JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment a u n012, Tome 45, d6cembre 1984 page C9-17
MOBILITY OF SODIUP.1 ON THE ( 1 1 0 ) FACE OF TUNGSTEN
R. Morin
CRMC2-CARS, C a q u s de Lwniny, Case 913, 13288 MarseiZZe Cedex 09, France
Rdsum6 - La m o b i l i t C e t l a c o m p r e s s i b i l i t d du sodium a d s o r b 6 s u r l a f a c e (110) du t u n g s t h e s o n t dCtermin6es p a r l a mesure d e l a f o n c t i o n d e c o r r 6 l a t i o n tem- p o r e l l e d e s f l u c t u a t i o n s du c o u r a n t l o c a l d ' g m i s s i o n de champ pour d e s recou- vrements e n sodium v a r i a n t d e 2x1013 3 3x1014 atomes p a r cm2 e t d e s tempdratu- r e s d e 170 5 500 K . Deux rdgimes d e t e m p s r a t u r e a p p a r a i s s e n t c l a i r e m e n t s d p a r m t d e s p h a s e s 06 l e s p a r a m b t r e s d e d i f f u s i o n s o n t complbtement d i f f 6 r e n t s . P a r comparaison avec l e s r g s u l t a t s o b t e n u s p a r d i f f r a c t i o n d q 6 1 e c t r o n s l e n t s l a t r a n s i t i o n e n t r e l e s deux rcgimes a p p a r a I t comme une t r a n s i t i o n c o n t i n u e o r d r e l o c a l - d d s o r d r e .
A b s t r a c t - The m o b i l i t y and t h e c o m p r e s s i b i l i t y o f sodium a d s o r b e d on t h e (110) f a c e o f t u n g s t e n have been d e t e r m i n e d by measuring t h e t i m e c o r r e l a t i o n f u n c t i o n of t h e l o c a l f i e l d e m i s s i o n c u r r e n t f l u c t u a t i o n s f o r c o v e r a g e s from 2x1013 t o 3x1014 atoms cm-2 and f o r t e m p e r a t u r e s from 170 t o 500K. Two t e m p e r a t u r e r e g i - mes c l e a r l y a p p e a r d i v i d i n g p h a s e s where t h e d i f f u s i o n p a r a m e t e r s a r e complete- l y d i f f e r e n t . By comparison w i t h slow e l e c t r o n d i f f r a c t i o n r e s u l t s t h e t r a n s i - t i o n between b o t h regimes a p p e a r s a s a c o n t i n u o u s l o c a l - a r d e r - d i s o r d e r
t r a n s i t i o n .
The i n t r o d u c t i o n of t h e f i e l d - e m i s s i o n - f l u c t u a t i o n method t o i n v e s t i g a t e t h e mobi- l i t y and t h e c o m p r e s s i b i l i t y of a d s o r b a t e s 11,2 1 h a s r e c e n t l y g i v e n v e r y i n t e r e s t i n g r e s u l t s c o n c e r n i n g g a s on m e t a l s y s t e m s ( 3 , 4 , 5 , 6 1 . The improvement compared w i t h c l a s s i c a l s p r e a d i n g methods i s based upon t h e f a c t t h a t i t i s a m i c r o s c o p i c method d i r e c t l y g i v i n g t h e d i f f u s i o n c o e f F - i c i e n t D ( t h r o u g h t h e c o r r e l a t i o n t i m e T~ of d e n s i t y f l u c t u a t i o n s and t h e probed r q i o n a r e a A) and t h e c o m p r e s s i b i l i t y x
( t h r o u g h t h e r e l a t i v e d e n s i t y f l u c t u a t i o n a m p l i t u d e ) of t h e a d p h a s e v e r s u s t h e mean coverage on a g i v e n f a c e w i t h o u t d e t a i l e d a n a l y s i s o f a c o n c e n t r a t i o n p r o f i l e . I n t h i s s h o r t p a p e r , u s i n g t h i s method; we p r e s e n t t h e r e s u l t s o b t a i n e d i n one c a s e (Na on W(110)) b e l o n g i n g t o a n o t h e r c l a s s o f a d s o r p t i o n system i . e . m e t a l on m e t a l system. One i n t e r e s t i n s t u d y i n g t h i s system comes from t h e knowledge of t h e adsorp- t i o n bond c h a r a c t e r i s t i c s ( i . e . i o n i c t y p e ) and of t h e phase diagram o b t a i n e d by slow e l e c t r o n d i f f r a c t i o n 17,8 1
.
