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Adsorption studies on Ar+ -sputtered MoS2(0001)
865
Surface Science 178 (1986) 865-871 North-Holland, Amsterdam
ADSORPTION STUDIES ON Ar +-SPUTTERED MoS,(0001) M. ~~A~TOS
and CA. FAPAGEORGOPOULOS
Deprtment of Physics, University of loannina. GR-453 32 Ioannina, Greece Received
13 March
1986: accepted
for publication
25 May 1986
In this work we report a study of Ar+-sputtered MoS~(OOO1) before and after its interaction with gas and metal adsorbates. The study took place in an UHV system with LEED, AES, EELS and WF me~urements. The adsorbates were Oa and Fe. Argon-ion bomb~dment of Mom caused a displacement of S atoms in the top layer thus exposing areas of the MO underlayer on the surface. The interaction of deposited oxygen with the MO areas exposed on the surface of the Ar’+-sputtered MoSz(OOO1) was quite similar to that of O2 with MO metal substrate. The Fe adsorbate was deposited on Ar+-sputtered MoS~(OOO1) in two different ways. It formed uniform layers on the MO areas while on the rest area covered by S, it formed small particles.
Recently, adsorption studies of metals on insulating and semiconducting substrates have received considerable interest [1,2]. Metals on metallic surfaces form initially uniform layers [3] while on surfaces of insulators and many semiconductors they form small particles [1,2]. Small metal particles in the 10-100 A size range are very interesting in heterogeneous catalysis [4-61. A very good support of metal particles is the basal plane (0001) of MO& [7,8]. The MoS, crystal is a layer compound and belongs to the family of TX, transition-metal dichalcogenides. It is a semiconductor with important properties 19-111. Its chemical bonds are all saturated within each layer [13]. So its basal plane is extremely inert to gas adsorption [12]. The outermost layer of MoS,(OOOl) consists of S atoms on top with the MO layer underneath. Auger measurements by Feng and Chen 1131have suggested that Ar f bombardment of MoS,(OOOl) does not remove uniformly the S atoms from the top layer of MoS,. The S atoms are removed only from some areas where the MO underlayer forms islands. However, the Auger measurements alone are not adequate to support this conclusion. In this work we will study the Arf-sputtered MoS,(OOOl) before and after its interaction with gas and metal adsorbates. The study will take place in an UHV system with LEED, AES, EELS and WF measurements. The adsorbates will be 0, and Fe. ~39-6028/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental The experiment was performed in an ultrahigh-vacuum (UHV) system with pressure smaller than 1 X lo-” Pa. The system was equipped with standard surface analysis techniques, such as LEED. AES. EELS and work function (WF) measurements. A more detailed description of the first three techniques has been reported previously [7]. The retarded potential method was used for the WF changes, which were measured at l/7 of the saturation current of the I- V curves. Iron of high purity was evaporated from a Fe source which was heated with electron bombardment. The Fe dosages were calibrated with Auger measurements. Spectroscopically pure 0, was admitted to the system through a bakeable leak-valve. The MoS, sample (10 x 9 x 0.4) mm-’ was cleaved in air without causing any appreciable contamination on the surface [12]. In the UHV system the MO& sample was cleaned by heating with electron bombardment of the back side of the sample. The specimen temperature Th was measured with a NiCr/NiAl thermocouple which was calibrated with an infrared radiation thermometer above 900 K. The Ar-ion bombardment of MoS,(OOOl) was done with ion energy E = 5 kV, ion current I = 4-6 PA. and with an Ar partial pressure of 1.4 X 10P4 Pa in intervals of 10 min.
