Adsorption studies of Ni on MoS2 and O2 on Ni-covered MoS2

Surface Science 164 (1985) 353 366 North-Holland, Amsterdam

ADSORPTION MoS 2

STUDIES

C. P A P A G E O R G O P O U L O S

353

O F Ni O N M o S 2 A N D O 2 O N N i - C O V E R E D

and M. K A M A R A T O S

Department of Physi(.v, Universi O, of loamuna, GR- 4,53 32 loannina, Greece

Received 26 November 1984; accepted for publication 5 August 1985

Deposition of Ni on the basal plane of MoS2 and the interaction of this system with subsequently adsorbed oxygen have been studied in an UHV system with LEED, AES, EELS and WF measurements. For substrate temperatures at or below room temperature the deposited Ni forms small islands, which change to 3D particles on heating. At elevated substrate temperature (450 K), Ni grows to 3D particles from the early stages of its deposition. The Ni adatoms do not interact with the surface S atoms of MoS~ as Fe does. The Ni particles thus remain clean on MoS> which is promising in heterogeneous catalysis. When the MoS, Ni system is exposed to oxygen the latter is adsorbed only on Ni. The interaction of the Ni adsorbate with oxygen is quite similar to that of oxygen with metallic Ni substrates.

1. Introduction Recent work [1] involved the study of the a d s o r p t i o n properties of Ee on the basal plane of MoS~ in the 80 1200 K t e m p e r a t u r e range. A c c o r d i n g to the results, the deposited Fe on MoS, formed small 3D particles, the size and n u m b e r of which d e p e n d e d on the substrate temperature. Beyond a certain average size the Fe particles began to act catalytically on all interaction of the s u b s e q u e n t l y deposited Fe with the S a to m s of the top layer of the Mo S,. Besides the interesting indication and conditions of catalytic action, the Fe particles on M o S , , after the F e - S interaction, would not be a p p r o p r i a t e to activate or e n h a n c e an interaction of gas adsorbates on the surface. Metallic elements which form islands on MoS~ and do not interact with substrate at o m s (Mo, S) would be more interesting in catalysis than Fe. Such an element may be Ni. As has been shown, nickel is a good catalyst for many interactions [2 5]. D e p o s i t i o n of Ni on mica formed small particles of a size which is interesting in h et er o g en eo u s catalysis [6]. T h e heat of f o r m a t i o n between S and Fe (22.7 c a l / m o l ) is greater than that between S and Ni (17.5 c a l / m o l ) . T h e r e f o r e the p r o b a b i l i t y of Ni interaction with S is smaller than that of Fe with S. In the h o p e that Ni forms clean small particles on M o S , , we have studied in this work the a d s o r p t i o n properties of Ni on the basal plane of M o S , , and the interaction of deposited Ni with subsequently a d s o r b e d oxygen. 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 ,~'~'Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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2. Experimental The e x p e r i m e n t was p e r f o r m e d in an ultra high vacuum ( U H V ) system v,i t h A u g e r electron s p e c t r o s c o p y (AES), electron energy loss s p e c t r o s c o p y (E[:,LS), low energy electron diffraction ( L E E D ) and work function m e a s u r e m e n t s (WF). Thc analysis of the A u g e r electrons took place with a semicylindrical m i r r o r energy analyzcr with resolution of d E / E ().3~. The L E E [ ) system consisted of four hemispherical grids with a solid angle of 104 °. The change in work function of the MoS~ surface during Ni or O d e p o s i t i o n was measured by a r e t a r d i n g potential m e t h o d with an electron gun and an error of + 0 . 0 2 eV. Nickel of high purity (99.9989[) was first melted in U I i V on a tungstcn filament and could be e v a p o r a t e d o n t o the s a m p l e afterwards. To i m p r o v e the u n i f o r m i t y of Ni deposition, a circular a p e r t u r c of 6 m m d i a m e t e r was placed in front of the s a m p l e and a b o u t 8 cm from the Ni source. The Ni was d e p o s i t e d on the MoS, surface in a sequence of similar dosages of 1 rain which wcre c a l i b r a t e d with A u g e r m e a s u r e m e n t s and were controlled by the tempera t u r e of the Ni source and the time of deposition. The MoS~ s a m p l e 9 x 9 × 0.4 m m -~ was cleaved m air and then cleaned in the U H V system by heating up to 1250 K. T h e heating was d o n e by electron b o m b a r d m e n t of the back side of the sample. The s a m p l e could be cooled to 80 K with liquid nitrogen. The s a m p l e t e m p e r a t u r e 1~ w'a~, measured with a N i C r / N i A I t h e r m o c o u p l e and it was c a l i b r a t e d with an infrared radiation t h e r m o m e t e r ['or t e m p e r a t u r e s between 500 and 1250 K. A r g o n ion b o m b a r d ment was inconvenient for cleaning of MoS~ because it caused irreparable d a m a g e to the surface structure [7].

