ADSORPTION OF HEAVY RARE-EARTH ATOMS ON (011) AND (111)W

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ADSORPTION OF HEAVY RARE-EARTH ATOMS ON (011) AND (111)W

S. Shakirova, M. Shevchenko

To cite this version:

S. Shakirova, M. Shevchenko. ADSORPTION OF HEAVY RARE-EARTH ATOMS ON (011) AND (111)W. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-129-C8-131. �10.1051/jphyscol:1989823�.

�jpa-00229921�

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COLLOQUE DE PHYSIQUE

Colloque C8, supplement au no ^11,Tome 50, novembre ¹⁹⁸⁹

ADSORPTION OF HEAVY RARE-EARTH ATOMS ON (011) AND (111)W

S.A. SHAKIROVA and M.A. SHEVCHENKO

I n s t i t u t e of Physics, Leningrad S t a t e University, Leningrad, 198904.

Petrodvorets, USSR

A b s t r a c t

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The r e g u l a r i t i e s o f t h e work f u n c t i o n v a r i a t i o n on W m d (111) f a c e s induced by a d s o r p t i o n of heavy r a r e - e a r t h ^{R E )} atoms ( e x c e p t i n g Y b ) a t u l t r a h i g h vacuum ( 1 0 ' ~ - 1 0 ' ~ 0 p a ) an

a

^{a t 77}^K h a s been s t u d i e d by f i e l d e l e c t r o n microscopy.The i n i t i a l adatom d i p o l e moments on t h e s e f a c e s have been determined.

I n v e s t i g a t i o n of t h e a d s o r p t i o n o f heavy RE atoms on s i n g l e c r y s t a l t u n g s t e n f a c e s i s of i n t e r e s t from t h e s t a n d p o i n t of b r o a d e n i n g of t h e c l a s s o f a d s o r b a t e s which, i n c o n t r a s t t o t h e a l k a l i and a l k a l i n e e a r t h m e t a l s s t u d i e d s u f f i c i e n t l y w e l l , p o s s e s s a more complet e l e c t r o n con- f i g u r a t i o n w i t h t h e , i n n e r 4 f - s h e l l becoming f i l l e d a s t h e atomic number Z i n c r e a s e s . Another remarkable f e a t u r e o f t h e s e e l e m e n t s i s t h a t t h e e l e c t r o n c o n f i g u r a t i o n s o f t h e RE i n t h e f r e e s t a t e a r e d i f f e r e n t from t h o s e i n t h e metal : i n d e e d , while i n t h e atomic s t a t e t h e y a r e diva- l e n t , w i t h t h e e x c e p t i o n of Gd and Lu which a r e t r i v a l e n t , i n t h e me- t a l a l l o f them a r e t r i v a l e n t because o f t h e 4f * 5d e l e c t r o n t r a n s i - t i o n , excepfi f o r yb which h a s a f i l l e d and, hence, more s t a b l e c o n f i - g u r a t i o n 4 f 4. It c o u l d t h u s be expected t h a t t h e a d s o r b e d s t a t e of t h e t h e RE atoms i s i n t e r m e d i a t e between t h e atomic one and t h e m e t a l o r , more p r o b a b l y , t h a t one c o u l d f o l l o w t h e t r a n s i t i o n from t h e atomic

( f o r n ^-+0 ) t o m e t a l l i c s t a t e a s t h e RE adatom c o n c e n t r a t i o n i n c r e a s e s . The v a r i a t i o n of t h e work f u n c t i o n of two W(011 and (111) f a c e s

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which d i f f e r s u b s t a n t i a l l y i n p o t e n t i a l r e l i e f and n i t i a l work func- t i o n was s t u d i e d as a f u n c t i o n o f adatom (Gd, Tb, Dy, Ho, E r , Tm, Lu) c o n c e n t r a t i o n n. The s t u d y was c a r r i e d out i n a s e a l e d o f f f i e l d emis- s i o n microscope with a probe h o l e i n an u l t r a h i g h vacuum (10'9-10~10 p a l and s u b s t r a t e t e m p e r a t u r e 77 K. A m e t a l b e l l o w s p r o v i d e d a p o s s i - b i i t y of d i r e c t i n g t h e W f a c e of i n t e r e s t toward t h e probe h o l e .

