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FIELD ION- AND ELECTRON-EMISSION
MEASUREMENTS ON SINGLE METAL CLUSTERS : Re ON W(110)
N. Ernst, G. Ehrlich
To cite this version:
N. Ernst, G. Ehrlich. FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS : Re ON W(110). Journal de Physique Colloques, 1986, 47 (C2), pp.C2-47-C2-51.
�10.1051/jphyscol:1986207�. �jpa-00225638�
JOURNAL DE PHYSIQUE
Colloque C2, supplement au n03, Tome 47, mars 1986 page c2-47
FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS:
Re ON ~ ( 1 1 0 ) *
N. ERNST and G. EHRLICH'
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 3 3 , F.R.G.
+ Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A.
A b s t r a c t
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Using a combined probe h o l e FIM-FEM, Fowler-Nordheim s t u d i e s have been c a r r i e d o u t on tungsten e m i t t e r s prepared by s p u t t e r i n a and f i e l d evapo- r a t i o n . The p r o j e c t i o n o f t h e i r i s probe hole onto a W(110) plane c o u l d be made as small as t h e u n i t c e l l o f W(110), mapped o u t by adatom d i f f u s i o n . A dependence o f (110) work f u n c t i o n values on plane s i z e as w e l l as on t i p r a - d i u s has been observed, a r i s i n g from v a r i a t i o n s i n t h e l o c a l f i e l d s t r e n g t h and from c o n t r i b u t i o n s t o t h e probe h o l e c u r r e n t from surrounding low work f u n c t i o n areas. A d i s t i n c t d i f f e r e n c e i n e l e c t r o n emission behaviour between Re5/W(110) and Rel/W(llO) was detected. Rel s l i g h t l y enhances t h e l o c a l f i e l d emission c u r r e n t ; Re5 causes a c u r r e n t decrease. Changes o f FN preexponential f a c t o r s and o f l o c a l work f u n c t i o n s caused by a d s o r p t i o n o f a s i n g l e Re atom and a Re c l u s t e r w i l l be presented.I
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I n t r o d u c t i o nLast year we described a combined f i e l d e l e c t r o n and f i e l d i o n microscope, w i t h which Fowler-Noraheim (FN) data on c l e a n W(110) as w e l l as on W(110) covered w i t h a s i n g l e Re adatom were obtained /l/. The l o c a l f i e l d e l e c t r o n emission c u r r e n t was found t o increase by about 20 % f o l l o w i n g the d e p o s i t i o n o f a s i n g l e r h e n i u r atom, i n agreement w i t h previous s t u d i e s /2,3/. The e f f e c t o f a s i n g l e adatom on f i e l d - e l e c t r o n emission has r e c e n t l y been discussed i n terms o f p h y s i c a l q u a n t i t i e s l i k e t h e l o c a l d e n s i t y o f e l e c t r o n i c s t a t e s as \.:ell as t h e l o c a l e l e c t r o n - p o t e n t i a l bar- r i e r /4/, and i t appears t h a t the nature o f t h e adspecies, t h a t i s chemical d i f - ferences o r c l u s t e r i n g of adatoms, should s i g n i f i c a n t l y i n f l u e n c e f i e l d - e m i s s i o n measurements, e s p e c i a l l y electron-energy d i s t r i b u t i o n s /4,5/. b!e have c a r r i e d o u t i n t r o d u c t o r y c u r r e n t - v o l t a g e measurements on a s i n g l e Re adatom and a Re pentamer (Re5) on W(110), which reveal marked d i f f e r e n c e s between t h e two. I n a d d i t i o n , f i e l d - e l e c t r o n emission was examined from clean Gl(110) surfaces prepared by neon s p u t t e r i n g and f i e l d evaporation t o assess the e f f e c t s o f plane s i z e as w e l l as t i p r a d i u s on our measurements.
