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ELECTRONIC AND STRUCTURAL
MODIFICATIONS OF POLYSELENOPHENE
DURING ELECTROCHEMICAL DOPING PROCESS OBSERVED IN SITU BY DISPERSIVE X-RAY
ABSORPTION SPECTROSCOPY
G. Tourillon, E. Dartyge, A. Fontaine, A. Jucha, C. Andrieux
To cite this version:
G. Tourillon, E. Dartyge, A. Fontaine, A. Jucha, C. Andrieux. ELECTRONIC AND STRUCTURAL
MODIFICATIONS OF POLYSELENOPHENE DURING ELECTROCHEMICAL DOPING PRO-
CESS OBSERVED IN SITU BY DISPERSIVE X-RAY ABSORPTION SPECTROSCOPY. Journal
de Physique Colloques, 1986, 47 (C8), pp.C8-551-C8-554. �10.1051/jphyscol:19868103�. �jpa-00225998�
JOURNAL D E PHYSIQUE
C o l l o q u e C8, s u p p l e m e n t a u n o 1 2 , Tome 47, dbcembre 1 9 8 6
ELECTRONIC AND STRUCTURAL MODIFICATIONS OF POLYSELENOPHENE DURING ELECTROCHEMICAL DOPING PROCESS OBSERVED IN SITU BY DISPERSIVE X-RAY ABSORPTION SPECTROSCOPY
G. TOURILLON, E. DARTYGE, A. FONTAINE, A. JUCHA and C. ANDRIEUX"
LURE ( L a b o r a t o i r e CNRS, CEA, M E N ) , B a t i m e n t 209D, F-91405 Orsay C e d e x , F r a n c e
' ~ a b o r a t o i r e des C o m p o s e s T h i o o r g a n i q u e s , u n i v e r s i t e de C a e n , F - 1 4 0 3 2 C a e n , France
ABSTRACT
The e l e c t r o n i c and s t r u c t u r a l m o d i f i c a t i o n s o f polyselenophene d u r i n g t h e e l e c t r o c h e m i c a l doping have been s t u d i e d by i n s i t u d i s p e r s i v e X-ray a b s o r p t i o n s p e c t r o s c o p y . XANES and EXAFS o b t a i n e d a t the C KAedge and t h e Se K-edge show t h a t i) e l e c t r o n s a r e e x t r a c t e d from t h e n bonding band w i t h a m o d i f i c a t i o n o f t h e n band d i s t r i b u t i o n , i i ) t h e Se7C bond l e n g t h i n c r e a s e s from 1.885 A (monomer) t o 1 .I95 A when t h e polymer is s y n t h g t i z e d , i i i ) no v a r i a t i o n i n t h e C-C and C = C bond l e n g t h v a l u e i s observed when polyselenophene i s doped i n o p p o s i t i o n with t h e b i p o l a r o n model and i v ) t h e i n t e r a c t i o n between t h e Se atom and t h e dopant is observed which l e a d s t o a r e o r d e r i n g of t h e polymeric c h a i n s .
RESUME
Les m o d i f i c a t i o n s s t r u c t u r a l e s e t e l e c t r o n i q u e s du polyselenophene l o r s de s o n dopage e l e c t r o c h i m i q u e o n t 6 t 6 d t u d i e e s i n s i t u ?I l q a i d e de l a
s p e c t r o s c o p i e d ' a b s o r p t i o n X e n mode d i s p e r s i f . Le XANES e t lvEXAFS o b t e n u s aux s e u i l s K du carbone e t du selenium montrent que i ) d e s e l e c t r o n s s o f t t e x t r a i t s de l a band n l i a n t e a v e c m o d i f i c a t i o n d e l a d i s t r i b u t i o n de l a bande r a n t i l i a n t e , i i ) l a longueur de l a l i a i s o n Se-C augmente de 1.885 A (pour l e monom8re) 3 1.915 A l o r s q u e l e polymere e s t s y n t h 6 t i g 6 , l i l ) aucune v a r i a t i o n de l o n g u e u r pour l e s l i a i s o n s C;-C et C = C n ' e s t observge l o r s q u e l e polym8r-e e s t dope c o n t r a i r e m e n t 3 c e q u i e s t prdvu p a r l e modsle b i p o l a r o n et i v ) l q i n t e r a c t i o n e n t r e l v a t o m e d e .
sdl6nium e t l e dopant e n t r a f n e une r e o r g a n i s a t i o n d e s c h a r n e s polym6riques.
