SULFUR SPREADING OVER A NICKEL FIELD EMITTER SURFACE

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SULFUR SPREADING OVER A NICKEL FIELD EMITTER SURFACE

M. Bžaszczyszynowa, R. Bžaszczyszyn, R. Måclewski

To cite this version:

M. Bžaszczyszynowa, R. Bžaszczyszyn, R. Måclewski. SULFUR SPREADING OVER A NICKEL FIELD EMITTER SURFACE. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-551-C6-556.

�10.1051/jphyscol:1987690�. �jpa-00226898�

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SULFUR SPREADING O V E R A NICKEL FIELD EMITTER S U R F A C E

M. Naszczyszynowa, R. Naszczyszyn and R. Meclewski

Institute of Experimental Physics, University of Wroclaw, ul. Cybulskiego 36, 50-205 Wrodaw, Poland

Abstract

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Sequences of field emission patterns corresponding to successive stages of the sulfur adsorption on the nickel emitter at 550, 700 and 1000 K are presented and discussed. It was found that: (i) the sulfur adsorbed at 700 and 1000 K is spread over the entire emitter tip contrary to the adsorption at 550 K

when accumulation of sulfur is observed in some regions;

(ii) sulfur clusters visible at 550 and 700 K can be partly res- ponsible for the observed poor reproducibility of the dependence of the average work function change vs the deposition time;

(iii) at 1000 K the penetration of sulfur atoms into the nickel surface occurs.

I

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INTRODUCTION

The poor reproducibility of the dependence of the average work function changes ( A ~ F - N ) vs the deposition time (t) for the S/Ni system was reported /I/. It has been suggested that the adsorption of sulfur on the nickel emitter at 550 and 700 K leads to a random distribution of molecules S x on the top of the c h e m i s ~ r b e d layer of sulfur. In the present paper this problem is discussed in terms of the analysis of the Field Emission (FE) patterns corresponding to the work function changes.

I1

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EXPERIMENTAL

The measurements were carried out using a sealed-off glass FE tube equipped with an ampule-type sulfur source. The tube was immersed in liquid nitrogen which secured a pressure below 10-9 Pa. The details of the experimental procedure were reported elsewhere / I / .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987690

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C6-552 JOURNAL DE PHYSIQUE

I11 - RESULTS AND SHORT DISCUSSION

Figs. 1 - 3 show FE patterns obtained after deposition of successive sulfur doses on the nickel emitter at 550, 700 and 1000 K, respective- ly. The patterns correspond to the b0F-N vs t curves in Fig. 3a of Ref. /I/.

Figs. la - Id show that the sulfur deposition at 550 K produces FE patterns which are a specific of the adsorbate uniformly distributed over the entire emitter surface but only for the coverage range corresponding to the increase of A ~ F - N . Further deposition leads to accumulation of the adsorbate on the exposed side of the emitter and to for- mation of the deposit boundary or edge (Fig. lh). The field emission from this area is markedly enhanced and, in contrast to the previous stages, the patterns become granular. This can be interpreted as a presence of bigger sulfur clusters on these regions. The size of the smal- lest visible cluster should be of th order of the resolution of the field emission microscope, i.e. 23

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/2/. The smaller clusters, with sizes less than 2 0

1,

are not visible on the screen but they should be expected even in the early stages of the adsorption. It was shown by Field Desorption Mass Spectroscopy that a sulfur multilayer on tun- gsten /3/ contains big and mobile molecules Sx (36x622). It is pro- bable that the granular structure of the S/Ni system at 550 K gives evidence of the presence of sulfur clusters containing such Sx molecules. As can be seen in Figs. lg

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lh the clusters are randomly distributed on a very small area comoarable to the size of the visible crvs- tal surface. Their positions on the crystal are different from spreading t o spreading. Such sulfur clusters adsorbed in this way may be res-

ponsible for the observed poor reproducibility of the A ~ F - N vs t curve /I/. This results from the fact that the total field emission current, depending exponentially on the work function and the applied voltage, is c llected from a relatively small surface area of the order of 106 %2. (The adsorbate concentration fluctuations are used for adsorption investigation by means of the Field Emission Flicker Noise method /e.g. 4/.1

The sulfur adsorption at 700 K produces FE patterns with a full symmetry for the entire used coverage range (Figs. 2a

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2h). The deposit edge and accumulations are not seen. However, for higher coverages (Figs. 29 and 2h) the patterns show a granular structure. This indica- tes that also at 700 K the sulfur clusters occur and migrate onto other regions of the substrate surface with a higher activation energy.

At 1000 K the FE patterns show also a full symmetry for the entire coverage range without any accumulations an edge. (Figs. 3a - 3h). However, in contrast to the adsorption at 700 and 550 K, the patterns lose their granular character and many high-index planes appear. Moreover, at 1000 K, as it is seen in Fig. 3a in Ref. /I/ the A ~ F - ~ vs t curve passes through a maximum higher than that at 550 and 700 K. It is there- fore possible, that the sulfur adsorption at 1000 K enables additional dissociation of the Sx molecules to occur and leads to rearrangement of certain regions and to the penetration of sulfur atoms into the substrate especially on the rough planes. (The latter effect was previous- ly observed for S/(001)Ni /5/.) As a consequence of this "smooth-out"

effect and the chemisorption of dissociated sulfur molecules, the additional increase of the work function change is observed.

