ATOM-PROBE STUDY OF CONDUCTING POLYMER : POLYPYRROLE

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ATOM-PROBE STUDY OF CONDUCTING POLYMER : POLYPYRROLE

O. Nishikawa, H. Kato

To cite this version:

O. Nishikawa, H. Kato. ATOM-PROBE STUDY OF CONDUCTING POLYMER : POLYPYRROLE.

Journal de Physique Colloques, 1986, 47 (C2), pp.C2-203-C2-208. �10.1051/jphyscol:1986230�. �jpa-

00225663�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n03, Tome 47, mars 1986 page C Z - 2 0 3

ATOM-PROBE STUDY OF CONDUCTING POLYMER : POLYPYRROLE

O. NISHIKAWA and H. KAT0

Department o f Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute o f Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Abstract

-

A thin layer of conducting polymer, polypyrrole, was deposited on a Pt tip and mass analyzed by the atom-probe. The variation of the mass spectrum of fragment ions of the poly- pyrrole with exposure time to air indicates that the first stage of oxidation proceeds fairly rapidly but the transition to the fully oxidized stage is rather slow. The estimated thickness of the fully oxidized section is about 45 nm in three months.

Conducting polymers, e.g. polypyrrole(PP)/l/ and polythiophene/2/, come to be intensively interested because such polymers may lead to a new technique of fabricating Molecular-sized Electron Devices(MED)/3/. One problem of these materials is the oxidation: it alters electronic and optical properties/l,4/. Polypyrrole takes up oxygen fairly actively but no detailed study on the oxidation process and the distribution of oxygen in the PP layer has been conducted because no analytical instru- ment has the capability of mass analyzing the polymers in a direct and microscopie way. The atom-probe/5,6/ is known to be one of the most powerful instrument for mass analyzing individual atoms and molecules

field evaporated £rom a specimen surface as positive ions. Since field evaporation proceeds atomic layer-by-layer quite orderly and the

detected ions reflect the binding States of evaporated atoms and fragments of molecules on the specimen surface and in the molecules, it was expected that the A-P analysis would be one of the most suitable approach for clarifying the oxidation process of the conducting

polymer S.

An A-P specimen is a sharp needle with a tip radius of less than 100 nm. Since it is impossible to make such a fine PP tip, a thin PP film was electrochemically deposited on a Pt tip. The electrolyte solution is acetonitrile containing 0.05 M pyrrole monomer and 0.1 M TEAPC[(Et)4 NC104] which provides the CIO4- anions. The background pressure of

the A-P was 3 x 10-8 Pa and the tip temperature was maintained at or below 100 K.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986230

C 2 - 2 0 4 JOURNAL DE PHYSIQUE

Many H+,

c2+

^{a n d}

cf

i o n s w e r e d e t e c t e d £rom a t i p s t o r e d i n a i r f o r o n e d a y , F i g . 1. The o t h e r m a s s p e a k s a r e i d e n t i f i e d a s t h e f r a g m e n t s o f PP s u c h a s N H + , CNf, CNH+, CO+ a n d COH+, a s shown i n F i g . 2 ( a ) , ( b ) a n d

( c ) . The d e t e c t i o n o f CO+ a n d COH+ i n d i c a t e s t h a t e v e n a f r e s h PP l a y e r i s p a r t l y o x i d i z e d . The d o t s i n t h e f i g u r e i n d i c a t e t h e e l e c t r o n u n p a i r e d by t h e o x i d a t i o n o f c a r b o n a t o m s . Many o t h e r m a s s p e a k s a r e s e p a r a t e d by o n e m a s s u n i t a c c o r d i n g t o t h e number o f a t t a c h e d h y d r o g e n a t o m s i n a f r a g m e n t .

Mass to charge ratio

F i g . 1

-

Mass s p e c t r u m o f a p o l y p y r r o l e f i l m e x p o s e d t o a i r f o r 1 d a y .

