Polarized X-ray absorption spectra of halogen-doped polyacetylene

Synthetic Metals, 17 (1987) 491-496

4 91

POLARIZED X - R A Y A B S O R P T I O N SPECTRA OF H A L O G E N - D O P E D P O L Y A C E T Y L E N E

HIROYUKI OYANAGI, MADOKA TOKUMOTO, TAKEHIKO ISHIGURO, HIDEKI SHIRAKAWA* HIROAKI NEMOTO*, TADASHI MATSUSHITA** and HARUO KURODA***

Electrotechnical

Laboratory,

Sakuramura,

Niiharigun,

Ibaraki

(Japan)

*Institute of Materials Science, U n i v e r s i t y of Tsukuba, Sakuramura,

Niiharigun,

Ibaraki

(Japan}

**National L a b o r a t o r y for High Energy Physics, Tsukubagun,

Ibaraki

Ohomachi,

(Japan)

* * * D e p a r t m e n t of Chemistry and Research Center for Spectrochemistry, The U n i v e r s i t y of Tokyo,

Hongo, Bunkyoku

(Japan}

ABSTRACT The

local

oriented

structure

spectroscopy. and

chemical states of halogens are

studied

by

doped

polarized

in

X-ray

stretchabsorption

Polarized EXAFS and near-edge spectra are m e a s u r e d for (CHBry) x

(CHIy) x

on

concentration.

the The

indicate that formed

and

trans-polyacetylene

Br

K- and

Fourier

I

L-edges

transform

in the lightly doped region

which

over

of EXAFS

a

wide

(0.01
are highly o r i e n t e d along the c-axis while

dopant (CHBry) x

p o l y b r o m i n e ions are bromines

It is e x p e c t e d that bromines are incorporated as isolated ions in the doping

(y<0.01).

indicate that iodines form also

The Fourier

transform

polyions which are

doped

form

bonds

of

increases on going to heavily

also

(y>0.2).

stage

(CH) x which

of

for

covalent

early

with

range

oscillations

analysis

region

(CH[y) x

c h e m i c a l l y less reactive.

INTRODUCTION Previous

p o l a r i z e d EXAFS

[1,2] and near-edge [1% spectra

of

bromine-doped

trans-polyacetylene, highly-oriented

t-(CHBr ) , have been interpreted as an evidence for yx p o l y b r o m i n e ions along the c-axis. Two different structure

models have been p r o p o s e d based on the different i n t e r p r e t a t i o n s of a short BrC

correlation

Oyanagi

et

observed

al.

in

the

Fourier

transform

of

EXAFS

a t t r i b u t e d this distance to the Br-C bonds

oscillations.

originating

from

bromines w h i c h are c o v a l e n t l y bonded with p o l y a c e t y l e n e chain by a substitution or

addition

reaction

[I] whereas Krone et al.

interpreted this peak

d i s t a n c e between the p o l y b r o m i n e ions and p o l y a c e t y l e n e chain

as

the

[2] assuming that

bromines exist only as p o l y b r o m i n e ions.

0379-6779/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

492 In

this

near-edge

paper, spectra

examined.

the d o p a n t - c o n c e n t r a t i o n d e p e n d e n c e of p o l a r i z e d EXAFS

and

of

are

Further,

( C H B r y ) x a r e studied and the two new

models

results of EXAFS and near-edge spectra for

(CHBr) yx Results for (CHBry) x are comparea

in the early stage of doping are presented. to

structure

those for (CHIy) x to e s t a b l i s h the structural basis for the

of electrical c o n d u c t i v i t y of (CHBry) x and (CHIy!x

interpretation

[3].

EXPERIMENTAL Trans-rich

(CH) x films were p r e p a r e d as p r e v i o u s l y d e s c r i b e d by Shirakawa e t

a_~l. [4Fr and carried

s t r e t c h - o r i e n t e d by uniaxial d r a w i n g [5].

out by exposing trans-(CH) x films to bromine or iodine vapor

temperature.

Halogen

after the doping. the

Halogen doping

content was d e t e r m i n e d by a weight uptake

room

immediately

P o l a r i z e d X-ray a b s o r p t i o n e x p e r i m e n t s were p e r f o r m e d with

EXAFS s p e c t r o m e t e r at the Photon Factory

near-edge

at

was

[6].

