Optical properties of highly oriented and disordered polyacetylene

Synthetic Metals, 28 (1989) D215-D223

OPTICAL

PROPERTIES

OF

HIGHLY

D215

ORIENTED

AND

DISORDERED

POLYACETYLENE

G. L E I S I N G Institut f~r Festk6rperphysik, Technische Universit~t Graz, Petersgasse 16, A-8010 Graz, AUSTRIA.

ABSTRACT Polarized optical reflectivity spectra of highly oriented nonfibrous crystalline trans-polyacetylene and reflectivity spectra of disordered trans-polyacetylene are presented. The oriented polyacetylene is produced by the stretch-conversion of a precursor polymer. This precursor route also allow the synthesis of a highly disordered polyacetylene when the conversion is carried out without applied stress (Durham route). From the reflectivity data we deduced the optical constants via Kramers-Kronig analysis. The anisotropy of the optical absorption coefficient for the crystalline polyacetylene is shown and compared to that of the disordered material. The band gap of crystalline trans-polyacetylene can be deduced from a ~2~2-versus ~ plot. The band edge absorption of disordered poylacetylene does not show a (~)i/a_ behaviour usually observed for amorphous semiconductors. The macroscopically isotropic dielectric function of disordered trans-polyacetylene is presented. While the shape of the interband transition is remarkably different for the two kinds of polyacetylene, the doping induced infrared active modes show a shift in frequency which is different for each mode. The highly doped oriented polymer exhibits a metallic behaviour in the optical reflectivity spectrum polarized parallel to the chain direction indicating a closed energy gap. 0379-6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

D216 EXPERIMENTAL

Highly o r i e n t e d n o n f i b r o u s p o l y a c e t y l e n e films with a thickness varying from I to 5 m i c r o m e t e r were p r e p a r e d by a route via a p r e c u r s o r p o l y m e r [I], which is c o n v e r t e d to p o l y a c e t y l e n e by heat treatment u n d e r a p p l i e d m e c h a n i c a l stress in vacuo [2]. The m o n o m e r 7,8-bis- (trifluoromethyl) tricyclo- [4.2.2.0] - d e c a - 3 , 7 , 9 - t r i e n e (BTFM-TCDT) is s y n t h e s i z e d by a D i e l s - A l d e r r e a c t i o n of h e x a f l u o r o 2-butyne and c y c l o - o c t a t e t r a e n e . With a p p r o p r i a t e c a t a l y s t s (WCI6 and ( C H 3 ) 4 S n ) this m o n o m e r is p o l y m e r i z e d to p o l y - ( B T F M - T C D T ) (precursor polymer) via a ring opening m e t a t h e t i c a l polymerization, in a c h l o r o b e n z e n e solution. To m i n i m i z e c a t a l y s t residues the p r e c u r s o r p o l y m e r is p u r i f i e d by several w a s h i n g and precipitation cycles . Films were cast from a s o l u t i o n (acetone) of the p r e c u r s o r polymer onto thoroughly cleaned p o l i s h e d s i l i c o n substrates and dried under a d u s t - f r e e pure argon gas flow. It a p p e a r e d that careful filtering of the p r e c u r s o r polymer solution prior to casting results in an improved surface q u a l i t y of the p o l y a c e t y l e n e films. The precursor polymer films were mounted in a vacuum assembly and c o n v e r t e d to p o l y a c e t y l e n e under applied uniaxial m e c h a n i c a l stress at a t e m p e r a t u r e of 80°C. The r e s u l t i n g conformation of p o l y a c e t y l e n e (cis or trans) is d e t e r m i n e d by the reaction time under the above conditions. A r e a c t i o n time of about 30 minutes gives highly o r i e n t e d c i s - p o l y a c e t y l e n e with a m a x i m u m ciscontent of 90% , calculated by the ratio of the integrated absorptions of the c h a r a c t e r i s t i c i n f r a r e d bands [3]. Highly oriented trans-polyacetylene is o b t a i n e d by a subsequent heat treatment of the o r i e n t e d c i s - p o l y m e r film at about 140°C still under applied stress. The s t r e t c h i n g ratios (relative to the original length of the p r e c u r s o r p o l y m e r film) of the o r i e n t e d cisand t r a n s - f i l m s varied from 10 to 20. After the s t r e t c h i n g and the conversion r e a c t i o n is completed, the o r i e n t e d p o l y m e r film was clamped between the two jaws of a s t a i n l e s s steel ring where it remains plane and flat during all s u b s e q u e n t m e a s u r e m e n t s keeping full orientation and a smooth flat surface for the optical investigations. To p r o d u c e n o n - f i b r o u s t r a n s - p o l y a c e t y l e n e with low crystallinity (disordered polyacetylene) the p r e c u r s o r p o l y m e r films were kept on the silicon s u b s t r a t e s and c o n v e r t e d to p o l y a c e t y l e n e at 80°C for about 8 to 10 hours under high vacuum conditions. The c o m p l e t e n e s s of the c o n v e r s i o n and the cis-trans i s o m e r i z a t i o n under the above conditions was checked by infrared spectroscopy (hexafluoroxylol and cis-bands). The AsFs doping of the t r a n s - p o l y a c e t y l e n e films was c a r r i e d out in a special stainless steel vacuum line at v a p o u r p r e s s u r e s of about 500 m b a r at maximum. The change of the e l e c t r i c a l c o n d u c t i v i t y upon doping was m o n i t o r e d in situ with a r e f e r e n c e sample of the same