3. Results and discussiun 3. ]. 14 l a g e r m e a , s u r o m e # t L s

Fig. 1 shows the variation of the Ni (61 eV), Mo (186 eV) and S (151 eV) A u g e r peak heights versus Ni dosages on the basal planc of M o S , . The specimen t e m p e r a t u r e T, during Ni d e p o s i t i o n was 300 K. As is sccn in this figure, the peak heights change linearly with increasing Ni coverage up to the fifteenth dosage, where the slopes of all three curves change. Initially. the Ni a t o m s are b o u n d directly to the substrate. The change of the slopes indicate that Ni a t o m s start to form a second layer [8]. However, the change in slope of the Ni curve is not as a b r u p t as in the case of characteristic l a y e r - b y - l a y e r growth of metals on metallic substrates [8]. Possibly, at T = 3 0 0 K thc Ni a d a t o m s form islands.

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Fig. 2 shows the variation of the Ni (61 eV) Auger peak height with Ni dosages on MoS, at 80. 300 and 450 K substrate temperatures. As is seen in this figure, the decrease in slope of the curves with increasing Ni deposition is greater at higher substrate temperatures. This behavior may be due to a decrease of the sticking coefficient of Ni on MoS, with increasing temperature a n d / o r the formation of islands and three-dimensional particles. Actually the curves, measured at room and higher temperature in fig. 2, are quite similar to those of Pd on oxygenated W(110) which have been considered as characteristic of island and particle formation [9]. The decrease in slope of these curves with increasing sample temperature has been attributed to the growth of particles which become fewer and larger at higher temperatures [1,9,10]. If we assume that, for the initial dosages, the Ni atoms were bound directly to the surface of MoS~ then the initial slopes of the curves in fig. 2 would be proportional to the sticking coefficient of Ni on MoS,. This means that the sticking coefficient of Ni on MoSt is greater at 80 K than at higher substrate temperatures. Fig. 3 shows the variation of the Ni (848 eV), Ni (61 eV), S (151 eV) and Mo (186 eV) Auger peak heights of Ni-covered MoS, surfaces with substrate temperature. The Ni was deposited at room temperature and corresponded to 45 dosages. As is seen in this figure, the Ni peaks decrease and the substrate (S, Mo) peaks increase analogously with increasing temperature. The Ni (848

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eV) peak decreases initially up to 600 K where it forms a plateau. At 800 K, it starts to decrease again. The initial decrease of the Ni (61 eV) peak is very drastic while the second decrease is not as obvious at that of Ni (848 eV). This difference is p r o b a b l y due to the difference in p e n e t r a t i o n depth of electrons between these two peaks [11]. The initial decrease of the Ni peaks, in the 300 600 K range is attributed to the formation of small Ni particles which coalesce into larger and fewer particles. At 800 K the Ni started to desorb. At 1200 K a small a m o u n t of Ni remained on the MoS,. The MoS 2 sample was not heated above 1200 K to avoid possible damage of the surface [7].

3.2. W o r k f u n c t i o n m e a s u r e m e n t s

Fig. 4 shows the work function change ,3~) versus dosage of Ni on MoS, at surface temperatures T, = 80, 300 and 450 K.

