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Oxygen induced surface segregation of Cu on the Au0.7Cu0.3(100) surface
Surface Science Letters North-Holland
247 (1991) L215-L220
Surface Science Letters
Oxygen induced surface segregation of Cu on the Au~.~Cu~.~(100) surface S. Nakanishi, Department Received
N. Fukuoka,
of Materials
1 August
IS. Kawamoto,
K. Umezawa,
Science, University of Osaka Prefecture,
1990; accepted
for publication
7 January
Y. Teraoka
and K. Nakahigashi
Mozu- Umemachi, Sakai 591, Japan
1991
The oxygen induced surface segregation of Cu on the Au ,,,Cu,,,(lOO) surface was investigated by means of LEED and APS techniques. The dissociative adsorption of Oz did not take place on this clean surface for a long time exposure at least up to 10“ L, and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the clean surface, the segregation of Au was clearly observed and the surface ~n~tration of Au was estimated to be about 86%, greater than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was observed. But, above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 x 4) LEED pattern was observed in a wide range of oxygen coverage. The maximum intensity of the (2 X 4) was observed at about 0.45 ML.
1. In~~uction The surface segregation of one constituent in a binary alloy system is a common phenomenon and has been extensively studied by many authors. As is well known, there is a tendency for the segregation of the constituent with a lower heat of vaporization, a lower surface energy or a smaller cohesive energy. On the other hand, the chemisorption of foreign atoms onto the alloy surface frequently changes the equilibrium concentration at the surface resulting in a chemisorption-induced surface segregation; in some cases, it results in an alternation of the segregation element. The binary alloy system of Cu-Au is one of the typical targets for a study of the segregation phenomena and widely investigated from experimental [l-5] and theoretical [6] standpoints. However, the chemisorption induced effects on this alloy surface have hardly been studied so far, especially in the Au rich regions such as the present Au 0.7Cu @.s,in spite of the importance for the systematic understandings of the segregation phenomena. In the case of the Au-rich sample, the surface top layer is thought to be occupied almost 0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers
completely by Au atoms due to a surface segregation of Au and, therefore, the oxygen chemisorption onto the surface is extremely depressed in contrast to the case of Cu-rich sample [7]. However, this does not mean the absence of the chemisorption-induced segregation; if we use the appropriate way to apply oxygen atoms forcibly onto the surface, we can reveal the chemisorption-induced segregation on this alloy surface. In the present work, we investigated the oxygen chemisorption-induced surface segregation of Cu on the (100) surface of AuO,,Cu,, single crystal alloy, using LEED (low energy electron diffraction), AES (Auger electron spectroscopy) techniques.
2, Experimental Experiments were performed using a standard LEED/AES system with hemispherical 4-grid electron optics. The base pressure of the working space was 2 x lo-’ Pa in optimum condition after a 15 h bake at 250°C. The AES measurements were performed at primary electron energy of 2
B.V. (North-Holland)
S. Nakanishi
et al. / Oxygen induced surface segregation of Cu on the Au, ,Cq,,(lOO)
kV, the current of 5 PA and the modulation of 1.5 to 5 v,,,. -I-heAuo.7Cuo.3 single crystal rod with a size of about 6 mm diameter x 20 mm length was grown by the Bridgman method using high quality source materials of Au(99.9998) and Cu(99.999W). The crystallization and the orientation of the pellet type sample (5 mm diameter X 1 mm thick) with the (100) surface cut from the single crystal rod were tested by the X-ray diffraction method. Some deviation (l-2 o ) of the orientation from the ideal (100) surface was recognized. The clean surfaces were prepared by the conventional Ar ion sputter-annealing procedure in vacuum. As the clean surface was very stable for the oxygen gas exposure, the surface oxygen coverage was controlled by the following procedures: (1) For the compulsive oxygen introduction onto the chemically stable clean surface, l-2 ML Cu was deposited onto the clean surface prior to the oxygen adsorption. (2) The Cu coated surface was fully exposed to oxygen gas and saturated with oxygen atoms. (3) In order to obtain the desired oxygen cover-
Electron
surface
age, the sample was heated up to an appropriate temperature leading to the reduction of the excess portion of oxygen due to the desorption from the surface. (4) To achieve the thermal equilibrium concentration, the sample was slowly cooled to room temperature. According to Canning [8], atomic oxygen created on the platinum hot filament can chemicsorb even on the pure gold surface. In the present work, however, this method was not employed, because of its inefficiency in our system.
