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LEED-Auger study of clean and carbon covered cobalt (100) surface
Surface Science 0 North-Holland
LEED-AUGER
7 1 (I 978) 495-502 Publishing Company
STUDY OF CLEAN AND CARBON COVERED COBALT (100)
SURFACE * Received
22 March 1977; manuscript
received
in final form
11 October
1977
In spite of the great interest of cobalt in catalytic reactions and its important role as a transition metal with properties intermediate between iron and nickel, no LEED study of its surface properties had been reported in the literature up to recent times. Only very recently, some results on the (0001) face of hcp cobalt have been published [ 11. The lack of studies is probably due to the great difficulties encountered in preparing a clean surface of a cobalt single crystal. The difficulties mostly derive from two circumstances: the transition from hcp to fee that cobalt undergoes at about 420°C and the contamination from carbon commonly present in the available samples, as in the case of nickel [21. Because of these difficulties, we tried various treatments to obtain a surface that could be considered “clean” at least from the operative point of view for a LEED study. The surface structures due to carbon diffusion from the bulk as a consequence of thermal treatments were also investigated. Measurements were carried out in standard three-grid LEED-Auger apparatus, equipped with quadrupole mass spectrometer. Typical residual pressures of less than I X lop9 Torr were obtained in the system. The tetnperature of the satnple was measured by means of a thermocouple spot-welded on the sample holder very near to the crystal. The sample, obtained from Semi Elements Inc., was cut from a cobalt single crystal, with 99.995% nominal purity (50 ppm metallic impurities, the carbon content being unspecified). The specimen surface was parallel, within *lo, to the (100) plane of the fee form, i.e. the form stable above 42O”C, retained at room temperature. Before installation in the vacuum chamber the sample was electropolished at high current density with a standard solution used for iron alloys. After this treatment the surface appeared smooth on an optical microscope scale. The contaminants present on the initial surface, as controlled by AES in the LEED chamber, were sulfur, carbon, chlorine, oxygen and nitrogen. As previously noted, at room temperature the sample was in the fee form, so that heating at temperatures above 420°C did not involve any risk of transformation. However, this risk did exist during the cooling of the sample. It has been reported in the literature that the complete transformation from the fee to the hcp form is observed only at very low cooling rates. As in our apparatus the cooling 495
496
M. Magktta,
G. Rovida / IXl:%)--Auger
study of clear1 and C-covered Co(lO0) surfbce
took place at the rate of SO-60”C/min in the temperature range of the transformation, the usual cleaning procedures were reasonably free from this risk in our case. A thermal treatment at high temperature was not effective to clean the surface. After a preliminary argon ion bombardment at 2.50 eV (2pA/cm2 current density: 4 h) no Auger signal of impurities other than carbon was detectable in the Auger spectrum. In the following the ratio of the peak-to-peak amplitude of the carbon peak to that of the L3M2,3M2,3 cobalt peak (at about 655 eV) will be conventionally indicated as “C/Co ratio”. The elimination of carbon by ion bombardment was found to be a slow process. To reduce the carbon content, the sample was heated in presence of oxygen at increasing temperatures. Although Berning has reported the efficiency of oxygen in carbon removal from cobalt at pressures as low as 2 X 10d8 Torr [l], treatments in oxygen at pressures up to 10e6 Torr and temperatures up to 800°C proved to be ineffective in our case. In fact, at temperatures from 450°C up to 750°C the oxygen reaction rate was not high enough to obtain significant results in a few hours. On the other hand, above 750°C oxygen could not react with carbon, as the latter was dissolved into the bulk. In this case carbon segregation to the surface was again observed during the cooling to room temperature.
r
400
200 T
t:ig. 1. C/Co ratio 1 min at increasing
(dotted line) and intensity temperatures.
