Adsorption studies of cobalt on tungsten (110), (100) and (111) planes by probe-hole field emission microscopy

surface science ELSEVIER

Applied Surface Science 94/95 (1996) 177-185

Adsorption studies of cobalt on tungsten (110), (100) and (111) planes by probe-hole field emission microscopy R.B. Sharma b, A.D. Ads0ol c, N. Pradeep a, D.S. Joag a.* a Center for Advanced Studies in Materials Science and Solid State Physics, Department of Physics, University ofPune, Pune-411 007, India b INS Shivaji, Lonavla-410 402, India c Y.M. College, Pune-411 038, India

Received 8 June 1995; accepted 13 October 1995

Abstract

The changes in the workfunction, A th, and in the Fowler-Nordheim pre-exponential factor, In B, have been investigated for cobalt adsorption on tungsten (110) (100) and (l 1l) planes by probe-hole field emission microscopy. The workfunction variations have been found to be strongly dependent on the temperature in the range 300-580 K on these planes. A~b decreases with submonolayer coverages of cobalt on the W(ll0) and W(100) planes. On the W ( l l l ) plane, Ark initially decreases and then exhibits a jump towards positive values at relatively higher doses. This abrupt increase in A~b and the corresponding zero crossover of In B have been attributed to surface reconstruction. The results are discussed in terms of the electronic structure and the degree of strain produced in the pseudomorphically grown cobalt layers on various crystallographic planes of tungsten.

1. Introduction

The study of adsorption of submonolayer and multilayer amounts of transition metals on refractory metal surfaces has been a subject of interest for many years [1-13]. In particular, the 3d transition metal adlayers on tungsten represent bimetallic catalyst systems demonstrating their diverse chemisorptive properties. The chemical properties of these supported catalysts are correlated with the geometrical and the electronic structure of the adlayer-substrate systems [14-17].

* Corresponding author. Fax: 91 212 353899; e-mail: [email protected].

Recently, we have reported investigations on iron adsorption on tungsten single crystal surfaces by probe-hole field emission microscopy [12]. This technique allows one to study the adlayer geometry and electronic properties as reflected in the workfunction variations with the adsorbate coverage on various single crystal surfaces in a single experiment. The present paper describes the study of cobalt (Co) adlayers on tungsten (W) by the probe-hole technique. To the best of our knowledge, no work on C o / W system by probe-hole field emission microscopy has been reported in the literature so far. It is surprising that in spite of the importance of this bimetallic catalyst system, little attention has been paid towards the field emission workfunction variation under various adsorption conditions.

0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0169-4332(95)00522-6

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R.B. Sharma et ul. / Applied Surface Science 94 / 95 (1996) 177-185

2. Experimental A probe-hole field emission microscope, the construction details of which have been described elsewhere [12], is used to carry out the adsorption studies in the present experiment. Clean vacuum conditions were achieved by a metal ultra high vacuum system comprising a diffusion pump and a sputter ion pump and were ensured by observing the stability of field emission current and field emission pattern. The estimated residual gas pressure in the probe-hole tube was well below 1 × 10-10 mbar. A high purity (99.99%, Goodfellow Metals, UK) cobalt adsorbate source in the form of a bead on a tungsten wire loop was resistively heated, in situ. An arbitrary dose (4.3 A, 60 s) of Co was deposited onto a [110]-oriented tungsten (W) field emitter tip held at two temperatures, 300 and 580 K. After depositing each dose, the adsorbate was equilibrated at 900 K for 45 s. Tip temperatures were calibrated by the usual four-probe method. The changes in the workfunction, a ~b, and in the pre-exponential factor, in B, were calculated from ten-point Fowler-Nordheim ( F - N ) plots using the relations:

crystal planes of tungsten, viz. W(110), W(100) and W ( l l 1). Fig. la shows that the average workfunction decreases with cobalt coverage in contrast to the previously reported results on iron/tungsten [12] system. The corresponding changes in the pre-exponential factor In B can be seen in Fig. lb. The average workfunction decreases rapidly in the low coverage region at the deposition temperature of 580 K. As the coverage approaches one monolayer, A ~b in the two cases attains almost identical value of ~ 0.60 eV. Fig. 2 shows a typical sequence of field emission micrographs recorded at different stages of adsorption of submonolayer amounts of cobalt deposited at 300 K and subsequently equilibrated at 900 K. Fig. 3a shows the A~b-0 curve corresponding to the W ( l l 0 ) plane. A~b is found to decrease in two steps and then to increase to - 1 . 1 eV. The corresponding changes in In B are shown in Fig. 3b. 0.15

t

(l)

CobaLt-Tungsten Cav = 4.54 eV

-0.15

\

-0.45

300 K

(2) '-0.7~

where m and m 0 are the slopes, A and A 0 are the intercepts of F - N plots corresponding to the adsorbate covered and the clean tip surfaces, and q50 is the workfunction of the clean surface. The estimated maximum error in the workfunction is not more than 0.05 eV. The reported values of A~b have been found to be highly reproducible. Cobalt coverages for the three planes have been quoted in terms of number of doses.