A c l a s s i c a l s t a i n l e s s s t e e l f i e l d e m i s s i o n microscope h a s been u s e d , a l l o w i n g a n u l t i m a t e u l t r a - h i g h vacuum o f 1 t o 3x10-l1 T o r r . L o c a l c u r r e n t measurements a r e a c h i e v e d u s i n g a probe-hole t e c h n i q u e w i t h a t i p h o l d e r movable on b e l l o w s . The t e m p e r a t u r e of t h e t i p c a n b e changed by c l a s s i c a l s e r v o - l o o p - c u r r e n t from 8 0 K t o 3000 K. The e v a p o r a t i o n s o u r c e i s a n a l k a l i m e t a l d i s p e n s e r f u r n i s h e d by S.A.E.S.
G e t t e r which i s u s u a l l y u s e d i n t h e p h o t o t u b e i n d u s t r y . I t i s a c o n t a i n e r f i l l e d up w i t h sodium chromate and a non e v a p o r a b l e a g e n t Zr 16% - A 1 a l l o y named S t 101 t o p r e v e n t from g a s e v a p o r a t i o n when h e a t i n g t h e s o u r c e .
L o c a l c u r r e n t c o r r e l a t i o n f u n c t i o n s a r e measured i n a s i m i l a r way a s t h a t u s e d by D i Foggio and Gomer t o s t u d y t h e d i f f u s i o n of hvdronen on W(110) ( F i g . 1)
The fowler-Nordheirn p a r a m e t e r s 19 1 governing t h e l o c a l f i e l d e m i s s i o n c u r r e n t i have been measured u s i n g a l i n e a r r e g r e s s i o n program on Log ( i / v 2 ) v s where V i s a p p l i e d t i p v o l t a g e . F i g . 2 shows v a r i a t i o n s of t h e s e p a r a m e t e r s w i t h Na coverage which i s o b t a i n e d from work f u n c t i o n v e r s u s c o v e r a g e measurements on a macroscopic c r y s t a l 18 1.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984904
JOURNAL DE PHYSIQUE
Fast screen Main screen Suppressor
*I
\Capacitive coupling
R e a l time correletor
F i g . 1 - Block diagram of t h e e l e c t r i c a l arrangement
F i g . 2 - P r e e x p o n e n t i a l Log B ( c u r v e 1) and s l o p e S (curve 2) of Fowler-Nordheim p l o t s v e r s u s coverage ( s t o c h i o m e t r i e s f o r observed slow e l e c t r o n d i f f r a c t i o n s t r u c - t u r e s a r e i n d i c a t e d by a r r o w s ) .
C o r r e l a t i o n f u n c t i o n s have been measured f o r coverages from 0.2 t o 3x1014 atoms cm-2 and f o r t e m p e r a t u r e s from 2. 170 t o 500 K. The long time behaviour e x h i b i t s a slower decay (T-0.7) than t h a t (r-1) p r e d i c t e d by t h e s i m p l e s t t h e o r e t i c a l models 11,2 1.
Fig. 3 shows a plot of D vs T in Arrhenius scales for different coverages.
Fig. 3 - Diffusion coef- ficient versus tempera- ture and coverage ( ( a ) :n=2x1013 at .cz1-2 ; (+) :n=1.25~1014 at . ~ m - ~ ;
(0) :n=2 .3x1014 at . ~ m - ~ ; (*) : n = 3 x 1 0 ~ ~ at .cm-2)
As can be clearly seen in Fig. 3 two temperature regimes appear, characterized by changes of Log D vs 1 / ~ parameters. Arrhenius fits of these curves give the relevant parameters values indicated in table 1 assuming D = Do exp(-E/kT).
Table 1 - Diffusion parameters in both temperature regimes and transition tempera- ture between these regimes.
coverage (x1014 atoms crn -2 )
---
High temperature log ~ ~ ( c d s - 1 )
I
regime E(eV)
--- Transition temperature (K)
.2 -7.6~0.5 0.2750.03 267215
...
-11.9+0.5 0.055+0.02 Low temperature
regime
log Do (cm2s-l) E (eV)
1.25
...
-5.450.5
...
0.3520.03
---
26025 -11.5+0.02
...
0.053+.001 2.3 -2.2~1.4 0.5850.1 30525 -1420.1 -1.075.04
3 -3.8521.2 0,442.07 27557 -14.220.5 -12.2
C9-20 JOURNAL DE PHYSIQUE
The diffusion parameters appear quite normal in the high temperature regime although the increase of Do with coverage already observed for oxygen and hydrogen on W(110) by Gomer and coworkers always stays an open question 110,11(. The numerical values of Do as well as the temperature dependance in the low temperature regime clearly indicate that the single adatom jump picture is a poor picture to describe the sys- tem.
The dependance of the relative fluctuation amplitude <An >/in> (which is proportio- 2 nal to the product of the compressibility times coverage and temperature) vs cove- rage n and temperature T is shown in Fig. 4.
Fig. 4 - Variations of the relative mean square fluctuation amplitude <6n >/<TI> 2 with temperature and coverage ; (a) : n = 2 x 1 0 ~ ~ at.cm-2 ; (+) : n = 1.25~101~ at.
