Electrical Switching and Memory Phenomena in ... - ACS Publications

17 Electrical Switching and Memory Phenomena in Semiconducting Organic Thin Films R. S. POTEMBER and T. O. POEHLER

Downloaded by UNIV LAVAL on June 26, 2014 | http://pubs.acs.org Publication Date: April 7, 1982 | doi: 10.1021/bk-1982-0184.ch017

Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20810

Switching and Memory Device:

Materials and Fabrication

This paper is a report on stable and reproducible current -controlled bistable electrical switching and memory phenomena observed in polycrystalline metal-organic semiconducting films. The effects are observed in films of either copper or silver complexed with the electron acceptors tetracyanoethylene (TCNE), tetracyanonapthoquinodimethane (TNAP), tetracyanoquinodimethane (TCNQ), (1) or other TCNQ derivatives shown below. The character of the switching in going from a high- to a low-impedance state in these organic charge-transfer complexes is believed to be com parable in many respects to existing inorganic materials. The basic configuration of the device, shown in Figure 1, consists of a 5-10 μm thick polycrystalline aggregate of a copper or a silver charge-transfer complex sandwiched between two metal electrodes. Electrical connection is made to the two metal electrodes through silver conducting paste or through liquid metals of mercury gal lium or gallium-indium eutectic. Fabrication of the device con sists of first mechanically removing any oxide layers and organic contaminants from either a piece of copper or silver metal f o i l . The cleaned metal foil is then placed in a solution of dry and degassed acetonitrile which has been saturated with a neutral acceptor molecule, for example, TCNQ°. The neutral acceptors used in a l l of these experiments are recrystallized twice from aceto n i t r i l and then sublimed under a high vacuum prior to their use. (2) When the solution saturated with the neutral acceptor is brought in contact with a metal substrate of either copper or silver, a rapid oxidation-reduction reaction occurs in which the corresponding metal salt of the ion-radical acceptor molecule is formed. The basic reaction is shown in Equation 1 for copper and TCNQ°. This technique of forming semiconducting films by direct oxidation-reduction is used to grow highly microcrystalline films directly on the copper or silver substrate. These films show a metallic sheen and can be grown to a thickness of 10 μm in a 0097-6156/82/0184-023 3 $ 0 5 . 0 0 / 0 © 1982 American Chemical Society

In Polymer Materials for Electronic Applications; Feit, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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Figure 1.

Schematic of an organic switching device.

.CN

NC

"NC Cu

Cu° + NC

+

'MX

NC

CN

CN CN

TCNQ Equation

1.


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matter of minutes. Once the p o l y c r y s t a l l i n e f i l m has been grown to the d e s i r e d t h i c k n e s s , the growth process can be terminated by simply removing the metal substrate c o n t a i n i n g the organic l a y e r from the a c e t o n i t r i l e s o l u t i o n ; t h i s terminates the redox react i o n . The two component s t r u c t u r e i s g e n t l y washed with a d d i t i o n a l a c e t o n i t r i l e to remove any excess n e u t r a l acceptor molec u l e s and d r i e d under a vacuum t o remove any t r a c e s o f s o l v e n t . Elemental a n a l y s i s performed on p o l y c r y s t a l l i n e f i l m s of Cu-TCNQ and Cu-TNAP removed from the copper substrate r e v e a l s that the metal/acceptor r a t i o i s 1:1 i n both complexes. (3) F i n a l l y , the three component s t r u c t u r e i s complete when a top metal e l e c t r o d e of e i t h e r aluminum or chromium i s evaporated or sputtered d i r e c t l y on the organic f i l m .


