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AMORPHOUS TRANSITION METAL OXIDES
J. Livage
To cite this version:
J. Livage. AMORPHOUS TRANSITION METAL OXIDES. Journal de Physique Colloques, 1981, 42
(C4), pp.C4-981-C4-992. �10.1051/jphyscol:19814215�. �jpa-00220844�
JOURNAL DE PHYSIQUE
Colloque C4, suppl&ment a u nO1O, Tome 42, o c t o b r e 1981 page C4-981
AMORPHOUS TRANSITION METAL O X I D E S
J . Livage
S p e c t r o c h i m i e du
Solide
( L A 3021, T.44-
U n i v e r s i t d P i e r r e e tMarie
Curie, 4, p l a c e J u s s i e u , 75230 Paris, F r a n c eA b s t r a c t
.-
The s e m i c o n d u c t i n g p r o p e r t i e s o f T r a n s i t i o n : l e t a l Oxide (T.M.O.) g l a s s e s have been known f c r a l m o s t 30 y e a r s , s i n c e t'ne p i o n e e r i n g work o f Denton e t a i i n
1954 ( 1 ) . They have been e x t e n s i v e l y s t u d i e d and r e c e n t l y r e v i e w e d by Wackenzie e t a 1 ( 2 ) ( 3 ) . T h e l r s p e c i f i c p r o p e r t i e s a r i s e from t h e f a c t t h a t t r a n s i t i o n m e t a l i o n s may e x h i b i t s e v e r a l v a l e n c e s t a t e s ( v 4 + - ~ 5 + , W ~ + - I @ ) s o t h a t e l e c t r o n t r a n s - f e r from low t o h i g h v a l e n c e s t a t e s can t a k e p l a c e . T h i s e l e c t r o n t r a n s f e r c a n be e i t h e r o p t i c a l l y o r t h e r m a l l y a c t i v a t e d and T.M.O. g l a s s e s w i l l e x h i b i t b o t h o p t i c a l and e l e c t r i c a l p r o p e r t i e s .
E l e c t r o n exchanges i n mixed v a l e n c e compounds a r e commonly d e s c r i b e d by u s i n g t h e diagramn shown on f i g u r e ( 1 ) . Such a diagrarnm g i v e s t h e p o t e n t i a l e n e r g y o f an e l e c t r o n hopping between two m e t a l l i c i o n s A and B a s a f u n c t i o n o f a s i n g l e c o n f i g u r a t i o n a l p a r a m e t e r q = q
-60,
c o r r e s p o n d i n g t o an a n t i s y m m e t r i c c o m b i n a t i o n o f t h e v i b r a t i o n a l modes o f as i t e s A anu B
( 4 ) ( 5 ) .
The e l e c t r o n c a n be t h e r m a l l y t r a n s f e r e d from one s i t e t,o t h e o t h e r w i t h an a c t i - v a t i c n e n e r g y E = W g i v e n by t h e h e i g h t o f t h e p o t e n t i a l e n e r g y b a r r i e r s c p a r a - t i n g t h e two t h c o n f i g u r a t i o n s . I t c a n a l s o be o p t i c a l l y e x c i t e d w i t h o u t moving t h e i o n s ( i . e . a c c o r d i n g t o tt.e Franck-Condon p r i n c i p l e ) w i t h a n e n e r g y hv.
According t o t h e harmonic a p p r o x i m a t i o n , t h e a c t i v a t i o n e n e r g i e s o f t h e two p r o c e s - s e s a r e r e l a t e d by
4
E = 2 a s shown by Hush( 6 ) .
t h o p t
P i g . 1 - P o t e n t i a l e n e r g y diagram o f a n e l e c t r o n hopping between two m e t a l l i c s i t e s .
a ) w i t h o u t any e l e c t r o n i c i n t e r a c t i o n J = 0 b ) w i t h a n e l e c t r o n i c i n t e r a c t i o n J
#
0E l e c t r o n exchange between A and 3 i o n s i s a c t u a l l y o n l y p o s s i b l e i f t h e o v e r l a p between t h e wave f u n c t i o n s Q A and
Q
i s d i f f e r e n t from z e r o . T h i s e l e c t r o - n i c i n t e r a c t i o n removes t h e d e g e n e r a c y a t 'the c r o s s i n g p o i n t o f t h e p o t e n t i a l energy c u r v e s l e a d i n g t o two e n e r g y l e v e l s s e p a r a t e d . by 2 J ( J = t r a n s f e r i n t e g r a l ) . The t h e r m a l a c t i v a t i o r i energy i s j h u s lowered (Eth = W - J ) , and t h e o p t i c a l and t h e r m a l p r o c e s s e s a r e r e l a t e d by2
E = E + J ( I + ) .o p t t h
Mixed v e l e n c e compounds w i l l e x h i b i t d i f f e r e n t b e h a v i o r s a c c o r d i n g t o t h e v a l u e o f J , l e a d i n g t o t h e w e l l known c l a s s i f i c a t i o n s w z e s t e d by Robin and Day(7).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814215
C4-982 JOURNAL DE PHYSIQUE
KO exchange occurs when J = 0, valence states remain localized and the corresponding compounds behave as insulators (class I). Electron hopping between localized states is observed when J is small (class 11). These compounds usually exhibit a deep blue color due to intervalence transfer (photochromism of amorphous WO ) together with semiconducting properties via a hopping process (vanadate glasses)? Delocalized states occur when J becomes large enough (class 111) leading to metallic conducti- vity as in the well known tungsten bronzes Iqa NO
X 3'
d-overlap is usually quite small in T.M.O. glasses, so that most of these compounds belong to the class 11. The most thoroughly investigated systems are the vanadate glasses for their semiconducting properties (8)(9) and the amorphous WO thin films for their electrochromic behavior (10) ( 1 1 )
.