Among t h e c l a s s o f conduction polymers, e l e c t r o c h e m i c a l l y s y n t h e t i z e d p o l y t h i o p h e n e , polyselenophene and d e r i v a t i v e s ( 1 ) a r e t h e f i r s t c l a s s of m a t e r i a l s which a r e c h e m i c a l l y s t a b l e i n a i r and m o i s t u r e i n b o t h doped and undoped s t a t e s . However t h e c o n d u c t i o n mechanism and t h e e l e c t r o n i c s t r u c t u r e of s u c h polymers a r e n o t w e l l known. S e v e r a l models were proposed t o e x p l a i n t h e l a r g e m o d i f i c a t i o n of t h e c o n d u c t i v i t y d u r i n g t h e doping : i ) f o r m a t i o n of l o c a l i z e d p o s i t i v e bands i n t h e gap ( b i p o l a r o n t h e o r y ) ( 2 ) , i i ) o b t e n t i o n of a d e g e n e r a t e d semiconductor o r i i i ) narrowing of t h e gap a s s o c i a t e d with a
q q m e t a l l i c l q l i k e behavior. We have r e c e n t l y shown, by u s i n g XPS, UPS ( 3 ) and X-ray a b s o r p t i o n s p e c t r o s c o p i e s ( 4 ) , t h a t e l e c t r o n s a r e e x t r a c t e d from t h e n bonding band yhen p o l y t h i o p h e n e and d e r i v a t i v e s a r e doped. Consequently, a s h i f t o f t h e n and T bands towards t h e Fermi l e v e l is observed which f i n a l l y r e s u l t s i n a narrowing o f t h e gap from 2.2 eV (undoped s t a t e ) t o
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0.3 r; 0.4 eV ( f u l l y doped s t a t e ) .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868103
C8-552 JOURNAL DE PHYSIQUE
These r e s u l t s a r e i n agreement with t h e d a t a a l r e a d y obtained by U V n V i s and ESR spectroscopy and confirm t h e appearance of a l l m e t a l l i c " l i k e behavior, i n opposition with t h e bipolaron theory. Thus it was very important t o check t h e v a l i d i t y of t h i s model on o t h e r systems. We r e p o r t i n t h i s paper i n v e s t i g a t i o n of polyselenophene a t t h e C K edge and Se K edge.
Polysel-enophene is electrochemicaJly g r a f t e d on P t o r g r a p h i t e e l e c t r o d e i n CH, CN t 5.10
'
M N (Bu), So,CF, -. 5.10'
M selenophene e l e c t r o l y t i c medium.The e l e c t r o d e i s a n o d i c a l l y p o l a r i z e d a t + 1.60 V/SCE ( s a t u r a t e d calomel e l e c t r o d e ) t o o b t a i n t h e doped f i l m and c a t h o d i c a l l y p o l a r i z e d a t + O.lV/SCE t o undoped t h e f i l m s . Two s e t s of s p e c t r a have be& recorded :
-
One e x - s i t u , a t t h e CK edge on a 600 A t h i c k f i l m deposited on Pt ( e l e c t r o n d e t e c t i o n mode i n UHV).-
The second a t t h e Se K edge on a 2 @-thick f i l m g r a f t e d on g r a p h i t e . Data were c o l l e c t e d using t h e d i s p e r s i v e scheme. The high value of t h i s edge (- 12658 eV) combined w i t h t h e f a s t a c q u i s i t i o n time of t h e d i s p e r s i v e method ( 5 ) allowed us t o follow i n r s i t u , t h e e l e c t r o n i c modifications of t h e polymer under t h e electrochemical treatment.1 rn EXAFS ON Se K-EDGE
The k3 weighted Fourier transforms of t h e monomer and t h e polymer a r e shown i n Fig. 1 ( a n a l y s i s done using a 400 eV window).
-
Two peaks a t 1.5 A and 2.45 A (non c o r r e c t e d from t h e phase s h i f t s ) a r e observed f o r t h e monomer, which a r e assigned t o t h e Se=C and Se-C bondsr e s p e c t i v e l y
-
The same peaks a r e d e t e c t e d f o r t h e undoped and doped polyselenophene..
aB
The i n t e n s i t y of t h e second one, however, d e c r e a s e s by a f a c t o r two with t h e appearance of a new one a t
-
2.8 A. The o r i g i n of t h e s e modification is l i k e l y due t o t h e aa coupling of t h e monomeric u n i t s during t h e polymerization. Theni n t e r f e r e n c e s should occur between t h e Se-C bond of one c y c l e with t h e Sere bond, C belonging t o t h e next one. This sh8uld modify t h e i n t e n s i t y r a t i o oP t h e s e t a o peaks.
iam'rw 4
-,".-
LO , m a B0.0 - 0
E N E R G Y - e V
-Figure 2
Figure 1 : R a d i a l d i s t r i b u t i o n s o f polyseleno- phene a t d i f f e r e n t doping l e v e l s a )
I selenophene, b ) undoped state,^) 2 %
0.0 1 0 . 0 doping l e v e l , d ) 15 % doped and e )
D I S T A N C E S f u l l y doped (30 % ) .
Figure 1 Figure 2 : F o u r i e r f i l t e r e d curve o f a f u l l y S03CF3 doped polyselenophene, ob- t a i n e d from t h e broad peak i n t h e range 4 - 5.5
a.