Polarized Br K-EXAFS

and

spectra were m e a s u r e d for (CHBry) x samples with y between 0.007

and

0.6

wherease I L-edge data were collected for (CHI ) with y between 0.02 and y x 0.2. For the m e a s u r e m e n t of Br K- and I L-edge a b s o r p t i o n spectra, a silicon (311 )

channel-cut

monochromator

m o n o c h r o m a t o r were used,

and

a

respectively.

silicon

( iii )

9 keV was o b t a i n e d with a photon flux of 1 0 8 p h o t o n s / s e c was o p e r a t e d at 2.5 GeV and i00 mA for a silicon measurement

of

double

crystal

A typical energy r e s o l u t i o n of 2eV at

I L-edge a b s o r p t i o n spectra,

when the storage

(311) m o n o c h r o m a t o r .

a silicon

(iii)

double

ring

For the crystal

m o n o c h r o m a t o r was d e t u n e d by 90% to remove the higher harmonics.

Polarization

dependence

sample

respect more

to

than

several

of

absorption

thermal

the

the electrical field vector E which is h o r i z o n t a l l y 95

%.

layers

concentration. throughout

spectra was m e a s u r e d by r o t a t i n g

(CHBry) x

Care

has

or been

the measurement.

disorder

polarized

O p t i m u m thickness for the sample was chosen

of

(CHIy) x

samples

depending

on

by the

taken not to expose the samples to

All samples were m e a s u r e d at 80 K so

is m i n i m i z e d by use of h e l i u m cryostat with

with

a

by

stacking dopant the

air

that

the

closed-cycle

refrigerator.

RESULTS AND D I S C U S S I O N P o l a r i z e d Br K-EXAFS of (CHBry) x As p r e v i o u s l y r e p o r t e d with highly

[i],

a n i s o t r o p i c Br K - E X A F S oscillations of (CHBry) x

respect to the p o l a r i z a t i o n d i r e c t i o n indicate that p o l y b r o m i n e ions oriented

along

the c-axis.

Figure 1 shows the

result

of

are

Fourier

493

transform

of Br K-EXAFS oscillations times k3for

(CHBr) yx

with

0.08,

y=0.02,

and 0.179 for the case with E parallel to the c-axis. o

Br-Br

The most prominent peak o b s e r v e d at 2.2 A is due to the first nearest distance

while

nearest

the

less intense peak located at 4.8 ~ is due to

Br-Br distance,

which are 2.55 ~ and 5.12 ~, respectively,

phase shift correction. the

direction

bromines

of

form

c-axis.

c-axis

in

Although

atoms

than

when

three,

the

1 d e m o n s t r a t e that polybromines are highly oriented along

overall

features

range

of

are similar for

bromine the

concentration.

three

samples,

m a g n i t u d e of Br-Br peak decreases with increasing dopant c o n c e n t r a t i o n the intensity of a short Br-C distance o b s e r v e d at l.l ~ increases, been

the

Since the second nearest peak appears only

trans-(CHBry) x over a wide

the

after

these peaks appear only in

linear polyions with larger number of

results shown in Fig. the

As reported earlier ±I],

second

the

the

whereas

which

has

a t t r i b u t e d to bromines covalently bonded to p o l y a c e t y l e n e chain by either

substitution performed

or a d d i t i o n reaction

by

weighting

backscattering

than

The Fourier t r a n s f o r m in Fig.

Br-Br while suppressing

amplitude

damps rapidly with k.

[7].

Br-C

scattering

1

was

since

the

of Br extends to a large k-region whereas that of

C

The contribution of Br-Br bonds is e s t i m a t e d to be less

30% for (CHBry) x with y=0.02 where Br-Br contribution

is most

prominent.

