SPECTRAL STUDIES ON THE ELECTRONIC STRUCTURE OF POLYACETYLENE

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SPECTRAL STUDIES ON THE ELECTRONIC STRUCTURE OF POLYACETYLENE

J. Tanaka, M. Tanaka, H. Fujimoto, M. Shimizu, N. Sato, H. Inokuchi

To cite this version:

J. Tanaka, M. Tanaka, H. Fujimoto, M. Shimizu, N. Sato, et al.. SPECTRAL STUDIES ON THE

ELECTRONIC STRUCTURE OF POLYACETYLENE. Journal de Physique Colloques, 1983, 44

(C3), pp.C3-279-C3-284. �10.1051/jphyscol:1983353�. �jpa-00222705�

JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-279

SPECTRAL STUDIES ON THE ELECTRONIC STRUCTURE OF POLYACETYLENE

J. Tanaka

, M. Tanaka*

, H. Fujimoto

, M. Shimizu"

", N. Sato

and H. Inokuchi

^Department of Chemistry, Faculty of Science and *'College of General Educa- tion, Nagoya University, Chikusa, Nagoya, 464 Japan

++Institute of Molecular Science, Myodaiji, Okazaki, 444 Japan

Résumé - Nous avons étudié l'effet du dopage des polyacétylè- nes sur les spectres visible, infrarouge et photoélectronique.

Un grand changement dans le spectre infrarouge résulte de l'abaissement de la température du film traité.

Abstract - The change of the optical spectra of polyacetylene upon doping is studied by measurements of visible, infrared and photoelectron spectra. The infrared spectra of the doped film showed new peaks with lowering the temperature.

The change of the molecular and the electronic structures of poly- acetylene (PA) upon doping is still a problem of active interest.

There remains several basic questions such as the bond length alter- nation of the conjugated system, the structure of the charged soliton state and the mechanism of the electronic conduction. We present in this paper on the spectral studies of the electronic structure of PA upon doping with bromine, iodine and arsenic pentafluoride. We have measured the visible, infrared and Raman spectra of the doped and undoped PA and also the ultraviolet photoelectron spectra of PA film before and after the doping. The change of the electronic structure by the doping is discussed to explain the results of these spectral measurements.

I - MATERIALS AND EXPERIMENTS

Thick films of PA were prepared by Shirakawa's method. Doping of thick film was performed by exposing to bromine or iodine vapor in the closed vessel, and the doping level was monitored by measuring the d.c. conductivity of another film placed close to the original one.

After the doping, the excess dopant was pumped out for at least 30 min.

For infrared transmission measurement, very thin film was polymerized on the teflon frame which has a small hole on it. The thin film was placed in the Dewar vessel and is doped in situ by ASF5 gas or I2 vapor. In the case of Br2 doping, the doping was carried out in a

separate vessel and the doped film was transferred to the Dewar vessel before the measurement. The infrared spectra' were measured with a HITACHI IR 260-50 (350 - 4000 c m " ! ) . For the visible absorption measurement, the thin PA film was polymerized on the inner surface of the 1 cm quartz cell and the absorption spectra were measured with a Carl-Zeiss spectrophotometer (4000 - 45000 c m

) . The Raman spectra of the doped film were obtained by rotating the doped PA film to avoid heating and the film was cooled in the flow of cold nitrogen gas in the quartz Dewar. A JEOL Raman spectrometer (Model JRS 400T) of the Instrument Center, Institute of Molecular Science, Okazaki, was used

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983353

C3-280 JOURNAL DE PHYSIQUE

w i t h an argon l a s e r a s a n e x c i t i n g l i g h t s o u r c e . U V p h o t o e l e c t r o n s p e c t r a were measured by u s i n g a H i n t e r e g g e r t y p e hydrogen d i s c h a r g e lamp a s a l i g h t s o u r c e and a n energy a n a l y z e r of a s p h e r i c a l r e t a r d i n g f i e l d t y p e . The energy r e s o l u t i o n of t h e system was < 0 . 2 eV.