3. Results and discussion 3.1. Clean Ar +-sputtered MoS,(OOOl) After Ar’ bombardment, the characteristic hexagonal LEED pattern of the clean MoS,(OOOl) surface disappeared completely. This is attributed to the non-uniform removal of S and MO from the surface during the Ar + bombardment. The Auger spectrum of MoS, was measured before and after Ar ’ bombardment. After Ar’. bombardment the height of the MO peaks increased substantially while the S peak decreased. The peak height ratio S(151 eV)/Mo(186 eV) before the bombardment was about 2.5 times greater than that after the bombardment. These measurements are in agreement with those reported by Feng and Chen [13]. The decrease of the S peak and the increase of MO peaks after Ar+ bombardment is attributed to the removal of S atoms in the top layer thus exposing MO areas on the surface. The work function (WF) of clean MoS,(OOOl) was measured before and after Ar+ bombardment. It has been found that the WF values measured after different Art bombardments were 0.3 to 0.8 eV smaller than that (4.8 eV) measured before the bombardment. The WF decrease of MoS, after the Ar’ bombardment is in agreement with the removal of S from some areas of the
M. K~ma~at~s, CA
PaPageorgoPoui~
/ ~~orptio~
on Ar +-sputtered Mo~~~~l~
867
surface. In these areas the uncovered electropositive MO layer has an electronegative S underlayer resulting in the decrease of the WF [14,15]. Fig. 1 shows the electron energy-loss spectra of MoS,(OOOl) (a) before and (b) after Arf bombardment. The energy losses were measured at the center of each peak. Table 1 shows the loss peaks of MO&, before and after Ar+ bombardment, and those of clean M~lOO). From fig. 1 and table 1 we may observe the following: The 4.8 eV peak is very close to the 4.9 eV peak of clean MO and is attributed to MO areas exposed on the surface after the Ar+-sputtering of MoS,(OOOl). The 9.6 eV peak may be due to a shift of the 8.4 eV peak toward the 10.5 eV peak of pure MO. Similarly, the 14.0 eV peak can be due to a shift of the 13.2 eV peak of MoS,. The 20.4 eV peak is close to 21.6 eV of Mo(100). A relatively large peak appears at 28.2 eV after Ar+ bombardment. It can be attributed directly to MO which is exposed on the surface. It is believed that this peak has the same origin as the 31.8 eV peak of crystalline MO. The relative shift of the 28.2 eV peak as compared to 31.8 eV is probably due to binding of the MO atoms exposed on the surface to the S atoms of the underlayer. This peak is attributed to a transition from the p band of MO to the conduction band [7]. The 40.8 eV (2 X 20.4 eV) peak may
84
Ep =102. eV
(0)
42.6 _I
f
Fig. 1. Electron
energy-loss
spectrum
of MO& (a) before and (b) after A? keV, i = 6 p A, t = 10 min.)
bombardment.
(E = 5
Table 1 Energy losses (in eV) of MO& before and after Ar’ MO& ”
Ar +-MO& a’
bombardment
and of Mo(100) Mo( 100) h’
3.0 4.8 4.9 5.7 8.4 9.6 10.5 13.2 14.0 15.6 18.0 20.4 ‘1.7 23.4 28.2 31.8 33.0 36.4 37.1 38.1 40.8 42.6 48.0 a) Present results ” From ref. [20].
be ascribed to a double-plasmon excitation. The 48.0 eV peak is also due to Ar+-sputtered MoS,. It appears that most of the peaks of MoS, after the Ar’ bombardment may be considered as peaks of MO& which are shifted toward the peaks of metallic MO or as shifted peaks of metallic MO. This is consistent with removal of S atoms from the top layer thus exposing MO areas on the surface after Ar+ sputtering of MoS,(OOOl).
3.2. 0, on sputtered MoS,(OOOl) From previous work 1121 and our measurements it is well known that the basal plane of MoS, is extremefy inert to oxygen even with oxygen pressure close to atmospheric. Fig. 2 shows the variation of the Auger peak height of O(512 eV) and the work function changes with oxygen exposure on Ar ‘sputtered MoS,(OOOl). As is seen in fig. 2. both the Auger peak height of oxygen and the WF increase substantially up to 5 langmuir (L), while in the 5-25 L exposure range they form a plateau. Above 25 L of oxygen exposure
M. Kamaratos, C.A. Papageorgopoulos / Adsorption on Ar ‘-sputtered MoS2(OO01)
312
-
5 ?I0 a
-
lb1 .A&
A_.‘-.-.-
%80' w I6l Y 1,
.-W.F.
l-AA.PPH.
l?z
la)
w2-\ 9
869
A------.