3.2.1. E f f e c t o f s u b s t r a t e t e m p e r a t u r e T on N i deposition

(a) T _< 300 K: W h e n the sample temperature was equal to or less than room temperature ( < 300 K), the W F initially decreased to a m i n i m u m value. The m i n i m a at 80 and 300 K correspond to work function changes A,f = - 0 . 2 6 and - 0 . 1 6 eV, respectively (lower frame of fig. 4). The initial decrease of the W F to the m i n i m u m value is not linear with Ni deposition. The deviation from linearity starts after the first or second dosage. The early deviation from

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linearity of the W F with d e p o s i t i o n of metals on surfaces is characteristic of island or particle f o r m a t i o n [8,98]. As seen in the insert of fig. 4, except for the initial one or two Ni dosages at T = 80 and 300 K. the W F change /t~b varics linearly' with the square root of the n u m b e r of Ni dosages ~/D until the W F m i n i m u m . The initial one or two Ni dosages c o r r e s p o n d to a d s o r p t i o n of isolated Ni a d a t o m s on the surface. A c c o r d i n g to Besocke's and W a g n e r ' s theory [12] the linear variation of J~b with { D suggests that the Ni a d a t o m s initially form t w o - d i m e n s i o n a l islands until the W F minimum. This theory, states that the n u m b e r of the edge a t o m s of the 2D islands is p r o p o r t i o n a l to ,H, d'0, where n, is the n u m b e r of islands and ~ the coverage. If w.'e assumc that, for constant subsirate t e m p e r a t u r e T and for early stages of Ni d e p o s i t i o m n, a n d the sticking coefficient of Ni oil MoS, remain a b o u t constant, then the n u m b e r of edge a t o m s is p r o p o r t i o n a l to {£7 and for the present case to { I ) . It is known that the dipole m o m e n t of the a t o m s inside the islands is smaller than that of the edge atonls [8,13]. ('onscquentl>., it may be assumed that for small islands, the c o n t r i b u t i o n of the a d a t o m s inside the islands to the surface d i p o l c m o m e n t is negligible as c o m p a r e d with that of edge atoms. Therefore the d i p o l e m o m e n t of the islands is a p p r o x i m a t e l y p r o p o r t i o n a l to the n u m b e r of edge a t o m s and c o n s e q u e n t l y to { I ) . Since the d i p o l e m o m e n t is p r o p o r t i o n a l to , ~ we may write that /t© o: {19. This is in agreement with the insert in fig. 4 and therefore will small island formation. Increasc of the Ni coverage on MoS~ a b o v e the W F m i n i m u m (lower frame of fig. 4) causes an increase in W F . A f t e r 30 dosages of Ni the W F remains a b o u t constant. F o r both 80 and 300 K the final W F of Ni on MoS~ is smallcr than that of a clean MoS~ s u b s t r a t e (4.80 ÷ 0.05 eV) [7]. On the other hand thc W F of bulk Ni (5.15 eV) [14] is greater than that of MoS,. "these results indicate that after the W F m i n i m u m Ni a t o m s tend to form a second layer on top of the 2[) islands. However, even at high coverage of Ni, at 80 and 300 K. the second layer on most of 2D islands was not cornplcted. ( ' o m p l e t i o n of thc second layer on every island wouM imply f o r m a t i o n of metallic Ni anct increase of the W F value between those of MoS, and bulk Ni. In other words, for s u b s t r a i e t e m p e r a t u r e s at or below RT and D < 4 0 . none or only some of the islands may change to 3D particles of Ni on MoS~. As seen in the lower frame of fig. 4, the W E m i n i m u m is low.er at 80 K than at 300 K. This is a t t r i b u t e d to the initial f o r m a t i o n of larger 2D islands. This is consistent with the higher sticking coefficicnt of Ni on MoS, at 80 than at 300 K, as was c o n c l u d e d from the A u g e r nleasurements (fig. 2). (b) T, = 450 K W h e n the t e m p e r a t u r c of the MoS, substrate was kept at 450 K, the W E started to increase from the beginning of Ni d e p o s i t i o n ( u p p c r frame of fig. 4). A f t e r five dosages of Ni the W F r e m a i n e d about c o n s t a n t at J ~ = 0 . 1 6 eV. This result suggests that, at 450 K substrate temperature, Ni started to form 3D particles from the early stages of deposition. A f t e r the fifth