For t h r e e d a y o l d t i p s , t h e n u m b e r s o f H + , c 2 + a n d C+ i o n s d r a s t i c a l l y d e c r e a s e d t o n e a r l y z e r o a n d t h e m a s s c o r r e s p o n d i n g t o cNOH2+, C N O H ~ + and

c2Nof

e m e r g e d , F i g . 3 , i n d i c a t i n g t h a t o x y g e n b r e a k s i n t o t h e p o s i t i o n b e t w e e n t h e C-H a n d N-H b o n d s a s shown i n F i g . 2 ( c ) . One month o l d PP l a y e r e x h i b i t e d f e w m a s s p e a k s o f CO+, COH+, C N O H ~ + a n d C N O H ~ + , F i g . 4. T h e s e f r a g m e n t i o n s a r e f i g u r e d o u t w i t h o u t b r e a k i n g a d o u b l e bond, F i g . 2 ( c ) . Many s m a l l p e a k s shown i n F i g . 1 a n d 3 s u b m e r g e d i n d i c a t i n g a l o w e r h y d r o g e n c o n c e n t r a t i o n i n t h e PP l a y e r d u e t o t h e a d v a n c i n g o x i d a t i o n . F o r t h e t h r e e month o l d l a y e r t h e i n i t i a l - l y d e t e c t e d 4600 i o n s e x h i b i t e d o n l y f o u r m a j o r p e a k s , 2 8 , 4 2 , 5 6 a n d 8 4 , F i g . 5 ( a ) . T h e s e p e a k s c o r r e s p o n d t o t h e f r a g m e n t s o f t h e f u l l y o x i d i z e d PP l a y e r , CO+, CNO+, c 2 0 2 + a n d c 2 ~ 2 o 2 + a s shown i n F i g . 2 ( b ) . The p e a k s a t 2 9 , 4 3 a n d 5 7 a r e t h e i o n s w i t h t h e i s o t o p e

I3c

^{and t h e}

i o n s w i t h a h y d r o g e n atom s u c h a s COH+ The s m a l l p e a k s a t 28 5 42.5 and 5 6 . 5 a r e t h e d o u b l y c h a r g e d i o n s , i 3 ~ 1 2 ~ 0 2 2 + , 1 2 ~ ~ 0

l3ci2cN2

~ ~ 2 ~ . 0 2 ~ ~ 1 1 2 ~ 2 ~ 2 0 2 ~ 2 + t 1 3 ~ 1 2 ~ 2 ~ 2 0 3 2 + a n d 1 2 ~ 3 ~ 2 ~ 3 ~ 2 + . T h i s s u g g e s t s t h a t

t h e m a j o r e e a k s a l s o c o n t a i n d o u b l y c h a r g e d i o n s s u c h a s ~ ~ The 0 ~ ~ ~ . number o f 3 ~ 0 ' i o n s e x p e c t e d f r o m t h e d e t e c t e d 2345 l 2 c 0 + i o n s is 0 n l y

26 w h i c h is n e a r l y o n e h a l f o f t h e d e t e c t e d number o f t h e i o n s , 48.

T h i s means t h a t t h e p e a k s a t 28 and 29 c o n t a i n C2O22+ a n d COH+ i o n s .

F i g . 2

-

S t r u c t u r e s o f p o l y p y r r o l e and p o s s i b l e f r a g m e n t s .

( a ) N o n - o x i d i z e d PP. ( b ) F u l l y o x i d i z e d PP. ( c ) PP a t t h e f i r s t o x i d a t i o n s t a g e .

Mass to charge ratio 100-

90

8 0

,

7 0 -

.- O

6 0 -

B

_O

%

^50-

-0

'i; 40-

D 5

s

^30-

F i g . 3

-

Mass s p e c t r u m o f a t h r e e d a y o l d PP f i l m .

I n t h e A-P a n a l y s i s o f m e t a l s a n d s e m i c o n d u c t o r s , 1 0 t o 20 i o n s a r e d e t e c t e d £rom o n e a t o m i c layer/6/. S i n c e t h e d i m e n s i o n s a n d t h e o r i e n - t a t i o n s o f t h e d e t e c t e d f r a g m e n t s a t t h e a n a l y z i n g a r e a a r e n o t known, i t i s assumed t h a t t h e d e p t h o f t h e a n a l y z i n g a r e a a d v a n c e s 0 . 1 nm by t h e d e t e c t i o n o f 1 0 i o n s , Then t h e t h i c k n e s s o f t h e h e a v i l y o x i d i z e d

z 2 0 -

118

-

CNH -

A* 149

CO

,167

CNOH,

CZ-206 JOURNAL DE _PHYSIQUE

186 105 492 122

t d

CO COH

CNOY CNOH3

H P

&O NO NOH

CNH C2No

G Y / NH

Mass to charge ratio

F i g . 4

-

Mass s p e c t r u m o f a o n e month o l d PP f i l m .

