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A STUDY OF NiCu+n AND NiAu+n (n < 10) CLUSTERS OBTAINED BY FIELD EVAPORATION
J. van de Walle, P. Joyes, P. Sudraud
To cite this version:
J. van de Walle, P. Joyes, P. Sudraud. A STUDY OF NiCu+n AND NiAu+n (n < 10) CLUSTERS
OBTAINED BY FIELD EVAPORATION. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-211-
C9-216. �10.1051/jphyscol:1984935�. �jpa-00224415�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n°12, Tome 45, décembre 1984 page C9-2H
A STUDY OF NiCu+ AND NiAu+ (n<lO) CLUSTERS OBTAINED BY FIELD EVAPORATION
J. Van de Walle, P. Joyes and P. Sudraud
Laboratoive de Physique des Solides , Univevsite Pavis-Sud, 9140S Crcsay Cedex, Prance
Résumé - Nous présentons les spectres de masse obtenus par évaporation de champ à partir de pointes liquides d'alliages ÇuNi et AuNi (méthode LMIS).
Ces spectres comportent des alternances très caractéristiques : les ions NiCu sont plus intenses pour n pair ; le même phénomène, quoique moins marqué, se produit pour les ions NiAu*. Ce résultat est interprété en utili- sant une règle de correspondance "Intensité -<-»• Stabilité" qui attribue les intensités plus élevées au fait que les stabilités des ions correspondants sont plus fortes. Nous montrons alors que les ions NiCun ou NiAun de n pair sont plus stables car le nickel se comporte comme un élément de configuration 3d24s donc de valence 1 en ce qui concerne la bande s. La configuration élec- tronique de la bande s est alors complète, donc plus stable, pour n pair.
Nous montrons aussi que ce comportement du nickel est cohérent avec d'autres études théoriques sur la structure électronique du nickel dans des systèmes nickel-cuivre.
Abstract - We present the mass spectra obtained by field evaporation from CuNi and AuNi alloys liquid tips (LMIS method). These spectra present a very speci- fic behaviour : NiCu ions are systematically more abundant for even n ; the same phenomenon though less stressed) occurs for NiAu ions. This result is interpreted by using a correspondance rule "Intensity *-*• Stability" according to which"the most stable clusters are the most abundant". Then, we show that NiCu and NiAu with even n are more stable because the nickel atom in the cluster behaves as a 3d24s element and thus gives one electron to the s band.
The electronic configuration of the s band is therefore complete, and more stable, when n is even. We also show that this behaviour of the nickel agrees with other theoretical studies on the electronic structure of nickel in various nickel-copper systems.
Laboratoire associe au CNRS.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984935
C9-2 12 JOURNAL DE PHYSIQUE
C l u s t e r s formed by atoms o f n i c k e l and a noble metal ( g e n e r a l l y copper) are an i n t e r e s t i n g s u b j e c t f o r a study, touching i n p a r t i c u l a r problems o f magnetism and catalyse.
When those c l u s t e r s a t t a i n e d a diameter o f about hundred
a ,
we were a b l e t o evidence a ferromagnetic behaviour /l/ f o r n i c k e l c o n c e n t r a t i o n where t h e massive a l l o y i s paramagnetic. This phenomenon was due, perhaps t o an e l e c t r o n i c e f f e c t o f a "surface magnetism" t y p e /23/, o r more simply, t o a surface segregation e f f e c t o f the n i c k e l , a phenomenon observed a l s o i n t h e Ni-A1 a l l o y s /4/. The e l e c t r o n i c s t r u c t u r e o f small c l u s t e r s Ni Cu (% 10 atoms) had been s t u d i e d from a t h e o r e t i c a lP n
p o i n t o f view, as w e l l as t h e e v o l u t i o n w i t h p o f t h e c a t a l y t i c p r o p e r t i 6 s o f these metal molecules a g a i n s t a hydrogenatic r e a c t i o n /5/.
Although t h i s concernes i n f i n i t e mediums, one may a l s o mention t h e existence o f a t h e o r e t i c a l study o f t h e e l e c t r o n i c s t r u c t u r e (magnetic moment, number o f "d"
e l e c t r o n s ) o f n i c k e l l a y e r s o f v a r i o u s t h i c k n e s s deposited on a copper surface /6,7/.
Several t e c h n i c a l means may be used f o r forming those f r e e s t a t e m e t a l l i c c l u s - t e r s which were studied, l a t e r on, by mass spectroscopy. We aim t o present i n t h i s paper t h e r e s u l t s o f one o f those mass spectroscopy studies, o r t o be more precise, t h e method o f f i e l d evaporation from l i q u i d t i p s (more commonly c a l l e d LMIS : L i q u i d Metal I o n Source) which, besides o t h e r uses o f f e r s t h e p o s s i b i l i t y o f o b t a i n i n g c l u s t e r s o f n i c k e l w i t h a noble metal.