359

C. Papageorgopoulos, M. K a m a r a t o s / ,,\'i on M o S , and O, on &loS, Ni

dosage, almost all the particles should be in the t h r e e - d i m e n s i o n a l form with a W E value close to that of bulk Ni (5.15 eV) [14]. The final W F of the surface was thus the average of the W F due to clean MoS~ (4.80 eV) [7] a n d that due to Ni particles. 3.2.2. Heating effect on Ni-co~ered M o S , Fig. 5 shows the variation of the W F with heating t e m p e r a t u r e of MoS~ which was covered with 40 dosages of Ni at T = 300 K and 450 K. In the case of Ni d e p o s i t i o n at 300 K, the W F increased with increasing t e m p e r a t u r e up to 450 K. In the 450 600 K t e m p e r a t u r e range the W F r e m a i n e d a b o u t constant. A f t e r 600 K the W F increased again to a m a x i m u m value with 'd,# = 0.29 eV at 800 K. A b o v e 800 K the W F started to decrease and at 1200 K it was 0.18 eV. The first W F increase in the 350 450 K t e m p e r a t u r e range is due to the f o r m a t i o n of 3D particles of Ni. The plateau in tile 4 5 0 - 6 0 0 K range c o r r e s p o n d s to a W F value which is very close to the final W F measured with Ni d e p o s i t i o n on MoS 2 at 450 K. The second increase of the W F in the 600 800 K range (fig. 5) may be a t t r i b u t e d to a partial diffusion of Ni into M o S , . This diffusion brings Ni under the more electronegative S layer with a c o n s e q u e n t decrease of the surface d i p o l e m o m e n t and therefore an increase in W F . A b o v e 800 K Ni starts to desorb from the surface resulting in a low, ering of W F . At 1200 K the W F remains higher than that of M o S , . This is due to an a m o u n t of Ni which remains diffused in M o S , , in a g r e e m e n t with the A u g e r m e a s u r e m e n t s (fig. 3). 3.3. E E L S measurements T h e energy loss s p e c t r u m of MoS, was affected s u b s t a n t i a l l y by' the Ni d e p o s i t i o n . Fig. 6 shows a series of energy loss spectra of MoS~ with different

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a m o u n t s of Ni on it. The spectra were taken with primary, electron beam energy Ep = 100 eV at 300 K specimen temperature. The energy, losses were m e a s u r e d at the center of each peak. The s p e c t r u m of clean MoS, showed the following peaks: 3.0, 5.7, 8.4, 13.2, 18.0 and 23.4 eV, which are indicated in fig. 6 with the letters A to F. T h e 3.0 eV peak did not change s u b s t a n t i a l l y with Ni deposition. Its small decrease is due to the masking of the Ni a d s o r b a t e . This peak is a t t r i b u t e d to an i n t e r b a n d transition in the d - b a n d of M o [1]. The presence of the S overlayer prevents a possible interaction of M o with the d e p o s i t e d Ni. Tile intensities of the 5.7, 8.4 and 18.0 eV peaks decreased g r a d u a l l y and at high coverages of Ni they, almost d i s a p p e a r e d . The 5.7 eV peak has been a t t r i b u t e d to an i n t e r b a n d transition in the p - b a n d of S [1,15]. Besides the masking of Ni, it is possible that the p - b a n d of S was c o m p l e t e d with electrons transfered from Ni a d a t o m s . This is in agreement with the W F lowering of MoS 2 with Ni d e p o s i t i o n (fig. 4). The 8.4 eV is a bulk p l a s m a peak and is due to o ,,r splitting of the c o n d u c t i o n b a n d of MoS, [1,16]. Except for the partial m a s k i n g of the surface with Ni, the drastic decrease of the 8.4 eV peak may also be a t t r i b u t e d to the charge transfer from Ni to the substrate. The charge transfer may cause a d e g e n e r a t i o n of the a vr b a n d of MoS~ and therefore the decrease of the