3. Results and discussion The sputter-annealed clean surface of Au,,, Cu,,(lOO) showed a p(1 x 1) LEED pattern. No marked impurity was observed in AES measurements as shown in fig. 1. The clean surface was very stable against a long time exposure of residual gases in a UHV chamber. For the estimation of the relative sensitivity factor for Cu(60 eV) and
Energy
(eV)
Fig. 1. Auger spectrum from the clean surface of the Au~,,CU~.~(~OO).
S. Nakanishi
et ai. / Oxygen induced surface segregation of Cu on the Au,,Cu,,(lOO)
Au{70 eV) Auger signal, 1 ML copper was deposited on the clean surface where the calibration of the deposition rate was monitored by a quartz oscillator. The monolayer-over-growth of the deposition time and the first break point of the linear slope agreed well with the expected value derived from the deposition rate. Fig. 2 shows the Auger spectrum at a monolayer coverage of Cu deposition. From fig. 2, the relative sensitivity S of the Auger signal height of Cu with respect to that of Au was estimated to be about 1.4 by using the following relation, S=(Zi,-
ZC”)/(Z,”
- ZL”>9
0)
where Z,, and I,,, Zd, and Z& represent Auger signal intensities of Au and Cu obtained from initial clean surface and from the Cu coated surface, respectively. It was assumed reasonably that the mean escape depths of the Auger electrons both for the A~(70 eV) and the Cu(60 eV) are the same in the alloy crystal. Using the relative
sensitivity S = 1.4, the mean concentration [Cu] near the surface was calculated by
Ful = ~C”/(SI,” + ZC”).
of Cu
(2)
Fig. 3 shows the variations of Auger signal intensities for the two cases as a function of the heat treatment temperature. In the first case represented by curves (a) and (b) corresponding to the Au and Cu Auger signal, respectively, about 2 ML Cu was initially deposited onto the clean surface. The sample was heated and held at the relevant temperature for about 30 min and almost the same time was spend to cool the sample down to room temperature where the measurements were made. In the second case indicated by the curves (c), (d) and (e) for the Au, Cu and oxygen Auger signal, respectively, the Cu coated surface was exposed to the oxygen gas until its saturation coverage was reached and then the measurements were made with the same procedure as in the first case. From the behavior of curves (a) and (b) in
LB1
[Al
50
surface
60
Electron
70 Energy
80 (eV1
50
60
Electron
70 Energy
80 (eV1
Fig. 2. Auger spectra [A] and syntheses [B] of the spectra: Syntheses were performed by using two reference spectra for Au and Cu with help of the least square method.