5 -

10

-

15

20

25

NO. OF DOSES

2.5

58O K

T

1.5

/

m

f

(bl

/

0.5

J 3. Results -OJ

'''' 0

The variations in the relative workfunction, Ark, and in the corresponding pre-exponential factor, In B, with the adsorbate coverage (0) of cobalt, have been obtained for the total tip surface as well as for single

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15

NO. OF DOSES

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25 •

Fig. 1. (a) Changes in the average workfunction and (b) changes in In B with cobalt doses at substrate deposition temperatures 300 and 580 K.

R.B. Sharma et al. / Applied Surjace Science 9 4 / 9 5 (1996) 177-185

179

Fig. 2. Sequence of field emission micrographs recorded from clean tungsten (a) to the state representing the saturation of the workfunction (f) at (a) 12.32 kV (b) 12.40 kV (c) 12.76 kV (d) 12.76 kV (e) 12.92 kV (f) 13.36 kV (deposition temperature 300 K).

180

R.B. Sharma et al. / Applied SurJace Science 9 4 / 9 5 (1996) 177-185

0-5

Co/W (110) ¢~110= 5.07 eV

4. Discussion

(a)

I -0.5°

The variations in the workfunction, A qS, and in the pre-exponential factor, In B, with the cobalt coverages on various single crystal planes of tungsten can be discussed in the light of geometrical and electronic structure of the adsurface. The effects of substrate deposition temperature and the surface texture of the individual plane on the adsorption behaviour are discussed below.

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~" -1.0 300 K J -1.5 -2.0

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=

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10

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15

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20

25

NO. OF DOSES

0.5 ColW(100) ~I00 = ~ 8 0 e V

3'

,f rn

(b)

0

300 K

-0.5
1

'~

''°''o''~ ~

,-o--.-o-~ ,---0.,~ 3 O0 K

~580K

-1.5

0L

-2.C _'

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10

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15

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20

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NO. OF DOSES

25

NO. OF DOSES

W BCC (110)

Co FCC (111)

(c)

T 2 ¢n

2.74A*.~ ~..-~--J.-~--JJ~

~L~ L

~_L~-2.5

.&.

cfio] l [OOf]

2.5 .~

©. co Fig. 3. (a) Changes in the W(110) workfunction, (b) variation of In B with cobalt doses at 300 and 580 K and (c) the model of the atomic arrangement of the W(110) plane and the Co(111) plane [17].

Figs. 4a,4b and 5a,5b show the A~b-0 and In B - O curves for W(100) and W(111) planes, respectively. In case of Co/W(100), the workfunction reduces by about 1.2 eV. For C o / W ( l l l ) , A~b suddenly increases to positive values above 4 doses after a small decrease at low coverage.

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NO. OF DOSES

W (100)// C o (100)

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(c)

3.16~ Fig. 4. (a) Changes in the W(100) workfunction, (b) variation of In B with cobalt doses at 300 and 580 K and (c) the model of the atomic arrangement of the W(100) plane and the Co(100) plane [17].

R.B. Sharma et al. / Applied Surface Science 9 4 / 9 5 (1996) 177-185

Co/W~

1.0

&/o

Table 1 Physical properties of cobalt and tungsten

~111 = 4.35eV

0.5

>

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Physical properties

Co

W

Crystal structure

fcc

bcc

Atomic radius (A) Ionization energy (eV) Electron affinity (eV) Electronegativity Electron configuration

1.25 7.86 0.662 1.8 3d74s 2

1.37 7.98 0.815 1.7 5d46s 2

Lattice constant (,~) Average workfunction (eV) Polarizability (in cc)

3.55 4.41 7.5X 10 -24

3.16 4.54 l l . I X I O -24

DOSES

1.0 0.5

I

I

t

t

8,

I

I

12

NO. OF DOSE5

W BCC(111)