('. Papageorgopoulos, M. K a m a r a t o s / Ni on M o S , and O , on M o S , - N i

361

peak. Besides the decrease in intensity, the 8.4 eV peak was shifted to grater energies with increasing Ni coverage. Specifically, at 40 dosages of Ni the peak was shifted from 8.4 to 9.3 eV. This shift may be attributed to the change of electron density in the conduction band of MoS, caused by the charge transfer from Ni. The 18.0 eV peak is associated with transitions from the 3s level of S to the second conduction band in MoS~ [1]. Probably, its disappearance at high coverage of Ni is due to a disturbance of the 3s level of S by the charge transfer from Ni. The 13.2 eV peak is attributed to anisotropic effects of MoS, [1]. The energy of this peak was shifted towards greater energies (14.2 eV). However, its intensity was not affected substantially by Ni deposition. This indicates that a surface plasmon interpretation proposed by Wilson [17] is unlikely. The decrease of the 23.4 eV peak with Ni coverage is relatively small. This supports the bulk plasmon interpretation of this peak [1,16]. Next we study the heating effect on the energy loss peaks of Ni-covered MoS~. Fig. 7 shows the variation of the height and energy shift of the 9.3 eV peak of MoS, covered at RT by Ni (60 dosages) with substrate temperature T,. According to the discussion of the preceding paragraph, deposition of Ni at RT caused a decrease in height and an energy shift of the peak from 8.4 to 9.3 eV. As seen in fig. 7, an increase of T, above 350 K causes an increase of the peak height up to about 650 K. While in the 350-450 K range the energy is

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shifted from 9.3 to 8.4 eV which is the value of clean MoS~. The reverse shift from 8.4 to 9.3 eV during Ni d e p o s i t i o n on clean MoS, has been a t t r i b u t e d to charge transfer from the Ni to the substrate in the direct interaction of Ni with the surface of MoS~. The present shift of peak energy from 9.3 to g.4 eV and the initial increase of the peak height with increasing T , in the 350 450 K range (fig. 7), m a y be a t t r i b u t e d to a reduction of the direct interaction of Ni with the s u b s t r a t e and c o n s e q u e n t l y to a change of the islands to 31) particles of Ni. This is consistent with the Auger (fig. 3) and W F (fig. 5) m e a s u r e m e n t s . F o r s u b s t r a t e t e m p e r a t u r e s a b o v e 450 K the height of the peak continues to increase while the energy remains at about 8.4 cV. The increase of the peak height a b o v e 450 K is due to a g g l o m e r a t i o n of Ni particles to fewer and larger sized particles and consequently to the d e s o r p t i o n of N i from the surface. 3.4. L E E D

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D e p o s i t i o n of Ni on the basal plane of MoS, at t e m p e r a t u r e s of 80, 300 and 450 K caused a diffusion of integer order beams and an increase of the b a c k g r o u n d in the L E E D pattern, without p r o d u c i n g any new o r d e r e d structure. Except for a decrease in diffusion and b a c k g r o u n d the p a t t e r n did not show any extra feature with a gradual heating of the Ni-covered MoS 2 up to 1200 K. The observed variation of the L E E D p a t t e r n of Ni on MoS~ is characteristic of island a n d / o r particle f o r m a t i o n [1,18], in a g r e e m e n t with the A u g e r and W F measurements. 3.5. O.vj'gen on N i - c m , e r e d M o S ,

