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SURFACE SELF-DIFFUSION OF Pt ON THE Pt(311) PLANE
G. Kellogg
To cite this version:
G. Kellogg. SURFACE SELF-DIFFUSION OF Pt ON THE Pt(311) PLANE. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-331-C2-336. �10.1051/jphyscol:1986250�. �jpa-00225684�
SURFACE SELF-DIFFUSION OF Pt ON THE Pt(311) PLANE
G.L. KELLOGG
Sandia National Laboratories, Albuquerque, New Mexico 87185, U.S.A.
Abstract - The s u r f a c e d i f f u s i o n of s i n g l e P t atoms and P t c l u s t e r s on t h e c h a n n e l e d (31 1 ) p l a n e of P t has been i n v e s t i g a t e d by f i e l d ion microscopy.
S i n g l e atoms were found t o migrate w i t h i n a given channel of t h e (31 1 ) plane a t t e m p e r a t u r e s above 215 K with an a c t i v a t i o n energy of 0.60 k 0.03 eV and a d i f f u s i v i t y o f 1 . 9 X 10-* c m 2 / s e c . No c r o s s - c h a n n e l d i f f u s i o n was o b s e r v e d f o r s i n g l e atoms. Two atoms w i t h i n t h e same channel a s s o c i a t e d i n t o a dimer which was immobile and s t a b l e up t o t e m p e r a t u r e s of 340 K . S u b s e q u e n t a d d i t i o n of atoms t o t h e same channel r e s u l t e d i n t h e formation of chains of atoms along t h e channel. The motion of two atoms d i f f u s i n g i n a d j a c e n t c h a n n e l s were n o t s t r o n g l y c o r r e l a t e d . However, o c c a s i o n a l occurrences of cross-channel processes between adjacent channels t o p r o d u c e t h e s t a b l e dimer c o n f i g u r a t i o n were o b s e r v e d . O b s e r v a t i o n s of atoms d i f f u s i n g i n n e x t - a d j a c e n t c h a n n e l s s u g g e s t e d a weak a t t r a c t i o n a t s e p a r a t i o n s from 9 . 2 t o 12.4A. Small c l u s t e r s of atoms l e f t on t h e plane a f t e r f i e l d evaporation were s t a b l e below 370 K , but a t 370 K and above, t h e atoms r e a r r a n g e d t o form a l o n g c h a i n w i t h i n one channel. These r e s u l t s suggest t h a t t h e mechanism f o r t h e formation of m i s s i n g rows i n t h e ( 1 x 2 ) r e c o n s t r u c t i o n of c h a n n e l e d s u r f a c e s of P t , I r , and Au involves s u r f a c e d i f f u s i o n of atoms which a r e s t a b i l i z e d by short-ranged atomic i n t e r a c t i o n s .
I - I N T R O D U C T I O N
Low Energy Electron D i f f r a c t i o n (LEED) s t u d i e s of t h e ( 1 1 0 ) - o r i e n t e d s u r f a c e s of P t , Ir, and Au i n d i c a t e a s u r f a c e r e c o n s t r u c t i o n which i s c h a r a c t e r i z e d by a (1x2) p a t t e r n / l - 5 / . Of the s e v e r a l models proposed f o r t h e r e c o n s t r u c t i o n , t h e "missing rowff model has- become t h e most p o p u l a r . A c c o r d i n g t o t h i s model, every o t h e r close-packed C1101 row i n t h e outermost l a y e r is missing. The missing-row model i s s u p p o r t e d by q u a n t i t a t i v e LEED a n a l y s i s / 2 / , Rutherford Ion Backscatterlng measurements / 6 / , and d i r e c t images of t h e s u r f a c e by S c a n n i n g T u n n e l i n g Microscopy (STM) / 7 / and Transmission E l e c t r o n Microscopy (TEM) / B / . I n a r e c e n t p a p e r Bonzel and F e r r e r / 9 / have q u e s t i o n e d t h e m i s s i n g row model b a s e d o n problems a s s o c i a t e d with mass t r a n s f e r . For t h e p a r t i c u l a r c a s e of P t ( l l 0 ) , they observe a t r a n s i t i o n between an unreconstructed (1x1) s u r f a c e and a r e c o n s t r u c t e d ( 1 x 2 ) s u r f a c e t a k i n g p l a c e w i t h i n 100 