A-. 0
I 25 o2 EXPOSURE
Fig. 2. Auger
peak height
(L)
of O(512 eV) and work function change Ar+-sputtered Mo~(OOOl), at 300 K.
versus
oxygen
exposure
on
both the peak height and WF increase again with the tendency to form a new plateau. Similar increases in Auger peak height of 0(512 eV) and WF have been reported in the case of oxygen adsorption on single crystals of MO [17,18]. We believe that, for the first 25 L, oxygen is adsorbed directly on MO in the areas where the S have been removed after the Arf bombardment of MoS,. However, the second small increase of the Auger peak height and WF value with oxygen adsorption above 25 L (fig. 2) has not been observed in the case of single crystals of MO. These increases may be attributed to a second adsorption state of 0, on the uncovered MO areas. Similar behaviour has been observed during adsorption of 0, on single crystals of Ni [19,20]. It has been reported that during 0, adsorption on single crystals of MO the WF decreases initially ( < 1 L) and subsequently increases. The initial decrease has been attributed to diffusion of oxygen atoms under the top layer of MO [17,18]. This initial decrease in WF has not been observed in the present work on MO&. This is due to the fact that there is already electronegative S under the layer of MO. 3.3. Fe deposition on Ar +-sputtered MoS, Reported results of Fe on the basal plane of MoS, [7] suggest that the deposited Fe forms initially 2D islands which with increasing Fe coverage change to 3D particles. The size and the number of these particles depend on the substrate temperature. When the Fe particles reach a certain size (- 12 A) they cause a displacement of the neighbouring S atoms of the surface.
Here we will study the deposition of Fe on Ar+-sputtered MoS,(OOOl) as compared to the results reported for Fe on MoS, [ll]. Fig. 3 shows the variation of the Auger peak heights of Fe(651 eV), S( 151 eV) and Mo(186 eV) and the WF change with time of Fe deposition on Ar+-sputtered MoS,(OOOl) at RT. We can make two interesting observations in fig. 3. First, the Auger peak height of Mo(186 eV) decreases very rapidly with Fe deposition. in contrast to the S( 151 eV) peak which decreases very little with Fe deposition. Whereas, in the case of Fe deposition on MoS,(OOOl) without sputtering, both Mo(186 eV) and S(151 eV) peaks were decreased almost similarly [7]; the decrease was relatively small. In the present work, the great difference of the decrease between the MO and S peak with Fe deposition is attributed to a different process of Fe deposition on uncovered MO areas and on S-covered areas of the surface. Iron is deposited uniformly on the uncovered MO areas while on the rest areas of the surface it forms islands and/or particles. This is in agreement with previous results of metals on MoS, [ll] and on metallic substrates [2,7]. The second interesting result in fig. 3 is the curve of WF change versus Fe deposition on Ar+-sputtering MoS,. This curve is quite similar to that of Fe on MoS, without sputtering [7]. This is understood. since, after the Art bombardment most of the S remains on the surface where Fe forms particles. The interpretation of the WF versus Fe deposition on MoS, has been reported [7]. The Auger peak height of Fe(651 eV), S(151 eV) and Mo(186 eV) and the WF changes were also measured versus heating of Ar+-sputtered MoS, which 15
0.0
2
k.
c 3
\.---l
32
B I-
lo-
5 w I
l
(’
’
A0
A-
-
o-
0 x00
--%.-i
Fe(651eV)Xlo
* >.a,:_:
Mo(136eV) X 5 s (151eV)X 1
.
_
.
0.5
-*-<
l
l
i
1
Y 2
. . .
.
0 Q
l
-1.0
Ub
0
Fig. 3. Auger
I
I
100
200 Fe DEPOSITION
I
I
300 TIME
LOO
500
(set)
peak height of Fe(651 eV). S(151 eV) and Mo(186 eV) and work function versus Fe deposition time on an Ar+-bombarded MO&, at 300 K.
change
M. Ka~~r~tos, C.A. PaFAgeorgoFo~os / Adrorpfion
on Ar ‘-sputtered ~0s~~~~)
871
was covered with Fe. The decrease of the Fe peak with heating is very similar to that of Fe on MoS, without sputtering [7]. The peak decreases in two steps. The first decrease is due to an agglomeration of the Fe particles to larger ones, and the second is due to the gradual desorption of Fe. The final increase of the MO peaks is relatively small. This may indicate that, after heating to 1200 K, a large amount of Fe remained on the MO areas as compared to that on the rest surface covered by S.
