Mesoscopic layered structure in conducting polymer thin film fabricated by potential-programmed electropolymerization

Synthetic Metals, 53 (1992) 1-10

1

Mesoscopic layered structure in conducting polymer thin film fabricated by potential-programmed electrop olymerization M a m o r u Fujitsuka, Reiko Nakahara, T o m o k a z u Iyoda* a n d T a k e o Shimidzu* Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-01 (Japan)

Shigehisa Tomita, Yayoi H a t a n o , F u s a m i S o e d a a n d Akira Ishitani Toray Research Center Co., Ltd., 3-3-7 Sonoyama Ohtsu, Shiga 520 (Japan)

Hajime T s u c h i y a Nitto Technical Information Center Co., Ltd., 1-1-2 Shimohozumi Ibaraki, Osaka 567 (Japan)

Akira Ohtani Central Research Laboratory, Nitto Denko Co., Ltd., 1-1-2 Shimohozumi lbaraki, Osaka 567 (Japan)

(Received November 15, 1991; accepted in revised form March 3, 1992)

Abstract Mesoscopic layered structures in conducting polymer thin films axe fabricated by the potential-programmed eleetropolymerization method. High lateral quality in the layered structure is realized by the improvement of polymerization conditions, i.e., a mixture of pyrrole and bithiophene as monomers, a silicon single-crystal wafer as a working electrode and propylene carbonate as a solvent. SIMS depth profiling of the resulting layered films indicates a significant linear correlation between the electric charge passed and the thickness of the individual layers on a 100/~ scale.

1. I n t r o d u c t i o n In m o d e r n s e m i c o n d u c t o r p h y s i c s a n d engineering, the t e c h n i q u e o f c o n f i n e m e n t a n d t u n n e l i n g o f e l e c t r o n s c o n t r o l l e d b y the potential m o d u l a t i o n as d e p t h s t r u c t u r e h a s a t t r a c t e d m u c h attention b e c a u s e o f its p e c u l i a r p r o p e r t i e s a n d i m p o r t a n c e in q u a n t u m m e c h a n i c s [1 ]. The q u a n t u m well lasers a n d the high e l e c t r o n mobility t r a n s i s t o r (HEMT) have b e e n d e v e l o p e d as the m o s t typical d e v i c e s b a s e d o n p r o p e r t i e s specific to m e s o s c o p i c a l l y d e s i g n e d s e m i c o n d u c t o r h e t e r o s t r u c t u r e s . It s e e m s w o r t h w h i l e to e m p h a s i z e t h a t brilliant p r o g r e s s in this field h a s b e e n driven b y the a d v e n t a n d p r o g r e s s o f g r o w t h t e c h n i q u e s o n the a t o m i c scale, e.g. m o l e c u l a r b e a m e p i t a x y (MBE) *Authors to whom correspondence should be addressed.

0379-6779/92/$5.00

© 1992- Elsevier Sequoia. All rights reserved

and metal organic chemical vapor deposition (MOCVD) [2]. Recently, these powerful techniques are being extended to the preparation of heterostructures of other crystal materials, such as rare earth metals [3]. Conducting polymers, a new class of materials, have fostered a wide variety of research with respect to their electrical, optical and magnetic properties [4]. These properties have been understood by adaptation of the so-called band theory [5], which has been successful in inorganic semiconductor crystals. The intrinsic difference between the concerned conducting polymers and the inorganic semiconductor crystals is that the structure of conducting polymers, such as polyacetylene and polypyrrole, is not a threedimensional lattice but amorphous. The conducting polymers consist of an assembly of pseudo-one-dimensional conjugate chains. The amorphous character of these polymers creates substantial disorder, which increases further during the doping process. This causes some difficulty for us in understanding critical aspect of the electronic structures and the properties of conducting polymers. However, recent works [6-8] on the confinement and tunneling of electrons in the microheterostructure of amorphous hydrogenated silicon encourage one to apply the concept of superlattice to amorphous materials. It is well known that in the energy gap of amorphous inorganic semiconductors a number of local states exist which result from dangling bonds and other misfits. A similar electronic structure holds in conducting polymers [4]. From this point of view several groups have studied theoretically the confinement and tunneling of carriers and the electronic structure of quasi-one-dimensional superlattices of conducting polymers [9-11 ]. 'Organic superlattices' are very attractive because they are not only an analogy of inorganic semiconductor engineering, but also useful for understanding the transport mechanism of carriers in polymer chains and exploring new phenomena specific to the designed structures. We have reported that conducting polymer thin films with any compositionally modulated depth structures, layered or graded structures, were realized by a new polymerization method, called the potential-programmed electropolymerization (PPEP) method [12]. This technique allowed compositional modulation on the order of 100/~. This resolution was confirmed with several profiling techniques [13], i.e. transmission electron microscopy (TEM), electron-probe microanalysis (EPMA), secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES). The PPEP method promises to be one of several methods to elucidate correlations between the depth structure and structure-specific properties. Therefore, the following goals should be attained: (i) adequate design of potential modulation, (ii) depth structure with high lateral quality, i.e. flatness of the film and layers and abrupt compositional change, and (iii) establishment of the way for observation of confinement of the electron wave. In this paper, we achieve high lateral quality of the mesoscopic layered structures in conducting poly(pyrrole-co-bithiophene) thin film by improving the conditions of the PPEP method. In particular, we evaluate a significant relationship between the electric charge passed and the thickness of the individual layers.