I1
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Experimental MethodThe combined f i e l d i o n and f i e l d e l e c t r o n probe h o l e microscope i s equipped w i t h an
*
Work performed a t t h e U n i v e r s i t y o f I l l i n o i s under NSF Grant DFlR 82-01884 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986207JOURNAL DE PHYSIQUE
a d j u s t a b l e i r i s and a computer aided data a c q u i s i t i o n system /l/. S i n g l e p a r t i c l e de- t e c t i o n i s made p o s s i b l e through t h e use o f a chevron channel p l a t e assembly coupled t o a conductive phosphor screen. The p o s i t i o n and s i z e o f t h e probe hole, p r o j e c t e d on a f i e l d evaporated W(110) plane, was determined by superposing a helium f i e l d i o n micrograph on t h e electron-emission through t h e probe hole, as shown i n Fig. l ( a ) . The b r i g h t spots o f t h e FIM p a t t e r n i n F i g . l ( a ) , photographed w i t h t h e i r i s probe- h o l e completely open, i n d i c a t e t h e p o s i t i o n o f edge-atoms a t t h e steps o f t h e f i r s t t h r e e l a y e r s f o r a f i e l d evaporated (110) o r i e n t e d tungsten e m i t t e r . A nlesh o f t h e surface u n i t - c e l l s o f t h e f i r s t atomic l a y e r was d e r i v e d by r e c o r d i n g t h e b i n d i n g s i t e s f o r a s i n g l e Re adatom d i f f u s i n g over t h e surface. I t i s displayed on t h e FIM image t o g e t h e r w i t h t h e edge o f t h e f i e l d e l e c t r o n emission spot photographed w i t h small i r i s diameter, shown as a c i r c l e i n the center o f F i g . l ( a ) . I n t h i s p a r t i c u l a r case, t h e probe-hole p r o j e c t i o n covers approximately t h r e e u n i t - c e l l s . T h i s area c o r - responds t o a c i r c l e w i t h 2.7
8
r a d i u s . The probe-hole p o s i t i o n c o u l d a d d i t i o n a l l y be checked by measuring t h e l o c a l minima o f t h e helium f i e l d i o n and electron-emission c u r r e n t s . This f e a t u r e turned o u t t o be e s s e n t i a l f o r t h e performance o f e x p e r i ~ e n t s on t h e c l e a n W(110) surface.I 1 1
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Results and Discussion ( a ) Clean W ( Z 1 O )F i g . l ( a )
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Superposition of e l e c t r o n e v i s s i o n F i n . l ( b )-
Results o f FN measure- through probe-hole on a f i e l d - i o n micrograph. ments on c l e a n W(110) obtained a t The mesh on t h e W(110) plane was d e r i v e d from d i f f e r e n t f i e l d evaporation v o l t - s i n g l e Re adatom d i f f u s i o n and a l l o w s d i r e c t aaes ( t i p r a d i i ) and maximum (110) d e t e r p i n a t i o n o f t h e p r o j e c t e d area o f t h e i r i s plane s i z e s .probe-hole, shown as a c i r c l e i n t h e center.
