Electronic properties of non-doped and doped polyacetylene films studied by E.S.R.

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Electronic properties of non-doped and doped

polyacetylene films studied by E.S.R.

P. Bernier, M. Rolland, M. Galtier, A. Montaner, M. Régis, M. Candille, C.

Benoit, M. Aldissi, C. Linaya, F. Schué, et al.

To cite this version:

P. Bernier, M. Rolland, M. Galtier, A. Montaner, M. Régis, et al.. Electronic properties of non-doped

and doped polyacetylene films studied by E.S.R.. Journal de Physique Lettres, Edp sciences, 1979, 40

(13), pp.297-301. �10.1051/jphyslet:019790040013029700�. �jpa-00231628�

Electronic

_properties

of

_non-doped

and

_doped

_{polyacetylene}

films

studied

_by

E.S.R.

P.

Bernier,

M.

Rolland,

M.

_Galtier,

A.

_Montaner,

M.

_Regis,

M.

Candille,

C. Benoit Groupe de _Dynamique des Phases Condensées ₍₊₎

M.

Aldissi,

C.

_Linaya,

F.

_Schué,

J.

Sledz,

J. M. Fabre

_(*)

and L. Giral

_(*)

Laboratoire de Chimie Macromoléculaire

(*) Laboratoire de Chimie Structurale _Organique

U.S.T.L., place Eugène-Bataillon, 34060

Montpellier

Cedex, France (Re~u le 30 mars _{1979, accepte} le 14 mai ₁₉₇₉₎

Résumé. 2014 Des films de

polyacétylène

ont été

_{synthétisés}

et _dopés avec l’iode et le

_{pentafluorure}

d’antimoine, à

l’aide des

_{procédés chimiques}

usuels. Le

_dopage

à saturation conduit à des valeurs de conductivité d’environ 120 et 25 _(03A9

_cm)-1

à _{température ambiante,}

_{respectivement}

_{pour I2} et

_SbF5.

La Résonance

_Electronique

de

_Spin

des échantillons cis et trans,

dopés

et

_non-dopés,

a été étudiée. Une analyse

qualitative

des résultats permet une

meilleure

_{compréhension}

des

_{propriétés électroniques}

de ces

_{polymères dopés,}

fortement

_inhomogènes

et

haute-ment conducteurs. Abstract. 2014

Cis-polyacetylene

films have been _synthetized and

_doped

with iodine and

_antimony

_penta fluoride,

using

usual chemical

_{techniques. Doping,}

leads, after saturation, to

_conductivity

values of about 120 and 25 (03A9

cm)-1

at room _temperature,

_respectively

for _I2 and

_{SbF5 dopants.}

The E.S.R. of cis- _{and trans-,}

_non-doped

and

_doped

_(CH)x

films has been studied. A

_{qualitative analysis}

of the results allows a better _{understanding} of the

electronic

_properties

of these

_extremely

non _homogeneous and

_highly

conductive _doped

_polymeric

materials. Classification

Physics Abstracts 76.30

1. Introduction. -

Doping

of

_{polyacetylene}

films with donor or

_acceptor

molecules

[1]

is known to

allow the

_conductivity

to _vary from

10-8-10-6

to

102-103

₍₀

_cm)-l

at room

_temperature,

the upper values

_{being comparable}

with those obtained on

polycrystalline organic

conductors such as

TTF-TCNQ

[2]

and

_{inorganic polymeric}

_systems such as

(SN)x [3].

Considerable interest is now focussed in several

_applied

research laboratories on the

_systematic

study

of such

materials,

varying

the nature of the

doping

element,

using copolymers

of

_acetylenes

and various other monomers, etc... This interest is

greatly

warranted

_by

the

_promising

mechanical and electrical

properties

in addition to the ease of the

_synthesis

and

_doping

_processes.

Beyond

these aims a fundamental interest concerns some

_aspects

of the

_transport

properties

connected with the

_polymeric

nature of the material.

Particularly,

anisotropy

of the

_conductivity

and the fact that

orientation

_by

stretching

increases the

_conductivity

(+) Laboratoire associe au C.N.R.S.

of the

_{doped samples,}

raise the

_question

of the

uni-dimensional character of the

_transport

_properties.