2. Experimental procedures 2.1. Chemicals Pyrrole (Py; Nacalai Tesque) was dried with Call2 overnight and then distilled fractionally under r e d u c e d pressure (44 °C, 4 . 0 × 1 0 a Pa). 2,2'Bithiophene (BT; Aldrich and Tokyo Kasei) was distilled fractionally under r e d u c e d pressure (75 °C, 2 . 7 × 102 Pa). Both m o n o m e r s were stored under N2 atmosphere. Propylene carbonate (PC; Nacalai Tesque) was dried with molecular sieves (5A, Wako Pure Chemical Industries) for several days and t h e n distilled u n d e r r educed pressure (68 °C, 1.5 × 102 Pa). Other chemicals were the best commercial grade available.

2.2. P o t e n t i a l - p r o g r a m m e d e l e c t r o p o l y m e r i z a t i o n The P P EP m e t hod is to electropolymerize a mixture of m o n o m e r s A and B of conducting polymer (such as Py, BT and their related species) u n d e r the potential sweep function (PSF). The applied PSF is p r o g r a m m e d on the basis of the current fraction-potential (CF-E) curve on m o n o m e r A given by

C F = iA(E)/[iA(E) + iB(E) l

(1)

and the individual c u r r e n t - p o t e n t i a l (i-E) curves on electropolymerization o f m o n o m e r s A and B. With a constant density specific to the film, growth rate of the film thickness is proportional to current. Therefore, the total thickness (d) is estimated by the am ount of electric charge passed:

d (x Q = J'i dt

(2)

G o od ag r eemen t between the CF value and the polymer composition [12] is attributed to the electrochemical stoichiometry between the polymerization yield and the electric charge passed in the electropolymerization:

C F ~ x value in the formula ((A)x(B)I-x)n

(3)

The resulting conducting p o l y m e r film has a compositionally modulated d e p t h structure corresponding to the applied PSF, e.g. a rectangular PSF for an alternating layered structure and a continuous PSF for a graded structure, respectively. The m o n o m e r feed and the potential sweep range a d o p t e d here were set so as to achieve a large compositional change between the u p p e r and lower electrode potentials.

2.3. Electrochemistry The P P EP was carried out in PC containing 7 . 5 × 1 0 -2 mol dm -3 Py and 5 . 0 × 10 -2 mol dm -a BT as m o n o m e r s and 1 . 0 × 1 0 - 1 mol dm -8 LiCIO4 as electrolyte. PC, a p o o r solvent for growing oligomers, was used here as an electrolytic solvent in o r d e r to prevent growing oligomers from flowing into bulk solution. This means that a m or e stoichiometric deposition should