S. Nakanishi et al. / Oxygen induced surface segregation
the first case, we can state that, for heat treatment temperatures above 300 o C, the excess Cu migrated into the bulk crystal far enough to reach the thermal equilibrium concentration near the surface, because both signals for Au (denoted by (a)) and Cu (by (b)) reach the values corresponding to that from the clean surface. In the second case, however, the Au (c) and Cu (d) Auger signals gradually change with increasing heat treatment temperature and, even above 300 o C, the intensity of the Cu signal was still kept at a high level away from that for the clean surface. Obviously, this indicates the presence of the oxygen chemisorption-induced surface segregation of Cu. The nearly parallel relationship between the Auger intensities of Cu (d) and oxygen (e) in the temperature range from 300” C to 600 o C means that the surface segregation is approximately proportional temperatures beyond 600 o C, Auger intensities for Au (c) and Cu (d) almost reach to that for the clean surface, in spite of the existence of the oxygen signal (e) and its change. This suggests the insensi-
0
0
I
I 100
I
, 200
I
of Cu on the Au,,,Cu,,,(IOO)
surface
bility of the induced segregation of Cu under low oxygen coverages. In order to confirm that oxygen atoms are located in the alloy surface, we also measure the variation of the Auger signal intensity as a function of the incident angle (defined as the angle with the surface) of the primary electron beam. The results are shown in fig. 4, where the data were taken after a mild heating ( > 200 o C) of the oxidized Cu coated sample. According to our previous report [9], the present results strongly suggest that the oxygen atoms are present at the topmost layer or very close to the surface, because the Auger signal becomes more surface-sensitive with a decreasing incident angle. In fact, if we assume oxygen atoms to be located at the surface, the calculated curve for the oxygen Auger intensity agreed well with the experimental result as shown by the dotted line in fig. 4, where the calculation was made by using our previous formula [9] with reasonable parameters. LEED observations were also carried out in the
, I I I I 400 500 300 TEMPERATURE('C)
I
I 600
I
700
Fig. 3. Variations of the Auger peak intensities as a function of the heat treatment temperature. In the case of curves (a) and (b), about 2 ML Cu was initially deposited on to the clean surface at room temperature. For curves (c), (d) and (e), oxygen was fully adsorbed after the 2 ML Cu deposition at room temperature.
S. Nakanishi et al. / Oxygen induced surface segregation of Cu on the Auo,,Cuo,,(IOO)
OL 0
I
I
10
#
I
INCIDENT &f&E
I
Q(deg.1
I
30
surface
1
Fig. 4. Variations of Auger intensity as a function of the incident angle (Y(in deg) of the primary electron beam.
course of the heat treatment. As indicated in fig. 5, (2 x 4) LEED patterns were mainly observed in a wide range of oxygen coverages. The maximum intensity of the (2 X 4) pattern was obtained at about 400” C, where the oxygen coverage was about 0.45 ML. Taking into account the weakness of the scattering factor for oxygen atoms alone, the diffraction of the (2 X 4) pattern suggests the
Fig. 5. LEED pattern of the p(2 X 4) single domain structure.
ordering of Au and Cu atoms in the surface layer, because of the fairly strong intensity of the observed extra spots. However, detailed structure analysis will be needed for the definite conclusion. Fig. 6 shows the concentration change of Cu and Au with respect to the oxygen coverage, where the oxygen coverage was estimated by using the reference signal for the monolayer coverage obtained by in situ measurements on the p(1 x 1)-O structure on .Fe(lOO) surface which is well established [lO,ll]. However, the difference of back scattering factors between Fe and the present alloy were not taken into account. Therefore, some errors are included in the oxygen coverage scale. The concentration for Cu and Au was derived from the formula (2) described above. From this figure, we can conclude that: (1) Au atoms segregate to the surface in the case of the clean surface; the surface concentration of Au is about 86%, greater than the bulk concentration of 70%. (2) At low oxygen coverages below 0.16 ML, the adsorbed oxygen does not change the segregation phenomena, or the oxygen induced surface segregation of Cu is negligibly small. (3) At high cover-
S. Nakanishi
et al. / Oxygen induced surface segregation of Cu on the Au,,Cu,,,(lOO)
90
-@-Q---o-.
0.0
80
0 +--
Au
\
1
Au
O "\O
O\
\
surface
energy competition between the decrease in surface energy due to the oxygen chemisorption and the increasde by the exchange of Au and Cu atoms. Teraoka [12] has recently treated the detailed chemisorption effect on the segregation phenomena on the fee binary alloy crystal surface (100) within a framework of the Bragg-Williams approximation and obtained very similar results in a qualitative manner to the present experimental results shown in fig. 6.
