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EFFECT OF PREADSORBED NICKEL ATOMS UPON THE POTASSIUM MOBILITY ON THE
W(112) PLANE
Ch. Kleint, R. Blaszczyszyn
To cite this version:
Ch. Kleint, R. Blaszczyszyn. EFFECT OF PREADSORBED NICKEL ATOMS UPON THE POTAS-
SIUM MOBILITY ON THE W(112) PLANE. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-
91-C8-96. �10.1051/jphyscol:1989816�. �jpa-00229914�
COLLOQUE DE PHYSIQUE
Colloque C8, supplkment au n o
11,Tome 50, novembre 1989
EFFECT OF PREADSORBED NICKEL ATOMS UPON THE POTASSIUM MOBILITY ON THE W(112) PLANE
CH. KLEINT and R. BLASZCZYSZYN'
Fektion Physik, Karl-Marx-Universitst Leipzig, DDR-7010 Leipzig, D.R.G.
Institute of Experimental Physics, University of Wroclaw, ul.
Cybulskiego 36, PL 50-205 Wroclaw, Poland
Ghstract - Spectral denrity functions Wff) of the field emisrion f l i eke? noi re (FEFN; of a potassium submanol dyer 10.*=C. 4) adsorbed on a nickel p r e c u v e r e d W ( 1 1 2 ! plane were investigated? A n analysis in terms of the Timm and Van der Ziel model was used to determine the K surface diffusion coefficient D. I t s dependence on the N i coverage is
presented for a t i p temperature of 383 K. With increasins coveraqe D
-9 -9 2 - 1
rises from about 1.6:<?0 to 9 x 1 0 rm 5 where the Ni concentration carrespends tc the work Qunction maximum of the W ! 1 1 2 : N i system, and then E derreases. The results are consistent with ~ r e v i o u s FEFN cross-cnrrelation ~ ~ a s u r e m e n t s +or the same coadsot-ption system. Q smoothing efiect caused by the Ni atoms is responritfe for the increair cf the diffusion ruefficient.
SLkali ~ v e t a l adsorption on solids never ceased to be of interest over decades and also alkali coadsorption with molecules is an object of current suriace rrsearch /i/. While molecular coadsorption is of practical importance because of the electronic aiparticie interactiun / 2 i tke main e + f e c t of the coadsorbed nickel is studied here in relati~n to the potassium mobility. The
W < 112INi-M system was alt-eady investigated by crass-correlation uf the field
gmi 551 on i 1 - ; i f i ? r : noire (FEFN) d.3' from two probed areas of t h e ! 1 1 Z f plane / 4 / . The crcs~.-.cort7eXatior; - f icnctio~s (CCFs! were measctred for eqtial K s ~ r b monolayer-, f0,=G. 4! depasi ted onto different preadsorbed Ni doses. The
3 ..
results wil? be discuzsed bglow in detail but we want to stress here their consequences: They clearly show the inilueoce o f a continunurly changed sitriace structure on the K mobility. In the present pzper we apply a ccmparison a+ experimental with th~aretical sgectral density functions of the same system which allows u s to determine the surface diffusion coefflrient directly.
I 1 - EXPERIMENTAL
The construction of the
FE
tube and the experimental conditions were the same as described in r e f . /4/. The emission current of the probed region with a diameter of aSaut 1008
was seiected by a probe-hole in the screen andArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989816
d e t e c t e d b y a F a r a d a Y c o l l e c t o r . T h e f r e q u e n c y a n a l y s i s n i t h o a c c o m p o n e n t of t h e c o l l e c t o r c u r r e n t w c s p e r f a r m e d b y a n " E r h t i e i t a n a l y s a t o r 01012"
l f l e s s e l e k t r n n i k D r e - % d e n ) . The d i r e c t p t - o l e - h o l e c u r r e n t o f a b o u t 1 3 nfi w a s k e p t c c n s t a n t . T h e FEFN s p e c t r a w e r e d e t e r m i n e d f o r a c o n s t a n t p o t a s s i u m c o v e r a g e o f 4 . 4 a d s o r b e d o n t h e t h e r m a l l y c l e a n e d W i l l Z ) p l a n e w i t h
K
p r e a d s o r b e d N i l a y e r s . T h e l a t t e r w e r e d e p o s i t e d f r o m a s m a l l N i b a r s p o t w e l d e d o n t o t w o c r o s s e d W l o o p s .
