Kinetics of iodine doping and dedoping processes in thin layers of ply-p-phenylene azomethine

Sensors

and Actuators,

7 (1985)

199

199 - 207

KINETICS OF IODINE DOPING AND DEDOPING PROCESSES IN THIN LAYERS OF POLYp-PHENYLENE AZOMETHINE 3

CHAGUE,

J P GERMAIN*,

C MALEYSSON

and H ROBERT

Laboratozre d’Electronzque et Rksonance Magnetzque, UA 830 du CNRS, CEermont II, 24, avenue des Landaus - B P 45, 63170 Aubzere (France) (Received

October

17, 1984,

m revised

form

February

7, 1985,

accepted

Unzversate’ de

June 13, 1985)

Abstract A new method for preparing thm films of a nitrogen-contammg aromatic polymer (polyparaphenylene azomethme) 1sreported Electrlcal properties of these thin films are reviewed when they are doped with halogen acceptors such as iodine gas The kinetics of conductlvlty 1s analysed m terms of adsorption and desorptlon rates of the doping species Thm layers of polyparaphenylene azomethme (PPCN) exhibit a good reverslblhty m the dopmg/dedopmg process and their conductlvlty response 1s sensltlve to the partial pressure of the doping gas. PPCN or electroactive polymers of this type may be used as vapour-sensltlve materials m gas detectors Introduction Since they were recognized as bemg ‘dopable’ by electron donors or acceptors via a charge transfer process, coqugated polymers have exhlblted interesting potential for numerous practical apphcatlons as conductors, Junctions, photovoltalc cells, batteries and sensors [l - 131 Among electroactlve polymers, special attention has been given to polyacetylene, both for its physical propertles [14 - 161 and Its capability to be processed into technrcally tractable films Some other coqugated polymers, such as polyaromatlc compounds [17 - 211, also have mterestmg properties They can become highly conductmg upon doping and have a relatively good stability agamst oxygen and moisture Moreover, It ISpossible to obtam a great variety of materials [19, 211 expected to be sensltlve to many different species of dopmg agents Unfortunately these materials are generally obtained as mfuslble and msoluble powders Thus their field of apphcatlon 1s at present still relatively narrow Recent studies [lS, 221, however, have developed new methods of syntheslzmg polyparaphenylene (PPP) films or thm layers We have developed an ongmal method of preparmg polyparaphenylene azomethme (here*Author 0250-6874/85/$3

to whom 30

correspondence

should

be addressed

0 EIsevler Sequola/Prmted

m The Netherlands

200

after referred to as PPCN) m the form of thin layers The purpose of the present paper 1s to report the properties of this form of the material and to mvestlgate the response of a layer to attempts at doping with halogen gases such as iodine One reason for focusing our attention on a polyaromatlc system with -CH=N(azomethme) groups between the aromatic nngs IS that theoretical computations have shown that this type of nitrogen-contammg polymer should have an lornzatlon potential a httle less than that of PPP (5 eV instead of 5 5 eV) 1211 This calculation predicts a sensltlvlty to doping agents weaker than those currently used for doping PPP Synthesis and charactenzatlon

of PPCN

PPCN 1s grown as thin layers by direct reactlon of the vapours of terephthahc aldehyde (I) and paraphenylene dlamme (II) n HOC

NH*-

(1)

(11)

(+?&

N = CH.-f, + n Hz0

CH=N-(@-

(III)

