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Oxygen adsorption on clean Mo (100) surfaces
Surface Science 47 (197.5) 183-190 0 North-Holland Publishing Company
OXYGEN ADSORPTION
ON CLEAN MO (100) SURFACES
R. RIWAN, C. GUILLOT
and J. PAIGNE
Service de Physique Atomique, 91190 Gifsur- Yvette, France
Centre d’Etudes Nuclhires
de Saclay, B.P. No. 2,
Oxygen adsorption on clean MO(100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2X 2) phase growing simultaneously with a (6X2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3x1) structure together with a p (1X1) structure, probably due to an oxide phase. Even in the low temperature range (370-500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4X4) structure which is followed by a (J5XdS)-R 26” 33 structure. The same occurs when the inital c (2X 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2X2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and MO.
1. Introduction Despite
the analogy
in the electronic
structure
and crystallographic
parameters
of
tungsten and molybdenum, significant differences have been observed in the carbon monoxide adsorption on the (100) face of these metals [ 1,2]. Therefore we extended our studies on this face to the adsorption of oxygen. In their LEED studies various authors [3-S] reported
that no new structures appeared at room temperature. Dooley and Haas f6], however, observed a ~(2x2) and ~(4x4) structure. Adsorption at high temperature (1020 K) was characterized by a successionofc(2X2j,p(lXl),(JjXJS)-R26”33,p(2X2),p(lXl)structuresat increasing exposures. Heating the saturation phase in vacua from 1070 to 1630 K leads to the following sequences: p(lXl),p(2X2),(JSXJJ),p(lXl),c(2X2),clean
[41,
R. Riwan et al.lOxyg,gen adsorption
184
(/5X&),
on clean MO (100) surfaces
(2X2), (2X1), c(4X4), clean [7]
A preliminary study of the effect of oxygen adsorption on the photoemission of MO(100) [8] has shown that besides states very sensitive to the surface condition, there exists a peak centered at about 6 eV below the Fermi level, which grows with oxygen adsorption. In energy loss spectroscopy (ELS) experiments Ballu and Lecante [9] have observed the appearance of new structures due to oxygen adsorption. They correspond to two energy losses centered at 7 eV and 5 eV.
2. Experimental The experimental details including the cleaning procedure were published in a previous paper [ 11. Energy loss spectroscopy measurements are performed with the three grid retarding field analyzer used for Auger spectroscopy [ 181. The results are compared with those obtained with a high resolution analyzer [9].
3. Results First the coverage and kinetics of adsorption at room temperature as by AES are described. Next LEED patterns resulting from adsorption at perature are presented, followed by LEED patterns obtained at elevated tures. Finally the effect of oxygen adsorption on the work function and ergy loss spectra are dealt with.
determined room temtemperaon the en-
3.1. AES results Determination of the oxygen coverage is rather difficult. As with tungsten, flash desorption is not well suited owing to desorption of reactive atomic oxygen and volatile oxides that can dissociate. The oxygen coverage 8 can be monitored by AES [ 111. Usually relative values can be obtained only. We determine the absolute value of f3 by comparing the oxygen Auger peak measured after exposure to oxygen with the corresponding peak observed after CO adsorption. A similar procedure was used by Viswanath and Schmidt [ 121. The oxygen peak for the c (2X2) 0 structure is twice that for the c (2X2) CO structure. Since there is no agreement whether CO adsorbed in the /I state is dissociated or not [ 131, we compared the carbon signal measured with CO adsorption with the value found for carbon layers, particularly with layers exhibiting the c (2 X 2) C structure. When the carbon Auger signal is assumed to be not affected by the presence of the neighbouring oxygen atom [ 141, the carbon content of the carbon c (2 X2) structure is twice that of the carbon monoxide c (2 X2) structure.
R. Riwan et aLlOxygen adsorption on clean MO(100) surfaces
185
COVERAGE
0
2
L
6
6
10
12
EXPOSURE
-
IL IL)
Fig. 1. Coverage versus exposure for room temperature adsorption.
Fig. 1 shows the intensity of the oxygen Auger peak during exposure to oxygen. Up to 8 = $(5X1O14 0 atom cmP2) a linear increase is observed. The saturation value is observed for an exposure of 12-l 3 Langmuir (1 L = lop6 torr set); the oxygen pressures were between 2X10Pg and 10e8 torr. 3.2. LEED results
Fig. 2. LEED patterns: (a) c (2~
2), 40
eV; (b) (6x2), 40 eV; (c) microfacets; (d) p (3X 1).