be achieved, in comparison with that of acetonitrile [14]. A mixture of Py and BT was ad o p t ed as m o n o m e r s since the resulting film surface looks flatter by TEM analysis than the copolymer film of Py and 3-methylthiophene (MTh), used in our previous reports [12, 13]. The electrochemical equipment consisted of a function generator (Tohogiken FG-2), a potentiostat (Tohogiken 2000) and a bipolar coulometer (Hokuto Denko HF-202D). The count er and the reference electrodes were a platinum coil and a saturated calomel electrode (SCE), respectively. A silicon single-crystal wafer (Si wafer) was used in order to improve lateral quality of the designed depth structures. Its surface is much flatter than that of a mechanically polished platinum electrode used in our previous reports [12, 13]. A Si-wafer electrode was p r e p a r e d as follows. A Si wafer (Shinetsu Handotai Co., Ltd.; As-doped, 0.0010--0.0030 ~ cm, 0.45 m m thickness) was cut into 7 x 7 l n l n 2 with a diamond cutter. After etching with 2% H F aqueous solution for 2 min under ultrasonics, a c o p p e r wire (100 m m length; 1 mm diameter) was contacted with indium to the reverse side. This contact was covered with e p o x y resin and then with silicon rubber (Shin-Etsu Chemical Co., Ltd.). The c o p p e r wire was sealed with a Teflon tube (1.35 mm inside diameter, 80 mm length). The working electrode was rinsed again with 2% HF aqueous solution for 2 min under ultrasonics, just prior to use.

2.4. Depth profile analysis The depth profiling of the resulting thin films by the PPEP m et hod was carried out by TEM and SIMS. The cross section of the film was directly observed with an H-800 TEM (Hitachi), where the accelerating voltage was 100 kV. The sample for a TEM was p r e p a r e d by using the method of ultrathin sections, as follows. The film peeled from the electrode was embedded in low-viscosity e p o x y resin (Spurr, Polysciences) and the resin was polymerized at 70 °C for 8 h. The e m bedded film was sectioned vertically to the film plane ( 8 0 0 / ~ thickness) with a diamond cutter of a microtome MT6000 instrument (Sorvall). A SIMS depth profile was m eas ur ed with a SIMS A-DIDA 3000 instrument (Atomica), where a Cs ÷ ion gun for sputtering was adjusted to 4 kV accelerating voltage. Secondary ions were collected from a 20% gate of 250 or 4 0 0 / ~ m square of raster scanning area sputtered by Cs + under 6.7 x 1 0 - ~ Pa. The sample for SIMS was provided by peeling the film off the electrode, and then by sputtering and analyzing it from the electrode-side surface. The sputtering rate was estimated from the depth of the sputtered crater with a DN-475 surface profile measuring instrument (Sloan).

3. R e s u l t s

and discussion

3.1. Improvement of layered structure Figure l ( a ) shows the / - E curves on electropolymerization of Py ( 7 . 5 X 1 0 -2 m o l d m -a) and BT ( 5 . 0 × 1 0 -2 mol dm -s) in PC containing

1.0 b 0.5

f

f

J

0.0

~ 1.0 0.0 1.0

1.2 1.4 E/Vvs. SCE

1.6

Fig. 1. (a) / - E c u r v e s o f e l e c t r o p o l y m e r i z a t i o n o f 7 . 5 X 1 0 -2 m o l d m -3 P y (solid line) a n d 5 . 0 x 1 0 -2 tool d m - a B T ( b r o k e n line) in P C c o n t a i n i n g 1 . 0 x 10 -~ m o l d m -2 LiCl04. P o t e n t i a l s w e e p rate w a s 8 . 3 m V s -1. ( b ) C F - E c u r v e o n B T o b t a i n e d f r o m t h e /--E c u r v e s (a) a n d e q n . ( 1 ) in t h e text.