E l e c t r i c a l Behavior Threshold and memory behavior i s observed i n these m a t e r i a l s by examining current as a f u n c t i o n o f voltage across the two terminal s t r u c t u r e . F i g u r e 2 shows a t y p i c a l dc c u r r e n t - v o l t a g e curve f o r a 3.75 pm t h i c k Cu/Cu-TNAP/Al system. The t r a c e i n Figure 2, as w e l l as a l l other I-V measurements presented i n t h i s paper, are made w i t h a 1 0 - £ l load r e s i s t o r i n s e r i e s with the device. F i g u r e 2 shows that there a r e two s t a b l e non-ohmic r e s i s t i v e s t a t e s i n the m a t e r i a l . These two s t a t e s , l a b e l e d "OFF" s t a t e and "ON" s t a t e , are e s s e n t i a l l y i n s e n s i t i v e to moisture, l i g h t , and the p o l a r i t y o f the a p p l i e d v o l t a g e . A r a p i d switching i s observed from the "OFF" to the "ON" s t a t e along the load l i n e when an a p p l i e d f i e l d across the sample surpasses a t h r e s h o l d value (Vth) o f 2.7 V. T h i s corresponds to a f i e l d strength of approximately 8.1xl0 V/cm. At t h i s f i e l d strength the i n i t i a l high impedance o f the device, 1.25x10** ohms, drops to a low impedance value of 190 ohms. T h i s r i s e i n current to 4 ma and concurrent decrease i n the v o l t a g e to approximately 1.2 V along the load l i n e i s observed i n the Cu-TNAP system. I t i s r e p r e s e n t a t i v e of the switching e f f e c t s observed i n a l l o f the metal charget r a n s f e r s a l t s examined and i s c h a r a c t e r i s t i c of a l l two terminal S-shaped o r c u r r e n t - c o n t r o l l e d n e g a t i v e - r e s i s t a n c e switches. (4) 2

3

In a d d i t i o n , i t has been observed i n a l l of the m a t e r i a l s i n v e s t i g a t e d that once the f i l m i s i n the "ON" s t a t e i t w i l l remain i n that s t a t e as long as an e x t e r n a l f i e l d i s a p p l i e d . In every case s t u d i e d , the f i l m e v e n t u a l l y returned to i t s i n i t i a l high-impedance s t a t e a f t e r the a p p l i e d f i e l d was removed. I t was a l s o found that the time r e q u i r e d to switch back to the i n i t i a l s t a t e appeared to be d i r e c t l y p r o p o r t i o n a l to the f i l m t h i c k n e s s , d u r a t i o n o f the a p p l i e d f i e l d , and the amount o f power d i s s i p a t e d i n the sample w h i l e i n t h i s s t a t e . Three general trends are noted i n the "ON" s t a t e character of the copper and s i l v e r complexes as r e l a t e d to the d i f f e r e n t acceptor molecules. The f i r s t i s that the copper s a l t s c o n s i s t e n t l y e x h i b i t e d greater s t a b i l i t y and r e p r o d u c i b i l i t y over the corresponding s i l v e r s a l t s o f the same acceptor. Second, i t i s


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p o s s i b l e to c o r r e l a t e the p r e f e r r e d switching behavior of the d i f f e r e n t complexes to the r e d u c t i o n p o t e n t i a l of the v a r i o u s acceptors. This p l o t i s shown i n F i g u r e 3 using copper as a donor i n each case. I t appears that f o r devices made from weak e l e c t r o n acceptors, the switching behavior i s u s u a l l y of the t h r e s h o l d type, i . e . , when the a p p l i e d v o l t a g e i s removed from a device i n the "ON" s t a t e , the device w i l l immediately r e t u r n to the "OFF" s t a t e . On the other hand, f o r strong e l e c t r o n acceptors a memory e f f e c t i s observed. T h i s memory s t a t e remains i n t a c t from a few minutes up to s e v e r a l days and can o f t e n be removed by the a p p l i c a t i o n o f a short pulse of current i n e i t h e r d i r e c t i o n . For intermediate s t r e n g t h acceptors, i t i s p o s s i b l e to operate the device as e i t h e r a memory switch or a t h r e s h o l d switch by v a r y i n g the s t r e n g t h or the d u r a t i o n of the a p p l i e d f i e l d i n the low-impedance s t a t e . T h i r d , i t a l s o recognized that the f i e l d s t r e n g t h o f the switching t h r e s h o l d tends to p a r a l l e l the s t r e n g t h of the acceptor. For i n s t a n c e , the copper s a l t of TCNQ(0Me>2 switches a t a f i e l d s t r e n g t h of approximately 2x10 V/cm, while the copper s a l t o f TCNQFi^ i s found to switch at a f i e l d s t r e n g t h of about 2x10** V/cm. I t i s c l e a r that these three trends a r e r e l a t e d to the r e d u c t i o n p o t e n t i a l of the acceptor c a l c u l a t e d from s o l u t i o n redox p o t e n t i a l s (_5). However, as these values do not always p a r a l l e l the values found i n the s o l i d phase, a more q u a n t i t a t i v e d e s c r i p t i o n r e l a t i n g to the switching behavior to the acceptor cannot be made unless the v a r i o u s c o n t r i b u t i o n s to the b i n d i n g energy of the d i f f e r e n t i o n - r a d i c a l s a l t s are considered. 3

