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Submitted on 1 Jan 1986
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ELECTRONIC STRUCTURE AND ORIENTATION OF PYRROLE, N METHYLPYRROLE, THIOPHENE,
3 METHYLTHIOPHENE AND THEIR POLYMERS ON Pt : A NEXAFS STUDY
G. Tourillon, A. Fontaine, M. Sagurton, N. Garrett, G. Williams
To cite this version:
G. Tourillon, A. Fontaine, M. Sagurton, N. Garrett, G. Williams. ELECTRONIC STRUCTURE AND
ORIENTATION OF PYRROLE, N METHYLPYRROLE, THIOPHENE, 3 METHYLTHIOPHENE
AND THEIR POLYMERS ON Pt : A NEXAFS STUDY. Journal de Physique Colloques, 1986, 47
(C8), pp.C8-579-C8-583. �10.1051/jphyscol:19868108�. �jpa-00226004�
ELECTRONIC STRUCTURE AND ORIENTATION OF PYRROLE, N METHYLPYRROLE, THIOPHENE, 3 METHYLTHIOPHENE AND THEIR POLYMERS ON Pt : A NEXAFS STUDY
G. TOURILLON, A. FONTAINE, M. SAGURTON* , N.R. GARRETT* and G. WILLIAMS*
LURE, Laboratoire CNRS, CEA, MEN, Bdtiment 2090, F-91405 Orsay Cedex, France
*NSLS, Brookhaven Upton, Long Island, NY 11973, U. S.A.
ABSTRACT
From the polarization dependence of the NEXAFS (XANES) spectra, pyrrole is found to be normal on the Pt ( 1 1 1 ) surface. Conversely undoped poly 3
methylthiophene lies flat on the surface.
Doped conducting,or undoped semiconducting polymer exhibits change in the intensity of the (3b, r ) antibonding band transition. In addition new empty states at lower energy are created. This is consistent with XPS and UPS spectroscopy.
RESUME
La dependance des spectres drabsorption avec la polarisation montre que le pyrrole est orient6 perpendiculairement la surface de Pt(ll1). Au contraire le polythioph&ne non dope est orient6 a plat la surface.
La forme dop6e et non dope du poly 3 me&hylthiopb&ne differ6 par l'intensite de la transition vers la bande antibante (3b,11 ). En outre cette d6croissance dans la forme dopee s'accompagne de la creation de nouveaux Btats vides 2 des Bnergies infBrieures, proches du niveau de Fermi. Ces resultats sont en bon accord avec 1' XPS et llUPS.
Five membered heterocycles are of great interest because they form organic conducting polymer of high purity when made by electrochemistry (1). Numerous problems remain concerning the conduction mechanism, the nature of the bonds between the electrode surface and the polymer both in doped and undoped states and the orientation of the first layers on the substrate.
The preliminary reported NEXAFS experiments concern i) the adsorption of pyrrole. N methylpyrrole, thiophsne on Pt ( 1 1 1 ) and ii) the electronic structure and orientation of thin doped and undoped poly 3 methylthiophene - PMeT films grafted on Pt.
All the data have been recorded in both partial and total yield modes at NSLS (Brookhaven). For the adsorption experiments, the spectra have been obtained at the NK and SL,, edges both after initial deposition of multilayer at 84 K on clean Pt ( 1 1 1 ) and after annealing treatments. Poly 3 methylthiophene - PMeTlis
electrochemically grafted on a flat Pt electrode immersed in a CH, CN. 5.10 M LiC104 - 5 10 M -1 3 methylthioph&ne electrolytic medium. The electrode is anodically polarized at + 1.35 V/SCE (saturated calonnel electrode) for obtaining the doped film and cathodically polarized at - 0.2 V/SCE for the undoped state. The thickness of PMeT is monitored by the electrolysis time such as a few monolayers up to thick films can be deposited on Pt.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868108
C8-580 JOURNAL DE PHYSIQUE
Adsorption of p y r r o l e , N methylpyrrole, thiophene on P t (111)
Five membered h e t e r o c y c l e s can be regarded a s being derived from benzene, t h e - CH = CH - group being replaced by - NH, - N C H , , - S -. The heteroatom provides two n e l e c t r o n s which i n a d d i t i o n t o t h e f o u r r e l e c t r o n s of t h e r i n g makes an aromtic system. I n a simple LCAO p i c t u r e ( 2 ) , combinations of t h e f i v e p atomic o r b i t a l s produce f i v e molecular o r b i t a l ; t h r e e of which a r e occupigd i n t @ e n e u t r a l ground s t a t e ( I b , n , 2 b , ~ , la 2 r ) and two which a r e unoccupied ( 3 Q l n , 2 a , ~ ) , t h e h i g h e s t occupied o r b i t a l ( l a 2 * ) and t h e lowest unoccupied one (3b1n ) having a pure n carbon c h a r a c t e r , Several i n t r a molecular resonance t r a n s i t i o n s a r e t h u s expected. S p e c t r a above t h e NK edge f o r a m u l t i l a y e r of p y r r o l e and N methylpyrrole adsorbed on P t a t 84 K a r e s i m i l a r and mainly composed of t h r e e peaks a t 403, 408 and 413 eV. These peaks a r i s e fsom t h e t r a n s i t i o n of t h e 1 s e l e c t r o n t o t h e 3b,n o r b i t a l , t h e o(C - N ) and t h e a ( C - C) shape resonances respectively.,After ,
annealing a t 156 K , t h e s t r o n g and opposite angular dependence of t h e n and a resonances c l e a r l y shows t h a t p y r r o l e i s o r i e n t e d i n such a way t h a t t h e molecular plane i s normal t o t h e P t s u r f a c e . I n marked c o n t r a s t , N methylpyrrole remains disordered a f t e r annealing t r e a t m e n t s . This d i s p a r i t y suggests t h a t p y r r o l e i s most l i k e l y adsorbed normal t o t h e s u r f a c e with t h e n i t r o g e n atom down. I f p y r r o l e was indeed edge bonded a s it was suggested by Sexton ( 3 ) from an EELS s t u d y , N methylpyrrole could t a k e t h e same ordered c o n f i g u r a t i o n s i n c e t h e CH, group would have no s t e r i c e f f e c t on t h e bonding.