Plan o f t h i s a r t i c l e
I n chapter I 1 we w i l l describe s u c c i n t l y t h e experimental methods and show, l a t e r on, how t h e i n t e n s i t i e s observed i n t h e mass spectra i n f o r m us on t h e e l e c - t r o n i c p r o p e r t i e s o f t h e c l u s t e r s themselves (chap. 111) ; th e r e s u l t s obtained w i l l be discussed i n chapter I V .
I 1
-
THE LMIS METHODThe l a s t t e n years have seen an i ~ p o r t a n t development i n t h e method o f analy- t i c a l i n t e r p r e t a t i o n o f t h e mass spectra o f m e t a l l i c i o n i s e d c l u s t e r s obtained by several methods (secondary i o n i c emission, spark source, l a s e r i r r a d i a t i o n ) /8,9/.
The LFlIS method appears as a complementary t e c h n i c a l means f o r t h e preceding methods, i t s p r i n c i p a l i n t e r e s t being t h e f a c t t h a t i t gives measurable i n t e n s i t i e s a t t a i n i n g very h i g h atom numbers. I n an experiment conducted t i l l about 1300 C > a tungsten t i p was wetted by melted metal, and i n f l u e n c e d by an e f f e c t on an extremely i n t e n s e
0
e l e c t r i c f i e l d (about one v o l t / A ) , t h e l i q u i d metal forming a T a y l o r cone a t t h e extreme t i p end.
This system, however, l a c k s s t a b i l i t y and t h e p o i n t o f t h e t i p i s a source o f an extremely i n t e n s e emission o f mono and polyatomic i o n s /10,11/, hence t h e reason f o r u s i n g t h i s method as an i o n source.
T i l l 1983, however, o n l y few metals had been examined, i n f a c t t h e d i f f i c u l t y was t o f i n d melted metals capable o f w e t t i n g t h e tungsten t i p , and o n l y t i n , g o l d and g a l l i u m c o u l d be analysed. A r e c e n t l y perfected /12/ t e c h n i c a l method opens the way f o r a g r e a t number o f experiences. Now many metals (copper, s i l v e r ) and a l l o y s are accessible, and t h e f o l l o w i n g chapter concerns one o f o f them : CuNi and AuNi.
% %
I11
-
EQUIVALENCE RULE "INTENSITY-STABILITYM.When t h e mass spectra presents a v e r y c l e a r behaviour ressembling t h e " t e e t h o f a saw" (as i t happens i n secondary i o n i c emission f o r Cun i o n s which a r e systema-
+
t i c a l l y more i n t e n s i v e f o r "nu odd) one i s l e a d t o formulate t h e p o s t u l a t e s o f a
" i n t e n s i t y - s t a b i l i t y " r u l e a l l o t i n g t h e systematic strenghtening o f t h e i n t e n s i t i e s t o a g r e a t e r s t a b i l i t y o f corresponding c l u s t e r s . #any authors have already claimed and used such a r u l e i n d i f f e r e n t works, wether t h e c l u s t e r s be produced i n a n e u t r a l (by supersonic j e t /13/), o r i n a i o n i s e d s t a t e /8,9,14/. On o t h e r hand, i n a secon- dary i o n emission f o r c l u s t e r o f two o r t h r e e atoms /15/, a beginning o f mathemati- c a l demonstration was a p p l i e d t o t h i s r u l e .
The emission mechanics o f polyatomic p a r t i c l e s i n LMIS a r e now being examined /16/. A study o f t h e p a t t e r n o f t h e energy d i s t r i b u t i o n r e v e a l s t h e existence o f two pinnacles : one correspondino t o p a r t i c l e s l e a v i n g t h e p i n n a c l e w i t h a p r a c t i - c a l l y nu1 energy, t h e o t h e r being remouved towards t h e losses o f energy corresponds, thus, t o p a r t i c l e s created a t about 10 angstr6ms i n f r o n t o f t h e t i p . The f i r s t o f t h e two pinnacles can be a t t r i b u t e d t o an e l e c t r i c a l f i e l d e x t r a c t i o n o f molecular i o n s from t h e l i q u i d t i p . The second i s due t o an e x p l o s i o n o f f l y i n g charged metastable d r o p l e t s confronted w i t h several causes o f excitment (emission o f a mono- mer o r a dimer, etc..). These mechanisms s h a l l be examined and discussed i n a paper t o be p r i n t e d l a t e r on /16/ and i t w i l l be shown t h a t i n b o t h cases one may expect t o see t h e " i n t e n s i t y - s t a b i l i t y " r u l e o f correspondance followed, i .e. t h a t t h e most s t a b l e molecular i o n s a r e those which are produced i n g r e a t e s t numbers.