The response to a very short pulse i s exemplified i n the next f i g u r e . F i g u r e 4 i s an o s c i l l o s c o p e t r a c e showing both the l e a d ing edge o f a v o l t a g e p u l s e and current pulse versus time f o r a Cu-TNAP sample i n response to a r e c t a n g u l a r v o l t a g e pulse with a 4 nsec r i s e time. T h i s v o l t a g e pulse switched the sample from the h i g h - to the low-impedance s t a t e and contained a 1.0 V overvoltage to e l i m i n a t e any current o s c i l l a t i o n s between the "OFF" and "ON" s t a t e s . Current o s c i l l a t i o n s a r i s e when the a p p l i e d v o l t a g e i s set very c l o s e to VthI t i s not p o s s i b l e from t h i s experiment to determine values f o r the conventional delay times and r i s e times because the combined delay and r i s e times appear to be l e s s than 4 nsec (the l i m i t i n g r i s e time o f the pulse generator). This experiment suggests that the mechanism of the switching phenomena i s not due to thermal e f f e c t s (6) which have been used to d e s c r i b e switching and memory phenomena i n many other systems. From F i g ure 4, i t appears that the delay time i s s h o r t e r than reported values f o r i n o r g a n i c semiconductors under the same experimental c o n d i t i o n s . A recent example of delay times i n an i n o r g a n i c amorphous m a t e r i a l i s given f o r the composition T e i 0 A S 3 s G e y S i i 7 P 1 (7), approximately 1 ym t h i c k , sandwiched between two molybdenum e l e c t r o d e s . A t y p i c a l delay time reported f o r t h i s device i n response to a s i n g l e 12 V pulse i s about 2 ysec. To reduce the delay time to a v a l u e of 10 nsec, a 30 V pulse (18 V overvoltage) was r e q u i r e d .


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Voltage 1V/div.

Time (5 nsec/div.) Figure 4.

Transient response to a 4-ns rise time rectangular pulse.


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An experiment was designed to determine i f the device generates an o p e n - c i r c u i t v o l t a g e o r electromotive f o r c e (emf) when r e t u r n i n g from the low- to the high-impedance mode. The appearance of a spontaneous emf (8) would i n d i c a t e that an electrochemi c a l r e a c t i o n was r e s p o n s i b l e f o r switching phenomena. In t h i s experiment: 1) an a p p l i e d v o l t a g e i n excess of the t h r e s h o l d v o l t a g e was used t o p l a c e a Cu-TNAP sample i n t o a low-impedance s t a t e where i t would remain f o r a short time a f t e r the a p p l i e d v o l t a g e was removed, i . e . , memory s t a t e ; 2) the sample was then e x t e r n a l l y s h o r t - c i r c u i t e d to e l i m i n a t e any c a p a c i t i v e e f f e c t s , and f i n a l l y ; 3) a high input impedance storage o s c i l l o s c o p e was used to measure o p e n - c i r c u i t discharge v o l t a g e when the sample spontaneously returned to i t s o r i g i n a l high-impedance s t a t e . The o s c i l l o s c o p e was s e t to t r i g g e r whenever a v o l t a g e exceeding a few m i l l i v o l t s appeared across the sample. The r e s u l t s are shown i n F i g u r e 5 where the spontaneous o p e n - c i r c u i t v o l t a g e measured by the o s c i l l o s c o p e i s reproduced and i s seen to have a maximum v o l t a g e discharge of approximately 0.3 v o l t s . The electromotive f o r c e (emf) of 0.3 v o l t s observed i n t h i s experiment does show that the mechanism by which the switching occurs i s c o n s i s t e n t with a f i e l d induced s o l i d - s t a t e r e v e r s i b l e e l e c t r o c h e m i c a l r e a c t i o n a s s o c i a t e d with the metal charge-transfer salts. I n f r a r e d Reflectance Spectra of Cu-TCNQ Semiconducting Films To i n v e s t i g a t e the formal charge of TCNQ i n the semiconducting f i l m s o f Cu-TCNQ, the i n f r a r e d r e f l e c t a n c e s p e c t r a was recorded at room temperature f o r c r y s t a l l i n e Cu-TCNQ f i l m s before and a f t e r an e x t e r n a l e l e c t r i c f i e l d was a p p l i e d to the sample. The a p p l i e d f i e l d i n t h i s experiment was of a strength comparable to t h a t i n switching device s t r u c t u r e s , i . e . , a f i e l d i n excess of 10 * V/cm was used. The r e s u l t s were then compared to the r e f l e c t i o n s p e c t r a measured f o r other c r y s t a l l i n e metal-TCNQ r a d i c a l - a n i o n s a l t s . These s a l t s a r e known to e x i s t as e i t h e r simple or complex s a l t s i n the s o l i d - s t a t e . The c r y s t a l l i n e m a t e r i a l s i n v e s t i g a t e d were lithium-TCNQ, cesium-TCNQ, copperTCNQ (prepared by a m e t a t h e t i c a l r e a c t i o n ) and copper-TCNQ grown on copper substrates i n the manner s i m i l a r to the switching d e v i c e s . S p e c i f i c a l l y , the r e g i o n o f the i n f r a r e d spectrum measured was between 2000 to 2500 cm" (0.25 to 0.3 eV). This s p e c t r a l r e g i o n corresponds to the V2 C=N s t r e t c h i n g mode i n TCNQ. Previous s t u d i e s have provided evidence to l i n k the f r e quency assignment of C=N s t r e t c h i n g and C=C s t r e t c h i n g modes to the degree o f charge t r a n s f e r i n complexes of TCNQ. (9) In these i n v e s t i g a t i o n s a frequency s h i f t to lower energy i s reported as charge d e n s i t y increases on TCNQ. 1