186
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
Fig. 3. LEED patterns: (a) disordered c (4X4); (b) ordered c (4X4); (c) disordered (fiXfi); (d) ordered (6X A).
At room temperature exposures up to 1 L lead to a rather diffuse c (2X 2) pattern. The broadening affects all integral order reflections. After 1 L exposure the intensity of the half order reflections attains a maximum. Simultaneously a weak (6X2) structure is observed at exposures of about 0.5 L (figs. 2a, 2b). After exposures from 1 to 3.5 L streaks running parallel to the [loo] directions, through the integral order reflections and (4 3) reflections, are observed (fig. 2~). At about 2.5 L the fractional order reflections and the lines passing through them begin to weaken. They disappear completely at 3 L. At this stage the integral streaks are slightly shortened. Exposures from 3.5 to 13 L cause the streaks to disappear progressively. A p (3X1) structure is developed (fig. 2d), which is transformed into a ~(1x1) structure at saturation. When adsorption is carried out between 3.50 and 500 K, instead of the c (2X2) a c (4X4) structure is observed, followed by a (6X &)-R 26”33 structure. These patterns are observed at temperatures near 350 K and higher. The structures are highly disordered, with a degree of order increasing with temperature. The order remains, however, far from the perfect order obtained by annealing in vacua at high temperatures (fig. 3). The disorder of the c (4X4) is revealed by
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
187
/<, 0 -0L
2
L e
6
8
10 12 EXPOSURE
IL (L)
Fig. 4. Work function change versus exposure for room temperature adsorption.
lines parallel to the [ 111, [ 1I] directions passing through the ( $ &), ( $ i), ( 3 3) reflections. At 500 K we observe the appearance of the c (4X4) structure with disorder lines after exposures of 0.1 L. The structure has its maximum intensity at 0.25 L; it begins to disorder at 0.5 L. After exposure of 1 L the (4.5X JS) structure appears. On heating the c (2X2) structure formed at room temperature transforms into the ~(4x4) structure. After an exposure of nearly ! L; the (45X 45) structure is formed. The threshold above which the formation of these two structures is observed (350 K), is the same as is found for adsorption at higher temperatures. 3.3 Work function changes The method used has been described in a previous paper [ 11. Fig. 4 shows the observed changes. Small discrepancies have been observed between the values obtained with the same face in different apparatus. The discrepancies are affecting the values of the maximum and minimum only and not their position; the overall variations are identical. The c (2X2) structure is characterized by a decrease in work function. In the second stage (1 to 3.5 L) a sharp initial increase is observed, which is followed by a plateau. Above 3.5 L exposure, the work function increases again up to the saturation value, 1.5 eV above the clean surface value. 3.4. Energy loss spectroscopy With this technique the plasmon energy can be measured together with some interband transitions [9,10,18]. New transitions resulting from chemisorption can be measured. They are related to induced electronic levels. This technique has been used with a retarding grid field analyzer, e.g. for adsorption on nickel [ 191. Owing to the low resolution of our retarding field analyzer (about 1%) only the main peaks (surface and volume plasmon peaks) are observed in the distribution curve. In the derivative curve (diV(E)/dE versus E), however, weaker peaks can be
188
R. Riwan et al/Oxygen
adsorption on clean MO (100) surfaces
measured in relatively good agreement with the results obtained with a high resolution electron spectrometer [9]. The results can be summarized as follows. Above 1 L the observed surface plasmon peak (10.5 eV) is progressively shifted to a constant value of 11.25 eV, which is attained at the end of the microfaceting stage. The derivative curve shows that up to 1.5 L a loss peak with a mean value of 7 eV increases its intensity. Next a new loss peak situated at 5.5 eV appears and, at saturation, only one peak at 6 eV remains.