I1 - PHOTOELECTRON SPECTRA

S i n c e we have used a hydrogen d i s c h a r g e lamp a s t h e VUV l i g h t s o u r c e , t h e photon e n e r g i e s a r e r e l a t i v e l y s m a l l a s compared w i t h t h e e a r l i e r s t u d i e s which used t h e r a r e g a s d i s c h a r g e t u b e s , t h e r e f o r e t h e

i o n i z a t i o n t h r e s h o l d e n e r g i e s were l e s s e f f e c t e d by secondary energy

F i g . 1 - P h o t o e l e c t r o n s p e c t r a of F i g . 2 - P h o t o e l e c t r o n s p e c t r a

n e a t trans-PA. o f Br2 doped trans-PA.

-- ^{Band Gap}

- - - T = 2 ( 0 , - 02)

Fig.4 - Charged s o l i t o n d i v i d e s l o n g PA c h a i n i n t o s m a l l segments having an i n v e r s i o n symmetry.

F i g . 3 - Band s t r u c t u r e o f PA by HMO

t h e o r y and p a r a m e t e r s .

loss processes. Typical photoelectron spectra are illustrated inFigs.

1 and 2 for the neat and Br2 doped trans-PA. The ionization threshold of the solid ( ~ s t h ) is determined by the analysis of the photo- electron energy distribution curves for various incident photon energies (7.29 - 9.18 eV) . ^2, The analysis showed that the ~ s t h of neat trans-PA is 5.24 eV and the Br2 doped PA is 5.59 eV; hence the depression of the ~ , t h is estimated as 0.35 eV. On standing the film in vacuo for a week, this figure was changed to 0.22 eV. The doping level of the present film was [(CH)(Br3)0.04Ix at the maximum.

A similar depression of the onset of photoemission was found by Salaneck e t aZ. 3 ) on [ (CH) (AsF5) 0 . 1 1 1 ~ as much as 1.2 eV, but no appreciable change was reported for the 12 doped PA film.4) The depression of the energy of the highest occupied valence band may be estimated from a simple ~ a c k e l MO theory. The orbital energies of the odd chain polyene composed of 2n carbon atoms are given as follows.5)

where the energy levels of higher orbitals are obtained by putting j = n, n - 1 and so on. The depression of the energy estimated by the above formula is about 0.01 eV/electron for the chain of 200 carbons. For the Br2 doped PA film, about four electrons were removed for 100

electrons, therefore the depression is estimated as 0.04 eV.

The value is apparently smaller than the observed one (0.2 - ^{0.3 eV.)} .

A more reasonable account of the depression of the ~ ~ will be given t h by the charge soliton model.6) By the doping of the odd chain polyeng the electron in the mid gap state (neutral soliton) will be most easily ionized,and the shorter polyenes will be produced. The positively charged carbon (charged soliton) will form a domain wall making shorter polyenes because the conjugation is interrupted, and the orbital energies are stabilized by the shortening of the conjugat- ed chain. Actually the estimated value of the depression for the 4

doped film (C25 chain) is 0.24 eV and for the 11 % doped film (Cg chain) is 1.07 eV. These values are in good agreement with the observed ones as mentioned above for the Br2 doped and AsF5 doped films.

I11 - VISIBLE AND NEAR INFRARED SPECTRA

The change of the visible and near infrared spectra of PA upon doping

with Br2 is shown in Fig.5 for ( I ~ D ) ~ . The starting film was cis-rich

form, but the cis-transisomerization takes place during doping and

the final state was Br2 doped trans form. After the doping a

characteristic band in the visible region disappeared and a new

band was found around 6000 cm-l . This band has been interpreted as

the charged soliton mid gap band.7) However, the energy gap still

existed in the lowest energy re ion at a 1000 cm-l. As the doping

proceeded, the peak at 6000 cm-7 moved to lower energy at 4000 cm-1

and the band gap seemed to be smeared out. A simulation of the broad

band with a Drude model of one-dimensional free electron picture

gives a calculated curve of Fig.6 with parameters of = 2.77,

up = 18200 cm-I and y = 8270 cm-l, where up = ( 4 ~ 1 ~ e ~ / m * ) l / ~ .