I V
-
RESULTS AND DISCUSSION.I n F i g . 1 a r e shown t h e normalized i n t e n s i t i e s o f t h e ions NiCu e m i t t e d by
+
an a l l o y (NiCu)
-
a t 5 % Ni i n weight. The i n t e n s i t i e s o f CU; e m i t t e d by a pure copper t i p are a l s o r e p o r t e d i n t h e Fig. 1. F i g . 2 shows t h e two curves corresponding res- p e c t i v e l y t o N~AU; ( a l l o y w i t h 15 % Ni i n weight) and t h e AU;.The o s c i l l a t i o n s which appear a r e v e r y c l e a r f o r t h e c l u s t e r s based on copper, t h e y l o o k l i k e a succession o f f a l l s and landings f o r t h e c l u s t e r based on gold. We n o t i c e , moreover t h a t t h e r e e x i s t s a g r e a t s i m i l i t u d e between t h e two curves presen- t e d i n Fig, 1 ; th e same may be observed f o r t h e curves i n Fig. 2. I n an e f f o r t t o s i m p l i f y , we would be tempted t o say, t h a t i t l o o k s as i f n i c k e l i n t r o d u c e d i n t o a copper c l u s t e r (resp. g o l d ) was behaving l i k e a copper atom (resp. gold).
'29-214 JOURNAL DE PHYSIQUE
According t o t h e equivalence law mentioned p r e v i o u s l y , those o s c i l l a t i o n s
+ +
r e f l e c t t h e s t a b l e behaviour o f t h e NiCun o r NiAun c l u s t e r s , b u t t h e r e remains the problem concerning t h e s t a b i l i t y o f these c l u s t e r s f o r a "n" p a i r , which has t o be solved. T h i s p o i n t has already been discussed i n an a r t i c l e mentioning t h e r e s u l t s obtained i n a secondary i o n emission /17/, where t h e same e f f e c t was obser- ved.
L e t us memorise them b r i e f l y .
I n those c l u s t e r s t h e n i c k e l behaves l i k e an element o f valency "one" w i t h an e x t e r n a l c o n f i g u r a t i o n o f t h e type 3d94s. L e t us a t f i r s t , d i s c u s s t h i s p o i n t .
It i s known t h a t i n a Xenon / l 8 1 and Carbon / l 9 1 m a t r i x , an i s o l a t e d n i c k e l places i t s e l f i n a c o n f i g u r a t i o n o f t h e type 3d94s. On t h e o t h e r hand, excepting the t h e o r e t i c a l r e s u l t s obtained by Johnson e t a l . /20/, (method SCFX) on NiCu18 showing t h a t t h e c o n f i g u r a t i o n o f t h e n i c k e l atom i s o f t h e t y p e dlO, most o f t h e mathematical e v a l u a t i o n s made are i n accordance w i t h t h i s scheme.
Thus, I t o h /21/ shows t h a t i n a NiCu7 c l u s t e r , t h e average e l e c t r o n i c populations on n i c k e l a r e o f t h e t y p e 3d94s. T h i s tendency o f t h e n i c k e l t o adopt a valence "1"
has a l s o been corroborated by c a l c u l a t i o n s o f t h e N i 2 molecule /22/23/ and even o f t h e g r e a t e r c l u s t e r s /24/ where t h e average p o p u l a t i o n on each Ni was 3d94s, and by t h e f a c t t h a t t h e i o n , w i t h o u t valency e l e c t r o n s , i s unstable /25/. We may i n c l u d e i n t h i s d i s c u s s i o n t h e mathematical r e s u l t s obtained w i t h t h e e s t i m a t i o n o f t h e e l e c t r o n i c s t r u c t u r e s o f copper surfaces covered w i t h a n i c k e l monolayer. I n t h i s case t h e number o f "d" e l e c t r o n s obtained i s 9, 21 /6/ ; ( i n massive n i c k e l , t h e value i s 9, 4). T h i s r e s u l t does n o t c o n t r a d i c t our hypothesis, because t h e m a t e r i a l s i n v o l v e d a r e massive, w i t h a h i g h number o f copper atoms near t h e n i c k e l , and where, on t h e o t h e r hand, t h e copper i s a g i v e r o f "d" e l e c t r o n s because o f i t s tendancy t o e q u a l i z e t h e number o f "d" e l e c t r o n s per atom o f t h e whole sample and i n i t s own p o p u l a t i o n i n the metal i.e. 10 e l e c t r o n s . I n t h e c l u s t e r s s t u d i e d w i t h t h e n i c k e l conforming t o t h e c o n f i g u r a t i o n 3d9, i t gives 1 e l e c t r o n t o t h e valence band popula- t i o n c o n s t i t u t e d by t h e 4s f u n c t i o n o f each atom (Ni, Cu o r Au). Since each atom o f copper o r g o l d g i v e s a l s o an e l e c t r o n i n N~CU; ( o r N~AU;). One f i n d s valence "nu e l e c t r o n s and f o r "nu p a i r t h e c l u s t e r shows a complete e l e c t r o n i c s t r u c t u r e which i s more s t a b l e than w i t h an 'n" odd.