602
(OC)
of the (l/2
0) spot (dashed
tine) after heating
for
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(100)
surface
491
Then we tried to clean the sample by means of cycles of ion bombardment at 250 eV (2pA/cm’ current density) followed by annealing above 45O’C. However, after processing the sample for many days the annealing, even at temperatures as low as 250-300°C caused a remarkable increase in the carbon peak amplitude (the C/Co ratio becoming as high as one). A small oxygen signal was also detected in the Auger spectrum after the annealing at all temperatures. This suggested that, at least partially, the increase in the carbon peak amplitude was due to chemisorption of carbon monoxide from the residual gases atmosphere. Auger spectra recorded at constant time intervals, beginning from the fresh bombarded surface, allows us to confirm this hypothesis. The amplitude of the carbon and oxygen peaks increased slowly at the same rate, maintaining a ratio of about 4/l, i.e. that expected for l/l stoichiometric ratio. It is also worth noting that, while heating the sample, we observed a desorption of CO from the sample holder, with a broad maximum between 150 and 2OO’C. Therefore, the choice of the most suitable annealing temperature was complicated by the CO presence and chemisorption, in addition to the mentioned carbon diffusion. For these reasons, we recorded the amplitude of the carbon Auger signal after successive heatings for one minute each at increasing temperature (fig. 1, dotted line). The peak height increased slowly up to the temperature of about 200°C (stage I), while above 200°C the increase was clearly more pronounced (stage II). We attributed the increase in stage I mainly to CO chemisorption. Taking into account also the corresponding observations of the LEED pattern (fig. 1, dashed line), we considered that the carbon diffusion becomes significant only beginning from stage II. It is worth noting that 1 min of heating at 400°C was found to be a sufficiently brief period to give no change in the crystal structure detectable in the LEED pattern, although 4OO’C is very near to the transition temperature. These results indicated that the annealing temperature could not be higher than 15O’C; therefore, we used low energy ion bombardments (150-180 eV) in order the surface damage and to be able to rearrange the surface even at those low annealing temperatures. This procedure gave rather encouraging results. Fig. 2 shows the Auger spectrum directly after ion bombardment (curve a) and either a successive heating at 150°C for 3 min (curve b). The C/Co ratio is about l/l0 (curve b) while the oxygen signal is at the noise level. The final surface corresponded to a fairly satisfactory LEED pattern in which only the cobalt spots were visible. The splitting of the spots present after ion bombardment, indicating preferential ejection of the metal atoms in the [OOl] and [OlO] directions, has been eliminated: their spreading was not too important and there was no trace of faceting. Therefore, heating at about 150°C appeared to be suitable to lead the surface to an acceptable long range order. We approximately evaluated the amount of carbon present on the surface at the end of the cleaning procedures on the basis of data obtained exposing the sample to CO up to formation of a c(2 X 2) superstructure [3]. As in these conditions the C/Co ratio is about 0.6, assuming a coverage 19= 0.5 for the c(2 X 2)CO, we ob-
498
M. Muglietia, c. Rovida /LEE&Auger
study of.ciean and C-covered &(!&?J
surface
dN(E) -i-z-
bT-----
0
200
“..__
400
600
8%
E (eV)
I-lg. 2. Auger spectra from Co(100) under different conditions: (a) directly after 40 min ion bombardment at 180 eV; (b) after 3 min annealing at 150°C; (c) after heating at 400°C for 2 min (surface covered by the p(2 X 2) superstructure). Ep = 2500 eV. Modulation: (a) and (b) 5 V rms; (c) 3 V rms.
tamed B = 0.04 after ion bo~nbardn~ent and 8 = 0.08 after annealing. In the most unfavorable case of dissociative adsorption of carbon and oxygen the total coverage from contaminants should be lower than two tenths of a monolayer. These surface conditions represented the best obtainabIe in our apparatus with our sample. However, at this point we were not yet sure that either the presence of impurities or an excessive surface roughness could not affect significantly the LEED intensities. Hence, we decided to correlate the evolution of the LEED intensity versus energy curves with that of the Auger spectrum and of the appearance of the LEED pattern after heating at increasing temperatures. This approach proved useful also in the study of the carbon surface segregation. Fig. 3. shows a series of intensity-energy curves for the (i0) beam at normal incidence, directly after ion bombardnlent and after heating for one minute at increasing temperature up to 400°C. In this temperature range the carbon diffusion to the surface gives origin to a bidimensional phase that is completely formed at about 400°C (fig. 3, curve 7). In agreement with fig. 1, fig. 3 shows that this phase causes structural alterations only after heating to at least 250°C. On the other hand it is apparent from fig. 3 that heating at 150°C already causes structural modifica-
i0
90
110
130
150
1
E teV)
Fig. 3. Intensity versus energy curves for the (TO) beam at normal incidence, directly after ion bombardment at 180 eV (curve 1) and after heating at increasing temperatures for 1 min up to formation of the p(2 X 2) superstructure (curves 2-7). For comparison the curve from the CO saturated surface is also reported (broken line).