COH ,

/

Mass to charge ratio

F + g . 5 - V a r i a t i o n o f t h e mass s p e c t r a o f a t h r e e month o l d PP f i l m w i t h d e p t h . ( a ) H i s t o g r a m o f i n i t i a l 4600 i o n s d e t e c t e d from t h e f u l l y o x i d i z e d s e c t i o n w i t h t h e t h i c k n e s s o f a b o u t 4 5 nm.

Mass to charge ratio

F i g . 5 ( b )

-

H i s t o g r a m o f 2 0 0 0 i o n s d e t e c t e d from t h e d e e p e r s e c t i o n . 100.

90.

80

Z 7 0 -

.- O

Mass to charge ratio

392 558 691

U1

.- O

$

3 0 -

O

F i g . 5 ( c ) - H i s t o g r a m o f t h e l a s t 560 i o n s d e t e c t e d £rom t h e non- o x i d i z e d d e e p e s t s e c t i o n .

-

H3

COH al

- _a:

H

D

'.-

O

20 - ^NH

&

n CO

z

5

CNOH

O 5 IO 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 C(,

20.

Hz0

O

O 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 160

A

CO

$ 5 0 - .+- O

40.

D à5

3

E 30.

COH Hz

A

CNO A

C&z

JOURNAL DE PHYSIQUE

s e c t i o n o f t h e PP l a y e r shown by F i g . 5 ( a ) is a p p r o x i m a t e l y 45 nm. The mass a n a l y s i s o f t h e t h r e e month o l d PP l a e r was c o n t i n u e d f u r t h e r d e t e c t i n g 2000 more i o n s . H+, H 2 + , H 3 + , C 2 I a n d C+ i o n s e m e r g e d a g a i n w i t h many s m a l l p e a k s , F i g . 5 ( b ) . F u r t h e r a n a l y s i s d e t e c t e d 560 i o n s . The m a s s s p e c t r u m , F i g . 5 ( c ) , i s q u i t e s i m i l a r t o t h a t o b t a i n e d f o r t h e f r e s h PP l a y e r e x c e p t t h e H2* and H3+ p e a k s .

The o b s e r v e d r e s u l t i n d i c a t e s t h a t a t t h e f i r s t s t a g e o f o x i d a t i o n o x y g e n b r e a k s i n t o t h e C-H a n d N-H b o n d s and t h e n d e t a c h h y d r o g e n . The f i r s t s t a g e o x i d a t i o n p r o c e e d s f a i r l y r a p i d l y s u b m e r g i n g h y d r o g e n p e a k s a s shown i n F i g . 3 . However, t h e t r a n s i t i o n t o t h e f i n a l o x i d a t i o n s t a g e i s r a t h e r s l o w and t h e t h i c k n e s s o f t h e f u l l y o x i d i z e d s e c t i o n i s o n l y 45 nm f o r t h e t h r e e month o l d PP l a y e r .

The m a s s e s o f C l a r e 35 a n d 37 a n d t h o s e o f C1O4 a r e 99 a n d 1 0 1 . U n e x p e c t e d l y n o i o n s w i t h t h e s e m a s s e s w e r e d e t e c t e d . The a n i o n may n o t b e e a s i l y f i e l d e v a p o r a t e d a s a p o s i t i v e i o n b u t n o p l a u s i b l e e x p l a n a t i o n is a v a i l a b l e a t p r e s e n t .

The a u t h o r s a r e g r a t e f u l t o P r o f . H . Kokado a n d M r . S. Yamamoto f o r s a m p l e p r e p a r a t i o n a n d t o M s . M . S h i b a t a and M r . M. Y a n a g i s a w a f o r t h e i r e x p e r i m e n t a l a s s i s t a n c e .

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( 1 9 8 2 ) 1 2 0 .

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