BULK CONCENTRATION I
References
20
10 0
Fig. 6. Surface
concentration changes of [Au] and function of oxygen coverage.
[Cu] as a
ages beyond 0.2 ML, oxygen atoms induce Cu atoms to segregate to the surface almost proportionally to the coverage. The first result indicates the strong tendency of the segregation of Au atoms and agrees with the segregation behavior observed at the Cu,Au(lOO) surface [3]. The second and the third results may be explained in terms of the
[l] R.A. van Santen, L.H. Toneman and R. Bouwman, Surf. Sci. 47 (1975) 64. [2] J.M. McDavid and S.C. Fain, Jr, Surf. Sci. 52 (1975) 161. [3] T.M. Buck and G.H. Wheatley and L. Marchut, Phys. Rev. Let. 51 (1983) 43. [4] R.-S. Li, T. Koshikawa and K. Goto, Surf. Sci. 129 (1983) 192. [5] M.J. Sparnaay and G.E. Thomas, Surf. Sci. 135 (1983) 184. [6] J.M. Sanchez and J.L. Moran-Lopez, Surf. Sci. 157 (1985) L297. [7] G.W. Graham, Surf. Sci. 137 (1984) L79. [8] N.D.S. Canning, D. Outka and R.J. Madix, Surf. Sci. 141 (1984) 240. [9] S. Nakanishi, N. Fukuoka, K. Nakahigashi, M. Kogachi, H. Sasakura, S. Minamigawa and A. Yanase, Jpn. J. Appl. Phys. 28 (1989) L71. [lo] T. Horiguchi and S. Nakanishi, Jpn. J. Appl. Phys. Suppl. 2, part 2 (1974) 89. [ll] K.O. Leg& F.P. Jona, D.W. Jepsen and P.M. Marcus, J. Phys. C8 (1975) L492. [12] Y. Teraoka, Surf. Sci., submitted.
247 (1991) L215-L220
Surface Science Letters
Oxygen induced surface segregation of Cu on the Au~.~Cu~.~(100) surface S. Nakanishi, Department Received
N. Fukuoka,
of Materials
1 August
IS. Kawamoto,
K. Umezawa,
Science, University of Osaka Prefecture,
1990; accepted
for publication
7 January
Y. Teraoka
and K. Nakahigashi
Mozu- Umemachi, Sakai 591, Japan
1991
The oxygen induced surface segregation of Cu on the Au ,,,Cu,,,(lOO) surface was investigated by means of LEED and APS techniques. The dissociative adsorption of Oz did not take place on this clean surface for a long time exposure at least up to 10“ L, and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the clean surface, the segregation of Au was clearly observed and the surface ~n~tration of Au was estimated to be about 86%, greater than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was observed. But, above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 x 4) LEED pattern was observed in a wide range of oxygen coverage. The maximum intensity of the (2 X 4) was observed at about 0.45 ML.
1. In~~uction The surface segregation of one constituent in a binary alloy system is a common phenomenon and has been extensively studied by many authors. As is well known, there is a tendency for the segregation of the constituent with a lower heat of vaporization, a lower surface energy or a smaller cohesive energy. On the other hand, the chemisorption of foreign atoms onto the alloy surface frequently changes the equilibrium concentration at the surface resulting in a chemisorption-induced surface segregation; in some cases, it results in an alternation of the segregation element. The binary alloy system of Cu-Au is one of the typical targets for a study of the segregation phenomena and widely investigated from experimental [l-5] and theoretical [6] standpoints. However, the chemisorption induced effects on this alloy surface have hardly been studied so far, especially in the Au rich regions such as the present Au 0.7Cu @.s,in spite of the importance for the systematic understandings of the segregation phenomena. In the case of the Au-rich sample, the surface top layer is thought to be occupied almost 0039-6028/91/$03.50
0 1991 - Elsevier Science Publishers
completely by Au atoms due to a surface segregation of Au and, therefore, the oxygen chemisorption onto the surface is extremely depressed in contrast to the case of Cu-rich sample [7]. However, this does not mean the absence of the chemisorption-induced segregation; if we use the appropriate way to apply oxygen atoms forcibly onto the surface, we can reveal the chemisorption-induced segregation on this alloy surface. In the present work, we investigated the oxygen chemisorption-induced surface segregation of Cu on the (100) surface of AuO,,Cu,, single crystal alloy, using LEED (low energy electron diffraction), AES (Auger electron spectroscopy) techniques.