seconds a t a temperature of 310 K . Their C a l c ~ l a t i O n S of t h e mean d i f f u s i o n d i s t a n c e f o r P t by mass t r a n s f e r s u r f a c e d i f f u s i o n using a c t i v a t i o n e n e r g i e s based on f i e l d emission measurements, however, i n d i c a t e t h a t t h e d i f f u s i o n d i s t a n c e i s much t o o small t o e x p l a i n t h e e m p t y i n g o f a l t e r n a t e rows of atoms. I n s t e a d , t h e y propose a new model which does not s u f f e r from t h e mass t r a n s f e r problem. I n an even more r e c e n t paper, Campuzano , e t . a l . / 1 0 / a r g u e t h a t t h e (1x1) - (1x2) t r a n s i t i o n i s an order-disorder t r a n s i t i o n and t h e r e f o r e does not r e q u i r e l a r g e mass t r a n s f e r . C l e a r l y , t h e r o l e of s u r f a c e atom migration i n t h e (1x2) r e c o n s t r u c t i o n needs t o be defined.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986250
JOURNAL DE PHYSIQUE
In t h i s paper we r e p o r t observations of s u r f ace d i f f u s i o n and atomic i n t e r a c t i o n s on t h e P t ( 3 1 1 ) p l a n e which s u g g e s t t h a t a (1x2) s u r f a c e r e c o n s t r u c t i o n could be driven by s u r f a c e d i f f u s i o n of atoms and s t a b i l i z e d by s h o r t - r a n g e d i n t e r a c t i o n s between t h e atoms. T h i s mechanism i s i n a c c o r d a n c e w i t h t h e o r d e r - d i s o r d e r explanation given f o r t h e t r a n s i t i o n s on t h e (1 1 0 ) p l a n e / 1 0 / . Our e x p e r i m e n t s were c a r r i e d o u t on t h e (311) plane of P t , which i s s i m i l a r t o t h e (110) i n t h a t i t c o n s i s t s of close-packed rows of atoms s e p a r a t e d by c h a n n e l s . The d i s t a n c e between rows i s 4.628, on t n e ( 3 1 1 ) p l a n e and 3.92 A on t h e (110) plane. Our m o t i v a t i o n f o r e x a m i n i n g s u r f a c e d i f f u s i o n on t h e ( 3 1 1 ) p l a n e stemmed f r o m o b s e r v a t i o n s of m i s s i n g rows of atoms i n t h e ( 3 1 1 ) p l a n e f o l l o w i n g t h e r m a l treatment of platinum t i p s . The occurrence of the missing rows i n t h e (31 1 ) plane was v e r y f r e q u e n t i f t h e a n n e a l i n g temperature was above 400 K . Unfortunately, s u r f a c e atom m o b i l i t y was s o g r e a t a t t h i s t e m p e r a t u r e ( t h e s u r f a c e would b e g i n i t s change from a f ield-evaporated endf orm t o a thermal endform) t h a t t h e d e t a i l e d atomic s t e p s l e a d i n g t o t h e f o r m a t i o n of m i s s i n g rows c o u l d n o t be d i r e c t l y o b s e r v e d . The o b s e r v a t i o n s did l e a d us t o s u s p e c t t h a t t h e chains of atoms were a c t u a l l y being formed by atoms d i f f u s i n g on t h e plane and s t a b i l i z i n g i n t h e c h a i n f o r m a t i o n s , r a t h e r t h a n rows of atoms w i t h i n t h e plane d i f f u s i n g o u t on t o t h e s t e p s . In o r d e r t o d e t e r m i n e w h e t h e r o r n o t s u c h a p r o c e s s was p o s s i b l e , we undertook an i n v e s t i g a t i o n of t h e s u r f a c e d i f f u s i o n of P t atoms and Pt c l u s t e r s on t h e (311) plane. In t h i s paper we r e p o r t t h e r e s u l t s of t h a t i n v e s t i g a t i o n which include s t u d i e s of t h e d i f f u s i o n of s i n g l e atoms within a given (31 1 ) channel, two atoms w i t h i n t h e same channel, two atoms i n adjacent channels, two atoms i n n e x t - adjacent channels, and s h o r t chains of atoms i n adjacent channels.