tions, whereas the subsequent curve (after heating at 200°C) is substantially unaltered. Taking into account the strong variations observed in the LEED pattern in correspondence with curves 1 and 2, the main difference between the two curves were attributed to the reordering of the surface. In order to exclude that the above mentioned differences could be due to CO &onta~nation, we compared the intensity-energy curves for the annealed and not annealed surface with those recorded after saturation with CO. Since for the (i0) beam the effect of CO adsorption (fig. 3, broken line) did not allow definitive con-
500
hf. Maglietta,
G. Rovida
(00)
I
30
50
Co(100)
surjhce
beam
90
70 E
Fig. 4. Intensity in fig. 3; broken
study of clean and C-covered
/ LEIYD-Auger
110
130
150
(eW
versus energy cursus for the (00) beam line, after saturation with CO.
at 10” incidence
angle:
(1) and (2) as
elusions to be reached, the comparison was extended to the (00) beam, as shown in fig. 4. In this case the difference between the curve after CO saturation and that after annealing is sufficiently enhanced to exclude any significant effect of CO contamination on the curves recorded after annealing. Therefore, ion bombardment at 150-180 eV followed by annealing at 150°C can be considered effective enough to yield a Co(100) surface suitable for a LEED intensity analysis [4]. In these conditions, the cobalt surface does not exhibit reconstruction, as expected. No comparison with the degree of cleanliness obtained by Berning is possible, since this author does not present Auger spectra. When heating the sample over 250°C the spots of a p(2 X 2) superstructure become visible in the LEED pattern. In the temperature range 2506350°C the appearance of this phase is very rapid. This is in agreement with the increase in the carbon peak amplitude (fig. 1, stage II). Between 350 and 450°C in the LEED pattern there is no appreciable variation and the increase in the carbon peak amplitude is considerably slower (fig. 1, stage III, and fig. 2). Above 450°C the ampli-
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(100)
surface
501
tude rises abruptly (fig. 1, stage IV), the C/Co ratio becoming rapidly of the order of 10/l. To determine more precisely the temperature at which the surface coverage with the p(2 X 2) phase is actually complete we measured the intensity of the (l/2 l/2) beam at normal incidence as a function of temperature (fig. 1, dashed line). This curve exhibits the same trend as that of the carbon peak amplitude up to 400°C. At this temperature the intensity curve exhibits a maximum, thus indicating the attainment of the maximum coverage. At higher temperatures the carbon diffusion causes a decrease in the spot intensity. Upon heating at about SOO”C, rings are observed in the LEED diagram, superimposed to the p(2 X 2) pattern. The spacing corresponds to that of the basal planes of graphite, indicating that crystallines of this solid are formed, with the basal plane parallel to the Co(100) surface and random azymuthal orientation. After heating at 600°C the intensity along the rings is mainly concentrated in twelve symmetric directions, indicating that azimuthal orientation also takes place at high temperatures, with the graphite [21.0] direction parallel to the cobalt [OlO] or [OOl]. Presence of graphite was also indicated by the fine structure of the carbon Auger peak, as previously reported [7]. On the other hand, the Auger line shape indicates that carbon in the p(2 X 2) superstructure is present as a surface carbide (fig. 2, lower curve). Since this phase is observed at temperatures higher than SOO”C, this surface carbide is more stable than the bulk carbides [5,6]. It is also worth noting that in the p(2 X 2) superstructure the (l/2 0) and symmetric beams are very weak, particularly at normal incidence. The extinction of these beams depends on the arrangement of the atoms on the surface, i.e. can be explained only with a structural analysis. In order to evaluate the carbon concentration in the p(2 X 2) phase, we compared its Auger peak amplitude with that found when the surface is saturated with CO. Since the tine structure of the carbon Auger peak is very different in the two cases, we integrated the derivative curve in order to obtain the N(E) versus E distribution. By comparing the areas of the two peaks we derived for the p(2 X 2) a carbon concentration of (1.4 +_0.3) X 10” atoms/cm*. The error in the area measurement is rather high because of the uncertainty in the position of the base-line. However, the reported value indicates that more than one carbon atom is present per unit mesh in the p(2 X 2)superstructure. This may be useful for the determination of the structure of this phase by LEED intensity analysis. In ref. [l] a superstructure due to carbon the (0001) face of hcp cobalt is reported. Even if some doubt could arise because of the presence of nitrogen on the surface, the hypothesis formulated to explain that superstructure may be supported by our observation of a p(2 X 2) phase on Co(lO0). M. MAGLIETTA Instituto di Chimica Fisica, Umiversitridi Firenze, Via G. Capponi 9, SO1 21 Firenze, Italy