2, Experimental Experiments were performed using a standard LEED/AES system with hemispherical 4-grid electron optics. The base pressure of the working space was 2 x lo-’ Pa in optimum condition after a 15 h bake at 250°C. The AES measurements were performed at primary electron energy of 2
B.V. (North-Holland)
S. Nakanishi
et al. / Oxygen induced surface segregation of Cu on the Au, ,Cq,,(lOO)
kV, the current of 5 PA and the modulation of 1.5 to 5 v,,,. -I-heAuo.7Cuo.3 single crystal rod with a size of about 6 mm diameter x 20 mm length was grown by the Bridgman method using high quality source materials of Au(99.9998) and Cu(99.999W). The crystallization and the orientation of the pellet type sample (5 mm diameter X 1 mm thick) with the (100) surface cut from the single crystal rod were tested by the X-ray diffraction method. Some deviation (l-2 o ) of the orientation from the ideal (100) surface was recognized. The clean surfaces were prepared by the conventional Ar ion sputter-annealing procedure in vacuum. As the clean surface was very stable for the oxygen gas exposure, the surface oxygen coverage was controlled by the following procedures: (1) For the compulsive oxygen introduction onto the chemically stable clean surface, l-2 ML Cu was deposited onto the clean surface prior to the oxygen adsorption. (2) The Cu coated surface was fully exposed to oxygen gas and saturated with oxygen atoms. (3) In order to obtain the desired oxygen cover-
Electron
surface
age, the sample was heated up to an appropriate temperature leading to the reduction of the excess portion of oxygen due to the desorption from the surface. (4) To achieve the thermal equilibrium concentration, the sample was slowly cooled to room temperature. According to Canning [8], atomic oxygen created on the platinum hot filament can chemicsorb even on the pure gold surface. In the present work, however, this method was not employed, because of its inefficiency in our system.
3. Results and discussion The sputter-annealed clean surface of Au,,, Cu,,(lOO) showed a p(1 x 1) LEED pattern. No marked impurity was observed in AES measurements as shown in fig. 1. The clean surface was very stable against a long time exposure of residual gases in a UHV chamber. For the estimation of the relative sensitivity factor for Cu(60 eV) and
Energy
(eV)
Fig. 1. Auger spectrum from the clean surface of the Au~,,CU~.~(~OO).
S. Nakanishi
et ai. / Oxygen induced surface segregation of Cu on the Au,,Cu,,(lOO)
Au{70 eV) Auger signal, 1 ML copper was deposited on the clean surface where the calibration of the deposition rate was monitored by a quartz oscillator. The monolayer-over-growth of the deposition time and the first break point of the linear slope agreed well with the expected value derived from the deposition rate. Fig. 2 shows the Auger spectrum at a monolayer coverage of Cu deposition. From fig. 2, the relative sensitivity S of the Auger signal height of Cu with respect to that of Au was estimated to be about 1.4 by using the following relation, S=(Zi,-
ZC”)/(Z,”
- ZL”>9
0)
where Z,, and I,,, Zd, and Z& represent Auger signal intensities of Au and Cu obtained from initial clean surface and from the Cu coated surface, respectively. It was assumed reasonably that the mean escape depths of the Auger electrons both for the A~(70 eV) and the Cu(60 eV) are the same in the alloy crystal. Using the relative
sensitivity S = 1.4, the mean concentration [Cu] near the surface was calculated by
Ful = ~C”/(SI,” + ZC”).