Being t h e c l o s e s t - p a c k e d and p o s s e s s i n g t h e smoothest p o t e n t i a l r e l i - e f , t h e (01 1 ) f a c e h a s t h e l a r g e s t i n i t i a l work f u n c t i o n ((p(110)=5,5eV) whereas t h e 11111 f a c e which i s l o o s e l y packed and e x h i b i t s t h e deep- e s t p o t e n t i a r e i e f i s c h a r a c t e r i s e d by a s u b s t a n t i a l l y lower i n i t i a l work f u n c t i o n ( l$~

The work f u n c t i d n o&=:h%V)aisorbate system was determined by compar- i n g t h e s l o p e of t h e Fowler-Nordheim p l o t s f o r t h e e m i t t e r a s whole and t h e p u r e o r a d s o r b a t e - c o a t e d f a c e t o w i t h i n *0,05eV. S i n g l e o r do- u b l e vacuum d i s t i l l e d m a t e r i a l of p u r i t y 99,96-99,99$ was u s e d i n de- p o s i t i n g t h e a d s o r b a t e s . The a b s o l u t e a d a t om c o n c e n t r a t i o n n was found by means of a q u a r t z microbalance with an e r r o r o f 7-10%- The adsorba- t e was d e p o s i t e d s t e p w i s e on t h e p o i n t maintained a t 77 K, t h u s p r e c l u - d i n g m i g r a t i o n of adatoms from one f a c e t o a n o t h e r . Only one a d s o r b a t e s p e c i e s was p r e s e n t i n each F E M a t a t i m e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989823

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Fig.1 d i s p l a y s v ( n ) p l o t s f o r s e v e n h e a v y RE a d s o r b a t e s on W(011).

The a b s o l u t e c o n c e n t r a t i o n s o f a l l t h e adatoms s t u d i e d t u r n e d o u t t o be t h e same a t t h e e x t r e m a o f n ) p l o t s , which s h o u l d n o t come a s a

f'

s u r p r i s e , s i n c e d e s p i t e t h e e f e c t o f l a n t h a n i d e c o n t r a c t i o n t h e a t o - mic r a d i i o f Gd t h r o u g h Lu d i f f e r by 0,065W o n l y . Note t h e q u a l i t a t i v e - l y s i m i l a r b e h a v i o r o f (P w i t h i n c r e a s i n g n f o r a l l t h e a d s o r b a t e s . The v ( n ) c u r v e s a r e nonmonotonic, t h e y p a s s t h r o u g h a deep minimum at n,3, 9x1014cm-2, a maximum f o r n=8,5x1014cm-2, a n d , f o r two a d s o r b a t e s o n l y (Gd and T b ) , a p l a t e a u . The a b s e n c e of a p l a t e a u f o r t h e o t h e r a d s o r b a t e s may be a t t r i b u t e d t o t h e f o r m a t i o n o f 3 D c r y s t a l l i t e s o n t h e s u r f a c e o f a m o n o l a y e r c o a t i n g which r e s u l t s i n a l o c a l i n c r e a s e o f e l e c t r i c f i e l d a n d , a c c o r d i n g l y , i n an anomalous d e c r e a s e o f

Cp ,

t h u s e v i d e n c i n g t h e g r o w t h of Dy, ^{9 0 ,} E r , Tm and Lu f i l m s by t h e mechanism o f S t r a n s k i i - K r a s t a n o v . A s f o r Gd a n d Tb, t h e y grow by t h e l a y e r - t o - l a y e r mechanism, a t l e a s t up t o two-monolayer c o v e r a g e , which a g r e e s w i t h Ref. /l/.Thf?

qmin

and Qmax i n c r e a s e l i n e a r l y w i t h a t o m i c number Z a s s e e n i n t h e l n s e t o f Fig.1. The d i f f e r e n c e i n t h e v a l u e s o f

9

f o r monolayer R E f i l m s between Gd a n d Lu i s 0,17eV.

Fig.1. W(011) work f u n c t i o n v s heavy RE adatom c o n c e n t r a t i o n a t s u b s t - r a t e t e m p e r a t u r e 77 K : 1

-

Gd, 2 - T b , 3

-

^{Dy, 4}

-

^{Ho, 5}

-

^{E r ,}

6

-

^Tm,⁷

-

^Lu.

I n s e t : Dependence of

ymin

and (pmax o f RE f i l m s v s a t o m i c number 2.