These materials are usual?y obtained as bulk glasses by adding a glass former or as thin film by sputtering or vacuum evaporation. Their properties will be reviewed in the first two parts of this paper. Amorphous T.M.O. such as V 0 or WO have recently been obtained as"gels" by chemical polymerization (12)?
heir
aain interest arises from the fact that they can easily be deposited onto a substrate giving semiconducting layers of large area (13), leading to new applications in the field of amorphous T.M.O.(14).
These mixed valence gels have mainly been studied by the french "groupe des gels"
during the past two years. They exhibit optical and electrical properties arising from electron transfer between transition metal. ions. The main results will be reviewed in the last part of this paper.
- SEVICONDUCTING PROPERTIES OF T.M.O. GLASSES
T.M.O. glasses can be obtained by quenching from the melt. A glass former has to be added to the molten oxide and most studies are dealing with binary sys- tems based on P 0 : P 0 -V 0 (16-lg), P205-WO (20) P 0 -MOO (21). Many other T.M.O. have bee2 'also2
' '
studied ( F ~ o , Tia C U O , ~ ':~~O,~COO, NiO,.. .
) . Theyhave been recently reviewed by Mackenzie et a1
( 5 ; .
Other glass formers such as Te02 or ge02 can also been used (15)(22). Despite the fact that conduction in T.M.O.glasses malnly depends on electron hopping between metallic ions, the nature of the glass former appears to be very important too (22). The electrical conducti- vity of V 0 -TeO glasses for instance is 3 orders of magnitude greater ( I o - ~ Q-' cm-1 at 5room temperature) than that of V 2 0 -P 0 glasses containing similar vanadium concentrations (15). V 0 -ge02 glasgez 5have also been reported to exhibit greater conductivities
'
(34).Amorphous VsOI1 WOI or MOO thin films can be obtained by vacuum evaporation (23)(24), r.f. sput e lng 25)(~6)~0r C.V.D. (27). Small ribbons of amorphous V 0 have been obtained by splat-cooling from the melt using the roller-quenching 2 5 method previously developped for metallic glasses ( 2 8 ) .
Two localization processes occur in T.M.O. glasses. The first one arises from the strong interaction of the unpaired electron with the polar network. It leads to a polarization of the lattice and a displacement of the oxygen ions around the low valence transition metal ion. In T.M.O. glasses, these distorsions are usually limited to the nearest neighbours and the unpaired electron becomes self- trapped in its own potential well. The electron and its accompanying polarization cloudmay be treated as a pseudo-particle called a "small polaron". Present treat- ments of small polarons in semiconducting T.M.O. glasses are based mainly on the theoretical work of Mott (28) and Austin and Mott ( h ) . According to these anthors, the small-polaron binding energy W in an ionic lattice can be expressed as : W = e2/2 c rp ; rp is the small 'polaron radius.
cp
= ( I/E, - 1 / ~ ~ ) - ~ , where E sP and E- ape respectively the static and high-frequency dielectric constants of the glass. The semiconducting properties of T.M.O. glasses are due to thermal- ly activated hopping of small polarons between metallic sites, as shown on figure
(2). Let us suppose that the unpaired electron is initially trapped on site A (fig. 2a). The hopping process will be thermally activated by the vibrations of the lattice. The smallest activation energy corresponds to the configuration (2b), when two adjacent wells have the same depth. It can be shown that the energy W necessary to produce such a configuration corresponds to half the polaron binding Henergy : W =
1
W(4).
In the state (2b), the electron can tunnel from A to B leading to a H 'lowering of the potential energy in ~(3b). Typical values for WH in T.M.O.F i g . 2 - P o l a r i z a t i o n w e l l s f o r two t r a n s i t i o n a e t a l i o n s d u r i n g t h e s n a l l p o l a r o n hopping i n a M O g l a s s ( a f t e r Mott ( 1 1 ) )
2 2 2
W = e / E r : P o l a r o n b i n d i n g p e n e r g y
a : b e f o r e hopping, b : t h e r m a l l y a c t i v a t e d
t r a n s i ~ i o n s t a t e c : a f t e r hopping
r a n g e j e t w e e n 0 . 2 end 0 . 6 CV ( 2 ) . A B
A second l c c a l i z a t i o n p r o c e s s o c c u r s i n T.M.O. g l a s s e s . I t a r i s e s from t h e s t r u c t u r a l d i s o r d e r and c o r r e s p o n d s t o t h e w e l l known "Anderson l o c a l i z a t i o n " . An a d d i t i o n a l t e r m WD , c o r r e s p o n d i n g t o t h e random d i s t r i b u t i o n o f t h e p o t e n t i a l energy among t h e ' i r a n s i t i o r ! n e t a l s i t e s , h a s t o , b e t a k e n i n t o a c c o u n t . The a c t i v a t i o n e n e r g y f o r hopping t h e n becomes W = W + L W i n t h e h i g h t e m p e r a t u r e
regime ( 4 ) . li 2 U
F o l l o w i n g MoLt
( h ) ,
t h e d . c . e l e c t r i c a l c o n d u c t i v i . t y i n T.M.O. g l a s s e s i s u s u a l l y e x p r e s s e d a s :voe?
U = -
,<,,,,
c ( l - c ) exp(-2ai3) exp-(W/kT) ( 1 )where :
v
i s a phonon Yrequency r e l a t e d t o t h e llebye temperature 0 by hVo = k0.R i s t h e 'average hoppir~g, d i s t a n c e , c i s t h e r a t i o o r low v a l e n c e s t a t e s r e p o r t e d t o t h e t o t a l c o n c e n t r a t i o n o f t r a n s i t i o n m e t a l i o n s ; a i s t h e r a t e o f t h e wave f u n c t i o n d e c a y . I t c c r r e s p o n d s t o a t u n n e l l i n g t r a n s f e r .