4. Discussion Three stages are deduced from the LEED structures formed during the oxygen adsorption at room temperature. 4.1. First stage: c (2X2) structure range This stage is characterized by the growth of a dominant c (2X2) structure and a weak c (6X 2) structure. The c (2X2) structure has its maximum intensity for 1 L exposure. If our coverage calibration (section 3.1) is correct, the density of oxygen atoms at 1 L corresponds to 19= 0.5 (fig. 1). Thus the formation of a c(2X2)O structure for an oxygen coverage of 0.5 implies that dissociation occurs on the surface. In addition, an initial sticking coefficient of 0.5 is deduced from fig. 1. It differs from the results obtained by Madey [ 15 ] on the (100) tungsten surface, where a continuous decrease from an initial value of unity was observed. Linnett and Lambert [l l] have also reported an initially linear uptake during the oxygen adsorption on the (111) molybdenum surface. The first stage is also characterized by a decrease of the work function (see fig. 4). Such a behaviour is consistent with the penetration of oxygen atoms into the surface layer of molybdenum. Thus we are led to consider the c (2X2) structure as reconstructed. This implies a high mobility of both oxygen and molybdenum atoms. The mobility is confirmed by the microfacets which are formed in the next stage. This mobility is also responsible for the structural modifications which appear at the relatively low temperatures used. The new structures obtained at higher temperatures are not easily interpreted in the present state of our investigations. 4.2. Second stage: microfaceting The behaviour of the streaks in the LEED patterns can be attributed to (110) microfacets, with a length of several hundred Angstroms in the [ 1001 direction and a very small width in the [ 1 lo] direction. At the beginning of this stage, a two-dimensional MO (110) p (2X2) 0 structure is (consistent with the behaviour of the (3 0) and (0 3) streaks. The disappearance of these half order streaks is attributed to a p (1X 1) 0 layer on the (110) facets. These
189
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
two-dimensional structures are among the structures which have been reported by Dooley and Haas [6] on the (110) face and are identical to the structures developed on the corresponding W face [ 161. Following Tracy and Blakely [ 171, the length of the reflections due to the facets is used to determine the width of the facets. As a result the width is evaluated to be about 334 times the lattice parameter in the [ 1 lo] direction. In ref. 17 the facet systems on the (loo), (110) and (111) W surfaces are observed above 540 K. The fact that at room temperature facets are formed on MO shows the previously mentioned mobility at the surface. The increase of the work function is probably related also to the higher work function of the (110) face (5 .l 1 and 4.28 eV for clean MO (110) and MO(100) surfaces
PI). 4.3. Third stage In this stage the surface flattens again. The observed features in ELS (surface plasmon energy and other losses) are very similar to those observed for heavily oxidized surfaces. Therefore we tentatively interpret the p (3X 1) and p (1X 1) phases as surface oxides. For the ELS results we shall assume that the 7 eV loss can be attributed to a transition involving the level 6 eV below the Fermi level which is deduced from photoemission results on W [2 l] and on MO (100) [8] ..This level is the initial state of the 7 eV transition, the final level being an unoccupied state above the Fermi level. The 5.5 eV loss may be tentatively attributed to oxygen adsorbed on (110) facets.
5. Conclusion The results of the present study indicate that in the early stage of adsorption (0 to 3.5 L) there is a penetration of oxygen atoms into the lattice, which causes reconstruction at room temperature of the molybdenum surface. This behaviour is very different from that of W (100) [22].
References J. Lecante, C. Guillot and R. Riwan, Surface Sci. 35 (1973) 271. P.J. Estrup and J. Anderson, J. Chem. Phys. 46 (1967) 563. K. Hayek, H.E. Farnsworth and R.L. Park, Surface Sci. 10 (1968) 429. H.K. Khan and S. Feuerstein, J. Chem. Phys. 50 (1969) 3618. D. Tabor and J. Wilson, J. Crystal Growth 9 (1971) 60. G.J. Dooley and J.W. Haas, J. Chem. Phys. 52 (1970) 461; 3. Vacuum Sci. Technol. (1970) 1570. [7] P.J. Estrup, private communication (1972).
[l] [2] [3] [4] [5] [6]
7
190
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
[S] (a) R. Cinti and N.E. El Khoury, unpublished results;
(b) T. Murotani, K. Fujiwara and M. Nishijima, Second Intern. Conf. on Solid Surfaces, Kyoto, 1974. [9] Y. Ballu and J. Lecante, Second Intern. Conf. on Solid Surfaces, Kyoto, 1974. [IO] L.N. Tharp and E.J. Scheibner, J. Appl. Phys. 38 (1967) 3320; G. Simmons and E.J. Scheibner, J. Appl. Phys. 43 (1972) 693. [ll] R.G. Musket and J. Ferrante, J. Vacuum Sci. Technol. 7 (1970) 14; R.M. Lambert, J. Linnett and J.A. Schwartz, Surface Sci. 26 (1971) 572. [12] Y. Viswanath and L.D. Schmidt, Chem. Phys. 59 (1973) 4184. [13] D.A. King, C.G. Goymour and J.T. Yates, Proc. Roy. Sot. (London) A 331 (1972) 331. [14] C. Guihot, J. Paignd, R. Riwan and J. Lecante, to be published. [15] T.E. Madey, Surface Sci. 33 (1972) 355. 1161 L.H. Germer and J.W. May, Surface Sci. 4 (1966) 452; B.J. Hopkins, C.B. Williams and PC. Wihmer, Surface Sci. 25 (1971) 633. [17] J.C. Tracy and J.M. Blakely, Surface Sci. 15 (1969) 257. [18] C. Guillot, J. Lecante and R. Riwan, Colloque de Physique et Chimie des Surfaces, Brest, 1973; Vide 164 (1973) 7. 1191 J. Kiippers, Surface Sci. 36 (1973) 53; F. Steinreisser and E.N. Sickafus, Phys. Rev. Letters 27 (1971) 992; S. Ohtani, K. Terada and Y. Murata, Phys. Rev. Letters 32 (1974) 415. [20] E. Chrzanowski, Symp. on Metal Surfaces, Gotherburg, 1973. [21] J.M. Baker and D.E. Eastman, J. Vacuum Sci. Technol. 10 (1973) 223. [22] J.L. Desplats, These de Doctorat, Paris, 1974.