In

_fact,

those materials are known to be

_highly

inhomogeneous,

as _appears from

_density

measure-ments and direct observation

_by

_scanning

electron

microscopy, leading

to a model of

entangled

fibrils whose characteristic dimensions are of the order of

a few 100

A. If the

_transport

_properties

_depend

on the

intrinsic

_properties

of the more or less

_crystalline

fibrils,

it has been shown that the

_conductivity

of

doped

samples

is

_principally

limited

_by

interfibrillar

contacts

_[1].

A similar conclusion

caii be

drawn from Hall effect measurements

_[4].

It has then been

_proposed

that the intrinsic _transport

_properties

of the fibrils

are much more effective than those

_appearing

from

macroscopic conductivity

measurements.

E.S.R. is a

technique

which can

_bring

information on the local

_properties

of a _system

_{containing unpaired}

localized electrons or conduction electrons.

In

_strongly

irradiated saturated

_polymers,

like

polyethylene, singlet

spectra

are

observed,

as a

consequence of the

high

number of

_conjugated

double bonds created

_by

irradiation

_[5].

_{Singlet ~spectra}

have

L-298 JOURNAL DE PHYSIQUE - LETTRES also been observed on cis- and

trans-(CH).,,

which

implies

that

_polymeric

chains are

_long,

while the

unpaired

electrons were determined to be distinct from the carriers

_responsible

for electronic conduc-tion

_[6].

Very

recently Goldberg et

al.

_[7]

have studied the

E.S.R. of

_undoped

and

_{AsF5 doped polyacetylene,}

utilizing

an

_apparatus

which allowed in-situ

_doping

in the microwave

cavity.

In the

_light

of their results

they proposed

that the

_signal

observed on

_undoped

samples

arises from a mobile defect in the 7c electron

system.

On

_{heavily doped}

_samples,

their

_experimental

data was consistent with the existence of Pauli

para-niagnetism resulting

from metallic behaviour.

Wein-berger et

al.

_[8],

with additional static

magnetic

susceptibility

measurements have evidence for such a

Pauli

_type

contribution in

_{heavily AsFs doped samples.}

We

_present

here some E.S.R.

experiments performed

on cis- and trans-,

₁₂

and

SbF s doped (CH)x,

in the

temperature

range 130 K to 300

K,

focussing

our

attention on the

magnetic properties

of the resonant

spins,

in order to

_identify

their nature.

2.

_Synthesis

and characterization of the

_(CH)~

films. -

Polyacetylene

films were obtained

by

the method described

_by

Ito et al.

_[9],

_using

the

Ti(OC4H9)4

+

_Al(C2H5)3

_homogeneous

_catalytic

sys-tem in toluene

(with

molar ratio

Al/Ti

=

4).

Poly-merization has been

_performed

at

_temperatures

between - 50 °C and - 78 °C

_favouring

the

cis-isomer.

Trans-isomerization

was obtained

by holding

the cis-isomer

_(prepared

at - 78

_°C)

in n-hexadecane

at 200 °C for 90 min. Various thicknesses

_(between

0.10 and 0.2

_mm)

have been

_obtained,

_depending

on the

_{polymerization}

time

₍₁₅

to 30

_min)

and

_acetylene

pressure (between 0.1 and 1

_atm).

In order to

_verify

the

_{sample quality,}

electrical

conductivity

at room

_temperature,

infrared and

visible

_spectroscopy

(by

reflection)

have been utilized.

The

_{dc-conductivity}

measurements were

performed

using

the four

_probe

method,

with

_gold

wires and

_gold

paints.

The results obtained on

_undoped

and

are in

_good

_agreement with values

given

_by

different

authors

_{[1], [6].}

Cis-(CH)x

films were

doped

in two ways.

Firstly,

by

direct contact with

₁₂

_{vapour when the}

_{conductivity,}

which was measured in-situ

during

the

doping

process,

reached a saturation value at 50

₍₀

_cm)-1

after five hours.

_Secondly,

_cis-(CH)x

films were

_doped

in saturated solutions of

₁₂

and

_SbF5

in

_pentane,

pro-viding conductivity

values of ~ 120

(0

cm)-1

and ~ 25

(0

cm)-1

respectively.