A s shown by e q ( l ) , t h e c o n d u c t j . v i t y depends on t h e c o n c e n t r a t i o n c o f low v a l e n c e t r a r i o i t i o n m e t a l i o n s . I t s h o u l d go t h r o u g h a m a x i m f o r c = 0 . 5 . Such a h e n a v i o r i s a l m o s t n e v e r o b s e r v e d and r e g o r t e d maximum f o r V 0 g l a s s e s l i e between 0.1 and 0 . 2 ( l 5 ) ( 1 7 ) . V a r i o u s e x p l a n a t i o n s have been prcpose?i,5taking i n t o a c c o u n t t r a p p i n g e f f e c t s ( i 7 ) o r p o l a r o n - p o l a r o n i n t e r a c t i o n s ( 6 ) ( 9 ) . I t seems morc p r o b a b l e Lhat t h e o b s e r v e d d i s c r e p a n c i e s a r i s e from p h a s e s e p a r a t i o n i n t h e g l a s s , b u t e x p e r i m e n t a l e v i d e n c e o f t h i s e f f e c t i s n o t abundant ( 3 0 ) .
T!!e i m p o r t a n c e OS t h e t u n n e l l i n g term e x p ( - 2 a ~ ) i s n o t o b v i o u s . S a y e r and Mansingi;
(8)
have shown t h a t i t was a l m o s t c o n s t a n t f o r WO -i' 0 and V 0 -P 03
2 5g l a s s e s . Wether t h i s r e s u l t c a n b e e x t r a p o l a t e d t o o t h e r T.M.O. g l a s s e z 5is2 5 a s u b j e c t o f controversy.Many a t t e n p t s h a v e b e ? n made t o c a l c u l a t e CI and r e p o r t e d v a l u e s r a n g e octween 0.14 and 4 A ( 2 ) .
The most d i f f i c u l t problem i n t h e a n a l y s i s o f c o n d u c t i o n p r o c e s s e s i n T.X.O.
g l a s s e s , would b e t o s e p a r a t e t h e measured a c t i v a t i o n e n e r g y W i n t o a p o l a r o n t e r m W. ar.2 a d i s o r d e r t e r m WD. According t o Schnakerlgerg ( 3 1 ) , a t h i g h t e m p e r a t u r e ('~>0/2), t h e s m a l l p o l a r o n hopping i s a c t i v a t e d by a n o p t i c a l multiphonon p r o c e s s and t h e a c t i v a t i o n e n e r g y i s g i v e n by W = W
+
- 1 W.
As t h e t e m p e r a t u r e i s l o w e r e d , t h e phonon spectrum f r e e z e s o u t and t h e o b s e r v e d 2 a c ? i v a t i o n e n e r g y c o n t i n u o u s l y I1 d e c r e a s e s . An a c o u s t i c a l phonon a s s i s t e d hopping t a k e s p l a c e a t low t e m p e r a t u r e (T<0/1) and t h e a c t i v a t i o n energy becomes W = - 1 W A non l i n e a r p l o t o f l n ( u T ) v e r s u s T-l t y p i c a l o f s m a l l p o l a r o n s i n T.M.O. 2 g l i i s e s i s r e p o r t e d o n f i g . ( 3 ) .JOURNAL DE PHYSIQUE
F i g . 3 - T e m p e r a t u r e dependence o f t h e c o n d u c t i v i t y o f amorphous V205 o b t a i n e d by s p l a t c o o l l n g ( 2 8 ) .
I n t h e l i m i t W , A u s t i n and Gamble ( 3 2 ) s t a t e d t h a t h' = WD, b u t Mott ( 2 9 ) p o i n t e d o u t t h a t a t v e r y low t e m p e r a t u r e , t h e o b s e r v e d v a l u e o f W s h o u l d a p p r o a c h
z e r o and a v a r i a b l e r a n g e hopping p r o c e s s s h o u l d t a k e p l a c e . D
Due t o t h e h i g h r e s i s t i v i t y o f T.M.O. g l a s s e s , t h e low t e m p e r a t u r e regime c a n u s u a l - l y n o t b e o b s e r v e d and c o n d u c t i v i t y measurements have r a r e l y been performed below 77K. An e x a c t e s t i m a t i o n of W i n t h e n r a t h e r d i f f i c u l t . An e v a l u a t i o n c a n b e made from :[iller-Abrahams t h e o r y ( 3 3 ) f o r i m p u r i t y c o n d u c t i o n i n doped and compensated s e m i c o n d u c t o r s . The d i s o r d e r t e r m i s g i v e n by W = K ~ ~ / E ? R where K i s a c o n s t a n t o f o r d e r 0 . 3 . Such a c a l c u l a t i o n l e a d s t o W "0.y eV i n t 6 e c a s e o f V 0 -P 0 g l a s s e s ( 1 8 ) . One o f t h e b e s t method f o r D e s t i m a t i n g WD would b e
S
'from low t e m p e r a t u r e measurements o f t h e t h e r m o e l e c t r i c power a s r e p o r t e d f o r V20 -Te02 g l a s s e s where a v a l u e o f WD = 0.02 eV h a s been f o u n d ( 1 5 ) . D i r e c tmeasure men?.^
o fW a t v e r y l o w t e m p e r a t u r e ( b e l o w 4 0 ~ ) have r e c e n t l y b e e n p e r f o r m e d by B u l l o t e t a1 (?3) o n amorphous V20 l a y e r s d e p o s i t e d from g e l s . They w i l l b e d e s c r i b e d l a t e r i n t h e s e c t i o n d e v o t e d
tz
T.M.O. g e l s .T a b l e I summarizes some o f t h e main d a t a s p u b l i s h e d i n t h e l i t e r a t u r e f o r T.M.O. g l a s s e s . As c a n b e seem t h e c o n d i t i o n s f o r a p p l y i n g s m a l l p o l a r o n t h e o r y ,
< R , a r e u s u a l l y f u l f i l l e d and t h e e l e c t r o n - p h o n o n c o u p l i n g p a r a m e t e r y i s q u i t e P h i g h ( y > 1 0 ) .
r e f .