4. Summary The Ar-ion bombardment of MoS,(OOOl) causes a displacement of S atoms in the top layer thus exposing MO areas to the surface. MoS~(~Ol) is completely inert to oxygen adsorption. Nowever, after Ar+ sputtering oxygen is adsorbed on the surface and specifically on the areas where MO is exposed on the surface. The interaction of the initially deposited oxygen with the uncovered MO areas is quite similar to that of 0, with metal substrates. Fe is deposited on the Ar+-sputtered MoS,(OOOl) in two different ways. It forms uniform layers on the uncovered MO areas while on the rest area of MoS,(OOOl), covered by S, it forms small particles.
References [l] A. Grant Elliot, J. Vacuum Sci. Technol. 11 (1974) 826. [Z] C. Papageorgopoulos and H. Poppa, in: 4th Intern. Conf. on Solid Surfaces and 3rd European Conf. on Surface Science, Vol. 1, Cannes, 1980, p. 688. [3] W. Schlenk and E. Bauer, Surface Sci. 93 (1980) 9. [4] D.L. Doering, H. Poppa and J.T. Dickinson, J. Vacuum Sci. Technol. 17 (1980) 198. ].5] M. Boudart, Topics Appl. Phys. 4 (1975) 275. [6] S. Ladas, H. Poppa and M. Boudart, Surface Sci. 102 (1981) 151. [7] M. Kamaratos and C. Papageorgopoulos, Surface Sci. 160 (1985) 451. [8] C. Papageorgopoulos and M. Kamaratos, Surface Sci. 164 (1985) 353. [9] J.A. Wilson and AD. Yoffe, Advan. Phys. 18 (1969) 193. [lo] P.M. Williams and F.R. Shephered, J. Phys. C6 (1973) L36. [ll] H.P. Hughes and W.Y. Liang, J. Phys. C7 (1974) 1023. 1121 CA. Papageorgopoulos, Surface Sci. 75 (1978) 17. [13] H.C. Feng and J.M. Chen, J. Phys. C? (1974) L75. 1141 C.A. Papageorgopoulos and J.M. Chen, Surface Sci. 39 (1973) 283. [15] CA. Papageorgopoulos, Surface Sci. 104 (1981) 643. 1161 J.M. Wilson, Surface Sci. 57 (1976) 499. [17] B.M. Zykov, D.S. Ikortnikov and V.K. Tskhakaya, Soviet Phys-Solid State 1’7 (1975) 163. [18] R. Riwan, C. Guillot and J. Paigne, Surface Sci. 47 (1975) 183. [19] CA. Papageorgopoulos and J.M. Chen, Surface Sci. 52 (1975) 40, 52. [20] M. Kiskinova, L. Surnev and G. Bliinakov, Surface Sci. 104 (1981) 240.
Surface Science 178 (1986) 865-871 North-Holland, Amsterdam
ADSORPTION STUDIES ON Ar +-SPUTTERED MoS,(0001) M. ~~A~TOS
and CA. FAPAGEORGOPOULOS
Deprtment of Physics, University of loannina. GR-453 32 Ioannina, Greece Received
13 March
1986: accepted
for publication
25 May 1986
In this work we report a study of Ar+-sputtered MoS~(OOO1) before and after its interaction with gas and metal adsorbates. The study took place in an UHV system with LEED, AES, EELS and WF me~urements. The adsorbates were Oa and Fe. Argon-ion bomb~dment of Mom caused a displacement of S atoms in the top layer thus exposing areas of the MO underlayer on the surface. The interaction of deposited oxygen with the MO areas exposed on the surface of the Ar’+-sputtered MoSz(OOO1) was quite similar to that of O2 with MO metal substrate. The Fe adsorbate was deposited on Ar+-sputtered MoS~(OOO1) in two different ways. It formed uniform layers on the MO areas while on the rest area covered by S, it formed small particles.