I1 - EXPERIMENTAL APPARATUS
The e x p e r i m e n t a l p r o c e d u r e s followed i n f i e l d i o n microscope s t u d i e s of s u r f a c e d i f f u s i o n a r e well-documented / l 1 / and a r e n o t r e v i e w e d h e r e . The e x p e r i m e n t s were performed i n an all-metal f i e l d i o n microscope pumped by a l i q u i d - n i t r o g e n - trapped, o i l d i f f u s i o n pump and a l i q u i d - n i t r o g e n - c o o l e d t i taniurn s u b l i m a t i o n pump. A f t e r e x t e n s i v e vacuum p r o c e s s i n g , t h e background p r e s s u r e with t h e t i p cooled and t h e d i f f u s i o n pump valved-off was 4 - 5 X 10-'l T o r r . The background p r e s s u r e was s t a b l e f o r s e v e r a l h o u r s . Neon, p u r i f i e d by d i f f u s i o n through a vycor bulb, was used f o r t h e imaging gas. Image i n t e n s i f i c a t i o n was a c c o m p l i s h e d w i t h an i n t e r n a l channel p l a t e cleaned by both ion and e l e c t r o n bombardment. P t adatoms were s u p p l i e d from d e p o s i t i o n c o i l s made from 0.005 inch Pt wire heated t o a t e m p e r a t u r e j u s t below t h e i r m e l t i n g p o i n t . The P t c o i l s along with t h e i r support loops were e x t e n s i v e l y outgassed a s a p a r t of the vacuum p r o c e s s i n g . The P t t i p was p r e p a r e d from 0.005 i n c h p o l y c r y s t a l l i n e w i r e u s i n g t h e p o l i s h i n g procedure described i n t h e l i t e r a t u r e /12/. I t was cleaned using a combination of neon cathode s p u t t e r i n g , thermal annealing a t 600 K and f i e l d evaporation.
The d a t a c o l l e c t i o n and a n a l y s i s were semi-automated. F i e l d i o n images were r e c o r d e d from t h e i n t e r n a l phosphor screen using a RCA model TC1005 video camera and a JVC model 6060U video r e c o r d e r . A home-buil t microprocessor c o n t r o l l e d t h e a l t e r n a t e a p p l i c a t i o n of t h e imaging v o l t a g e , h e a t i n g of t h e t i p , and s t a r t i n g and stopping of the video r e c o r d e r . The a n a l y s i s of the recorded images c o n s i s t e d o f marking t h e adatom p o s i t i o n with a cursor generated by a Colorado Video Model 622 X-Y D i g i t i z e r . The d i g i t i z e r t r a n s m i t t e d t h e c o o r d i n a t e s t o an Dec LSI 11 /03 minicomputer f o r a n a l y s i s . B e t t e r - q u a l i t y images were o b t a i n e d f o r d i s p l a y purposes by photographing t h e phosphor s c r e e n w i t h a 35 mm camera e q u i p p e d w i t h Kodak t r i - X f i l m . A l l of t h e f i e l d i o n images d i s p l a y e d i n t h i s paper were recorded with t h e 35 mm camera.
I11 - EXPERIMENTAL RESULTS
S i n g l e P t atoms on t h e (31 1 ) plane of P t were f o u n d t o m i g r a t e a t t e m p e r a t u r e s above 215 K . The m i g r a t i o n was c o n f i n e d t o a g i v e n channel, i . e . , no cross- channel d i f f u s i o n such a s t h a t o b s e r v e d on t h e (1 1 0 ) p l a n e / l 3 / was d e t e c t e d . Mean square displacements were obtained f o r s i x temperatures i n t h e range from 217 K t o 256 K. The d a t a a r e p l o t t e d i n A r r h e n i u s form i n F i g . 1. The s l o p e and
p r e v i o u s s t u d y o f P t d i f f u s i o n on Pt(311) by B a s s e t t and Webber /13/ i n which an a c t i v a t i o n energy f o r s u r f a c e d i f f u s i o n of 0.69 2 0.2 eV was o b t a i n e d by a s s u m i n g
Fig. 1. An Arrhenius p l o t f o r a s i n g l e P t atom d i f f u s i n g w i t h i n a c h a n n e l of a Pt (31 1 p l a n e .
Fig. 2 Two P t atoms m i g r a t i n g i n t h e same Pt(311) channel ( a , b ) combine t o form a dimer ( c , d ) which i s s t a b l e t o 340K.
JOURNAL DE PHYSIQUE
t h e s t a n d a r d p r e f a c t o r / l 1 /. The frequency occupation f o r various s i t e s along a givenchannel measured f o r approximately 300 d i f f u s i o n i n t e r v a l s i n d i c a t e d t h a t t h e b i n d i n g a l o n g t h e channel was nonuniform, t h e adatoms were held more s t r o n g l y a t s i t e s near t h e edge of the plane than a t t h e c e n t e r . T h i s phenomenon h a s been s t u d i e d i n q u a n t i t a t i v e d e t a i l by Fink and E h r l i c h / 1 4 / f o r W and Re adatoms migrating on t h e channeled W(211) plane.