8 , 1 8 , 1 9 15 35 36 37 a ( ? )
2.9-4 0 . 9 7 2.h-2.8 0.45-0.8 T.M.0 g i a s s
V 0 -P 0- 2 5 2 2 V 0 -Te02
2
5
WO3 -P205 MOO -P O3
2 5 T i O - P O2 2 5
i
W ( e v )
0.29-0.42 0.25-0.34 0.29-0.35 0.5-0.69 0.48-0.54
W D ( e v ) 0.1-0.4 0 . 0 2
0 . 1
0.05
2.1-2.9
l . 2 1-2 1 . 7
-
ELECTROClIROMISM IN AMORPXOUS T.M.O. THIN FILMSPhotochromism and electrochromism of amorphous T.M.O. have received much attention during the last decade. Most of the studies are dealing with amorphous WO thin films obtained by vacuum evaporation or r.f. sputtering. Such films could have considerable application in digital display devices (10). The first paper was 3 published in 1973 by Deb (38) who reported that amorphous WO3 could exhibit two stable states. One is transparent and highly resistive while the other is deep blue and less resistive. Coloration can be obtaincd by optical irradiation, application of an electric field, or proton injection from an electrolyte ( 1 1 ) .
Photochromism occurs when the film is U.V. irradiated (hv>3.4 eV) for seve- ral hours (38). The blue color develops gradually and grows nore intecse with pro- longed exposure (39)(40). This process is not reversible and the blue coloration remains after exposure.
Electrochromism is performed by application of an electric field of about 10 V/cm. The blue color first appears at the cathode and then propagates toward the anode. This coloration persists after the current has been removed, the trans- parent state being restored by inversing the polarity of the current. The blue color then migrates back and disappears into the anode while new coloration begins at the cathode (38).
These two processes are rather long and cannot therefore be applied for display devices. Most of the studies are now dealing with electrochemichromism (10). The WO thin film is placed into contact of a suitable electrolyte, usually and acid aqueous solution or gel (41 3 ) , in an electrochemical cell (figure 4).
counter electrode
I
electrolyte
Fig. 4
-
Electrochemichromism of amorphous T.M.O. thin films.+
-Coloration WO + If + e -> H WO
3 X 3
(transparent ) (blue )
+
- Bleaching HxWo3-
> W 0 3 + H + e (blue) (transparent)The application of a negative potential (>0.7
V)
causes the WO film to turn deep blue within 0.5-1.0 seconds. The blue state remains after the 3voltage has been removed but the initial transparent state can be restored by applying a reverse+
0.8 V potential. The coloring and bleaching cycles have been repeated more than 106 times without significant degradation (41). Non aqueous solvents (propylene carbonate) with LiC104 electrolyte can also been used, but the coloring process is slower owing to the lower rate of diffusion of ~ i + (40)(42). Display devices based on solid state electrolytes have recently been proposed (43).C4-986 J O U R N A L DE PHYSIQUE
I t i s now well. a d m i t t e d t h a t t h e b l u e c o l o r a t i o n o f u o r p h o u s U0 t h i n f i l m s a r i s e s f r o m a b r o a d a b s o r p t i o n b a n d c c n t e r c d a r o u n d 900 nm ( 1 . 3 8 e V ) ( k 3 ) . But
3
a c o n t r o v e r s y r e m a i n s a b o u t t h e d e s c r i p t i o n o f t h e c o l o r a t i o n a n d b l e a c h i n g p r o c e s s e s i n t h e s e f i l m s . S p e c t r o s c o p i c e x p e r i m e n t s ( = S , ESH...
) show t h a t Lpon c o l o r a t i o n i s o l a t e d W?+ i o n s a r e f o r m e d w h i l e t h e y d i s a p p e a r upon ' c l e a c h i n g ( 4 3 ) ( 4 4 ) . T h o s e o b s e r v a t i o n s r u l e o u t t h e f i r s t mechanism p r o p o s e d by Deb, s u g g e s t i n g t h c f o r m a t i o n o f F c o l o r c e n t e r ( 3 8 ) .I n f r a - r e d e x p e r i m e n t s show t h e p r e s e n c e o f w a t e r m o l e c u l e s a d s o r b e d o n UO, t h i n f i l m s . Therefore C h a r ~ g c t a1 ( 1 0 ) su6:gested t h e f o r m a t i o n o f non- s t o f c h i o m e t r i c V10 a c c o r d i n g t o t h e f o l l o w i n g mechanism :
3
T h e c u r r e n t l y a d o p t e d m o d e l f o r e l e c t r o l y t i c c o l o r a t i o r . i s t h a t o f a d o u b l e i n j e c - t i o n p r o c e s s i n w h i c h t h e n e g a t i v e c h a r g e c a r r i e r s a r c e l e c t r o n s w h i l e IY+ i o n s ( M = H , L i ) a r e t h e c h a r g e compensators ( 1 0 ) ( 3 9 ) . A t u n g s t e n brorize v o u l d t h e n be formed b y p r o t o n i n j e c t i o n d u r i n g t h e c o l o r a t i o n p r o c c s s ( 4 6 - 9 )
6 +
.