D217 thickness

w i t h a four probe g e o m e t r y and

platinum

d o p i n g time was about 8 hours on the average. stored

u n d e r high v a c u u m

contacts.

The

The samples were then

(10 -6 mbar) until an e q u i l i b r i u m value of

the conductivity has been o b s e r v e d (after about 12 hours). The doped samples a p p e a r e d to be very s e n s i t i v e to air e x p o s u r e of even seconds, w h i c h results in a f l a t t e n i n g of the p l a s m a edge in the parallel r e f l e c t i v i t y spectrum. Therefore, all our measurements were carried out exclusively under high vacuum or inert gas conditions. The p o l a r i z e d reflectivity measurements in the range b e t w e e n 200 cm -I (0.025 eV) to 4000 cm -I (0.5 eV) were p e r f o r m e d on a Perkin Elmer 684 grating instrument e q u i p p e d w i t h A g B r and polye t h y l e n e w i r e - g r i d polarizers. A Perkin Elmer Lambda 9 spectrophotometer covered the range from 3800 cm -~ (0.47 eV) to 33000 cm -I (4.1 eV) u t i l i z i n g d e p o l a r i z e r s and p r i s m p o l a r i z e r s in the sample and r e f e r e n c e beam. Special s p e c u l a r r e f l e c t a n c e accessories d e s i g n e d in our l a b o r a t o r y for both i n s t r u m e n t s a l l o w e d to carry out the m e a s u r e m e n t s in inert a t m o s p h e r e w i t h o u t any contact of the samples w i t h air. The angle of incidence of the r e f l e c t a n c e a c c e s s o r i e s is about 10 ° in both instruments. The d e v i a t i o n of the m e a s u r e d r e f l e c t a n c e from the value for normal incidence is about 2%, which has been v e r i f i e d by measurements of the r e f l e c t i v i t y as a function of the angle of incidence on some of the samples. The r e f l e c t i v i t y of the samples is m e a s u r e d r e l a t i v e to r e f l e c t i v i t y of a l u m i n u m mirrors, w h i c h are corrected with respect to the literature values of nickel, chromium and p l a t i n u m [4]. This is neccessary, since a l u m i n u m shows a p r o n o u n c e d dip in the r e f l e c t i v i t y s p e c t r u m at about 1.5 eV (arising from a weak i n t e r b a n d t r a n s i t i o n [5]), w h i c h coincides w i t h the interband a b s o r p t i o n edge in t r a n s - p o l y a c e t y l e n e . The c a l c u l a t i o n of the r e f l e c t i v i t y spectra and the optical constants from the m e a s u r e d spectra is carried out via a KramersKronig analysis taking into account the m u l t i p l e reflection and i n t e r f e r e n c e effects in a self c o n s i s t e n t way [6]. Such interference effects always show up for the nicely p l a n e p a r a l l e l films in the frequency region below the interband transition. RESULTS

AND DISCUSSION

Highl Y o r i e n t e d n o n - f i b r o u s c r y s t a l l i n e t r a n s - p o l y a c e t y l e n e In Fig. I w e p r e s e n t the optical reflectivity spectra of oriented trans-polyacetylene for p o l a r i z a t i o n s of the v e c t o r of the elctromagnetic wave being parallel or p e r p e n d i c u l a r to the chain axis. A c c o r d i n g to the interband t r a n s i t i o n a r e f l e c t i v i t y maximum is o b s e r v e d slightly below 2 eV for light polarized p a r a l l e l to the chain axis with a value of 0.63 at the maximum. The

D218

0

1.0

1.0

,

,

,

h~(eV} 2.0 a

,

3.0 ,

4.0

0.5

E/c ' 1.0

'

Wavenumber

............... "-.... 2'0.