Recently, adsorption studies of metals on insulating and semiconducting substrates have received considerable interest [1,2]. Metals on metallic surfaces form initially uniform layers [3] while on surfaces of insulators and many semiconductors they form small particles [1,2]. Small metal particles in the 10-100 A size range are very interesting in heterogeneous catalysis [4-61. A very good support of metal particles is the basal plane (0001) of MO& [7,8]. The MoS, crystal is a layer compound and belongs to the family of TX, transition-metal dichalcogenides. It is a semiconductor with important properties 19-111. Its chemical bonds are all saturated within each layer [13]. So its basal plane is extremely inert to gas adsorption [12]. The outermost layer of MoS,(OOOl) consists of S atoms on top with the MO layer underneath. Auger measurements by Feng and Chen 1131have suggested that Ar f bombardment of MoS,(OOOl) does not remove uniformly the S atoms from the top layer of MoS,. The S atoms are removed only from some areas where the MO underlayer forms islands. However, the Auger measurements alone are not adequate to support this conclusion. In this work we will study the Arf-sputtered MoS,(OOOl) before and after its interaction with gas and metal adsorbates. The study will take place in an UHV system with LEED, AES, EELS and WF measurements. The adsorbates will be 0, and Fe. ~39-6028/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental The experiment was performed in an ultrahigh-vacuum (UHV) system with pressure smaller than 1 X lo-” Pa. The system was equipped with standard surface analysis techniques, such as LEED. AES. EELS and work function (WF) measurements. A more detailed description of the first three techniques has been reported previously [7]. The retarded potential method was used for the WF changes, which were measured at l/7 of the saturation current of the I- V curves. Iron of high purity was evaporated from a Fe source which was heated with electron bombardment. The Fe dosages were calibrated with Auger measurements. Spectroscopically pure 0, was admitted to the system through a bakeable leak-valve. The MoS, sample (10 x 9 x 0.4) mm-’ was cleaved in air without causing any appreciable contamination on the surface [12]. In the UHV system the MO& sample was cleaned by heating with electron bombardment of the back side of the sample. The specimen temperature Th was measured with a NiCr/NiAl thermocouple which was calibrated with an infrared radiation thermometer above 900 K. The Ar-ion bombardment of MoS,(OOOl) was done with ion energy E = 5 kV, ion current I = 4-6 PA. and with an Ar partial pressure of 1.4 X 10P4 Pa in intervals of 10 min.
3. Results and discussion 3.1. Clean Ar +-sputtered MoS,(OOOl) After Ar’ bombardment, the characteristic hexagonal LEED pattern of the clean MoS,(OOOl) surface disappeared completely. This is attributed to the non-uniform removal of S and MO from the surface during the Ar + bombardment. The Auger spectrum of MoS, was measured before and after Ar ’ bombardment. After Ar’. bombardment the height of the MO peaks increased substantially while the S peak decreased. The peak height ratio S(151 eV)/Mo(186 eV) before the bombardment was about 2.5 times greater than that after the bombardment. These measurements are in agreement with those reported by Feng and Chen [13]. The decrease of the S peak and the increase of MO peaks after Ar+ bombardment is attributed to the removal of S atoms in the top layer thus exposing MO areas on the surface. The work function (WF) of clean MoS,(OOOl) was measured before and after Ar+ bombardment. It has been found that the WF values measured after different Art bombardments were 0.3 to 0.8 eV smaller than that (4.8 eV) measured before the bombardment. The WF decrease of MoS, after the Ar’ bombardment is in agreement with the removal of S from some areas of the
M. K~ma~at~s, CA
PaPageorgoPoui~
/ ~~orptio~
on Ar +-sputtered Mo~~~~l~
867