and G. ROVIDA
502
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(lO0) surface
References [l] [2] [3] [4]
G.L.P. Berning, Surface Sci. 61 (1976) 673. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. G. Rovida and M. Maglictta, to be published. M. Maglietta, E. Zanazzi, F. Jona, D.W. Jepsen and P.M. Marcus, (I 977) 355. [ 51 M.M.G. Drain and A. Michel, Bull. Sot. Chim. Franc. (195 1) 5 17. [6] S. Nagakura, J. Phys. Sot. Japan 16 (1961) 1213. [7] J.T. Grant and W. Haas, Surface Sci. 24 (1971) 332.
Bull. Am. Phys.
Sot.
22
LEED-AUGER
7 1 (I 978) 495-502 Publishing Company
STUDY OF CLEAN AND CARBON COVERED COBALT (100)
SURFACE * Received
22 March 1977; manuscript
received
in final form
11 October
1977
In spite of the great interest of cobalt in catalytic reactions and its important role as a transition metal with properties intermediate between iron and nickel, no LEED study of its surface properties had been reported in the literature up to recent times. Only very recently, some results on the (0001) face of hcp cobalt have been published [ 11. The lack of studies is probably due to the great difficulties encountered in preparing a clean surface of a cobalt single crystal. The difficulties mostly derive from two circumstances: the transition from hcp to fee that cobalt undergoes at about 420°C and the contamination from carbon commonly present in the available samples, as in the case of nickel [21. Because of these difficulties, we tried various treatments to obtain a surface that could be considered “clean” at least from the operative point of view for a LEED study. The surface structures due to carbon diffusion from the bulk as a consequence of thermal treatments were also investigated. Measurements were carried out in standard three-grid LEED-Auger apparatus, equipped with quadrupole mass spectrometer. Typical residual pressures of less than I X lop9 Torr were obtained in the system. The tetnperature of the satnple was measured by means of a thermocouple spot-welded on the sample holder very near to the crystal. The sample, obtained from Semi Elements Inc., was cut from a cobalt single crystal, with 99.995% nominal purity (50 ppm metallic impurities, the carbon content being unspecified). The specimen surface was parallel, within *lo, to the (100) plane of the fee form, i.e. the form stable above 42O”C, retained at room temperature. Before installation in the vacuum chamber the sample was electropolished at high current density with a standard solution used for iron alloys. After this treatment the surface appeared smooth on an optical microscope scale. The contaminants present on the initial surface, as controlled by AES in the LEED chamber, were sulfur, carbon, chlorine, oxygen and nitrogen. As previously noted, at room temperature the sample was in the fee form, so that heating at temperatures above 420°C did not involve any risk of transformation. However, this risk did exist during the cooling of the sample. It has been reported in the literature that the complete transformation from the fee to the hcp form is observed only at very low cooling rates. As in our apparatus the cooling 495
496
M. Magktta,
G. Rovida / IXl:%)--Auger
study of clear1 and C-covered Co(lO0) surfbce
took place at the rate of SO-60”C/min in the temperature range of the transformation, the usual cleaning procedures were reasonably free from this risk in our case. A thermal treatment at high temperature was not effective to clean the surface. After a preliminary argon ion bombardment at 2.50 eV (2pA/cm2 current density: 4 h) no Auger signal of impurities other than carbon was detectable in the Auger spectrum. In the following the ratio of the peak-to-peak amplitude of the carbon peak to that of the L3M2,3M2,3 cobalt peak (at about 655 eV) will be conventionally indicated as “C/Co ratio”. The elimination of carbon by ion bombardment was found to be a slow process. To reduce the carbon content, the sample was heated in presence of oxygen at increasing temperatures. Although Berning has reported the efficiency of oxygen in carbon removal from cobalt at pressures as low as 2 X 10d8 Torr [l], treatments in oxygen at pressures up to 10e6 Torr and temperatures up to 800°C proved to be ineffective in our case. In fact, at temperatures from 450°C up to 750°C the oxygen reaction rate was not high enough to obtain significant results in a few hours. On the other hand, above 750°C oxygen could not react with carbon, as the latter was dissolved into the bulk. In this case carbon segregation to the surface was again observed during the cooling to room temperature.