The value obtained at room

_temperature,

as well as the activation _energy

Ea ~

10 meV deduced from the

_temperature

depen-dence of the

conductivity,

with

₁₂

_dopant

are in

_good

agreement

with

_published

results

[1],

[10],

while

SbFs

doping

is not cited.

Infrared and visible

_spectroscopy

_performed

at

200 K to avoid any transformation of the

samples,

were

respectively

utilized to check the cis and trans

content and the _{energy gap}

_Eg

of the

_samples.

Fresh

(CH)x

films

_prepared

at - 78 °C have been shown

to have a cis-isomer content of the order of 90

_%.

Thermal treatment of the cis-isomer as described

above,

has led to a trans content better than 80

%.

Working

with

_samples

of

_large

thickness,

we have

used the

_anomaly

of the

_reflecting

power to estimate

Eg.

Values of - 1.3 eV for

_{non-doped cis-(CH)x}

and ~ 1.5 eV for iodine

_doped

_samples

are in

_good

agreement

with those obtained

by

different authors

[1], [6].

Thermal treatment in the _range 290 to 390 K as

well as

_aging

at room

_temperature

and

atmospheric

pressure gave

strongly degraded samples :

in addition

to the

_expected

cis-trans isomerization we observed an

important

oxidation

(as

shown

_by

IR

spectra)

and a decrease of the

conductivity.

Therefore,

fresh

samples

were sealed in

_glass

tubes

under vacuum and

kept

at T ~ 255 K. E.S.R. results have been obtained

with such

_samples.

The essential characteristics of our

_samples

are

summarized in table I.

_Impurity

contents have been

determined

_by

elemental

_{analysis and/or weight}

increase of the

_samples

after the

_doping

_process. The

_cis/trans

content of the

_{doped samples}

is unknown as it seems that the

_doping

_{process induce} an

isome-rization from cis to trans

_[1].

Table 1. - Essential characteristics

of thefilm samples used for

E.S.R.

_experiments.

6 room

cis/trans Impurity contents _{(W %) Eg} temp. Ea content Oxygen catalyser 12 SbFs (eV)

(S~ cm)-1

(meV)

cis-(CH)x 90 % -5% Ti Al 2013 2013 -1.3 ~ 2 x 10-9 2013 3% 3% trans-(CH)x 20 % 2013 2013 2013 2013 - 6 x 10-6 2013 cis-(CH)x doped 12 ? 2013 - 20% - 1.5 -120 -10 cis-(CH)., doped SbFs ? - 2013 - N 15 % - -25

-3.

_Experimental

results. - E.S.R.

spectra have been recorded on an ERIO Brucker

_spectrometer

working

at 9 GHz. Maximum

_{hyperfrequency}

power

was 30 mW.

Temperature

could be varied in the range 100 K

to 300 K

_using

a standard

nitrogen

_gas flow _cryostat.

Temperature

stability

and _accuracy were of the order of 1 K.

3.1 NON-DOPED cis- AND

_trans-(CH)x.

- The

observed

_spectra

had the

_following

characteristics

_{[11] :}

a)

A Lorentzian

_shape

with a

g-value

(1)

_very close

to the free electron

_{value g}

= 2.002 3.

b)

The

_intensity

I of the

_{signal corresponds roughly}

to

l O i 9

_{g -1}

resonant electrons. Its

_dependence

on

the

_{hyperfrequency}

field

_amplitude

_Hi

at room

tempe-rature

_presents

a saturation effect as

_dI/dH1

is not constant above a

hyperfrequency

_power of 8 mW

(Fig.

1).

Such an effect is

_expected

_when,

in the saturation factor

₍₁

_{+ g2 Jlb}

b- 2

_Hi

Ti

T2) -’

which

enters the measured

_intensity,

the second term in the brackets becomes of order

_{unity (T1}

and

_T2

are the

spin

lattice and

_spin-spin

electronic relaxation times

respectively).