111
-
RESULTSF i g s . 1 a n d 2 s h o w t h e FEFN s p e c t r a l d e n s i t y f u n c t i o n s o b t a i n e d f o r c o a d s o r p t i n n i a y e r s K-Ni o n t h e W I 1 12) p l a n e f a r v a r i o u s N i d e p o s i t i o n t i m e s t N i f m i n ) a t a t i p t e m p e r a t u r e o f 393 K ( d o t s ) . T h e s o l i d l i n e s c o r r e s p o n d t o t h e t h e o r e t i c a l s p e c t r a l d e n s i t y f u n c t i o n f a r u n b o u n d e d d i f f u . s i o n a c c o r d i n g t o t h e T i m m a n d Van der Z i e l m o d ~ l J5/ a n d t a k e n Fram t h e p a p e r o f G e s l e y a n d Sk*anson /6/. T h e r e l e v a n t Q u n c t i o n a n d t h e f i t t i n g p r o c e d u r e w i l l b e d i s c u s s e d b e l ow.
-
F o r a n e x p l a n a t i o n o f t h e m o d e l u s e d f o r d a t a e v a l u a t i o n w e a l s o p r e s e n t a s p e c t r a l d e n s i t y f u n c t i o n M(f 3 o f t h e W ( 1 1 2 ) K s y s t e m a t r o o m t e m p s r a t u r e w i t h o u t n i c k e l d e p o s i t i o n . D t h e r c o n d i t i o n b e i n g e q u a l t h e m e a s u r e d v a l u e s { d o t s f are h e r e c o m p a r e d a n d f i t t e d t o t h e k h e o r e t i c a f G e s l e y - S w a n s o n s p e c t r a l d e n s i t y f u n c t i o n 4 a r I - d i m e n S i o n a 1 d i f f u s i o n /A/.
F i n a l l y f i g . 4 i n d i c a t e s t h e t y ~ i c a l d e p e n d e n c e o f b o t h t h e W(I1P) w o r k f u n c t i a n @ t 1 2 a n d v o l t a g e a t c o n s t a n t d c c o l l e c t o r c u r r e n t o n t h e i n v e s t i g a t e d n i c k e l d e p u s i t i o n . T h i s d e p o s i t i o n d e p e n d e n c e o f t h e v o l t a g e i s u s e d f a r t h e n i c k e l s o u r c e c a l i b r a t i o n . @112 is o b t a i n e d b y m e a s u r i n g t h e F a w i e r - N o r b h e i m c h a r a c t e r i s k i c.
I V
-
EVkLUkTION AND DISCUSSIONT h e o r i g i n of t h e f 1 i c k : e r n o i s e i n t h e c o a d s a r p t i o n s y s t e m W!1121K-Ni i s d i s c u s s e d e l s e w h e r e / 4 / . B r i e f l y , t h e m o b i l i t y o f %: a d a t o m s a n t h e W(112;
c r y s t a l p l a n e i s e x p e c t e d t o b e much h i g h e r t h a n t h a t o f N i a d a t o m s a n d t h e f l i c k e r n o i s e f r o m t h e Ni a d l a y e r i n t h e u s e d t e m p e r a t u r e r a n g e c a n b e n e g l e c t e d r e l a t i v e t o t h e p o t a s s i u m n o i s e . T h i s s t a t e m e n t r e s u i t s f r o m a c o m p a r i s o n u f t h e r e s p e c t i v e e x p e r i m e n t a l c o n d i t i o n = f o r d i f f u s i o n a n d d e s o r p t i o n o f N i a n d K a t o m s o n t u n g s t e n , t h e p a r a m e t e r s u f t k : s s e p r o c e s s e s a n d a l s o w o r k f u n c t i o n c h a n g e s u p o n a d s o r p t i o n o f t h e s e s p e c i e s o n t h e W I 1 12j p l a n e /4/.