r
400
200 T
t:ig. 1. C/Co ratio 1 min at increasing
(dotted line) and intensity temperatures.
602
(OC)
of the (l/2
0) spot (dashed
tine) after heating
for
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(100)
surface
491
Then we tried to clean the sample by means of cycles of ion bombardment at 250 eV (2pA/cm’ current density) followed by annealing above 45O’C. However, after processing the sample for many days the annealing, even at temperatures as low as 250-300°C caused a remarkable increase in the carbon peak amplitude (the C/Co ratio becoming as high as one). A small oxygen signal was also detected in the Auger spectrum after the annealing at all temperatures. This suggested that, at least partially, the increase in the carbon peak amplitude was due to chemisorption of carbon monoxide from the residual gases atmosphere. Auger spectra recorded at constant time intervals, beginning from the fresh bombarded surface, allows us to confirm this hypothesis. The amplitude of the carbon and oxygen peaks increased slowly at the same rate, maintaining a ratio of about 4/l, i.e. that expected for l/l stoichiometric ratio. It is also worth noting that, while heating the sample, we observed a desorption of CO from the sample holder, with a broad maximum between 150 and 2OO’C. Therefore, the choice of the most suitable annealing temperature was complicated by the CO presence and chemisorption, in addition to the mentioned carbon diffusion. For these reasons, we recorded the amplitude of the carbon Auger signal after successive heatings for one minute each at increasing temperature (fig. 1, dotted line). The peak height increased slowly up to the temperature of about 200°C (stage I), while above 200°C the increase was clearly more pronounced (stage II). We attributed the increase in stage I mainly to CO chemisorption. Taking into account also the corresponding observations of the LEED pattern (fig. 1, dashed line), we considered that the carbon diffusion becomes significant only beginning from stage II. It is worth noting that 1 min of heating at 400°C was found to be a sufficiently brief period to give no change in the crystal structure detectable in the LEED pattern, although 4OO’C is very near to the transition temperature. These results indicated that the annealing temperature could not be higher than 15O’C; therefore, we used low energy ion bombardments (150-180 eV) in order the surface damage and to be able to rearrange the surface even at those low annealing temperatures. This procedure gave rather encouraging results. Fig. 2 shows the Auger spectrum directly after ion bombardment (curve a) and either a successive heating at 150°C for 3 min (curve b). The C/Co ratio is about l/l0 (curve b) while the oxygen signal is at the noise level. The final surface corresponded to a fairly satisfactory LEED pattern in which only the cobalt spots were visible. The splitting of the spots present after ion bombardment, indicating preferential ejection of the metal atoms in the [OOl] and [OlO] directions, has been eliminated: their spreading was not too important and there was no trace of faceting. Therefore, heating at about 150°C appeared to be suitable to lead the surface to an acceptable long range order. We approximately evaluated the amount of carbon present on the surface at the end of the cleaning procedures on the basis of data obtained exposing the sample to CO up to formation of a c(2 X 2) superstructure [3]. As in these conditions the C/Co ratio is about 0.6, assuming a coverage 19= 0.5 for the c(2 X 2)CO, we ob-
498
M. Muglietia, c. Rovida /LEE&Auger
study of.ciean and C-covered &(!&?J
surface
dN(E) -i-z-
bT-----
0
200
“..__
400
600
8%
E (eV)
I-lg. 2. Auger spectra from Co(100) under different conditions: (a) directly after 40 min ion bombardment at 180 eV; (b) after 3 min annealing at 150°C; (c) after heating at 400°C for 2 min (surface covered by the p(2 X 2) superstructure). Ep = 2500 eV. Modulation: (a) and (b) 5 V rms; (c) 3 V rms.