Fig. 1. - The derivative of the E.S.R.

signal intensity at room

temperature with _{Hl amplitude} is _plotted versus _Ht. It is _expected to be constant as saturation does not occur. _Any deviation from this behaviour arises when

(1) The g-value of the _spectra of a _liquid solution of the Ziegler-Natta _catalyst used _{during polymerization} was close to 1.97, which indicates that contributions from Ti3+ residual _impurities, if _present in such a state in _{(CH}~ films,} are more than 100 gauss away from the observed _{signal. Any attempt} to observe a _signal close to the

Ti3+ g-value in the solid _polymer has been _{unsuccessful, probably} due to the oxidation of traces of the _catalyst.

c)

The width

_AHpp

(defined

as the distance in gauss between the

_peaks

of the

_absorption

_derivative)

at

room

_temperature

is

(8 ±

0.5)

G for the cis-isomer and

_(1.4

+

0.2)

G for the trans-isomer. These values do not

_{vary significantly}

as the

_temperature

is lowered

to 150 K.

d)

The

_{spin susceptibility X(T)}

as deduced from

the

_{spectrum parameters}

(x

oc

(linewidth)2

x

_(amplitude))

presents

a Curie

_{type temperature}

T

_{dependence :}

xocT-1.

Concerning

the

_shape,

_{g-value, intensity}

and

line-width,

our results are in

_good

_agreement

with those

reported

by

some authors

[5],

[6],

[7],

while some

disagreement, probably

due to different

_sample

pre-paration

and treatment, exists with others

[12],

[13].

3.2 DOPED

_cis-(CH)x.

-

a)

One essential feature

of the observed

_spectra

is their evolution with time which

_depends

on the

_type

of

_doping

element. If no

signal

could be observed on fresh iodine

_{doped (CH)x,}

the

_sample

_kept

in standard room conditions pre-sented after a few

_days

a

_signal

close to g =

2,

whose

amplitude slowly

increased with time. The

_spectrum

was non

_{symmetric (peak amplitude}

in low

_field/peak

lamplitude

in

_high

field ~

_2).

This non

symmetric

character

_{slowly disappeared}

with time. On the other hand

_{SbF5 doped}

_samples

showed at _any time a

signal

near g = 2 which _was

slowly

decreasing

with

time. The

_spectrum

was also non

_symmetric

(ampli-tudes ratio -

1.3).

b)

Intensities

_correspond

to numbers of resonant

electrons 10 to

102

_(for

_{SbFs dopant)}

and

102

to

103

(for 12 dopant)

times smaller than in

_non-doped

samples.

No saturation effect occurs as

_dI/dH1

is

still a constant _up to our 30 mW maximum

hyper-frequency

power

(Fig.1

for

SbF5

case).

c)

The widths

_AHpp

are

_{(14 ±}

₂₎

G and

_(2.4

±

1 ) G

for

_I2

and

_SbFs

_doped

_{(CH)x respectively.}

We have

observed in the case

_{of SbF5 doped samples}

an increase

of a factor - 3 of

_ðJIpp

when T decreases from 300 to

150 K. These results do not show _any evolution with time.

d)

The

_{spin susceptibility}

deduced as above does

not show _any

_significant

_temperature

_dependence

between 300 K and 130 K

_(Fig.

2 for the

SbF5

case).

4. Discussion. - The

preceding

results

_clearly

show that the behaviour of the resonant electrons are

fundamentally

different in

_non-doped

and

_doped

(CH)~ posing

distinct

_questions

for the two cases.

The characteristics and

_temperature

_dependence

of the

_spectra

obtained on

non-doped (CH)x

are due

L-300 JOURNAL DE PHYSIQUE - LETTRES

Fig. 2. - The E.S.R.

susceptibility X(T) normalized to the room

temperature value is _plotted versus the inverse temperature for

non-doped and _{SbFs doped cM-(CH)~.}

from the

_temperature

_{dependence of x (Fig.}

_2).

Ohnishi

_{[5], by}

a

_comparison

with E.S.R. _spectra

obtained on irradiated satured

_polymers,

_suggested

the existence of radicals of the form :

here n refers to the number of elements in a

_given

isomer

_{configuration}

_(the

_length

of the chain is

evi-dently

expected

to

_correspond

to a _greater number of

_elements).

Goldberg et

al.

_[7]

also

_supposed

the existence of neutral defects of the same

_{type :}

spread

over a domain wall

_containing

several atomic units but of undetermined thickness.