F i e l d - e l e c t r o n emission o r i g i n a t i n g from W(110), as w e l l as from t h e t i p as a whole, was analysed according t o FN t h e o r y (see e.g. Ref. / 4 / ) . E x t r e r e l y sharp t i p s were prepared by neon s p u t t e r i n g and surfaces were c h a r a c t e r i z e d by FEV and FIM. Values
f o r t h e s l o p e s mave and mpro, obtained from FN p l o t s of t o t a l and probe-hole cur- r e n t s , a r e p l o t t e d i n Fig. l ( b ) a s a function of f i e l d evaporation voltage i . e . i n - creasing t i p r a d i u s . A s i g n i f i c a n t reduction of t h e v o l t a g e - t o - f i e l d s t r e n g t h con- version f a c t o r
B,,,,
derived from mave d a t a i n t h e usual way 141, i s apparent i n Fig. l ( b ) a s t h e t i p r a d i u s i n c r e a s e s . Data f o r t h e work f u n c t i o n of t h e (110) plane were c a l c u l a t e d from measured FN s l o p e s , assuming t h e value f b r t h e t i p a s a whole t o be 4 . 5 eV. In Fig. 2 ( a ) , values a r e p l o t t e d versus t h e d . c . f i e l d evaporation voltage required f o r t h e removal of one W(110) l a y e r within about f i v e seconds. The derived work function i n c r e a s e s from 4.9 eV t o 5.1 eV a s t h e FEV v o l t - ape i s r a i s e d from 5.7 kV t o 14.1 kV. This i s expected 15-101. As t h e t i p r a d i u s i n - c r e a s e s t h e c e n t r a l (110) plane i n c r e a s e s i n s i z e , and t h e c o n t r i b u t i o n of roupher low work f u n c t i o n regions t o t h e probe-hole c u r r e n t diminishes. However, a pro- nounced s c a t t e r i s a l s o e v i d e n t i n t h e d a t a , well beyond t h e e r r o r l i m i t s f o r i n d i - vidual measurements. This s c a t t e r becomes s t r o n g e r a s t h e t i p r a d i u s becomes l a r o e r .To Probe Hole
lpro ~ F ~ ~ , r l l o , R , ~ ~ l
...,.-.
5 7 9 11 13 15
F~eld Evaporation Voltage (kV) for One W(110) Layer/Five Seconds .,.,a,,
Fig. 2 ( a )
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Work f u n c t i o n d a t a of c l e a n , Fig. 2 ( b )-
Scherratic s e c t i o n through f i e l d evaporated W(110) obtained from FN t h e apex of a f i e l d e w i t t e r and quanti- a n a l y s i s of f i e l d - e l e c t r o n emission. t i e s determining t h e c u r r e n t a c t u a l l yd e t e c t e d i n a probe hole experiment.
As o u t l i n e d schematically i n Fig. 2 ( b ) , t h e probe-hole c u r r e n t ipro, measured during f i e l d - e l e c t r o n emission, e s p e c i a l l y from r e l a t i v e l y sharp t i p s , i s poverned by a t l e a s t f o u r physical q u a n t i t i e s , such a s t h e l o c a l f i e l d s t r e n g t h d i s t r i b u t i o n , t h e geometry o f t h e t i p apex and t h e t a n g e n t i a l v e l o c i t y of those e l e c t r o n s o r i g i n a t i n g from surroundin? low work function a r e a s 151. Computer simulations of t h e probe-hole c u r r e n t have been performed following e a r l i e r work by Liu /10/. The c a l c u l a t i o n s sugcest a s i g n i f i c a n t i n f l u e n c e not only of plane s i z e , b u t a l s o of t i p r a d i u s on FN d a t a . E f f o r t s were made i n t h e experiments t o c r e a t e (110) planes of maximum diame- t e r by means of FIM c o n t r o l l e d FEV, but plane s i z e s were not reproduced e x a c t l y , and i t i s presumably t h i s e f f e c t t h a t causes t h e s c a t t e r . For t h e p r e s e n t t h e important
C Z - 5 0 JOURNAL DE PHYSIQUE
conclusion i s t h a t absolute work f u n c t i o n values cannot be safely d e r i v e d from these experiments, b u t t h a t work f u n c t i o n changes due t o a d s o r p t i o n are a c c e s s i b l e .