'

t0'

{lO~cm -1)

Fig. I. Polarized optical reflectivity spectra for oriented transpolyacetylene (solid lines) and unpolarized spectrum of disordered trans-polyacetylene (dotted line).

perpendicular reflectivity is very low, featureless and nearly constant throughout the entire frequency range. Due to the high stretching ratios (10-20), the alignment of the polymer chains is within 5 ° as determined by rocking-curves using x-ray and electron-diffraction. Together with the good surface quality of our samples this explains the high reflectivity value of 0.63 at the maximum for parallel polarization compared to values of 0.5 or less for other oriented polyacetylenes [7,8]. The C-H vibrational modes of the pure trans-polyacetylene have been neglected for this investigation. The frequency dependence of the optical absorption coefficient which is determined via the Kramers-Kronig calculation is shown in Fig. 2 again for both polarization directions. The maximum for the parallel polarization appears at 1.9 eV with a value of ~lj= 7.4 * 105 cm -I and an anisotropy of about 26. These values are in excellent agreement with those determined by electron-energy loss-spectroscopy on samples prepared by the same route [9]. Band edge analysis of the parallel absorption due to the interband transition is carried out by extrapolating the linear part of an ~ i ~ 2 versus ~ plot and leads to a value of 1.5 eV for the energy gap [6]. Due to the nonfibrous morphology and perfect alignment of the polymer chains the absorption is already very low in the gap region and can be reduced even more by compensation with NH3.

D219 ~,.,, (eV) 1.0 I

10

2.0

I

I

40

].0

I

I

I

I

E ~o

m

~0s o

g

..///'""

c-

"'" "'...

x:l

I0 20 Wavenumber (10~cm-I) Fig.

Optical

2.

polyacetylene (dotted

non-fibrous

unpolarized

reflectivity in

disordered

0

The

Fig.

I0 ,

t

,

L

3.

analysis.

Real

and

in

oriented

trans-

trans-polyacetylene

(dotted

~(eV) 2.0 ,

L

~

optical

30 ,

I

i

imaginary

I (dotted eV w i t h

absorption

As one

disordered line).

a value

of

The 0.32

coefficient

would

coefficient

shows

expect

is

for

a very

a

broad

&0

,

L

2.0 (10~cm-I)

trans-polyacetylene

2.1

line).

the a b s o r p t i o n

of n o n - f i b r o u s

Fig.

at a b o u t

calculated

10 Wavenumber

disordered

spectrum

shown

appears

2

material

L

Fig.

for

for d i s o r d e r e d

trans-polyacetylene

is a l s o

maximum

the m a x i m u m .

depicted

10

coefficient

and

reflectivity

trans-polyacetylene at

lines)

line).

Disordered The

absorption

(solid

30

3.0

part as

of the d i e l e c t r i c derived

from

function

for

Kramers-Kronig

D220 peak with a 2.5 eV. The

maximum value of 3.9 * 105 cm -~ located at about position of the m a x i m u m is s l i g h t l y preparation