When two P t adatoms were deposited within t h e same channel of t h e Pt(311) plane, t h e adatoms m i g r a t e d f r e e l y u n t i l a c l o s e - p a c k e d d i m e r was f o r m e d . The a s s o c i a t i o n o f two atoms i n t o a dimer is shown i n Fig. 2. Once formed t h e dimer did not migrate or d i s s o c i a t e u n t i l the s u r f a c e was heated t o temperatures of 340 K o r a b o v e . A t t h e s e temperatures, displacements of t h e dimer were o c c a s i o n a l l y noted, but more o f t e n t h e dimer would simply be gone a f t e r t h e d i f f u s i o n i n t e r v a l i n d i c a t i n g t h a t t h e dimer had d i s s o c i a t e d and t h e s i n g l e atoms migrated o f f t h e plane. I t i s w o r t h n o t i n g t h a t a t t h e s e t e m p e r a t u r e s t h e r e was c o n s i d e r a b l e s u r f a c e atom rearrangement occurring on t h e s u r f a c e a s a whole. This observation means t h a t t h i s dimer c o n f i g u r a t i o n was more s t a b l e than t h e atoms a t t h e v a r i o u s plane edges. Additional atoms deposited i n a channel with a dimer a l r e a d y present would e v e n t u a l l y a s s o c i a t e with t h e dimer and form a chain s t r u c t u r e .
In previous FIM s t u d i e s of W and Re adatoms migrating on t h e channeled (211) plane of tungsten, an i n t e r a c t i o n between atoms i n a d j a c e n t rows was d i s c o v e r e d / 1 1 / . T h i s a t t r a c t i v e i n t e r a c t i o n c a u s e d t h e atoms' d i f f u s i o n t o be c o r r e l a t e d , and e x t e n s i v e a n a l y s i s of t h e d i f u s i o n d a t a y i e l d e d t h e i n t e r a t o m i c p o t e n t i a l s i n v o l v e d / 1 1 / . I n o u r measurements of P t d i f f u s i o n on P t ( 3 1 1 ) , t h i s type of c o r r e l a t e d motion was not observed. Although our d a t a a r e s t i l l somewhat l i m i t e d , i f t h e r e i s any a t t r a c t i v e i n t e r a c t i o n a t a l l , i t occurs a t d i s t a n c e s g r e a t e r than t h e " s t r a i g h t " o r qlstaggeredf* c o n f i g u r a t i o n s . On s e v e r a l o c c a s i o n s , however, we n o t e d a d i f f e r e n t t y p e of i n t e r a c t i o n . Atoms d i f f u s i n g i n a d j a c e n t channels were observed t o combine and form a close-packed dimer w i t h i n t h e same c h a n n e l . T h i s behavior is c u r r e n t l y being i n v e s t i g a t e d i n more d e t a i l .
Our observation t h a t missing rows of atoms i n t h e ( 3 1 1 ) p l a n e a r e produced by thermal annealing of P t suggested t h a t an a t t r a c t i v e i n t e r a c t i o n may e x i s t between atoms s e p a r a t e d by a c h a n n e l . To check t h i s p o s s i b i l i t y , we e x a m i n e d t h e d i f f u s i o n o f adatoms i n c h a n n e l s one channel a p a r t and looked f o r c o r r e l a t i o n . The measurements were performed a t a temperature of 239 K . Fig. 3 shows a p l o t of t h e f r e q u e n c y of occupation a t a given s e p a r a t i o n vs. t h e s e p a r a t i o n d i s t a n c e f o r 320 d i f f u s i o n i n t e r v a l s . Also shown i n Fig. 3 is t h e e x p e c t e d d i s t r i b u t i o n f o r n o n - i n t e r a c t i n g atoms. Although t h e number of observations a r e l i m i t e d , t h e d a t a suggest a weak a t t r a c t i o n a t s e p a r a t i o n s of 9.2 t o 12.48.
I n a l l of t h e above m e a s u r e m e n t s , t h e d i f f u s i n g atoms were d e p o s i t e d from an e x t e r n a l sublimation source. I t was a l s o p o s s i b l e t o f i e l d e v a p o r a t e t h e (31 1 ) p l a n e t o a v e r y s m a l l s i z e c o n s i s t i n g of 4-8 atoms and examine t h e d i f f u s i o n p r o p e r t i e s of t h e s e atoms. The r e s u l t s were q u i t e s t r i k i n g . No motion of s u r f a c e atoms g e n e r a t e d i n t h i s fashion was detected a t temperatures below 370 K. Again, i t is worth n o t i n g t h a t s i g n i f i c a n t d i f f u s i o n o c c u r s a t t h i s t e m p e r a t u r e on t h e s u r f a c e a s a whole. A t temperatures of 370 K and above, t h e small c l u s t e r s were observed t o r e a r r a n g e and form a long chain i n a s i n g l e channel. Fig. 4 shows an example of t h i s process. Between photographs t h e sample was heated t o 370 K f o r 2 min. This rearrangement p r o c e s s was n o t an i s o l a t e d o b s e r v a t i o n , i t happened e v e r y t i m e t h e e x p e r i m e n t was performed ( - 10 t i m e s ) , The observation again p o i n t s t o t h e very high s t a b i l i t y of t h e chain c o n f i g u r a t i o n s .