1.0 X 10 -1 mol dm -a LiCIO4 by using a Si-wafer electrode. The CF-E curve on BT (Fig. 1 (b)) was obtained by eqn. (1). The sweep range of the applied rectangular PSF was arranged to be between 1.1 and 1.5 V, at which the pure polypyrrole (PPy) and the copolymer of Py and BT (PPy-PBT) are to be obtained, respectively, i.e. x = 0 . O 0 and x = 0 . 4 8 in the formula of ((PY)I _~(BT)~)n. BT is a dimer of thiophene, so that the fraction of thiophene unit in the copolymer is 0.65. Figures 2(b) and 2(c) show TEM cross sections of the film prepared by the rectangular PSF (Fig. 2(a)). The film thickness is quite uniform, appearing flat over distances of many micrometers (Fig. 2(c)). The total thickness of the film is 1.18 × 1 0 4 J?k with standard deviation (SD) of 3.7 X 102 /~. In the case of the previously reported poly(Py-MTh) layered film, the SD of the total thickness (c. 1.2 x 104/~) was estimated to be 1.8 x 10 ~/~*. The lateral quality, which should be noticed in such a mesoscopic layered film, was remarkably improved by the modification of the PPEP method. All the sections have a layered structure, which can be seen as five vertical stripes, corresponding to the applied PSF. The light and the dark layers of the stripes with 0 . 8 × 10 a and 1.4X l 0 s /~ of thickness are assigned .to the PPy and PPy-PBT layers, respectively. Additionally, no scratches made on cutting the film were observed, quite different from the poly(Py-MTh) layered film [12 ]. The disappearance of the scratches results from similar hardness between the film and epoxy resin used for embedding. Figure 3 0a) is a SIMS depth profile of the film prepared by the rectangular PSF (Fig. 3(a)). The oscillatory profiles of the sulfur and the nitrogen, which appear 180 ° out-of-phase, verified the layered nature of the film clearly from the electrode-side surface to the opposite side of the film with little decrease *TEM c r o s s s e c t i o n s o f t h e p o l y ( P y - M T h ) l a y e r e d films w e r e r e p o r t e d in t h e first p a p e r [12].

1.5

I

I

H I

l

I

40

80

12O

160

20O

0 oO >

1.3 1.1

(a)

30

t / sec

(c) (b) Fig. 2. (a) Applied rectangular PSF. (b) TEM cross section ( x 50 000, reduced in reproduction 79%) of the PPy-PBT composite thin film. (c) TEM cross section ( x 5000, reduced in reproduction 79%) of the same film. The left interfaces in (b) and (c) are the electrode-side surfaces. of t h e i r amplitudes. Although similar oscillatory profiles w e r e obtained in the SIMS d e p t h profiling o f the p o l y ( P y - M T h ) layered films, the a m p l i t u d e s of the profiles gradually d e c r e a s e d with Increasing d e p t h f r o m the e l e c t r o d e side surface mainly b e c a u s e o f d e g e n e r a t i o n o f lateral quality of the l a y e r e d s t r u c t u r e [13l. T h e r e f o r e , the p r e s e n t fine profiles illustrate t h a t a highquality l a y e r e d s t r u c t u r e was f a b r i c a t e d t h r o u g h o u t the film and in a m u c h larger r e g i o n t h a n t h e analyzIng area (50 X 50 /~m2), as we d e s i g n e d b y the PSF. The SIMS d e p t h profiling gave the s a m e profiles after four m o n t h s f r o m p r e p a r a t i o n . This result shows the s t r u c t u r e d film is stable u n d e r n o r m a l a t m o s p h e r i c c o n d i t i o n s b y the P P E P m e t h o d . G o o d stability o f the d e p t h s t r u c t u r e is significant f o r e x a m i n i n g a reliable s t r u c t u r e - p r o p e r t y correlation.

3.2. Controllability of a layered structure with 100 ~ resolution In o r d e r t o e x a m i n e the controllability o f the periodicity of the l a y e r e d film b y the P P E P m e t h o d , we d e s i g n e d several kinds o f layered films b y

sputteringtime/ 103sec 1.0 2.0

0.0 i

i

i

i

S

e

LU 0

15! 1.3

1 1 2

LU 1.1

~

30 0

(a)

c

l 40

i

i

80 120 t / sec

i

;

I

0.0

160 200 (b)

i

I

I

5.0 10.0 depth/ 103A

i

15.0

Fig. 3. (a) Applied rectangular PSF. Co) SIMS depth profile on the sulfur and the nitrogen of the film prepared by the rectangular PSF (a) under sputtering from the electrode-side surface. The area of raster scanning was 250 /zm square. Short vertical bars show the boundaries between PPy and PPy-PBT layers, which were estimated from haft-intensities of neighboring peaks and valleys of the sulfur profile. The individual layer is defined as a set of the order of periodicity from the electrode-side surface and 'c' or 'p' which is a poly(Py-BT) or PPy layer.