Thc b l u e c o l o r a t i o n a r i s e s from i n t e r v a l e n c e t r a n s f e r s b e t w e e n W?+ a n d W l o n s . The small p o l a r o n m o d e l c o u l d a p p l i e d t o t h e s e b l u e ainorphcus t h i n f i l m s .
E l e c t r i c a l c o r i d u c t i v i t y m e a s u r e r n c n t s show t h a t t h e y e x h i b i t s e m i c o n d u c t i n g p r o p e r - t i e s d u e t o a h o p p i n g p r o c e s s betweer] l o c a l i z e d s t a t e s . C o n d ! ~ c t i v i t y d e p e n d s on t h e number o f c h a r g e c a r r i e r s a n d t h e a c t i v a t i o n e n e r g y d e c r c a s c s s s t h e n m b e r o f i n j e c t e d p r o t o n s i n c r e a s e s . F a u g h n a n e v e n f o u n d a n i n s u l a t o r - m e t a l t r a n s i t i o n a t X = 0 . 3 2 when h y d r o g e n c o r i c e n t r a t i o n i n c r e a s e s from 0 t o 0 . 5 ( 5 0 ) .
A n o t h e r d e s c r i p t i o r ~ o f t h e c o l o r e d a n d b l e a c h e d s t a t e s h a s b e e n r e c e n t l y p u s h e d f o r w a r d b y D e n e u v i l l e e t a1 ( 5 1 ) . They snowed t h a t c o l o r a t i o n c f t r a n s p a r e n t v i r g i n WO f i l m s c o u l d t)c o b t a i n c d w i t h o u t a n y p r o t o n i n j e c t i o n , by a n n e a l i n g u n d e r vacuum o r 3 u . v . i r r a d i a t i o n . A c c o r d i n g t o t h c s e a u t h o r s , t h e a b i l i t y o f WO f i l m s t o u n d e r g o 3.V. c o l o r a t i o n r e q u i r e s b o t h a d e f i c i e n c y o f oxygcn a n d t h e p r e s 2 n c e o f h y d r o g e n i n t h e i r t r a n s p a r e n t s t a t e . The c o l o r a t i o n was assumed t o o r i g i n a t e from l o c a l t r a n s f e r o f h y d r o g e n from p a s s i v e t c n c t i v e s i t e s i n t h e m a t e r i c l . A b i p o l a - r o n model was t h e n p r o p o s e d suggesting p a i r e d d i a m a g n e t i c ~ 5 ~ i o r i s ( a s s e e n by X-ray p h o t o e m i s s i o n ) i n t h e t r a n s p a r e n t s t a t e a n d i s o l a t e d
v5+
p a r a m a g n e t i c i o n s i n t h e b l u e o n e .D e s p i t e t h e c o n t r o v e r s y a b o u t t h e c o l o r a t i o n n e c h a n i o n ; i.n m o r p h o u s t h i n f i l m s , t h e i r a l m o s t i n s t a n t a n e o u s r e v e r s i b l e c o l o r a t i o r l - b l e a c h i n g p o s s i b i l i t i e s o p e n c o n s i d e r a b l e a p p l i c a t i o n s as d i g i t a l d i s p l a y d e v i c c s .
They a r e i n e x p e n s i v e a n d c o m p a c t , a n d c a n b e e a s i l y s e e n i n b r i g h t s u n l i g h t . C o l o r a t i o n o c c u r s m o s t e f f i c i e n t l y when t h e f i l m a r e a n o r p h o u s r a t h e r t h a n c r y s t a l - l i n e . I t a p p e a r s a l s o t o b e s e n s i t i v c t o m b i e n t c o n d i t i o n s , m a i n l y t h e p r e s e n c e o f m o i s t u r e a n d t h e t e m p e r a t u r e ( a maximum i s o b s e r v e d arourid 80°C).
- T3ANSITION METAL OXIDE GELS
Amorphous T.M.O. (V 0 WO
,
MOO...
) h a v e r e c e n t l y b e e n o b t a i n e d as g e l s by c h e m i c a l p o l y m e r i z a t i o n 5 i 1 2 ) ? l ' h i s 3 p o l y m e r i z a t i o n o c c u r s t h r o u g h a c o n d e n s a - t i o n p r o c e s s i n which w a t e r m o l e c u l e s a r e e l i m i n a t e d . Vanadium p e n t o x i d e g e l s f o r i n s t a n c e c a n b e formed by p o l y m e r i z a t i o n o f v a n a d i c a c i d s a c c o r d i n g t o t h e f o l l o -P o l y v a n a c i i c a c i d s o l u t i o n s a r e o b t a i n e ? by i o n e x c n z n g e i n a r e s i n e from sodium m e t a v a n a d a t e s o l u t i o n s ( 5 2 ) . The f r e s h l y p r e p a r e d a c i d i s y e l l o w c o l o r e d a n d d e c a c o n d e n s e d . High p o l y m e r s a r e s p o n t a n c o u s l y formed a t room t e m p e r a t u r e . Dark r e d c o l l o i d a l s o l u t i o n s o r g e l s a r e o b t a i n e d , d e p e n d i n g o n t h e vanadium con- c e n t r a t i o n C. C e l a t i o n o c c u r s f o r C>0.1 "01.1-'. O t h e r s m c t h o d s o f p r e p a r a t i o n h a v e b e e n d e v e l o p p e d . V 0 g e l s c a n a l s o b e e n o b t a i n e d by q u e n c h i n g t h e m o l t e n o x i d e d i r e c t l y i n t o wate? 'or by h y d r o l y s i s o f v a n a d i c e s t e r s ( 1 3 ) .