Fig. 8 shows the variation of the A u g e r peak height of O (512 eV) with oxygen exposure on Ni-covered MoS, with different Ni coverages. It is well k n o w n that the basal plane of MoS, is extremely inert to gas a d s o r p t i o n [1,11,19]. T h e r e f o r e the following discussion refers mainly to the interaction of oxygen with the Ni a d s o r b a t e on the surface of MoS~. Both curves in fig. 8, with different initial coverages of Ni. have the same characteristic shape as those of the A u g e r peak height of O (512 eV) versus oxygen a d s o r p t i o n on single crystals of Ni [20,21]. As shown in fig. 8, initially the sticking coefficient of oxygen is relatively great up to a b o u t 25 L of oxygen exposure. A b o v e 25 L, it levels off for a while and increases again. Based on the i n t e r p r e t a t i o n of similar results of 02 on single crystals of Ni [20,21]. we m a y c o n c l u d e that the oxygen is initially c h e m i s o r b e d on the islands a n d / o r particles of Ni. A b o v e 25 L a gradual f o r m a t i o n of N i O starts. The change in W F d u r i n g ( L a d s o r p t i o n on N i - c o v e r e d MoS, at RT is shown in fig. 9. T h e changes in W F were measured i m m e d i a t e l y after each A u g e r m e a s u r e m e n t (fig. 8). The curves in fig. 9 have the characteristics of those of ()~ a d s o r p t i o n on Ni single crystals [20,21]. The W F increases n t ally

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Fig. 8. A u g e r p e a k height of O (512 eV) versus o x y g e n e x p o s u r e o n N i - c o ~ c r e d M o S , at 3(10 K.

with increasing oxygen exposure up to about 25 L. Oxygen is initially chemisorbed on Ni and the difference in electronegativity of O (3.5) and Ni (1.80) explains the increase of the W F [20]. Above 25 L of oxygen exposure, the W F decreases and tends to reach the W F of NiO [20]. It is worth noting that Benndorf el al. [22] have studied 02 adsorption on thin films (100 A) of Ni and found that the results are quite similar to those of O, on single crystals of Ni. 0.2

I,

0.1

0.0

> @

• --

Ni

60

DOSAGES

• --

Ni

15

D O S A G E S

"dk.. A

o -0.1

-0.2

-

I

0.3

0

200

I

400 0 2 EXPOSURE

I

600 (L)

i

800

Fig. 9. W o r k f u n c t i o n c h a n g e of N i - c o v e r e d M o S 2 versus o x y g e n e x p o s u r e , at 300 K.

1300

364

(". Papa g<,or.gopo do~, M. l
15 d~

x

.....

~---

--X

/J

D X jx

_d

•~--,----4

L U

x y

10 32

©

oo

W I

j

~-

o

,--

°/

Ni ( 6 1 e V ) X 4

o-- Mo(186eV)X5

W5 \ 0~ Ld

©

•

x--

S (151eV)Xl

• -

0 2 (512 e V ) X l 0

\

•

250

500 ANNEALING

750 TEMPERATURE

1000

i

1250

(K)

Fig. 10. Auger peak height of Ni (61 cV), () (512 cV). S (151 ¢V) and Mo (186 c\'l ot' (Ni - (),)-co'~cred N'IoS~ versus annealing temperature (2 rain almcaling each [iillc).

Fig. 10 shows the variations of the Auger peaks of O, Ni. S and M o with the heating t e m p e r a t u r e of MoS~ covered with Ni and oxygen, at RT. Each heating took place for 2 rain. U p to 400 K the peaks did not change substantially. In the 400 650 K range, the heights of Ni and O, peaks decreased drastically, while the peaks of the s u b s t r a t c c o m p o n e n t s (S, Mo) increased c o r r e s p o n d ingly. A b o v e 650 K the changes of the peak heights were very small. After heating at 1200 K, small quantities of Ni and oxygen r e m a i n e d on the crystal. These results arm consistent with the idea that oxygen is a d s o r b e d only on Ni. Fig. 11 shows the W F change of MoS, covered with Ni and oxygen with heating t e m p e r a t u r e of MoS,. The initial coverages with Ni were 15 and 60 dosages, while the oxygen exposures were the same (1300 L) and occurred tit RT. As has been explained in the previous sections of this paper, the 15 dosages c o r r e s p o n d to 2 D islands while the 60 dosages c o r r e s p o n d to islands with more than one layer of Ni. The heating of the surface with 15 dosages of Ni caused a small initial increase of the W F w.'hich, a b o v e 750 K, r e m a i n e d c o n s t a n t close to the value of clean MoS~. W h e n the initial coverage of Ni was 60 dosages the W F decreased with increasing t e m p e r a t u r e up to 400 K where it f o r m e d a m i n i m u m with A@ = - 0 . 4 2 eV. A b o v e 400 K the W F increased and at 550 K it formed a m a x i m u m with A@ = + 0 . 1 2 eV. W i t h further t e m p e r a t u r e increase the W F decreased again. A n initial decrease of the W F in the 300 400 K t e m p e r a t u r e range has been