This is estimated from the coordination number of bromine d e t e r m i n e d for the Br K-EXAFS

with E parallel

bromines

doped region. y=0.179 If

to the c-axis

in the form of p o l y b r o m i n e

is

In Fig. 1.92

~

I,

It is expected that

a short Br-C distance

after the phase shift

the ratio of heavily

o b s e r v e d for CHBry) x

with

correction.

there is only p o l y b r o m i n e ions i n t e r c a l a t e d between p o l y a c e t y l e n e chains

as p r o p o s e d by Krone et al. be

[i].

ion further decreases in the more

independent

longer

[2], the local structure of bromines is e x p e c t e d to

on the dopant concentration.

Br-C distance

Oyanagi et al.

(3.4-3.8 ~) to the correlation

between

have

attributed

polybromine

ion

and the p o l y a c e t y l e n e chain, which is roughly the sum of van der Waals radii of bromine and carbon On its

[i].

going to the lightly doped region, intensity

indicating region.

and for (CHBry) x with

that

These

the results

forms with

y<0.01,

only

polybromine

bromines

are

reaction [8].

is

observed,

small

in

S u m m a r i z i n g the EXAFS results for (CHBry)x, y
(Region

ions with further doping in the more heavily (Region

substituted

region with y>0.2

peak

decreases

this

imply that bromines are doped as i s o l a t e d ions in

d o p e d as ions in the lightly d o p e d region with

0.Ql
Br-C

ratio of Br-Br bond to the Br-C bond is

early stage of doping. are

the Br-Br peak intensity

II).

Throughout

these two

with hydrogens whereas in the

most

I),

doped

regions,

a

the

bromines which region part

heavily

of

doped

(Region III), most of bromines form Br-C bonds by an addition

494 i

i

,

J

Br K- edge ( CH8ry )x

0.01( (a) y = 0.020 E//c

0.00~

(b) y : 0.080 E//c

0.010

,'T0.005

/

0.005 f

(c) y=0.179 E//c

00025~ , 0

1

3 4 5 6 7 8 RAD[AL D[STANCE (~)

2

9

lO

Fig. l. Fourier transform of polarized Br K-EXAFS oscillations E parallel to the c-axis.

for (CHBry) x with

Polarized Br K near-edge spectra of (CHBry) x In

Fig. 2, the near-edge spectra of (CHBry) x are shown with E parallel

perpendicular to the c-axis. in

all

A sharp peak observed 9-10 eV below the threshold

spectra is due to the transition from Br Is state to the

bound states.

and

unfilled

4p

In the Region II, this peak decreases its intensity and broadens

for (CHBry) x with E parallel to the c axis as shown in Fig. 21(c). These results indicate that there is two species of bromine, The

i.e.

Br-Br bond and Br-C bond.

broadening of this peak results from the increased higher energy component

which is clearly observed with E perpendicular to the c-axis as shown in Fig. 2 (b).

The

with

E

increased intensity of higher energy peak in Fig. perpendicular

to the c-axis indicates that the

increases on going to heavily doped region, results The

and

two-species model

near-edge

(d) for

ratio

of

(CHBry) x Br-C

bond

which is consistent with the EXAFS

[i].

spectra of (CHBry) x in Region I is characterized by a

anisotropy with respect to polarization as indicated in Fig.

3.

small

The near-edge

495

1

I

I

I

Br - K - edge

8r - K - edge 03

(CHBry)x z {e) y=O020 E//c

(o) C6H58r

(b) y=O02__O EJ_c

( CHBry)x (b} E//c

f ~ I--

[33

E//c

_J

(c) D_c

(3:

J

(d)y=0179 E±c

.J

_P

_o

o

_J

J

c

J

J -40

y=O007

,

0 210 40 ENERGY (eV)

' - 2'0

,

6LO

8 I0

, - 2'0

-40

0' 2tO ' ENERGY (eV)

4~0

'

_6~0

810

Fig. 2. Polarized Br K near-edge spectra for ( C H B r ) with y=0.02 and 0.179 (left figure), y x Fig. 3. P o l a r i z e d (right figure).

Br K near-edge spectra for

(CHBry) x

with

y=O.O07

structures within 40 eV above the threshold o b s e r v e d in (CHBry) x are similar to those of bromobenzene, Region I.

which suggests that most of bromines form Br-C bonds in

In this region ,

the dip between the sharp spike and the threshold

d i m i n i s h e s as indicated in Fig. with

the

early for

a s s u m p t i o n that

3 (b) and (c).