From a consideration of

electron density in the PA film, the number

of free carrier is estimated as one-sixth of all

electrons, if we

assume that m* = m e . The origin of the gapless transition might be

due to the overlap of the charged soliton wave function, because the

gap is lost at high density of the charged soliton. An appearance of

a metallic state was suggested by Mele and ~ i c e 8 ) b ~ this mechanism.

JOURNAL DE PHYSIQUE

F i g . 5 -

V i s i b l e and n e a r i n f r a r e d s p e c t r a of n e a t and B r 2 doped PA f i l m .

1. n e a t PA 2. a = 0 . 1 S/cm 3. a = 2 S/cm 4 .

= 4 S/cm.

5 10

15 20 25

WAVE NUMBER

lo3

~ b s o r ~ t i o n s p e c t r a

of n e a t and Br2

o

doped PA. .

n e a t c i s - r i c h * -

--- ^-

Br2 doped,

... c a l c u l a t e d by 8 ,... ---__

I... >..:

....

a Drude model.

0

. . " ' " , "

0 5 10 15 20 25 30

I V - TEMPERATURE DEPENDENCE OF INFRARED AND RAMAN BANDS OF DOPED PA.

The i n f r a r e d a b s o r p t i o n s p e c t r a of t h e doped PA have been c h a r a c t e r - i z e d by t h e a p p e a r a n c e of s e v e r a l v i b r a t i o n a l bands of t h e C=C, C-C and C-H v i b r a t i o n s a t 870, 1280 and 1370 cm-I i n ( C H ) , and 780, 1100 and 1260 cm-1 i n (CD),.g) The s h i f t and t h e hancement of t h e s e bands have been i n t e r p r e t e d by Mele and R i c e l g f a s due t o t h e c o u p l i n g of t h e s e v i b r a t i o n s w i t h t h e charged s o l i t o n s t a t e .

The measurement of t h e i n f r a r e d a b s o r p t i o n s a t low t e m p e r a t u r e showed a d r a s t i c i n c r e a s e of a b s o r p t i o n i n t e n s i t y below 900 cm-1 r e g i o n . The measurement of t h e a b s o r p t i o n was c a r r i e d o u t w i t h a f r e e s t a n d i n g t h i n f i l m , t h e r e f o r e t h e y were n o t a f f e c t e d by t h e s u b s t r a t e a b s o r p - t i o n , b u t t h e t e m p e r a t u r e of t h e f i l m might be somewhat modified d u r i n g t h e measurement from t h e v a l u e s d e s c r i b e d . I n Fig.7 t h e change of a b s o r p t i o n w i t h t e m p e r a t u r e i s i l l u s t r a t e d f o r Br2 do ed (CH),.

S t r o n g new peaks were found a t 420, 570, 610 and 650 cm-e and t h e

i n t e n s i t y of whole broad bands below 900 cm-1 was i n c r e a s e d . I t i s

remarkable t h a t t h e 1370 cm-1 and 1280 cm-1 bands were n o t i n f l u e n c e d

by t h i s t e m p e r a t u r e change. A s i m i l a r s p e c t r a l change was found w i t h

B r 2 doped (CD), ( F i g . 8 ) . The new broad bands were found below 800 cm-l

and t h e i n t e n s i t y was i n c r e a s e d a t low t e m p e r a t u r e , w h i l e t h e 1260

and 1100 cm-1 bands were n o t a f f e c t e d . With I 2 d o p i n g , s i m i l a r new

bands were found a t 570, 660 and 800 cm-1, and t h e i n t e n s i t y was

i n c r e a s e d a t low t e m p e r a t ~ r e ~ w h i l e t h e 1370 and 1280 cm-1 bands were

n o t changed by t h e t e m p e r a t u r e . ( ~ i ~ . 9 )

Fig. 7 - Temperature dependence of IR spectra of Br2 doped (CHIx.