1

The Cu-TCNQ switching m a t e r i a l was subjected to e l e c t r i c f i e l d s by clamping a t h i n h i g h l y i n s u l a t i n g f i l m of e i t h e r t e f l o n or polyethylene between the surface of the Cu-TCNQ f i l m on a cop-



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> Time (0.1 msec/cm) Figure 5.

Spontaneous open-circuit potential generated in a sample at room temperature.

Cu/Cu-TNAP/Al


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per s u b s t r a t e and an e x t e r n a l top metal e l e c t r o d e . The r e f l e c tance spectrum was recorded a f t e r removing the f i e l d and separating the Cu-TCNQ (on the copper substrate) from the top e l e c t r o d e and the i n s u l a t i n g p l a s t i c f i l m . A l l o f the samples were f r e s h l y prepared and the s o l i d - s t a t e d i f f u s e r e f l e c t a n c e s p e c t r a was recorded on a Perkin-Elmer 621 Grating IR Spectrometer. Wherever p o s s i b l e , elemental a n a l y s i s was performed on the samples to v e r i f y t h e i r composition. The upper t r a c e i n F i g u r e 6 i s a r e f l e c t a n c e spectrum of a c r y s t a l l i n e f i l m o f Cu-TCNQ b e f o r e the a p p l i c a t i o n of an e l e c t r i c field. A moderately strong i n f r a r e d a c t i v e mode f o r CN i s observed t o dominate the r e g i o n c h a r a c t e r i z e d by a s i n g l e l i n e center a t approximately 2320 cm" . The lower t r a c e (Figure 6) i s a r e f l e c t a n c e spectrum of the same f i l m o f Cu-TCNQ, but i n t h i s spectrum an e l e c t r i c f i e l d has been a p p l i e d to the sample f o r 72 hours. In t h i s t r a c e there a r e two r e f l e c t a n c e maxima. One l i n e can be assigned a value o f 2321 cm" which i s n e a r l y i d e n t i c a l to the maximum v a l u e seen a t 2320 cm" i n the o r i g i n a l spectrum. However, a second l i n e has appeared that i s s h i f t e d to a higher frequency by 21 cm" . T h i s a d d i t i o n a l peak i s i n d i c a t i v e o f a decrease i n the e l e c t r o n charge on the CN moiety o f some f r a c t i o n of the TCNQ molecules. (10) In Table I, the r e s u l t s o f t h i s experiment a r e compared to r e f l e c t a n c e s p e c t r a measured f o r other simple and complex metalTCNQ s a l t s . We found that the CN s t r e t c h i n g mode i n r e f l e c t a n c e measurements s h i f t e d to higher frequency by about 100 cm" from absorption measurements made on the same m a t e r i a l . The peak i n the r e f l e c t a n c e band a t 2320 cm" f o r the Cu-TCNQ f i l m p r i o r to the a p p l i c a t i o n of a f i e l d i s c o n s i s t e n t w i t h the values measured f o r the simple (1:1) s a l t s o f Li+(TCNQ ) and Cu+(TCNQ" ) tabulated i n Table I . These c r y s t a l l i n e m a t e r i a l s a r e simple s a l t s which do not c o n t a i n n e u t r a l TCNQ°. On the other hand the s p e c t