This vapour phase polymerlzatlon 1s conducted m a Veeco 770 high vacuum evaporator under a pressure of 5 X lo-’ Torr (I) and (II) are slmultaneously sublimated and (III) 1s deposited on a substrate kept at room temperature The layers are then treated under vacuum for one hour at about 200 “C m order to eliminate a possible excess of the components (I) or (II) Glass, Sl + SIOZ and alumma substrates are used The thickness of the layers that we have studied 1s about a few thousand a Good adhesion to the substrates 1s observed The thin films of PPCN are not soluble m usual solvents and do not melt below 300 “C The structure of the layers 1s essentially amorphous, as shown by X-ray experiments The 1 r spectrum of a layer deposited on a crystallme substrate of KBr (Fig l(a)) presents absorption bands at 850, 1190 and 1480 cm-’ characterlstlc of dlsubstltuted benzene and also a line at, 1630 cm-l arising from the C=N vibration of -@--CH=N-q+ The 1 r band at 2250 cm-’ may arise from CEN groups We believe that these groups occurrmg at the end of the polymer chains do not have an Important effect on electrical properties of the mam conjugated backbone The u v -vlslble spectrum (Fig l(b)) sh ows a strong absorption for wavelengths lower than 500 nm This transltlon can be attnbuted to the ‘optical gap’ between 7r and 7r* electronic bands The electrical characterlzatlon IS made by measurmg the d c conductlvlty, Od c , of the layers The measurements are performed by means of an mterdlgltated gold electrode structure deposlted under vacuum and consisting of six pairs of electrodes with an area of 25 mm X 25 mm, each electrode 1s 1 mm wide and 20 mm long and the gap between every other

201

18

16

14

12

10

8

10 “xa(cmr’

-‘!! 300

400

500

600

700

(b)

Fig

800 Xnml

l(a)

I r spectrum

of PPCN,

(b) u v spectrum

of PPCN

electrode 1s also 1 mm The contacts between the gold and polymer samples are purely ohmic For od c measurements, the voltage used 1s 1 volt and the numerical values of od c are determmed from the calculated resistance V/I of the devtce od c of PPCN 1s less than lo-’ (52 cm)-l The layers are stable to the ambient atmosphere and we do not observe any change of their proper-ties with time

Iodme dopmg and dedopmg of PPCN Expermen

tal

The following experimental condltlons were used (a) doping mixture Nz (dilution gas) + I2 vapour (dopant), (b) dedopmg gas pure Nz , (c) maxunum partial pressure of I2 (PI, max) 0 3 Torr (saturated lodme vapour at 295 K), (d) flow rate of Nz + Iz or pure N2 150 l/h, (e) substrate temperature 325 K, (f) lodme concentration m the polymer measured by the penodlc vmatlon of a quartz oscillator on which a thin layer of PPCN ISdeposited Results

and drscussmn

Upon exposure to I2 + N2 mixture at PI, = 0 3 Torr, the mtlally yellow PPCN layers become dark grey Their conductlvlty (7dc = 0 quickly increases

202

and reaches an equlllbrlum value 0, that depends on the number of cycles to which the material 1s exposed (m our experiments, one cycle consists of a 5 mm doping followed by a 5 mm dedopmg) 0, mcreases during the first 50 - 60 cycles and then becomes stable at least for the 150 next cycles, where our measurements stopped (Fig 2) The iodine concentration m the polymer, y, also increases during the first 50 - 60 cycles and remams constant (y,-,) for the followmg cycles (Fig 3), note that the layer 1s then stabilized In order to explam the increase of CJ‘, during the first 50 - 60 cycles, It may be assumed that for the first dopmg cycles the dopant 1sonly adsorbed on a very thin layer at the ‘surface’ of the grams This layer prevents further penetration of iodine At this stage, the measured current 1s stable for several hours The adsorbed lodme may be responsible for drastic mlcroscoplc changes of the polymer morphology like those observed in (CH), ~231 on dedopmg, the lodme desorptlon leaves cavltles which allow lodme to reach new doping sites for the next cycle In the course of the followmg cycles, the number of these sites increases (as well as CJ)until all the avallable sites are filled The results and dlscusslon presented below will concern stabilized layers only Figure 4 shows the dependence of (T, on the partial pressure of Iodine, % 0,

VI2

Y

where x 1sabout 1

Increasing PI, increases the slope of the Log CY, uersus Log PI, curves above that obtained by decreasing PIZ. This hysteresis may arise from several causes the time constant of dedopmg 1s longer than that of doping and the time between two step changes of PIi, 5 mm, 1s too short Moreover, PI, 1s o(urn) ' lo-'

-

I '

20

22 24

26 28

30 32

JJ r 60 61 62

Numberofcycles

l?lg 2 doping

u vs

time

for

different

cycles,

each conslstmg

of

5 mm

dopmg

and 5 mm de-

203

i

c

60toZOOcycles

Mcycles

e

/ cl0cycles

/ i

,

1

s.