F i g . i a n d 2 s h o w , t h a t t h e s p e c t r a W ( f 1 d e p e n d o n t h e Ni c o v e r a g e a t c o n s t a n t p o t a s r i u m c a v e r a g e a n d t e m p r r a t ~ i r e . T h e e x p e r i m e n t a l r e r u i t s are c o m p a r e d w i t h t h e t h e o r e t i c a l s p e c t r a ? d e n s i t y f u a c t i o n S!u 1 d e r i v e d f u r a c i r c u l a r
D F
p r o b e h o l e b y G e s i e y a n d Swa-nsan I b i i n t h e f r a m e w o r k o f t h e Timm a n d Van der Z i e l model / 5 / . I t is a s s u m e d t h e r e i n t h s t t h e f l u ~ t u a t i o n i n t h e n u m b e r a+
t h e a d p a r t i c l e s o n t h e s m a l l p r o b e d area i r c a u s e d by t w o - d i m e n r i u n a l s u r f a c e d i f + u s i o n af t h e a d s o r b a t e . C o n s e q s e n t l y , t h e a d s o r b a t e f i u c t u a t i o n i n d u c e s a FE c u r r e n t f i u c t u a t i o n . R c c o r d i n q t o r e $ = . i 5 / a n d /6/ t h e c a r r e s p u n d i n g s p e c t r a l d e n s i t y f u n c t i o n f l u 3 i s g i v e n b y
p
w n e r e u = 2 n i r 2 / D i 5 t h e d i m e n s l o n f e s s n o r m a l l z e d f r e q ~ ~ e n c y . i ( u ) i s a n
P P
e x p r e c s i a n c o m p o s e d a f K e l v i n f u n c t i o n s (cf. G e s l e y a n d S w a n s o n /bi, Eq. 483, r t h e r a d i u s o f t h e p r a t e d a r e a i g SO
8
i n t h i s e x p e r i m e n t ) ,D
t h e s u r f a c e d l { - f u s i o n c o e Q f i c i e n t a n d CFN t h e F o w j e r - N o r d n e i m term.F i g . 1 a n d 2 s h o w t h a t W ! i > c a n n o t b e d e s c r z b ~ d b y t h e Timm a n d Van d e r Z i e l model i n a w i d e f r e q u e n c y b a n d . T h e p r e s e n t s p e c t r a c o n f i r m a n e a r f i e r e x p e r i m e n t a l r e s u l t /7/ w h e r e n o s l o p e E = 3i2 a s t h e h i g h - f r e q u e n c y a s y m p t o t e o f Eq. < I f c o u l d b e f o u n d f o r p o t a s s i u m o n t h e W ! 1 1 2 > p l a n e . k l s n t h e l o w - f r e q u e n c y p a r t o f t h e s p e c t r a resists t h s f i t b y t h e s i m p l e f u n c t i o n s
Fig.? 10-2$,
. . . .
I. . .
I. . . . I \ I
lo2 ld
FREQUENCY f lo1
IHzl
Fig. 2
F i g s . 1 a n d 2. E x p e r i m e n & a l s p e c t r a l d e n s i t y f u n c t i o n s W ( f 1 o f a c o n s t a n t p o t a s s i u a s u b m o n e l a y e r iOk: = 0.4) a d s o r b e d o n t h e n i c k e l p r e c o v e r e d W ( 1 1 Z f p l a n e , d e t e r m i n e d a t a t i p t e m p e r a t u r e o f 393 K ! d o t s ) . T h e n i c k e l d o s e s 't4i a r e g i \ - e p i n m i n u t e s . T h e c u . r v e s a r e d i s p l a c e d i n t h e v e r t i c a l d i r e c t i o n f o r e a s e o f v i e w i n g w h i c h is e x p r e s s e d i n t h e ' f a c t o r ' a . T h e s o l i d l i n e s r e p r e s e n t t k e o r e t i n z a l s p e c t r a l d e n s i t y f u n c t i o n s S(u ) / & / f i t t e d t o t h e e x p e r i m e n t a l
v a l u e s . P
1 0 2 ~ 1 1 a
I
10' lo2 lo3 10' [HZ]
FREQUENCY f
F i g . 3. S p e c t r a l d e n s i t y f u n c t i o n o f a p o t a s s i u m s u b m o n o l a y e r ( 0 K = 0 . 4 )
-
.
. d e p o s i t e d o n t h e %(I121 a n d m e a s u r e d a t room t e m p e r a t u r e I d o t s ) . T h e d a s h e d l i n e r e p r e s e n t s t h e t h e o r e t i c a l s p e c t r a l d e n s i t y f u n c t i o n S o r t h e 1 -d i m e n s i o n a l d i f f u s i o n madel a t t a i n e d h y G e s l e y a n d Swanson i b f .
d e r i v e d f r o m Eq. I . D i f f e r e n t p r o c e s s e s h a v e t o b~ t a k e n i n t o a c c o u n t i n t h e c a s e o f t h e c o ~ p l e x s p e c t r a , e s p e c i a l l y $ o r n o n - i s u t r o p i c p l a n e s o f t h e b c c
1 2 1 2 ) s t r u c t u r e c o v e r e d w i t h t h e s t r o n g l y i n t e r a c t i n g K a d s o r b a t e .