C. Papageorgopoulos. M. Kamarator / NI on M o S , and O, on M o S , - 3,'t

365

0.4 0.2

0.0

> -

-

O 2 EXPOSURE

-0.2

O ,<1

-0,4

%',.., °__/ ~°/

,x--

1300L

N~ 60 b O S A G E S Ni 15 D O S A G E S

- 0.6 I

250

500 ANNEALING

I

750 TEMPERATURE

I

1000 (K)

1250

Fig. 11. Work function change of (Ni + O e )-covered MoS~ versus a n n e a l i n g t e m p e r a l u r c (2 rain a n n e a l i n g each time).

also observed in the case of the oxidized surface of Ni single crystals at RT [231. It is believed that the a d s o r b e d oxygen at RT forms an oxide only with the top Ni layers of the islands. H e a t i n g of the substrate increases the m o b i l i t y of Ni a t o m s of the inner layers, which diffuse out and interact with oxygen atoms. M e a n w h i l e the islands change to 3D particles, which subsequently coalesce to fewer and larger sized particles. In the 400 450 K t e m p e r a t u r e range the coalescence of the oxidized Ni particles p r e d o m i n a t e s . A b o v e 550 K, Ni and oxygen start to d e s o r b from the surface. In the case of 15 dosages, the single layers of 2 D islands are oxidized during oxygen a d s o r p t i o n . The increase in t e m p e r a t u r e caused the f o r m a t i o n a n d coalescence of 3D particles. 4. S u m m a r y

This p a p e r concerns the study of Ni deposition on the basal p l a n e of the M o S 2 layer c o m p o u n d and the interaction of this surface system with subseq u e n t l y a d s o r b e d oxygen. The study was p e r f o r m e d in an U H V system with the use of L E E D , AES, E E L S and W F m e a s u r e m e n t s in the 80 1250 K s u b s t r a t e t e m p e r a t u r e range. Nickel was first melted in U H V on a tungsten filament a n d could be e v a p o r a t e d afterwards. The nickel dosages were c a l i b r a t e d with A u g e r measurements. The conclusions of this study are as follows. W h e n the M o S : surface was kept at r o o m t e m p e r a t u r e or lower, the deposited Ni formed initially 2D islands. W i t h increasing a m o u n t s of Ni the Ni a t o m s were mainly d e p o s i t e d oil top of the 2 D islands. However, the second layer was not c o m p l e t e d on most of the islands, at least for the m a x i m u m dosages used in this work. At the

366

( . Papa,'4cor~gopoul,,,. M. Kamarazo.~ / :".* olt .'~loN, at~U O ,

o#l

'~.1o£, ,'Vi

elevated substratc temperature, 450 K, nickel started from the early stages of its d e p o s i t i o n to f o r m 3 D p a r t i c l e s . H e a l i n g o f MoS~ c o x e r e d b> N i at o r bclo~v room substrate temperature caused a change of thc islands to 3D particles, w h i c h c o a l e s c e d to l a r g e r p a r t i c l e s u p o n f u r t h c r h e a t i n g . The Ni adatoms

d i d n o t i n t e r a c t ~ i i h t h e s u r f a c e S a t o m s o f MoS~ a s F c

does. This may be attributed

to t h c fact t h a t t h e h e a t o f f o r m a t i o n

a n d F e is h i g h e r t h a n t h a i b e t w e e n

remain

clean

on

the

MoS,

c o n c l u s i o n t h a t the M o S , neous calalysis. When

t h e MoS~

bclwccn S

S a n d N i . In o t h e r w o r d s , iho Ni p a r t i c l e ' ,

surface.