2

3

5

10

y(%)

Fig 3 IJ as a function of lodme concentration y as the number of cycles increases 1s the number of lodme atoms per monomer unit of PPCN )

1o-7

d

‘0 i 0 03

006

0 12

Fig 4 0 us partial pressure of lodme

,

(y

204

vaned by dilution of an lodme-staturated mixture of N2 + I, at 295 K, iodine sorbed by the walls of the experimental apparatus probably affects the PI, values, especially when PI, IS decreasing Let y0 be the residual lodme concentration m the stablhzed layer dedoped for 2 h and ypIl the lodme concentration after 5 mm of exposure at an I2 pressure of PI1, then to PI, m the range 0 03 Torr f PI2 d 0 3 Torr AY = YQ - y ,, 1s proportional Wg 5) As the temperature of the layers increases, a decrease of (T, 1s seen (for example, If P,, = 0 3 Torr, (J, = 4 55 X 10d7 (S2 cm)-’ and Ay = 1 15% at T=293 K, CJ, = 2 15 X 10e7 (S2 cm)-’ and Ay = 0 7% at T= 323 K) This result disagrees with the thermally activated process of conduction expected from the semlconductmg nature of the iodine-doped PPCN [24], assuming that electronic properties are well described m terms of conventional band theory [ 251 Thus, we have to assume that the number of iodine molecules mvolved m a charge transfer process with the polymer decreases when the temperature increases These miscellaneous results can be explained m terms of a process of adsorption and desorptlon of iodine 1261 glvmg rise to charge transfer phenomena at the contact with the polymer layer A phenomenologlcal description of what happens can be made within the theory proposed by Langmulr [ 271 Let N be the number of entitles actually adsorbed and No the total number of adsorption posslbllltles for these molecules Then, under an lodme pressure P -

dt

0,Ol

=

yP(N,-N)-+N

-I 003

Fig

5 Variation

006

012

of lodme concentration

l

Ay us partlal pressure of iodine

205

Here y stands for the adsorption rate and contains an actlvatlon term accountmg for the potential barrier E that a molecule must overcome before adsorption occurs

where S 1s the effective area of iodine action, and M the molecular mass of dopant molecules /31s the desorptlon rate, assumed to be of the form P=j30exp-(~~Eo) where E, represents the bonding energy between the adsorbed and the adsorbing species The hmlt of iV for t going to mfmlty 1s given by exp(--E/kT)

No

exp(-

E/hT) + POexp(-

(E + E,)/kT)

Such a model allows a quahtatlve explanation of most of our experlmental results (a) An increase of lodme pressure causes an increase m the number of adsorbed molecules, that 1s an increase m the number of species really efficient m the charge transfer process (b) An mcrease of temperature favours the desorptlon process This can account for the correspondmg decrease of conductlvlty (c) The reverslblhty m the dopmg and dedopmg cycles can be explamed m terms of reversible adsorption and desorptlon processes This latter reverslblhty is quite plausible at the experimental temperatures m view of the relatively small value of Eo, the bondmg energy correspondmg to a charge transfer phenomenon from our data for Ay(T), E. - 0 15 eV Under the expenmental condltlons,

po@iZiBiT SP

exp(--E,)/kT)

Z+1

Then, AyaN(~)/NoccPIz IS m good agreement with the expenmental results (Fig 5) Conclusion These preliminary results on a nitrogen-contammg polyaromatlc, PPCN, show good reverslblllty of the dopmg-dedopmg process m the case of