; (0) : n = 2.3 x 1014 at .em-2 ; (*) : n = 33x01~ at . ~ m - ~ .
The t r a n s i t i o n t e m p e r a t u r e observed i n d i f f u s i o n c o r r e s p o n d s t o a n i n f l e x i o n p o i n t from p o s i t i v e t o n e g a t i v e c u r v a t u r e on t h e < 6 n 2 > / < n > v s T c u r v e . The i n c r e a s e of
< 6 n 2 > / < n > w i t h t e m p e r a t u r e i n t h e h i g h t e m p e r a t u r e regime i n d i c a t e s a n e f f e c t i v e r e p u l s i v e i n t e r a c t i o n 112 1 between adatoms (whereas t h i s c h a r a c t e r i s t i c i s o n l y p r e s e r v e d f o r h i g h c o v e r a g e s i n t h e low t e m p e r a t u r e r e g i m e ) .
One i n t e r e s t i n g p o i n t can b e u n d e r l i n e d c o n c e r n i n g some c h a r a c t e r i s t i c s of t h e t r a n s i t i o n . F i r s t l y b e c a u s e o f t h e c o n t i n u i t y o f t h e d i f f u s i o n c o e f f i c i e n t t h e t r a n - s i t i o n i s c o n t i n u o u s . Then a u s e f u l comparison i s p o s s i b l e w i t h s l o w - e l e c t r o n d i f - f r a c t i o n r e s u l t s 181 where o r d e r - d i s o r d e r t r a n s i t i o n s a r e observed f o r s u c h covera- g e s a t low t e m p e r a t u r e s . These t r a n s i t i o n s a r e i n d i c a t e d b y s t ~ e p d e c r e a s e s of d i f - f r a c t i o n s p o t i n t e n s i t y w i t h t e m p e r a t u r e which a r e f o l l o w e d by h i g h t e m p e r a t u r e t a i l s i n d i c a t i v e of s h o r t - r a n g e - o r d e r p e r s i s t e n c e . Although, u s i n g t h i s t e c h n i q u e , no q u a n t i t a t i v e a n a l y s i s c a n be o b t a i n e d t h e t a i l s of t h e s e c u r v e s go t o z e r o f o r
t e m p e r a t u r e s between 250 and 300 K which i s i n t h e r a n g e where we f i n d o u r d i f f u s i o n t r a n s i t i o n t e m p e r a t u r e s o t h a t t h e i d e n t i f i c a t i o n of b o t h t r a n s i t i o n s a p p e a r s n a t u - r a l . The s c a l e on which o ~ d e r i s d i s t r o y e d can b e p r e c i s e d a s b e i n g s m a l l e r t h a n t h e probed r e g i o n s i z e (s 50 A) b e c a u s e t h e c o r r e l a t i o n f u n c t i o n i s t h e same i n b o t h t e m p e r a t u r e r e g i m e s . The t r a n s i t i o n we o b s e r v e i n d i f f u s i o n t h e n p r o b a b l y c o r r e s - ponds t o a c o n t i n u o u s s h o r t - r a n g e - o r d e r - d i s o r d e r t r a n s i t i o n .
REFERENCES
11 I GOMER, R., S u r f . S c i . 2 (1973) 373
121 MAZENKO, G . , BANAVAR, J.R. and GOMER, R., S u r f . S c i . 107 (1981) 459 131 CHEN, J . R . and GOMER, R., S u r f . S c i . 79 (1979) 413
141 CHEN, J.R. and GOMER, R., S u r f . S c i . 81 (1979) 589 151 CHEN, J.R. and GOMER, R . , S u r f . S c i . 94 (1980) 456 161 D I FOGGIO, R., and GOMER, R., Phys. Rev. B 5 (1982) 3490 171 KLIMENKO, E.V., and NAUMOVETS, A.G., S u r f . S c i . 14 (1969) 141
181 MEDVEDEV, V.K., NAUMOVETS, A.G., and FEDORUS, A.G., S o v i e t Phys. - S o l i d S t a t e - 12 (1970) 301
191 -R, R . , F i e l d Emission and F i e l d I o n i z a t i o n (Harvard U n i v e r s i t y P r e s s , Cambridge, Mass, 1961)
11.0! BANAVAR, J.R., COHEN, M.H., and GOMER, R., S u r f . S c i . 107 (1981) 113 1111 VEDULA, Yu.S., LYUKSYUTOV, I . F . , NAUMOVETS, A.G., and POPLAVSKII,V.V.,
Pis'ma Zh. Eksp. Teor. F i z . 36, NQ 3 , (1982) 7 3
112 1 BELL, B . , GOMER, R., and REISS, H . , S u r f . S c i . I1 (1976) 494