dependent with a smallest observed value of about 2.3 eV. The band tailing in the region b e l o w the gap energy due to the d i s o r d e r is clearly visible. A band edge analysis by p l o t t i n g ( ~ ) ~ / 2 versus ~, as it is u s u a l l y done for amorphous s e m i c o n d u c t o r s [10], showed no linear part in this kind of plot. However, a plot of ~ 2 ~ 2 versus ~, as for the c r y s t a l l i n e m a t e r i a l reveals a linear part, which e x t r a p o l a t e s to a value of the band gap of 1.72 eV. The real (£i) and imaginary parts (E2) of the d i e l e c t r i c f u n c t i o n w h i c h are again d e r i v e d from the K r a m e r s - K r o n i g a n a l y s i s are shown in Fig. 3. For the low frequency value of ~i we d e r i v e d 6.1, which is slightly below 6.75, the average of the parallel (10.5) and perpendicular (3) values of the o r i e n t e d t r a n s - p o l y a c e t y l e n e [6]. The i m a g i n a r y part shows a broad m a x i m u m at about 2 eV w h i c h arises from a broad l e n g t h - d i s t r i b u t i o n of u n d i s t u r b e d c o n j u g a t e d sequences with a strong c o n t r i b u t i o n from short s e q u e n c e - l e n g t h s . Due to the strong d i s o r d e r and defects in this m a t e r i a l the b a n d tail covers the whole gap region. Infrared active v i b r a t i o n a l modes (IRAV) It is k n o w n from p r e v i o u s studies on p o l y a c e t y l e n e that doping creates new and strong modes in the infrared region [11-13]. These modes appear at about 900 cm -I, 1260 cm -I and 1400 cm -I. They show an i n c r e a s i n g intensity with increasing doping. We carried studies of the IRAV for both o r i e n t e d and d i s o r d e r e d transpolyacetylene. The doping has been carried out with AsFs in the gas phase and only up to a m e d i u m d o p i n g c o n c e n t r a t i o n s (a few mole %) to ensure m e a s u r a b l e t r a n s p a r e n c y in the i n f r a r e d region. In Fig.4 we p r e s e n t a b s o r p t i o n spectra for AsF, doped trans-polyacetylene. The upper curve was o b t a i n e d from o r i e n t e d polyacetylene for parallel polarization (in the p e r p e n d i c u l a r polarization these modes are still absent or very low for this doping level). The lower curve denotes to d i s o r d e r e d trans-polyacetylene with a similar d o p i n g concentration. The overall features of the two spectra are similar, but the p o s i t i o n of the bands are d i f f e r e n t for the two materials. Whereas the mode at 1400 cm -I is only about 10 cm -~ softer in the o r i e n t e d polyacetylene c o m p a r e d to the doped d i s o r d e r e d system, the broad low frequency mode is shifted by about 40 cm -~ to h i g h e r frequency for the d i s o r d e r d polyacetylene, which suggests a s t r o n g e r pinning of this mode due to the disorder. A more d e t a i l e d a n a l y s i s of these modes comparing their b e h a v i o u r in crystalline and disordered p o l y a c e t y l e n e will be p u b l i s h e d elsewhere.

D221

l

oriented-(EH)x

1

1

==

'o

1600

l& 0

' ' ' 1200 10'o 0 800 600 Wavenurnber (cm -1 )

' 400

200

Fig. 4. Infrared a b s o r p t i o n spectra for A s F s - d o p e d o r i e n t e d transp o l y a c e t y l e n e (upper curve) and for d i s o r d e r e d t r a n s - p o l y a c e t y l e n e (the arrows indicate the v i b r a t i o n a l modes of AsF6). Metallic polyacetylene The samples d e s c r i b e d in this section were doped with AsFs r e s u l t i n g in an average electrical D C - c o n d u c t i v i t y p a r a l l e l to the chain direction of 6"I I= 1200 Q - I c m - 1 at room temperature (the m a x i m u m value of ~II we could achieve was about 4 0 0 0 Q - I c m - 1 ) . In Fig. 5 we present the m e a s u r e d optical r e f l e c t i v i t y spectra of an A s F s - d o p e d o r i e n t e d t r a n s - p o l y a c e t y l e n e film. For the p a r a l l e l r e f l e c t i v i t y we observe a f r e e - e l e c t r o n like spectral b e h a v i o u r and the reflectivity approaches a value of I in the long wavelength

1.0

0

i

1.0

i

,

1~(eV} 2.0

J

a

3.0

i

4.0

J

,

£11[

= c m oJ

0.5

ed

0

Fig.

0

1.0 2.0 Wavenumber (lO~/rn -1)

'

30

5. P o l a r i z e d optical r e f l e c i v i t y of m e t a l l i c p o l y a c e t y l e n e

D222 limit.

The

IRAV

at 0.17 eV (1400 cm -I) is still p r e s e n t

in

the

highly c o n d u c t i n g m a t e r i a l and we also o b s e r v e d a c o n t r i b u t i o n from the 900 cm -I IRAV a l t h o u g h the latter mode is d i f f i c u l t to extract from the steep rise of the reflectivity. C a l c u l a t i o n s u s i n g a freee l e c t r o n model are in progress, w h i c h should a l l o w us to the v i b r a t i o n a l c o n t r i b u t i o n s from the e l e c t r o n i c part.

subtract

h=(eV)

10

0

1.0

I

2.0

I

I

I

3.0

I

&O

//... ................. "'-........ y'"

"".... ,

/

~o

"...

.