of Cu
(2)
Fig. 3 shows the variations of Auger signal intensities for the two cases as a function of the heat treatment temperature. In the first case represented by curves (a) and (b) corresponding to the Au and Cu Auger signal, respectively, about 2 ML Cu was initially deposited onto the clean surface. The sample was heated and held at the relevant temperature for about 30 min and almost the same time was spend to cool the sample down to room temperature where the measurements were made. In the second case indicated by the curves (c), (d) and (e) for the Au, Cu and oxygen Auger signal, respectively, the Cu coated surface was exposed to the oxygen gas until its saturation coverage was reached and then the measurements were made with the same procedure as in the first case. From the behavior of curves (a) and (b) in
LB1
[Al
50
surface
60
Electron
70 Energy
80 (eV1
50
60
Electron
70 Energy
80 (eV1
Fig. 2. Auger spectra [A] and syntheses [B] of the spectra: Syntheses were performed by using two reference spectra for Au and Cu with help of the least square method.
S. Nakanishi et al. / Oxygen induced surface segregation
the first case, we can state that, for heat treatment temperatures above 300 o C, the excess Cu migrated into the bulk crystal far enough to reach the thermal equilibrium concentration near the surface, because both signals for Au (denoted by (a)) and Cu (by (b)) reach the values corresponding to that from the clean surface. In the second case, however, the Au (c) and Cu (d) Auger signals gradually change with increasing heat treatment temperature and, even above 300 o C, the intensity of the Cu signal was still kept at a high level away from that for the clean surface. Obviously, this indicates the presence of the oxygen chemisorption-induced surface segregation of Cu. The nearly parallel relationship between the Auger intensities of Cu (d) and oxygen (e) in the temperature range from 300” C to 600 o C means that the surface segregation is approximately proportional temperatures beyond 600 o C, Auger intensities for Au (c) and Cu (d) almost reach to that for the clean surface, in spite of the existence of the oxygen signal (e) and its change. This suggests the insensi-
0
0
I
I 100
I
, 200
I
of Cu on the Au,,,Cu,,,(IOO)
surface
bility of the induced segregation of Cu under low oxygen coverages. In order to confirm that oxygen atoms are located in the alloy surface, we also measure the variation of the Auger signal intensity as a function of the incident angle (defined as the angle with the surface) of the primary electron beam. The results are shown in fig. 4, where the data were taken after a mild heating ( > 200 o C) of the oxidized Cu coated sample. According to our previous report [9], the present results strongly suggest that the oxygen atoms are present at the topmost layer or very close to the surface, because the Auger signal becomes more surface-sensitive with a decreasing incident angle. In fact, if we assume oxygen atoms to be located at the surface, the calculated curve for the oxygen Auger intensity agreed well with the experimental result as shown by the dotted line in fig. 4, where the calculation was made by using our previous formula [9] with reasonable parameters. LEED observations were also carried out in the
, I I I I 400 500 300 TEMPERATURE('C)
I
I 600
I
700
Fig. 3. Variations of the Auger peak intensities as a function of the heat treatment temperature. In the case of curves (a) and (b), about 2 ML Cu was initially deposited on to the clean surface at room temperature. For curves (c), (d) and (e), oxygen was fully adsorbed after the 2 ML Cu deposition at room temperature.