This

obser\ation

nla\

lead

N i s u r f a c e s y s t e m 111~.i\ b c p r o m i s i n g

Ni s u r f a c e s y s t c m

is c x p o s e d

to o x y g e n ,

us to i h c

in h e t e r o g e the oxygen

is

a d s o r b e d o n l y o n Ni i s l a n d s o r p a r t i c l e s . T h e a d s o r p t i o n c h a r a c t e r i s t i c s a r c q u i t e s i m i l a r to t h o s e o f o x y g e n o n Ni s i n g l e c r y s t a l s , f t i g h o x y g e n e x p o s u r e s r e s t l l t c d in t h c t ' o r m a t i o n o f N i O . F i l m i l y , at a s u b s t r a l c t c m p c r a t u r e s o m e o f t h e Ni a n d o x y g e n r e m a i n e d o n t h e surl'acc.

o f 1200 K

References

[1] [21 13] [4] 15] 16] [71 18] [9] (101 [11] ]12] [13] 114] [15J [16] [17] [18] [191 12(/] 121] ]22] [23]

M. Kamaratos and C. Papagcorgopoulos, Surface Sci.. l o b c published R.A. Zuhr and J.B. ltudson, J. Vacuum Sci. lcchnol. 14 (19771 431. ('. 13enndorf, ('. Noble. M. R[isenberg and F. Thicmc, Appl. Surface Sci. 11/12 il982i 803 I).W. Goodman. Surface Sci. 123 (1982t 1.679. M. Boudari. Topics Appl. Phys. 4 (19751 275. I).L. l)oering, PhD Thesis. Washington ,qt:.ltc Univcrsit\' I1981 ). C Papageorgopoulos, Surface Sci. 75 (1978) 17. W. Schlenk and 1-]. Bauer. Surface Sci. 93 (198111 9:94 (198(t) 528. ('. Papageorgopoulos and It. Poppa, in: 4th Intern. ('onf. on Solid Surfaces and 3rd t+,uropcan Conf. on Surface Science, Vol. 1, (anncs, 1980. p. 688. H. Poppa and F. Sofia. Surface Sci. 115 (19821 L105. ('.(_'. ('hang. in: ('haractcrization of Solid Surfaces. Ed. K. Larrabc (Plenum, Nc~ York. 19761. K. Besockc and tt. Wagner, Phys. Rc~. B8(197314597. P,. Smoluchowski, Phys. Re,,. 60 (19411 661. J. Holzl and F.K. Schulte. m: Springer Iracts in modern phy~,ics. Vol. 85 (Swinger. Berlin, 19791. P.M. Williams and F.R. Shephered, J. Phy~,. ('(Solid State Phys.)6(1973) L36. R.H. Williams and A.J. Mclivo~,, J. Phys. D(Appl. Phys.)4(1971)456. J.M. Wilson, Surface Sci. 57 (19761 499. M. Henzler, Appl. Surface Sci. 11/12 11982) 45(I. J.A. Wilson and A.I). Yoff¢. Advan. Phys. 18 (19691 193. C.A. Papageorgopoulos and J.M. ('lien, Surface Sci. 52 (19751 4(/, 52. M. Kiskinova. I. Surnev and G. Bilsnakov, Surface Sci. 104(1981)240. C. Benndorf, B. Egerl. C. N,Sble, H. Seidel and F. Thieme, J. Vacuum Sci. Technol. 17 (19801 115. C. Benndorf, B. figeri. ('. Noble, H. Seidel and F. Thicme. Surface Sci. 92 (19801 636.