OXYGEN ADSORPTION
ON CLEAN MO (100) SURFACES
R. RIWAN, C. GUILLOT
and J. PAIGNE
Service de Physique Atomique, 91190 Gifsur- Yvette, France
Centre d’Etudes Nuclhires
de Saclay, B.P. No. 2,
Oxygen adsorption on clean MO(100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2X 2) phase growing simultaneously with a (6X2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3x1) structure together with a p (1X1) structure, probably due to an oxide phase. Even in the low temperature range (370-500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4X4) structure which is followed by a (J5XdS)-R 26” 33 structure. The same occurs when the inital c (2X 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2X2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and MO.
1. Introduction Despite
the analogy
in the electronic
structure
and crystallographic
parameters
of
tungsten and molybdenum, significant differences have been observed in the carbon monoxide adsorption on the (100) face of these metals [ 1,2]. Therefore we extended our studies on this face to the adsorption of oxygen. In their LEED studies various authors [3-S] reported
that no new structures appeared at room temperature. Dooley and Haas f6], however, observed a ~(2x2) and ~(4x4) structure. Adsorption at high temperature (1020 K) was characterized by a successionofc(2X2j,p(lXl),(JjXJS)-R26”33,p(2X2),p(lXl)structuresat increasing exposures. Heating the saturation phase in vacua from 1070 to 1630 K leads to the following sequences: p(lXl),p(2X2),(JSXJJ),p(lXl),c(2X2),clean
[41,
R. Riwan et al.lOxyg,gen adsorption
184
(/5X&),
on clean MO (100) surfaces
(2X2), (2X1), c(4X4), clean [7]
A preliminary study of the effect of oxygen adsorption on the photoemission of MO(100) [8] has shown that besides states very sensitive to the surface condition, there exists a peak centered at about 6 eV below the Fermi level, which grows with oxygen adsorption. In energy loss spectroscopy (ELS) experiments Ballu and Lecante [9] have observed the appearance of new structures due to oxygen adsorption. They correspond to two energy losses centered at 7 eV and 5 eV.
2. Experimental The experimental details including the cleaning procedure were published in a previous paper [ 11. Energy loss spectroscopy measurements are performed with the three grid retarding field analyzer used for Auger spectroscopy [ 181. The results are compared with those obtained with a high resolution analyzer [9].
3. Results First the coverage and kinetics of adsorption at room temperature as by AES are described. Next LEED patterns resulting from adsorption at perature are presented, followed by LEED patterns obtained at elevated tures. Finally the effect of oxygen adsorption on the work function and ergy loss spectra are dealt with.
determined room temtemperaon the en-
3.1. AES results Determination of the oxygen coverage is rather difficult. As with tungsten, flash desorption is not well suited owing to desorption of reactive atomic oxygen and volatile oxides that can dissociate. The oxygen coverage 8 can be monitored by AES [ 111. Usually relative values can be obtained only. We determine the absolute value of f3 by comparing the oxygen Auger peak measured after exposure to oxygen with the corresponding peak observed after CO adsorption. A similar procedure was used by Viswanath and Schmidt [ 121. The oxygen peak for the c (2X2) 0 structure is twice that for the c (2X2) CO structure. Since there is no agreement whether CO adsorbed in the /I state is dissociated or not [ 131, we compared the carbon signal measured with CO adsorption with the value found for carbon layers, particularly with layers exhibiting the c (2 X 2) C structure. When the carbon Auger signal is assumed to be not affected by the presence of the neighbouring oxygen atom [ 141, the carbon content of the carbon c (2 X2) structure is twice that of the carbon monoxide c (2 X2) structure.