S. Nakanishi et al. / Oxygen induced surface segregation of Cu on the Auo,,Cuo,,(IOO)
OL 0
I
I
10
#
I
INCIDENT &f&E
I
Q(deg.1
I
30
surface
1
Fig. 4. Variations of Auger intensity as a function of the incident angle (Y(in deg) of the primary electron beam.
course of the heat treatment. As indicated in fig. 5, (2 x 4) LEED patterns were mainly observed in a wide range of oxygen coverages. The maximum intensity of the (2 X 4) pattern was obtained at about 400” C, where the oxygen coverage was about 0.45 ML. Taking into account the weakness of the scattering factor for oxygen atoms alone, the diffraction of the (2 X 4) pattern suggests the
Fig. 5. LEED pattern of the p(2 X 4) single domain structure.
ordering of Au and Cu atoms in the surface layer, because of the fairly strong intensity of the observed extra spots. However, detailed structure analysis will be needed for the definite conclusion. Fig. 6 shows the concentration change of Cu and Au with respect to the oxygen coverage, where the oxygen coverage was estimated by using the reference signal for the monolayer coverage obtained by in situ measurements on the p(1 x 1)-O structure on .Fe(lOO) surface which is well established [lO,ll]. However, the difference of back scattering factors between Fe and the present alloy were not taken into account. Therefore, some errors are included in the oxygen coverage scale. The concentration for Cu and Au was derived from the formula (2) described above. From this figure, we can conclude that: (1) Au atoms segregate to the surface in the case of the clean surface; the surface concentration of Au is about 86%, greater than the bulk concentration of 70%. (2) At low oxygen coverages below 0.16 ML, the adsorbed oxygen does not change the segregation phenomena, or the oxygen induced surface segregation of Cu is negligibly small. (3) At high cover-
S. Nakanishi
et al. / Oxygen induced surface segregation of Cu on the Au,,Cu,,,(lOO)
90
-@-Q---o-.
0.0
80
0 +--
Au
\
1
Au
O "\O
O\
\
surface
energy competition between the decrease in surface energy due to the oxygen chemisorption and the increasde by the exchange of Au and Cu atoms. Teraoka [12] has recently treated the detailed chemisorption effect on the segregation phenomena on the fee binary alloy crystal surface (100) within a framework of the Bragg-Williams approximation and obtained very similar results in a qualitative manner to the present experimental results shown in fig. 6.
BULK CONCENTRATION I
References
20
10 0
Fig. 6. Surface
concentration changes of [Au] and function of oxygen coverage.
[Cu] as a
ages beyond 0.2 ML, oxygen atoms induce Cu atoms to segregate to the surface almost proportionally to the coverage. The first result indicates the strong tendency of the segregation of Au atoms and agrees with the segregation behavior observed at the Cu,Au(lOO) surface [3]. The second and the third results may be explained in terms of the
[l] R.A. van Santen, L.H. Toneman and R. Bouwman, Surf. Sci. 47 (1975) 64. [2] J.M. McDavid and S.C. Fain, Jr, Surf. Sci. 52 (1975) 161. [3] T.M. Buck and G.H. Wheatley and L. Marchut, Phys. Rev. Let. 51 (1983) 43. [4] R.-S. Li, T. Koshikawa and K. Goto, Surf. Sci. 129 (1983) 192. [5] M.J. Sparnaay and G.E. Thomas, Surf. Sci. 135 (1983) 184. [6] J.M. Sanchez and J.L. Moran-Lopez, Surf. Sci. 157 (1985) L297. [7] G.W. Graham, Surf. Sci. 137 (1984) L79. [8] N.D.S. Canning, D. Outka and R.J. Madix, Surf. Sci. 141 (1984) 240. [9] S. Nakanishi, N. Fukuoka, K. Nakahigashi, M. Kogachi, H. Sasakura, S. Minamigawa and A. Yanase, Jpn. J. Appl. Phys. 28 (1989) L71. [lo] T. Horiguchi and S. Nakanishi, Jpn. J. Appl. Phys. Suppl. 2, part 2 (1974) 89. [ll] K.O. Leg& F.P. Jona, D.W. Jepsen and P.M. Marcus, J. Phys. C8 (1975) L492. [12] Y. Teraoka, Surf. Sci., submitted.