206

nitrogen-lodme mixtures The electrlcal conductlvlty of thin layers 1s sensltlve to the partial pressure of the dopmg gas, with a response time of the order of one minute These results, which are quahtatlvely explained wlthm the framework of Langmulr’s theory, show the potential of this material as the basis of gas sensors The sensltlvlty to other dopants (H2S, NH,, ) has to be tested, as well as the reproduclblhty, reverslblhty and stabilCl2 ity m ambient air Acknowledgement This research was m part supported by a grant m ald from the Mmlst&-e de 1’Industne et de la Recherche References 1 A G MacDlarmld and A J Heeger, Molecular Metals, Plenum Press, New York, 1979, p 161 2 C K Chlang, Y M Park, A J Heeger, H Shlrakawa, E J LOUIS and A G MacDlarmld, Conductmg polymers J Chem Phys , 69 halogen doped polyacetylene, (1978) 5098 3 T Tam, W D Gill, P M Grant, T C Clarke and G B Street, Photoconductlvlty and Junction propertles of polyacetylene films, Synth Met, 1 (1979) 301 4 P M Grant, T Tam, W D Gill, M Krounbl and T C Clarke, Propextles of metal/ 869 acetylene Schottky barriers, J Appl Phys , 52 (1981) 5 A G MacDlarmld, A J Heeger and P Nlgrey, Patent 81-101464 6 6 R L Van Ewlk, A V Chadwick and J D Wright, Electron donor-acceptor mteractlons and surface semlconductwvlty m molecular crystals as a function of ambient gas, J Chem Sac , Faraday Trans , I 76 (1980) 2194 7 B Bott and T A Jones, A highly sensltlve NO1 sensor based on electrlcal conductlvlty changes in phthalocyanme films, Sensors and Actuators, 5 (1983) 43 8 J I Jm, S Antoun, C Ober and R W Lenz, Brevets du Pont de Nemours, EI, BF (2) (1976) 310-426 9 B Mlllaud and C Strazlelle, Dilute solution properties of thermotroplc polymers aromatic polyazomethmes, Polymer, 20 (1979) 563 10 S D Sentuna, Fabrlcatlon and evaluation of polymeric early warning fire alarm devices, NASA Contact Report CR 134764 (1975) 11 N R Byrd and M I3 Sheratte, Synthesis and evaluation of polymers for use in early warnmg fire alarm devices, NASA Contact Report CR 134693 (1975) 12 J E Katon, Organic Semrconductzng Polymers, Marcel Dekker, New York, 1968, p 105 13 J E Katon, Encyclopedza of Polymer Science and Technology, 10, p 659 14 W P Su, J J Schrleffer and A J Heeger, Sohtons m polyacetylene, Phys Rev 1698 Letf, 42 (1978) 15 W P Su, J R Schrleffer and A J Heeger, Sollton excltatrons in polyacetylene, Phys Rev B, 22 (1980) 2209 16 H Shlrakawa and S Ikeda, Infrared spectra of polyacetylene, Polym J 2 (1971) 231 17 D M Ivory, G G Mdler, J M Sowa, L W Shacklette, R R Chance and R H Baughman, Highly conductmg charge-transfer-complexes of poly(paraphenylene), J Chem Phys, 71 (1979) 1506

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Blographles B Chague received his Docteur degree m Electromcs Engmeermg from the Umverslty of Clermont-Ferrand (1984) He works at the Laboratolre d’Electromque et Rksonance Magn&que In the field of organic semlconductor gas sensors P P Germazn, Professor at Clermont University, received his Doctorat d’Etat (1977) for his research m the field of liquid crystals Since 1982 his topic of interest have been organic solid state sensors m the Laboratolre d’Electromque et RGsonance Magn&lque C Maleysson received her Engineer Degree from Ecole Cent&e de Lyon (1981) She JOIned the Laboratolre d’Electromque in order to prepare her D I Thesis m the field of organic gas sensors H Robert, Maftre de Recherches at the Centre Natlonal de la Recherthe Sclentlflque received his D I from the Unlverslty of Clermont-Ferrand (1968) Since 1964, he has worked at the Laboratolre d’Electromque et His current research actlvltles are m the area of Rksonance Magn&que organic conductors and semiconductors