R. Riwan et aLlOxygen adsorption on clean MO(100) surfaces
185
COVERAGE
0
2
L
6
6
10
12
EXPOSURE
-
IL IL)
Fig. 1. Coverage versus exposure for room temperature adsorption.
Fig. 1 shows the intensity of the oxygen Auger peak during exposure to oxygen. Up to 8 = $(5X1O14 0 atom cmP2) a linear increase is observed. The saturation value is observed for an exposure of 12-l 3 Langmuir (1 L = lop6 torr set); the oxygen pressures were between 2X10Pg and 10e8 torr. 3.2. LEED results
Fig. 2. LEED patterns: (a) c (2~
2), 40
eV; (b) (6x2), 40 eV; (c) microfacets; (d) p (3X 1).
186
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
Fig. 3. LEED patterns: (a) disordered c (4X4); (b) ordered c (4X4); (c) disordered (fiXfi); (d) ordered (6X A).
At room temperature exposures up to 1 L lead to a rather diffuse c (2X 2) pattern. The broadening affects all integral order reflections. After 1 L exposure the intensity of the half order reflections attains a maximum. Simultaneously a weak (6X2) structure is observed at exposures of about 0.5 L (figs. 2a, 2b). After exposures from 1 to 3.5 L streaks running parallel to the [loo] directions, through the integral order reflections and (4 3) reflections, are observed (fig. 2~). At about 2.5 L the fractional order reflections and the lines passing through them begin to weaken. They disappear completely at 3 L. At this stage the integral streaks are slightly shortened. Exposures from 3.5 to 13 L cause the streaks to disappear progressively. A p (3X1) structure is developed (fig. 2d), which is transformed into a ~(1x1) structure at saturation. When adsorption is carried out between 3.50 and 500 K, instead of the c (2X2) a c (4X4) structure is observed, followed by a (6X &)-R 26”33 structure. These patterns are observed at temperatures near 350 K and higher. The structures are highly disordered, with a degree of order increasing with temperature. The order remains, however, far from the perfect order obtained by annealing in vacua at high temperatures (fig. 3). The disorder of the c (4X4) is revealed by
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
187
/<, 0 -0L
2
L e
6
8
10 12 EXPOSURE
IL (L)
Fig. 4. Work function change versus exposure for room temperature adsorption.
lines parallel to the [ 111, [ 1I] directions passing through the ( $ &), ( $ i), ( 3 3) reflections. At 500 K we observe the appearance of the c (4X4) structure with disorder lines after exposures of 0.1 L. The structure has its maximum intensity at 0.25 L; it begins to disorder at 0.5 L. After exposure of 1 L the (4.5X JS) structure appears. On heating the c (2X2) structure formed at room temperature transforms into the ~(4x4) structure. After an exposure of nearly ! L; the (45X 45) structure is formed. The threshold above which the formation of these two structures is observed (350 K), is the same as is found for adsorption at higher temperatures. 3.3 Work function changes The method used has been described in a previous paper [ 11. Fig. 4 shows the observed changes. Small discrepancies have been observed between the values obtained with the same face in different apparatus. The discrepancies are affecting the values of the maximum and minimum only and not their position; the overall variations are identical. The c (2X2) structure is characterized by a decrease in work function. In the second stage (1 to 3.5 L) a sharp initial increase is observed, which is followed by a plateau. Above 3.5 L exposure, the work function increases again up to the saturation value, 1.5 eV above the clean surface value. 3.4. Energy loss spectroscopy With this technique the plasmon energy can be measured together with some interband transitions [9,10,18]. New transitions resulting from chemisorption can be measured. They are related to induced electronic levels. This technique has been used with a retarding grid field analyzer, e.g. for adsorption on nickel [ 191. Owing to the low resolution of our retarding field analyzer (about 1%) only the main peaks (surface and volume plasmon peaks) are observed in the distribution curve. In the derivative curve (diV(E)/dE versus E), however, weaker peaks can be
188
R. Riwan et al/Oxygen
adsorption on clean MO (100) surfaces
measured in relatively good agreement with the results obtained with a high resolution electron spectrometer [9]. The results can be summarized as follows. Above 1 L the observed surface plasmon peak (10.5 eV) is progressively shifted to a constant value of 11.25 eV, which is attained at the end of the microfaceting stage. The derivative curve shows that up to 1.5 L a loss peak with a mean value of 7 eV increases its intensity. Next a new loss peak situated at 5.5 eV appears and, at saturation, only one peak at 6 eV remains.