A g e l i s a n i n t e r m e d i a t e s t a t e o f t h e m a t t e r , l y i n g b e t w e e n s o l i d s a n d l i q u i d s . Vanadium p e n t o x i d e g e l s f o r i n s t a n c e a r e made o f e n t a n g l e d p o l y m e r i c vanadium-oxygen f i b r e s , s t r o n g l y a s s o c i a t e d w i t h w a t e r m o l e c u l e s .
E l e c t r o n m i c r o g r a p h o f a g e l shows t h a t t h e s e f i b r e s a r e a b o u t 1 m i c r o n l o n g a n d 5 0 t o 100 A w i d e f i g . ( 5 ) .
F i g .
5
- E l e c t r o n m i c r o g r a p h o f a V20 g e l s h o w i n g t h e f i b r o z s t e x t u r e o f t h i s i n o r g a n i c p o l y m e r .The main i n t e r e s t o f t h e s e g e l s i s t h a t t h e y b e h a v e l i k e a p a i n t i n g and c a n b e e a s i l y d e p o s i t e d o r s p r a y e d o n t o g l a s s o r p o l y m e r i c s u b s t r a t e s g i v i n g l a y e r s o f l a r g e a e r a . A f t e r d r y i n g , r a t h e r h a r d c o a t i n g s a r e o b t a i n e d t h e t h i c k n e s s o f which r a n g e s from 500 t o 10.000
X.
T h e s e T.M.O. g e l s a r e m i x e d - v a l e n c e compounds. They e x h i b i t o p t i c a l a n d e l e c t r i c a l p r o p e r t i e s d u e t o e l e c t r o n t r a n s f e r b e t w e e n m e t a l l i c i o n s . T h e s e p r o - p e r t i e s h a v e b e e n s t u d i e d d u r i n g t h e l a s t t w o y e a r s by f r e n c h g r o u p s . We s h a l i f o c u s h e r e o n l y o n t h e s e m i c o n d u c t i n g p r o p e r t i e s o f amorphous V205 l a y e r s , b u t e l e c t r o c h r o m i s n o f WO g e l s i s a l s o u n d e r s t u d y .
3
JOURNAL DE PHYSIQUE
- SEiflICONDUCTING COATINGS
The s e m i c o n d u c t i n g p r o p e r t i e s o f amorphous V 0 l a y e r s d e p o s i t e d from g e l s have been s t u d i e d by B u l l o t e t a 1 ( 1 3 ) . L i n e a r c u r r e n z - q o l t a g e c h a r a c t e r i s t i c s were o b t a i n e d and t h e t e m p e r a t u r e dependence o f l n ( a T ) between 300;: and 20i; i s shown on f i g u r e ( 6 ) .
Temperature dependence o f a:
V 0 l a y e r d e p o s i t e d fro; ( 1 3 ) .
T h i s c u r v e i s t y p i c a l o f s m a l l p o l a r o n c o n d u c t i o n i n amorphous T.i.1.O. T'ne room t e m p e r a t u r e c o n d u c t i v i t y a p p e a r s t o be s u r p r i n s i n g l y h i g h , a r o u n d 1 2-1 cm-l.
Such a v a l u e i s much g r e a t e r t h a n 1;hose a l r e a d y r e p o r t e d f o r V 0 b a s e d g l a s s e s and even f o r c r y s t a l l i n e V 0 ( T a b l e 11). The meagured conduct?v?ty d o e s n o t depend on t h e t h i c k n e s s o f 2 t z e l a y e r ( 6 0 0 t o 104 A ) , s o t h a t we may a s c e r t a i n t h a t we a r e d e a l i n g w i t h b u l k c o n d u c t i v i t y r a t h e r t h a n s u r f a c e e f f e c t s .
T a b l e I1
The r e a s o n o f such a h i g h c o n d u c t i v i t y i s n o t y e t c l e a r l y u n d e r s t o o d . I t 4 -
p a r t l y a r i s e s from t h e h i g h V + / V r a t i o ( a b o u t 0 . 0 6 ) . A p o s s i b l e i o n i c c o n d u c t i v i t y due t o w a t e r had t o b e examined, t h e r e f o r e , t h e l a y e r h a s been a n n e a l e d i n a i r a t 160°c f o r 16 h i n o r d e r t o remove p h y s i c a l l y a d s o r b e d w e t e r . The c o n d u c t i v i t y o f t h e a n n e a l e d sample d e c r e a s e d i r r e v e r s i b l y by a f a c t o r o f 1 , 5 . The a u t h o r s t h e n
3 e f e r e n c e
5 3
2
3
2 5 1715 13 C r y s t a l l i n e V 0
2 5 Vapor d e p o s i t e d V205
r . f . s p u t t e r e d V 0 2
5
V 0 -P 0 g l a s s e s2 5 2 5 V 0 -Te02 g l a s s e s
2 5
V 0 g e l s 2 5
3001;
(Q-' cm-1 ) 10-2- 0-4 1
o - ~
I O - ~ - I O - ~ 1 0 - ~ - 1 0 - ~
1
o - ~
1
w ( e v )
0.21 0.66 0 . 7 0.32-0.44 0.25-0.32 0.17
concluded t h a t t h e high c o n d u c t i v i t y of t h e V 0 g e l was not due t o an i o n i c conduc- t i o n of w a t e r , but r a t h e r t o t h e i n t r i n s i c na$u?e o f t h e m a t e r i a l ( 1 3 ) .