1. 300 K, 2. 260 K, 3. 230 X, 4. 200 K and 5. 160 K.

OO

c

WAVE NUMBER 1 lo2 an-l

Fig.8 - Temperature dependence of IR spectra of Br2 doped (CD) x.

1. 300 K,2280 X I 3.260 K,4. 240 K 5. 220 K, and 6. 160 K.

Fig.9 - Temperature dependence Fig. 10 - Temperature dependence of of IR spectra of I2 doped (CH),. IR spectra of AsF5 doped (CH) x.

1. 300 K, 2. 270 K, 3. 240 K 1. 300 K, 2. 280 K , 3.260 K.

and 4. 230 K. 4. 220 K, 5.200 K and 6. 140 K.

The AsF5 doped film showed a characteristic peak of A S F ~ ion at 696 cm-1 (Fig.lO), and it was overlapped with several other new peaks, which were enhanced at low temperature. Thus these new bands at low

temperature were found with different dopant as well as (CD)x, therefore they should be electronic origin rather than the skeltal vibrations.

A most plausible explanation of these bands may be due to the oscillation of the charged soliton around the negatively charged dopant ion as was suggested by SU, Schreiffer and Heeger.6)

Following their mechanism the pinning of the charged soliton might

be occurred at low temperature and they showed enormous intensity

because the large dipole oscillation of the charged soliton took

place.

JOURNAL DE PHYSIQUE

Fig.11 - Temp rature,dependence of Raman spec 1 ra of Br2 doped

(CH)x with 514.5 nm excitation.

1. 300 K

2. 130 K .

t

1

1 , , , , 1 , , , , 1 . , . . .

moo

RAMAN SHIFT 1 cm-1

In order to find the structural changes accompanied by cooling of the doped film, the Raman spectra of the Br2 doped PA film were measured as shown in Fig.11. The intensity of the Raman bands due to the Br3 ion was anomalously enhanced and the asymmetric vibration of the

~ r 5 ion was strongly found at 197 cm-1 region.ll) The overtones and combination tones were observed in the 300 - 600 cm-1 region. Such a tremendous intensity change was also found with the I2 doped film.

The structural basis of such a significant change of the spectra is difficult to understand unless the interaction between the PA chain and the dopant was increased at low temperature. Further studies will be required to clarify these points.

References

SHIRAKAWA H. and IKEDA S., PoZymer J . 2 (1971) 23L

SAT0 N., SEKI K. and INOKUCHI H., J. Chem. Soc., Faraday Trans. I1 77 (1981) 1621.

SALANECK W.R., THOMAS H.R., DUKE C.B., PATON A., PLUMMER E.W., HEEGER A.J. and MACDIARMID A.G., J.Chem.Phys. 71 (1979) 2044.

SALANECK W.R., THOMAS H.R., BIGELOW R.W., DUKE C.B.,PLUMMER E.W., HEEGER A.J. AND MACDIARMID A.G. ,J. Chem.Phys.72 (1980) 3674.

COULSON C.A., Proc. Roy. Soo. A 1 64 (1938) 383.

SU W.P., SCHRIEFFER J.R. and HEEGER A.J., Phys.Rev. 822 (1980) 2099.

SUZUKI N., OZAKI M., ETEMAD S., HEEGER A.J. and MACDIARMID A.G., Phys.Rev.Lett. 45 (1980) 1209.

MELE E. J. and RICE M.J., Phys. Rev. B 2 3 (1981) 5397.

FUJIMOTO H., TANAKA M. and TANAKA J., BuZZ.Chem.Soc.Jpn. to be published.

MELE E.J. and RICE M.J., Phys.Rev. Lett. 45 (1980) 926.

GABES W. and GERDING H., J. MoZ.Structure 14 (1972) 267.