tamed B = 0.04 after ion bo~nbardn~ent and 8 = 0.08 after annealing. In the most unfavorable case of dissociative adsorption of carbon and oxygen the total coverage from contaminants should be lower than two tenths of a monolayer. These surface conditions represented the best obtainabIe in our apparatus with our sample. However, at this point we were not yet sure that either the presence of impurities or an excessive surface roughness could not affect significantly the LEED intensities. Hence, we decided to correlate the evolution of the LEED intensity versus energy curves with that of the Auger spectrum and of the appearance of the LEED pattern after heating at increasing temperatures. This approach proved useful also in the study of the carbon surface segregation. Fig. 3. shows a series of intensity-energy curves for the (i0) beam at normal incidence, directly after ion bombardnlent and after heating for one minute at increasing temperature up to 400°C. In this temperature range the carbon diffusion to the surface gives origin to a bidimensional phase that is completely formed at about 400°C (fig. 3, curve 7). In agreement with fig. 1, fig. 3 shows that this phase causes structural alterations only after heating to at least 250°C. On the other hand it is apparent from fig. 3 that heating at 150°C already causes structural modifica-
i0
90
110
130
150
1
E teV)
Fig. 3. Intensity versus energy curves for the (TO) beam at normal incidence, directly after ion bombardment at 180 eV (curve 1) and after heating at increasing temperatures for 1 min up to formation of the p(2 X 2) superstructure (curves 2-7). For comparison the curve from the CO saturated surface is also reported (broken line).
tions, whereas the subsequent curve (after heating at 200°C) is substantially unaltered. Taking into account the strong variations observed in the LEED pattern in correspondence with curves 1 and 2, the main difference between the two curves were attributed to the reordering of the surface. In order to exclude that the above mentioned differences could be due to CO &onta~nation, we compared the intensity-energy curves for the annealed and not annealed surface with those recorded after saturation with CO. Since for the (i0) beam the effect of CO adsorption (fig. 3, broken line) did not allow definitive con-
500
hf. Maglietta,
G. Rovida
(00)
I
30
50
Co(100)
surjhce
beam
90
70 E
Fig. 4. Intensity in fig. 3; broken
study of clean and C-covered
/ LEIYD-Auger
110
130
150
(eW
versus energy cursus for the (00) beam line, after saturation with CO.