( b ) Rhenium on W ( 1 1 O )
Fig. 3(a) shovrs a sequence o f f i e l d - i o n micronraphs w i t h f i v e Re adatoms i n i t i a l l y l o c a t e d on t h e f l a t (110) plane. A rhenium pentamer (Re5) was created a f t e r several h e a t i n g c y c l e s a t 400 K f o l l o w i n g a r e c e n t l y r e p o r t e d procedure /11/. As demonstrat- ed i n F i g . 3(a), t h e i o n beam e m i t t e d from adsorbed Re5 was a l i g n e d w i t h t h e i r i s probe-hole covering t h e center o f t h e rhenium c l u s t e r . A f t e r pumping o f f t h e helium gas, t h e f i e l d - e l e c t r o n c u r r e n t s through t h e probe h o l e as w e l l as from t h e t i p as a whole were recorded as a f u n c t i o n o f t i p v o l t a g e . F o l l o w i n g t h i s , FIM o f t h e sur- face showed t h e c l u s t e r i n t h e same p o s i t i o n and o r i e n t a t i o n as observed p r i o r t o electron-emission measurements. I n an attempt t o f i e l d desorb t h e rhenium pentamer, the t i p v o l t a g e was t h e n c a r e f u l l y r a i s e d w h i l e watching t h e FIM p a t t e r n . One spe- cies, appearinq as a dim image spot ( F i g . 3 ( a ) ) , remained a t t h e s u r f a c e a f t e r f i e l d desorption o f p a r t o f t h e c l u s t e r . Some o f the FN data, taken t h e r e a f t e r , are p l o t - ed i n F i g . 3(b) and displayed t h e same f e a t u r e s as measured a f t e r d e p o s i t i o n o f a s i n g l e rhenium atom /l/: A t a t i p v o l t a g e of 1 kV the a c t u a l probe-hole c u r r e n t ex-
A f t e r H e a t ~ n g
After FD. Re,
Fig. 3 ( a )
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FIN o f Re/!.!(llO). F i g . 3 ( b )-
Section o f t h e FN p l o t o f t h e f i e l d - emission c u r r e n t from W(110), covered w i t h a rhe- nium pentamer (lower d a t a ) , one rhenium atom (up- per data) and clean surface (data i n t h e m i d d l e ) . ceeds t h e emission-current measured f o r t h e c l e a n surface by about 20 %. The pres- ence o f a rhenium pentamer, however, causes a decrease o f t h e probe-hole c u r r e n t by about 20 % r e l a t i v e t o t h e emission from t h e clean surface. I t should be noted t h a t FN data, measured f o r t h e t i p as a whole, d i d n o t show any s i g n i f i c a n t d i f f e r e n c e before and a f t e r rhenium f i e l d desorption.Since the measured c u r r e n t - v o l t a g e curves obey t h e o r d i n a r y FN behaviour over sev- e r a l orders o f magnitude, we have s u b n i t t e d the present data t o an FN a n a l y s i s , even
Table 1
Summary FN a n a l y s i s : Re/W(110)
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A+= workfunction change, B=ln(bc,/bRe), change o f preexponential f a c t o r and ~ i ~ ~ ~ = probe-hole c u r r e n t change r e l a t i v e t o data obtained f o r clean t!(110).
though FN theory cannot r i g o r o u s l y account f o r electron-emission changes due t o a s i n g l e adatom /4,5/. As one v i g h t expect, l o c a l work f u n c t i o n changes a r e noderate, b u t they a r e d i f f e r e n t i n s i g n f o r t h e two adspecies. A wore dramatic effect i s n o t - ed i n t h e change i n t h e preexponential f a c t o r and t h e l o c a l f i e l d emission c u r r e n t . More data, such as electron-energy d i s t r i b u t i o n s , a r e r e q u i r e d b e f o r e any f i n a l con- c l u s i o n s can be drawn about t h e p h y s i c a l foundations o f these observations. However, t h e present measurements a l r e a d y serve t o demonstrate a q u i t e unusual e f f e c t o f rhenium c l u s t e r s on t h e emission p r o p e r t i e s o f t h e surface.
Acknowledgements
We a r e indebted t o many i n t h i s l a b o r a t o r y f o r t h e i r h e l p and advice d u r i n g t h i s i n - v e s t i g a t i o n . Special thanks a r e due t o Robert B. Bales, Thomas L. G i l b e r t s o n , Donald
J.
Holmgren, W i l l iam L. Lawrence, and R. Liu, now o f t h e B e l l Labs.REFERENCES
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