"7

5

.

.

.

3~

oLllc

.

i " i~.¸

"...

z~ """......

"...........,..' I

I

I

1.0

2O

3',0

o

Wavenumber (104cm -I

Fig.

6.

P a r a l l e l a b s o r p t i o n c o e f f i c i e n t and optical

conductivity

for m e t a l l i c polyacetylene. The o s c i l l a t i o n s which show up in the p e r p e n d i c u l a r p o l a r i z a t i o n are due to i n t e r f e r e n c e effects because of the p l a n e p a r a l l e l nature of the sample and the low a b s o r p t i o n for this polarization. Samples of d i f f e r e n t thickness show a d i f f e r e n t periodicity of these oscillations. For the parallel r e f l e c t i v i t y s p e c t r u m we have carried out a K r a m e r s - K r o n i g calculation. The results for the optical conductivity and the optical absorption are shown in Fig. 6. We c o n c l u d e from this picture that optically, metallic p o l y a c e t y l e n e behaves like a o n e - d i m e n s i o n a l metal. For low wavenumbers the c a l c u l a t e d values of the p a r a l l e l c o n d u c t i v i t y by far exceed the m e a s u r e d D C - v a l u e because the D C - t r a n s p o r t necessarily involves an i n t e r c h a i n h o p p i n g r e s t i s t a n c e and is t h e r e f o r e determined by this process. S U M M A R Y AND C O N C L U S I O N S The optical c o n s t a n t s and d i e l e c t r i c f u n c t i o n of n o n - f i b r o u s highly oriented crystalline trans-polyacetylene, non-fibrous d i s o r d e r e d t r a n s - p o l y a c e t y l e n e and m e t a l l i c p o l y a c e t y l e n e have been d e r i v e d from optical r e f l e c t i v i t y m e a s u r e m e n t s via a K r a m e r s - K r o n i g analysis. We have shown that the a v a i l a b i l i t y of n o n - f i b r o u s transpolyacetylene in an highly o r i e n t e d and a d i s o r d e r e d form allows

D223

to gain an insight into the intrinsic properties of the so called simplest conjugated polymer and to distinguish them from the extrinsic properties. Doping studies on the crystalline and disordered form of polyacetylene by optical spectroscopy will allow to learn more about the properties of the defects in the pure material as well as about the doping induced defects. In chain direction highly doped oriented polyacetylene behaves like a one-dimensional metal. ACKNOWLEDGEMENTS I would like to thank my teacher and friend H. Kahlert for his stimulating participation and extremely motivating support. The careful preparation of the precursor polymers by B. Herz is gratefully acknowledged. This work was supported by the Austrian Science Research Fund under project No. 6198. REFERENCES 1 J.H. Edwards and W.J. Feast, Polymer Commun., 21 (1980) 595. 2 G. Leising, Polymer Bulletin, I_!I (1984) 401. 3 H. Shirakawa, T. Ito and S. Ikeda, Polym.J., 4 (1973) 460. 4 M.v. Ardenne, Tabellen zur Angewandten Physik, Bd.3 (VEB Deutscher Verlag der Wissenschaften, Berlin, 1973). 5 H. Ehrenreich, H.R. Philipp and B. Segall, Phys.Rev., 132 (1963) 1918. 6 G. Leising, Phys.Rev.B, (submitted). 7 C.R. Fincher, M. Ozaki, M. Tanaka, D. Peebles, L. Lauchlan, A.J. Heeger and A.G. MacDiarmid, Phys.Rev.B, 20 (1979) 1589. 8 P.D. Townsend and R.H. Friend, Synth.Met., 17 (1987) 361. 9 J. Fink and G. Leising, Phys.Rev.B, 34 (1986) 5320. J. Fink, H. Fark, N. N~cker, B. Scheerer, G. Leising and R. Weizenh~fer, Synth.Met., I/7 (1987) 377. 10 J. Tauc, R. Grigorovici and A. Vancu, Phys. Status Solidi, 15 (1966) 627. 11 C.R. Fincher, M. Ozaki, A.J. Heeger and A.G. MacDiarmid, Phys.Rev.B, 19 (1979) 4140. 12 J.F. Rabolt, T.C. Clarke and G.B. Street, J.Chem. Phys., 71 (1979) 4614. 13 S. Etemad, A. Pron, A.J. Heeger, A.G. MacDiarmid, E.J. Mele and M.J. Rice, Phys.Rev. B, 23 (1981) 5137.