4. Discussion Three stages are deduced from the LEED structures formed during the oxygen adsorption at room temperature. 4.1. First stage: c (2X2) structure range This stage is characterized by the growth of a dominant c (2X2) structure and a weak c (6X 2) structure. The c (2X2) structure has its maximum intensity for 1 L exposure. If our coverage calibration (section 3.1) is correct, the density of oxygen atoms at 1 L corresponds to 19= 0.5 (fig. 1). Thus the formation of a c(2X2)O structure for an oxygen coverage of 0.5 implies that dissociation occurs on the surface. In addition, an initial sticking coefficient of 0.5 is deduced from fig. 1. It differs from the results obtained by Madey [ 15 ] on the (100) tungsten surface, where a continuous decrease from an initial value of unity was observed. Linnett and Lambert [l l] have also reported an initially linear uptake during the oxygen adsorption on the (111) molybdenum surface. The first stage is also characterized by a decrease of the work function (see fig. 4). Such a behaviour is consistent with the penetration of oxygen atoms into the surface layer of molybdenum. Thus we are led to consider the c (2X2) structure as reconstructed. This implies a high mobility of both oxygen and molybdenum atoms. The mobility is confirmed by the microfacets which are formed in the next stage. This mobility is also responsible for the structural modifications which appear at the relatively low temperatures used. The new structures obtained at higher temperatures are not easily interpreted in the present state of our investigations. 4.2. Second stage: microfaceting The behaviour of the streaks in the LEED patterns can be attributed to (110) microfacets, with a length of several hundred Angstroms in the [ 1001 direction and a very small width in the [ 1 lo] direction. At the beginning of this stage, a two-dimensional MO (110) p (2X2) 0 structure is (consistent with the behaviour of the (3 0) and (0 3) streaks. The disappearance of these half order streaks is attributed to a p (1X 1) 0 layer on the (110) facets. These
189
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
two-dimensional structures are among the structures which have been reported by Dooley and Haas [6] on the (110) face and are identical to the structures developed on the corresponding W face [ 161. Following Tracy and Blakely [ 171, the length of the reflections due to the facets is used to determine the width of the facets. As a result the width is evaluated to be about 334 times the lattice parameter in the [ 1 lo] direction. In ref. 17 the facet systems on the (loo), (110) and (111) W surfaces are observed above 540 K. The fact that at room temperature facets are formed on MO shows the previously mentioned mobility at the surface. The increase of the work function is probably related also to the higher work function of the (110) face (5 .l 1 and 4.28 eV for clean MO (110) and MO(100) surfaces
PI). 4.3. Third stage In this stage the surface flattens again. The observed features in ELS (surface plasmon energy and other losses) are very similar to those observed for heavily oxidized surfaces. Therefore we tentatively interpret the p (3X 1) and p (1X 1) phases as surface oxides. For the ELS results we shall assume that the 7 eV loss can be attributed to a transition involving the level 6 eV below the Fermi level which is deduced from photoemission results on W [2 l] and on MO (100) [8] ..This level is the initial state of the 7 eV transition, the final level being an unoccupied state above the Fermi level. The 5.5 eV loss may be tentatively attributed to oxygen adsorbed on (110) facets.
5. Conclusion The results of the present study indicate that in the early stage of adsorption (0 to 3.5 L) there is a penetration of oxygen atoms into the lattice, which causes reconstruction at room temperature of the molybdenum surface. This behaviour is very different from that of W (100) [22].
References J. Lecante, C. Guillot and R. Riwan, Surface Sci. 35 (1973) 271. P.J. Estrup and J. Anderson, J. Chem. Phys. 46 (1967) 563. K. Hayek, H.E. Farnsworth and R.L. Park, Surface Sci. 10 (1968) 429. H.K. Khan and S. Feuerstein, J. Chem. Phys. 50 (1969) 3618. D. Tabor and J. Wilson, J. Crystal Growth 9 (1971) 60. G.J. Dooley and J.W. Haas, J. Chem. Phys. 52 (1970) 461; 3. Vacuum Sci. Technol. (1970) 1570. [7] P.J. Estrup, private communication (1972).
[l] [2] [3] [4] [5] [6]
7
190
R. Riwan et aLlOxygen adsorption on clean MO (100) surfaces
[S] (a) R. Cinti and N.E. El Khoury, unpublished results;
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