at 10” incidence
angle:
(1) and (2) as
elusions to be reached, the comparison was extended to the (00) beam, as shown in fig. 4. In this case the difference between the curve after CO saturation and that after annealing is sufficiently enhanced to exclude any significant effect of CO contamination on the curves recorded after annealing. Therefore, ion bombardment at 150-180 eV followed by annealing at 150°C can be considered effective enough to yield a Co(100) surface suitable for a LEED intensity analysis [4]. In these conditions, the cobalt surface does not exhibit reconstruction, as expected. No comparison with the degree of cleanliness obtained by Berning is possible, since this author does not present Auger spectra. When heating the sample over 250°C the spots of a p(2 X 2) superstructure become visible in the LEED pattern. In the temperature range 2506350°C the appearance of this phase is very rapid. This is in agreement with the increase in the carbon peak amplitude (fig. 1, stage II). Between 350 and 450°C in the LEED pattern there is no appreciable variation and the increase in the carbon peak amplitude is considerably slower (fig. 1, stage III, and fig. 2). Above 450°C the ampli-
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(100)
surface
501
tude rises abruptly (fig. 1, stage IV), the C/Co ratio becoming rapidly of the order of 10/l. To determine more precisely the temperature at which the surface coverage with the p(2 X 2) phase is actually complete we measured the intensity of the (l/2 l/2) beam at normal incidence as a function of temperature (fig. 1, dashed line). This curve exhibits the same trend as that of the carbon peak amplitude up to 400°C. At this temperature the intensity curve exhibits a maximum, thus indicating the attainment of the maximum coverage. At higher temperatures the carbon diffusion causes a decrease in the spot intensity. Upon heating at about SOO”C, rings are observed in the LEED diagram, superimposed to the p(2 X 2) pattern. The spacing corresponds to that of the basal planes of graphite, indicating that crystallines of this solid are formed, with the basal plane parallel to the Co(100) surface and random azymuthal orientation. After heating at 600°C the intensity along the rings is mainly concentrated in twelve symmetric directions, indicating that azimuthal orientation also takes place at high temperatures, with the graphite [21.0] direction parallel to the cobalt [OlO] or [OOl]. Presence of graphite was also indicated by the fine structure of the carbon Auger peak, as previously reported [7]. On the other hand, the Auger line shape indicates that carbon in the p(2 X 2) superstructure is present as a surface carbide (fig. 2, lower curve). Since this phase is observed at temperatures higher than SOO”C, this surface carbide is more stable than the bulk carbides [5,6]. It is also worth noting that in the p(2 X 2) superstructure the (l/2 0) and symmetric beams are very weak, particularly at normal incidence. The extinction of these beams depends on the arrangement of the atoms on the surface, i.e. can be explained only with a structural analysis. In order to evaluate the carbon concentration in the p(2 X 2) phase, we compared its Auger peak amplitude with that found when the surface is saturated with CO. Since the tine structure of the carbon Auger peak is very different in the two cases, we integrated the derivative curve in order to obtain the N(E) versus E distribution. By comparing the areas of the two peaks we derived for the p(2 X 2) a carbon concentration of (1.4 +_0.3) X 10” atoms/cm*. The error in the area measurement is rather high because of the uncertainty in the position of the base-line. However, the reported value indicates that more than one carbon atom is present per unit mesh in the p(2 X 2)superstructure. This may be useful for the determination of the structure of this phase by LEED intensity analysis. In ref. [l] a superstructure due to carbon the (0001) face of hcp cobalt is reported. Even if some doubt could arise because of the presence of nitrogen on the surface, the hypothesis formulated to explain that superstructure may be supported by our observation of a p(2 X 2) phase on Co(lO0). M. MAGLIETTA Instituto di Chimica Fisica, Umiversitridi Firenze, Via G. Capponi 9, SO1 21 Firenze, Italy
and G. ROVIDA
502
M. Maglietta, G. Rovida / LEED-Auger
study of clean and C-covered Co(lO0) surface
References [l] [2] [3] [4]
G.L.P. Berning, Surface Sci. 61 (1976) 673. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. G. Rovida and M. Maglictta, to be published. M. Maglietta, E. Zanazzi, F. Jona, D.W. Jepsen and P.M. Marcus, (I 977) 355. [ 51 M.M.G. Drain and A. Michel, Bull. Sot. Chim. Franc. (195 1) 5 17. [6] S. Nagakura, J. Phys. Sot. Japan 16 (1961) 1213. [7] J.T. Grant and W. Haas, Surface Sci. 24 (1971) 332.
Bull. Am. Phys.
Sot.
22