r a of a Cu-TCNQ f i l m a f t e r the a p p l i c a t i o n of an a p p l i e d f i e l d c l o s e l y resembles the s p e c t r a o f CS2(TCNQ*) w i t h two CN s t r e t c h i n g modes separated by ~ 20 cm" . C S 2 ( T C N Q ) 3 i s a complex s a l t which cont a i n s n e u t r a l TCNQ° and r a d i c a l - a n i o n TCNQ~. (11) The d i f f u s e r e f l e c t a n c e s p e c t r a reported i n Table I show that i t i s p o s s i b l e to a s s i g n a CN s t r e t c h i n g frequency to both n e u t r a l and r a d i c a l - a n i o n TCNQ i n c r y s t a l l i n e samples o f metal-TCNQ complexes because the r e f l e c t a n c e peak f o r n e u t r a l TCNQ i s s h i f t e d ~ 20 cm" higher i n frequency than f o r r a d i c a l - a n i o n TCNQ". Spec i f i c a l l y , the r e f l e c t a n c e data f o r Cu-TCNQ when compared to other metal-TCNQ s a l t s of known composition s t r o n g l y suggests that n e u t r a l TCNQ° i s not present i n the unswitched Cu-TCNQ f i l m s . On the other hand, the a d d i t i o n a l peak t h a t appears i n the s p e c t r a of Cu-TCNQ subjected to an a p p l i e d f i e l d shows a peak superimposable w i t h the peak recorded f o r n e u t r a l TCNQ° i n C s ( T C N Q ) 3 . T h i s evidence suggests that n e u t r a l TCNQ° i s formed i n a s o l i d s t a t e f i e l d induced phase t r a n s i t i o n when e l e c t r i c f i e l d s a r e app l i e d to c r y s t a l l i n e f i l m s o f Cu-TCNQ grown on copper s u b s t r a t e s .


1

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T

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Figure 6.

Reflectance spectra of a crystalline film of Cu-TCNQ and after the application of an electric field.

on copper before


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2320

2321 A N D 2340

COMPLEX 2:3 S A L T SIMPLE 1:1 S A L T PREPARED BY M E T A T H E T I C A L REACTION BEFORE APPLICATION OF ELECTRIC F I E L D A F T E R APPLICATION OF ELECTRIC F I E L D

Cu TCNQ

Cu TCNQ SWITCH

Cu TCNQ SWITCH

3

Cs T C N Q

2

2320

SIMPLE 1:1 S A L T

1

Li TCNQ

COMMENTS

REFLECTION MAXIMUM (cm" )

Comparison of Reflectance Maximum for the CN Stretching Mode in TCNQ for Various Metal-TCNQ Salts.

TCNQ S A L T

Table I.


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Conclusions I t i s p o s t u l a t e d that mixed-valence species o r complex s a l t s (12) formed as a r e s u l t o f t h i s f i e l d induced redox r e a c t i o n cont r o l the semiconducting behavior of these f i l m s and these complex s a l t s e x i s t i n a s o l i d - s t a t e e q u i l i b r i u m with the simple 1:1 s a l t . Since n o n - i n t e g r a l o x i d a t i o n s t a t e s are common i n s o l i d s , i t i s d i f f i c u l t to p r e d i c t exact s t o i c h i o m e t r y i n the e q u i l i b r i u m equat i o n , but a l i k e l y equation f o r switching i n Cu-TCNQ, f o r example, may i n v o l v e