Our data suppose the existence of one such defect

for

roughly

103

monomer units.

_{Unfortunately}

the number of

_conjugation

n, as well as the real

_length

of the chain are not

known,

due to the non

_solubility

of the

_polymer

in _any solvent. Nevertheless Shira-kawa

_[6]

was able to estimate that n would be

_surely

greater than - 100. We then have a

_strong

deloca-lization of the

_unpaired

electrons, coherent with the

absence of

_hyperfine

structure. We _suggest that the observed decrease of the width

_AHpp

as the trans

content of the

_(CH)x

increases from

_{10 %}

to 80

_%

is a _consequence of the better

_stability

of the

trans-isomer. The mean

_length

of sequences of the

trans-type

is

_expected

to be _greater than that of the

cis-type :

both n and the delocalized character of the 7c

electrons

_increase,

which leads to a decrease in

_AHpp.

It is then clear that the E.S.R.

_signal

on

non-doped

samples

is due to semi-delocalized

_unpaired

_electrons,

on free radicals in resonant interaction with the

conjugated

yc electron _system.

Results obtained on

_doped

samples

are much more

difficult to

_interpret,

as the exact nature of the inter-action between the

_{doping species}

and the

_polymeric

chain is not well known. Nevertheless

_{doping of (CH)x}

films with

_halogens

is known to result in

_charge

transfer with the chain and not in a direct chemical

bond. The delocalized electrons _system is not then

disrupted, allowing

for a

_{relatively high mobility}

of

the excess carrier

_[1].

In order to

_identify

the resonant

system, we can consider two

_{possibilities :}

the E.S.R.

spectra

are due to neutral defects of the _{type described}

above,

in interaction with the mobile excess

carriers,

or

_they

concern the conduction carriers themselves.

Considering

the first

_possibility,

a weak interaction between the neutral defect and the conduction carriers would

_give

a behaviour _very close to the

_non-doped

case, in contradiction with the observation of a

temperature

independent

X for the

doped

case and Curie

_type

X for the

non-doped

(Fig. 2).

The

strong

interaction _presents some

_analogy

with the situation of localized moments in metals

_[14]

and would be coherent with the observed

_{susceptibilities (Fig.}

₂₎

as well as decrease of relaxation times _upon

doping

(Fig.

1).

Such a

_suggestion

has been

_already

made

_by

Heeger

and Mc Diarmid

_[15]

but has been shown to

fail for

_{lightly doped samples [8].}

An E.S.R.

_spectrum

due to conduction carriers is also

_expected

if their relaxation times

_Tl

and

_T2

are

_{sufficiently long,}

which is

generally

not the case

in a metallic state at room

_temperature

_[11~].

In fact

uni-dimensional character of the

_transport

_properties,

which is one essential feature of these

polymeric

materials,

is known to lead to a _strong

_narrowing

of the

_spectra

in

_comparison

with tri-dimensional

cases

_[16].

Our observed linewidth of a few _gauss can

be

_interpreted

in that sense. The

_temperature

inde-pendent

susceptibility (Fig. 2)

as well as the

_7B

decrease _upon

_{doping (Fig. 1)}

are coherent with the existence of conduction electrons

_coupled

with the

lattice.

_Finally

the non

_symmetric

character of the

spectra

if

_interpreted

at the

_light

of the

Dyson theory

of E.S.R. in metals

_[17]

would

_{qualitatively}

suppose

a

considerably

_high

intra-fibril

conductivity.

This

conductivity

which _{governs the} skin effect in these

conducting

systems is

probably

several orders of

magnitude

greater than that measured which is not

a true bulk

_conductivity

but results from conduction

through

a loose tri-dimensional network of

entangled

fibrils. Such a conclusion is in _agreement with

conduc-tivity

measurements on stretched

_{doped (CH)x}

where

the fibrils are

partially

oriented

[15].

A definitive choice between the two

_preceding

possibilities

is not

_possible

as this

_stage.