u s i n g P S F s with different periodicities. As Fig. 4(a) shows, t¢ a n d tp a r e t h e p o l y m e r i z a t i o n t i m e s for the P P y - P B T a n d P P y layers, r e s p e c t i v e l y , w h e n 1.5 a n d 1.1 V e l e c t r o d e p o t e n t i a l s w e r e applied. T h e tp' v a l u e is t h e polym e r i z a t i o n t i m e f o r t h e final P P y buffer layer. T h e s e l a y e r e d films a r e a b b r e v i a t e d h e r e b y u s i n g re, tp a n d t h e n u m b e r o f cycles (n): e.g. t h e l a y e r e d film, tp30/tc6:5, w a s p r e p a r e d b y the r e c t a n g u l a r P S F c o n s i s t i n g o f five cycles o f t p - - 3 0 s a n d t¢ = 6 s. F i g u r e 4(b) s h o w s the SIMS d e p t h profiles o n t h e sulfur o f t h e tp9/t¢3:5, tp6/t¢2:lO and tp3/tcl:lO films. T h e film in Fig. 2(b) is tp30/tc6:5. The tp9/t¢3:5 film a l s o s h o w e d a n o s c i l l a t o r y profile, indicating a l a y e r e d s t r u c t u r e , f r o m t h e e l e c t r o d e - s i d e s u r f a c e to the o p p o s i t e side, a l t h o u g h its a m p l i t u d e d e c r e a s e d with i n c r e a s i n g depth. T h e t h i c k n e s s e s of the individual l a y e r s w e r e e s t i m a t e d to b e 4.2 × 102 /~ f o r t h e P P y l a y e r s a n d 4.6 X 102/~ f o r t h e P P y - P B T l a y e r s ( a b b r e v i a t e d as P4.2 × 102/C4.6 × 102). In t h e tp6/t¢2:lO film, a similar o s c i l l a t o r y profile w a s o b s e r v e d u p t o t h e third p e a k s a n d valleys. T h e t h i c k n e s s e s o f the individual layers w e r e e s t i m a t e d to b e P 2 . 7 × 1 0 2 / C 2 . 3 × 1 0 2 . T h e tp3/tcl:lO film s h o w e d n o indication o f layered structure. F r o m t h e s e results, w e c o n c l u d e t h a t t h e p r e s e n t P P E P m e t h o d c a n c o n t r o l t h e p e r i o d i c i t y o f t h e l a y e r e d s t r u c t u r e with a r e s o l u t i o n o f o r d e r 100/~. A s h o r t e r - p e r i o d i c l a y e r e d s t r u c t u r e will b e f a b r i c a t e d o n t h e achievem e n t o f a l a r g e r c o m p o s i t i o n a l c h a n g e b e t w e e n layers, as well as t h e i m p r o v e m e n t of the depth resolution of the measuring technique.

8 sputtering time / 103 sec 0.0

5.0

10.0

15.0

20.0

I

i

I

I

g ~2

o~ 1.3 tp

tp'

~

(b)

t / sec

6/tc2' 10

~

~c 1:10

0.0

(a)

-

._ =

.........

> Lu 1.1

/-~

-~

ncycles

i

i

i

I

i

1.0

2,0

3.0

4.0

5.0

6.0

depth / 103 ~,

Fig. 4. (a) Applied rectangular PSFs. Co) SIMS d e p t h profiles o n the sulfur of the films p r e p a r e d by rectangular PSFs (a): tp9/tc3:5 where tp = 9, t¢ = 3, tp' = 210 and n = 5; tp6/t¢2:10 where tp = 6, t¢=2, t p ' = 1 8 0 and n = 1 0 ; tp3/tcl:lO where t p = 3 , t e l l , t p ' = 2 4 0 and n = 1 0 . The area of raster sputtering was 400 ~ m square. Short vertical bars show the boundaries of the layers, which were estimated in the same m a n n e r as Fig. 3. The individual layer is defined as a set of the order of periodicity from the electrode-side surface and 'c' or 'p' which is a poly(Py-BT) or PPy layer.

2000

1500

t¢30/icl 0:5

4~ c

4p

1000 .E

500

0

0

i

i

i

10

20

30

40

electricity / mCocm -2

Fig. 5. Correlation between the electric charge passed and the thicknesses of the PPy-PBT ( 0 ) and the PPy (O) layers.