This argument o f a general c h a r a c t e r concerning t h e complete o r incomplete c h a r a c t e r o f t h e e l e c t r o n i c s t r u c t u r e was made more p r e c i s e (and confirmed) by a more accurate q u a n t i c a l chemistry e v a l u a t i o n , a l l o w i n g t o determine t h e band ener- qies f o r a l l the d i f f e r e n t forms o f c l u s t e r s ( w i t h a number o f atoms i n f e r i o r t o 9) /16/. This d i s c u s s i o n i s a r e p l y t o t h e q u e s t i o n o f t h e o r i g i n o f t h e s t a b i l i t y a l t e r n a t i o n s .
The-work presented i n t h i s a r t i c l e shows t h a t t h e LMIS method i s w e l l adapted f o r o b t a i n i n g i n f o r m a t i o n on t h e behaviour o f n i c k e l i n t h e noble metal c l u s t e r s .
C9-215
We aim t o extend t h i s work and i n c l u d e o t h e r elements i n our studies.
Figure 1
Normalized i n t e n s i t i e s 1 ( c u n N i + / ~ i + ) and I(cu~+~/cu+) obtained by LMIS.
F i g u r e 2
Normalized i n t e n s i t i e s l(AunNi+/Ni+) and I(Au~+~/Au+) obtained by LMIS.
JOURNAL DE PHYSIQUE
REFERENCES
1. CHIZHOV P.E., PETINOV V . I . , GRIGORrVSKI A.V., S o l i d S t a t e Com., 42 (1982) 327.
2. WUSAT J.P., BEAL-MONOD M.T., NEWNS D.H., SPANJAARD D., Phys. Rev. B11 (1975) 1437.
3. JOYES P., Phys. Rev. B28 (1983) 4006.
-
4. IGATA N., SAT0 S., SAWAI T., NISHIKAWA O., SHIBATA M., Proceedings o f t h e 29th I n t . F i e l d Emission Symposium, Gateberg (Sweden) 9/13 August 1982.
5. ITOH H., Proc. Japan, J. Appl. Phys. Suppl. 2, P t 2 (1974) 497.
6. WANG D.S., FREEMAN A.J., KRAKAUER H., Phys. Rev. B24 (1981) 1126.
-
7. TERSOFF J., FALICON L.M., Phys. Rev. B26 (1982) 6186.
-
8. JOYES P., J, Phys. Chem. Sol. 32 (1971) 1269.
-
9. MARTIN T.P., J. Chem. Phys. 80 (1984) 170.
-
10. GAUBI H., SUDRAUD P-, TENCE M., VAN DE WALLE J., P r o c e e d i n ~ s o f t h e 29th I n t . F i e l d Emission Symposium, G6teberq (Sweden), 9/13 August 1982.
11. SUDRAUD P., BENASSAYAG G., JOUFFREY B., CASTAING R., t o be published (1984).
12. BENASSAYAG G., SUDRAUD P., Proceedings o f t h e 30th I n t . F i e l d Emission Symposium, P h i l a d e l p h i a , 1/5 August 1983.
13. MUHLBACH J., SATTLER K., PFAN P., RECKNAGEL E., Phys. Rev. 87A (1982) 415.
14. STACE A.J., t o be published J. Am. Chem. Soc.
15. JOYES P., J. Physique - 45 (1984) C2-129.
16. BENASSAYAG G., SUDRAUD P., JOYES P., VAN DE UALLE J., CASTAING R., t o be published Phys. Rev.
17. JOYES P., LELEYTER M., J. Phys. 516 (1983) 671.
-
18. JACOBI K., SCHWEISSER D., KOLB D.Y., Chem. Phys. Let. 69 (1980) 113.
-
19. GIBBS R.A., llINOGRAD N., YOUG V.Y., J. Chem. Phys.
72
(1980) 4799.20. JOHNSON K.H., VVEDENSKY D.D., MESSMER R.P., Phys. Rev. 19 (1979) 1519.
-
21. ITOH H., J. Phys. F. 4 (1974) 1930.
22. ANDERSON A.B., 3 . Chem. Phys. 66 (1977) 5108.
-
23. MELIUS C.F., MOSKObIITZ J.V., MORTOLA A.P., BAILLIE M.B., RATNER M.A., Surf. Sci.
59 (1976) 279.
-
24. MELIUS C.F., UPTON T.H., W.A. Goddard 111, S o l i d S t a t e Com. 28 (1978) 501.
25. TOMANEK D., YUKHERJEE S., BENNEMANN K.G., Phys. Rev. B.28 (1983) 665.