F u r t h e r annealing l e a d s t o t h e d i s s o c i a t i o n of t h e p y r r o l e molecule a s evidenced by t h e l o s t of p o l a r i z a t i o n dependence and by t h e appearance of a new t r a n s i t i o n a t low energy. With N methylpyrrolke, no modification i s d e t e c t e d which confirm t h e s t a b i l i z a t i o n of t h e r i n g by t h e methyl group.
S i m i l a r d i s s o c i a t i o n processes under annealing is observed when thiophene is adsorbed on c l e a n P t (111). T h i s h a s been r e p o r t e d on c l e a n N i a l s o .
E l e c t r o n i c s t r u c t u r e and o r i e n t a t i o n of t h i n doped and undoped poly 3 methythiophene (PMeT) f i l m s
Upon polymerization, t h e two d e e p e s t , o c c u p i e d g a l e n c e o r b i t a l s l b l n and 2b,n and t h e two lowest unoccupied o r b i t a l s 3bln and 2a2n a r e not modified and g i v e t h e corresponding bands a t t h e same energy. The modification comes only from t h e la,n occupied o r b i t a l which l e a d s t o t h e formation of a
7bonding band l o c a t e d a t 1 eV below EF. NEXAFS spectroscopy is t h u s a well adapted method t o probe t h e unoccupied s t a t e s . I n a d d i t i o n t h e energy shape resonance p o s i t i o n i s c o r r e l a t e d with t h e bond l e n g t h ( 4 ) and g i v e s c l e a r evidence of eventual s t r u c t u r a l
modification of PMeT between t h e doped and undoped s t a t e s . I t should be p o s s i b l e i n t h e s e c o n d i t i o n s t o determine t h e n a t u r e of t h e conduction mechanism of t h e s e organic conducting polymers :
i ) bipolaron model, i i ) degenerated semiconductor o r i i i ) o b t e n t i o n of a m e t a l l i c l i k e behavior by c r e a t i o n of enough h o l e s i n t h e valence band.
Spectra (Fig. l a and 2a) recorded a t t h e CK edge and SL,, edge f o r a t h i n undoped PMeT g r a f t e d on Pt a r e very s i m i l a r with those obtained on thiophene ( 5 ) i n gas phase and i n s o l i d s t a t e which c l e a r l y evidences t h e same s t r u c t u r a l
a r c h i t e c t u r e f o r t h e polymeric backbone. The assignments of t h e t r a n s i t i o n s a r e r e p o r t e d on on t a b l e 1 . When t h i s , t h i n f i l m is eaectrochemically doped (Fig. I b , 2b) t h e energy p o s i t i o n s of t h e a (C - C ) and a (C
=C) shape resonances a r e e x a c t l y a t t h e same values than i n t h e undoped form. One can i n f e r t h a t no chemical and s t r u c t u r a l modifications take place which is t h e r e v e r s e of what i s expected with t h e b i p o i a r o n model where t h e i n i t i a l aromatic s t r u c t u r e should be replaced by t h e quinoid l i k e one.
The s e c o n 4 i n t e r e s t i n g point concerns f o r both t h e C and S edges, t h e decrease of t h e n band i n t e n s i t y with t h e appearance on t h e C: K edge of a shoulder a t low energy a t t r i b u t e d t o t h e formation of empty s t a t e s i n the gap confirmed with o t h e r experiments done a t Bessy. These d a t a combined with t h e XPS and UPS r e s u l t s
( 6 ) confirm t h e narrowing of t h e gap (from 2.2 eV i n t h e undoped s t a t e t o - 0.3 -
.4