surface. In these areas the uncovered electropositive MO layer has an electronegative S underlayer resulting in the decrease of the WF [14,15]. Fig. 1 shows the electron energy-loss spectra of MoS,(OOOl) (a) before and (b) after Arf bombardment. The energy losses were measured at the center of each peak. Table 1 shows the loss peaks of MO&, before and after Ar+ bombardment, and those of clean M~lOO). From fig. 1 and table 1 we may observe the following: The 4.8 eV peak is very close to the 4.9 eV peak of clean MO and is attributed to MO areas exposed on the surface after the Ar+-sputtering of MoS,(OOOl). The 9.6 eV peak may be due to a shift of the 8.4 eV peak toward the 10.5 eV peak of pure MO. Similarly, the 14.0 eV peak can be due to a shift of the 13.2 eV peak of MoS,. The 20.4 eV peak is close to 21.6 eV of Mo(100). A relatively large peak appears at 28.2 eV after Ar+ bombardment. It can be attributed directly to MO which is exposed on the surface. It is believed that this peak has the same origin as the 31.8 eV peak of crystalline MO. The relative shift of the 28.2 eV peak as compared to 31.8 eV is probably due to binding of the MO atoms exposed on the surface to the S atoms of the underlayer. This peak is attributed to a transition from the p band of MO to the conduction band [7]. The 40.8 eV (2 X 20.4 eV) peak may
84
Ep =102. eV
(0)
42.6 _I
f
Fig. 1. Electron
energy-loss
spectrum
of MO& (a) before and (b) after A? keV, i = 6 p A, t = 10 min.)
bombardment.
(E = 5
Table 1 Energy losses (in eV) of MO& before and after Ar’ MO& ”
Ar +-MO& a’
bombardment
and of Mo(100) Mo( 100) h’
3.0 4.8 4.9 5.7 8.4 9.6 10.5 13.2 14.0 15.6 18.0 20.4 ‘1.7 23.4 28.2 31.8 33.0 36.4 37.1 38.1 40.8 42.6 48.0 a) Present results ” From ref. [20].
be ascribed to a double-plasmon excitation. The 48.0 eV peak is also due to Ar+-sputtered MoS,. It appears that most of the peaks of MoS, after the Ar’ bombardment may be considered as peaks of MO& which are shifted toward the peaks of metallic MO or as shifted peaks of metallic MO. This is consistent with removal of S atoms from the top layer thus exposing MO areas on the surface after Ar+ sputtering of MoS,(OOOl).
3.2. 0, on sputtered MoS,(OOOl) From previous work 1121 and our measurements it is well known that the basal plane of MoS, is extremefy inert to oxygen even with oxygen pressure close to atmospheric. Fig. 2 shows the variation of the Auger peak height of O(512 eV) and the work function changes with oxygen exposure on Ar ‘sputtered MoS,(OOOl). As is seen in fig. 2. both the Auger peak height of oxygen and the WF increase substantially up to 5 langmuir (L), while in the 5-25 L exposure range they form a plateau. Above 25 L of oxygen exposure
M. Kamaratos, C.A. Papageorgopoulos / Adsorption on Ar ‘-sputtered MoS2(OO01)
312
-
5 ?I0 a
-
lb1 .A&
A_.‘-.-.-
%80' w I6l Y 1,
.-W.F.
l-AA.PPH.
l?z
la)
w2-\ 9
869
A------.
A-. 0
I 25 o2 EXPOSURE
Fig. 2. Auger
peak height
(L)
of O(512 eV) and work function change Ar+-sputtered Mo~(OOOl), at 300 K.
versus
oxygen
exposure
on
both the peak height and WF increase again with the tendency to form a new plateau. Similar increases in Auger peak height of 0(512 eV) and WF have been reported in the case of oxygen adsorption on single crystals of MO [17,18]. We believe that, for the first 25 L, oxygen is adsorbed directly on MO in the areas where the S have been removed after the Arf bombardment of MoS,. However, the second small increase of the Auger peak height and WF value with oxygen adsorption above 25 L (fig. 2) has not been observed in the case of single crystals of MO. These increases may be attributed to a second adsorption state of 0, on the uncovered MO areas. Similar behaviour has been observed during adsorption of 0, on single crystals of Ni [19,20]. It has been reported that during 0, adsorption on single crystals of MO the WF decreases initially ( < 1 L) and subsequently increases. The initial decrease has been attributed to diffusion of oxygen atoms under the top layer of MO [17,18]. This initial decrease in WF has not been observed in the present work on MO&. This is due to the fact that there is already electronegative S under the layer of MO. 3.3. Fe deposition on Ar +-sputtered MoS, Reported results of Fe on the basal plane of MoS, [7] suggest that the deposited Fe forms initially 2D islands which with increasing Fe coverage change to 3D particles. The size and the number of these particles depend on the substrate temperature. When the Fe particles reach a certain size (- 12 A) they cause a displacement of the neighbouring S atoms of the surface.