Figure 5 shows the correlation between the electric charge passed and the thickness of the individual layers. The electric charge passed corresponding to each layer was estimated by integrating current over each t, or tc value. There is a good linear relationship between them. This shows that the thickness of the individual layer can be controlled quantitatively by the electric charge passed, as eqn. (2) predicts. The slopes of these lines are estimated to be 53/~ mC-~ cm 2 for the PPy-PBT layer and 46/~ mC-1 cm 2 for the PPy layer, respectively. The slope is the film growth coefficient per

unit electric charge density, relating to the film density. The ratio of these values is 1.2. On the contrary, the P P y - P B T layer prepared at 1.5 V has the composition o f x = 0.48 In the formula of ((PY)I -x(BT)x)n from the C F - E curve (Fig. 1). BT is a dimer o f thiophene, so that this ratio would be 1.5, assuming that the film density is independent of the composition of pyrrole and thiophene units in the composite*. This deviation leads us to the conclusion that the P P y - P B T layers should have a smaller composition of BT units than was e x p e c t e d from the C F value at 1.5 V. This composition deviation from the C F value was also observed in the poly(Py-MTh) layered film by the AES depth profiling as the composite layer becam e thin** [13]. The reason for the smaller BT composition in the P P y - P B T layers than that e x p e c t e d by the C F value when the thickness of layers b e c o m e s smaller might be the result o f some transient stage before the steady-state electrocopolymerization proceeds. A detailed study on the transient stage is in progress in o r d e r to control layered structure with abrupt compositional changes.

4. C o n c l u s i o n s In conducting pol ym er thIn films, we fabricated mesoscopic layered structures with high lateral quality by the P PE P method. Remarkable imp ro vemen t o f the flatness and u n i f o r m i t y of the layered structure was achieved by careful choice of an appropriate workIng electrode, m o n o m e r and solvent. In particular, g o od controllability of the PPEP m et hod could be obtained, as shown by the linear relationship between the electric charge passed and the thickness o f the individual layers.

Acknowledgements Three o f the authors (T.I., M.F. and R.N.) thank Professor Hiroyuki Sakaki, Institute of Science and Technology, Tokyo University, for his valuable and stimulating discussions. The authors are grateful to Messrs Tetsuyuki Saika and Hiroki Imoto, Daiso Co., Ltd., for their preparation of Si plates. This work was partly s uppor t ed by a Grant-in-Aid from the Ministry of Education of Japan. One of the authors (T.I.) is m uch indebted to the financial support of the Toray Science Foundation.

References 1 For example, R. Dingle (ed.), Semiconductor and Semimetals, Application of Multiquantum Wells, Selective Doping, and Superlattice, Vol. 24, Academic Press, San Diego, CA, 1987. *By the floatation technique, densities of PPy and polybithiophene (PBT) were estimated to be 1.54 and 1.50 g cm -s, respectively. The copolymer (PPy-PBT) would have almost the same density as those of PPy and PBT. **When the film was thick enough, we confirmed by elemental analysis that the film composition is almost the same as the CF value [12].

10 2 H. Sakaki, M. Tanaka and J. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L415. 3 A. Maeda, T. Satake, T. Fujimori, H. Ta~ima, M. Kobayashi and H. Kuroda, J. Appl. Phys., 65 (1989) 3845. 4 For example, Synth. Met., 28 (1989); 29 (1989); 30 (1989). 5 For example, T. A. Skotheim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 6 B. Abels and T. Tiedje, Phys. Rev. Lett., 51 (1983) 2003. 7 S. Miyazaki, Y. lhara and M. Hirose, Phys. Rev. Lett., 50 (1987) 125. 8 K. Hattori, T. Mori, H. Okamoto and Y. Hamakawa, Phys. Rev. Lett., 60 (1988) 825. 9 A. Saxena and J. D. Gunton, Synth. Met., 30 (1989) 729. 10 M. Seel, C. M. Liegener, W. Forner and J. Ladik, Phys. Rev. B, 37 (1988) 956. 11 A. K. Bakhshi, C. M. Liegener and J. Ladik, Synth. Met., 30 (1989) 79. 12 T, Iyoda, H. Toyoda, M. Fujitsuka, R. Nakahara, H. Tsuchiya, K. Honda and T. Shimidzu, J. Phys. Chem., 95 (1991) 5215. 13 T. Iyoda, H. Toyoda, M. Fujitsuka, R. Nakahara, K. Honda, T. Shimidzu, S. Tomita, Y. Hatano, F. Soeda, A. Ishitani and H. Tsuchiya, Thin Solid Films, 205 (1991) 258. 14 C. K. Baker and J. R. Reynolds, J. Electroanal. Chem., 251 (1988) 307.