figure I : Evolution of the EPR spectra of rubidium doped polyacetylene as a function of the doping time, t. g = gain of the EPR spectrometer

Figure 2 : Evolution of the EPR spectra of potassium doped polyacetylene as a function of the doping time, t. g = gain of the EPR spectrometer

is more rapid than thermal isomerisation : indeed, the latter should be done at 150°C for twe hours to obtain a total isomerisation12. Our EPR measurements carried out on samples weakly doped with Na, K, Rb and Cs furnish signals in which AHp_p is in the neighbourhood of 1 G, characteristic of fully isomerised samples 13. Durin~ the doping, the intensity of the signal increases in relationship with the increase in the number of solitons created during the isomerisation of the cis portions of the polymer. Simultaneously, the dissymetry ratio A/B of the signal rises as expected from the theory of Dyson 14 in which two conditions were established to explain the distorsion : first, the sample thickness must be large with respect to the skin depth, 8, and secondly, the time constant for the diffusion of the paramaonetic centers must be low with respect to the electron spin relaxation time. In the case of the materials studied here, the first moments of doping are characterized by an inhomogeneous distribution of the dopant within the polymer, the latter thus possessing majority regions which are heavily doped (metallic) and other, either little or undoped regions. In this case, the electromagnetic microwave field cannot penetrate uniformly into

the material because of the skin effect associated with the metallic regions. The narrow signal which is observed thus corresponds to species with unpaired spins (solitons) and the disto~ ~ sion is attributed to the doped portions o f ~ h e film. A comparable situation has already been observed in the case of copper doped with mangane, se 15 : the EPR signal of the manganese undergoes a distorsion because of the nonuniform distribution of the electromagnetic wave caused by the skin effect of the conduction electrons contributed by the copper. The third region is characterized by a decrease in the ratio A/B from the maximum value of 4.5 to 2.7, whereas the width of the signal, AHp_p, remains almost unchanged. This is in qualitative agreement with the work of Kodera 16 who studied the variation of A/B as a function of the different EPR parameters of the material. The behaviour is typical of a degenerate semiconductor in which the electrical conductivity is fairly high whereas the mobility of the spin carriers is low : in other words, T D is high. An intermediary region exists (figure 2b) in which the narrow signal indicated above is superposed on a wider one for which AHp_p increases with the atomic number of the dopant.

1

Vol. 64, No. I

EPR STUDIES OF BINARY ALKALI ME.TAL DOPED POLYACETYLENE

The origin of the wider signal will be discussed below. Upon further doping, the narrow signal decreases in intensity and finally disappears. An unusual evolution in its shape can be seen on figures ld and 2c with the appearance of a negative peak at low fields. This anomaly is not seen instantaneously but involves several intermediate states as seen in figure Ic. Based on the curves given by Kodera 16 relating A/B to d/6 (d = sample thickness), a ratio (TD/T21/2 >> I leads to an increase in the value of A/B from 1 to 4.5 for a value of d/~ which increases from 0 to 2.5. However, experimentally when the negative peak appears at low fields, A/B rises above the limiting value of 4.5 and increases to 8 (figure 2c). This is possible only if (TD/T2)1/2 becomes less than 1, which would imply a drastic change in mobility of the paramagnetic centers within the skin depth. Kodera has shown that in this situation the product a : (d/~)(TD/T2)I/2 should be less than I and that the EPR signal should present, as in the case for our samples, a negative peak at low fields. The intensity of this peak diminishes when a increases. At the end of the doping process only a single wide asymmetric signal is observed characteristic of the metallic state of the sample. Note that the unpaired spins which would persist in highly doped samples do not give rise to a signal. This is due to the fact that the skin effect leads to significant absorption of microwave energy near the sample surface so that the unpaired spins found well inside the sample where the intensity of the microwave field is very low, do not allow them to participate in the resonance phenomena. The variation in width of this wide signal as a function of the nature of the dopant allows determining the types of interactions responsible for the widening ; however, because of the dissymetry of the signal, its intearation does not reproduce the true absorption curve. The experimental values of AH _ and of the g factor should thus be correctedPu~inq the following formulas : Ax l

AH = AH a

~

xa

AN

Ha

; g = ga (I + A-'~"''F ) =ga ~'-

a a o o resonance field apparent g factor, apparent width of signal, normalized apparent resonance field= 2~T 2 --_-_-_-_-_-_-_-_-_~-(Ha-H o) Ax a = n 6 F m a l i z e d a p p a r e n t l i n e w i d t h =

w h e r e Ho ga 6Hp_p xa

= = = =

2BT 2

T

(AHa)

Ax I = normalized linewidth : in the case of a Lorentzian absorption line = 1.15 The following table contains the corrected values of AHp_p and o corresponding to the polyacetylene heavily doped (15 %) with Na, K, Rb ~nd Cs. Figure 3 represents the variation of AH __ as a function of the atomic number Z of th~ ~opant for the material doped to saturation. It is seen that AHp_p varies as Z~ with = 2.9 ~ 0.5. These-results are very similar to those indicated by Rachdi et al. 17 for the case of polyacetylene doped in a solvent milieu with Li, K or Rb. A similar relationship has also been reported for graphite intercalation com-

Na AHpp 4±1

71

K

Rb

Cs

25±4

190±20

500±50

(G) g

2.0028 2.0031 + ÷

1.98 +

0.0~02 0.~003 010002

AH pp(G) Cs

1000 ~

Rb 100 K

10.. ,Na o~ = 2.9*-0,5

1-

.1 .1

1

10

100Z

Figure 3 : Room temperature EPR linewidth AHpp of alkali metal doped polyacetylene versus the atomic number Z of the dopant

pounds 18. This evolution of AHp-p with Z shows that the spin-orbit interaction at the dopant site must be the principal source of this widening. Elliott 19 and Yafet 20, who studied the influence of the spin-orbit coupling on the electron spin relaxation processes in metals and semiconductors indicated a dependence of AHp_p on Z4. Study of the temperature dependence of the EPR parameters AHp_ 0 and A/8 during the doping processes should furnish supplementary information on the nature of the spins responsible for the EPR spectrum and may be further correlated with our measurements of electrical conductivity as a function of temperature 7. Acknowledgements, We would like to express our thanks to Dr. P. Bernier for helpful discussions.

72

EPR STUDIES OF BINARY ALKALI METAL DOPED POLYACETYLENE

Vol. 64, No. I

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