Here we will study the deposition of Fe on Ar+-sputtered MoS,(OOOl) as compared to the results reported for Fe on MoS, [ll]. Fig. 3 shows the variation of the Auger peak heights of Fe(651 eV), S( 151 eV) and Mo(186 eV) and the WF change with time of Fe deposition on Ar+-sputtered MoS,(OOOl) at RT. We can make two interesting observations in fig. 3. First, the Auger peak height of Mo(186 eV) decreases very rapidly with Fe deposition. in contrast to the S( 151 eV) peak which decreases very little with Fe deposition. Whereas, in the case of Fe deposition on MoS,(OOOl) without sputtering, both Mo(186 eV) and S(151 eV) peaks were decreased almost similarly [7]; the decrease was relatively small. In the present work, the great difference of the decrease between the MO and S peak with Fe deposition is attributed to a different process of Fe deposition on uncovered MO areas and on S-covered areas of the surface. Iron is deposited uniformly on the uncovered MO areas while on the rest areas of the surface it forms islands and/or particles. This is in agreement with previous results of metals on MoS, [ll] and on metallic substrates [2,7]. The second interesting result in fig. 3 is the curve of WF change versus Fe deposition on Ar+-sputtering MoS,. This curve is quite similar to that of Fe on MoS, without sputtering [7]. This is understood. since, after the Art bombardment most of the S remains on the surface where Fe forms particles. The interpretation of the WF versus Fe deposition on MoS, has been reported [7]. The Auger peak height of Fe(651 eV), S(151 eV) and Mo(186 eV) and the WF changes were also measured versus heating of Ar+-sputtered MoS, which 15
0.0
2
k.
c 3
\.---l
32
B I-
lo-
5 w I
l
(’
’
A0
A-
-
o-
0 x00
--%.-i
Fe(651eV)Xlo
* >.a,:_:
Mo(136eV) X 5 s (151eV)X 1
.
_
.
0.5
-*-<
l
l
i
1
Y 2
. . .
.
0 Q
l
-1.0
Ub
0
Fig. 3. Auger
I
I
100
200 Fe DEPOSITION
I
I
300 TIME
LOO
500
(set)
peak height of Fe(651 eV). S(151 eV) and Mo(186 eV) and work function versus Fe deposition time on an Ar+-bombarded MO&, at 300 K.
change
M. Ka~~r~tos, C.A. PaFAgeorgoFo~os / Adrorpfion
on Ar ‘-sputtered ~0s~~~~)
871
was covered with Fe. The decrease of the Fe peak with heating is very similar to that of Fe on MoS, without sputtering [7]. The peak decreases in two steps. The first decrease is due to an agglomeration of the Fe particles to larger ones, and the second is due to the gradual desorption of Fe. The final increase of the MO peaks is relatively small. This may indicate that, after heating to 1200 K, a large amount of Fe remained on the MO areas as compared to that on the rest surface covered by S.
4. Summary The Ar-ion bombardment of MoS,(OOOl) causes a displacement of S atoms in the top layer thus exposing MO areas to the surface. MoS~(~Ol) is completely inert to oxygen adsorption. Nowever, after Ar+ sputtering oxygen is adsorbed on the surface and specifically on the areas where MO is exposed on the surface. The interaction of the initially deposited oxygen with the uncovered MO areas is quite similar to that of 0, with metal substrates. Fe is deposited on the Ar+-sputtered MoS,(OOOl) in two different ways. It forms uniform layers on the uncovered MO areas while on the rest area of MoS,(OOOl), covered by S, it forms small particles.
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