..-
s h e Z s w € ? t . . i ~ g 0 i t h e t i p t e m p e r a k i t r e t o room t e m p e r a t i t r e e s s e n t i a z l y c h a n g e s t h e s h a ~ e o i t h e Wff 1 's ( F i g . 3 ) w h i c h is i n a g r e e m e n t w i t h t h e t h e o r e t i c a l f u n c t i o n S c u>
f o r a 1 - d i m e n s i o n a t d i f f u s i o n modef f c f . / b / , F i y . : j . A t t h i sP
t e m p e r a t u r e d i i f u s i o n a l o n g t h e C 1 1 1 1 d i r e c t i o n o u t w e i g h s t h e p r u c e s s i n t h e p e r p e n d i c u l a r d i r e c t i o n ,
Assuming t h a t s u r f a c e d i i f u s i o n p l a y s t h e d o m i n a n t r o l e I n t h e adsarbate i n d u c e d n o i s e a 2 t i e a p p l i e d t e m p e r a t u r e , t h e s u r f a c e d i f f u s i o n c o e f f i c i e n t 9 c a n b e d e t e r m i n e d b y f i t t i n g t h e t h e o r e t i c a l
-
S < u P j t o t h e e x p e r i m e n t a l Wff! 's. Then D = 2 n i r L , w h e r e f c o r r e s p o n d s t o t h e n o r m a l i z e d i r e q u e n c yP P
u =I, f is marked b y a n a r r o w i n F i g s . 2 - 3. F i g . 5 p r e s e n t s t h e o b t a i n e d
P P
N i c o v e r a g e d e p e n d e n c e o f t h e p o t a s s i u m d i i f u s i o n coefficient. I t r e v e a l s a p r o n o u n c e d maximum a t t = 2.7 min by a d r a s t i c c h a n g e o f D. The
Ni
r o m p a r i s , o n w i t h t h e a c c o m p a n y i n g work i u n r t i a n c h a n g e CFig.4) s h o w s t h a t t h e n i c k e l c o v e r a g e d e ~ e n d e n c e o f D r e i f e c t s a s i m i l a r d e p e n d e n c e o f t h e work f u n c t i o n
er
L & A
The r e s u l t i s c o n s i s t e n t w i t h p r e v i o u s c r a s s - c o r r e l a t i o n m e a s u r e m e n t s o f t h e f i e l d e m i s s i o n f l i c k e r n a i s e f o r t h e s a m e s y s t e m
( / a / ,
F i g s . L a n d 7;. T h e c r o s s - c o r r e i a t i o n f u n c t i o n s (CCFs? a + t h e W - I 1 121 p l a n e were a l s o m e a s u r e d f o r a c o n s t a n t p o t a s s i u m s u b m o n o l a y e r iQbr = 0 . 4 ) d e p o s i t e d o n t o d i f f e r e n t p i e a d s o r b e d n i c k e l d o s e s . I n g e n e r a l , t h e ZCF's show a d i s t i n c t maximum w i t h i n c r e a s i n g d e l a y t i m e T f o r t h e E l l l f a n d C 1 1 0 7 c r y s t a l l o g r a p h i c d i r e c t i o n s .F i g . 4 V o l t a g e f o r c o n s t a c t c o l l e c t o r i z u r r e a t of P h e N C 1 1 2 i p i a n e < i l l Z = 3 cp,) a n d w o r k f u n c t i o n v e r s u s W! 112) n i c k e l c o v e r a g e ( f r u m a s i m i i a r e x p e r i m e n t ) . T h e sai i G I i n e s r e p r e s e n t 5th o r d e r E a u s s i a n p o l ynomi a2
+
i i-5.F i g . 5. D i f f u r i c n c o e f f i c i e n t B a n d f r e q a e n c y 6 v e r s u s d e p o s i t i o n t i m e o f P
n i c k e l t N i . D w a s cairuiated f r o = t h e r e l a t i a n D = ~ n + r ? u _ / h / w h e r e i P , w h i c h
P F
c o r r e s ~ o n d s t o u _ = i , i s t a k e n f r o m F i g s . 1 a n d 2. The s o P i d l i n e s r
r e p r e s e n t 7 t h o r d e r G a u s s i a n p o l y n o m i a l f i t s .
The delay time r corresponding to the maximum of the CCF, reaches a deep max
minimum at a nickel coverage
e
indicating the work function maximum of the mW(I12)Ni system. The dependence of the activation energy of the crass-correlation process, determined from the Arrhenius
-
like temperature dependence of the delays Tmas exhibits also a minimum at the nickel coverage Om. Such a behaviour of T
mas and Q with the nickel coverage has been interpreted in detail in ref. /4/ by the increase of adatom mobility in the potassium submonolayer as a result of the smoothing effect by the nickel adatoms on the (112) plane /B/. At higher Ni coverages a roughening effect occurs which is also present in the Ni - K coadsorption layers nn the W(110) plane /9/.The cross-correlation picture of the Ni adsorbate influence on the potassium mability is supported by the presented spectral density function measurements.
Work sponsored by the Polish Ministry of Education within the Central Project of Basic Research CPBP O1. OE. A.
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M. A. Gesley and L. W. Swanson, Phys. Rev.
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32 (1985) 7703.Ch. Kleint,
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