16 ,~

Co FCC (111)

(c)

cobalt surfaces. Cobalt exists in a hexagonal close packed (hcp) phase below 720 K and face centered cubic (fcc) phase above this temperature. Also, if the size of the supported cobalt crystallites is small ( ~ 200 ,~,), it has the fcc structure even at low temperatures [ 18]. The strain in the pseudomorphic cobalt layers on W(110), W(100) and W(111) can be calculated from the lattice constants in the two directions on a particular single crystal plane. Table 2 shows the misfit parameters for Co (fcc) layers on tungsten. These Table 2 Misfit parameters Substrate

7~ C]I03

181

,~ ~, ~4-47

2.6 d ~-,4o 2.5A

Fig. 5. (a) Changes in the W ( I l l ) workfunction, (b) variation of in B with cobalt doses at 300 and 580 K and (c) the model of the atomic arrangement of the W(I 11) plane and the Co(11 i) plane.

Various physical properties of cobalt and tungsten are given in Table 1. 4.1. Geometrical consideration

Considering the atomic sizes of tungsten (atomic radius 1.37 ,~) and cobalt (atomic radius 1.25 ,~), the adsorbed layers of cobalt on tungsten single crystal surfaces are expected to exhibit a strained surface arrangement in comparison with the free standing

Adsorbate

Misfit (percentage)

Tungsten (bcc)

Cobalt (fcc)

(110)

(111)

a = 3.16 ~,

a = 2.51 ~,

+25.90

b = 2.74 ~, 0 = 109.47 ° nrnax = 1.22× 1019 atoms/m z

b = 2.51 ,~ 0 = 60 ° nmax = 1.82;< 1019 atoms/m 2

+09.16

(1oo)

(lOO)

a = 3.16 ~,

a = 2.51 ~.

+25.90

b = 3.16 ~. 0 = 90 ° nma x = 1.01 × 1019 atoms/m 2

b = 2.51 ,~, 0 = 90 ° nmax = 1.59× 1019 atoms/m 2

+25.90

(111)

(111)

a = 4.47 ,~

a = 2.51 /~

+43.80

b = 3.87 A 0 = 90 °

b = 2.51 ,~ 0 = 60 ° nmax = 1.82X 1019 atoms/m 2

+35.10

nma x = 5.78X

atoms/m 2

1018

--49.20

-57.40

-214.9

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R.B. Sharma et al. / Applied Surface Science 9 4 / 9 5 (1996) 177-185

misfit parameters indicate the degree of strain produced in the pseudomorphic layers of cobalt grown on the above mentioned single crystal planes. The superposed planes of (bcc) substrate and (fcc) adsorbate are determined by the best geometrical fit at the adsurface. As seen from Table 2, the strain in the cobalt overlayers on the various single crystal planes of tungsten decreases in the order W(111) > W(100) > W(110).

4.2. Electronic structure consideration Being a transition metal cobalt is expected to show different electronic behaviour under different electronic environments. When a cobalt atom is brought near the substrate W atoms, it tends to lose the electron to attain a more stable half filled shell configuration. The electron affinities of Co and W are 0.662 and 0.815 eV respectively, the electronegativities being nearly the same. Thus the reduction in the workfunction of Co covered W surfaces can be attributed to the electronic charge transfer from adsorbed Co atom to the W substrate, in case of close packed planes.

4.3. Average workfunction (ch~v) As is evident from Fig. l a, the reduction in the average workfunction suggests that cobalt is adsorbed mainly as electropositive layer on tungsten substrate. The average workfunction decreases and attains a constant value of about 0.6 eV as coverage approaches one monolayer. The decreasing trend is found to be almost the same for the deposition temperatures of 300 K and 580 K. The decrease in the workfunction is accompanied by a large increase in In B.

The overall reduction in the average workfunction of cobalt covered tungsten surface may be understood in terms of a significant reduction in the workfunction of high workfunction planes viz. W ( I I 0 ) and W(100). The large reduction in the workfunction can be understood in terms of the degree of strain produced on these single crystal planes due to the lattice mismatch (Table 2) as well as the electronic structure as discussed above. Cobalt adsorption on W(111) plane causes a sudden increase in the workfunction at higher numbers