Nevertheless we note that whatever the case _may

_be,

the E.S.R.

spectra

are

_directly

or

indirectly

influenced

_by

the

existence of

conducting

carriers. More

_qualitative

and

quantitative

measurements are needed in order to

investigate

the electronic

_properties

of these carriers. Such an extended

_{investigation}

of the electronic

properties

of

_doped

_{polyacetylene, varying}

several

experimental

parameters

(catalyst, experimental

con-ditions

_{during polymerization,}

monomers,

doping

elements)

is in _progress. Of course a better control of

the

_{polymerization}

conditions as well as a

_precise

characterization of the

_samples

is needed in order to

consider a

_{quantitative analysis}

of the E.S.R. results.

Acknowledgments.

- The authors thank Dr. Collet

(U.S.T.L.

Montpellier)

for his technical aid in E.S.R.

measurements. The _support for the E.S.R.

_equipment

(Brucker

Minispec

ER

_{10) by}

Brucker

_Spectrospin

France is

_{gratefully acknowledged.}

One of us

_(P.B.)

thanks Dr. D. Jerome for _very

_helpful

discussions.

References

[1] CHIANG, C. K., PARK, Y. W., HEEGER, A. J., SHIRAKAWA, H., LOUIS, E. J., MC _DIARMID, A. G., J. Chem. _Phys. 69 ₍₁₉₇₈₎ 5098.

[2] COLEMAN, L. B., Ph. D. thesis _{Philadelphie (1975).}

TOOMBS, G. A., Phys. Rep. 40 ₍₁₉₇₈₎ 181.

[3] BRIGHT, A. A., COHEN, M. J., GARITO, A. F., HEEGER, A. J.,

Appl. Phys. Lett. 26 ₍₁₉₇₅₎ 612.

For a _comparison between _(SN)x and _(CH)x see : STREET,

G. B., CLARKE, T. _C., Advances in _Chemistry, ACS series,

to be published.

[4] SEEGER, K., GILL, W. D., CLARKE, T. C., STREET, G. B., Solid State Commun. 28 (1978) 873.

[5] OHNISHI, S. I., IKEDA, Y., SUGIMOTO, S. _{I., NATTA, I.,} J. _Polym. Sci. 47 (1960) 503.

[6] SHIRAKAWA, H., ITO, T., IKEDA, S., Makromol. Chem. 179

(1978) 1565.

[7] GOLDBERG, I. _{B., CROWE,} H. R., NEWMAN, P. R., HEEGER,

A. J., MC DIARMID, A. G., J. Chem. _Phys. 70 (1979) 1132.

[8] WEINBERGER, B. R., KAUFER, J., HEEGER, A. J., PRON, A.,

MC DIARMID, A. _G., Phys. Rev. B, to be published. [9] ITO, T., SHIRAKAWA, H., IKODA, S., J. Polym. Sci. Polym.

Chem. Ed. 12 ₍₁₉₇₄₎ 11; 13 (1975) 1943.

[10] HSU, S. L., SIGNORELLI, A. J., PEZ, G. P., BAUGHMAN, R. H.,

J. Chem. Phys. 69 (1978) 106.

[11] For a _general review on E.S.R. see : _a) ESR _{spectroscopy in}

polymer Research, RANBY, B. and RABEK, J. F. (Springer-Verlag) 1977; b) Electron _{Spin Resonance, POOLE,} C. P.

(J. Wiley Ed.) 1967.

[12] FLANDROIS, S., C.R. Hebd. Séan. Acad. Sci. Paris 264 (1967)

1244. Linewidths cited in this paper are in _{contradiction}

with our results. We note that Flandrois has used powder samples which can have _very different behaviour from film _samples.

[13] SNOW, A., YANG, N. L., BRANDT, P., WEBER, D., to be

published.

[14] See the review on this subject : ALLOUL, H., BERNIER, P.,

Ann. Phys. 8 ₍₁₉₇₄₎ 169.

[15] HEEGER, A. J., MC DIARMID, A. G., DUBROVNIK, 1978, in press.

[16] BLOCH, A. _{N., CARRUTHERS,} T. _{F., POEHLER,} T. O., COWAN,

D. O. in Chemistry and Physics of one-dimensional metals,

Ed. Keller H. J. (Plenum Press) 1977.

[17] FEHER, G., KIP, A. F., Phys. Rev. 98 ₍₁₉₅₄₎ 337.