Such, a h i g h c o n d u c t i v i t y , t o g e t h e r w i t h t h e p o s s i b i l i t y of making e a s i l y l a y e r s o f l a r g e a e r a l e d an i n d u s t r i a l company t o p a t e n t t h e s e g e l s i n o r d e r t o u s e t h e n a s a n t i s t a t i c c o a t i n g s d e p o s i t e d on t h e back of photographic f i l m s ( 1 4 ) . Such c o a t i n g s appear t o e x h i b i t a much b e t t e r behavior toward m o i s t u r e t h a n t h e u s u a l polymeric a n t i s t a t i n g c o a t i n g s ( 5 4 ) .
The high c o n d u c t i v i t y of t h e amorphous V205 l a y e r s d e p o s i t e d from g e l s allowed B u l l o t e t al ( 5 5 ) t o perform c o n d u c t i v i t y measurements down t o 28ii f i g . ( 6 ) . They c o u l d t h e n r e a c h t h e low t e m p e r a t u r e regime ( ~ < 0 / 4 ) where, according t o Schnakenberg ( 3 1 ) an a c o u s t i c a l phonon a s s i s t e d hopping p r e v a i l s , l e a d i n g t o an a c t i v a t i o n energy W = W
.
A l i n e a r p l o t i s a c t u a l l y found below4 3 ~ ,
g i v i n g t h e f i r s t d i r e c t measureme& Dof WD i n amorphous T.M.O. (WDhO. l elr).E l e c t r o n m o b i l i t y i n V 0 g e l s has a l s o been s t u d i e d by E.S.R. A w e l l r e s o l v e d V
4+
h y p e r f i n e s t r u c t u r e is2 5observed up t o room t e m p e r a t u r e f i g . ( 7 ) , showing t h a t t h e charge c a r r i e r s a r e l o c a l i z e d on vanadium s i t e s . T h e i r hopping frequency remains s m a l l e r t h a n t h e ESR l i n e w i d t h ( i . e . vh<lOO MHZ) up t o 300K.
Fig.
7 .
l The most s t r i k i n g f e a t u r e of t h eESR s p e c t r a of V 0 g e l s comes from t h e i r s t r o n g 5frequency dependence f i g .
( 7 ) .
T h i s has been a t t r i b u t e d t o a d i s t r i b u t i o n of g v a l u e s among t h e d i f f e r e n t vana- dium s i t e s , a r i s i n g from f l u c t u a - t i o n of t h e l o c a l c r y s t a l f i e l d A( 2 8 ) . These two q u a n t i t i e s a r e r e l a t e d by g = g (l-nX/A) where g = 2.0023,
X i g
t h e s p i n - o r b i t c$upling c o n s t a n t and n a parame- t e r depending on t h e ground s t a t e wave f u n c t i o n and t h e magnetic f i e l d o r i e n t a t i o n . An a n a l y s i s of t h e ESR l i n e w i d t h a s a func- t i o n o f t h e microwave frequency (hv = L OH) l e d t o a v a l u e of t h e c r y s t a l f i e l d f l u c t u a t i o n s dA = 0.08 eV. These random f l u c - t u a t i o n s c o n t r i b u t e t o t h e d i s o r - d e r term WD involved i n t h e a c t i v a t i o n energy f o r hopping and t h e bA v a l u e found by ESR appears t o be i n good agreement w i t h t h e W v a l u e deduced from conductivi?y measurement S.-
SWITCHING DEVICESNon d e s t r u c t i v e breakdown,otherwise c a l l e d s w i t c h i n g has been observed i n a wide v a r i e t y of amorphous m a t e r i a l s s i n c e it was d i s c o v e r e d i n 1959 by Ovshinsky i n chalcogenide g l a s s e s ( 5 6 ) . The s u b j e c t h a s been reviewed r e c e n t l y by F r i t z s c h e ( 5 7 ) . Under t h e l o o s e l y d e f i n e d name o f "switching" a v a r i e t y of phenomena a r e r e f e r r e d t o :
a ) n e g a t i v e r e s i s t a n c e d e v i c e , b ) n e g a t i v e r e s i s t a n c e w i t h memory, c ) s w i t c h i n g , d) s w i t c h i n g w i t h memory ( 5 7 ) .
They a l l o c c u r when t h e v o l t a g e V a p p l i e d a c r o s s a t h i n f i l m exceeds a t h r e s h o l d v a l u e Vt. A t y p i c a l I n t e n s i t y - V o l t a g e c h a r a c t e r i s t i c o f a s w i t c h i n g d e v i c e ( t y p e c ) i s shown on f i g u r e ( 8 ) . For V<V t h e d e v i c e i s i n t h e high r e s i s t a n c e "OFF" s t a t e w h i l e f o r V>Vt it s w i t c h e s t o F;e low r e s i s t a n c e "ON" s t a t e . When t h e c u r r e n t i s
C4-990 JOURNAL DE PHYSIQUE
d e c r e a s e d below t h c h o l d i n g c u r r e n t l
,
t h e d e v i c e s w i t c h e s back t o i t s o r i g i a 1 OFF s t a t e . Under p u l s e c o n d i t i o n s , some Hamorphoils a l l o y s have been o p e r a t e d 1 0 l e t i m e s w i t h o u t f a i l u r e .The t h e o r i c a l d e s c r i p t i o n o f s w i t c h i n g h a s been s u b j e c t o f much c o n t r o v e r s y o v e r t h e p a s t few y e a r s . F o l l o w i n g some a u t h o r s , s w i t c h i n g i s due t o t h e r m a l i n s t a -
On
state'
\ b i l i t y ( 5 7 ) whereas o t h e r s p u t f o r w a r d a d o u b l e i n j e c t i o n pheno-X menon ( 5 8 ) ( 5 9 ) . S w i t c h i n g h a s been
\ o b s e r v e d i n o x i d e g l a s s e s ( 6 0 ) and i n p a r t i c u l a r i n T.M.0 g l a s s e s
c o n t a i n i n g W , V , .Q, Fe and ~ ~ ( 6 1 ) . V 0 -P 0 g l a s s e s o f v a r i o u s com-
2 5 . 2 5
Off
state p o s x t l o n s were found t o have v e r y s t a b l e s w i t c h i n g v o l t a g e s and low h o l d i n g c u r r e n t s ( 6 2 ) ( 6 3 ) , whereas CuO-V 0 -P 0 g l a s s e s were found"t capab?e5 50f p a s s i n g a c u r r e n t
Potential (V)
o f 1 A i n t h e ON s t a t e w i t h o u t p h y s i c a l damage ( 6 4 ) . I n s u c h d e v i c e s , t h e OFF t o ON r e s i s t a n c e- 7 .