of doses except for a small initial decrease. This increase in the workfunction after 4 doses is outweighed by a large decrease on the smoother planes, causing a reduction in the average workfunction. A comparison of this behaviour with that of Fe and Ni is interesting. The average workfunction of tungsten increases in case of F e / W [12] and N i / W [14] systems. Nickel also has fcc structure and has electron affinity 1.156 eV exceeding that of tungsten (0.815 eV). Nickel has a d s configuration and tries to acquire electrons to reach the full d shell state. Hence, nickel is expected to show an increase in the average workfunction due to an electronic charge transfer from the tungsten to the nickel adlayer. In this case also, strained overlayers are formed which cause significant changes in the workfunction of the adsorbed surfaces. It can be seen that the lattice mismatch in case of N i / W system is smaller than that for C o / W system. Iron, on the other hand, is closer to the half-filled electronic configuration and it has bcc structure which is homomorphic to tungsten, Lattice mismatch in this case is minimum as compared to C o / W and N i / W systems. Hence, the strain in the pseudomorphic layer of iron on tungsten is small. This is reflected in the smaller values of A4~ and In B as observed in the case of the F e / W system [12].

4.4. Co/W(110) As is evident from Fig. 3a, Aq5 is strongly temperature dependent on this close packed plane. At the deposition temperature of 300 K, the workfunction decreases monotonically and reaches a minimum value of A4, ~ - 1.5 eV at a coverage equivalent to 8 doses. Thereafter, it increases to reach a saturation value of ~ - 1 eV. This could be attributed to the shrinking of the W(110) plane as the other brighter areas extend into this region. The value of the workfunction minimum in our case (see Fig. 3a) is found to be different from the value obtained by Johnson et al. [17]. The value of the minimum in the latter case is about 40% less than our value. This discrepancy may be attributed to different experimental conditions, namely, different annealing temperatures, and the difference in size and shape of the substrate surfaces. In spite of this

R.B. Sharma et al. / Applied SurJhce Science 9 4 / 9 5 (1996) 177-185

discrepancy, the trend in the workfunction variation is remarkably similar. Similar results have also been reported for N i / W ( I I 0 ) by Kolaczkiewicz and Bauer [14] and for Cr/W(110) by Berlowitz and Shinn [16]. The large decrease in the workfunction of the W(110) plane can be understood on the basis of the strain in the pseudomorphic layer of cobalt along with the electronic factor discussed earlier. The geometry of the underlying W(110) plane is seen in Fig. 3c. The superposed planes in this case are C o ( I l l ) and W ( l l 0 ) as determined by the best possible geometrical fit of the adsorbate and the substrate. The misfit parameters in the two directions are found to be about + 26% and + 9%. This lattice mismatch is the cause for a strained overlayer of CO(111) on W(ll0). The calculated atomic density of Co/W(110) layer is about 49% less than Co(111). The decrease in the workfunction indicates electronic charge transfer from the adsorbate to the substrate. The zero coverage dipole moment using the Topping model is 1.53 X 10 -29 C m. It is also noteworthy that the decrease in the workfunction upon Co adsorption on W(110) plane is large ( ~ - 1.2 eV) in comparison to Fe adsorption (+0.25 eV). This decrease in the workfunction is accompanied by a large increase in the pre-exponential term (ln B = 3). The value of In B remains nearly unchanged in the case of iron overlayers [12], which are pseudomorphic. At 580 K, there is no significant variation in the values of A4' and In B with coverage suggesting rapid surface diffusion of cobalt adatoms across this plane.

4.5. Co/W(IO0) As seen in Fig. 4a, at the deposition temperature of 300 K, A 4' decreases linearly and attains a constant value of about 1.2 eV. This decrease in the workfunction is significantly larger for Co/W(100) as compared to the Fe/W(100) system [12]. Johnson et al. [17], reported that the A4', in case of Co/W(100), makes a brief excursion to the positive values and then decreases to - 0 . 2 eV. This behaviour has been explained in terms of surface reconstruction of the Co layer which transforms into the c(2 x 2) structure at a coverage between 1 and 2

183

monolayer (ML). We believe that in the present experimental conditions the stage beyond 1 ML could not be reached. A decrease in the workfunction is also observed for the Cu/W(100) system [1]. Berlowitz and Shinn [ 16] reported a similar behaviour for Cr/W(100). The linear decrease in the Co/W(100) workfunction (see Fig. 4a) may be attributed to the growth of islands of cobalt on the W(100) plane. As the coverage increases these islands coalesce to form a layer. The value of A 4' saturates above 5 doses. The degree of strain in the pseodomorphic layer in both directions is about +26%. Hence, the Co(100) lattice must be stretched in matching the substrate lattice spacings. This is also evident that the calculated atomic density of Co/W(100) layer is about 57% less than CO(100). In general the behaviour of Co on the W(100) is similar to that of Co on the W(110). The value of zero coverage dipole moment in this case is 1.1 X 10 -29 C m. Thus the strength of the dipole is larger on the smoother W ( l l 0 ) plane as expected. At 580 K, the variations of A4' and In B with 0, are small in comparison with those at 300 K. It is believed that at this deposition temperature, rapid surface diffusion across the plane does not allow cobalt atoms to accumulate on this plane.