F o r t h e h e a v y RE e l e m e n t s s t u d i e d , t h e v ( n ) r e l a t i o n s f o r W(111) have a s h a p e t y p i c a l o f e l e c t r o p o s i t i v e a d s o r b a t e s ( F i g . 2 ) , n a m e l y , a f - t e r p a s s i t h r o u g h a minimum (n,4,9x1014cm-2) t h e y r e a c h a p l a t e a u

( n = 8 , 5 ~ 1 0 ~ ' c m - 2 ) . On t h i s f a c e one o b s e r v e s a l s o an i n c r e a s e o f b o t h t h e work f u n c t i o n a t minimum, (P,in, and o f i t s p l a t e a u v a l u e s , (Pp, w i t h i n c r e a s i n g a t o m i c number Z s e e t h e i n s e t i n F i g . 2 ) .

The s i m i l a r v a r i a t i o n of +(n) f o r a l l a d s o r b a t e s a t a g i v e n f a c e shows t h e s e a d s y s t e m s t o h a v e common p r o p e r t i e s , o n l y t h e d i f f e r e n t v a l u e s of i n t h e r a n g e of s u b m o n o l a y e r ((pmin) a n d m o n o l a y e r ( (f)max o r Y p ) c o v e r a g e i n d i c a t i n g they are a c t u a l l y d i f f e r e n t a d s o r b a t e s . The n o t i c e a b l y d i f f e r e n t b e h a v i o r o f q ( n ) on t h e ( 0 1 1 ) and ( 1 1 1 ) f a c e s

o f W c r e a t e d by a d s o r b e d h e a v y RE a t o m s , a s w e l l a s t h e a b s o l u t e v a l u - e s of t h e work f u n c t i o n o f m o n o l a y e r c o a t i n g s c o n f i r m t h e e s s e n t i a l r o l e p l a y e d by t h e s u b s t r a t e s t r u c t u r e and e v i d e n c e a p p a r e n t l y p s e u d o -

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C8-131

m o r p h i c g r o w t h o f t h e f i l m s .

F i g . 2 . W ( 1 1 1 ) w o r k f u n c t i o n v s h e a v y RE a d a t o m c o n c e n t r a t i o n a t s u b s t - r a t e t e m p e r a t u r e 7 7 K s 1 - Gd, 2 - T b , 3 - Dy, 4 - H o , 5 - E r , 6 - Tm, 7 - L u .

I n s e t : D e p e n d e n c e o f CD . a n d M RE f i l m s v s a t o m i c n u m b e r Z . ' m m >p

T h e f ( n ) r e l a t i o n s o b t a i n e d f o r t h e a d a t o m s u n d e r s t u d y on b o t h HIT f a c e s h a v e b e e n u s e d t b d e t e r m i n e t h e d i p o l e m o m e n t s JJL. i n t h e l i m i t o f z e r o c o v e r a g e ( n - » - 0 ) . I t h a s b e e n f o u n d t h a t f o r a g i v e n f a c e t h e i n i t i a l d i p o l e m o m e n t s do n o t d e p e n d on Z f o r a l l t h e s e v e n a d a t o m s a n d a r e 4 , 0 D f o r t h e ( 0 1 1 ) , a n d 2 , 0 D f o r t h e ( 1 1 1 ) f a c e t o w i t h i n

± 0 , 1 D ( F i g . 3 ) . T h e a v a i l a b l e d a t a on t h e v a l u e s o f ) * . f o r Gd a n d Tb o n t h e W(0111 / 1 / a n d W ( 1 1 1 ) f a c e s / 2 / a g r e e w e l l w i t h o u r r e s u l t s . A s - s u m i n g d i p o l e l e n g t h s f o r a l l a d a t o m s on a g i v e n f a c e t o b e t h e s a m e , t h e i n d e p e n d e n c e o f ^ i . on Z o b t a i n e d b y u s s u g g e s t t h a t t h e a d s o r b e d RE a t o m s h a v e t h e s a m e c h a r g - e s t a t e on t h e g i v e n f a c e i r r e s p e c t i v e o f t h e i r e l e c t r o n c o n f i g u r a t i o n i n a t o m i c s t a t e .

F i g . 3 . I n i t i a l d i p o l e m o m e n t s jx. o f h e a v y RE a d a t o m s v s a t o m i c n u m b e r Zs • - d a t e o f R e f . / ? / , x - d a t e o f R e f . / 2 / .

REFERENCES

/ 1 / KOLACZKIEWICZ, J . , S u r f a c e S c i e n c e I 7 5 ( 1 9 8 6 ) 4-87

/ 2 / GOFCHAR, F . M . , SMEREKA, T . P . , S T E P A l O V S K I I , S . I . , BABKIS, G . V . , F i z . T v . T e l a 2 0 ( 1 9 8 8 ) 3 5 4 1 .