As the maximum of the A ~ F - N vs t curves obtained at 550, 700 and 1000 K appears after a similar deposition time (Fig. 3a in Ref. / I / ) we conclude that the sticking coefficient for S on Ni does not change under these conditions and according to the result of /6/ it appro- aches unity.

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s t a n t e m i s s i o n c u r r e n t i = 1 . 5 x A , U = 7 5 1 0 V; ( b ) s u l f u r d e p o s i - t i o n t i m e , t = 20 m i n , U = 6 1 4 2 V ; ( c ) t = 4 0 m i n , U = 6 3 8 8 V ;

( d l t = 50 m i n , U = 6 4 9 4 V ; ( e ) t = 60 m i n , U = 6 5 8 0 V ; ( f ) t = 70 min U = 6 6 1 0 V ; (g) t = 80 m i n , U = 6 4 8 5 V ; ( h ) t = 1 0 0 m i n , U = 5 8 4 1 V . The p a t t e r n s c o r r e s p o n d t o t h e c o v e r a g e d e p e n d e n c e o f t h e work f u n c t i o n c h a n g e s shown i n F i g . 3 a o f R e f . /1/

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c u r v e 3 .

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C6-554 JOURNAL DE PHYSIQUE

I;"

h

Fig. 2

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Field emission patterns of sulfur on a thermally equilibrated nickel emitter at 700 K : ( a ) clean nickel, applied voltage for a con- stant emission current i = 1.5 x 10-7 A , U = 6140 V; ( b ) sulfur deposition time, t = 20 min, U = 6610 V; (c) t = 40 min, U = 6900 V;

(d) t = 50 min, U = 7006 V ; (e) t = 60 rnin, U = 7075 V; (f) t = 7 0 mi", U = 7075 V; ( g ) t = 80 min, U = 6212 V; (h) t = 140 min, U = 4877 V.

The patterns correspond to the coverage dependence of the work function changes shown in Fig. 3a of Ref. /1/

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curve 2.

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s i t i o n t i m e , t = 20 m i n , U = 7 2 8 2 V ; ( c ) t = 40 m i n , U = 7700 V ;

( d ) t = 50 m i n , U = 7 8 3 8 V ; ( e ) t = 6 0 m i n , U = 7 9 2 0 V ; ( f ) t = 70 min, U = 7 7 8 2 V ; ( g ) t = 8 0 m i n , U = 7 3 4 2 V ; ( h ) t = 90 m i n , U = 6 8 2 0 V ; ( i ) t = 1 4 0 m i n , U = 6 8 3 1 V ; (j) a f t e r h e a t i n g t h e n i c k e l t i p o f F i g .

3i

t o 1100 K f o r 6 m i n , U = 7 8 2 0 V ; ( k ) t h e s a n e f o r 1 2 m i n , U = 7 9 9 0 V (1) f o r 18 min, U = 7 9 9 0 V . The p a t t e r n s c o r r e s p o n d t o t h e c o v e r a g e

d e p e n d e n c e o f t h e work f u n c t i o n c h a n g e s shown i n F i g . 3a o f R e f . /1/

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c u r v e 1.

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C6-556 JOURNAL DE PHYSIQUE

F i g s . 3 j

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3 1 show t h e r e s u l t o f s u c c e s s i v e h e a t i n g a t 1 1 0 0 K i n t h e e n d s t a g e o f t h e s u l f u r a d s o r p t i o n a t 1 0 0 0 K i l l u s t r a t e d i n F i g . 3 i . I t i s s e e n i n F i g s . 3 f a n d 3 1 t h a t t h e t h e r m a l d e s o r p t i o n a t 1 1 0 0 X r e p r o d u c e s t h e f i e l d e m i s s i o n p a t t e r n s o b t a i n e d a f t e r s u l f u r d e p o s i t i o n a t 1.000 K , f o r t h e e o v e r a g e r a n g e c o r r e s p o n d i n g t o t h e d e c r e a s e o f t h e

A ~ F - N vs t d e p e n d e n c e . A s t h e r e p r o d u c t i o n o f t h e c l e a n N i s u r f a c e r e q u i r e s h i g h e r t e m p e r a t u r e ( a b o u t 1 6 0 0 K ) t h e d e s o r p t i o n e n e r g y o f s u l f u r f r o m n i c k e l s h o u l d d e c r e a s e w i t h i n c r e a s i n g s u l f u r c o v e r a g e . A C K N O W L E D G E M E N T

T h i s work was s u p p o r t e d by t h e P o l i s h Academy o f S c i e n c e s , The I n s t i t u - t e o f C a t a l y s i s a n d P h y s i c o - C h e m i s t r y o f S u r f a c e s , Krak6w; P r o j e c t No. 3 . 2 0 .

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/ 3 / D a v i s , T . R . , B e c h t o l d , E . a n d B l o c k , J . H . , S u r f a c e S c i . 4 5 ( 1 9 7 4 ) 5 8 5 ; C o c k e , O . L . , Abend, G . a n d B l o c k , J . H . , J. P h y s . ~ h = . 8 0 ( 1 9 7 6 ) 5 2 4 .

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