4.6. Co/W(lll) In case of Co/W(111), A 4' initially decreases by 0.5 eV and then suddenly increases to positive values above a coverage equivalent to 4 doses (see Fig. 5a). At this coverage, the surface layer is thought to reconstruct as seen by the abrupt increase in the workfunction to a positive value accompanied by a zero crossover of In B [19]. The absence of a saturation in the workfunction can be attributed to the growth of 3D crystallites at coverages much above the first pseudomorphic layer. This conjecture is supported by the observation of 3D cluster growth in a separate experiment performed under identical conditions. A typical field emission micrograph recorded after deposition and subsequent equilibration of 50 doses of cobalt is shown in Fig. 6. The initial decrease in the workfunction at 300 and 580 K can be understood in terms of electronic

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R.B. Sharma et al. / Applied Surf'ace Science 9 4 / 9 5 (1996) 177-185

Fig. 6. Field emission micrograph recorded after 50 doses of cobalt at 9.9 kV (deposition temperature 300 K).

charge transfer from adatom to the substrate at low coverage. The zero coverage dipole moment has been calculated as 4.2 × 10 -30 C m. The W(111) plane has an open structure (Fig. 5c), and cobalt atoms fit very well into the valleys existing on this plane. In this position, the adatom has four nearest neighbours. These adatoms increase the density of sites in this plane to make it smooth. Thus an increase in the workfunction is expected based on these geometrical considerations. The adsorption behaviour on this plane is temperature independent as opposed to other smooth planes. Similar behaviour has been observed in case of gold adsorption on W(111) [3]. The strain in the pseudomorphic layer of cobalt on W ( l l l ) is about + 4 4 % and + 3 5 % along [llO]and [112]-directions. This layer is, therefore, highly strained.

5. Summary and conclusions (1) The average workfunction decreases monotonically with cobalt coverage in the submonolayer range. In the low coverage region, the workfunction decreases with a faster rate at 580 than at 300 K. (2) At 300 K, adsorption of cobalt on W(110) and W(100) planes causes a decrease in the workfunction as in case of metallic adsorption on high workfunction planes. Cobalt overlayers grow pseudomorphi-

cany on the W ( l l 0 ) and W(100) planes up to the first monolayer. The results in case of the W ( l l 0 ) plane are in overall agreement with those reported by Johnson et al. [17]. (3) At 580 K, the workfunction and In B change sign as the coverage increases, indicating the possibility of surface reconstruction of the adlayer covered W(110) and W(100) planes. (4) On the more open W ( I l I ) plane, the workfunction increases abruptly with cobalt coverage above 4 doses at 300 and 580 K. The corresponding zero crossovers of In B suggest reconstruction on this plane, too. (5) The probe-hole field emission microscopy studies show that cobalt is mainly adsorbed as an electropositive element causing a decrease in the workfunction of densely packed planes. This decrease in the workfunction may be understood in terms of the difference in the electron affinities of cobalt and tungsten. The large variations in the workfunction are attributed to the geometrical effects in terms of the strain in the cobalt overlayers on the tungsten substrate planes.

Acknowledgements D.S.J. thanks DAE, Government of India, for financial support. R.B.S. wishes to thank DRDO, Government of India, for granting study leave and also for financial support. Thanks are also due to Professor P.L. Kanitkar, Head, Department of Physics, for the facilities provided. Excellent glass blowing work by Mr. N.D. Mali is duly acknowledged.

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[15] P.J. Berlowitz and D.W. Goodman, Surf. Sci. 187 (1987) 463. [16] P.J. Berlowitz and N.D. Shinn, Surf. Sci. 209 (1989) 345. [17] B.G. Johnson, P.J. Berlowitz and D.W Goodman, Surf. Sci. 217 (1989) 13. [18] R.C. Reuel and C.H. Bartholomew, J. Catal. 85 (1984) 63. [19] C. Darmadhikari and R. Gomer, Surf. Sci. 143 (1984) 223.