- r l g .
8
- r a i o t y p i c a l l y r a n g e s between ,OS-107. Depending o n t h e a p p l i e d S w i t c h i n g i n amorphous semiconductors power, V 0 -P 0 g l a s s e s wereshown t o 5 e x h i b i t e i t h e r t h e r e s h o l d o r memory s w i t c h i n g ( 6 5 ) ( 6 6 ) .
Up t o now, s w i t c h i n g i n p u r e amorphous V 0 h a s n o t been r e p o r t e d . 2 5 .
T h e r e f o r e , B u l l o t e t al, t e s t e d t h e s w i t c h i n g c a p a b l l l t y o f amorphous V 0 l a y e r s 2 5
d e p o s i t e d from g e l s . Samples a b o u t 1 IJrn t h i c k were d e p o s i t e d o n t o m i c r o s c o p e s l i d e s and g o l d e l e c t r o d e s 100 urn a p a r t were vacuum e v a p o r a t e d i n a c o p l a n a r geometry f i g . ( 9 ) .
I
G l a s s substratc ( /F i g . 9
S w i t c h i n g e f f e c t i n a l a y e r o f amorphous V 0 d e p o s i t e d from g e l . 2 5
A d . c . v o l t a g e was a p p l i e d t h r o u g h a c u r r e n t - l i m i t i n g r e s i s t o r R The v o l t a g e d r o p
vx
a c r o s s t h e sample and t h e v o l t a g e d r o p V a c r o s s a s e r i e s o f L ' r e s i s t o r s R were r e c o r d e d on a xy r e c o r d e r . A t y p i c a l I-V ' c h a r a c t e r i s t i c i s shown on f i g ( 9 ) : S w i t c h i n g o c c u r s f o r V r a n g i n g between 10 and 2 0 v o l t s , t h e t h e r e s h o l d v o l t a g e d e c r e a s i n g when t h e C t r a t i o ( C =v4+/v)
i n c r e a s e s . No s w i t c h i n g i s o b t a i n e d f o r C>0.06. The h o l d i n g c u r r e n t v a r i e s between 1.6 and 2.5 mA. Some o f t h e s e d e v i c e s were c a p a b l e o f p a s s i n g c u r r e n t s up t o 50 mA. Upon a p p l i c a t i o n of a 50 1Iz a . c .v o l t a g e , a s t a b l e c h a r a c t e r i s t i c was o b t a i n e d even a f t e r s e v e r a l days of continuous o p e r a t i o n . These t h r e s h o l d v o l t a g e d e c r e a s e s l i n e a r l y when t h e t e m p e r a t u r e . i n c r e a - s e s , s w i t c h i n g d i s a p p e a r s above 350K.For a c = 0.01 sample, Vt = 1 1 v o l t s a t room temperature and r e a c h e s a maximum v a l u e a t 2 6 0 ~ . A p l a t e a u i s observed f o r lower temperature.
- CONCLUSION
Amorphous T r a n s i t i o n Metal Oxides e x h i b i t a wide v a r i e t y o f p r o p e r t i e s . Semiconductivity electrochromism and switching e f f e c t have been reviewed h e r e , b u t many o t h e r a s p e c t s could be i n t e r e s t i n g t o o . Lithium c o n t a i n i n g t u n g s t a t e and molybdate g l a s s e s e x h i b i t v e r y high i o n i c c o n d u c t i v i t i e s ( 10-5 cm-' a t 300K), much g r e a t e r t h a n t h e corresponding c r y s t a l l i n e phases ( 6 7 ) . They could be used a s r e v e r s i b l e cathodes i n l i t h i u m b a t t e r i e s . Some semiconducting t r a n s i - t i c n Metal Oxide such a s Ti02, W.l o r V205 a r e found t o be good c a n d i d a t e s f o r t h e p h o t o e l e c t r o l y s i s of water
( 8
3 ( 6 8 ) .A s i n t h e c a s e of s i l i c o n , inexpensive amorphous p h o t o e l e c t r o d e s could compete w i t h s i n g l e c r y s t a l s . T.M.0 c o l l o i d a l suspensions appear t o be very pro- mising ( 6 9 ) . Magnetic p r o p e r t i e s of amorphous T.M.0 a r e a l s o e x t e n s i v e l y s t u d i e d ,
and from a chemical p o i n t of view, most of t h e t r a n s i t i o n m e t a l o x i d e s a r e k n o w t o be v e r y good c a t a l y s t s , o f t e n used i n t h e i r amorphous s t a t e by t h e chemical inciustry.
Another g r e a t advantage of amorphous T.M.0 i s t h a t t h e y can be o b t a i n e d i n d i f f e r e n t s t a t e s : bulk g l a s s e s , t h i n f i l n i s , powders o r g e l s . T h e i r composition can be v a r i e d smoothly over a wide s c a l e , allowing an o ~ t i m i s a t i o n of t h e m a t e r i a l according t o t h e planned a p p l i c a t i o n .
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