I V - DISCUSSION AND SUMMARY
S i n g l e P t atoms f r e e l y migrate along channels i n t h e (311) p l a n e a t t e m p e r a t u r e s w e l l below room t e m p e r a t u r e . I f another atom is encountered, a dimer is formed and t h e migration proceeds only i f t h e temperature is 340 K o r above. T h i s h i g h s t a b i l i t y r e s u l t s i n t h e formation of chains i f a number atoms a r e deposited i n a given (31 1 ) channel. Even atoms i n adjacent channels w i l l d i f f u s e c r o s s - c h a n n e l
Fig. 3 The number of o b s e r v a t i o n s of atoms a t a given s e p a r a t i o n vs. s e p a r a t i o n d i s t a n c e f o r two atoms migrating i n next-adjacent channels of t h e P t ( 3 1 1 ) p l a n e . The h i s t o g r a m i s t h e d a t a and t h e l i n e r p l o t i s t h e expected d i s t r i b u t i o n f o r non-interacting atoms.
Fig. 4. A c l u s t e r of atoms on t h e P t (311) plane ( a ) r e a r r a n g e s t o form a l o n g chain ( b ) upon annealing a t 370K f o r 2 min.
JOURNAL DE PHYSIQUE
i n o r d e r t o a c h i e v e t h i s s t a b l e c o n f i g u r a t i o n . (The a c t u a l mechanism f o r t h i s c r o s s - c h a n n e l d i f f u s i o n was n o t i d e n t i f i e d i n t h i s s t u d y , i t may i n v o l v e t h e same s o r t of exchange p r o c e s s proposed f o r c r o s s - c h a n n e l d i f f u s i o n on t h e P t ( l l 0 ) p l a n e / 1 3 / . ) The o b s e r v a t i o n of c h a i n s t r u c t u r e s w i t h m i s s i n g rows produced by t h e r m a l a n n e a l i n g a t t e m p e r a t u r e s above 400 s u g g e s t s t h a t 400 K is t h e minimum t e m p e r a t u r e r e q u i r e d f o r atoms t o d i f f u s e on t o t h e (311) p l a n e from s t e p s below o r i t i s t h e t e m p e r a t u r e a t w h i c h t h e t o p s u r f a c e l a y e r of t h e (311) p l a n e d i s s o l v e s . T h i s same t y p e of p r o c e s s could l e a d t o t h e flmissing-rowll r e c o n s t r u c t i o n s o b s e r v e d on m a c r o s c o p i c ( 1 1 0 ) s u r f a c e s . Atoms i n c h a n n e l s o f t h e (110) p l a n e t h e have t h e same close-packed s e p a r a t i o n (2.77 8 ) a s atoms i n t h e (311) p l a n e a n d i n d i v i d u a l a t o m s h a v e b e e n f o u n d t o d i f f u s e on t h e (110) p l a n e below room t e m p e r a t u r e /13/.
The h i g h m o b i l i t y o f s u r f a c e atoms i m p l i e s t h a t mass t r a n s f e r w i l l n o t l i m i t a r e c o n s t r u c t i o n p r o c e s s . A mechanism i n which atoms from e d g e s and k i n k s d i f f u s e o u t on t o t h e t e r r a c e s and a r e s t a b i l i z e d by t h e s h o r t - r a n g e d i n t e r a c t i o n s d i s c u s s e d h e r e is c l e a r l y p o s s i b l e . I n o r d e r t o f u r t h e r e x p l o r e t h i s p o s s i b i l i t y we a r e i n i t i a t i n g a s t u d y of t h e t y p e r e p o r t e d h e r e i n v o l v i n g Pt d i f f u s i o n on t h e P t ( l l 0 ) p l a n e .
ACKNOWLEDGMENTS
The t e c h n i c a l a s s i s t a n c e of L . M. Karkiewicz was g r e a t l y a p p r e c i a t e d . T h i s work was performed a t Sandia N a t i o n a l L a b o r a t o r i e s s u p p o r t e d by t h e U . S. Department of E n e r g y u n d e r c o n t r a c t #DE-ACO4-76DP00789 and i n p a r t by ONR under N00014-84-F- 0151.
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