These results are consistent

bromines are incorporated as isolated ions

in

the

stage of doping since the threshold of the sharp resonance peak o b s e r v e d bromine

(CHBry) x

as

ions is located between the 4p peak and

continuum

threshold

of

indicated in Fig. 3 (d).

Polarized EXAFS and near-edge spectra of Figure angle

4

(CHI ) Y× shows the I L 1 near-edge spectra of (CHIy) x as a function

between

peak

E and the d i r e c t i o n p e r p e n d i c u l a r to the c-axis

[9].

of A

A which results from the transition from I L 1 core states to u n f i l l e d

bound states increases its intensity while peak B decreases its rotating

an

sharp

magnitude

5p with

the sample indicating that iodines also form polyiodine ions oriented

along the c-axis. The

Fourier

transform

of

polarized I

L-EXAFS

shows

that

iodines

are

o

s e p a r a t e d with each other by 2.88 A, which is the intermediate value between I F O

(2.76

~)

and

characterized

I3

(2.92

A).

The

by a long I-C distance

radial

are

oriented

(CHBry)x, respect

around

iodine

Since the c o n t r i b u t i o n of

due to the i n c o m m e n s u r a t e geometry of p o l y i o d i n e

to polymer backbone.

I-C

most of iodines form linear polyions which

along the c-axis with a less degree of o r i e n t a t i o n than that

partly

is

(4.06-4.68 ~) which might be the distance

between the p o l y i o d i n e ion and polyacetylene. bond is e s t i m a t e d to be very small,

distribution

of

ions

with

The Fourier t r a n s f o r m results for (CHI ) yx

are

496 I

i 90~ 8O-?0--

~ i--

,

(CHIy)x

!

[ Ll-edge y. 0020

40~.

8

Ioe

i

.

i

0

- 2 0

20

40

ENERGY {eV)

Fig. 4. Polarization dependence of I L 1 near-edge spectra for (CHIy) x with y=0.02. Numerical values indicate the angle between E and the direction perpendicular to the c-axis.

not sensitive to the dopant concentration, indicating that most of

iodines are

intercalated with a linear chain structure. These (CHIy) x amount

results are consistent with the fact that electrical conductivity is of

always

greater than that of (CHBry) x in magnitude

for

dopant and gradually increases with further doping in

the

the

of same

heavily

doped region.

ACKNOWLEDGEMENTS The

authors wish to express their thanks to Dr.

discussions.

They

are

also

indebted to Prof.

T.

Fujikawa for

Kohra

for

his

valuable continuous

encouragement throughout this work.

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H.Oyanagi, M. Tokumoto, T. Ishiguro, H. Shirakawa, H. Nemoto, T. Matsushita,

2

W. Krone, G. Wortman, K.H. Frank, G. Kaidal, K. Menke and S. Roth,

3

H. Shirakawa, E.J. Louis, A.G. MacDiamid, C.K. Chaing and A.J. Heeger,

M°Ito and H. Kuroda, J. Phys. Soc. Jpn., 53 (1984) 4044.

Solid State Commun., 52 (1984) 253.

J° Chem. Soc. Chem. Commun. (1977) 57. 4

T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci. Polym. Chem., 12 (1974) ii.

5

H. Shirakawa and S. Ikeda, Synth. Met., I (1979/80) 175.

6

H. Oyanagi, T. Matsushita, M. Ito and H. Kurada, KEK Report 83-30 (1984).

7

M.Tokumoto, H. Oyanagi, T. Ishiguro, H. Shirakawa, H. Nemoto, T. Matsushita, M. Ito, H. Kuroda and K. Kohra, Solid State Commun. 48 (1983) 861.

8

H. Oyanagi, M. Tokumoto, T. Ishiguro, H. Shirakawa, H. Nemoto, T.Matsushita, and H. Kuroda, to be submitted to J. Phys. Soc. Jpn.

9

M. Tokumoto, H. Oyanagi, T. Ishiguro, H. Shirakawa, H. Nemoto, T. Matsushita and H. Kuroda, Mol. Cryst. Liq. Cryst., 117 (1985) 139.