-
During t h e o x i d a t i o n of polyselenophene, a broad peak appears i n t h e range 4-5.5 A whose i n t e n s i t y i n c r e a s e s with t h e doping l e v e l . The F o u r i e r f i l t e r e d curve of t h i s peak is s p l i t t e d i n two domains (Fig. 2 ) with ac h a r a c t e r i s t i c b e a t mode i n between. The low k o s c i l l a t i o n s could be a s s o c i a t e d t o
a light element backscatter. Conversly, the oscillations over the node of the
"beat" are relevant to high Z element which can be identified only as Se in this system.
These characteristics reveal that i) the dopant interacts with the Se atom which is consistent with the appearance of positive charge on the heteroatom and ii) the polymeric chains rearange during the doping.
However, we cannot decide if the Se2Se interaction comes from two adjacent monomeric units or from two different polymeric chains.
The fit of the Fourier filtered spectra reveals an increase of the Se-C bond length when the polymer is synthetized (1.915 A instead of 1.885 A for the
"
monomer) and the number of neighbors remains constant. When the polymer is doped, no variation in the Se-C bond length is observed.
XANES
-
Five membered heterocycles (pyrrole, thiophene, selenophene) are planar molecules which have C2" symmetry with an inAplane axis containing the heteroatom.
A LCAO calculation shows that the highest occupied orbital and the lowest unoccupied orbital correspond to pure n orbitals (6). No theoretical calculation is available gor polyselenophene. However we expect the formation of a pure r bonding and r antibonding bands upon coupling as polypyrrole and polythiophene.
Intramolecular resonance transitions are frhus expected when a carbon or Se core electron is excited into these unfilled T bands.
C K EDGE
For an undoped thin polyselenophene film (Fig. 31, several peaks are observed at 283,,285, 291 and 301 ex. These peaks jtrise from the transition of the 1s electron to r band, o (C-Se), o (C
-
C) and a (C = C) shape resonances (Table 1 1.The same bound states and shape resonances are observed on a CJ.O,* doped film, except an additionnal shoulder at 286 eV. and a decrease of the T
transition intensity.
This decrease is directly linked to the extraction of electrons fromthe r,band and to the modifications of the n and n band distribution. In addition the Y
a
285 295 305
ENERGY OV
b
ENERGY -0V
Figure 3 : XANES spectra at the CK edge of a thin polyselenophene film grafted on Pt a) undoped state b) fully doped
JOURNAL DE PHYSIQUE
TA3!.E 1 : Energies and pro;osed assignoents of feztares i n the carbo;, I(
spectra of poLyse1enophene pSe
UNDOPED PSe DOPE3 PSe ASSIGflSbZlT
E(e.1; E(e:i)
F i g u r e 4 : Se K-edge XANES of polyseleno- phene
.
a) selenophene, b) undoped,
C ) 2 % doping level, d ) 15 %, e) 30 %.
o (C
-
C) and o (C t = C) shape resonances are observed at the same energy for both doped and undoped states, which means that no variation in the C-
C (or C = C) bond length value occurs in opposition with the bipolaron model where the initial aromatic structure should evolve to a quinoid like structure.Se K EDGE
-
The XANES spectra for the monomer (selenophene) and for the undoped polyselenophene are identical and characterized by a broad white line centered at-
12660 eV. ThAs band arises from the excitonic transitAon of the 1s electron i) to the empty ?T prbital (selenophene) or to the empty r antibonding band (polymer and ii) to the o shape resonances associated with the Se-C bond (1.885 A ) .-
During the doping process, this white line is splitted into two components : one on the low energy side of the absorption edge and the other one at higher energy ( - 15 eV above this edge), the intensity of both transitions increasing with the doping level. These features should be due to the creation of empty states near the Fermi level and/or to the appearance of localized charge on the Se atom which modifies the potential barrier distribution near this atom.Four conclusions can be drawn from this X-ray absorption study : i) the electrochemical polymerization mechanism agrees with the aa coupling of monomeric units, ii) during the,doping electons are extracted from the r bonding band with a modification of the u band distribution, iii) the aromatic structure does not evolve to a quinoid-like structure as expected with the bipolaron theory iv) the interaction between the Se atom and the dopant is detected by EXAFS, which induces a reorganization of the polymeric chains.
REFERENCES
(1) Handbook on "conducting polymers" Ed. by T. SKOTHEIM, Vol 1 and 2, Marcel Dekker (1986). New York
(2) J.L. BREDAS, B. THEMANS, J.G. FRIPIAT and J.M. ANDRE Phys. Rev. B 29, 6761 (1984)
(3) Y. JUGNET, G. TOURILLON and TRAN MINH DUC, Phys. Rev. Lett. April, 1862 (1986) (4) G. TOURILLON, A. FONTAINE, M. SAGURTON, R. GARRET and G. WILLIAMS
4rd International EXAFS conference, Fontevraud (France), July 1986
(5) E. DARTYGE, C. DEPAUTEX, J.M. DUBUISSON, A. FONTAINE, A. JUCHA, P. LEBOUCHER and G. TOURILLON, SRI Stanford, July 85, NIM to be published
(6) A. MORDELLI, M. GUENA, D. JONES, G. DISTEFANO, K.J. IRGOLIC, K. FRENCH and G.C. PAPPALARDO, Chem. Phys. 88, 455 (1984)