+


[Cu (TCNQ~)] n

+

Cu° + [Cu (TCNQ~)] + (TCNQ°) . x n—x x

In a d d i t i o n , an i o n i c or a molecular displacement a s s o c i a t e d w i t h t h i s e q u i l i b r i u m would e x p l a i n the observed memory phenomena and the f a c t that a l l the devices show only two s t a b l e r e s i s t i v e states. Since conduction i n these narrow band semiconducting s a l t s o f TCNQ i s b e l i e v e d to be l i m i t e d by the motion of unpaired e l e c t r o n s along the stacks of TCNQ molecules, t h i s i n t e r p r e t a t i o n i s i n accordance w i t h the e l e c t r i c a l behavior reported i n these f i l m s when f a b r i c a t e d i n t o switching d e v i c e s . (13, 14, 15) In a simple s a l t l i k e Cu (TCNQ~) there i s roughly one unpaired e l e c t r o n per molecule which tends to keep e l e c t r o s t a t i c r e p u l s i o n i n the ground s t a t e c o n f i g u r a t i o n a t a minimum. The low c o n d u c t i v i t y reported i n these simple s a l t s i s due i n part to an i n c r e a s e i n the energy r e q u i r e d to overcome the r e p u l s i v e coulomb f o r c e s that r e s u l t when a conduction e l e c t r o n i s removed from one TCNQ' and placed i n t o a higher energy o r b i t a l o f another TCNQ* molecule. In the case o f a mixed-valence s a l t c o n t a i n i n g n e u t r a l TCNQ° there a r e more TCNQ molecules than there a r e unpaired e l e c t r o n s and, t h e r e f o r e , e l e c t r o s t a t i c r e p u l s i o n of charge c a r r i e r s i s kept at a minimum by a l l o w i n g conduction e l e c t r o n s to occupy the empty molecular o r b i t a l s o f TCNQ°. T h i s i s a lower energy pathway compared to p u t t i n g more than one e l e c t r o n on the same TCNQ s i t e and i t may e x p l a i n how mixed-valence semiconducting s a l t s l i k e Csa(TCNQ') and the "switched" form of Cu-TCNQ can e x h i b i t greater c o n d u c t i v i t y than s i m i l a r s a l t s with 1:1 s t o i c h i o m e t r y . +

3

Acknowledgment s We g r a t e f u l l y acknowledge support by the N a t i o n a l Science Foundation (DMR 80-15318) and the Department of the Navy (N0002481C-5301).


246

POLYMER MATERIALS FOR ELECTRONIC APPLICATIONS

Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Potember, R. S.; Poehler, T. O.; Cowan, D. .O. App. Phys. Lett. 1979, 34, 405. Gemmer, R. V.; Cowan, D. O.; Poehler, T. O.; Bloch, A. N.; Pyle, R. E.; Banks, R. H. J. Org. Chem. 1975, 40, 3544. Elemental analysis was performed by Galbraith Laboratories, Inc., Knoxville, Tennessee 37291. Owen, A. E.; Robertson, J. M. IEEE Trans. Electron Devices 1973, 20, 105. Values for the reduction potential of acceptor were taken from Wheland, R. C.; Gillson, J. L. J. Am. Chem. Soc. 1976, 98, 3916. Buckley, W. D.; Holmberg, S. H. Solid-State Electron. 1975, 18, 127. Reinhard, D. K. App. Phys. Lett. 1977, 31, 527. A spontaneous electrochemical reaction is reported in magnesium-TCNQ salts. See Gutmann, F.; Herman, A. M.; Rembaum, A. J. Electrochem. Soc. 1967, 114, 323. Matsuzaki, S.; Kutwata, R.; Toyoda, K. Solid State Commun. 1980, 33, 403. Khatkale, M. S.; Devlin, J. P. J. Chem. Phys. 1979, 70, 1851. Fritchie, C. J.; Arthur, Jr., P. Acta Cryst. 1966, 21, 139. For a discussion of complex TCNQ salts see LeBlanc, Jr., O. H. J. Chem. Phys. 1965, 42 4307. Torrance, J. B.; Scott, B. A.; Kaufman, F. B. Solid State Commun. 1975, 17, 1369. Soos, Z. G. Ann. Rev. Chem. 1974, 25, 121. Hubbard, J. Phys. Rev. 1978, B17, 494.

RECEIVED November 16, 1981.


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